Our long-term objective is to formulate quantitative models of auditory perception in both normal and impaired hearing. Such models improve our understanding of normal and impaired auditory processes. They also aid significantly in providing better diagnostic tools for hearing Impairment and better rehabilitation of hearing-impaired listeners. The proposed project aims to investigate how the auditory system processes time- varying excitation patterns and forms multiband decision rules to perceive loudness, roughness, and modulation patterns (envelopes) of complex sounds. Because the Input to these processes is conceived as originating from excitation patterns, the proposed work provides further testing of the concept that excitation patterns and multi-band decision rules provide a general framework for quantitative modeling of normal and impaired hearing. Model predictions will be compared to psychoacoustic data obtained in forced-choice experiments with cochlearly impaired listeners and with normal listeners tested in the quiet and under Equivalent-Threshold Masking, ETM. ETM is produced by presenting a continuous masker that is spectrally shaped to produce thresholds equal to those of an impaired listener. These experiments also will provide an extensive set of sorely needed data on auditory processing of brief amplitude fluctuations, which is an important prerequisite for the perception of speech.
The specific aims of the experiments are: i) To test the hypothesis that the loudness contribution of stimulus components at threshold increases with threshold and is greater than zero even for components that are below their thresholds in isolation, loudness matches will be obtained between pure tones and multitone complexes in the quiet and under two levels of masking. 2) To test the hypothesis that temporal integration for loudness depends on level and has a simple relation to the internal intensity code, loudness matches between tones of different durations will be obtained as a function of level and frequency. 3) To test the hypothesis that the perceived magnitude of roughness is inversely related to temporal integration for loudness, roughness matches between partially masked amplitude-modulated tones will be obtained as a function of level and frequency of the carrier tones. 4) To test the hypothesis that across-channel processing facilitates the perception of relatively fast, partially correlated envelope fluctuations in different frequency regions, psychometric functions for envelope-frequency discrimination and roughness matches will be obtained with noise bands produced by filtering a sinusoidally amplitude modulated broadband noise. All these experiments are motivated by specific models of the auditory system and discrimination processes in it, but regardless of our models' success, the data obtained in the proposed project will add significantly to our understanding of normal and impaired hearing and will be important for the development and testing of general models for the processing of dynamic signals by the auditory system.
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