Projections of auditory cortex to the medial geniculate body of the cat

The corticofugal projection from 12 auditory cortical fields onto the medial geniculate body was investigated in adult cats by using wheat germ agglutinin conjugated to horseradish peroxidase or biotinylated dextran amines. The chief goals were to determine the degree of divergence from single cortical fields, the pattern of convergence from several fields onto a single nucleus, the extent of reciprocal relations between corticothalamic and thalamocortical connections, and to contrast and compare the patterns of auditory corticogeniculate projections with corticofugal input to the inferior colliculus. The main findings were that (1) single areas showed a wide range of divergence, projecting to as few as 5, and to as many as 15, thalamic nuclei; (2) most nuclei received projections from approximately five cortical areas, whereas others were the target of as few as three areas; (3) there was global corticothalamic-thalamocortical reciprocity in every experiment, and there were also significant instances of nonreciprocal projections, with the corticothalamic input often more extensive; (4) the corticothalamic projection was far stronger and more divergent than the corticocollicular projection from the same areas, suggesting that the thalamus and the inferior colliculus receive differential degrees of corticofugal control; (5) cochleotopically organized areas had fewer corticothalamic projections than fields in which tonotopy was not a primary feature; and (6) all corticothalamic projections were topographic, focal, and clustered, indicating that areas with limited cochleotopic organization still have some internal spatial arrangement. The areas with the most divergent corticothalamic projections were polysensory regions in the posterior ectosylvian gyrus. The projection patterns were indistinguishable for the two tracers. These findings suggest that every auditory thalamic nucleus is under some degree of descending control. Many of the projections preserve the relations between cochleotopically organized thalamic and auditory areas, and suggest topographic relations between nontonotopic areas and nuclei. The collective size of the corticothalamic system suggests that both lemniscal and extralemniscal auditory thalamic nuclei receive significant corticofugal input.