Point-light walkers and stick-figures rendered orthographically and without self-occlusion do not contain any information as to their depth. For instance, a frontoparallel projection could depict a walker from the front or from the back. Nevertheless, observers show a strong bias towards seeing the walker as facing the viewer (FTV, Vanrie and Verfaille, 2006 Perception & Psychophysics 68 601-612). A related stimulus, the silhouette of a stationary human figure (Kayahara, 2003, http://www.procreo.jp) does not seem to show a FTV bias. We created stimuli representing gradual transitions from the silhouette figure to a stick-figure. Rotating them slowly about a vertical axis, we asked observers to indicate if they saw clockwise or counterclockwise rotation. Measuring frequency and angle of perceptual reversals we derived a unbiased measure of the FTV bias. Results reveal that the FTV bias is not due to the presence or absence of walking behaviour and does not depend on the posture of the figure. It is a direct consequence of transitioning from a silhouette to the stick figure. The FTV bias can be explained assuming that the visual system perceives the marks (dots or sticks) to be on the surface of an opaque body and that it assumes a higher probability of them being on the front than on the back.

We investigate the pigeon's ability to discriminate the direction into which a biological motion point-light walker is facing. We do that for two reasons. First, it has been shown that pigeons have great difficulty to distinguish between mirror symmetric versions of the same static object. Here, we want to demonstrate that this is not the case for biological motion displays which they can readily distinguish, even if they are mirror symmetric versions of one another. Second, we ask whether pigeons discriminate facing direction based on motion-mediated form, or whether they rather rely on local cues.Using a two-alternative forced choice paradigm in which pigeons had to peck on one of two stimuli presented on a screen, eight pigeons were trained to discriminate between a right and a left facing walker depicting either a human or a pigeon. We then tested them with non-reinforced catch trials inserted into the continuing training sessions.In Experiment 1, these catch trials were walkers played backwards. These stimuli provide conflicting cues. The global structure points in one direction while the local motion cues the other direction. Six out of the eight birds clearly chose the direction indicated by the local motion. The other two birds based their decision on the global shape. In a number of additional experiments, we presented upright and inverted spatially scrambled versions of the displays. Only the birds that had indicated to use local motion cues in Experiment 1 could handle these tasks, the other two birds responded at random.The results show interesting individual differences between pigeons. While most of the birds rely on local motion to derive facing direction, others can handle global, motion-mediated shape. Each individual bird makes a clear decision for one or the other strategy, though. Results also show that pigeons have no problems to distinguish between mirror symmetric versions of the same stimulus as long as motion is involved.

Biological motion stick-figures rendered orthographically and without self-occlusions do not contain any information about the order of their elements in depth and therefore are consistent with at least two different in-depth interpretations. Interestingly, however, the visual system often prefers one over the other interpretation. In this study, we are investigating two different sources for such biases: the looking-from-above bias and the facing-the-viewer bias (Vanrie et al. 2004). We measure perceived depth as a function of the azimuthal orientation of the walker, the camera elevation, and the walker's gender, which have previously been reported to also affect the facing bias (Brooks et al, 2008). We also compare dynamic walkers with static stick-figure displays. Observers are required to determine whether 0.5 s presentations of stick-figures are rotating clockwise or counter-clockwise - basically telling us in that way which of the two possible in-depth interpretations they are perceiving. In contrast to previous work, this measure is entirely bias-free in itself. Data collected with this method show that the facing-the-viewer bias is even stronger than previously reported and that it entirely dominates the viewing-from-above bias. Effects of walker gender could not be confirmed. Static figures which imply motion result in facing biases which are almost as strong as obtained for dynamic walker. The viewing-from-above bias becomes prominent for the profile views of walkers, for which the facing-the-viewer bias does not apply, and for other depth ambiguous stimuli (such as the Necker cube). In all these cases, we find a very strong bias to interpret the 2D image in terms of a 3D scene as seen from above rather than from below. We discuss our results in the context of other work on depth ambiguous figures and look at differences between the initial percept as measured in our experiments and bistability observed during longer stimulus presentations.

Even when a display of a person walking is presented only as dots following the motion of the major joints, human observers can readily determine both the facing direction of locomotion and higher-level properties, including the gender of the individual. We investigated the spatial concentration of direction and gender cues, by tracking the eye movements of 16 participants while they judged either property of a point-light display. The walkers had different levels of ambiguity of both their direction and their gender, which affected the difficulty of the tasks. Fixation locations were recorded throughout the 2 s presentation times. We analyzed the fixation data in two ways: first by creating fixation maps for the different conditions, and second by finding the average number of fixations that fell into three ROIs, representing the shoulders, pelvis and feet. In accordance with past literature emphasizing the role of lateral shoulder sway in gender identification, participants on average fixated more on the shoulders in the gender task than in the direction task. Analysis of individual differences showed that more fixations in the shoulder region predicted slightly better performance in the gender task. On the other hand, the number of fixations on the pelvis, an area also known to contain gender information, was not significantly different between tasks. In accordance with studies showing that the motion of the feet contains cues to direction, participants fixated significantly more often on the feet in the direction task. The feet were rarely fixated in the gender task. In general, task difficulty did not have an effect on fixation patterns, except in the case of walkers viewed from the side, which produced on average slightly fewer feet fixations in the direction task.

Thornton et al. (2002, Perception) showed that detection of point-light walkers (biological motion) embedded in noise is impaired in the presence of a competing change detection task (dual-task). Gurnsey et al. (2010, Journal of Vision) argued that the presence of masking noise dots was critical to this effect. Therefore, we measured walker direction discrimination thresholds with variable noise levels under single- and dual task conditions (change detection). For most subjects-as predicted--direction thresholds were identical under single- and dual-task conditions in the absence of noise, but thresholds increased with noise in the dual task condition.Hebb Award AbstractThornton et al. (2002) asked observers to judge whether point-light walkers were heading ±90° from the line of sight while simultaneously performing a change detection task. They found walker direction discrimination was impaired in the presence of the competing task, suggesting that attention is critical to the detection of biological motion. However, a subsequent study by Gurnsey et al. (2010) showed that neither the addition of a colour discrimination task nor a radial frequency discrimination task impaired walker azimuth thresholds (the absolute angular difference from straight ahead). Although it is arguable that colour is separable from the form or motion information needed to discriminate walker direction the same cannot be said about the radial frequency task.It is possible that the discrepancy between these two results is related to Thornton et al.'s (2002) use of masking dots. Because the standard direction discrimination task is trivially easy in the absence of noise, Thompson et al. embedded their walkers in noise to limit performance. (This is common practice in the biological motion literature.) In the study by Gurnsey et al. (2010) azimuth thresholds at large sizes were ±1.5° on average. If biological motion is susceptible to resource competition then ±90° walkers should make fewer demands on attention than ±1.5° walkers. It may be that in the Thornton et al. study the observers' difficulty was with segregating walker from noise rather than with encoding the properties of the walker itself.In the present task we measured walker azimuth thresholds with and without noise in single- and dual-task conditions (as per Thornton et al., 2002). For four of our six observers we found that the competing task had no effect on thresholds in the absence of noise but thresholds increased linearly with noise in the dual task condition. This result is consistent with the idea noise impairs segmentation rather than sensitivity to properties of the walker. Three of these four subjects had extensive experience judging walkers in noise, and all four were avid video gamers. Two of our observers were impaired in the dual task even when no noise was present and exhibited much higher thresholds overall in the dual-task condition. One of these subjects had extensive experience judging point-light walkers without noise. Although we have evidence that noise alters the ability to segment walkers from noise, it seems that subject variables have a very large effect on performance in dual task experiments.

Theoretischer Hintergrund: Embodiment Theorien postulieren eine enge reziproke Wechselwirkung zwischen dem motorischen System und emotionalen Prozessen. In einer Serie von mehreren Studien wurde untersucht, ob sich (1) Unterschiede in den Gangmustern von akut und niemals depressiven Personen zeigen; (2) ob sich solche Gangcharakteristika auch bei nicht-depressiven Personen nach Induktion von trauriger Stimmung zeigen; (3) ob sich auch bei ehemals depressiven Patienten Auffälligkeiten im Gangmuster zeigen und (4) ob Mindfulness-based Cognitive Therapy (MBCT) einen normalisierenden Einfluss auf die Gangmuster ehemals depressiver Patienten hat. Methoden: Gangmuster von 14 akut depressiven Patienten, 23 ehemals depressiven Patienten, die an einem MBCT-Kurs teilnahmen und 29 niemals depressiven Versuchspersonen wurden analysiert. Ergebnisse: Die Gangmuster ehemals depressiver und akut depressiver Patienten unterscheiden sich von niemals depressiven VPn. MBCT hat einen teilweise normalisierenden Effekt auf das Gangmuster. Diskussion: Depressive Personen zeigen Auffälligkeiten im Bereich des motorischen Systems. MBCT verändert diese Auffälligkeiten und hat somit möglicherweise günstige Auswirkungen auf propriozeptives Feedback, das im Rückfallgeschehen eine wichtige Rolle spielen könnte.

According to embodiment theories, emotional states affect somatovisceral and motoric systems, whereas bodily states affect methods by which emotional information is processed. In the present research we investigated (1) whether dynamic gait patterns of currently and formerly depressed patients differ from never depressed people and (2) whether mindfulness-based cognitive therapy (MBCT) normalize gait patterns of formerly depressed patients. Motion data of 23 formerly depressed patients participating in MBCT, 14 currently depressive inpatients and 29 never depressed participants were collected with an optical motion capture system. The data was analyzed by fourier-based descriptions and computation of linear classifiers. Gait patterns of currently depressed patients as well as formerly depressed patients differed form never depressed people. Moreover, MBCT had some normalizing effect on the way patients walk. We conclude that training in mindfulness might change proprioceptive-bodily feedback that is important in the generation of depressive states.

The silhouette illusion depicts a rotating dancer. Published online (Kayahara, 2003) it has since travelled the internet. As any silhouette, the display is depth-ambiguous. Consequently, the direction of rotation is ambiguous as well. The online community has noticed that perceived rotation direction is biased toward one direction, and a number of hypotheses have been provided to explain this. Here, we systematically test the hypothesis that our visual system prefers to see the silhouette from above rather than from below. We varied camera elevation and show that the resulting biases do indeed account for the ones observed in the original illusion.

Meta-analysis and reviews on sexual recidivism consistently report thatsexual preference for children is one of the strongest predictors for reoffendingin child molesters. The aim of our study is to introduce a newpowerful implicit stimulus for the detection of sexual preference in childmolesters with and without the clinical diagnosis of paedophilia.During the early 1970s Gunnar Johansson (1973) introduced Point-Light-Walkers (PLWs) as a new stimulus to perception research. Hefound that 12 lights fixed at the major joints of a moving person aresufficient to identify socially relevant features and the kind of actionsdisplayed. Looking at the vast amount of literature on biological motionperception, we think that the ecological power combined with the ambiguityof PLWs make them appropriate as a new stimulus for forensicresearch dealing with sexual preference.The current study was performed in order to determine differences inattractiveness-ratings of PLWs between child molesters with (n = 44) andwithout (n = 21) the ICD-10 diagnosis of paedophilia and a controlgroup of non-sex offenders (n = 68). We assume that sexual preferencefor underage girls and boys respectively is associated with higher attractiveness-ratings for female or male walking-patterns of children.We used logistic regression analysis to differentiate between differentsubgroups of child molesters and non-sex offenders. Attractivenessratingsfor female walking-patterns of children were the strongest predic77tor for the diagnosis of paedophilia. Overall correct-classification ratesvaried from 74.6-76.1%, positive predictive values from 65.0-80.0%and negative predictive values from 68.8-77.6%. Our results on predictivepower are at least comparable to the findings of phallometric measures(Abel et al., 1998) and implicit association methods (Gray et al.,2005).Nevertheless any single psychometric measure can never be sufficientenough to verify or rule out an individual clinical diagnosis of paedophilia.

Our visual system can extract directional information even from spatially scrambled point-light displays (Troje & Westhoff, 2006). In three experiments, we measured saccade latencies to investigate how local features in biological motion affect attentional processes. Participants made voluntary saccades to targets appearing on the left or the right of a central fixation point, which were congruent, neutral or incongruent with respect to the facing direction of a centrally presented point-light display. In Experiment 1, we presented two kinds of human point-light walker stimuli (coherent and spatially scrambled) with three different viewpoints (left-facing, frontal view, right-facing) at two different stimulus durations (200 and 500 ms) to sixteen observers. The saccade latency of the incongruent condition was significantly longer compared to that of the congruent condition for the 200-ms coherent point-light walker stimuli, but not for the spatially scrambled stimuli. In Experiment 2, a new group of observers (N = 12) were presented with two point-light walker displays. The only difference with respect to Exp. 1 was, that in the scrambled version of the stimulus, the location of the dots representing the feet, was kept constant. Different from the results of Experiment 1, the saccade latency in the incongruent condition was significantly longer than that in the congruent condition irrespective of the stimulus types. In Experiment 3, we put into conflict the facing direction indicated by the local motion of the feet and the facing direction as indicated by the global structure of the walker by presenting newly recruited observers (N = 12) with backwards walking point-light walkers. In agreement with the results of Experiment 2, the modulation of the saccade latency was dependent on the direction of feet motion, irrespective of the postural structure of the walker. These results suggest that the local motion of the feet determines reflexive orientation responses.

The walking direction of a biological entity is conveyed by both global structure-from-motion information and local motion signals. Global and local cues also carry distinct inversion effects. In particular, the local motion-based inversion effect is carried by the feet of the walker. Here, we searched for a "super foot", defined as the motion of a single dot that conveys maximal directional information and carries a large inversion effect, by using a psychophysical procedure driven by a multi-objective evolutionary algorithm (MOEA). We report on two rounds of searches involving the evolution of 25-27 generations each (1000 trials/generation) conducted via a web-based interface. The search involved an eight-dimensional space spanned by amplitudes and phases of a 2nd-order fourier representation of the dot's motion in the image plane. On each trial, observers were presented with multiple copies of a "foot" chosen from a population of feet stimuli for the current generation and were required to indicate whether the perceived stimulus was right- or left- facing. The stimuli were shown at upright and inverted orientations. Upon completion of a generation, each stimulus was evaluated for its "fitness" based upon its ability to convey direction and carry an inversion effect from observer accuracy rates. The fittest stimuli were then selected to form a subsequent generation for testing via methods of crossover and mutation. We show that the MOEA was effective at driving increases in accuracy rates for the upright stimuli and increases in the inversion effect, quantified as the difference between upright and inverted stimuli, across generations. We show further that the two rounds of searches, beginning at different points in space, converge towards the same region. We characterize the "super foot" in relation to current theories about the importance of gravity-constrained dynamics for biological motion perception.

Human ability to perceive biological motion pattern is well established. Furthermore, it has been shown that older observers can be quite efficient at detecting biological motion. Recently, Legault & Faubert (VSS 2008) showed that young adult biological motion perception is influenced by distance in virtual space. Observers obtained good performance when a 1.8 meter biological motion target was located 1 meter or further but performances decreased dramatically at the shorter distance (less than a meter). The purpose of the present study was to determine if there is a difference between younger and older adult's performances when biological motion patterns are presented at different distances in virtual space. To create our setup, we used a full immersive virtual reality environment (CAVE), giving the observers an immersive 3D experience. We used a biological motion pattern composed of 13 dots, walking left or right on a treadmill. The size of the walker was 1.80 meters and it was shown at virtual distances from the observer of 0.50, 1, 2, 4 and 16 meters. The observer's task was to identify the walker's direction (left or right) in upright and inverted conditions. The walker was presented in a scrambled mask which was generated by randomly selecting dots with biological motion patterns and repositioning them in 3D space. Threshold mask density was determined using an adaptive staircase procedure. The results showed that older adults are influence by distance and that their performance begins to decrease at a distance of 4 meters, compared to young adults who perform well down to a distance of 1 meter. In other words, biological motion detection in noise, in upright and inverted conditions, depends on how far the walker is positioned in 3D virtual space and the critical distance where biological motion judgements break down highly depends on observer age.

Purpose: Size scaling compensates for eccentricity-dependent sensitivity loss in a point-light-walker (PLW) direction discrimination task (Gurnsey et al., Vision Research, 2008) and PLW direction discrimination thresholds reach similar asymptotically low levels at large sizes for eccentricities of 0 to 16° (Gurnsey et al., submitted). Here we ask how PLW direction discrimination thresholds change as a function of stimulus size and eccentricity for first and second order stimuli.
Methods: On each trial a PLW was shown moving left or right at an angle (±?°) from straight ahead. An adaptive threshold procedure was used to determine threshold ? at a range of stimulus sizes (uniform magnifications) at eccentricities from 0 to 16° in the right visual field. Second order walkers comprised uniform luminance dots embedded in dynamic noise (SO1) or vice versa (SO2). First order walkers were structurally identical to the second order walkers but had a higher mean luminance in the uniform luminance region; FO1 and FO2, respectively.
Results: Within each condition dividing stimulus size at each eccentricity (E) by an appropriate F = 1 + E/E2 (where E2 is the eccentricity at which stimulus size must double to achieve equivalent-to-foveal performance) collapsed all thresholds onto a single psychometric function. The average E2 values were: E2(S01) = 2.85, E2(S02) = 2.03, E2(F01) = 1.50 and E2(FO2) = 0.80; asymptotic thresholds averaged ±3.91°, ±3.83°, ±3.90° and ±4.17° respectively. However, SO1 stimuli could not be discriminated at 8 and 16° and had to be much larger at fixation in order for thresholds to be measured.
Conclusions: Second order signals can elicit PLW direction discrimination thresholds similar to first order signals. For second order stimuli, noise dots in a uniform background convey information about walker direction at much smaller sizes than do uniform dots in a noise background.

