Auditory thalamocortical projections in the cat: Laminar and areal patterns of input

Thalamocortical projections were studied in adult cats using biotinylated d extran amines, wheat germ agglutinin conjugated to horseradish peroxidase, and autoradiography with tritiated leucine and/or proline. The input from 7 architectonically defined nuclei to 14 auditory cortical fields was charac terized qualitatively and quantitatively. The principal results were that 1 ) every thalamic nucleus projected to more than 1 field (range, 4-14 fields ; mean, 7 fields); 2) only the projection from the ventral division to some primary fields (primary auditory cortex and posterior auditory cortex) had a periodic, clustered distribution, whereas the input from other divisions to nonprimary areas was continuous; 3) layers III-V received >85% of the t otal axonal profiles; 4) in most experiments, five or more layers were labe led; 5) the projections to nonprimary auditory areas had many laterally ori ented axons; 6) the heaviest input to layer I in all experiments was usuall y in its upper half, suggesting a sublaminar arrangement; 7) the largest ax onal trunks (up to 6 mum in diameter) arose from the medial division and en ded in layer Ia, where they ran laterally for long distances; 8) there were three projection patterns: type 1 had its peak in layers III-TV with littl e input to layer I, and it arose from the ventral division and the dorsal s uperficial, dorsal, and suprageniculate nuclei of the dorsal division; type 2 had heavy labeling in layer I and less in layers III-TV, arising from th e dorsal division nuclei primarily, especially the caudal dorsal and deep d orsal nuclei; and type 3 was a trimodal concentration in layers I, III-TV, and VI that originated chiefly in the medial division and had the lowest de nsity of labeling; and 9) the quantitative profiles with the three methods were very similar. The results suggest that the subdivisions of the auditor y thalamus have consistent patterns of laminar distribution to different co rtical areas, that an average of five or more layers receive significant in put in a specific area, that a given thalamic nucleus can influence areas a s far as 20 mm apart, that the first information to arrive at the cortex ma y reach layer I by virtue of the giant axons, and that several laminar patt erns of auditory thalamocortical projection exist. The view that the audito ry thalamus land perhaps other thalamic nuclei) serves mainly a relay funct ion underestimates its many modes for influencing the cortex on a laminar b asis. (C) 2000 Wiley-Liss, Inc.