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

The corticofugal projection from 12 auditory cortical fields onto the media l geniculate body was investigated in adult cats by using wheat germ agglut inin conjugated to horseradish peroxidase or biotinylated dextran amines. T he chief goals were to determine the degree of divergence from single corti cal fields, the pattern of convergence from several fields onto a single nu cleus, the extent of reciprocal relations between corticothalamic and thala mocortical connections, and to contrast and compare the patterns of auditor y corticogeniculate projections with corticofugal input to the inferior col liculus. The main findings were that (1) single areas showed a wide range o f divergence, projecting to as few as 5, and to as many as 15, thalamic nuc lei; (2) most nuclei received projections from approximately five cortical areas, whereas others were the target of as few as three areas; (3) there w as global corticothalamic-thalamocortical reciprocity in every experiment, and there were also significant instances of nonreciprocal projections, wit h the corticothalamic input often more extensive; (4) the corticothalamic p rojection was far stronger and more divergent than the corticocollicular pr ojection from the same areas, suggesting that the thalamus and the inferior colliculus receive differential degrees of corticofugal control; (5) cochl eotopically organized areas had fewer corticothalamic projections than fiel ds 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 arr angement. The areas with the most, divergent corticothalamic projections we re polysensory regions in the posterior ectosylvian gyrus. The projection p atterns were indistinguishable for the two tracers. These findings suggest that every auditory thalamic nucleus is under some degree of descending con trol. Many of the projections preserve the relations between cochleotopical ly organized thalamic and auditory areas, and suggest topographic relations between nontonotopic areas and nuclei. The collective size of the corticot halamic system suggests that both lemniscal and extralemniscal auditory tha lamic nuclei receive significant corticofugal input. J. Comp Neurol. 430:27 -55, 2001. (C) 2001 Wiley-Liss, Inc.