Traditional studies of acceleration perception have measured acceleration sensitivity in terms of the ratio of final to initial velocity or the proportion of change in velocity relative to the average velocity. From these studies, it is unclear as to how sensitivity to visual acceleration is affected by stimulus properties such as motion orientation, base velocity, and size. Here, we measured visual sensitivity to acceleration by parameterizing acceleration as it is defined: the change in velocity per unit time. Observers (n = 18) were asked to discriminate an accelerated stimulus from a constant velocity stimulus equated for mean velocity and size. Acceleration was adjusted according to the QUEST staircase procedure and thresholds, defined as the acceleration discriminated at the 82% correct-level, were obtained for positive and negative acceleration, horizontal and vertical motion, two base velocities, and two trajectory sizes. Consistent with previous findings, thresholds, if expressed according to proportion of velocity change relative to the base velocity were relatively constant across base velocities and sizes. Critically, we show that absolute acceleration thresholds varied in a manner analogous to Weber’s law. We show also that thresholds were better for motions along the horizontal axis than the vertical axis, but only at the high base velocity and smaller size. Furthermore, acceleration sensitivity was not affected by the sign of acceleration or stimulus direction within the principle axes. These findings are discussed in the context of predictions of acceleration sensitivity from previous data for the perception of animate and inanimate motions.

The perception of both biological motion and faces is widely reported to be orientation-dependent (i.e., is impaired when the stimulus is inverted). For the perception of biological motion, there are at least two distinct inversion effects: one that is based upon the retrieval of form from motion and another that is based on the local motion of the limbs. The orientation of a visual stimulus can however be described with respect to a variety of allocentric (e.g., gravity, visual-environment) and egocentric (e.g., head-based) frames of reference. In the standard experimental setting, such reference frames are all aligned. Here, we investigated the role of different reference frames for the perception of faces and biological motion by testing observers (n = 12) on face recognition and biological motion tasks inside the York University "tumbling room". Independent rotations of the room, observer, and stimulus enabled comparisons of retinal, visual-environmental, and gravitational frames of reference. The biological motion task required observers to discriminate the facing direction (left or right) of a treadmill point-light walker shown in sagittal view. The face recognition task required observers to determine whether two consecutively presented faces were of the same or different identities. Performances on both the biological motion and face recognition tasks were best when the stimulus was aligned with the observer, as compared to gravity or the room. Interestingly, performances were also better when the stimulus was aligned with gravity as compared to the room, but for the biological motion task only. The results suggest that the perception of both biological motion and faces operates in large part in accordance with an egocentric frame of reference.

We investigated the reference frames for the face and biological motion inversion effects by testing observers on face recognition and biological motion direction discrimination tasks inside the York University "tumbling room". Rotations of the room and the observer enabled comparisons of retinal, visual-environmental, and gravitational frames of reference. Performances on the biological motion task were best when the stimulus was aligned with the observer, and better when the stimulus was aligned with gravity rather than the room. Face recognition was best when the stimulus was aligned with the observer but did not differ between congruency with gravity or the room. The results suggest that the perception of both biological motion and faces operates in large part in accordance with an egocentric frame of reference.

Recently, Troje (2008; Troje & Westhoff, 2006) has suggested that the local motion contained in upright, scrambled biological motion displays can trigger a simple "life detection" mechanism. The goal of the present study was to further characterize this mechanism. In two experiments, we assessed participants' ability to make accurate direction-facing judgments about point-light displays presented very briefly in central vision. In both experiments, the walkers varied in terms of the amount of the configural information that was available in the displays, and with regard to their orientation (upright or inverted) and facing direction. In the first experiment (in which stimuli were unmasked) we found that heading could be discerned from upright, scrambled displays even with brief (170 ms) exposure durations. In the second experiment, we showed that local motion cues could support accurate heading judgments, regardless of the species depicted (human, cat or pigeon). In contrast, when viewers had to rely solely on global cues to make their heading judgments, their performance was disproportionately better with upright human displays. Exposure times in this experiment were 500 ms, and all stimuli were masked. Whether they had to rely on local or global cues to make their heading judgments, viewers in Experiment 2 (unlike those in Experiment 1) tended to show a bias to report seeing a right-facing walker. We speculate that the right-facing bias may be more apparent when longer exposure durations are used, or in situations where greater attentional resources are required (as is the case when a target must be disembedded from a mask). The right-facing bias is discussed in relation to the literature on attentional biases and specialized scanning habits associated with reading.

Pigeons (Columba livia) and their mutual courtship display represent a good animal model of interactive communication. Here we ask the question – Is pigeon courtship behaviour based on anticipation of social reactions and controlled by social feedback? In other words, is it inter-subjective in nature? Using a closed-circuit TV setup that allows the manipulation of real-time interaction between two pigeons, we manipulated social contingency and inter-individual timing, to test for the perception of social influence and social synchrony respectively. To test social synchrony perception we delayed the interaction by 0, 1, 3, or 9 sec.  To test social contingency perception subjects courted another pigeon either in real-time (contingent interaction) or on pre-recorded video (non-contingent). We then repeated this experiment, extending the duration of each condition from 2 to 6 minutes.  In all cases, our dependent measure was courtship intensity.  Our results show that pigeons adjust their display intensity depending on the presence of social contingencies during mutual visual display, an effect observed only when conditions last 6 minutes.  Pigeons, representing a highly social species, appear to possess true inter-subjectivity, a capacity that is known to be omnipresent in humans.

Innate sensitivity to characteristics of social stimuli, such as faces and biological motion, may facilitate learning about caregivers and other conspecifics. To date, most research has focused on early preferences for faces. Within two hours of birth newborns look preferentially towards three blobs arranged as facial features (e.g., Mondloch et al., 1999); likewise, visually inexperienced chicks preferentially approach the head and neck region of a hen (Johnson & Horn, 1988). These early preferences are not tuned to species-specific details of faces but serve two important functions: they facilitate rapid recognition of conspecifics (i.e., potential care-givers) and, at least in humans, allow for the later development of expert face processing (Le Grand et al., 2001). The present symposium will examine whether similar developmental principles apply to biological motion. The first two speakers will present evidence that both dark-reared chicks (Regolin) and human newborns (Simion) demonstrate a preference for biological motion over other patterns of motion. Like face perception, this preference is not species-specific; newborns look preferentially towards walking hens and chicks show a preference for walking cats. The ability of both chicks and human newborns to detect biological motion is impaired when stimuli are inverted, indicating that perception of biological motion is constrained by core knowledge of gravity. Despite similar patterns of sensitivity to faces and biological motion in early development, the two systems are differentially affected by early visual deprivation. The third presentation (Maurer) will draw on studies of children treated for bilateral congenital cataract to show that, unlike most visual functions, sensitivity to biological motion develops normally in the absence of early visual experience. The discussant (Troje), who is well known for his extensive studies of adults’ sensitivity to biological motion, will consider the implications of these findings for understanding social development and the origins of a “life detector”

If biological motion point-light displays are presented upside down, performance on most tasks is strongly impaired. We have recently shown that this inversion effect has two entirely different and independent causes. One is due to the inversion of the familiar upright shape. The second is related to a visual filter tuned to the gravity-constrained local motion of the feet of a human or animal in locomotion. Here, we are investigating whether the two inversion effects operate in retinal coordinates or in gravitational coordinates. We designed two different tasks isolating the structure-from-motion aspect of biological motion, on the one hand, and the mechanism tuned to local motion, on the other hand, and conducted experiments in which either the stimulus or the observer were turned upside down. The results clearly indicate that both inversion effects operate in retinal coordinates and are not affected by vestibular input. Apparently, a heuristic that gravity is aligned with retinal coordinates is replacing a reality check that would require visual-vestibular sensory integration.

There is a growing body of literature investigating point-light biological motion perception. Based solely on the kinematics of a handful of dots representing the body and major joints of a human actor, observers can extract complex information such as gender from point-light displays. Many previous studies have used artificially generated point-light animations to investigate critical features for gender discrimination (Cutting, 1978; Mather & Murdoch, 1994). Here we investigate the diagnostic cues for gender discrimination of natural point-light walkers using a technique similar to temporal "bubbles" (Thurman & Grossman, 2007), an adaptation of the "bubbles" technique (Gosselin & Schyns, 2001). We presented three full cycles of a point-light walker, randomly chosen from a set of 25 male and 25 female actors (Troje, 2002), while observers made forced-choice gender discriminations. On each trial, we removed a randomly chosen subset of frames from the animation and assessed performance as a function of frames present and absent. We reason that performance is best when a non-critical interval is removed, but declines when a critical interval is removed. Hence, our experiment identifies the temporal windows and diagnostic features that most often lead to a correct gender discrimination. Preliminary results suggest that hip sway, as reflected by the distance between the hip dots over time in the profile view, is a primary critical feature for discriminating gender in natural point-light displays. This result is consistent with previous studies using artificially generated point-light animations (Barclay et al., 1978; Cutting 1978). This interpretation is supported by the observation that male walkers in our data set with high levels of hip movement are consistently misclassified as female, and that females with low hip movement are typically misclassified as male.

Biological motion perception involves two distinct visual mechanisms. One is based on deriving the global shape of the agent, while the other is based on the local motion of individual dots. In this study, we present a new method that allows us to further characterize the two mechanisms. By measuring azimuth discrimination thresholds for point-light walkers that contain either only local information or only global structure, we avoid a number of confounds which make other methods less reliable. Our results confirm the dissociation between the two proposed mechanisms.

The question of whether individuals with autism spectrum disorder (ASD) are impaired in the perception of biological motion is as yet unresolved. Here adults with high-functioning autism and neurotypical controls judged the direction of motion of a normal or spatially scrambled point-light walker which, on each trial, walked from the centre of the screen either leftward or rightward at ~3 deg s-1 in a strip of scrambled walker noise of variable density. The walker appeared with variable onset time between 0 and 500 ms after the noise onset. While the ASD group showed slower reaction times and more errors in judging the direction of motion, their performance was otherwise comparable to controls; specifically, they showed superior performance for normal over scrambled walkers, for delayed over immediate onset, and they showed comparable increases in reaction time and error with noise density. Finally, error rates differed on left and rightward trials, an effect which distinguished ASD and control performance: this is discussed with reference to hemispheric asymmetry in local and global processing.

Objectives: (1) Do dynamic gait patterns of currently and formerly depressed patients differ from never depressed people (2) Does mindfulness-based cognitive therapy (MBCT) normalize gait patterns of formerly depressed patients? Methods: Gait patterns of 30 formerly depressed patients participating in MBCT, 14 currently depressive inpatients and 30 never depressed participants were analyzed by fourier-based descriptions and computation of linear classifiers. Results:  Gait patterns of currently depressed patients and formerly depressed patients differ form never depressed people. MBCT has some normalizing effect on the way patients walk. Conclusions: Mindfulness might change proprioceptive-bodily feedback important in the generation of depressive states.

Background and Aims: Biological motion can be conveyed through capturing videos of humans walking with point lights attached to their major joints, thus eliminating the appearance of mass, depth, bodily features. Studies consistently show that adult observers immediately identify biological motion displays as a human walking (Johansson, 1973). Additionally, for adults, scrambled biological motion which is completely devoid of structural information not only retains information about the direction of a walking human, but also is subject to a pronounced inversion effect such that the direction of inverted walkers is difficult to determine (Troje & Westhoff, 2006). Infants as young as 3-months of age are also sensitive to biological motion (e.g, Bertenthal et al., 1984; 1985); we tested here whether they can also detect the direction of walking, and if this is subject to an inversion effect. Procedure: 6-month-old infants were habituated to movies of an upright (Experiment 1) or inverted (Experiment 2) point-light walker who walked as if on a treadmill (i.e., there was no actual displacement across the screen). For half of the infants, the walk was to the right, and for half the walk was to the left. In test, infants saw the familiar direction of walking on one trial, and the new direction on a second, with order counterbalanced. Looking time to each display was recorded and analyzed. Results and Conclusions: When presented with an upright walker, infants were sensitive to a switch in the direction of walking from habituation to test. Infants looked longer at the new direction than the old in test trials (familiar direction M=7.64s, new direction M=14.03s; t(19)=2.86, p=.01). In contrast, when the walker was inverted, infants did not seem to notice the switch in direction, and looking times were equal across both trials (familiar direction M=10.06s, new direction M=8.61s; t(18)=.510, p=.616). Thus, young infants not only seem to recognize the difference between upright and inverted walkers and visually prefer the former (Bertenthal et al., 1984), but they also can detect the direction of motion of upright walkers. The detection of direction does not, however, extend to inverted walkers, suggesting the existence of an inversion effect.

Zusammenfassung: Der menschliche Gang dient nicht alleine der Fortbewegung, sondern ist darüber hinaus ein effektives Medium der sozialen Kommunikation. Auch anhand stark reduzierter Stimuli ist unser Wahrnehmungssystem aufgrund hoch spezialisierter Hirnareale in der Lage, eine Vielzahl an relevanten Informationen aus biologischen Bewegungsmustern effizient zu enkodieren. Zur Identifizierung von Geschlecht, Alter und Emotionslage des Gegenübers reichen uns meist wenige Sekunden. Mittels der in den 70er Jahren von Gunnar Johansson entwickelten Methode der Point-Light-Displays lässt sich die dynamische Information des Ganges von strukturellen anatomischen Eigenschaften trennen. Auf diese Weise wurden aus 15 Lichtpunkten bestehende prototypische kindliche und erwachsene 3D-Modelle beiderlei Geschlechts erzeugt und computergestützt einer Stichprobe von Kindesmissbrauchern (n = 65) und Kontrollprobanden (n = 68) präsentiert. In Abhängigkeit vom Alter der Point-Light-Walker zeigen sich unter anderem signifikante Gruppenunterschiede in den Attraktivitätsbeurteilungen der Stimuli. Basierend auf den Attraktivitäts-, Geschlechts- und Altersbeurteilungen der Probanden sollen mit Hilfe der Classification Regression Tree Analysis (CART) erste Klassifikationsergebnisse hinsichtlich unterschiedlicher Probandenmerkmale (z.B. sexuelle Orientierung, ICD-10-Diagnose Pädophilie, Opfergeschlecht und Opferalter) dargestellt werden.

A significant amount of past research has studied person identification from point light displays of walking humans, investigating parameters such as viewing angle and the differential contributions of structural and kinematic information. However, little is known about the ability of human observers to generalize identity across different activities. In this study we use a same/different paradigm to compare observers' ability to identify point light displays within and across activities. We drew from a database of 100 motion-captured humans, each of which encompassed both walking and running activities. Subjects were shown successive paired stimuli and had to indicate whether the stimuli represented the same or different person. In either case, the two displays were at slightly different viewpoints. Two independent factors were examined: stimulus pairing (walker/walker, runner/runner, walker/runner) and information content (structural only, kinematic only, full information). For all information contents for stimulus pairing of matching activities (walker/walker, runner/runner) subjects performed significantly better than chance (t(5)=2.71, p0.05). The main effect of Pairing was significant (F(2, 30)=35.7, p[[lt]]0.001), with the walker/runner pairing being the most difficult. Information was not a significant factor. However, there was a significant interaction between Pairing and Information (F(4, 30)=4.03, p[[lt]]0.01) that manifested in performance on the runner/runner task in particular being better for full information than for structural or kinematic only. Results are discussed in light of a principal components-based linear model that estimates a runner time series from a given walker time series by equating principal component coordinates.

The ability to discriminate direction from spatially scrambled point-light displays relies on the orientation of the foot dot motions (Troje & Westhoff, 2006). We present two experiments that investigated this local motion-based inversion effect by testing direction discrimination from novel biological motion displays that exaggerate and display solely foot-specific information. In Experiment 1, we isolated the foot motion of a treadmill human walker, human runner, cat, and pigeon and presented observers (n = 20) with 1000 ms displays consisting of 10 copies of two foot dots that traced 150 ms segments at counterphase positions of the gait cycle. For each foot type, we derived left and right signalling displays from five such segment pairs that collectively sampled the entire gait cycle and presented them at both upright and inverted orientations. Direction discrimination accuracies varied with foot type, orientation, and segment pair. Significantly, the decrease in accuracies due to inversion was most substantial for the runner stimuli which exhibit the most pronounced vertical velocity changes and smallest for the cat stimuli which carry little vertical motion. In Experiment 2, a new group of observers (n = 20) were presented with the natural human walker stimuli of Experiment 1 and with stimuli that were spatiotemporally-matched to the natural stimuli but moved with constant velocities. Here, overall discrimination accuracies did not differ per foot type, decreased with inversion, and varied with segment pair. Critically, performances were higher for upright than for inverted displays for the natural stimuli only. Upright and inverted versions of the constant velocity stimuli did not differ. The results suggest that the local inversion effect in biological motion perception is carried by the velocity gradients of the foot motions. We conjecture that the visual system is sensitive to characteristic velocity changes exhibited by biological movements in a gravity-driven environment.

We tested direction discrimination from biological motion stimuli that display only fragments of full foot trajectories at upright or inverted orientations. Results from observers presented with displays derived from counterphase fragments of different types of foot motions showed an inversion effect that was largest for stimuli derived from the human runner which exhibit pronounced vertical accelerations. Results from new observers presented with veridical human walker stimuli and stimuli that were identical but had accelerations removed showed an inversion effect for the veridical stimuli only. These findings suggest that the local inversion effect is carried by acceleration cues in foot motions.

