Shape recognition alters sensitivity in stereoscopic depth discrimination.

A fundamental question in visual perception is to characterize how information from sensory input is integrated with prior probabilities. The role of prior probabilities is controversial for elementary visual processes, which are often believed to be immune from higher-level influences. In this paper, we demonstrate such influences. We tested human observers' abilities to discriminate stereoscopic depth defined by points embedded in a biological pattern--a human figure. Our results indicate that the internal representation of a walking human figure imposed constraints on depth discrimination of a static stimulus. This constraint was manifested when the stimulus was recognized as a human figure. When the expected human figure configuration (forearms having equal length) was inconsistent with the sensory input information, discrimination of forearm lengths was impaired. In contrast, when there was no inconsistency (the hand-hip distance on the left was not expected to be equal to that on the right), discrimination between the two distances was improved, presumably because the human figure configuration provided a more accurate frame of reference for stereoscopic depth. Both the impairment and improvement were due to changes of discrimination sensitivity rather than decision bias. Our findings support the view that visual perception is an inference process constrained by Bayesian prior probabilities.