Older adults who wear hearing aids often report that speech is too loud, yet they have difficulty with the clarity of the message. Part of this difficulty is rooted in the well-known phenomenon of recruitment. However, changes in speech representation in the central auditory system also play a role.

The effects of hearing loss on auditory processing of a speech syllable were recently evaluated using the cABR the ABR to complex sounds. Brainstem responses were recorded to a 40-ms speech syllable /da/ in quiet and in noise in two groups of older adults. One group had clinically normal hearing and the other had mild to moderate sensorineural hearing loss and had never worn hearing aids. The group with hearing loss had higher representation of the low frequencies that comprise the speech envelope. In response to the temporal fine structure (TFS), there was no difference in frequency representation between the groups. Overall, in the group with hearing loss, there was a greater ratio of envelope to temporal fine structure than there was in the group with normal hearing, suggesting that the augmented response to the speech envelope swamps out the temporal fine structure, leading to a deficit in fine structure coding.

Work with cochlear implants has taught us that access to the speech envelope is adequate for hearing in quiet situations, but temporal fine structure cues may be important for hearing in background noise. Therefore, a relative deficit in fine structure coding in individuals who wear hearing aids may account for difficulty understanding speech in background noise. Work is underway to examine the effects of training and amplification on the balance of envelope and temporal fine structure representation of the speech signal.