Cortical Connections to Area TE in Monkey: Hybrid Modular and Distributed Organization

To investigate the fine anatomical organization of cortical inputs to visual association area TE, 2–3 small injections of retrograde tracers were made in macaque monkeys. Injections were made as a terminal procedure, after optical imaging and electrophysiological recording, and targeted to patches physiologically identified as object-selective. Retrogradely labeled neurons occurred in several unimodal visual areas, the superior temporal sulcus, intraparietal sulcus (IPS), and prefrontal cortex (PFC), consistent with previous studies. Despite the small injection size (<0.5 mm wide), the projection foci in visual areas, but not in IPS or PFC, were spatially widespread (4–6 mm in extent), and predominantly consisted of neurons labeled by only one of the injections. This can be seen as a quasi-modular organization. In addition, within each projection focus, there were scattered neurons projecting to one of the other injections, together with some double-labeled (DL) neurons, in a more distributed pattern. Finally, projection foci included smaller “hotspots,” consisting of intermixed neurons, single-labeled by the different injections, and DL neurons. DL neurons are likely the result of axons having extended, spatially separated terminal arbors, as demonstrated by anterograde experiments. These results suggest a complex, hybrid connectivity architecture, with both modular and distributed components.