Abstract
Humans extract depth information from differences between the images in the two eyes (binocular disparities), a process known as stereopsis. The visual system is highly sensitive to these differences, a remarkable accomplishment given that disparity signals change continually on the retina. This happens because of fixational eye movements (FEM), the relatively large instability of fixation that causes the two retinal images to move largely independently from each other. Previous work has suggested that, rather than being detrimental, FEM may actually contribute to stereopsis. These experiments focused on the disparity modulations introduced by fixational vergence, which are particularly suited to be used by the visual system since they do not depend on the stimulus. Here we show that disparity modulations due to fixational version are also beneficial. In a forced-choice task, observers reported the vertical slant of a planar surface relative to the frontoparallel plane (top closer than bottom or vice versa). Random-dot stereograms were examined through a stereoscope, while eye movements were recorded at high-resolution via Dual Purkinje Imaging. A custom system for gaze-contingent control updated the display in real-time to either counteract all FEM (retinal stabilization) or selectively eliminate disparity modulations caused by fixational vergence or version. Results confirm that stereoscopic discrimination is impaired under retinal stabilization (d’ was reduced from an average of 2.8 to 1.4) and that fixational vergence alone is sufficient to re-establish normal performance. Critically, however, version alone is also sufficient to recover performance as long as it introduces disparity modulations on the retina. Furthermore, we show that performance in all conditions is proportional to the strength of the disparity modulations caused by retinal image motion (p<0.001) with stronger modulations resulting in higher sensitivity. These findings support the proposal that stereopsis relies on transient disparity signals caused by eye movements.