Prefrontal connections of the parabelt auditory cortex in macaque monkeys

In the present study, we determined connections of three newly defined regi ons of auditory cortex with regions of the frontal lobe, and how two of the se regions in the frontal lobe interconnect and connect to other portions o f frontal cortex and the temporal lobe in macaque monkeys. We conceptualize auditory cortex as including a core of primary areas, a surrounding belt o f auditory areas, a lateral parabelt of two divisions, and adjoining region s of temporal cortex with parabelt connections. Injections of several diffe rent fluorescent tracers and wheat germ. agglutinin conjugated to horseradi sh peroxidase (WGA-HRP) were placed in caudal (CPB) and rostral (RPB) divis ions of the parabelt, and in cortex of the superior temporal gyrus rostral to the parabelt with parabelt connections (STGr). Injections were also plac ed in two regions of the frontal lobe that were labeled by a parabelt injec tion in the same case. The results lead to several major conclusions. First , CPB injections label many neurons in dorsal prearcuate cortex in the regi on of the frontal eye field and neurons in dorsal prefrontal cortex of the principal sulcus, but few or no neurons in orbitofrontal cortex. Fine-grain label in these same regions as a result of a WGA-HRP injection suggests th at the connections are reciprocal. Second, RPB injections label overlapping prearcuate and principal sulcus locations, as well as more rostral cortex of the principal sulcus, and several locations in orbitofrontal cortex. Thi rd, STGr injections label locations in orbitofrontal cortex, some of which overlap those of RPB injections, but not prearcuate or principal sulcus loc ations. Fourth, injections in prearcuate and principal sulcus locations lab eled by a CPB injection labeled neurons in CPB and RPB, with little involve ment of the auditory belt and no involvement of the core. In addition, the results indicated that the two frontal lobe regions are densely interconnec ted. They also connect with largely separate regions of the frontal pole an d more medial premotor and dorsal prefrontal cortex, but not with the exten sive orbitofrontal region which has RPB and STGr connections. The results s uggest that both RPB and CPB provide the major auditory connections with th e region related to directing eye movements towards stimuli of interest, an d the dorsal prefrontal cortex for working memory. Other auditory connectio ns to these regions of the frontal lobe appear to be minor. RPB has connect ions with orbitofrontal cortex, important in psychosocial and emotional fun ctions, while STGr primarily connects with orbital and polar prefrontal cor tex. (C) 1999 Elsevier Science B.V. All rights reserved.