Binocular strategies in reading and non-reading visual tasks: from oculomotor behaviours to higher cognition
Humans use two eyes to extract selected information from the real world and deliver it to the human brain for higher cognition and the perceptual system. However, the two eyes are not always aligned. Eye movements are produced and the eyes accommodate in response to different visual stimuli. Studies on binocularity have reported its importance in accord with specific visual processing requirements. The main aim of this thesis is: how does binocular performance correspond to higher cognition in visual perception? This thesis explores how the two eyes coordinate their efforts to respond and adapt to cognitive activity, through oculomotor behaviours. The physical disposition of the eyes can be a part of cognitive processing, as an ‘embodied’ process of visual perception, that facilitates visual information processing. Therefore, we investigate binocular strategies in both reading and non-reading visual tasks, from oculomotor behaviours to higher cognition, to understand its implications for systematic and strategic processing in visual perception. In the reading part, we first investigate the two left-to-right orthographies, English and Chinese. We analyse small temporal non-alignments between the two eyes’ with nine patterns of temporal disjugacy and find that the distribution of small asynchronies of binocular fixation accord with ocular prevalence (Chapter 2). Such asynchronies are predictive of ocular prevalence, in which input to the left eye is prioritised in conscious perception of a fused visual stimulus for targets in the left visual field and right-eye input is prioritised for targets in the right visual field. Then, this typology is further explored in two right-to-left orthographies, Arabic and Hebrew (Chapter 3). We found reading behaviours of these Semitic languages resemble this advantageous switching of ocular prevalence, but mirror the pattern of left-to-right orthographies due to the change of reading direction. In the non-reading tasks, we first investigate binocular adaptive behaviours in two reading-like tasks (i.e. reading lines of numbers). We specifically explore how the visual system reacts and adapts to continuous text-background contrast (chapter 4) and how the visual system reacts and adapts to the level of faintness of the stimulus (chapter 5). In the contrast experiment (chapter 4), we found systematic adaptation of binocular vision in response to the contrast change, with an overall crossed tendency of binocular fixations (i.e. right eye’s fixation to the left of the left eye’s), as well as disparity-related behaviours as the contrast became lower and reading difficulty increased. On the other hand, in the experiment with variation of the level of faintness of the stimulus (chapter 5), we also found systematic adaptive binocular behaviours with an overall crossed tendency of fixations, but with increasing distribution of uncrossed fixation pairs (i.e. right eye’s fixation to the right of the left eye’s) as the level of blurriness of the text stimulus increases and reading difficulty increases. Our findings are consistent with previous research but reveal that the adaptation is a strategy rather than the effect of fatigue, and that this strategy operates very flexibly and specifically with respect to location. The overall effect of the visually adaptive behaviours in these two experiments suggests that the two eyes coordinate to adjust through very peripheral muscle-driven movements of the eyes and all the way to higher cognitive processing, in response to contrast change and blurring of the stimuli, for better visual quality and performance in different conditions for visual perception. Finally, we conducted two illusion-related experiments, including a visual illusion created by depth information (chapter 6) and a motion-based illusion with Plateau’s Spiral (chapter 7). We replicated Murray et al.’s (2006) experiment with far and near spheres and investigated how the visual system reacts and responds to a visual illusion created by depth information as a cue for judgement of perceived size (chapter 6). Our results support Murray et al.’s (2006) interpretation of their data only partially and suggest an alternative interpretation to their influential experiment: Murray et al.’s apparent VI correlate of the size illusion may be partially attributable to larger binocular fixation disparities on the back sphere (i.e. sizeable non-overlap between the retinotopic mappings from the left and right eye). In another visual illusion experiment, we investigate how the two eyes would respond to stimuli that produce a motion-based illusion of depth, given that such stimuli produce not just an aftereffect but different perceptions during the stimulus. We found general similarities in processing clockwise and anticlockwise spinning spirals, in terms of vergence movements. However, our experiment shows a remarkable difference in the binocular strategies in processing at different spatial locations. It indicated hemispheric specialisation and projection during viewing an illusional stimulus, and corresponding binocular strategies for visual processing. Perception can be manipulated by hemisphere-based higher cognition controlling the oculomotor musculature, allowing the hemispheres to generate their own most appropriate input, when viewing a visually challenging stimulus, Plateau’s Spiral. In summary, this thesis investigates binocular strategies in visual perception in response to different stimuli and conditions in both reading and non-reading tasks. We found systematic and harmonious behaviours from peripheral muscle-driven movements of the eyes that correspond to the cortical processing that contributes to cognition, showing the flexibility of the visual system in different modes of operation, from peripheral binocular movement all the way to higher cognitive processing.
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