Laminar Termination Patterns of Thalamic, Callosal, and Association Afferents in the Primary Auditory Area of the Rhesus Monkey

Thalamic, callosal, and association afferents to the primary auditory koniocortex, area KA, were investigated in rhesus monkeys with the aid of anterograde tract tracing techniques (radioactively labeled amino acids, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), and the Fink-Heimer silver impregnation method). Different patterns of anterograde label in KA were produced by radioactively labeled amino acid injections in the contralateral primary auditory area, the ipsilateral medial geniculate body (MGB), or auditory association cortex of the superior temporal gyrus (STG). Thalamic afferents from the MGB were found throughout KA in layers IV and III where the density waxed and waned, forming regular patches of higher density label separated by areas of less dense label. Callosal afferents from the contralateral auditory cortices were found in layers IV, III, and II in larger, more spatially separated, and irregular patches. In contrast, association afferents from the auditory association cortex of the STG were found in a continuous band in layer I. To determine the extent to which the patches of callosal afferents overlap with or interdigitate between the high-density thalamic clusters, two types of double anterograde labeling experiments were undertaken. In one set of experiments isotope injections of the MGB were made in conjunction with transection of the corpus callosum in the same animal. In another set of experiments isotope injections of the MGB were made in one hemisphere and an injection of WGA-HRP was made in the opposite supratemporal plane, including KA, in the same animal. Both approaches demonstrate that the patches of callosal afferents in KA mainly occur between the high-density patches of thalamic afferents, although there is a variable degree of overlap at their periphery. Thus it seems that thalamic and callosal inputs to KA are organized in a largely complementary fashion while input from the lateral association cortex is continuous and overlaps both. When considered in light of pertinent anatomical and physiological studies in a variety of species, it appears that the areas mainly occupied by callosal or thalamic inputs may be differentially involved in frequency analysis and sound localization, respectively.