The mechanisms of crossed and uncrossed disparities in coarse stereopsis

Stereopsis, our ability to perceive depth, is a fundamental aspect of vision. It allows us to judge whether objects are “in front of” or “behind” each other. High variability in the perception of stereopsis has been reported in the population. Previous studies suggest that this variability may results from different mechanisms subserving the perception of crossed and uncrossed stereopsis. In our previous study, we focused on fine stereopsis. Here we focused on coarse stereopsis. We investigated the difference between crossed and uncrossed stereopsis mechanisms using an identification-at-threshold paradigm. We used a 2-by-2 forced choice procedure where participants had to both report the interval with the stimulus and judge whether the stimulus was crossed or uncrossed. The stimulus consisted of a gaussian bump (size) in a filtered noise texture (0.7 cycles per degree). Preliminary data revealed that typical observers were able to consistently achieve perfect discrimination. However, at extremely large disparities, some individuals could not discern polarity, probably because they could not fuse anymore. As observed before for fine stereopsis, these results suggest that crossed and uncrossed disparities are mediated by 2 different channels for coarse stereopsis.