This study had two purposes. The first was to assess the prevalence of cochlear dead regions (DRs) among listeners with moderate to severe hearing loss that is typical of a large proportion of adult hearing aid wearers. The second was to determine whether subjects who tested positive for DRs differed from those without DRs in their ability to utilize high-frequency speech cues in a laboratory test.
One hundred and seventy adults (307 ears) were tested for DRs at frequencies from 0.5 to 4 kHz using the threshold equalizing noise (HL) test. Speech recognition ability was measured for high-frequency emphasis (HFE) stimuli and for low-pass filtered HFE (HFE-LP) stimuli using the Quick Speech In Noise test. Results obtained from the HFE and HFE-LP conditions were compared to examine changes in word recognition when more speech cues were provided above 2.5 kHz. Possible effects of audiogram differences between DR-no and DR-yes groups were examined by estimating the change in audibility for the two Quick Speech In Noise conditions using calculated differences in Speech Intelligibility Index for each condition for every subject.
Thirty-one percent of subjects (23% of ears) were found to have a DR at one or more test frequencies. Sixty-eight percent of subjects who tested positive for DR had DRs in one ear only. DRs were most prevalent at frequencies above 1.5 kHz. Comparison of word recognition scores obtained with the HFE and HFE-LP conditions revealed that, on average, both groups scored significantly better when more high-frequency cues were provided. The magnitude of the benefit was small for both groups, but the computed effect size was larger for listeners without DRs than for those with DRs. Further, subjects with contiguous DRs at 2 to 3 frequencies obtained less benefit than subjects with DRs at isolated frequencies. It was determined that the improved audibility of high-frequency cues in the HFE condition was significantly less for listeners with DRs, and this accounted for some, but not all, of the difference in effect sizes.
Although about one-third of listeners with flat or sloping moderate to severe hearing losses tested positive for at least one DR, there was no evidence to support a proposal for reducing high-frequency gain in hearing aid fittings for these types of listeners. Making high frequencies more audible was helpful, on average, regardless of DR status. It is recommended that field trials be undertaken in which subjects with and without DRs wear hearing aids in daily life. This type of study would produce higher level evidence about best practice in hearing aid fitting for patients with flat or sloping moderate to severe hearing loss who test positive for DRs.