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Adrien Chopin, David C. Knill, Dennis M. Levi, Daphne Bavelier; Stereoscopic depth from absolute and relative disparities. Journal of Vision 2014;14(10):969. doi: https://doi.org/10.1167/14.10.969.
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There has been a long-standing debate about the mechanisms underlying the human perception of stereoscopic relative depth. Relative depth between visual objects could be recovered in two different ways. The first one is using the difference of their absolute disparities, which are the differences of the monocular distances between each object and fixation point. A second more direct route consists of using relative disparities, i.e. differences in monocular distances between objects. Studies have claimed the existence of an independent relative disparity system from better performances in two-alternative forced choice discriminations between simultaneously presented stimuli compared to two-interval forced choice (2IFC) between successively presented stimuli, designed to isolate absolute disparities. However, memory noise and vergence noise can substantially reduce performance in the 2IFC task. Further, no previous study has controlled for visual references, leaving open the possible use of relative disparities in 2IFC tasks. We measured depth performance from absolute and relative horizontal disparities with a single stimulus method, involving a single memory component for both conditions. No fixation point was presented and the screen-border shape was in binocular rivalry. Participants were asked to judge the distance to the screen of two lines at the same depth (absolute condition) or the distance between two lines at different depths (relative condition). Vergence noise was also measured using nonius lines. If relative disparity is computed from absolute disparities, one can predict performance in the relative condition from performances in the absolute condition and from the vergence noise. Performance in the relative condition was significantly better than predicted performance, and better than absolute disparity performance. Interestingly, dress-makers displayed significantly better stereoscopic and vergence performance compared to a group of control participants. We conclude that either the relative disparity system is independent from the absolute disparity system, or that absolute disparities cannot be accessed directly.
Meeting abstract presented at VSS 2014
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