Max Planck Institute for Dynamics and Self-Organization -- Department for Nonlinear Dynamics and Network Dynamics Group
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Arbeitsgruppen-Seminar

Wednesday, 13.07.2005 14 c.t.

Object segregation and learning in the auditory cortex

by Dr. Holger Schulze
from Leibniz Institute for Neurobiology, Magdeburg

Contact person: Fred Wolf

Location

Seminarraum Haus 2, 4. Stock (Bunsenstr.)

Abstract

The perceptual quality associated with amplitude modulated tones (AM) varies as a function of modulation frequency (fm): AM of low fm (up to about 100 Hz) evoke percepts of rhythm and roughness, AM of higher fm evoke percepts of periodicity pitch. Recent findings provided evidence that (1.) these different perceptual qualities are paralleled by differences in neuronal coding in primary auditory cortex (AI), where high fm are represented by a spatial rate-place code in a circular periodicity map and low fm are represented by a temporal synchrony code, and (2.) learning performance also differs for low and high fm in an AM discrimination task in gerbils. The latter results give rise to the hypothesis that different learning mechanisms might be recruited in AM discrimination tasks involving low fm or high fm, respectively. By recording from AI before and after AM discrimination learning of low fm in individual animals we demonstrated both on the population level and the single-unit level in chronically implanted animals that neuronal plasticity in the auditory cortex induced by AM discrimination learning is highly specific with respect to both the spectrum and the time structure of the training stimuli. Our data suggest a neuronal mechanism of differential alteration of the timing of excitatory and inhibitory afferent input to auditory cortical neurons due to AM discrimination learning. In a second set of experiments, the functional role of the circular periodicity map in AI was investigated. We follow the hypothesis that the periodicity of speech sounds, which is characteristic for a given speaker, is one of the main cues for the auditory system to solve the so-called cocktail-party problem by a competitive interaction mechanism (“winner-take-all”-algorithm) within the circular map. Using pharmacological blocking of auditory cortical inhibition, we present evidence against the classical view of locally acting cortical inhibition, showing that sharpness of tuning of frequency receptive fields does not depend on GABAA-mediated inhibition. Furthermore, we present data from simultaneous recordings in primary auditory cortex (AI) under conditions of stimulation with concurrent auditory objects, demonstrating that GABAA-mediated inhibition acts in a more global fashion in AI. These latter observations are interpreted in favor of a global rather than local contrast enhancing mechanism in AI that may be suitable for object segregation, e.g. in cocktail-party situations.

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