The projection from medial geniculate to field AI in cat: organization in the isofrequency dimension.

The topography of the anatomical projection from isofrequency contours (IFCs) in auditory thalamus to IFCs in primary auditory cortex (field AI) was investigated in the cat. In each experiment, a best-frequency map of AI was obtained with electrophysiological recording techniques. Then, different retrogradely transported tracers (HRP, fluorescent dyes) were introduced into AI. In some experiments, different parts (e.g., dorsal, central, and ventral) of a previously mapped IFC were injected, each part with a different tracer. In other experiments, 2 or 3 rows of tracer injections were made at different dorsoventral levels of AI, over a large frequency range (5-38 kHz); each injection row was oriented orthogonal to the IFCs and contained a different tracer. The main mass of the labeled thalamic cells was found in the ventral nucleus of the medial geniculate body (MGv). The MGv cells projecting to a limited sector (1-2 mm in length in most experiments) of an IFC in AI form one or several densely packed neuron clusters of variable shape. The cells labeled by a given tracer are largely separated in space from cells labeled by a different tracer. Thus, different sectors of a cortical IFC receive input from different portions of the corresponding thalamic IFC. As a general rule, cells labeled from dorsal (ventral) injections are centered rostrally (caudally) in the part of MGv innervating AI. However, the topographic details are variable between individuals, and the rostrocaudal gradient is complicated by numerous irregularities and gradients. Previous studies of the auditory thalamocortical projection in the cat have not recognized the topographic order in the isofrequency dimension. Instead, it was believed that different sectors of a cortical IFC were innervated by coincident thalamic populations.