Elsevier

Hearing Research

Volume 271, Issues 1–2, January 2011, Pages 133-146
Hearing Research

Research paper
Information flow in the auditory cortical network

https://doi.org/10.1016/j.heares.2010.01.011Get rights and content

Abstract

Auditory processing in the cerebral cortex is comprised of an interconnected network of auditory and auditory-related areas distributed throughout the forebrain. The nexus of auditory activity is located in temporal cortex among several specialized areas, or fields, that receive dense inputs from the medial geniculate complex. These areas are collectively referred to as auditory cortex. Auditory activity is extended beyond auditory cortex via connections with auditory-related areas elsewhere in the cortex. Within this network, information flows between areas to and from countless targets, but in a manner that is characterized by orderly regional, areal and laminar patterns. These patterns reflect some of the structural constraints that passively govern the flow of information at all levels of the network. In addition, the exchange of information within these circuits is dynamically regulated by intrinsic neurochemical properties of projecting neurons and their targets. This article begins with an overview of the principal circuits and how each is related to information flow along major axes of the network. The discussion then turns to a description of neurochemical gradients along these axes, highlighting recent work on glutamate transporters in the thalamocortical projections to auditory cortex. The article concludes with a brief discussion of relevant neurophysiological findings as they relate to structural gradients in the network.

Section snippets

Auditory and auditory-related areas in cortex

Areas that process sound have been discovered in every lobe of the brain, but are all of these areas part of auditory cortex? The answer depends on how one defines auditory cortex. From an anatomical perspective, auditory cortex can be defined as those areas of the cerebral cortex that receive significant thalamic input from one or more divisions of the medial geniculate complex (MGC). By this definition, the auditory cortex of mammals is confined to a group of adjoining areas in the temporal

Information flow in the auditory cortical network

As one reviews the myriad studies of auditory cortex over the last 40 years, a number of common themes stand out. These are based on the structural and functional properties that tend to be the most robust, and are referred to herein as principles of auditory cortical organization (Table 1). Each is related in some way to the flow of information into, within and out of auditory cortex. Perhaps the most fundamental organizational feature is that, in nearly all studied mammals, auditory cortex

Laminar aspects of information flow

In addition to the establishment of a simple ‘connection’ between two areas, the laminar patterns of retrogradely-labeled cells and anterogradely-labeled axonal terminations represent an additional level of refinement that is needed to achieve a more accurate understanding of information flow within these circuits. In the discussion of information flow presented above, some of these details were factored into the classification of connection type (e.g., feedforward, feedback, lateral) and used

Neurochemical influences on information flow

In consideration of the thalamocortical, corticocortical, and laminar patterns of connections involving auditory cortex reviewed so far, there is rather clear evidence of anatomical gradients along at least two axes in cats and monkeys. Connectivity is only part of a much more elaborate picture, however. Differences between areas are also related to the neurochemical properties of their intrinsic and extrinsic circuitry, but we know relatively little about the neurochemistry of the networks

Neurophysiological correlates of information flow

A wide range of neurophysiological findings are consistent with the various anatomical schema described above. Briefly highlighted here are results from studies in which systematic changes in neuronal response properties were observed along the major axes of information flow in auditory cortex.

Conclusions and future directions

In this article, a broad range of anatomical and physiological data were compiled in an effort to track information flow into, within, and out of auditory cortex. The patterns and trends observed indicate that while information processing in auditory cortex is strongly parallel at all levels, information tends to move along axes that are hierarchically ordered. These form the basis of the processing streams that impact numerous auditory-related areas downstream. At present, our models of

Acknowledgements

The author gratefully acknowledges the support of NIH/NIDCD Grant RO1 DC04318 to T.A. Hackett and T32 MH075883 to Vanderbilt Kennedy Center for support of the confocal microscope. We also thank Lisa de la Mothe, Corrie Camalier and the reviewers for their helpful and insightful comments.

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