Gunnar Johansson (1973) was the first to demonstrate that human observers can perceive animate activity solely from information about the movements of dots attached to the joints of an otherwise invisible figure. From even brief exposure to these dynamic "point light" displays, viewers are able to extract surprisingly detailed information, including information about the actor's gender and mental state (e.g., Troje, 2002). Recently, Troje and Westhoff (2005) have suggested that several independent processes are involved in biological motion perception, the most basic of which is a simple form of "life detection" that is automatically triggered by low-level, local motion cues. In support of this, they found that, even when configural information was disrupted through spatial scrambling of the dots comprising a point light walker, participants were still able to judge the direction the walker was facing quite accurately, provided the moving dots were presented in their normal (upright) orientation. Stimuli in this study were presented in central vision and viewing times were unlimited. In the present study, we replicated this basic result using very brief exposure durations (200 and 170 ms). We also went on to show that viewers could achieve above-chance direction discrimination performance with upright, scrambled displays when the targets were presented in peripheral vision. Performance in the periphery, however, varied as a function of the side of presentation and the direction the walker was facing. Specifically, while participants were better at processing right-facing (compared to left-facing) walkers in the right visual field, they were equally accurate at processing right- and left-facing walkers in their left visual field. This interaction was seen both with configural and scrambled displays. This interesting result is discussed with reference to recent studies examining hemispheric differences in spatial attention and body representation, and in the operation of the "mirror neuron" system.

Joint action is behaviour that requires coordination between animals to achieve a desired goal. We investigated the perception of social feedback in mutual courtship of the pigeon, Columba livia, in a double closed-loop teleconferencing setup. Pigeons could interact in real time with the life-sized video image of the other bird. We manipulated social feedback in two ways; 1) by altering temporal contiguity by three delays of 1s, 3s and 10 s and 2) by altering social contingency, by playing back a video of the subject’s partner from a previous interaction. Courtship intensity decreased in all three temporal delay conditions. Courtship intensity did not differ between a zero-delay condition and the non-contingent playback condition. We conclude that pigeon courtship is sensitive to temporal contiguity in social feedback, implying visual coordination in joint action. Pigeons did not show social contingency perception, and may lack a representation of social causality.  

Visual perception of biological motion is a complex process that involves several independent mechanisms. Particularly, two such mechanisms have to be distinguished. One responds to the local motion of the feet of a moving animal and signals both the presence and the facing direction of the animal. The second integrates the global configuration of a set of moving dots into the coherent, articulated shape of a human or animal body. We hypothesize that the first one is evolutionary old, not specific to human motion, and not sensitive to learning, while the second requires individual learning and is therefore specific to human motion. Here, we conducted two experiments. The first one required an observer to derive the direction in which a stationary walker was facing. The walker depicted either a human walker, a walking pigeon or a walking cat masked by a varying number of stationary flickering dots. Walkers were shown either spatially intact or scrambled. Five blocks of 60 trials each were run to probe for learning effects. The second experiment was a 2AFC detection experiment. In each trial, two displays were shown. One contained only a mask of scrambled walkers while the other one also contained a coherent walker. Walkers depicted a human, a pigeon, or a cat. Again, five blocks with 60 trials each were run to test for learning effects. Results confirmed our hypotheses: For the first task which focused on the local mechanism, we found effects of the number of masking dots, and an effect of scrambling, but neither an effect of the nature of the walker, nor an effect of learning. In contrast, for the second task (requiring global shape-from-motion processing) we found much better performance for the human walker as compared to the non-human walkers, and a strong effect of learning.

Background. It is currently believed that the cortical deficit associated with amblyopic vision extends beyond striate cortex into extrastriate areas. Biological motion perception has been localized to a specific extrastriate cortical region (STS) which receives input from both dorsal and ventral visual processing streams. We used a variety of biological motion perception tasks to assess the function of this extrastriate region in amblyopia. Methods. Amblyopic observers viewed biological motion stimuli with either their amblyopic or fellow fixing eye. A range of tasks were used to better characterize the ability of amblyopic eyes to perceive biological motion. Detection of a point light walker was measured using both scrambled walker masks and linear motion masks to modulate task difficulty. Walking direction discrimination was also measured using both scrambled walkers, which provided only motion information, and unscrambled walkers. These stimuli were embedded in linear dot masks of various densities. Results. Amblyopic eyes showed a deficit in biological motion detection. Amblyopic eyes did not however show a similar deficit for walking direction discrimination and could perform this task with both unscrambled and scrambled walkers. Conclusion. Amblyopic eyes are impaired at segregating a point light walker from a noise mask. However the ability to extract information from the biological motion of the walker dots showed little impairment.

In 1978, James Cutting published an algorithm to generate point-light displays that resemble the movements of the joints of a human walker. The method has since been used frequently to create stimuli for research on biological motion perception. More recently, Troje and Westhoff (2006) found that pattern of local movement of the feet was used to derive the direction in which a point-light walker is facing, even when structural information is removed. The results of previous studies that direction could not be determined using a scrambled version of Cutting's walker, may be explained by the significantly different motion of the feet between Cutting's walker and motion-captured humans. To compare the two stimuli, 14 participants performed a detection task and a direction task. Walkers consisted of 11 points presenting a sagittal view. In the detection task, walkers were embedded in a scrambled walker mask consisting of 50, 100, or 200 dots. Participants had to decide which of two successive intervals contained the walker. In the direction task, participants judged whether the walking figure was oriented towards the left or the right. The mask consisted of randomly appearing stationary dots (50, 200, or 750) with limited lifetime. Half of the walkers were spatially coherent and half of them were scrambled. Observers performed equally well for the two walkers in the detection task. However in the direction task, the error rate for Cutting's walker was significantly higher than for the motion-captured walker. Most of the difference came from the scrambled walker condition, where error rate increased from 39% to 48%. We conclude that Cutting's walker lacks critical features which signal direction in real walking motion, and suggest that studies which have presented the local motion of the Cutting walker as a stimulus need to be revisited.

Biological motion perception refers to the ability to perceive and interpret the movements of animate objects, in the absence of form cues (Johansson, 1973). To date, only one study has examined this ability in elderly observers (Norman et al., 2004). These authors reported that, particularly at longer exposure durations (400 ms), biological motion perception was well-preserved in older adults. Norman et al. assessed participants' ability to recognize specific activities that, in some instances, were partially occluded. In Experiment 1 of the present study, a walker was displayed (for 200 ms) in a frontal view on half of the trials; on the remaining trials a spatially scrambled walker was presented. Stimuli were shown either in isolation, or in a scrambled walker mask comprised of 25 to 150 dots. The task on each trial was to decide whether a coherent walker was present. In Experiment 2, a walker appeared on each trial, in a profile view, and participants indicated whether it was facing left or right. The stimulus was presented alone or in a field of masking dots, as described above. Healthy elderly and young adults performed essentially at ceiling levels on both tasks when no mask was added. The presence of masking dots, however, had a much more deleterious effect on the performance of elderly participants than on that of young adults. Indeed, elderly participants' performance fell to chance levels as more masking dots were added. While the two groups differed in terms of education, and scores on both the Token Test and Digit Symbol, these differences could not account for the impairment seen in elderly participants on the biological motion perception tasks. We conclude that healthy elderly show a marked impairment in their ability to perceive biological motion in the presence of visual noise, at short exposure durations.

Einleitung: Störungen der Wahrnehmung biologischer Bewegungen wurden bei Menschen mit autistischen Störungen mehrfach beschrieben. Im Rahmen einer größeren Studie zeigten sich Unterschiede der Hirnaktivierung in temporo-parietalen und frontalen Regionen von Jugendlichen mit autistischen Störungen bei der Beobachtung komplexer biologischer Bewegungen. Ziel dieser morphometrischen Studie ist es, mögliche anatomische Grundlagen der gestörten Verarbeitung der Beobachtung komplexer Bewegungen zu untersuchen. Methode: Dreizehn männlichen und zwei weiblichen Jugendliche mit autistischen Störungen nach DSM IV und eine gleiche Anzahl alters-, geschlechts- und IQ-gematchten Kontrollpersonen (Altersmittel 18 Jahre) wurden in einem 1,5 Tesla MRT untersucht (Siemens Sonata, Erlangen, Germany). Neben funktionellen Messungen (siehe Freitag et al. DGPPN 2007) wurden sagitale T1-gewichtete MPRAGE Sequenzen mit isotropen Voxeln mit 1 mm Kantenlänge anatomische Bilder aufgenommen. Das Volumen der grauen Substanz wurde mittels optimierter voxel-basierter Morphometrie (VBM) untersucht. Diskussion/Ergebnisse: Jugendliche mit autistischen Störungen zeigten eine signifikante Reduktion des Volumens der grauen Substanz in der Region des rechten intraparietalen Sulcus, die in enger räumlicher Beziehung zu funktionellen Aktivierungen bei der Betrachtung biologischer Bewegungen stand. Eine signifikante Reduktion des Volumens der grauen Substanz wurde außerdem bilateral im mittleren frontalen Gyrus gefunden. Frühe strukturelle parietale Veränderungen könnten eine Ursache für die Beeinträchtigung der Wahrnehmung komplexer biologischer Bewegungen bei Jugendlichen mit autistischen Störungen sein.

Im Verlauf der Kindheit werden Unterschiede zwischen Jungen und Mädchen im Körperbau und der Bewegungsdynamik sichtbar. Die Entwicklung von Körperproportionen und der Gangdynamik wurde an einer Querschnittsstichprobe von 27 Mädchen und 27 Jungen im Alter von 4 bis 16 Jahren analysiert. Zur Synthese digitalisierter Point-Light Modelle wurden die Gangmuster der Probanden im dreidimensionalen Raum erfasst. Lineare Diskriminanzfunktionen der Bewegungsinformation erlauben die überzufällig korrekte Zuordnung individueller Gangmuster nach Alter und Geschlecht, wobei die Präzision mit dem Alter ansteigt. Berechnet man die Diskriminanzfunktion als altersspezifische Funktion innerhalb von drei Altersgruppen, ergibt sich zusätzlich zur dynamischen Identifikation eine korrekte Zuordnung auf Grund der strukturellen anatomischen Information. Korrelative Zusammenhänge zwischen altersspezifischen Körperproportionen und dynamischen Aspekten treten dabei für Mädchen und Jungen in unterschiedlichen Entwicklungsphasen auf. Diese Ergebnisse werfen ein neues Licht auf die bisherigen Erkenntnisse der anthropometrischen Forschung und die Entwicklung von geschlechtsspezifischen Bewegungsmustern.

Biological motion perception, having both evolutionary and social importance, is performed by the human visual system with a high degree of sensitivity. It is unclear whether peripheral vision has access to the specialized neural systems underlying biological motion perception; however, given the motion component, one would expect peripheral vision to be, if not specialized, at least highly accurate in perceiving biological motion. Here we show that the periphery can indeed perceive biological motion. However, the periphery suffers from an inability to detect biological motion signals when they are embedded in dynamic visual noise. We suggest that this peripheral deficit is not due to biological motion perception per se, but to signal/noise segregation.

Biological motion point-light walkers convey information about the sex of a walker. As has been shown earlier, retrieving this information depends on the viewpoint: Frontal views are easier to classify than profile views. However, what happens if a walker is shown from a varying viewpoint as is the case when we see a walker walking on a circle? Multiple viewpoints should facilitate activation of a three-dimensional representation which might help classification. On the other hand, the additional rotation might mask intrinsic (that is, relative) motion diagnostic for the sex of the walker and therefore hinder classification. In the current study, observers had to indicate perceived sex of point-light displays of individual walkers shown either in frontal view (0 deg), half profile view (30 deg), profile view (90 deg), or in a condition in which the viewpoint rotated from -50 to 50 deg over the display time of 2 sec. In addition, we manipulated the information provided. Walkers contained either only structural information, only kinematic information, or all information. The results replicated earlier findings showing that performance at frontal and half profile view is much better than at profile view and that kinematic information is required for sex classification whereas structural information has very little diagnostic value. In addition, we could show that rotating views of a walker are clearly resulting in worse classification than frontal or half-profile views, but were classified much better than profile view walkers. We conclude that three-dimensional representations do not facilitate sex classification from biological motion. Diagnostic information about sex is primarily contained in the kinematics within the fronto-parallel plane and the motion due to rotation of the walker aggravates retrieval of this information.  

Einleitung: Eine Dysfunktion des dorsalen visuellen Verarbeitungsweges wurde in verschiedenen Studien bei Personen mit autistischen Störungen (ASD) gefunden. Da die Bewegungswahrnehmung unter anderem auch im dorsalen visuellen Weg verarbeitet wird und in einer Studie Störungen der Bewegungswahrnehmung bei Kindern mit Autismus beschrieben worden sind, ist in der vorliegenden Untersuchung die neurale Aktivierung bei der Beobachtung einfacher und komplexer biologischer Bewegungen bei Personen mit ASD und gematchten Kontrollpersonen untersucht worden. Methode: Die BOLD-Antwort bei der Beobachtung zweier Stimuli, einer einfachen und einer komplexen biologischen Bewegung, wurde bei 13 männlichen und 2 weiblichen Jugendlichen mit ASD und 13 männlichen und 2 weiblichen alters- und IQ-gematchten Kontrollpersonen verglichen (Alter MW 18 Jahre, Spanne 14-28). Diskussion/Ergebnisse: Die Aktivierung durch den einfachen Bewegungsstimulus unterschied sich nicht zwischen den Gruppen. Bei der komplexen biologischen Bewegung zeigten sich deutliche Unterschiede in temporo-parietalen und frontalen Regionen. Diskussion Die Ergebnisse weisen auf eine grundlegende Beeinträchtigung der Wahrnehmung komplexer biologischer Bewegungen hin, die mit einer verminderter Aufmerksamkeitsaktivierung einherzugehen scheint.

Directional information can be extracted from scrambled point-light displays that are devoid of all structural cues prompting the suggestion of a distinct local mechanism in biological motion perception that may serve as a general "life detector" (Troje & Westhoff, 2006). We investigated this hypothesis by testing the perception of both animacy and direction from point-light stimuli. Coherent and scrambled point-light displays of humans, cats, and pigeons that were upright or inverted were embedded in a random dot mask and presented in saggital view to two groups of naïve observers (n = 12/grp). The first group assessed the animacy of the walker on a six-point Likert scale and the second group discriminated the direction of walking. Across blocks, stimulus duration varied from 200 - 1000 ms. Coherent stimuli appeared more animate than scrambled stimuli (p [[lt]] 0.001) and inversion decreased animacy ratings (p [[lt]] 0.001), although more substantially for coherent than for scrambled walkers (p = 0.007). Similarly, discrimination accuracies were higher for coherent versus scrambled stimuli (p [[lt]] 0.001) and inversion decreased performance (p [[lt]] 0.001), but more substantially for coherent than for scrambled walkers (p = 0.004). Both animacy ratings and discrimination accuracies did not differ for animal type (ps [[gt]] 0.200) nor stimulus duration (ps [[gt]] 0.300). The results indicate that like the ability to discriminate direction, the perception of animacy from scrambled displays is orientation-specific. We suggest that the responsible mechanism uses a dynamic, gravity-dependent framework to assess the presence of life in the environment and is remarkably robust, operating efficiently at limited exposure times.

Directional information can be extracted from upright scrambled point-light displays that are devoid of all structural cues prompting the suggestion of a distinct local mechanism in biological motion perception that may serve as a “life detector” (Troje & Westhoff, 2006).  Whether the proposed mechanism conveys information beyond direction is unknown.  We present three experiments that investigated the perception of both animacy and direction from point-light stimuli.  Coherent and scrambled point-light displays of humans, cats, and pigeons that were upright or inverted were embedded in a random dot mask and presented in sagittal view to three groups of naïve observers.  Observers assessed the animacy of the walker on a six-point Likert scale in Experiment 1, discriminated the direction of walking in Experiment 2, and completed both the animacy rating and direction discrimination tasks in Experiment 3.  Stimulus duration varied from 200 – 1000 ms across blocks in Experiments 1 and 2, and was fixed at 500 ms in Experiment 3.  Coherent stimuli appeared more animate than scrambled stimuli and inversion decreased animacy ratings, although more substantially for coherent than for scrambled walkers.  Similarly, discrimination accuracies were higher for coherent versus scrambled stimuli and inversion decreased performance, but more substantially for coherent than for scrambled walkers.  Both animacy ratings and discrimination accuracies did not differ for animal type or stimulus duration.  Experiment 3 showed further, a linear correlation between animacy ratings and discrimination accuracies.  The results indicate that like the ability to discriminate direction, the perception of animacy from scrambled displays is orientation-specific.  The linear relationship between the animacy and direction data suggests that they address a similar mechanism within our context.  We propose that the responsible mechanism uses a dynamic, gravity-dependent framework to interpret terrestrial articulated locomotion and is remarkably robust, operating efficiently at limited exposure times.  

The arithmetic mean of the same number of male and female biological motion point-light walkers represented in a morphable, linear walking space is perceived to be male. The perceptually neutral walker corresponds to a point in the female part of the space. It is not clear, though, if this “male bias” is a genuine phenomenon or an artifact of the specific walker space and its underlying metric. Here, we present a number of experiments in which observers reported the perceived sex of a series of walkers while we varied the range and the distribution from which the walkers were sampled. We observe a pronounced range effect: If we sample from a distribution which is shifted towards the female part such that it is now centered around the walker that appeared to be sexually neutral before, observers adapt to the range and still perceive more walkers to be male than female. On the other hand, if we sample from a larger range with the same mean the observed “male bias” changes only marginally. We conclude that the male bias is not an artifact of the motion space used but a genuine phenomenon. We discuss different possible causes and particularly the question of whether the “male bias” is stimulus-specific or rather a more general phenomenon.

Biological motion perception has long been treated as one single phenomenon. Science often moves forward by trying to simplify complex ideas. However, I will suggest that in the case of biological motion perception the “unitary view” has hindered research and slowed down progress. In my talk I will identify a number of dissociable processing levels that need to be distinguished carefully to improve our understanding of the complex phenomenon of biological motion perception.

