Callosal connections of the parabelt auditory cortex in macaque monkeys

Auditory cortex of macaque monkeys is located on the lower bank of the late ral sulcus and the adjoining superior temporal gyrus. This region of cortex contains a core of primary-like areas surrounded by a narrow belt of assoc iated fields. Adjacent to the lateral belt on the superior temporal gyrus i s a parabelt region which contains at least two subdivisions (rostral and c audal). In previous studies we defined the parabelt region as cotter with t opographic cortical connections with the belt areas surrounding the core, a nd connections with the dorsal and magnocellular divisions of the medial ge niculate 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 si x distinguishable tracers, into different locations of the parabelt region in each of four macaque monkeys. The results indicated that the strongest c allosal projections arise from homotopic areas in parabelt cortex, and they roughly matched the rostrocaudal levels of the medial and lateral belt cor tex. Weaker callosal inputs to the parabelt originate from the correspondin g levels of the superior temporal gyrus and superior temporal sulcus. The c ore region does not contribute significant callosal projections to the para belt region. The results provide further support for the conclusion that th e parabelt region represents a third level of auditory cortical processing beyond direct activation by primary subcortical and cortical auditory struc tures.