[EN] We aimed at investigating why hearing impaired (HI) listeners vary widely in their ability to understand speech in noisy environments, even when their hearing loss is compensated for with amplification. We hypothesized that outer hair cell (OHC) dysfunction affects the intelligibility of audible speech in noise, and that HI listeners with identical audiometric loss can vary widely in their degree of OHC dysfunction. To test these hypotheses, we inferred the proportion of the audiometric loss that is due to cochlear mechanical gain loss (HLOHC), and investigated the correlation between HLOHC and residual cochlear compression with the speech reception thresholds (SRTs) in noise. HLOHC and residual compression were estimated from comparisons of cochlear input/output (I/O) curves for HI listeners with reference curves for normalhearing listeners. I/O curves were inferred using a psychoacoustical technique known as the temporal masking curve (TMC) method. Therefore, for completeness, a third objective was to validate the assumptions of this method.
68 listeners with symmetrical sensorineural hearing losses participated in the study. For each of them, we measured air and bone conduction thresholds, temporal masking curves, and distortion product otoacoustic emissions at
multiple test frequencies (0.5, 1, 2, 4 and 6 kHz). We also measured SRTs for speech-shaped noise and a (time-reversed) two-talker masker.
Results showed that (1) it is reasonable to use TMCs for inferring cochlear input/output curves in humans; (2) the large majority of HI listeners suffer from mixed inner hair cell and OHC dysfunction; (3) HLOHC contributed
between 30–40 and 60–70% to the total audiometric loss, and the contribution was approximately constant across the frequency range from 0.5 to 6 kHz; (4) the contribution of HLOHC to audiometric loss varied largely across listeners, particularly at low frequencies or for mild-to-moderate hearing losses; (5) estimated OHC dysfunction is uncorrelated with SRT in steady-state noise; (6) residual cochlear compression, however, is correlated with SRT in speechshaped
steady noise but not in a (time-reversed) two-talker masker; (7) age per se reduces the intelligibility of speech in any of the two maskers tested here, regardless of absolute thresholds or cochlear mechanical dysfunction.
