CALLOSALLY PROJECTING NEURONS IN THE MACAQUE MONKEY V1 V2 BORDER ARE ENRICHED IN NONPHOSPHORYLATED NEUROFILAMENT PROTEIN/

Previous immunohistochemical studies combined with retrograde tracing in macaque monkeys have demonstrated that corticocortical projections can be differentiated by their content of neurofilament protein. The p resent study analyzed the distribution of nonphosphorylated neurofilam ent protein in callosally projecting neurons located at the V1/V2 bord er. All of the retrogradely labeled neurons were located in layer III at the V1/V2 border and at an immediately adjacent zone of area V2. A quantitative analysis showed that the vast majority (almost 95%) of th ese interhemispheric projection neurons contain neurofilament protein immunoreactivity. This observation differs from data obtained in other sets of callosal connections, including homotypical interhemispheric projections in the prefrontal, temporal, and parietal association cort ices, that were found to contain uniformly low proportions of neurofil ament protein-immunoreactive neurons. Comparably, highly variable prop ortions of neurofilament protein-containing neurons have been reported in intrahemispheric corticocortical pathways, including feedforward a nd feedback visual connections. These results indicate that neurofilam ent protein is a prominent neurochemical feature that identifies a par ticular population of interhemispheric projection neurons at the V1/V2 border, and suggest that this biochemical attribute may be critical f or the function of this subset of callosal neurons.