Extraction of depth from opposite-contrast stimuli: transient system can, sustained system can't.

The ability of observers to extract depth from opposite luminance-contrast-polarity stimuli was investigated. The stimuli consisted of two dichoptic-pairs of Gaussians, with one of the Gaussians in each pair having a positive contrast-polarity and the other a negative contrast-polarity. Stimulus durations ranging from 0.2 to 4 s were used. This range of durations was employed to reveal stereo mechanisms that were preferentially sensitive to transient or sustained stimuli. Stimuli were presented in a raised-cosine temporal envelope. Performance with stimuli of the same contrast-polarity was also tested. Observers could easily perceive depth with the same-polarity stimuli, at both long and short durations. Depth could be perceived with low-contrast opposite-polarity stimuli only at short durations. However, depth could be perceived with long-duration stimuli presented within a raised cosine temporal-envelope if a high contrast was used. Depth could also be perceived with low-contrast long-duration stimuli if they were presented within a rectangular temporal-envelope. These findings suggest there are separate sustained and transient mechanisms for stereopsis and that the transient-stereoscopic system can extract depth from opposite-contrast stereograms while the sustained system cannot. Further, it is likely that depth perception with opposite-contrast stereograms found in many previous studies was mediated by the transient-stereopsis system.