According to a classic theory by Morton and Johnson [1], humans and chicks (and probably other animals as well) share an important principle in the design of the developing face and conspecific recognition system. In humans, an innate mechanism (CONSPEC) based on a very coarse template of a face is reponsible for guiding attention to face-like stimuli, ensuring that a second mechanism (CONLEARN) receives ample input to learn about the subtle cues that carry information about the identity of another person. In newly hatched chicks, the two mechanisms have similar roles and control filial imprinting on their mothers.

Based on experiments investigating the inversion effect in biological motion [2], I will present evidence for the dissociation of two mechanisms for biological motion perception. One of them attracts attention to a particular signature in biological motion that is largely independent of the particular nature of the animal generating it. This mechanism works well in the visual periphery and functions as a general “life detector”. I will argue that it is evolutionary old and that humans share it with other animals (including chicks [3]). The other mechanism is based on learning. It relies on the first one, which makes sure that sufficient visual input is provided for this learning process. Evolutionary, it is a more recent acquirement and its particular implementation might strongly vary between species.

[1] Morton, J., and Johnson, M.H. (1991). CONSPEC and CONLERN: a two-process theory of infant face recognition. Psychological Review 98, 164-181.
[2] Troje, N.F., and Westhoff, C. (2006). The inversion effect in biological motion perception: evidence for a "life detector"? Curr Biol 16, 821-824.
[3] Vallortigara, G., and Regolin, L. (2006). Gravity bias in the interpretation of biological motion by inexperienced chicks. Curr Biol 16, R279-280.  

Animal courtship in many species is not only an evaluation process, but also a two-way communicative interaction. Although pigeon courtship has been extensively described, it is not known whether male and female pigeons coordinate their body movements as a form of communication. We used simultaneous motion capture recordings of both partners during courtship to examine correlations between kinematic measurements reflecting behaviors at several different time scales. The behaviors included head bobbing, movement speed, body orientation, and direction of turning. Each was modeled using semi-markov processes previously used for modeling parallel streams of human nonverbal behaviors during conversation. The analysis results in a quantitative ethogram of the choreography of pigeon courtship and elucidates the different levels of coordination, as well as the role of leading, following, and behavioral synchronization between the two partners.

People are more than faces, and much of our perception of others derives from visual appraisal of bodies and their movement -- rich sources of information as to gender, identity, etc. We find that, even ignoring overt courtship displays (eg, dancing), the mere act of walking, a ubiquitous human activity, provides observers a compelling percept of attractiveness. Previously, we demonstrated the influence of sexual dimorphism and prototypicality on attractiveness of human gait; here we extend this to examine the role of symmetry. To do so, we obtain attractiveness ratings for motion-captured women, displayed as point-light walkers, and for their perfectly symmetric counterparts. Our results show that making symmetric an individual's body and movement can indeed increase attractiveness, although this benefit might not be seen for less attractive individuals. Moreover, a key feature of our approach (Troje, 2002) is the ability to independently manipulate the symmetry of either the body or its movement and thus investigate the contribution of each to attractiveness. Whereas previously examined anatomical asymmetries may be quite small and difficult to measure and to perceive visually, we propose that asymmetries in movement may be more readily observed and salient. Our results thus far indicate that, at least for more attractive individuals, symmetry of movement has a greater bearing on attractiveness than does anatomic symmetry. In conclusion, we suggest that explicitly and independently manipulating anatomic and kinematic symmetry (and sexual dimorphism, prototypicality, etc) of motion-captured individuals provides an important complement to existing correlational and video-based methods in the study of person perception.

Point-light walkers are ambiguous stimuli when no explicit depth cues are provided. Recently it has been shown that frontal views of such walkers are more often interpreted as facing towards the viewer than facing away from the viewer (Vanrie, Dekeyser & Verfaillie, 2003). As human walkers are social stimuli, we investigated the influence of social motives on participants’ perception. Participants were shown moving point-light figures from 80 different male and female walkers and they had to decide for each walker whether he or she seemed to move towards or away from them. Again moving-towards interpretations were more frequent than moving-away interpretations. This result, however, was qualified by participants’ dispositional need for affiliation and their gender. Among women the preference for the towards-interpretation could only be shown when their need for affiliation was high but not when it was low. In contrast, men’s need for affiliation had no impact on their perception. High affiliation women decided, however, more often than high affiliation men that the walkers moved towards them. The influence of motivation on perception is discussed.  

Human gate patterns give a lot of information about person walking. For example, it is possible to derive information about gender or mood merely from dynamic gate features. Mindfulness-based approaches stress the importance of getting in contact with the here-and-now experience of the body. Moreover, recent theories of emotion highlight the role of proprioceptive-bodily information in the generation of emotional states. Therefore, investigation of bodily processes might be a relevant target for the study of mindfulness-based approaches. In our current research we analyzed the gate patterns of 30 patients participating in a mindfulness-based cognitive therapy (MBCT), 18 acute depressive inpatients and 30 never depressed participants by fourier - based descriptions and computation of linear classifiers. The following research questions were guiding our research: (1) Do dynamic gate patterns of acute und formerly depressed patients differ from never depressed people (2) Does MBCT normalize gate patterns of formerly depressed patients?  

Human gait contains a huge amount of socially relevant information and even highly reduced visual stimuli such as Point-Light-Walkers (PLWs) are sufficient to transmit information about a walking person’s gender and age. The aim of our study is to develop a new perception-based method for the detection of paedophilic interests using PLWs as stimulus material. The advantage of PLWs is that they are ambiguous and do not show any explicit sexual content.

Seven prototypical female and 7 male PLWs covering an age range from 4 to 30 years were presented to a group of child molesters (n = 21) and a control group (n = 30). The experiment consisted of a gender-, attractiveness and age-rating-block. In each block all PLWs were presented twice for 4 s in a random order.Discriminant Analysis is able to classify 82.4% of all cases correctly as child molesters or control participants, using individual attractiveness- and gender-ratings as predictive variables (?2 (6, N = 51) = 28.55, p = .00007). Currently we are collecting additional data to investigate interactions of “victim age” and “victim gender” with visual ratings of child molesters.

[1] Morton, J., and Johnson, M.H. (1991). CONSPEC and CONLERN: a two-process theory of infant face recognition. Psychological Review 98, 164-181.
[2] Troje, N.F., and Westhoff, C. (2006). The inversion effect in biological motion perception: evidence for a "life detector"? Curr Biol 16, 821-824.
[3] Vallortigara, G., and Regolin, L. (2006). Gravity bias in the interpretation of biological motion by inexperienced chicks. Curr Biol 16, R279-280.  

A central question in motor control is how the CNS dealswith redundant degrees of freedom inherent in musculoskeletalsystems. The human arm is a prime example for such a system. It showsa vast variety of behavior and plays a key role in most aspects ofour life. However, because of its biomechanical complexity, the armposes a formidable control problem to our CNS. In this study weanalyzed movements performed by human subjects, which were asked tocatch a ball launched towards them on 16 different trajectories.Subjects had to initiate movements from 2 different startingpositions. Motor activity of the right arm was recorded using opticalmotion capture and was transformed into 10 joint angle time coursesby a model-based optimization algorithm. The resulting time series ofarm postures were analyzed by principal components analysis (PCA). Wegenerally found that more than 90% of movement variance was capturedmostly by as few as 2 principal components (PCs). Furthermore,subspaces spanned by PC sets associated with different catchingpositions varied smoothly across the arm’s workspace. When wepooled complete sets of movements, 3 PCs were still sufficient toexplain 80% of the data’s variance. This indicates strongkinematic couplings between the joints of the arm. We hypothesizethat flexible and context-dependent behavior like multijoint humanarm movement does not necessarily require complex neural algorithms.Instead, we show that catching movements towards diverse targets canbe generated efficiently by linear combinations of a small set ofcardinal movement synergies.

Die Fähigkeit, andere Personen anhand ihrer Bewegungen identifizieren zu können, ist eine geläufige Beobachtung aus dem Alltag. Bereits gegen Ende der 70er Jahre wurden erste psychophysische Experimente zur Personenidentifikation durch sog. Point-Light Displays (PLDs) durchgeführt, bei denen Personen nur durch eine geringe Anzahl von Lichtpunkten dargestellt wurden. Mit Hilfe moderner Aufnahmemethoden (Motion Capturing) und die Anwendung von Fourier-Analysen ist es möglich, Gangmuster einzelner Personen in strukturelle und kinematische Komponenten zu zerlegen und dabei nur wenig individuelle Informationen einzubüßen. Diese Technik wurde in zwei Experimenten benutzt, um den Einfluss verschiedener Parameter auf die Identifikation individueller Gangmuster zu überprüfen. In Studie 1 lernte eine Gruppe von Versuchspersonen, PLDs von sieben verschiedenen männlichen Personen zu benennen. Im Verlauf des Experiments wurden die Informationen über individuelle Größe, Körperstruktur und Gangfrequenz aus den PLDs entfernt und durch die Mittelwerte der gesamten Gruppe ersetzt (Normalisierung). Es konnte gezeigt werden, dass Körperstruktur und Gangfrequenz, nicht aber die Körpergröße, einen signifikanten Einfluss auf die Erkennungsleistung ausüben. Studie 2 überprüfte die Rolle von kinematischen Faktoren. Ein erstes Experiment zeigte, dass die erste und zweite Harmonische der Fourier-Analyse hinreichend zur Identifikation beitragen, während Harmonische höherer Ordnung nicht berücksichtigt werden müssen. In einem zweiten Experiment wurden die PLDs hinsichtlich ihrer Amplituden- bzw. Phasen-Spektra normalisiert. Beide Spektra tragen signifikant zur Erkennung individueller Gangmuster bei, jedoch zeigte sich für das Amplituden-Spektrum ein wesentlich stärkerer Einfluss.

KörperbildkomponenteVocks, S., Legenbauer, T., Kiszkenow, S.,Troje, N.F., Schulte, D.Körperbildstörungen sind wesentlich an derEntstehung und Aufrecherhaltung der Anorexia und Bulimia nervosabeteiligt. Dennoch wurden Interventionen zur Verbesserung desKörperbildes in Essstörungstherapien zumeistvernachlässigt. Auch wurde das Körperbild oft nurunidimensional erfasst; tatsächlich ist es jedoch als einmultidimensionales Konstrukt anzusehen, welches sich aus vierinterindividuell unterschiedlich ausgeprägten Komponentenzusammensetzt. So beinhaltet es eine perzeptive (Überschätzungder eigenen Körperdimensionen), kognitive (negative Bewertungendes eigenen Körpers), affektive (negative körperbezogeneGefühle) und behaviorale Komponente (körperbezogenesVermeidungs- und Kontrollverhalten). Daher wurde eine Untersuchungzur Evaluation einer Körperbildtherapie durchgeführt,welche Interventionsbausteine bezüglich aller vierKörperbildkomponenten integriert. Entsprechend wird derTherapieerfolg multidimensional erfasst. 

Spatially scrambled point-light displays of humans oranimals in locomotion contain unambiguous information about the direction in which the agent is facing. Observers are well able to retrieve this information, but only if the displays are presented right-side up. Even though spatial integrity is not required for direction discrimination the temporal relations between dots may still be important. Here, we report the results of an experiment in which we manipulated the temporal integrity of spatially scrambled point-light displays in two different ways. In the first condition we apply random offsets to the phase of the single dots. Whereas this manipulation changes the “beat” of the pattern which defines the particular gait of the agent, it leaves it's general rhythmicity intact. In a second condition, we also changed the playback speed individually for each dot, which results in completely uncorrelated dot movements.
The results show a small but consistent effect of the temporal integrity on the strength of the inversion effect. As the degree of temporal scrambling increases the inversion effect decreases. Even though temporal integrity apparently plays a role, observers can still determine the direction of the upright, fully scrambled point-light agent with an accuracy that is still much higher than the spatially and temporally intact, but inverted walker.
The results can be modeled by means of a simple linear model and they are discussed in terms of a basic, yet reliable and form invariant visual filter designed for the general detection of animate motion in the visual environment.

Animate motion patterns are a rich source of biologically significant information. They quickly and reliably signal the presence of an animate agent and convey its identity, actions and intentions. Observing other people in motion, our visual system is able to retrieve information about sex, age and weight of a person and can even detect signatures that identify a familiar individual. The relevant information is encoded on different levels in the motion patterns ranging from purely local motion information to complex global correlation patterns. In my talk, I will present empirical and computational data from two different studies. In the first, we investigated the nature of the general saliency of biological motion. Based on findings from experiments designed to explore the cause of the inversion effect in biological motion, we propose a simple, yet reliable and fast sensory filter that can detect the presence of a living animal in the visual environment. The proposed mechanism is based on local motion trajectories and does not require any form processing. On the other hand, I will present a computational framework that explores the complex correlation pattern of a moving body to retrieve information about a person's sex and other attributes that define his or her identity. It is based on a morphable representation of human walking data which in turn makes it possible to formulate linear classifiers for the attributes of interest. Results on sex classification obtained from our model are being compared to behavioural data and the relevance of our framework as a model for visual information processing in the human brain will be discussed.

Biological motion has long been treated as one single phenomenon. Here, we want to suggest a dissociation between a mechanism responsible for a non-specific, shape independent detection of biological events, on the one hand, and a mechanism that retrieves shape from motion and recovers the articulation and general structure of a specific body, on the other hand. Evidence for this dissociation is presented in terms of experiments designed to explore the nature of the inversion effect in biological motion. As long as it is presented upright, scrambled biological motion still contains information about the facing direction of a walker. We isolate the invariants responsible for this inversion effect as being representative for the ballistic movement of limbs under conditions of gravity. The consequences of using scrambled motion to mask biological motion in detection experiments and the usage of scrambled motion as control stimulus in imaging studies are discussed.

While the study of facial attractiveness has explored a number of factors such as familiarity, symmetry, and sexual dimorphism, perhaps the most popular notion to emerge has been that the mean of a population is what is considered most attractive. In contrast to this concept of "averageness," however, exaggerations of sex differences have been shown to play a key role in attractiveness -- a finding now mirrored in the domain of biological motion (i.e., point-light walkers) where, in men's ratings of female walkers, attractiveness correlates very well with a gender axis (Troje, 2003). We should like to clarify that this is not due to merely approaching a hypothetical average female walker but more specifically to the relative display of sexually dimorphic characteristics, even to the detriment of averageness.
As with averages of faces, synthetic walkers made by averaging two or more individuals do generally appear to be attractive. This is certainly the case with the full population average, and, indeed, even averaging all walkers of below-average attractiveness can yield a walker that is above-average. However, even the maximally average walker is nevertheless less attractive than a number of real, individual walkers. Moreover, the most attractive individuals are not necessarily nearer to the average; direction of deviation from the mean may be more meaningful than distance, so that, e.g., far-from-average walkers may be very attractive if they exaggerate female characteristics.
Thus, the most average walker is not the most attractive, the most attractive walkers are not the most average, and walkers equidistant from the average may be very attractive or unattractive depending on their relative expression of sexually dimorphic traits. For biological motion, then, the perception of attractiveness (and perhaps of gender) might be guided not simply by prototypes anchored at averages of categories but by representations specifically attuned to salient variation between categories.

In a simple, sparse display of a few moving dots on a plain background, one's perception of a human actor can be remarkably compelling and rich; individual dots spontaneously cohere into a single object that relays extensive information about both actor and action. Here we show subjects' high-level categories (male vs. female) and related judgments (ratings of femininity or attractiveness)suggest representations of vectors or axes exploiting differences between classes, not simply averages, norms or prototypes defined within classes. Thus, for example, the most average female is not the one judged most feminine or most attractive.

Head-bobbing in pigeons comprises of a hold phase and a thrust phase. Visual feedback required for fine tuning the head movement probably relies on continuous retinal image flow. In two experiments, we investigated the temporal properties for acquiring such optic flow information by measuring locomotion behaviour understroboscopic illumination. In Experiment 1, pigeons were trained to actively walk back and forth between two food hoppers. Locomotion behaviour was measured by relating travelled distance and subtended rotation angles to the number of steps performed. In Experiment 2, birds were restrained and moved passively on the belt of a treadmill while we measured the number of performed head-bobbing cycles. In both experiments strobe frequencies varied in steps from 1.0 Hz to 100 Hz on an equidistant logarithmic scale. Locomotion behaviour was normal at strobe frequencies above 20 Hz. Between 8 Hz and 20 Hz pigeons displayed a significant level of activity. However, all movements were executed on the spot leading to rotation but not to translation. Within the same range of strobe frequencies, head-bobbing in passively moved birds is suppressed, but rotational saccadic head-movements are unimpaired. At frequencies below 8 Hz activity ceases. We discuss the observed behaviour in the context of corollary discharge processing in the rotundus/triangularis complex of the pigeon’s brain.

Whereas pigeons learn to discriminate moving point-light displays of different animate objects (human versus pigeon, Exp.1) as well as different movement categories (pecking versus walking, Exp.2) rather easily, it is a much harder task for them to discriminate the movement direction (to the left versus to the right) of a stationary walking point-light pigeon (Exp.3). Whereas in the first and second experiment discrimination can be explained by extraction of form or posture, a different process must have been at work in the third. Once the pigeons had acquired the discriminationin Exp. 3, they were presented (1) with versions of the training stimuli in which certain points were kept static (i.e., those forming the head, the torso, or the feet), so that only parts of the point-light pigeon were presented dynamically, and (2) with the original training stimuli presented upside down. For most pigeons dynamic presentation of the feet was most informative for detection of movement direction. When the dynamic stimuli were presented upside down, discrimination broke down completely. The present findings are in keeping with recent results in human subjects showing that assumptions about the direction of gravity play a role in interpreting biological motion (Shipley 2003, Troje 2004).

