Collateralized divergent feedback connections that target multiple cortical areas.

Nonreciprocal feedback connections from ventromedial areas TE and TF have previously been reported to visual areas V1 and V2 (Kennedy and Bullier, 1985; Rockland and Van Hoesen, 1994). The present report confirms these earlier observations by utilizing anterograde label in conjunction with serial section analysis. Furthermore, it directly demonstrates the divergent configuration and range of these terminal fields. Thirteen axons were analyzed from ventromedial TE (4) or area TF (9) to occipitotemporal areas, and two from area TF to the upper bank of the intraparietal sulcus (IPS). All these axons have narrow, elongated fields that range from 4.0-21.0 mm. Terminations are distributed linearly along the axon or, in some cases, concentrated in irregularly spaced clusters. Most of these axons have terminations concentrated in layer 1. The two axons in the IPS have a bistratified terminal distribution (in layers 1-3 and 6) in their anterior field, but a distinctly different laminar pattern (with terminations concentrated in layer 1) in their distal 2.0 mm. These fields probably correspond to different areas, most likely MIP and PO. Axons projecting from higher order to early visual areas may contribute to extraperceptual, complex processes within area V1, such as activation in response to visual imagery, and are a possible substrate for synchronous linkage of spatially discrete assemblies of neurons. In summary, these results demonstrate 1) that some neurons in ventromedial TE and TF are in direct communication with early visual areas, including V1 and V2, and 2) that some feedback axons target several areas, sometimes with different laminar termination patterns. These results emphasize that cortical areas are interrelated by multiple direct and indirect pathways, not all of which are strictly hierarchical.