One of the central challenges in the field of audiology is the verification of optimal hearing aid fitting in infants and young children who cannot reliabl provide detailed subjective feedback about hearing aid settings. The audiologist often must depend on parental report to guide the selection of hearing aid settings and obtaining that information may take weeks or months. Recently, it has been established that cortical auditory evoked potentials (CAEPs), objective recordings of activity in the auditory cortex, may serve as an alternative to parental report for fitting hearing aids. While CAEPs can be used in clinical settings to determine audibility of sound for infants using various hearing aid gain characteristics, at this time, no objective measure exists to guide the selection of optimal frequency compression parameters for an individual hearing aid user. Frequency compression is a hearing aid signal-processing strategy for systematically lowering high frequency sounds that is now commonly enabled for adult and child hearing aid users. It can be effective at increasing the audibility of high frequency speech sounds for individuals with sloping, high frequency hearing loss because it moves those speech sounds from a region of hearing impairment to a region of better hearing. Unfortunately, frequency compression algorithms can also cause a reduction in the spectral contrast between different speech sounds - particularly when more aggressive frequency compression settings are used. The acoustic change complex (ACC) is a variant of traditionally recorded CAEPs and the presence of an ACC is correlated with detection of change in an ongoing stimulus. The ACC may be ideal to evaluate objectively the effects of frequency compression parameters on the representation of spectrally contrasted sounds in the auditory cortex in hearing aid users who are unable to provide subjective feedback on frequency compression settings.
The specific aim of this project is to determine the relationship between cortical evoked potentials and psychophysical performance in conditions of frequency compression and hearing loss. A series of three experiments are proposed to address this specific aim. In the first experiment, ACC responses, psychophysical spectral discrimination thresholds, and vowel identification scores will be obtained from normal hearing participants listening to stimuli that have been compressed in frequency using custom software. In the second experiment, these same measures will be completed in normal hearing participants listening to stimuli additionally processed to simulate high frequency hearing loss. Finally, in the third experiment, a heterogeneous group of participants with high frequency sensorineural hearing loss will be tested. The results of these three experiments will describe the relationship between perception and cortical evoked responses in a range of frequency compression conditions, filling a critical gap in our knowledge base and potentially establishing a needed evidence-based objective outcome measure applicable to clinical pediatric hearing aid fitting.
The proposed research will serve to establish relationships between perception and the encoding of sounds in the auditory cortex in conditions of frequency compression. An objective measure of the type proposed may address an unmet need in clinical pediatric hearing aid fitting, resulting in improved audibility and discriminability of sounds by young hearing aid users.
|Kirby, Benjamin James; Brown, Carolyn J (2015) Effects of Nonlinear Frequency Compression on ACC Amplitude and Listener Performance. Ear Hear 36:e261-70|