Perception of biological motion is a fundamental property of the human visual system. It is as yet unclear which role the cerebellum plays with respect to the perceptual analysis of biological motion represented as point-light displays. Imaging studies investigating biological motion perception revealed inconsistent results concerning cerebellar contribution. The present study aims to explore the role of the cerebellum in the perception of biological motion by testing the performance of biological motion perception in patients suffering from circumscribed cerebellar lesions and comparing their performance with an age-matched control group.
Perceptual performance was investigated in an experimental task testing the threshold to detect biological motion masked by scrambled motion and a control task testing detection of motion direction of coherent motion masked by random noise. Results show clear evidence for a differential contribution of the cerebellum to the perceptual analysis of coherent motion compared to biological motion. Whereas the ability to detect biological motion masked by scrambled motion was unaffected in the patient group, their ability to discriminate direction of coherent motion in random noise was substantially affected. We conclude that intact cerebellar function is not a prerequisite for a preserved ability to detect biological motion. Since the dorsal motion pathway as well as the ventral form pathway contribute to the visual perception of biological motion, thequestion remains open, whether cerebellar dysfunction affecting the dorsal pathway is compensated for by the not affected ventral pathway or whether perceptual analysis of biological motion is performed completely without cerebellar contribution.

Everyday situations demand high flexibility from neural mechanisms underlying motor control. Movement of the human arm is such an example for the control of a multijointed limb with redundant degrees of freedom. In this study we analyzed the movement patterns performed by the right arm of 9 different subjects. The observed task was to catch an approaching ball traversing the workspace of the arm on 16 different trajectories. Movement started from one of two different initial postures. Subjects were instructed to catch the ball at a convenient point. Only successful catches were evaluated. Time courses of 9 joint angles of arm and shoulder were computed from Cartesian marker coordinates obtained from an optical motion capture system. We subsequently applied principal components analysis to average sets of angular time courses of each one of the 32combinations of start posture and catch position. Though our kinematic model comprises 9 degrees of freedom we can show that 2principal components (PCs) account mostly for over 90% of the variance. PCs thereby vary smoothly across the arm’s workspace. When pooling all movements from one initial posture to any of the 16 catching positions, 2 to 3 PCs still explain 75 – 80% of the data’s variance. This indicates a strong coupling between the observed joint angles. Based on these findings we hypothesize that flexible and variable behavior like multijointed human arm movement does not necessarily require complex neural algorithms. Instead we show that catching movements within the entire workspace can be generated efficiently by linear combinations of a small set of basic movements.

With rigid objects, the visual system showssome degree of viewpoint invariance. Information obtained about anobject from one view can be used to identify the object from a novelview. Little is known whether we can also generalize to new views ofnon-rigid objects, such as human bodies. Here, we examine this questionusing biological motion. Stimuli were Fourier represented point-lightwalkers decomposed into an average posture and the first fiveharmonics. We investigated the role of these harmonics for personidentification from biological motion, and measured performance whenviewing angles were varied between learning and test. Three groups ofobservers were trained to identify seven male walkers, previouslyunknown to them, shown from different views: 0 deg (frontal view), 30deg, or 90 deg. Non-reinforced test stimuli were generated by firstcomputing an average walker and then replacing either only its first,second, or third to fifth harmonics with the respective harmonics ofthe individual walkers. In the test session these walkers were showneither from the same viewing angle as in the training sessions, or fromone of the two other viewpoints. Results show that walkers can beidentified best if shown from the same view as during training. Therewas also a significant transfer to other angles, but performancedeclined with increasing difference between learning view and testview. There was a marginal effect of test view, with the 30 deg viewproducing best performance. The first harmonic contributes mostinformation to the identification of the walkers. The second harmonicalone is still sufficient for recognition, but this is not the case forthe higher order harmonics. There were no significant interactionsbetween the type of harmonic and the training view or the testviewpoint, respectively. We conclude that there is a clear viewpointeffect for recognition of biological motion. Still, the visual systemis able to generalize to a considerable degree across viewpoints.This research is funded by the VolkswagenFoundation.

Object recognition across changing viewpoints requires sophisticated neural processing. We examine recognition of point-light walkers and investigate the contribution of different Fourier components to viewpoint generalization performance. Observers were trained to identify seven walkers in one of three learning groups differing according to the viewpoint: frontal, half-profile, or profile view. Test stimuli were generated by replacing either the first, second, or third to fifth harmonics of an average walker with the respective harmonics of the individual walkers, and were shown from all three viewpoints. No differences in performance were found between the learning groups. The test view had a marginal effect with the half-profile view being recognized best. Walkers can be identified best if shown from the same angle as during training, but there is also significant transfer to other views. There were no significant interactions between the type of harmonic and the training view or the test viewpoint, respectively.

Human observers are able to learn to discriminate individuals shown as point light walkers. The walking pattern of a person can be represented very accurately using discrete Fourier analysis. Here, we represent walking data by decomposing them into an average posture and the first five harmonics, and we investigated their role for person identification from biological motion. Observers learned to identify seven male walkers from one of three different viewpoints: 0 deg (frontal view), 30 deg, or 90 deg. The displays were previously normalized with respect to the shape of the walker and his walking frequency. Non-reinforced test stimuli were generated by first computing an average walker and then replacing either only the first, second, or third to fifth harmonics of this average walker with the respective harmonics of the individual original walkers. Results show that the first harmonic contributes most information to the identification of the walkers. The second harmonic alone is still sufficient for the task, but this is not the case for the higher order harmonics. No differences in performance were found between the three viewpoint groups. Walkers can be identified best if shown from the same viewing angle as in the training sessions, but there is also a significant transfer to other angles. There was a marginal effect of the test view, with the 90 deg view producing the worst performance. There were no significant interactions between the type of harmonic and the training view or the test viewpoint, respectively. We conclude that individual walking patterns can be represented adequately with the use of two Fourier-harmonics and that the visual system is not able to detect subtle differences between point light walkers beyond this representation.

This research is funded by the Volkswagen Foundation.  

Background: As it was shown before,professional ballet dancers are at an enhanced risk of developingdisturbances of body image and eating disorders, because of the highpressure to maintain a low body weight. However, there is almost nodata on the dynamic body image which might be an important factorespecially in this group. Studies with non-professional ballet dancersare very rare, though they might be of a similar risk in developing aneating disorder.

Method: We compared a group of 25 non-professional ballet dancers with 33 control persons concerning several aspects of body image and eating behaviors with the "Multidimensional Body-Self Relations Questionnaire", the "Eating Disorder Examination Questionnaire", the "Eating Disorder Inventory" and the "Body Image Avoidance Questionnaire". In addition to those questionnaire measures, we included a digital distortion technique to get information with respect to the static and dynamic body image. Results: Results indicate that ballerinas show a higher preoccupation with overweight (p<.05) and food (p<.05) than controls, while eating behaviors were not different in both groups. Additionally, ballerinas scored higher on fitness orientation (p<.05) and fitness evaluation (p<.001). No group differences were found for the other components of body image as measured with the digital distortion technique.

Conclusion: These results indicate that non-professional ballerinas have at the most a slightly enhanced risk of developing eating disorders and body image disturbances as compared with professional ballet dancers. We assume that the pressure to be thin is not as strong on non-professionals as on professionals.  

Background: Besides pathological eating behaviour, body image disturbances are a main charateristicum of anorexia and bulimia nervosa. In the past, only static body image was examined; dynamic aspects as perception and evaluation of one's own motion patterns have not been studied yet in eating disorders.

Method: To assess static body image, patients with anorexia and bulimia nervosa (n=22) and a healthy control group (n=58) estimated their 'real', 'felt' and , 'ideal' figure with a digital distortion technique. Assessment of the dynamic body image was realized by a computer programme based on the 'Biomotion-Technique' (Troje, 2002). Patients were asked to adjust motion patterns shown on the screen along the body mass index axis, representing at best their ,real', ,felt' and ,ideal' motions.

Results: Concerning static body image, patients with eating disorders show a significantly stronger overestimation of their ,real' (p=.014) and ,felt' (p>.001) body dimensions than control subjects, while for 'ideal' body image, no differences were found. Additionally, for dynamic body image, there was a trend to significant group difference with respect to the estimation of ,real' (p=.077) and a highly significant difference for the estimation of ,felt' (p>.001) motions. Patients estimated their motion patterns in the direction of a higher body mass index. Again, the 'ideal' motion pattern was not different in both groups.

Discussion: It was demonstrated for the first time that in patients with anorexia and bulimia nervosa, the body image disturbances includes, in addition to the static aspect, a dynamic component, too.

References: Troje, N.F. (2002). Decomposing biological motion. Journal of Vision, 2(5), 371-387.  

Fragestellung: Neben dem pathologischenEssverhalten stellen Körperbildstörungen einHauptcharakteristikum der Anorexia und Bulimia nervosa dar. In derVergangenheit wurde allerdings ausschließlich das statischeKörperbild beforscht; dynamische Aspekte wie die Wahrnehmung undBewertung von eigenen Bewegungen bei Patientinnen mit Essstörungenwurden bisher nicht berücksichtigt.Methode: Zur Erhebung des statischenKörperbildes sollten Patientinnen mit Anorexia und Bulimia nervosa(n=22) und eine gesunde Kontrollgruppe (n=58) ein Digitalfoto von ihrerPerson in einem enganliegenden Anzug am Computer entsprechend ihrer,tatsächlichen', ,gefühlten' und ,idealen' Figur einstellen.Die Erfassung des dynamischen Körperbildes erfolgte anhand einesvon unserer Arbeitsgruppe entwickelten Computerprogramms, welches aufder ,Biomotion-Technik' (Troje, 2002) basiert. Hierbei stellen dieProbandinnen ein per Bildschirm dargebotenes Bewegungsmuster entlangeiner Body-Mass-Index-Achse so ein, dass dieses ihren,tatsächlichen', ,gefühlten' und ,idealen' Bewegungenentspricht. Ergebnisse: Bezüglich des statischenKörperbildes zeigten die Patientinnen mit Essstörungen imVergleich zur Kontrollgruppe eine signifikant stärkereÜberschätzung ihrer ,tatsächlichen' (p=.014) und,gefühlten' (p>.001) Körperdimensionen, während sichbeim ,idealen' Körperbild keine Unterschiede ergaben. Auch beimdynamischen Körperbild zeigten sich tendenziell signifikanteUnterschiede zwischen beiden Gruppen hinsichtlich der Einschätzungihrer ,tatsächlichen' (p=.077) sowie ein hochsignifikanterUnterschied hinsichtlich der ,gefühlten' Bewegungsmuster(p>.001), wobei die Patientinnen mit Anorexia und Bulimia nervosaihre Bewegungen in Richtung eines höheren Body Mass Indexeinstuften. Das ,ideale' Bewegungsmuster unterschied sich nichtzwischen den beiden Gruppen. Schlussfolgerung: Es konnte erstmalsnachgewiesen werden, dass bei Patientinnen mit Anorexia und Bulimianervosa neben einer Störung des statischen auch eine Störungdynamischen Körperbildes vorliegt. Literatur: Troje, N.F. (2002). Decomposingbiological motion. Journal of Vision, 2(5), 371-387.  

Biological motion point-light displays produce a strong inversion effect which shows similarities to the well known inversion effect in face recognition. In face recognition, the inversion effect has been interpreted in terms of a distinction between configural processing and featural processing. There is evidence that turning faces upside-down hinders configural processing. The inversion effect for biological motion may also be based on impaired configural processing. The effects of turning a walker upside-down should then be comparable to the effects observed with scrambled motion stimuli, i.e. with stimuli that keep local motion intact but displace the trajectories of the single dots randomly. An alternative explanation for the inversion effect in biological motion is based on recent findings that assumptions about the direction of gravity play a role in interpreting biological motion. If inverted gravity is the reason for the inversion effect, upright scrambled motion should produce less effects than intact but inverted motion.We tested the two alternative predictions using a task in which subjects had to determine the apparent walking direction of human and animal point-light walkers shown in sagittal view. The displays maintained stationary on the screen, i.e. they looked like being recorded on a treadmill, and they were masked with a dynamic random dot background.Both response times and accuracy clearly showed that scrambling the motion had only a very minor impact on direction perception. Even though subjects had no idea what kind of animal they might be seeing they could indicate its walking direction fast and accurately. On the other hand, turning the displays upside-down had a strong impact on the perceived direction with performance being almost at chance.We conclude that the inversion effect in biological motion is not due to disturbance of configural processing but is rather a result of prior assumptions about the direction of gravity becoming invalid.

This research is funded by the Volkswagen Foundation. 

Biological motion perception and Marey’s work. NikolausF. Troje Department of Psychologie, Queen’s University, Kingston. Besides many other achievements, Etienne Jules Marey was a pioneer in data visualization. He realized very early that complex physiological data can only be understood if they can be represented and displayed in away that suits human perception. The ease with which our visual system can interprete complex animate motion patterns in order to retrieve information about both a performed action as well as the performing actor has fascinated many researchers since. I will outline the impact that Marey’s work had on studying visual perception of biological motion and I will sketch the main results obtained in this field. I will finally introduce you to a framework that can be used to analyse human motion patterns with respect to biological, psychological attributes of the agent. I will further show how the same framework can be used to synthesize human motion patterns with well defined attributes to be used for computer animation and robotics, and I will finally discuss it as a model for visual motion processing in the human brain.

Animate motion contains information about the identity of an agent as well as about his or her actions, intentions, emotions, and personality. The human visual system is highly sensitive to biological motion and has evolved a remarkable capability to extract this socially relevant information from the way a person moves. The effortlessness with which we can adequately interpret other peoples motion grossly contrasts the fact that we still know very little about the mechanisms underlying information encoding in, and retrieval from, biological motion patterns. In this contribution, I want to outline a framework that enables us to successfully search for the effects of a number of different traits on human gait patterns. The proposed model is based on the statistics of a database of motion capture data. Based on linearization of the motion data, a motion space is defined which is spanned by the first few principal components obtained from the database of input walkers. Using biological and psychological traits attributed to the input walkers, linear discriminant functions are computed which define vectors in the motion space that generalize the respective trait. The framework will be used to explore variances in human walking patterns with respect to sex, age, attractiveness, a number of different emotions and identity. In doing so, I will stress the fact that successful retrieval of the respective information from the input data is only possible if we treat the moving person in a holistic way which retains the complex correlations between different moving parts of the articulated body as well as between the kinematics and static, structural properties of the body. The framework used to analyze animate motion is a generative model, which can also be used for motion synthesis. In order to create psychologically convincing motion it is crucial to preserve the correlative nature of animate motion. The human visual system is extremely sensitive to violations of the complex correlation patterns and I will present examples on how it uses implicit knowledge about the biomechanics (and general physics) of articulated motion causing these correlations.

The human visual system is very sensitive tothe detection of animate motion patterns. We can efficiently recognizehuman action patterns and attribute many features of psychological,biological and social relevance to other persons. An experimentalapproach for studying information from biological motion (BM) withreduced interference from non dynamic cues is to represent the mainjoints of a person's body by bright dots against a dark background.

In the present study we investigated the influence of viewing angle on recognition performance of walking patterns from familiar persons represented as point-light displays (PLD). Classical work on individual recognition (Cutting & Kozslowski, 1977; Beardsworth & Buckner, 1981) provided empirical evidence that cues from BM contain information enabling to recognize familiar persons and to identify one's own walking pattern when it is represented as PLD.

We tested viewpoint-dependent recognition performance in two groups of twelve persons respectively knowing each other very well. Motion data of the participants were acquired by recording their walking patterns in 3D space using a motion capture system. The location of the major joints were computed from the trajectories of the original markers. Size normalized PLDs of these walking patterns were presented to the same group members on a computer screen in three different orientations (frontal view, half profile view and profile view). `Before the experiment observers were shown a list of all occurring names, including their own. Observers were requested to press a button if they had recognized the person's gait pattern and to indicate afterwards the person's name by clicking on the corresponding name button in a list containing all names. Observers did not receive feedback on their response.

Analysis revealed a significant effect of viewing angle on recognition performance. Displays presented in frontal view and in half profile view were significantly more often correctly identified than displays presented in profile view. Whereas recognition performance was found to be significantly above chance level, BM failed to provide a high reliable cue for individual identification if walking patterns of familiar persons had not been seen before as PLD.

We conclude that individual features of gait dynamics can be more efficiently extracted when seen in frontal or half profile view. This viewpoint dependent recognition effect might be due to the fact that the attention towards another person is triggered if this person is approaching us resulting in increased exposure to frontal views of gait patterns.    

The human visual system is very sensitive to the detection of animate motion patterns. We can efficiently recognize human action patterns and attribute many features of psychological, biological and social relevance to other persons. In the present study we investigated the influence of viewing angle on recognition performance of walking patterns of one's own person and familiar individuals such as friends or colleagues represented as point-light displays (PLD).We tested viewpoint-dependent recognition performance in two groups of twelve persons who know each other very well. Participants' motion data were acquired by recording their walking patterns in 3D space using a motion capture system. Locations of major joints were computed from the trajectories of the original markers. Size normalized PLDs of these walking patterns were presented to the same group members on a computer screen in frontal view, half profile view and profile view. Before the experiment, observers were shown a list of names of all people to be presented, including their own. Observers were requested to press a button if they had recognized the person's gait pattern and to indicate afterwards the person's name by clicking on the corresponding name button in the names' list. No feedback was given to the observers.Whereas recognition performance of the own walking patterns was viewpoint independent, recognition rate for other familiar individuals was better for frontal and half profile view than for profile view. We conclude that the viewpoint dependent recognition effect for other people might be due to attention being triggered if another person is approaching us, resulting in increased exposure to frontal and half profile views of gait patterns. The finding of viewpoint independent recognition effects for own movement patterns might be related to a crossmodal transfer from motor to visual representations.This research is funded by the Volkswagen Foundation. 

