Sub-Compartments Within Orientation Columns of Primary Visual Cortex: A Proposal for a Contour Building Architecture

Abstract

PURPOSE: In the mammalian visual system, early stages of visual form perception begin with orientation selective neurons in primary visual cortex (V1). In many species (including humans, monkeys, tree shrews, cats, and ferrets), these neurons are organized in beautifully arrayed orientation columns, which shift in orientation preference across V1 and are highlighted by orientation pinwheels. However, to date, the relationship of orientation architecture to the encoding of elemental aspects of visual contours is still unknown. METHODS: Using a novel highly accurate method of targeting electrode position, combining with optical image and single-unit recording, we report for the first time the presence of three functionally distinct zones within single orientation domains. RESULTS: We found evidence for three concentric sub-regions centered on the orientation-pinwheel. The central-most region contains neurons with small receptive fields and strong suppressive surrounds, while the outermost region contains neurons with larger receptive fields and weak suppressive surrounds. CONCLUSIONS: We suggest that these zones subseries computation of distinct aspects of visual contours (linear orientation, local curvature, and contour singularities). The orientation domain thus embodies a three stage visual contour processor in V1.