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Neural Responses in the Retinotopic Representation of the Blind Spot in the Macaque V1 to Stimuli for Perceptual Filling-In

When visual stimuli that cover the entire blind spot are presented monocularly, the color and brightness of the surrounding field are seen within the blind spot, although it receives no retinal input. Important questions about such perceptual filling-in are whether neurons in the visual system representing visual field locations within the blind spot are activated when filling-in occurs and, if so, what the properties of these neurons are. To address these questions, we recorded the activities of single neurons in the primary visual cortex (V1) of the awake monkey. We first identified the area of V1 representing the region of the blind spot and then assessed neural responses to stationary visual stimuli of various size. We found that there are neurons in layer 4 and deeper laminae, particularly layer 6, that respond to large stimuli covering the blind spot which induces perceptual filling-in. Most of these neurons had very large binocular receptive fields that extended outside the blind spot. These neurons also preferred relatively large stimuli and exhibited color selectivity. These results indicate that when a large uniform surface is presented on the blind spot, neurons at the V1 region representing the blind spot transmit signals essential for filling-in that inform of the presence of a large surface as well as the absence of smaller stimuli at the blind spot.