Many birds show a characteristic backward and forward head movement called head-bobbing. It is composed of a hold phase, where the head remains static in space, and a thrust phase, where the head is moved forward. Three main functions for head-bobbing have been proposed:biomechanical function, image stabilization and depth perception through motion parallax. However its function is not yet well understood. Although head-bobbing behaviour has often been discussed in the literature, the birds that bob their head are not listed. It has been reported that head-bobbing occurs in at least 8 of the 27 orders of birds and in 28 species such as pigeons, doves, hens, starlings, pheasants, coots, rails, sand-pipers, phalaropes, parrots, magpies and quail. At the moment, we are collecting data about head-bobbing birds by an exhaustive field observation of different species of birds. Until now we found head-bobbing birds in 10 orders, 25 families and more than 60 species. In contrast we observed non-head-bobbing birds in 9 orders, 20 families and almost 100 species. The list indicates that around 40% of the birds show head-bobbing. We discuss whether head-bobbing is a monophyletic or apolyphyletic trait and we analyze the ecological and behavioural factors under head-bobbing behaviour, such as predatory pressure, source of feeding, habitat, etc. Based on these findings, we will discuss the functional significance of avian head-bobbing.  

The human visual system shows an impressive sensitivity to subtleties in animate motion patterns carrying biologically relevant information. Frontal views of biological motion point-light walkers can be classified with respect to the gender of the walker withhigh accuracy. Here, we document pronounced adaptation effects that alterthe perceived gender of a point-light walker.

Stimuli were generated using a morphing technique which provides smooth transitions along a linear discriminant function classifying a set of 80 walkers according to their sex/sup/1/sup/. Thirteen different walkers were sampled along the male-female walking axis covering a total range of 7 standard deviations of the walker distribution. In a first experiment we determined the location of a perceptually neutral walker on the male-female axis. Five different presentation times between 350 and 7000 ms were used. Using a two-alternative-fourced-choice procedure, subjects had to indicate for each display, whether it showed a man or a woman. The data were fitted by logistic psychometric functions. A morph half way between an average male and an average female walker was rated to be male. To obtain a perceptually neutral walker about one standard deviation of femaleness has to be added. Those results were independent of the presentation time.

In a second experiment, observers were first presented with 7000 ms point-light displays of either an exaggerated male walker, an exaggerated female walker or a perceptually neutral walker. After this adaptation period, they were tested with short presentations of walkers sampled along the male-female walking axis. The presentation time of the test stimulus was either 350, 700, or 1400 ms. Adaptation results in a pronounced shift of perceived gender of the test stimulus. A neutral walker is perceived to be female after adaptation with the exaggerated male and male after adaptation with the exaggerated female walker.

Our data demonstrate that adaptation can occur not only within low-level vision processes but also at high-level information processing stages. In the case of biological motion perception the aftereffects are affecting stages at which the complex information from the series of single moving light-dots is integrated into a coherent percept of walking person.

This research is funded by the Volkswagen Foundation.

Troje, N. F. (2002). Decomposing biological motion: A framework for analysis and synthesis of human gait patterns. Journal of Vision, 2:371-387, http://journalofvision.org/2/5/2  

Adaptation is a very general and basic phenomenon in biological information processing, covering a broad range from gain control to "fatigue". Adaptation provides an active mechanism for efficient data compression by removal of redundancy: encoding changes of properties rather than the properties themselves allows the visual system to acquire, transmit, process and store information in a highly economical manner while minimizing losses. However, besides its functional significance, adaptation has also proven to be a valuable scientific instrument to non-invasively investigative, characterize and isolate sensory information processing pathways. In the visual domain, adaptation has traditionally been used mainly to study early visual processing. During the last few years, however, it has become evident that adaptation and corresponding after-effects also play a major role in high-level cognitive processing and that it can be employed to study phenomena such as face recognition or biological motion perception. In this symposium, we want to trace this development spanning the whole range between low-level vision and high-level cognitive processes, on the one hand, while emphasizing the dualistic nature of adaptation as a neural mechanism and as an investigative tool, on the other hand. J. Zanker will open the series of presentations by providing a general introduction into the concepts of spatio-temporal visual signal coding that lead to the phenomena of after-effects in time as well as to simultaneous contrast enhancement in space. Using the example of motion boundaries in time and space he will illustrate this point in more detail by comparing results from a computational motion detection model to psychophysical observations. The next two talks will provide illustrative examples of the use of adaptation for probing the properties of low level visual filters. In the contribution of M. Fahle selective adaptation is used as a tool to study the effects of perceptual learning on the characteristics of orientation selective visual filters. M. Bach uses a complex double adaptation paradigm to isolate direction specific motion responses in VEP from direction unspecific flicker responses. M. Greenlee's contribution is particularly interesting because he shows that contrast gain control, a mechanism that implements adaptation to varying light intensities, itself can be highly adaptive, therefore demonstrating "second order adaptation" in the visual system. In the last two contributions it is shown that adaptation is not only a low-levelvisual phenomenon. D. Leopold presents data on aftereffects in face recognition and N.Troje finds similar effects for biological motion perception.

Human motion contains a wealth of information about actions and intentions, but also about identity and personal attributes of the moving person. Our visual system can retrieve information about a person's gender, age, emotional state and personality traits and we can individually recognize a good friend -- solely based on his or her walking patterns. What our visual system seems to solve so effortlessly is still a riddle in vision research and an unsolved problem in computer vision. Little is known about exactly how biologically and psychologically relevant information is encoded in visual motion patterns. Here, I will outline a general framework that can be used to retrieve information from biological motion patterns and I will present a number of examples which will demonstrate the capabilities as well as the limits of the proposed approach.

The approach is based on transforming biological motion data into a representation that subsequently allows for analysis using linear statistics and pattern recognition techniques. The required linearization is achieved by using a Fourier-based approach. The DC part of the transform encodes structural information, i.e. the geometry of the moving body, whereas its dynamic parts encode the motion itself. The major property of the resulting representation is, that it is morphable: linear combinations of existing walking patterns result in proper walking patterns which represent smooth transitions between the original walkers.

Using this Fourier-based representation we apply principal component analysis to reduce the dimensionality of the resulting space to an extent that allows to compute efficient linear classifiers. The classifiers can be trained with different properties of interest. These properties are either obtained from the subjects themselves (like the sex of a walker) or they can reflect the perception of observers who were asked to rate point-light displays of the walkers (for instance, according to their attractiveness). I will show examples for extracting information about sex, weight, attractiveness and a number of emotional and personality traits. I will furthermore compare some of the computational results with psychophysically obtained data from human observers in order to discuss the relevance of our approach as a model for the mechanisms underlying the recognition of animate motion in the human visual system.  

Biological motion contains plenty of visual information about several attributes of biological andpsychological significance. In particularly, we can accurately determinethe sex of a walker from the way he or she moves. Furthermore, motion patternscan vary to a large extent in perceived sexual attractiveness. In this study,we investigate the relation between perceived attractiveness and gender usingdynamic point-light displays from 40 male and 40 female walkers. Using alinear, morphable stimulus space, we determined discriminant functions basedon attractiveness ratings of 12 male and 12 female participants. In a firstblock observers were shown with displays of the other sex and asked to ratethe walkers in terms of their sexual attractiveness. In the second blockthey were presented with walkers of their own sex and asked to rate the assumedattractiveness on the other sex. The resulting discriminant functions arevisualized in terms of caricatured walking displays and compared with thelinear discriminant function that best classifies the sex of a walker. Theresults show that female attractiveness as rated by male observers highlycorrelates with gender -- i.e. with the projection of a walker onto the linearsex classifier. In contrast, female attractiveness as rated by female observersis virtually independent of gender and rather appears to display a vivacious,energetic character. Male attractiveness as rated by male and female observersshows a similar tendency. Whereas male observers assume themselves to berated attractive by females when displaying masculinity, the discriminantfunction based on the ratings of female observers is in fact almost perpendicularto the gender discriminant function.

For an online demonstration, see http://www.biomotionlab.de/Demos/attractivity.html.

Male pigeons show species-specific courtship displays in front of female pigeons. The present study examined whether visual information, without auditory or tactile input, could trigger such courtship displays. The study also examined which technical measures could be used to present the visual stimuli. We studied the behaviors of male pigeons in response to video-taped and computer-animated stimuli presented on a computer monitor. The subjects did show courtship displays in front of video-taped and computer-animated females. However, they showed little or no such behavior when empty cages or upside-down views of females were presented on the monitor. Thus, subjects selectively reacted to the visual stimuli, suggesting that the artificial pigeons can be used as stimuli to represent a viable potential mate.  

The human visual system is very sensitive to animatemotion patterns. Humans can detect efficiently another living being in avisual scene and retrieve many features of psychological, biological andsocial relevance. By representing the main jointsof a person's body by bright dots against a dark background, observers caneasily recognize a human walker and determine his/her gender, recognize variousaction patterns and identify individual persons. The importance of the perceptionof biologically relevant motion patterns is reflected by the identificationof a specific neural circuitry as shown by brain imaging studies. Whereasbasic principles of the neural basis of perception of biological motion areunderstood, many issues concerning the temporal characteristics of the processingof such kind of information are as yet unclear. In the present study we investigatedhow inversion of biological motion stimuli affects components of event relatedpotentials (ERP). ERPs were recorded in response to point-light displaysof an upright walking person, point-light displays of an inverted walkingperson and displays of scrambled motion, in which the moving dots had thesame motion vectors as in biological motion displays with their initial startingpositions being randomized. Analysis yielded a N170 componentat parieto-occipital electrodes, which was more pronounced for upright walkersthan for inverted walkers and scrambled motion. A later component in thetime window between 300 and 400 ms after stimulus onset had a larger amplitudefor upright walkers and inverted walkers as compared to scrambled walkers.We hypothesize that the N 170 component reflects the holistic recognitionof prototypical configurations of a human body, whereas the later componentis associated with the integration of the dots' interrelations to a coherentpercept.  

The human visual system is very sensitive to the detection of animate motion patterns. Humans can detect efficiently another living being in a visual scene and retrieve many features of psychological, biological and social relevance.

An experimental approach for studying information from biological motion without interference from form is to represent the main joints of a person's body by bright dots against a dark background. Using this procedure, observers can easily recognize a human walker and determine his/her gender, recognize various action patterns and identify individual persons. The importance of the perception of biologically relevant motion patterns is reflected by the identification of a specific neural network as shown by brain imaging studies. Whereas basic principles of the neural basis of perception of biological motion are understood, many issues concerning the temporal characteristics of the processing of such kind of information are as yet unclear.

In the present study we investigated how different processing stages involved in the perception of biological motion are reflected by modulations in event related potentials (ERP). ERPs were recorded in response to point-light displays of a walking person, point-light displays of an inverted walking person and displays of scrambled motion, in which the moving dots had the same motion vectors as in biological motion displays with their initial starting positions being randomized.

Analysis yielded a N200 component at parieto-occipital electrodes, which was more pronounced for upright walkers than for inverted walkers and scrambled motion. A later component in the time window between 300 and 400 ms after stimulus onset had a larger amplitude for upright walkers and inverted walkers as compared to scrambled walkers. We hypothesize that the N 200 component reflects the holistic recognition of a human person, whereas the later component is associated with the integration of dot patterns to a coherent percept.  

The human visual system is very sensitive tothe detection of animate motion patterns. We can efficiently detectanother living being in a visual scene, recognize human action patternsand attribute many features of psychological, biological and socialrelevance to other persons. An experimental approach for studyinginformation from biological motion (BM) with reduced interference fromnon dynamic cues is to represent the main joints of a person's body bybright dots against a dark background.In the present study we investigated howdifferent processing stages involved in the perception of BM arereflected by modulations in event related potentials (ERP) in order toelucidate the time course and location of neural processing in BMperception. ERPs were recorded in response to point-light displays of awalking person, an inverted walking person and displays of scrambledmotion. Analysis yielded a pronounced negativity with a peak at 180 msafter stimulus onset (N180) at parieto-occipital electrodes which wasmore pronounced for upright walkers than for inverted walkers andscrambled motion. A later negative component between 230 and 360 msafter stimulus onset (N2) had a larger amplitude for upright andinverted walkers as compared to scrambled walkers and reveals a shiftof maximum negativity to temporo-parietal areas. In the latercomponent, negativity was more pronounced over the right hemisphererevealing asymmetries in BM perception. The early component mightreflect the global recognition of a human person, whereas the latercomponent might be associated with the local integration of dotpatterns into a coherent percept. We conclude that visual processingneeded to perform the highly demanding task to discriminate between BMand scrambled motion can be achieved within a time period of merely 180ms.This evidence for fast, efficient processingunderlines the importance of perception of BM and provides furtherevidence for a specific neural network involved in processingbiologically relevant motion signals. Furthermore, theright-hemispheric dominance associated with the perception of BM showsclear analogies to asymmetries in face perception and probably reflectsthe social relevance of animate motion perception. 

Computing the distance of an object from motion parallax involves the comparison of the displacement or velocity of the observer's eye with the hereby induced displacement or velocity of the object on the retina. Motion parallax computation therefore requires knowledge about the relative speed between the observer and the object. In many situations this information is not available to the observer -- either because the observer or the object moves with an unknown velocity. Theoretically, one could still determine distance by moving with two different speeds and employing only knowledge about the difference between them. We refer to this mechanism as "differential motion parallax" and we assume that many birds use this mechanism to monocularly measure distance in the lateral visual field. Here, we examine whether humans are capable of using differential motion parallax. Observers had to indicate whether a central, horizontal array of small squares was before or behind a plane represented by two flanking horizontal arrays. We measured depth discrimination thresholds for the monocularly viewed patterns with and without adding a constant, but from trial to trial unpredictibly varying motion component to the stimulus. Since motion parallax was the only cue, subjects had to make lateral translational movements with their upper body in order to solve the task. If the stimulus did not move, subjects demonstrated a high accuracy (in average 0.2 % of the viewing distance). Adding a constant speed of the same magnitude to both the central pattern and the flanking arrays did only slightly impair this performance. However, when adding constant, randomly chosen speeds independently to both patterns, the threshold increased dramatically, suggesting that the human visual system is not able not take advantage of differential motion parallax.This research is funded by the Volkswagen Foundation. 

Animals are consitently faced with theproblemof recognizing objects from various perspectives in the naturalenvironment,either because of their own movements or because of the movements ofsomeexternal object. For instance, depth rotation dramatically alters theinformationpresent in any two-dimensional view of an object; yet humans readilyrecognizemost objects despite enormous variations in vantage point. Do non-humananimalsalso show generalization over rotation in depth? Here we report anexperimentthat tested the perspective processing in the pigeon when presentedwithpictures of human heads rotated in depth. In the training, thediscriminationperformance of pigeons presented with pictures of two male headsrotatingin depth did not differ from the performance of pigeons trained on asingleview of each head or from the performance of pigeons trained onmultiplestatic views of the heads. In the transfer test, novel views of theheadswere then tested for recognition. The pigeons of all three groupsshowedviewpoint dependence, with their performance declining systematicallywithdegree of rotation from the nearest training view. These findingssuggestthat pigeons--in cotrast to humans-- do not integrate the multipleviewsof the rotating heads into the percept of a 3D-object but rather encodetheheads in the specific poses in which they are seen. This implies thattheheads are represented as a collection of familiar views that reflecttheidiosyncratic morphological aspects of these particular viewpoints.  

The human visual system shows an impressive sensitivity to subtleties in animate motion patterns carrying biologically relevant information. Frontal views of biological motion point-light walkers can be classified with respect to the gender of the walker with high accuracy. Here, we document pronounced adaptation effects that alter the perceived gender of a point-light walker. Stimuli were generated using a morphing technique which provides smooth transitions between male and female walking patterns. Observers were first presented with 5 walking cycles of either an exaggerated male walker, an exaggerated female walker or a neutral walker. Subsequently, they were tested with 700 ms presentations of walkers sampled along the male-female walking axis. Their task was to indicate whether the test walker was a man or a woman. A psychometric function was fitted to the data. Adaptation to the male walker results in a pronounced shift of perceived gender of the test stimulus. A neutral walker is perceived to be female after adaptation with the exaggerated male and male after adaptation with the exaggerated female walker. This demonstrates that adaptation can occur not only within low-level vision processes but also at high-level information processing stages.

This research is funded by the Volkswagen Foundation.  

Both face recognition and biological motion perception are strongly orientation dependent. Recognition performance decreases if the stimuli are rotated with respect to their normal upright orientation. Here, we examine the question of whether this effect operates in egocentric coordinates or in external coordinates. Two different tasks were employed. In the first task observers had to indicate whether two successivly presented images of human faces were same or different. In the second task subjects had to indicate whether a display of 50 moving dots contained a point-light walker or not. The stimuli were either shown right side up or rotated 90 degrees clockwise. The observer was either sitting upright or lying on his left side.In the face recognition task, error rates were effected neither by the observers position nor by the stimulus orientation. A strong interaction (F(1,7)=23.7, p<0.005) between the two factors indicated, that the performance is only determined by the relative orientation of stimulus and observer. Performance is best if the stimulus has the same orientation as the observer. The same result is obtained for the biological motion task (interaction: F(1,7)=22.6, p<0.005).We conclude that the frame of reference within which both inversion effects operate is an egocentric, probably a retinal frame of reference.

This research is funded by the Volkswagen Foundation.  

Biological motion patterns contain information about the identity of a person including biological attributes such as gender and age as well as personality traits. The human visual system is highly sensitive to biological motion and capable to extract this information from it. How is this information encoded in biological motion patterns and how can it be retrieved from it? I will outline a framework that transforms biological motion into a representation within which it can be analysed by means of linear methods from statistics and pattern recognition. Based on this framework the attributes that transmit information about gender and about basic personality traits are analysed. Furthermore, an ideal observer model for gender classification is constructed and compared to the performance of human observers. Although the artificial classifier performs generally much better than human observers, it turns out that it picks up on similar features as human observers do. 

