The human visual brain devotes most of its neuronal resources to process the part of the visual scene that we see with both eyes. To achieve this goal, afferents from the two eyes representing the same binocular point in visual space become close neighbors in the primary visual cortex, the first cortical recipient of visual input. In turn, the primary visual cortex carefully allocates its neuronal resources to represent stimuli within each binocular point as efficiently as possible. The primary visual cortex of different species achieve this goal using different strategies. In humans and macaques, the cortex splits the map of visual space in intercalated pairs of stripes for the left and right eyes forming a Zebra pattern. In carnivores, the cortex splits the map in blobs forming a Dalmatian pattern. In rodents and lagomorphs, the afferents from the two eyes mix and do not form any specific pattern. For decades, the origin of these diverse ocular cortical patterns remained a controversial puzzle.
In a recent study that will be published in the Journal of Neuroscience on November 14th, researchers found evidence that ocular dominance patterns are diverse because the amount of cortex available to represent each binocular point varies greatly across species and individual animals of the same species. In humans, the primary visual cortex devotes large cortical rectangles to represent each binocular point, allowing the afferents from the two eyes to form stripes running parallel along the shortest axis of the rectangle. However, in cats, the cortex devotes smaller cortical squares to represent each binocular point and the afferents are constrained to form blob patterns. Finally, in mice, the cortex is too small and the few afferents representing the same binocular point mix and form no specific pattern. The researchers also found that, when cortical resources decrease to represent points that are increasingly farther from the point of visual fixation, the half eye that is closest to the nose (nasal retina) dominates and gains access to more cortical space than the other half eye (temporal retina). Therefore, just as the right hand dominates motor processing in right-handed humans, the nasal retina dominates visual processing and this dominance increases with the distance from the point of fixation. Taken together, these results support the notion that the primary visual cortex optimizes its neuronal resources to encode as efficiently as possible the different stimulus combinations available at each binocular point of visual space. The work was done by Sohrab Najafian in the laboratory of Jose Manuel Alonso at the State University of New York College of Optometry.
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Significance Statement from the Original Paper in the Journal of Neuroscience:
Thalamocortical afferents segregate in primary visual cortex by eye input and light-dark polarity. This afferent segregation forms cortical patterns that vary greatly across species for reasons that remain unknown. Here we show that the formation of ocular dominance patterns follows a common organizing principle across species that aligns the cortical axis of ocular dominance segregation with the axis of slowest retinotopic gradient. Based on our results, we propose a model of visual cortical topography that sorts thalamic afferents by eye input and stimulus polarity along orthogonal axes with the slowest and fastest retinotopic gradients, respectively. This organization maximizes the binocular retinotopic match needed for depth perception and the light-dark retinotopic mismatch needed to process stimulus orientation in carnivores and primates.
Abstract from the Original Paper in the Journal of Neuroscience:
The primary visual cortex contains a detailed map of retinal stimulus position (retinotopic map) and eye input (ocular dominance map) that results from the precise arrangement of thalamic afferents during cortical development. For reasons that remain unclear, the patterns of ocular dominance are very diverse across species and can take the shape of highly organized stripes, convoluted beads or no pattern at all. Here, we use a new image-processing algorithm to measure ocular dominance patterns more accurately than in the past. We use these measurements to demonstrate that ocular dominance maps follow a common organizing principle that makes the cortical axis with the slowest retinotopic gradient orthogonal to the ocular dominance stripes. We demonstrate this relation in multiple regions of the primary visual cortex from individual animals, and different species. Moreover, consistent with the increase in the retinotopic gradient with visual eccentricity, we demonstrate a strong correlation between eccentricity and ocular-dominance stripe width. We also show that an eye/polarity grid emerges within the visual cortical map when the representation of light and dark stimuli segregates along an axis orthogonal to the ocular dominance stripes, as recently demonstrated in cats. Based on these results, we propose a developmental model of visual cortical topography that sorts thalamic afferents by eye input and stimulus polarity, and then maximizes the binocular retinotopic match needed for depth perception and the light-dark retinotopic mismatch needed to process stimulus orientation. In this model, the different ocular dominance patterns simply emerge from differences in local retinotopic cortical topography.