ORIENTATION SELECTIVITY AND THE ARRANGEMENT OF HORIZONTAL CONNECTIONSIN TREE SHREW STRIATE CORTEX

Horizontal connections, formed primarily by the axon collaterals of py ramidal neurons in layer 2/3 of visual cortex, extend for millimeters parallel to the cortical surface and form patchy terminations. Previou s studies have provided evidence that the patches formed by horizontal connections exhibit modular specificity, preferentially linking colum ns of neurons with similar response characteristics, such as preferred orientation. The issue of how these connections are distributed with respect to the topographic map of visual space, however, has not been resolved, Here we combine optical imaging of intrinsic signals with sm all extracellular injections of biocytin to assess quantitatively the specificity of horizontal connections with respect to both the map of orientation preference and the map of visual space in tree shrew V1. O ur results indicate that horizontal connections outside a radius of 50 0 mu m from the injection site exhibit not only modular specificity, b ut also specificity for axis of projection, Labeled axons extend for l onger distances, and give off more terminal boutons, along an axis in the map of visual space that corresponds to the preferred orientation of the injection site. Inside of 500 mu m, the pattern of connections is much less specific, with boutons found along every axis, contacting sites with a wide range of preferred orientations. The system of long -range horizontal connections can be summarized as preferentially link ing neurons with co-oriented, coaxially aligned receptive fields. Thes e observations suggest specific ways that horizontal circuits contribu te to the response properties of layer 2/3 neurons and to mechanisms o f visual perception,