Direct temporal-occipital feedback connections to striate cortex (V1) in the macaque monkey.

Although there have been reports of sparse projections from temporal areas TE, TF, and even TH to area V1, it is generally believed that cortical afferents to V1 originate exclusively from prestriate areas. Injections of anterograde tracers in anterior occipital and temporal areas, however, consistently produce labeled terminals in area V1. In order to confirm these results and display the full range of foci projecting to V1, we injected V1 in two monkeys with the retrograde tracer fast blue. Feedback connections were found, as expected, from several prestriate areas (V2, V3, V4, and MT). These originate from neurons in layers 3A and 6. Connections were also found from several more distal regions, namely, areas TEO, TE, TF, TH, and from cortex in the occipitotemporal and superior temporal (STS) sulci. Filled neurons occurred in two small foci in the caudal intraparietal sulcus. These more distal feedback connections tend to originate only from layer 6. An additional injection of the retrograde tracer diamidino yellow in area V2 of one animal revealed a similarly widespread network of feedback connections. In some areas (in the STS and in TEO), 10-15% of fluorescent neurons were double-labeled. These results indicate that feedback connections to early visual cortex derive from a widespread network of areas, including limbic-associated cortices. These connectional patterns testify to the massive recursiveness of anatomical pathways. As there are no reports of projections from V1 to anterior temporal cortices, our results also indicate that some cortical feedback connections may not be strictly reciprocal.