Understanding the role of reduced basilar membrane compression in masking release.

Abstract

Hearing-impaired (HI) listeners often show poorer performance on psychoacoustic tasks than do normal-hearing (NH) listeners. For example, listeners with cochlear hearing loss often show less benefit, or masking release (MR), than do listeners with NH when a steady-state masker is replaced by a temporally fluctuating one, even when overall audibility is equated. It is possible that some of this degraded performance may be due to reduced cochlear compression. To compare performance of HI and NH listeners, the sensation level (SL) and overall SPL of the stimuli must be equated. Masking noise can be used to accomplish this by raising the thresholds of NH to equal the thresholds in quiet of HI listeners. However, such noise may have other effects, including changing peripheral response characteristics, such as the compressive input-output function of the basilar membrane in the normal cochlea. To test this, the first study estimated compression behaviorally across a range of background-noise levels in six NH listeners at a 4-kHz signal frequency, using a growth-of-forward-masking (GOM) paradigm. For signals 5 dB or more above threshold in noise, no significant effect of broadband noise level was found on estimates of compression. Due to limitations with the use of forward GOM for low-frequency signals, and the desire to estimate compression across a wide range of frequencies, additivity of forward masking (AFM) was evaluated in the second study. AFM was measured in six NH listeners for signal frequencies of 500, 1500, and 4000 Hz in the presence of the same TEN background noise used in the first study. Results consistent with compressive BM responses were found for all listeners for the 500 Hz signal, 5 listeners at 1500 Hz, and only 2 listeners at 4000 Hz. Further measurements in the absence of background noise also indicate a lack of consistent compression at 4000 Hz at higher signal levels, in contrast to earlier results collected at lower levels. Better understanding of this issue will be required before AFM can be used as a general behavioral estimate of BM compression. The final study tests whether changes in cochlear compression can predict the degree of MR loss. Behavioral estimates of compression, using temporal masking curves (TMCs), were compared with MR for speech and pure tones in hearing-impaired (HI) individuals and age-matched, noise-masked normal-hearing (NMNH) listeners. Compression estimates were made at 500, 1500, and 4000 Hz. Pure-tone masking period patterns and MR for band-limited (500-4000 Hz) IEEE sentences were measured in a 10-Hz square-wave-gated noise. In addition, an estimate of temporal resolution was calculated using the off-frequency curve from the TMC measurements. No strong relationship was found between estimates of cochlear compression and MR for either speech or pure tones. There was a nonsignificant correlational trend between temporal resolution estimates and MR for speech overall, and NMNH had significantly steeper recovery slopes than did HI. The results suggest either that the effects of hearing impairment on MR are not determined primarily by changes in peripheral compression, or that the TMC does not provide a sufficiently reliable measure of compression.