Perception of limited-lifetime biological motion from different viewpoints.

Studies with time-limited point-lights suggested that biological motion does not require local motion detection. These studies used walkers seen from the side, but biological motion perception excels also when walkers are oriented toward the observer, or in intermediate, half-profile views. In perspective projection, the local motion of points on the body provides a cue to the 3D structure of the walker. Thus, local point motion that was irrelevant for walkers in profile view may become important for biological motion perception in perspective projection. We compared performance on forward/backward walking discrimination of walkers in orthographic and perspective projection when view orientations and with point lifetime was varied. We found no difference between orthographic and perspective projections. Walkers with point lifetime 1 allowed forward-backward discrimination reliably in non-profile views, suggesting that local image motion is not required. Discrimination performance became extremely difficult in the frontal view, however. Follow-up experiments that tested lifetime, view orientation, and specific information from the feet indicated that this dependence on viewing angle can be explained by the reliance of the forward/backward discrimination on information about the movement of the lower legs, which is difficult to ascertain in the frontal view.