Nonlinear frequency compression in hearing aids: impact on speech and language development

The research questions of this study were: (1) Are children using nonlinear frequency compression (NLFC) in their hearing aids getting better access to the speech signal than children using conventional processing schemes? The authors hypothesized that children whose hearing aids provided wider input bandwidth would have more access to the speech signal, as measured by an adaptation of the Speech Intelligibility Index, and (2) are speech and language skills different for children who have been fit with the two different technologies; if so, in what areas? The authors hypothesized that if the children were getting increased access to the speech signal as a result of their NLFC hearing aids (question 1), it would be possible to see improved performance in areas of speech production, morphosyntax, and speech perception compared with the group with conventional processing.

Participants included 66 children with hearing loss recruited as part of a larger multisite National Institutes of Health-funded study, Outcomes for Children with Hearing Loss, designed to explore the developmental outcomes of children with mild to severe hearing loss. For the larger study, data on communication, academic and psychosocial skills were gathered in an accelerated longitudinal design, with entry into the study between 6 months and 7 years of age. Subjects in this report consisted of 3-, 4-, and 5-year-old children recruited at the North Carolina test site. All had at least at least 6 months of current hearing aid usage with their NLFC or conventional amplification. Demographic characteristics were compared at the three age levels as well as audibility and speech/language outcomes; speech-perception scores were compared for the 5-year-old groups.

Results indicate that the audibility provided did not differ between the technology options. As a result, there was no difference between groups on speech or language outcome measures at 4 or 5 years of age, and no impact on speech perception (measured at 5 years of age). The difference in Comprehensive Assessment of Spoken Language and mean length of utterance scores for the 3-year-old group favoring the group with conventional amplification may be a consequence of confounding factors such as increased incidence of prematurity in the group using NLFC.

Children fit with NLFC had similar audibility, as measured by a modified Speech Intelligibility Index, compared with a matched group of children using conventional technology. In turn, there were no differences in their speech and language abilities.