It is VSS time again! This year I came here with two graduate students and two posters. Seamas Weech and Sophie Kenny both present studies relating to some recent discussions of the "facing-the-viewer bias" that we observe in biological motion perception: Even though standard point-light displays and related depictions of the human body in action do not contain any cues to the order of the elements in depth, observers seem to prefer one of the two theoretically possible percepts: The walker seems to be facing the observer rather than facing away. While this observation has been demonstrated multiple times, it is not clear why it occurs. The poster by Weech and Troje offers an explanation. Troje, Kenny and Weech explore whether linear perspective can really disambiguate depth in these displays. I am posting both posters here for you to explore.

Weech, S., Troje, N. F. (2013) Does a convexity prior explain the facing-the-viewer bias in the perception of biological motion?

Troje, N. F., Kenny, S. Weech, S. (2013) Can we perceive linear perspective in biological motion point-light dispays?

If someone would present you with the display of a person stepping backwards on a treadmill and you were asked “Which way is the person going“ you might ask for more specific instructions: “The person is facing in one direction, she is moving as if walking into the other direction, but overall she remains stationary. So what do you mean with 'direction'?" How would a pigeon respond?

Human are able to retrieve information from biological motion through at least two different channels: The global articulated structure as revealed by the non-rigid, yet highly constrained deformation of the dot pattern, and the characteristics of local motion trajectories of individual dots. In the example above, the articulated structure indicates the facing direction, and the local motion indicates the intended walking direction of the backwards walking walker.

In a new study that has just come out in the journal Vision Research, we tested eight pigeons on a task in which they had to discriminate stationary left-facing from right-facing biological motion point-light figure. We then challenged them with a number of test trials introduced into the sequence of the normal training trials.

Tested on backwards moving walkers, the majority of the birds indicated that they used local motion cues to solve the training task, while the remaining birds obviously used global, configural cues. Testing the pigeons on different versions of scrambled biological motion confirmed that each individual bird had made a clear decision for one of the two potentially available strategies. While we confirm a previously described local precedence in processing visual patterns, the fact that some birds used global features suggests that even the birds who relied on local cues probably dispose of the perceptual abilities to use global structure, but “chose” to not use them.