The long-term aim of this project is an understanding of the causes for the hearing disability experienced by cochlear-impaired listeners. The project has two main areas of focus. The first area addresses the importance of comodulation in across-frequency processing of speech. Here we test the hypothesis that speech understanding at low signal-to-noise ratios may depend critically upon the ability of the auditory system to integrate information present at a given frequency and time with other information that has occurred recently at other spectral regions. This issue will be addressed by investigating vowel perception for nonsimultaneous formants, and the recognition of speech that is interrupted either synchronously or asynchronously across frequency. Another main goal in this area is to determine whether part of the advantage for speech perception in modulated noise is related to comodulation masking release (CMR). New methods are proposed that should realize important advances in this area. In addition, we will determine whether the magnitude of CMR for speech is related directly to the difficulty/redundancy of the speech material, a relation that is predicted from recent suprathreshold psychoacoustical results. The initial studies in this area will involve normal-hearing listeners. The second area of study will investigate the effect of cochlear hearing loss on the coding of fine temporal information (in terms of temporal fine structure and high-frequency envelope). Such temporal coding may be critically important for pitch perception, binaural analysis, and speech perception. Sensitivity to fine temporal cues will be measured via pitch discrimination for unresolved harmonics and binaural interaural time discrimination (ITD). We will also establish a new pitch perception method using unresolved harmonics to estimate sensitivity as a function of frequency to temporal fine structure and temporal envelope in the human auditory system. Once established, this method will be used to determine effects of hearing impairment on the ability to temporally code the fine structure and envelope of the stimulus. A main goal will be to test the hypothesis that there is a relation between monaural tasks of pitch discrimination and binaural ITD in cochlear-impaired listeners, and that both of these measures will be related to speech perception performance. A related goal is to test the hypothesis that the enhanced speech recognition found for Meniere's patients during the glycerol test is due to an improvement in the coding of the fine temporal features of the stimulus.
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