Intrinsic connections and architectonics of the superior temporal sulcus in the rhesus monkey

Abstract

The intrinsic connections of the superior temporal sulcus (STS) in the rhesus monkey were studied by anterograde and retrograde tracer techniques and correlated with a reevaluation of cortical cytoarchitecture. The polymodal region in the upper bank (area TPO) is divisible into four rostral‐to‐caudal architectonic sectors, exhibiting increasing degrees of laminar differentiation and cellularity as one proceeds caudally. These sectors, including the sulcal proisocortex (area Pro), are tied together in a sequence of reciprocal connections. Each rostrocaudal sector of area TPO also has reciprocal connections with the laterally adjacent area TAa, at the upper rim of the sulcus, and medially adjacent areas PGa and IPa, near the depth. A similar architectonic/ connectional organization exists for unimodal vision‐related cortex in the lower bank of the STS. Here a rostrocaudal sequence of reciprocal connections unites area Pro, rostral and caudal divisions of area TEa, and the extrastriate visual area OAa (MT). Area TEa also has reciprocal connections with adjacent segments of area TEm laterally, at the lower rim of the sulcus, and area IPa, medially, in the depth. In both upper and lower banks, caudal‐to‐rostral “for‐wardgoing” connections begin in supragranular layers of cortex and terminate in and around layer IV. Reciprocal, “backgoing” connections take origin from cells in infragranular layers and terminate mainly over the first layer of the caudally adjacent target zone. Orthogonally directed, “side‐to‐side” projections originate in both supra‐ and infragranular layers and terminate diffusely over all layers of cortex.