A primary operation of the inner ear is the division of acoustic stimuli into the activities of separate bands, often called cochlear filters. A history of experimentation directed at measurement of these filters has shown them to be wider in listeners with cochlear impairments than in subjects with normal hearing, an important consideration in explaining diminished speech intelligibility in noise generally found with such impairment. Historically, studies of cochlear filter have concentrated on their amplitude spectra, i.e., their frequency responses, but the present program of research has joined cutting edge research on phase responses of the filters. These data will have a dual function. First, they have considerable value for our basic understanding of auditory function; secondly, in providing information about the dynamic processing of components both within and across channels, they address issues related to the lister's ability to utilize information in the intensity and frequency contours of speech. The traditional way to tailor a hearing aid for the individual was to match the gain characteristics of the aid to threshold data from the patient. However, with the advent of powerful digital techniques for modifying the processing algorithms for new aids, it will be possible to take advantage of knowledge of the phase characteristics of cochlear filters in the patient to increase the effective signal-to-noise ratios and hence the discriminability of speech signals in everyday listening. With this in mind, the experiments on phase effects in normal and abnormal cochleas will ask how temporal processing within the auditory channels enhances spectral contrast and sensitivity to change. Both normal hearing and impaired listeners will be tested using psychophysical methods as well as tests of the acoustic reflex, a tool commonly used in audiology to measure sensitivity.
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