Organization of intrinsic connections in owl monkey area MT.

Area MT (middle temporal) is a well-defined visual representation common to all primates, which shows a clear selectivity to the analysis of visual motion. In the present study we examined the architecture of the intrinsic connections in area MT in an attempt to reveal its organizing principles and its potential relationship to the functional domains in area MT. Intrinsic connections were studied by placing small injections of the tracer biocytin in area MT of seven adult owl monkeys (Aotus nancymae). The injections were targeted at well-defined orientation domains revealed using optical imaging of intrinsic signals. The distribution of axons labeled by these injections was related both to the cytochrome oxidase histochemistry and to the layout of functional domains in area MT and surrounding tissue. Tracer injections in the superficial layers of area MT produced a complex network of extrinsic and intrinsic axonal connections. Clear instances of extrinsic connections were observed between area MT proper and the MT crescent situated postero-medially to it. The intrinsic connections were laterally spread and organized in patch-like clusters with an average distance from injection center to the furthest patch of 1.8 +/- 0.55 mm (+/-SD, n = 9). The overall axonal distribution tended to be anisotropic, i.e. the patches were distributed within an elongated ellipse [average anisotropy ratio: 1.86 +/- 0.66 (+/-SD)] and were asymmetrically distributed about either side of the injection site [average asymmetry ratio: 2.3 +/- 0.7 (+/-SD)]. Finally, there was a tendency for the intrinsic connections to connect to functional domains of similar orientation preference in area MT. However, this tendency varied substantially between individual cases. The highly specific nature of MT lateral connections puts clear constraints on models of surround influences in the receptive fields of MT neurons.