Cytoarchitecture and thalamic afferents of the sylvian and composite posterior gyri of the canine temporal cortex.

The composite posterior and sylvian gyri of the canine temporal cortex show cytoarchitectonic features of poorly differentiated isocortex. Quantitative evaluation of connections examined with retrogradely transported fluorescent tracers indicated that both gyri received strong thalamic projections from the medial geniculate body (MG) and the lateromedial-suprageniculate (LM-Sg) complex, and a weaker projection from the posterior (Po) nuclei. On the basis of the connectivity patterns and cytoarchitectonic features we distinguished the anterior (CPa) and posterior (CPp) areas in posterior composite gyrus and the anterior (SA), dorsal (SD) and posterior (SP) sylvian areas. Afferents from individual thalamic nuclei were focused in distinct areas, forming dominant projections, and diminished gradually in the adjacent areas as non-dominant projections. The most prominent MG projection arose from the dorsal caudal (MGdc) nucleus. Its ventral subdivision sent a dominant projection into the SP and CPa, whereas the dorsal MGdc subdivision was connected with the SA, SD and CPp areas. The most substantial connections from the LM-Sg complex were directed to areas SA, SD and CPp, with weak connections to areas CPa and SP. A gradient of density of LM-Sg afferents was distributed in the opposite direction to that sent from the MGdc. The origin of the CPa and SP afferents in the ventral MGdc, like connections reaching the posterior ectosylvian cortex, suggest that these areas are related to processing of auditory information. In contrast, areas SA and CPp, receive dominant projections from the polymodal LM-Sg, and therefore may constitute successive steps in a hierarchy of cortical areas.