Biological motion contains information about the identity of an agent as well as about his or her actions, intentions, and emotions. The human visual system is highly sensitive to biological motion and capable of extracting socially relevant information from it. Here we investigate the question of how such information is encoded in biological motion patterns and how such information can be retrieved. A framework is developed that transforms biological motion into a representation allowing for analysis using linear methods from statistics and pattern recognition. Using gender classification as an example, simple classifiers are constructed and compared to psychophysical data from human observers. The analysis reveals that the dynamic part of the motion contains more information about gender than motion-mediated structural cues. The proposed framework can be used not only for analysis of biological motion but also to synthesize new motion patterns. A simple motion modeler is presented that can be used to visualize and exaggerate the differences in male and female walking patterns.  

The information provided in a point-light display of a walking figure conveys information both about the geometry and structure of the moving body as well as about the dynamics itself. Human observers can estimate the sex of a point-light walker with a reasonable accuracy. In this study we examine whether information about gender is carried rather by motion mediated structural information or by dynamic cues.

Three groups of observers were presented with point-light displays of 80 individual walkers and had to attribute a gender to each of them. To the first 8 observers the walkers were shown in frontal view (0 deg), another 8 observers saw them in half profile (30 deg) and yet another 8 observers were presented with profile views (90 deg). After a first block of sex ratings of the veridical walkers a second block was shown which contained modifications of the walkers. Those stimuli were either normalized by their structure, thus containing only dynamic information to be used for gender classification or they were normalized by their dynamics, thus containing only structural information.

Both factors effected the number of mis-classifications (Orientation: F(2,21)=31.9, p<0.001; Information: F(2,42)=27.2, p<0.001). The interaction was not significant. With respect to the orientation of the walker, performance was best in the 0 deg condition (27% errors on average) and worst in the profile view condition (44% errors). Error rates on the veridical walkers were on average across all orientations 30%. Performance on the dynamic-only stimuli was much better (32% errors) than on the structure-only stimuli (39% errors).

Dynamical information seems to be more important for gender discrimination than structural information and is best accessible in the frontal view.

This research is funded by the Volkswagen Foundation.  

Marlies Pinnow, Dagmar Engelke, Nikolaus Troje, & Axel SchölmerichDepartment of Psychology, Ruhr-University Bochum, Germanymarlies.pinnow@ruhr-uni-bochum.deFaces, like most objects, change appearance when they are rotated. Adult viewers identify human faces from different viewpoints with remarkable accuracy. While the general interest of infants in faces is well documented, we know little about their ability to identify rotated faces.

Our study investigated three- and six-month-old infants' recognition performance using images of laser-scanned three-dimensional head models preserving the nartural texture (Troje & Bülthoff, 1996) as stimuli. 27 infants were habituated in six consecutive trials to the full-face view of either a female or a male face. Following habituation, infants were presented with two dishabituation trials, during which all infants viewed the habituated face in a new angle and a new face in the identical angle. The rotating angles used in this study varied from ±10 deg to ±50 deg.

The 6-months-old infants looked longer at the new face than at the habituated face in the 10 and 20 deg rotation when compared with the 3-months-olds, whereas higher degrees of rotation yielded no differences between the new and the rotated face for both age-groups.

Our results must be viewed with caution, since many infants did not show the expected drop in fixation time during habituation. Currently, we are modifying the protocol to gather additional data.

Troje, N.F. & Bülthoff, H.H, (1996), Face recognition under varying poses: the role of texture and shape. Vision Research, 36 (12), 1761-1771.  

A familiar person can be recognized by the way he or she moves. We investigated this ability using point-light displays of seven different walkers shown from three different viewpoints. Each observer was presented with only one viewpoint and was trained to name the walkers. During training, diagnostic information was gradually reduced by normalizing the stimuli with respect to their size, their shape, and their speed. We evaluated the influence of those manipulations on the learning curve and on the performance in separate non-reinforced test sessions. Finally, the fully normalized walkers were shown in different orientations in order to test the observer's ability to generalize to new viewpoints. Starting at chance level (14% correct responses) subjects learned quickly to associate the correct names to the stimuli (77% correct on average across all observers after 60 presentations of each walker). Normalizing the walkers by their size did not impair performance neither in the learning curve nor in the testing sessions. Normalization of the walkers with respect to their shape did cause a slight drop in performance (from 88% to 77%). An additional normalization with respect to walking speed had a stronger effect (from 86% to 71%). After relearning, observers still reached a performance of 85% correct. Performance in the test sessions was somewhat smaller. Results confirmed the ones obtained from the learning curves: Size is not used as a cue and shape does only play a minor role. The last test on viewpoint generalisation showed that although performance drops considerably even with fully normalized walkers, a 90 deg viewpoint changed walker is correctly identified in 40% of all trials. This research is funded by the Volkswagen Foundation. 

The ways in which we move our faces and bodies are the source of much biologically important information. Such movements can be exaggerated by extending techniques developed for static facial caricature into the temporal domain. Spatial exaggeration of movement is accomplished by first time-normalising the sequences to be exaggerated and then exaggerating the differences between individual frames and an average frame. We can also exaggerate the temporal properties of movement by reversing and extrapolating the time-normalisation step. Previous findings from a variety of domains where exaggeration has been shown to enhance the perception of task-relevant information are reviewed, together with new data showing that spatial exaggeration of emotional utterances relative to an averaged utterance can enhance their perceived happiness, sadness, or angriness. Conversely, it is argued that exaggerating emotional or other individual differences may actually interfere with the recovery of information common to all the sequences being averaged, in this case the lexical content. Lastly, motion exaggeration, like facial caricature, may reflect general underlying principles involved in the encoding and discrimination of biological movement, an as yet poorly understood process.  

Biological motion contains many forms of information. Observers are usually able to tell `what' action is being performed (eg. walking versus running), `how' it is being performed (eg. quickly versus slowly),and by `whom' (eg. a young versus an old actor). We used visual search to explore the perception of gender-from-motion. In the first experiment, we used computer-animated, fully rendered human figures in which the structural and dynamic information for gender were factorially combined. In separate blocks, observers were asked to locate a figure walking with a male or female gait among distractors having the same form but opposite motion. In the second experiment, point-light walkers moved along random paths in a 3-D virtual environment. Observers were asked to locate a figure walking with a male or female gait among distractors with the opposite motion. In both experiments, the set size was varied between 1 and 4, and targets were present on 50% of the trials. The results suggest that (a) visual search can be used to explore gender-from-motion, (b) extraction of gender-from-motion is fairly inefficient (search slopes often exceed 500 ms item-1), and (c) there appears to be an observer-gender by target-gender interaction, with male observers producing lower RTs for female targets and vice versa.  

Menschen und Tiere bewegen sich in einer physikalischen Welt nach physikalischen Gesetzen. Eine konstante Größe, welche dabei zeitliche und räumliche Parameter in Beziehung setzt, ist die Gravitation. Dies gilt zum Beispiel bei Pendelbewegungen, in denen bei konstanter Gravitation die Pendellänge und die Schwingungsdauer in einer festen Beziehung stehen. Biologische Bewegung enthält Elemente von Pendelbewegung sowie von weiteren Bewegungskomponenten in denen räumliche und zeitliche Größen in definierten Beziehungen zueinander stehen. Wir untersuchten in unseren Experimenten, ob diese Information vom visuellen System dafür verwendet werden kann, die absolute Größe eines sich bewegenden Lebewesens zu bestimmen. Wir zeigten unseren Versuchspersonen Punktlichtquellen-Displays von echten Hunden und von synthetischen Vierbeinergängen, die mit unterschiedlicher Geschwindigkeit abgespielt wurden, und ließen sie Angaben über deren Größe machen. Ein Tier mit langsamen Bewegungen wird tatsächlich größer wahrgenommen, als ein Tier mit schnellen Bewegungen. In der Beziehung zwischen Bewegungsgeschwindigkeit und wahrgenommener Größe drücken sich dabei die physikalischen Beziehungen zwischen Trägheitskräften und Gravitationskräften aus.

The human visual system is highly sensitive to animate motion patterns. It is able to classify and identify motion patterns on several levels ranging from the recognition of an action to the identification of the actor. How is such information encoded in visual motion data? We present a computational model that transforms visual motion data into a representation that allows identification of diagnostic invariants and we test the model for its ability to discriminate between human walking and running.The model is based on an algorithm that transforms visual motion data such that they can be successfully analyzed with linear methods from statistics and pattern recognition. The input data can be either three-dimensional trajectories of feature points on the body or their two dimensional projections. The transformation is based on a linear decomposition of postural data into a few components whose coefficients change with sinusoidal temporal patterns. Different aspects of this representation are diagnostic for different aspects of the motion patterns. We employ a linear discrimination function to classify different gait patterns.We tested the model for its ability to discriminate between 10 walking and 10 running sequences using both 3D motion capture data as well as 2D projections from 12 different viewpoints. The algorithm is robust with respect to viewpoint and with respect to the position of the marker points on the body. Classification errors are below 4%. Discrimination is mainly based on phase relations between the postural components.The algorithm does not employ any a-priori knowledge about the articulation of the body or the labels of the features. Together with a video based tracking algorithm, it can therefore be extended to work directly on video data. Hence, it is not only a powerful model for biological information processing but has also implications for both computer vision and computer graphics.

Animate motion contains not only information about the actions of a person but also about his or her identity, intentions and emotions. We can recognize a good friend by the way he or she moves and we can attribute age, gender and other characteristics to an unfamiliar person. How is such information encoded in visual motion data and how can it be retrieved by the visual system? We present an algorithm that transforms visual motion data such that they can be successfully approached with linear methods from statistics and pattern recognition. The transformation is based on a linear decomposition of postural data into a few components that change with sinusoidal temporal patterns. The components repeat consistently across subjects such that linear combinations of existing motion data result in smooth, meaningful interpolations. Examples are presented how this model can be used to discriminate between different types of motion (here: walking vs. running) and how it can be used to classify instances of the same type of motion (e.g. walking) in terms of characteristics of the actor (here: gender classification). Since the transformation is reversible the model can also be used for synthesis and modeling of animate motion. Therefore, it serves not only as a model for biological information processing but has also implications for both computer vision and computer graphics.

Efficient detection of the presence of another animate creature has an immense evolutionary significance. Animate motion, however, not only signals the presence of another creature but also carries information about its individual characteristics, actions and intentions. Humans are well able to recognize a good friend by the way he or she moves and an unknown person can easily be classified according to attributes such as gender and age based only on his or her gait patterns.

How is this kind of information encoded in animate motion patterns and how can it be retrieved from them? We present a computational model for the analysis of human gait. This model is used as a tool to analyze animate motion patterns in order to extract diagnostic invariants. It also serves as a model for animate motion perception in the visual system.

Animate motion can be described as a set of feature points (e. g. joints of an articulated body) that change their position in time. The core of the method described here is an algorithm that maps animate motion defined as a set of feature trajectories into a linear space such that linear operations become meaningful. Analogous to methods used to linearize image information (Troje and Vetter, 1998; Vetter and Troje, 1997) linearization of motion data is achieved by dissociating the information into a range-specific part and a domain-specific part (Ramsay and Silverman, 1997). For motion data, the domain-specific part contains information about the timing of the motion, whereas the range-specific part contains information about the position of the features.

The resulting linearized representation allows access to methods from linear statistics and classical pattern recognition that can be used to decode information from the motion patterns. Since the representation is lossless and thus invertible it can also be used to synthesize new, artificial motion patterns with well defined and parameterized features. This not only allows us to generate well-controlled animate motion stimuli for psychophysical and neuroethological investigations but also has applications in computer animation industry.

Ramsay, J. O. and Silverman, B. W. (1997). Functional Data Analysis. New York: Springer.

Troje, N. F. and Vetter, T. (1998). Representations of human faces. In Downward processing in the perception representation mechanism (ed. C. Taddei-Ferretti and C. Musio), pp. 189-205. Singapore: World Scientific.

Vetter, T. and Troje, N. F. (1997). Separation of texture and shape in images of faces for image coding and synthesis. Journal of the Optical Society of America A 14, 2152-2161.  

Pigeons as well as many other birds show a characteristic head-bobbing motion during walking, pecking and other behaviours: Hold-phases during which the retraction of the head compensates the forward motion of the body alternate with thrust-phases during which the head is saccadically extended forward. Based on the observed stabilization of the head during the hold-phase of the walking pigeon head-bobbing is interpreted to be an optokinetic response.

Head-bobbing also occurs during flight. In that case, however, the bird travels with a speed that can not be compensated by the retraction of the head. Our hypothesis is that head-bobbing is not only an optokinetic reaction that serves image stabilization but that it is used to derive depth information from differential motion parallax. A flying bird has the problem that it does not have direct access to information about its own velocity over ground -- a prerequisite needed to compute depth from standard motion parallax. Proprioceptive systems can provide information about velocity relative to the surrounding air, but motion of the medium itself (wind, convection) can add a further velocity component. Differential motion parallax computation might be the solution to that problem: The eye is moved with two different velocities. Comparing the difference between these velocities with the difference between the corresponding retinal image velocities can reveal distance.

We tested this hypothesis by measuring the pigeons head-motion during the approach to a perch. In this situation precise distance information is particularly important to the bird. However, to induce optic flow of the landing site on the retina the motion of the eye has to have a component perpendicular to the line along which the pigeon travels. The data show that head-bobbing does indeed contain such a component. The birds move their head not on a straight line from one hold position to the next but on a curved, U-shaped trajectory that would induce substantial optical flow of the landing site on the retina. This research is funded by the Volkswagen Foundation  

Johansson's (1973) point light walkers remain one of the most compelling demonstrations of how motion can determine the perception of form. Most studies of biological motion perception have presented isolated point-light figures in unstructured environments. Recently we have begun to explore the perception of human motion using more naturalistic displays and tasks. Here, we report new findings on the perception of gender using a visual search paradigm. Three-dimensional walking sequences were captured from human actors (4 male, 4 female) using a 7 camera motion capture system. These sequences were processed to produce point-light computer models which were displayed in a simple virtual environment. The figures appeared in a random location and walked on a random path within the bounds of an invisible virtual arena. Walkers could face and move in all directions, moving in both the frontal parallel plane and in depth. In separate blocks observers searched for a male or a female target among distractors of the opposite gender. Set size was varied from between 1 and 4. Targets were present on 50% of trials. Preliminary results suggest that both male and female observers can locate targets of the opposite gender faster than targets of the same gender.

In laboratory experiments, pigeons (Columba livia) have shown to categorize a variety of visual stimulus classes. Our aim is to investigate which aspects of the class members pigeons employ for successive discrimination. Recently, we found that the pigeons' ability to discriminate between photorealistic frontal images of human faces on the basis of sex, was predominantly based on information contained in the visual texture of the images rather than in their configural properties. The subjects' substantial failure to utilize the shape information contained in human faces was surprising. However, in these experiments we only used static images, so an important attribute of natural stimuli was excluded: parallactic motion. At least in humans motion is one of the most efficient cues for shape perception. Here we report experiments conducted in order to investigate whether or not motion supports the use of shape information in pigeons. Using laser scanned 3D-models of human faces, we compared the classification performance of three groups of pigeons. One group was trained to discriminate faces rotating around the vertical axis. A second group was trained with a single static view of each face. The subjects of a third group were trained with multiple static images showing the faces from all the viewpoints visible in the rotating face stimuli, so they were presented with all the information inherent in these stimuli except coherent motion itself. The results of these experiments will be presented and its implications for further work on this topic will be discussed.

Keywords: visual categorization, pigeon, motion, configural shape, perception

Gravitation ist eine konstante Kraft, welche auf alle Bewegung in der Umwelt einwirkt. In vielen Fällen definiert Gravitation dabei eine feste Beziehung zwischen zeitlichen und räumlichen Bewegungsgrößen (z.B. zwischen der Länge und der Periodendauer eines Pendels). Terrestrische Bewegung von Lebewesen ist ebenfalls von dieser Beziehung betroffen. Lokomotion erfordert kostbare Energie und um deren Verbrauch gering zu halten, zeichnen sich Gangmuster dadurch aus, dass sich die Gesamtenergie periodisch zwischen kinetischer, potentialer und elastischer Energie verschiebt. In einem psychophysischen Experiment untersuchten wir, inwieweit das menschliche visuelle System in der Lage ist, durch Geschwindigkeit vermittelte Größeninformation aus biologischen Bewegungsmustern von Tieren zu gewinnen.Mit Hilfe eines Bewegungserfassungssystems (Vicon 512, Oxford Metrics) wurden 3D-Trajektorien von Markerpunkten vermessen, die an den Gelenkpunkten von drei Hunden verschiedener Größe befestigt waren, welche sich mit mittlerer Geschwindigkeit durch das Messvolumen bewegten. Point-light Displays von diesen biologischen Bewegungsmustern wurden generiert, indem Projektionen der Markerpositionen in sagittaler Ansicht auf einem Computermonitor animiert wurden. Neben der Originalgeschwindigkeit wurden zwei langsamere und zwei schnellere Wiedergabegeschwindigkeiten gezeigt. Die Position der Animation auf dem Bildschirm blieb dabei unverändert, so dass der Eindruck entstand, als würde der Hund auf einem Laufband laufen. Die Größe aller Animationen blieb konstant (8.5 Grad Sehwinkel). Aufgabe der Versuchspersonen war es, die absolute Größe des Hundes (Schulterhöhe) in Zentimetern anzugeben.Sowohl die absolute Größe des Hundes als auch die Wiedergabegeschwindigkeit hatten einen signifikanten Einfluss auf die Größeneinschätzung in der erwarteten Richtung. In Animationen größerer Hunde wurden diese tatsächlich als größer wahrgenommen als in solchen von kleineren Hunden (F(1,30) = 30.8, p<0.001). In mit verlangsamter Geschwindigkeit präsentierten Animationen wurden die Hunde größer wahrgenommen als in solchen mit erhöhter Geschwindigkeit. (F (4,60) =37.4, p<0.001).Wir schließen daraus, dass menschliche visuelle Wahrnehmung die Wirkung von Gravitationskräften als Hinweisreiz für Größenwahrnehmung in biologischen Bewegungsmustern nutzen kann. Weitere Untersuchungen sind notwendig, um die exakte psychophysische Beziehung zwischen Wiedergabegeschwindigkeit und wahrgenommener Größe zu bestimmen.  

