fMRI measures of interocular luminance masking reflect rapid binocular plasticity

Recent behavioral studies suggest that rapid short-term plasticity in adult binocular vision may occur over several minutes. In the example of adult monocular patching, a difference in luminance between the eyes may cause plasticity. Analogous work in animal models exists, but relevant physiological data from humans is sparse. We measured human binocular function with fMRI by determining how the BOLD signal evoked by test stimuli in one eye is modified by the effects of fellow eye stimulation. The tested eye viewed moving checkerboard patterns that varied in contrast. Three fellow eye viewing conditions were compared over various time periods: black screen, gray screen, or the same checkerboard as the tested eye. Overall, the same checkerboard in both eyes elicited the highest brain activity in early visual cortices and area MT+. In addition, we found higher brain activity for a black screen compared to gray screen in early visual areas, consistent with luminance masking. However, this effect is absent if the luminance presented to the fellow eye changes several times per minute, rather than remaining constant for several minutes. We suggest that longer viewing periods allow for greater fellow eye adaptation effects to accumulate over time. Results are discussed along with previous studies of monocular patching and contrast normalization models of vision.