Depth perception from point-light biological motion displays

Humans have a clear impression of facing in depth for point-light biological motion. However, this has not been measured systematically nor is it known on which cues humans rely for their judgment. In the present study subjects judged the facing orientation-in-depth of point-light displays. The displays represented natural walking and modified versions in which the time sequence was reversed, action was perturbed, the limbs and joints were nonrigid, the temporal sequence was scrambled, or the joint positions were scrambled. We found that the subjects were best at judging the facing direction of normal and reversed walking with an accuracy of 6 degrees and 10 degrees precision. The results show that pendular motion of the limb segments and the implicit knowledge of the human body play an important role for the precision of the judgment. Three further factors were relevant for the judgment of facing direction: (a) the discrimination of the front and back side, (b) the facing bias, and (c) the impression of depth from the display, probably due to the kinetic depth effect. The latter influences the accuracy, which differed strongly between subjects. The results suggest that the facing bias, to perceive the figure as facing toward the observer rather than away, is not related to the recognition of a human figure but rather to the presence of oscillating movements of the dots in the display.