Cortical connections of the middle temporal visual area (MT) and the superior temporal cortex in owl monkeys

The connections of the middle temporal visual area (MT) and an adjoining superior temporal visual region (ST) were investigated with injections of 3H-proline and horseradish peroxidase (HRP) in adult owl monkeys. Injections in MT revealed an extensive pattern of intrinsic connections that appeared to be organized in a series of bands across MT. MT was found to project ipsilaterally to an array of visual areas including the first (V I) and second (V II) visual areas, the dorsolateral visual area (DL), and the dorso-medial visual area (DM). When injection sites and projection zones were related to previously described retinotopic maps, it was apparent that the projections of MT to these fields were largely homotopic. Other projections were to posterior parietal cortex, the dorsointermediate visual area (DI), the ventral visual area (V), and possibly the medial visual area (M). In addition, dense projections to cortex on the rostral border of MT were used to define a new subdivision of visual cortex, the superior temporal visual region (ST), as the major projection zone of MT. Callosal connections of MT were mainly to MT, with some terminations in DL and ST. Injections of 3H-proline into ST revealed a diffuse projection to largely layer I of MT, and dense projections to posterior parietal cortex, cortex in more rostral parts of the superior temporal sulcus, ventromedial inferior temporal cortex, and the region of the frontal eye fields. Callosal projections were largely to ST cortex of the other cerebral hemisphere. The results place MT and V II in distinctly different processing chains. While both MT and V II receive the major cortical outputs of V I, V II relays to DL and DL relays to subdivisions of inferior temporal cortex. In contrast, the most significant cortical target of MT appears to be to ST, which relays to posterior parietal cortex and other targets.