Subcortical projections of area MT in the macaque

Abstract Area MT is a visuotopically organized area in extrastriate cortex of primates that appears to be specialized for the analysis of visual motion. To examine the full extent and topographic organization of the subcortical projections of MT in the macaque, we injected tritiated amino acids in five cynomolgus monkeys and processed the brains for autoradiography. The injection sites, which we identified electrophysiologically, ranged from the representation of central through peripheral vision in both the upper and lower visual fields and included, collectively, most of MT. Projections from MT to the superior colliculus are topographically organized and in register with projections from striate cortex to the colliculus. Unlike projections from striate cortex, those from MT are not limited to the upper layer of the stratum griseum superficiale but rather extend ventrally from the upper through the lower layer of the stratum griseum superficiale and even include the stratum opticum. Projections from MT to the pulvinar are organized into three separate fields. One field (P 1 ) is located primarily in the inferior pulvinar but extends into a portion of the adjacent lateral pulvinar. The second field (P 2 ) partially surrounds the first and is located entirely in the lateral pulvinar. The third and heaviest, projection field (P 3 ) is located posteromedially in the inferior pulvinar but also includes small portions of the lateral and medial pulvinar that lie dorsal to the brachium of the superior colliculus. While projections from MT to P 1 and P 2 are topographically organized, there appears to be a convergence of MT inputs to P 3 . Projections from MT to the reticular nucleus of the thalamus are located in the ventral portion of the nucleus, approximately at the level of the caudal pulvinar. There was some evidence that MT sites representing central vision project more caudally than do those representing peripheral vision. Projections from MT to the caudate, putamen, and claustrum are localized to small, limited zones in each structure. Those to the caudate terminate within the most caudal portion of the body and the tail. Similarly, projections to the putamen are always to its most caudal portion, where the structure appears as nuclear islands. Projections to the claustrum are located ventrally, approximately at the level of the anterior part of the dorsal lateral geniculate nucleus. Projections from MT to the pons terminate rostrally in the dorsolateral nucleus, the lateral nucleus, and the dorsolateral portion of the peduncular nucleus. A topographic organization of these projections was not apparent, but there may be a heavier input from the part of MT representing peripheral vision than from the part representing central vision. The results indicate that while subcortical projections of MT in the macaque are more extensive than those of either striate cortex or V2, they are not more extensive than those of V4 and overlap them considerably. The lack of a unique set of subcortical projections from MT suggests that MT's contribution to subcortical visual processing lies in the unique information it supplies.