Journal of Neuroscience, Vol 10, 50-61, Copyright © 1990 by Society for Neuroscience
The projection from medial geniculate to field AI in cat: organization in the isofrequency dimension
S Brandner and H Redies
Department of Neurobiology, Max-Planck-Institute for Biophysical Chemistry, Gottingen, Federal Republic of Germany.
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.
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