Adapting to low contrast; how the remarkable flexibility of binocular fixation helps us read better

We report a new, unexpected effect in which the visual system responds binocularly to text becoming harder to read because of reduced contrast. Participants read a row of white numbers against a static background that changed from black to white, from left to right. There was pervasive binocular disparity between the fixation points of the left and right eye. The lines of sight typically crossed in front of the stimulus plane. The proportion of such disparities increased systematically, as did their size, as reading difficulty increased. We explain this novel effect in terms of the projection of the visual hemifields to the cortex. First, the central part of the binocular perceptual window benefits from contralateral hemispheric projection for both eyes. Second, the larger this central zone, the more high quality context is available against which to compare the digit currently being read. Third, placing the stimulus beyond the horopter reduces the probability of one eye’s input being suppressed. Thus, we see that binocularity in human vision is remarkably flexible. Optimal perception for higher cognitive processing is ensured by strategies embodied in the peripheral musculature, cashed out neuropsychologically: apparent failures to fixate conjointly help us to read in difficult conditions.