INDEPENDENCE AND MERGER OF THALAMOCORTICAL CHANNELS WITHIN MACAQUE MONKEY PRIMARY VISUAL-CORTEX - ANATOMY OF INTERLAMINAR PROJECTIONS

An important issue in understanding the function of primary visual cor tex in the macaque monkey is how the several efferent neuron groups pr ojecting to extrastriate cortex acquire their different response prope rties. To assist our understanding of this issue, we have compared the anatomical distribution of V1 intrinsic relays that carry information derived from magno- (M) and parvocellular (P) divisions of the dorsal lateral geniculate nucleus between thalamic recipient neurons and int erareal efferent neuron groups within area V1. We used small, iontopho retic injections of biocytin placed in individual cortical laminae of area V1 to trace orthograde and retrograde inter- and intralaminar pro jections. In either the same or adjacent sections, the tissue was reac ted for cytochrome oxidase (CO), which provides important landmarks fo r different efferent neuron populations located in CO rich blobs and C O poor interblobs in laminae 2/3, as well as defining clear boundaries for the populations of efferent neurons in laminae 4A and 4B. This st udy shows that the interblobs, but not the blobs, receive direct input from thalamic recipient 4C neurons; the interblobs receive relays fro m mid 4C neurons (believed to receive convergent M and P inputs), whil e blobs receive indirect inputs from either M or P (or both) pathways through layers 4B (which receives M relays from layer 4C alpha) and 4A (which receives P relays directly from the thalamus as well as from l ayer 4C beta). The property of orientation selectivity, most prominent in the interblob regions and in layer 4B, may have a common origin fr om oriented lateral projections made by mid 4C spiny stellate neurons. While layer 4B efferents may emphasize M characteristics and layer 4A efferents emphasize P characteristics, the dendrites of their constit uent pyramidal neurons may provide anatomical access to the other chan nel since both blob and interblob regions in layers 2/3 have anatomica l access to M and P driven relays, despite functional differences in t he way these properties may be expressed in the two compartments.