The inversion effect in biological motion: Maybe there are two of them!

If biological motion point-light displays are presented upside down, performance in almost any visual task will decline. The inversion effect in biological motion shows characteristics that make it comparable to the inversion effect described for face recognition. Particularly, there exists evidence that this inversion effect is due to impaired processing of the familiar shape of an articulated body.

Recently, we have demonstrated that a scrambled biological stimulus which is completely devoid of structural information not only retains information about the direction of a walking human or animal, but is also subject to a pronounced inversion effect. Furthermore, we could show that the information conveying direction and carrying the inversion effect is entirely due to the local motion of the feet.

Apparently, our visual system responds sensitively to an invariant signature contained in the movement of the feet of a locomoting, terrestrial animal constrained by gravity and the attempt to move energetically efficient. We speculate, that a simple visual filter tuned to the resulting velocity profile functions as a general “life detector”. It might be evolutionary old and shared with other animals, a speculation corroborated by recent work on filial imprinting behaviour in chicks (Vallortigara & Regolin, 2006).

Direction from scrambled motion

This animation contains stimuli that demonstrate our ability to derive information about the direction from scrambled biological motion. If the stimulus is inverted performance declines to chance level.

Since our initial demonstration of an inversion effect in biological motion perception that is entirely local in nature, we have carried out a series of projects that have been dedicated to investigating how the visual system retrieves information from individual local motions, and in particular, the motions of the feet.

Local biological motion containing a signature of terrestrial “life”?

Can information about animacy be retrieved from local biological motion? Our data show that like the perception of direction, the perception of animacy from scrambled displays is orientation-dependent such that upright scrambled walkers are perceived to be more “alive” than inverted scrambled walkers.

Characteristics of the local motion mechanism

How is the local motion mechanism different from the mechanism responsible for retrieving the global, articulated structure of the agent? We acted on predictions driven by a hypothesis that the local motion mechanism reflects a system that is hard-wired and the global mechanism reflects a system that is acquired through experience. Our data show that the local motion mechanism is not sensitive to the familiarity of the agent (i.e.,human vs. another animal), is not sensitive to training across time, and is robust to visual noise. By contrast, the global structure-from-motion mechanism is affected by walker familiarity, can improve with training, and is largely susceptible by visual noise.

Why is the foot motion so special?

In this project, we investigated the critical cues in the foot’s motion that are exploited by the visual system. We generated novel stimuli derived from solely fragments of the foot trajectory and showed that the inversion effect is carried by vertical acceleration in the foot’s motion! See a sample stimulus here:

What is the adequate local biological motion?

Observers from around the world participated in our search for the “super foot”, defined as one that carries salient directional information and a pronounced inversion effect.Check out the feet that have evolved! [BMLevolution]


  • Troje, N. F. and Westhoff, C. (2006). Inversion effect in biological motion perception: Evidence for a “life detector”? Current Biology 16:821-824.
  • Johnson, M. H. (2006). Biological motion: A perceptual life detector? Current Biology 16:R376-R377
  • Chang, D. H. F. and Troje, N. F. (2008) Perception of animacy and direction from local biological motion signals. Journal of Vision 8(5,3), 1-10.
  • Chang, D. H. F. and Troje, N. F. (2009) Acceleration carries the local inversion effect in biological motion perception. Journal of Vision 9(1,19):1-17.
  • Chang, D. H. F., Troje, N. F. (2009) Characterizing global and local mechanisms in biological motion perception. Journal of Vision 9(5):8, 1-10.

Gravity and its Role in Biological Motion Perceptions

Gravity links spatial and temporal aspects of the behaviour of objects in our visual environment. Gravity also imposes constraints on moving animals. Our visual system employs knowledge about these constraints in order to detect, perceive and adequately interpret biological motion. Psychophysical experiments show how this is achieved.

Size and speed demo

This demo simulates an experiment (Jokisch and Troje, 2003) in which a quadruped point-light animal, walking with different gait frequencies through a structured environment, was presented to observers. They had to adjust its size to look “as natural as possible”. In doing so, they basically reproduced the inverse quadratic relation between size and frequency as formulated in the pendulum law.


      • Jokisch, D. and Troje, N. F. (2003) Biological motion as a cue for the perception of size. Journal of Vision, 3:252-264, Full Text
      • Troje, N. F. (2003) Reference frames for orientation anisotropies in face recognition and biological-motion perception.Perception 32:201-210. Full Text
      • Troje, N.F. (2004) Inverted gravity, not inverted shape impairs biological motion perception. Paper presented at the Vision Science Society meeting, Sarasota, FL.  Poster / Abstract
      • Hirai, M, Chang, DHF, Saunders, DR, Troje, NF (2011) Body configuration modulates the usage of
        local cues to direction in biological-motion perception Psychological Science(22):1543-1549. Full Text
      • Troje, N. F., Chang, D. H. F. (2013) Shape-independent processes in biological motion perception. In: Johnson, K. L. & Shiffrar, M. (eds.) People Watching: Social, Perceptual, and Neurophysiological Studies of Body Perception. Oxford University Press, pp. 82-100. Full Text