Corticothalamic connections of auditory-related areas of the temporal lobe in the rhesus monkey.

Corticothalamic connections of auditory areas of the superior temporal regions (STR) were investigated in the rhesus monkey. These connections are organized according to the recently described architectonic parcellation of the STR. The core line regions of the supratemporal plane (STP) project to the medial geniculate nucleus (MGN). All regions except the primary auditory area also have projections to additional thalamic nuclei. The rostral STP has strong connections with the caudal part of the ventral subdivision (MGv) of MGN as well as with medial pulvinar (PM). In contrast the primary auditory area projects mainly to rostral MGv. The caudalmost STP projects mainly to the dorsal subdivision (MGd) and to the magnocellular subdivision (MGmc) as well as to the PM and the lateral (PL) and oral (PO) pulvinar, nucleus limitans (Li), and mediodorsal (MD) nucleus. The belt line regions of the superior temporal gyrus (STG) project mainly to the pulvinar but also have projections to MGd and MGmc. Specifically, rostral STG projects to the caudal part of PM, to MGmc, and to the suprageniculate (SG) nucleus, whereas caudal STG projects to the rostral part of PM and to PL, PO, MGd, MGmc, SG-Li and MD nuclei. The root line areas in the circular sulcus of the Sylvian fissure project mainly to MGmc but also to MGd, PM, and SG-Li nuclei. These connections originate mainly from neurons in cortical layer VI, with some from layer Vb. It is suggested that these connections may be involved in different aspects of auditory information processing.