Thalamic and callosal connections of the rat auditory cortex.

This study was designed to assess the relative distributions of two extrinsic afferent fiber systems in the rat auditory cortex as indicated by the patterns of specific lesion-induced degeneration evident in Fink-Heimer preparations. The auditory cortex consists of cytoarchitectural areas 41, 20 and 36. Lesions were made in the medial geniculate body (MGB) or the corpus callosum in some rats, while in other rats, lesions were made in both the MGB and the corpus callosum. Following the thalamic lesions, degenerating terminals occur throughout the auditory region of cortex, principally in layer IV and deep layer III, but also in layer VI and in the superficial part of layer I. With the exception of the band of degeneration in layer I, the density of the thalamic degeneration is uneven, such that patches of increased density of degeneration are separated by regions with few degenerating terminals. Following lesions of the corpus callosum, degenerating callosal terminals are also evident throughout the auditory region of cortex and they occur in deep layer I through layer III, superficial layer V and in layer VI. The density of the degenerating callosal terminals is not uniform throughout most of area 41, to the extent that there are radially-oriented bands of increased density which appear within the continuous callosal projection. Following the double lesions, degenerating terminals throughout the auditory region are distributed homogeneously within all cortical layers with the exception of deep layer V which is relatively free of degeneration. The results indicate that all regions within the rat auditory cortex are subject to both thalamic and callosal influence, although the input is not completely uniform, for the zones in layers IV and VI which have decreased thalamic input appear to have increased callosal input.