The internal representation of spectral contrast in hearing-impaired listeners.

Abnormal frequency resolution associated with sensorineural hearing impairment produces a smearing of spectral detail in the internal representation of complex acoustic stimuli. As a result, listeners with hearing loss may have difficulty locating spectral peaks (e.g., vowel formants) within stimuli which cue their identity. This study examined the relationship between frequency separation of peaks in a complex spectrum and the degree of spectral contrast preserved in the internal representations in normal and impaired auditory systems. Hearing-impaired and normal-hearing subjects discriminated a flat-spectrum bandpass stimulus from a stimulus containing a sinusoidal ripple across its frequency range. The peak-to-valley amplitude (in dB) necessary for detection of the ripple was measured for ripple frequencies ranging from 1 to 9 cycles/oct. Auditory filter characteristics were also measured at 1 and 3 kHz in order to examine the internal representations of the stimuli after cochlear processing. There were clear differences between groups in both auditory filter characteristics and spectral contrast detection. However, the amount of contrast in the internal representations predicted from these measurements was nearly the same for all subjects, suggesting that the reduced frequency resolution of the hearing-impaired group was largely responsible for differences in required peak-to-valley amplitude in the input spectra. Further, for all subjects, there was a trade-off between the absolute level of internal contrast necessary for ripple detection and the number of samples of this contrast available to the listener.