Van Summers Marjorie R. Leek
Army Audiol. and Speech Ctr., Walter Reed Army Med. Ctr., Washington, DC 20307-5001
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 that provide important cues to speech understanding. This study examined the relationship between frequency separation of peaks in a complex sound and the degree of spectral contrast preserved in normal and impaired auditory systems. Five hearing-impaired subjects (HI) and five normal-hearing subjects (NH) 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 in notched-noise were measured at 1 and 3 kHz in order to assess the relationship between frequency resolution and the ability to detect spectral contrast in complex spectra. There were clear differences between groups in both auditory filter characteristics and spectral contrast detection. Auditory filters tended to be both broader and more asymmetric for HI listeners than for NH listeners. Mean ripple amplitudes at threshold were approximately 4 dB lower for the NH group than the HI group at all ripple frequencies. However, excitation patterns based on auditory filter characteristics and threshold stimuli specific to individual listeners exhibited levels of peak-to-valley contrast that were nearly identical across listeners. This suggests that increased ripple detection thresholds of the hearing-impaired listeners were due to spectral smearing resulting from impaired frequency resolution. [Work supported by NIDCD Grant No. DC00626.]