Feedback connections from area MT of the squirrel monkey to areas V1 and V2.

Area MT/V5 is reciprocally connected with both V1 and V2; but, despite extensive anatomical and physiological investigations, detailed information on the feedback component of these connections is still not available. The present report uses serial section reconstruction of single axons, labeled by anterograde tracers injected in area MT of squirrel monkeys, to characterize these connections further. As with other feedback systems, MT axons terminating in both areas V1 (n = 9) and V2 (n = 6) are widely divergent. In area V1, MT fields are larger than those from V2 and are about comparable to those from V4 or TEO. Terminations in V1, unlike other feedback connections described so far, terminate in several laminar combinations: only layer 1 (n = 2); only layer 4B (n = 3); layers 1 and 4B (n = 1); and layers 1, 4B, and 6 (n = 3). In V2, they occur mainly in layers 1 and 5 or 6. Terminations have two patterns even within a single axon: strung along collateral segments and grouped within small clusters. There are no apparent differences in the size, shape, or density of terminal specializations in V1 or V2, and, consistently with previous double-labeling experiments (Kennedy and Bullier [1985] J Neurosci 5:2815-2830), some axons can branch to both areas. This result, along with the laminar evidence for subtypes of feedback connections, argues against an exclusively hierarchical organization based on "pairwise" connectivity. For V1 and MT, there may be directly reciprocal loops between feedforward and feedback projecting neurons, but this is less likely to be so for V2 and MT.