INTENSITY DLS AND MASKING IN NORMAL AND IMPAIRED HEARING

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Our long-term objective is to reach an understanding of the auditory system that will allow quantitative modeling of the perception of tones, noise, and speech in both normal and impaired hearing. Such models not only will improve our understanding of normal auditory processes, but also will significantly aid in providing better diagnostic tools for hearing impairment and better rehabilitation of hearing-impaired listeners. The proposed project aims to obtain some sorely needed data and to continue testing the general concept that excitation patterns may provide a framework for quantitative modeling of normal and impaired hearing. Based on 21, 2AFC experiments in normal listeners, impaired listeners, and listeners with impairments simulated by masking a normal ear, the proposed project will establish a firm psychoacoustic basis for refining our excitation-pattern model of intensity discrimination and testing several hypotheses related to this and other models. Our specific aims are (1) To test two hypotheses stating how sensitivity, d', depends on the level difference between two sounds being discriminated by measurements of psychometric functions for intensity discrimination, primarily in conditions with large DLs. (2) To test the hypothesis that Weber's Law does not hold within a single auditory channel. Intensity DLs as a function of level will be measured for sounds whose excitation is limited by random- level maskers to comprise a small number of auditory channels. (3) To test the hypothesis that detection of tonal signals in noise and intensity discrimination depends on similar decision rules and employ an unweighted (non-optimal) sum of information from an optimum selection of channels. Measurements of detection and intensity discrimination of a 1-kHz tone and a 16-tone complex will be made in fixed conditions and in conditions in which these two types of signals are presented at random from trail to trial. (4) To begin extending the model to account for discrimination of sounds that vary over time. The manner in which the auditory system can use excitation-level information over time will be assessed through measurements of sensitivity, d', for intensity discrimination as a function of stimulus duration. (5) The hypothesis is that excitation-level information decreases over time due to adaptation processes in the auditory system will be tested in several experiments. Intensity DLs for multiple tone bursts will be measured as function of number of the bursts and the inter-burst interval. In other experiments, intensity DLs for brief tones will be measured in the presence of a preceding, adapting tone of various durations. (6) To further investigate the processing of time-varying sounds and the flexibility of decision processes, the release from masking caused by masker--envelope fluctuations will be investigated in several experiments. These data will significantly add to our knowledge of normal and impaired auditory processing and will be important for the development and testing of models of the auditory system.