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

Abstract

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.

Branched projections in the auditory thalamocortical and corticocortical systems

“…If a point-to-point connectivity model proposed for AI (Brandner and Redies, 1990) extended throughout AC, double labeling would vary inversely with the distance between injection sites, as confirmed in cat area 17 (Salin et al, 1989). We cannot definitively confirm this principle in AC because different areas were injected in each experiment.…”

“…(B) CF-guided interareal BAs provide a limited (<6%) contribution to the TC projections . (C) TC BAs may participate in creating binaural bands within areas (Middlebrooks and Zook, 1983) and they are not inconsistent with models of point-topoint TC projections (Brandner and Redies, 1990). (D) Extensive interlaminar and intralaminar TC branching (Huang and Winer, 2000) is superimposed upon sparse interareal branching (Lee et al, 2004a).…”

“…There is a substantial representation of binaural response class in both the MGBv and AI (Middlebrooks and Zook, 1983;Middlebrooks et al, 1989;Brandner and Redies, 1990), and, if a similar representation was accessible elsewhere in AC, we might match injection sites on the basis of binaural responses (Fig. 4C).…”

Spatial Organization of Frequency Response Areas and Rate/Level Functions in the Developing AI

“…Projections from the MGBv to the AI do appear to be topographically arrayed in the dimension of bandwidth (as well as the dimension of frequency). That is, ventral injections into the AI tend to label cells caudally positioned within the MGBv and dorsal injections tend to label rostrally positioned cells (Brandner and Redies 1990). However, if cortical tuning were merely a result of topographic projection from the MGBv, one would expect that tuning in the dorsal region of the AI would tend to be sharper than tuning in the ventral region, which is not the case.…”

Two thalamic pathways to primary auditory cortex

“…MGBv neurons in turn project to AI, where these features are represented in layer III and layer IV (Read et al, 2002), forming a two dimensional projection plane (Huang and Winer, 2000;Smith and Populin, 2001). Previous studies in cat and rabbit found a single low-to-high BF organization or tonotopy in MGBv (Aitkin and Webster, 1972;Imig and Morel, 1985;Brandner and Redies, 1990;Cetas et al, 2002;Cetas et al, 2003). Neurons with similar BFs form isofrequency laminae which extend ~3 mm in the dorsal-ventral dimension of MGBv in both species and which correspond anatomically to fibrodendritic laminae (Morest, 1965).…”