The long-term goal of the proposed experiments is to identify the fundamental processing principles and strategies that underlie auditory cortical receptive field transformations and the representations of complex sounds. We propose to contrast and compare the organization of two primary auditory fields in adult carnivores (Felis catus) and the awake squirrel monkey (Saimiri sciureus) with specific reference to functional domains and laminar organization. A main premise is that core areas receive largely independent inputs and exhibit specific functional differences that may constitute task-specific processing streams. The questions are how do the inputs differ in source location and functional properties;what do the thalamic inputs contribute to the generation of cortical receptive field (RF) diversity;and how do the functional characteristics at the cortical input level compare to those in the output layers? The principal hypotheses underlying the proposed work are that (i) neurons in primary auditory cortical fields express novel, emergent processing properties compared to subcortical stations (the """"""""where"""""""" question);(ii) these emergent features are generated by thalamocortical and corticocortical interactions (the """"""""how"""""""" question);(iii) multiple stimulus dimensions are expressed in each cortical neuron and govern nonlinear interactions that contribute to transformations toward a more robust, less variant sound representation (the """"""""what"""""""" question). Information-theory based methods of spectro-temporal receptive field (STRF) estimation suggest the emergence of novel stimulus processing properties in the auditory forebrain. These properties bear some similarities to emergent processing features in simple and complex cells of primary visual cortex. The goal is to characterize these features and compare their properties across cortical fields and across species in order to extrapolate their general properties and their potential contributions to human hearing.
Aim 1 will assess the parameter distributions of the multiple-feature receptive fields and their nonlinearities in auditory thalamus and auditory cortical core areas.
Aim 2 will determine the main contributing sources to the generation of multiple feature dimensions in auditory cortical neurons.
Aim 3 will determine how well a multi-feature, nonlinear model of neuronal processing predicts responses to combinations of input stimuli and to signals in noise. Combined these studies will establish a functional framework of signal processing in primary auditory cortical fields. Understanding these principles is crucial for a systematic evaluation of concurrent cortical ascending and descending pathways, their role in normal auditory processing, in perceptual learning, and their contributions to auditory disorders.
Peripheral hearing loss has many consequences for the processing of sounds by the central nervous system. Examples of central auditory consequences of peripheral hearing disorders are a reduction of speech comprehension for patients even if they that have well-fitted hearing aids, or the difficulty to understand speech in background noise even for patients with only a mild cochlear hearing loss. We attempt to explore the normal function of the auditory cortex for processing complex signals and signals in different types of background noise in order to ascertain the mechanisms that lead to the normally very robust ability to process signals in noise. Once the normal functions are better understood, the detrimental consequences of hearing loss on these central mechanisms can be examined and may lead to the development of new therapeutic approaches to communicative disorders.
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