Topographic population codes in primate V1

Abstract:

What is the nature of the neural code in primate primary visual cortex (V1)? Two key properties of primate V1 are relevant to this question. First, V1 representations are widely distributed. Because receptive fields of neighboring V1 neurons largely overlap, the cortical point image (the V1 area that encompasses all the neurons whose receptive fields contain a given point in the visual field) spreads over multiple square millimeters and contains millions of neurons. Second, V1 is topographically organized. It contains a large-scale (multiple mm) homeomorphic map of the contralateral visual field (‘retinotopic map’), and finer scale (sub-mm) semi-periodic topographic maps of stimulus features such as edge orientation (columnar ‘orientation map’). The distributed and topographic nature of V1’s representations raises the possibility that in some visual tasks, downstream areas that decode V1 signals in order to mediate perception could combine V1 signals at the relevant topographic scale—e.g., at the scale of orientation columns. If this were the case, then the fundamental unit of information would be individual columns rather than single neurons, and to account for the subject’s behavior in a perceptual task, it would be necessary and sufficient to consider the summed activity of the thousands of neurons within each column. In this presentation, I will discuss several studies that begin to test this topographic-code hypothesis by using optical methods to measure and perturb neural population responses in V1 of behaving macaques.