Connectional analysis of the ipsilateral and contralateral afferent neurons of the superior temporal region in the rhesus monkey.

The interhemispheric and ipsilateral afferents of the superior temporal region (STR) were investigated with the aid of fluorescent retrograde tracers (Diamidino Yellow and Fast Blue). Different tracers were injected in selected cortical areas of the STR of each hemisphere of four rhesus monkeys. The results show that the interhemispheric afferents originate not only from the homotopic but also from heterotopic areas. The heterotopic areas giving rise to interhemispheric projections correspond to cortical areas of the origin of the ipsilateral projections. Although there is considerable overlap of labeled neurons of both afferent systems, only occasional double-labeled neurons are found. Whereas the laminar patterns of ipsilateral neurons of origin vary considerably, the interhemispheric projection neurons are located mainly in cortical layer III. This study provides additional information about the ipsilateral connectional organization of the superior temporal region. That is, the primary auditory area receives projections not only from adjacent lateral and medial cortical regions but also from adjoining rostral and caudal cortical regions. Thus, the highly differentiated primary auditory cortical area receives strong projections from the surrounding less-differentiated cortical regions. This connectional pattern is discussed from the perspective of the growth ring concept of cortical development.