Dynamic Random-Dot Stereograms Reveal Up-Down Anisotropy and Left-Right Isotropy Between Cortical Hemifields

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Abstract
With the use of dynamic random-dot stereograms (which are devoid of all monocular depth cues), the temporal duration for detecting a small, briefly presented test square of different depth than the surround varied as a function of its location in the central portion of the visual field. Test squares presented in the upper hemifield were detectable at consistently shorter durations than those in the lower hemifield when the fixation marker was in front of the surround, and vice versa when the marker was behind. No such anisotropy was found for left and right hemifields. Exploratory studies suggested a similar up-down anisotropy and left-right isotropy in spatial resolution. Thus, the upper hemifield representation at the cortex shows a general superiority over the lower one for binocular detectors tuned to uncrossed disparities, and the lower hemifield shows superiority for those tuned to crossed disparities.