Gravity is a constant force that affects motion in the physical world. This is particularly true for animate motion, since animals try to energetically optimize their gait by shifting energy between different states such as kinetic, potential, and elastic energy. Constant gravity defines a fixed relation between temporal and spatial measures in motions such as pendulum motion and ballistic motion. In a psychophysical experiment, we tested whether the human visual system can retrieve size information mediated by gravity from dynamic point-light displays of animal locomotion. We used a motion capture system (Vicon 512, Oxford Metrics) to acquire the 3D-trajectories of marker points attached to the major joints of three dogs with different sizes. Biological motion point-light displays were created by animating these data on a computer screen in a sagittal view. Animation was presented for one second on a monitor at five different playback speed. The position of the animation remained in the center of the screen. The retinal sizes for all dog animations were identical (8.5 degrees of visual angle). Subjects had to estimate the absolute size of the dog in terms of its shoulder height in centimeters. Both the true size of the dog and the playback speed significantly affected the size estimate in the expected direction. Animations from larger dogs were perceived to be larger than animations from smaller dogs (F(1,30)=30.8, p<0.001). Dogs presented in slow motion were perceived to be larger than dogs in fast motion (F(4,60)=37.4, p<0.001). We conclude that human visual perception is able to use gravitational acceleration as a cue for size perception in biological motion displays. Further research is necessary to find out the exact psychophysical relationship between speed of biological motion and perceived size.

The retinal image is a two-dimensional projection of the three-dimensional world. Information about the third dimension, i.e. the depth of an object or a scene, originates from several cues. One of them is the shading caused by directional light. Here, we demonstrate a compelling visual illusion that is probably caused by a misinterpretation of shading information. A three-dimensional head model was stretched along the axis perpendicular to the symmetry plane by a factor of cos(25)/cos(35) and was compressed along the horizontal axis in the symmetry plane by a factor of sin(25)/sin(35). If this head is rendered in orthographic projection with an orientation of 35° (with respect to the frontal view) using only ambient light, it results in exactly the same image as the projection of the undistorted head projected with 25° orientation. Instead of ambient light we use directional light. Surprisingly, rather than revealing the distortion in depth, directional illumination causes the projected image of the distorted head to appear considerably wider. We quantitatively measured this illusion by determining the point of subjective equivalence (PSE) using a fixed-interval nulling paradigm. From both images (the undistorted and the distorted head) we derived a series of images with constant height but with a width that ranged in nine steps from 93% to 107% of the original width. In a series of 180 trials these images were simultaneously shown together with the reference head (undistorted, 25í orientation, 100% width). Subjects had to indicate which of the heads appeared to be wider. Psychometric functions were fitted to the data and PSEs were determined. The mean PSE across 15 subjects was 3.2%. As revealed by the psychometric function for the undistorted head, identical images that differ in their width by that amount were correctly discriminated at a rate of 90%. We discuss the observed illusion in the context of shape-from-shading models.

In einem Experiment zur Lateralisation der Gesichterwahrnehmung zeigten wir in einem visuellen Halbfeldparadigma 16 Männern und 16 Frauen Gesichter mit einem same-different-task. Mit diesem Experiment gingen wir der Frage nach, welchen Einfluss die Merkmale FORM und TEXTUR auf die Lateralisation der Gesichterwahrnehmung ausüben. Dazu verwendeten wir Gesichter, die sich bezüglich dieser beiden Merkmale unterschieden. Wir erwarteten für die rechte Hemisphäre einen Vorteil für das Merkmal FORM und für die linke Hemisphäre einen Vorteil für das Merkmal TEXTUR. Es zeigte sich jedoch keine direkte Interaktion von Stimulusmerkmalen (FORM/TEXTUR) und visuellem Halbfeld (RVF/LVF), sondern eine mit dem Geschlecht der Versuchsperson. Männer zeigten im RVF weniger Fehler für das Merkmal FORM und im LVF weniger Fehler für das Merkmal TEXTUR. Bei Frauen kehrte sich das Lateralisationsmuster um. Diese Ergebnisse sprechen dafür, dass es keinen einfachen Geschlechtsunterschied in der funktionellen cerebralen Asymmetrie gibt, sondern dass diese durch Stimulusmerkmale bestimmt wird.

The information contained in the image of a human face can be subdivided into components attributed to visual texture ("texture") and components attributed to configural relations ("shape"). Here, we present evidence for a dissociation of texture and shape processing in a same-different face recognition task by showing that hemispheric laterality is different for those two domains. Ten students participated in two experiments. In both experiments the two images of one trial differed only in their visual texture or they differed only in their shape. In Exp. 1 the first image was shown 6.3 degrees either to the left or to the right of a fixation point and the second image was shown centrally (lateralized encoding). In Exp. 2 the first image was shown centrally whereas the second was presented peripherally (lateralized retrieval). In Exp. 1 shape processing shows a left-hemispheric dominance whereas texture processing shows a right-hemispheric dominance. In Exp. 2 the opposite is the case. 

The head movement of a walking pigeon Columba livia is characterized by two alternating phases, a thrust phase and a hold phase. While the head is rapidly thrust forward during the thrust phase, it has been shown repeatedly that it remains virtually motionless with respect to translation along a horizontal axis (roll axis) during the hold phase. It has been shown that the stabilization during the hold phase is under visual control. This has led to the view that the pigeon's head-bobbing is an optokinetic response to stabilize the retinal image during the hold phase. However, it has never been shown explicitly that the head is really held stable in space with respect to other translatory or rotatory dimensions. Using videography, we show here that this is in fact the case: except for a small but systematic slip that presumably serves as an error signal for retinal image stabilization, the head of the pigeon remains locked in space not only with respect to the horizontal (roll) axis but also with respect to vertical translation (along the yaw axis) and with respect to rotation around the pitch and yaw axes.

Tauben ( Columba livia ) sind bekannt für ihre Fähigkeit, in Lernexperimenten eine große Anzahl verschiedenster visueller Stimulusklassen kategorisieren zu können. Es ist jedoch bis heute nicht klar, welche die dabei für die Taube perzeptuell relevanten Merkmale in den Bildern sind. Wir untersuchten anhand einer Kategorisierungsaufgabe mit komplexen natürlichen Bildern, welche Rolle die in den Bildern enthaltene Form- und Texturinformation spielt. Die Tauben (n=24) wurden darauf trainiert, Bilder von menschlichen Gesichtern nach dem Geschlecht zu unterscheiden. Wir verwendeten eine Art der Bildrepräsentation, die es erlaubt, Textur- und 2D-Forminformation in den Gesichtern zu trennen ("correspondence-based representation"; siehe Vetter, T. & Troje N.F. 1995. In: Mustererkennung. Springer, 118-125). Der erste Teil unserer Studie bestand in einem Vergleich der Klassifikationsleistungen dreier Gruppen von Tauben (je 8 Tiere), wobei jede dieser Gruppen mit einer anderen Version der Gesichterbilder trainiert wurde, die sich bezüglich Textur- und Forminformation unterschieden. Eine Gruppe (ORIGINAL) wurde mit den Originalbildern trainiert, die zweite Gruppe (TEXTUR) mit Bildern von Gesichtern, die alle identische Form besaßen und sich nur hinsichtlich der Texturinformation unterschieden, und die dritte Gruppe (FORM) wurde mit Bildern von Gesichtern trainiert, die sich nur in der Form unterschieden, jedoch alle dieselbe Texturinformation aufwiesen. 100 Bilder (50 männliche und 50 weibliche Gesichter) wurden für dieses Klassifikationstraining verwendet, 100 weitere für die anschließenden Klassifikationstests. Die Ergebnisse zeigen deutlich, daß die Tauben hauptsächlich die in den Bildern enthaltene Texturinformation verwendeten. Während die Tiere der ORIGINAL- und der TEXTUR-Gruppe im Training rasch eine hohe Klassifikationsleistung erreichten und im Generalisationstest mit neuen Bildern gute Transferleistungen zeigten, waren die Ergebnisse bei der FORM-Gruppe signifikant schlechter. Wir konnten zeigen, daß Texturmerkmale für Tauben nicht nur genügend Information bieten, um eine komplexe natürliche Kategorie leicht klassifizieren zu können, sondern darüber hinaus auch, daß sie - im Gegensatz zum Menschen - Texturmerkmale gegenüber Formmerkmalen deutlich bevorzugen. Offensichtlich haben Tauben ein hoch entwickeltes Texturerkennungssystem, das für standpunktunabhängige Objekterkennung von Bedeutung sein könnte.  

Studies based on anthropocentric cognitive models of categorisation and computer simulations have failed to explain the ways in which animals analyse and group natural stimuli. Among animal researchers, it is commonly agreed that stimuli that may appear complex to the experimenter might be classified by the subjects using very simple image properties. Furthermore, a fundamental problem posed by natural categorisation is how organisms such as the pigeon, which are devoid of language and presumably also of further high-level capacities, can rapidly extract abstract invariances from stimulus classes containing instances so variable that neither the class rule nor the exemplar can be physically described. An example of such a class is the human face. Despite innumerable attempts, it remains to be determined how humans sort faces into psychologically relevant classes. However,it is possible that birds categorise this class of stimuli in much simpler, though still successful ways. Our pigeons rapidly sorted two hundred photo-realistic frontal views of human faces according to sex. Using a correspondence-based representation of faces, we dissociated the information about an item into two parts; one coding for the spatial configuration of its features, and the other for its particular appearance. The results of the original discrimination training clearly indicated that pigeons preferred to exploit the surface properties of faces to their spatial properties. Furthermore,subsequent transfer tests revealed that within the surface domain they used overall brightness, colour, brightness gradients (ratio between the top and bottom half of the face), and shading.

Purpose: Judging the orientation of bilateral objects, e.g. faces, is a task that humans perform everyday. In order to achieve some understanding of the mechanisms underlying this ability, we investigated the influence of directional light on perceived orientation. Changing the illuminant's position induces a substantial apparent orientation shift. We documented this phenomenon and measured its size as a function of the mean orientation of the face and the angle between the two light directions. To assure that the phenomenon is based on perceived orientation of the face rather than on gaze direction, we repeated the experiment using surface models with constant albedo.

Methods: Using a 2AFC paradigm, the participants decided whether a sequence of two images appeared to rotate to the left or to the right. The images showed the same face in different orientations and illuminated from different directions. Between the two images, a distractor was shown for 2 sec. The physical rotation nullifying the illumination induced apparent orientation shift was determined. Results: Depending on the set of parameters used, an apparent orientation shift of up to 10 degs can be induced. The effect is minimal when the face is shown in a frontal view and maximal if the face is oriented 30 to 45 degs from frontal. The increase of the apparent orientation shift with the angle between the two light directions saturates at an angle of 60 degs between the light directions. The phenomenon does not require photorealisticly textured stimuli. Using surface models with constant albedo yields similar results.

Conclusions: The results are discussed in the context of possible strategies for orientation judgement. The finding that the magnitude of the effect is dependent on the mean orientation of the face implies that the effect is unlikely to be based only on local surface attitude judgements. Rather, we favor a model which assumes that first the symmetry plane of the face is detected, and then a comparison is made between the visible parts on both sides of this plane to estimate orientation.

How do people estimate the orientation of other people's faces? We observed that two images of a face seen from the same orientation, but illuminated from different angles, appeared to have different orientations. The first experiment was designed to document and quantify this phenomenon with respect to the average orientation of the face. The images were rendered on a black background that made it impossible to discriminate the shadowed facial parts from the background. We determined the physical orientation shift necessary to compensate for the illumination-induced effect. Results showed that the measured illumination-induced apparent orientaion shift (IAOS) correlates positively with the average orientation of the face and reaches values of up to 9°. This correlation implies that the mechanism is not based on local surface attitude judgements. We propose a model in which the symmetry plane of the face is detected, and then a comparison is made between the visible parts on both sides of this plane. The effect of the shadow occluding parts of the faces would then be responsible for the apparent orientation shift. To test this hypothesis we repeated the first experiment using a background colour that allowed subjects to perceive the true outline of the faces. We found that the IAOS was reduced to values of less than 2° and no longer depended on the average orientation of the faces. The results imply that orientation may be judged by comparing the size of the visible parts of the left and right halves of the face.

In human perception, as well as in machine vision, a crucial step in solving any object recognition task is an appropriate description of the object class under consideration. We emphasise this issue when considering the object class `human faces'. We discuss different representations that can be characterised by the degree of alignment between the images they provide for. The representations used span the whole range between a purely pixel-based image representation and a sophisticated model-based representation derived from the pixel-to-pixel correspondence between the faces [Vetter and Troje, 1995, in Mustererkennung Eds G Sagerer, S Posch, F Kummert (Berlin: Springer)]. The usefulness of these representations for sex classification was compared. This was done by first applying a Karhunen -- Loewe transformation on the representation to orthogonalise the data. A linear classifier was trained by means of a gradient-descent procedure. The classification error in a completely cross-validated simulation ranged from 15% in the simplest version of the pixel-based representation to 2.5% for the correspondence-based representation. However, even with intermediate representations very good performance was achieved.

Purpose: Recently, we investigated human performance to generalize to novel views of a learned face (Troje & Bülthoff, 1996, Vision Research, in press). Among other results, we made the observation that generalization to views that are symmetric with respect to the frontal view is much better than to otherwise different views. Here, we present new psychophysical experiments investigating the nature of this performance. In particular, our question is, whether this performance is based on the bilateral symmetry of the 3D-object or on the resulting mirror symmetry of the images.

Methods: Two experiments were performed. Both used a SAME/DIFFERENT recognition paradigm in which two images of faces were shown in immediate succession. The subject then decided, whether or not the two images showed the same person. The images were made from 3D head models and showed the face without its natural texture, only applying a Lambertian shading model. This allows decoupling the symmetry of the view from the mirror symmetry of the image. Symmetric views yield mirror symmetric images only if the simulated light source in both images comes from the direction of the camera or if it is also symmetric with respect to this direction. However, if the light comes from the same side, the images taken from symmetric viewpoints are no longer mirror symmetric. In experiment 1 training and testing views where either identical or symmetric. The light direction was also either identical or symmetric, yielding four different conditions. In experiment 2 the light always came from the direction of the camera. Training and testing views were either identical, symmetric or otherwise different. In a fourth condition we used instead of the symmetric view the flipped, perfectly mirror symmetric image for testing.

Results: Experiment 1. If both viewpoint and illumination were symmetric, performance was much better (p<0.005) than when only the view or the illumination changed. Experiment 2. Generalization to the perfectly mirror symmetric image was even better (p<0.05) than to the symmetric view. Conclusions: The better generalization to the symmetric view of a head model is not based on knowledge about the almost bilaterally symmetric three-dimensional structure of the head but rather on the simple image operation of identifying mirror symmetric images.

Purpose: We compared quality of information available in 3D surface models versus texture maps for classifying human faces by sex. Methods: 3D surface models and texture maps from laser scans of 130 human heads (65 male, 65 female) were analyzed with separate principal components analyses (PCAs). Individual principal components (PCs) from the 3D head data characterized complex structural differences between male and female heads. Likewise, individual PCs in the texture analysis contrasted characteristically male vs. female texture patterns (e.g., presence/absence of facial hair shadowing). More formally, representing faces with only their projection coefficients onto the PCs, and varying the subspace from 1 to 50 dimensions, we trained a series of perceptrons to predict the sex of the faces using either the 3D or texture data. A "leave-one-out" technique was applied to measure the gen-eralizability of the perceptron's sex predictions. Results: While very good sex generalization performance was obtained for both representations, even with very low dimensional subspaces (e.g., 76.1% correct with only one 3D projection coefficient), the 3D data supported more accurate sex classification across nearly the entire range of subspaces tested. For texture, 93.8% correct sex generalization was achieved with a minimun subspace of 20 projection coefficients. For 3D data, 96.9% correct generalization was achieved with 17 projection coefficients. Conclusions: These data highlight the importance of considering the kinds of information available in different face representations with respect to the task demands.

Purpose:How do observers recognize objects despite dramatic image variations that arise from changes in illumination? Some evidence suggests that changes in illumination direction influence object recognition (Kersten et al., ARVO 1995). We examine whether illumination dependency extends to face recognition. A corollary issue is whether cast shadows improve performance by providing information about light source direction, or hinder performance by introducing spurious edges that must be discounted prior to recognition. Methods:The 3-D geometry and color texture maps of 80 human faces were digitized using a 3-D laser scanner (Cyberware™). Each face was rendered with the light source located in the upper right or upper left quadrant in front of the face, and with cast shadows present or absent. On each trial, 2 faces were presented sequentially, each followed by a mask (a random array of face features). The two faces were illuminated from either the same or different directions. Observers judged whether the two faces were the same or different individuals. Half of the observers viewed faces with cast shadows, and half viewed faces without cast shadows. Results:When there was no change in the direction of illumination, recognition was faster (877 vs. 930 ms, p<.001) and more accurate (92% vs. 85%, p<.001) than when there was a change. Recognition was slower when cast shadows were present than when they were absent (973 vs. 833 ms, p<.05). Conclusions:Our finding that face recognition is illumination dependent is consistent with the use of "information-rich" representations for recognition. The results indicate that 1) face recognition processes are sensitive to either the direction of lighting or the resultant pattern of shading, and 2) rather than aiding recognition by providing information about the illuminant, cast shadows hinder recognition, possibly by masking out informative features or leading to spurious contours.