How does the visual system adapt to 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 as legibility decreased. Overall, the lines of sight crossed more frequently in front of the stimulus plane (“crossed fixation disparities”) than behind (“uncrossed fixation disparities”). The proportion of crossed fixation disparities increased systematically with reading difficulty. Absolute size of fixation disparity changed differentially in the two disparity types when contrast reduced, implicating different subsystems. We claim that hemisphericity provides the most insightful understanding of these behaviours. The viewer has flexible control of (a) advantaged contralateral projections from retina to cortex, (b) the size of the perceptual window, and (c) binocular fusion. Thus, an apparent failure of the eyes to fixate precisely conjointly is revealed as an adaptation of embodied cognition.

We analyse binocular eye-tracking data from multiline Arabic and Hebrew reading. We describe distributions of small temporal asynchronies between the two eyes as each fixation starts and ends. We test the theory, derived from research on left-to-right orthographies, that these asynchronies reflect ocular prevalence for the left eye in the left hemifield and the right eye in the right hemifield. Ocular prevalence means one eye’s input is prioritised in the fused binocular percept. The overall pattern of asynchronies in Arabic and Hebrew resembles that seen in the left-to-right orthographies, English and Chinese, but with some very specific differences. We discuss the implications of the hemispheric asymmetry in parafoveal lookahead between the two orthographic directions. We consider orthographic conventions associated with reading direction and we conclude that a language tends to get the orthographic conventions that the reading direction and the hemispheres deserve.

We explore low-level, behavioural universals in reading,across English and Chinese. We investigated binocularcoordination in terms of the small non-alignments betweenthe two eyes’ fixations in time. We define a typology of ninesuch asynchronies and report the different spatial distributionsof these types across the screen of text. We interpret them interms of their implications for ocular prevalence—theprioritizing of the input from one eye over the input from theother eye in higher perception/cognition, after binocularfusion. The results show striking similarities of binocularreading behaviours across the two very differentorthographies. Asynchronies in which one eye begins thefixation earlier and/or ends it later occur most frequently inthe hemifield corresponding to that eye. We propose that suchsmall asynchronies in binocular fixations prioritize the higherprocessing of the input from that eye, after binocular fusion.