Long-range clustered connections within extrastriate visual area V5/MT of the rhesus macaque

Visual area V5/MT in the rhesus macaque has a distinct functional organization, where neurons with specific preferences for direction of motion and binocular disparity are co-organized in columns or clusters. Here, we analyze the pattern of intrinsic connectivity within cortical area V5/MT in both parasagittal sections of the intact brain and tangential sections from flatmounted cortex using small injections of the retrograde tracer cholera toxin subunit b. Labeled cells were predominantly found in cortical layers 2, 3, and 6. Going along the cortical layers, labeled cells were concentrated in regularly spaced clusters. The clusters nearest to the injection site were approximately 2 mm from its center. In flatmounted cortex, along the dorsoventral axis of V5/MT, we identified further clusters of labeled cells up to 10 mm from the injection site. Quantitative analysis of parasagittal sections estimated average cluster spacing at 2.2 mm; in cortical flatmounts, spacing was 2.3 mm measured radially from the injection site. The results suggest a regular pattern of intrinsic connectivity within V5/MT, which is consistent with connectivity between sites with a common preference for both direction of motion and binocular depth. The long-range connections can potentially account for the large suppressive surrounds of V5/MT neurons.