Jose-Manuel Alonso, MD, PhD, SUNY College of Optometry
Visual processing in the brain is mediated by two major thalamocortical pathways that signal local light increments (ON) and decrements (OFF) in the visual scene. In carnivores and primates, ON and OFF channels segregate in the retina and thalamus but combine in visual cortex. Our work demonstrates that this ON-OFF cortical mixing is not complete; it is partial and unbalanced. OFF thalamic afferents make stronger connections and occupy more territory in primary visual cortex than ON afferents, OFF-dominated cortical receptive-fields largely outnumber ON-dominated cortical receptive-fields, and cortical responses to dark stimuli are stronger, faster, more linearly related to luminance contrast and have better spatial and temporal resolution than responses to light stimuli, making the cortex OFF-dominated. In addition, our work demonstrates that ON and OFF afferents segregate in primary visual cortex, forming ON and OFF cortical domains that run orthogonal to ocular dominance columns and make the cortical maps OFF-centric. In these OFF-centric cortical maps, ON retinotopy tends to rotate around OFF retinotopy and this retinotopic rotation generates cortical maps for stimulus orientation. We propose that this surprising OFF-dominated/OFF-centric organization of visual cortical topography is a consequence of a size distortion for lights that originates already at the photoreceptor and has important implications in the construction of cortical receptive fields, cortical maps, visual function and visual disease.
