Efferent cortical connections of multimodal cortex of the superior temporal sulcus in the rhesus monkey

The cortex of the upper bank of the superior temporal sulcus (STS) in the rhesus monkey contains a region that receives overlapping input from post-Rolandic sensory association areas and is considered multimodal in nature. We have used the fluorescence retrograde tracing technique in order to answer the question of whether multimodal areas of the STS project back to post-Rolandic sensory association areas. Additionally, we have attempted to answer the question of whether the projections from the multimodal areas directed to the parasensory association areas originate from common neurons via axon collaterals or from individual neurons. The results show that multimodal area TPO of the STS projects back to specific unimodal parasensory association areas of the parietal lobe (somatosensory), superior temporal gyrus (auditory), and posterior parahippocampal gyrus (visual). In addition, a substantial number of projections from area TPO are directed to distal parasensory association areas, area PG-Opt in the inferior parietal lobule, areas Ts1 and Ts2 in the rostral superior temporal gyrus, and areas TF and TL in the parahippocampal gyrus. These latter regions are themselves considered to be higher-order association areas. It was also noted that the majority of the projections to these higher-order association areas originate from the middle divisions of area TPO (TPO-2 and TPO-3). These neurons are organized in a significantly overlapping manner. Despite this overlap of the projection neurons, only an occasional double labeled neuron was observed in area TPO. Thus, our observations indicate that the multimodal region of the superior temporal sulcus has reciprocal connections with the unimodal parasensory association cortices subserving somatosensory, auditory and visual modalities, as well as with other post-Rolandic higher-order association areas. These connections from area TPO to post-Rolandic association areas may have a modulating influence on the sensory association input leading to multimodal areas in the superior temporal sulcus.