Top-down influence of recognition on stereoscopic depth perception

Purpose. Last year we demonstrated that the recognition of biological motion sequences is consistent with a view-based recognition framework. We found that anomalies in the depth structure of 3D objects had an intriguing lack of influence on subject ratings of its figural goodness. In the present work, we attempt to explain this result by showing a strong top-down influence from high-level vision (object recognition) on early vision (stereoscopic depth perception). Methods. We used biological motion sequences of the kind first described by Johansson (Percep. & Psychophysics, 14, 201-211, 1973) to study the perception of 3D structure of human-like versus randomly moving dots displayed in stereo. The depth structure of the human sequence was altered by adding controlled amounts of depth noise (that left the 2D projections largely unchanged). "Random" sequences were created by adding x-y positional noise to the "Human" sequences. In a 2AFC task, participants had to decide whether 3 randomly chosen dots from stereoscopically displayed dot motion sequence appeared at the same distance from the observer. Results. Subject performance was significantly (p < 0.005) better with "random" sequences than with "human" ones. In a human sequence triples drawn from the same limb were often perceived as being in one depth plane irrespective of their actual "distorted" 3D configuration. Conclusions. Those results indicate the existence of top-down object-specific influences that suppress the perception of deviations from the expected 3D structure in a motion sequence. The absence of such an influence for novel structures might account for subjects' better performance with the random sequences.