Cochlear Dead Regions and Hearing Aid Fittings
Cox, Johnson, and Alexander (2012) report cochlear dead regions (DRs) occur when inner hair cells at a given frequency are lost or malfunctioning. Previous publications have suggested hearing-aid high-frequency gain should be attenuated in the presence of high frequency DRs. To further evaluate this premise (see Cox, Alexander, Johnson amd Rivera, 2011), the authors performed a double-blind, non-randomized intervention study with 18 pairs of matched adult subjects with mild-to-moderate severe hearing loss. Across the 18 pairs of subjects, one member of each pair demonstrated high frequency DRs, the other member of each pair did not have DRs. Each subject was fitted with two programs. One program was the NAL program and the alternative program was NAL with reduced gain in the high frequencies.
In general, across all subjects (those with and those without DRs) in the lab, the NAL program produced the best speech recognition in quiet. In noisy lab conditions, NAL produced the best results for subjects without DRs. For subjects with DRs the results were equivalent (for NAL and for NAL with reduced gain in the high frequencies). In general, across all subjects in "daily life," NAL was given superior ratings for speech understanding. Interestingly, when asked about their "preference," about one-third of the subjects chose the NAL program with reduced high-frequency gain. However, the most often repeated reason for choosing the NAL program with reduced high frequency gain was that NAL was too loud.
Cox, Johnson, and Alexander note their study is limited to adults subjects with mild-to-moderate severe hearing loss (typical of most hearing-aid candidates). Further, they note a positive DR result (that is, the subject/patient demonstrates DRs) "…does not call for a priori modification in that patient’s target prescription for amplification…" and patients/subjects with one or two demonstrated DRs is "…likely to benefit from high-frequency gain…" as would be expected of subjects/patients without DRs.
For More Information, References, and Recommendations
Cox RM, Johnson JA, Alexander GC. (2012) Implications of High-Frequency Cochlear Dead Regions for Fitting Hearing Aids to Adults With Mild to Moderately Severe Hearing Loss. Ear & Hearing 33(5):573-587.
Cox RM, Alexander GC, Johnson J, Rivera I. (2011) Cochlear dead Regions in Typical Hearing Aid Candidates: Prevalence and Implications for Use of High-Frequency Speech Cues. Ear & Hearing 32(3):339-348.