Thalamic projections to fields A, AI, P, and VP in the cat auditory cortex

Abstract

Thalamocortical projections to four tonotopic fields (A, AI, P, and VP) of the cat auditory cortex were studied by using combined microelectrode mapping and retrograde axonal transport techniques. Horseradish peroxidase (HRP) or HRP combined with either tritiated bovine serum albumin or nuclear yellow was injected into identified best‐frequency sites of one or two different fields in the same brain. Arrays of labeled neurons were related to thalamic nuclei defined on the basis of their cytoarchitecture and physiology. In some cases, patterns of labeling were directly compared with thalamic best‐frequency maps obtained in the same brain. We compared only patterns of labeling resulting from injections into similar parts of the frequency representation in different fields to insure that observed differences in patterns of labeling did not simply reflect differences in the frequency representation at the injection sites. The thalamic projection to the four fields is divided among seven nuclei, three tonotopic nuclei (ventral nucleus, V; lateral part of the posterior group of thalamic nuclei, Po; and dorsal cap nucleus, d) and four nontonotopic nuclei (caudodorsal nulceus, cd; ventrolateral nucleus, vl; and small, Ms, and medium‐large, Mg, cell regions of the medial division). Projections to each field differ, and each field receives inputs from tonotopic and nontonotopic nuclei. Field A receives its major inputs from Po and Mg, and a minor input from V. Field AI receives its major inputs from V, Po, and Mg, although Po and Mg have heavier projections to field A. Field P receives its major inputs from V, d, and vl; and minor inputs from cd, Ms, Mg, and Po. Field VP receives major inputs from V, vl, and cd; and minor inputs from d, Ms, and Mg. There are segregated territories in V and Po in which most neurons project to one cortical field (major projection), and a smaller proportion projects to one or more other fields (minor projections). Field VP receives a major projection from the caudal pole of V. Field P receives a major projection from the caudal half of V, and from a thin band along the dorsal border of rostral V. Field AI receives a major projection from most of the rostral one‐half of V, and smaller areas in Po and the caudal half of V exclusive of its caudal pole. Field A receives a major projection from most of Po. Neurons with collateral projections to at least two cortical fields have been found in small proportions in most areas projecting to tonotopic cortex. In V and Po at least one‐third of the minor‐projection fibers appear to be collaterals of major‐projection fibers.