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2025
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"Binocular vision plays an essential role in mammalian perception and is critical for tasks such as visual navigation, feeding, and survival. In animals with frontally positioned eyes, such as humans and carnivores, binocular vision dominates visual perception. This dominance reflects the brain’s reliance on the precise integration of signals from both eyes through a complex network of neurons, circuits, and cortical regions. Any impairment within this complex network can lead to significant binocular deficits, disrupting perception. Despite decades of research, key questions remain about how the brain coordinates inputs from the two eyes to achieve stable binocular fusion and depth perception, and how specific failures in this coordination give rise to perceptual distortions. This thesis addresses these questions through two investigations into the neural mechanisms underlying binocular vision and image processing in the early visual cortex.
The primary visual cortex (area V1) is the first cortical stage where inputs from the two eyes converge, forming binocular receptive fields that support unified perception. These receptive fields must balance two competing requirements: diversity to adequately sample the visual world, and similarity across the two eyes to achieve binocular fusion. However, the neural mechanisms by which the cortex achieves this balance have remained unclear. In the first chapter of my thesis, I analyzed an extensive dataset comprising thousands of multielectrode array recordings from anesthetized cats collected over nine years. My results demonstrate that receptive fields in the cat visual cortex exhibit exquisite binocular matching in multiple dimensions, including retinotopy, orientation/direction preference, orientation/direction selectivity, response latency, and ON-OFF polarity/structure. Specifically, the average binocular mismatches in retinotopy and ON-OFF structure are tightly constrained to approximately 1/20 and 1/5 of the average receptive field size, respectively. These small mismatches are sufficient for generating diverse binocular disparity tuning, crucial for depth perception. Consequently, I conclude that cortical receptive fields are binocularly matched with exquisite precision to facilitate binocular fusion while allowing subtle mismatches essential for processing visual depth.
The second chapter of my thesis investigates the neuronal mechanisms underlying perceptual distortions in humans with binocular deficits caused by amblyopia (lazy eye). Previous research has suggested that errors in neural coding of orientation in primary visual cortex may be responsible for distorted perception in amblyopes. However, the precise mechanisms by which these errors occur are not yet fully understood. In the second chapter of my thesis, I introduce a computational model that effectively simulates a wide variety of perceptual distortions experienced by individuals with amblyopia. Using this model, I quantify the magnitude of visual distortions and uncover robust correlations among distortion magnitude, contrast sensitivity deficits, and the predicted spread of cortical activation. Based on my findings, I propose a neuronal mechanism for amblyopia in which weak cortical responses are compensated by increased cortical spread. This compensatory mechanism activates neurons with mismatched stimulus preferences, leading to perceptual distortions analogous to those induced by reduced contrast. Altogether, my thesis offers new insights into the neural basis of binocular vision, from the precise cortical balance that enables fusion and depth perception to the subtle disruptions that give rise to visual distortions in amblyopia. These findings underscore the critical role of balanced binocular integration in the early visual cortex to generate a coherent and unified perceptual experience."