Callosal connections of the parabelt auditory cortex in macaque monkeys.

Auditory cortex of macaque monkeys is located on the lower bank of the lateral sulcus and the adjoining superior temporal gyrus. This region of cortex contains a core of primary-like areas surrounded by a narrow belt of associated fields. Adjacent to the lateral belt on the superior temporal gyrus is a parabelt region which contains at least two subdivisions (rostral and caudal). In previous studies we defined the parabelt region as cortex with topographic cortical connections with the belt areas surrounding the core, and connections with the dorsal and magnocellular divisions of the medial geniculate complex, but minimal connections with the core region and ventral division of the medial geniculate complex. The callosal connections of the parabelt auditory cortex were determined by placing injections, of up to six distinguishable tracers, into different locations of the parabelt region in each of four macaque monkeys. The results indicated that the strongest callosal projections arise from homotopic areas in parabelt cortex, and they roughly matched the rostrocaudal levels of the medial and lateral belt cortex. Weaker callosal inputs to the parabelt originate from the corresponding levels of the superior temporal gyrus and superior temporal sulcus. The core region does not contribute significant callosal projections to the parabelt region. The results provide further support for the conclusion that the parabelt region represents a third level of auditory cortical processing beyond direct activation by primary subcortical and cortical auditory structures.