Hearing Aids and the Brain What’s the Connection? Features
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Features  |   May 01, 2007
Hearing Aids and the Brain
Author Notes
  • Curtis J Billings, associate professor, Department of Speech and Hearing Sciences, University of Washington, can be contacted at curtisjb@u.washington.edu.
    Curtis J Billings, associate professor, Department of Speech and Hearing Sciences, University of Washington, can be contacted at curtisjb@u.washington.edu.×
  • Kelly L Tremblay, associate professor, Department of Speech and Hearing Sciences, University of Washington, can be contacted at tremblay@u.washington.edu.
    Kelly L Tremblay, associate professor, Department of Speech and Hearing Sciences, University of Washington, can be contacted at tremblay@u.washington.edu.×
Article Information
Hearing Aids, Cochlear Implants & Assistive Technology / Features
Features   |   May 01, 2007
Hearing Aids and the Brain
The ASHA Leader, May 2007, Vol. 12, 5-23. doi:10.1044/leader.FTR3.12072007.5
The ASHA Leader, May 2007, Vol. 12, 5-23. doi:10.1044/leader.FTR3.12072007.5
Hearing aids may serve as a useful model for studying the effects of auditory stimulation on the brain, and experiments designed to examine the effect of amplification on the human central auditory system (CAS) are beginning to emerge. Understanding the effects of acoustic stimulation on the auditory system has implications for both basic science and the clinic. We could ask: Are there long-term changes in CAS activity when an individual is fit with a hearing aid? Does hearing aid amplification affect maturation of the auditory system? How does the CAS contribute to performance variability, and are neural responses for successful hearing aid users different from those of unsuccessful hearing aid users?
The term “auditory plasticity” is sometimes used to describe the brain’s capacity to change; that is, the effect of periods of auditory deprivation (i.e., attenuation or removal of auditory input) and/or stimulation (i.e., reintroduction or exposure to auditory input) on the way frequency, intensity, and timing information is coded in the brain. The typical person being evaluated by an audiologist for hearing aids or a cochlear implant has an auditory system that has likely undergone significant deprivation-related physiological changes, and introducing sound to a previously deprived auditory system via hearing aids likely alters the way spectral and temporal information is represented in the central auditory system. Irvine and Rajan (1996) refer to processes similar to this as “deprivation-related” and “stimulation-induced” or “use-related” neural plasticity.
Although the effects of hearing aid amplification on the brain is an interesting topic, it is important to note that some of the basic effects of the hearing aid on the brain—and in particular, the effect of the signal processing parameters—are relatively unknown. For this reason, we completed a series of experiments to characterize how amplified sounds are represented in the CAS. Without such studies, conclusions about CAS changes related to hearing aid amplification might not be valid.
For example, when sound is amplified by a hearing aid, neural response patterns should be larger in amplitude and shorter in latency when compared with unaided neural responses. Recent experiments conducted in our laboratory, however, indicate that this response is not the case. Surprisingly, when 20 dB of gain was provided by a hearing aid, there were no significant differences between unaided and aided neural responses patterns. In other words, 20 dB of hearing gain affects neural responses differently than 20 dB of stimulus intensity change. We don’t know the reason for this difference, but it is likely that signal alterations introduced by the signal processing of the hearing aid (e.g., modification of stimulus rise characteristics, alteration of the signal-to-noise ratio, or introduction of an amplitude overshoot caused by activation of compression circuitry) are interacting with how the auditory system encodes intensity changes.
So what’s the connection between the hearing aid and the brain? We don’t know. We first need to understand the interaction between the device-related variables and the auditory system so we can move on to the more clinically applicable questions. The same is probably true for cochlear implants. And with these points in mind, additional research, using various types of measures, might someday help us understand the effect of amplification on the CAS, as well as the auditory system’s capacity to change with amplification.
Special Division 6, Research and Diagnostics, contributed this article. The division offers affiliates the opportunity to earn CEUs through self-study of its peer-reviewed publication, Perspectives; an exclusive e-mail list and Web forum; and other benefits. Learn more about Division 6.
References
Irvine, D. R. F., & Rajan, R. (1996). Injury-and use-related plasticity in the primary sensory cortex of adult mammals: Possible relationship to perceptual learning. Clinical and Experimental Pharmacology and Physiology, 23, 939–947. [Article] [PubMed]
Irvine, D. R. F., & Rajan, R. (1996). Injury-and use-related plasticity in the primary sensory cortex of adult mammals: Possible relationship to perceptual learning. Clinical and Experimental Pharmacology and Physiology, 23, 939–947. [Article] [PubMed]×
Tremblay, K. L., Billings, C. J., Friesen, L. M., & Souza, P. E. (2006). Neural representation of amplified speech sounds. Ear and Hearing, 27, 93–103. [Article] [PubMed]
Tremblay, K. L., Billings, C. J., Friesen, L. M., & Souza, P. E. (2006). Neural representation of amplified speech sounds. Ear and Hearing, 27, 93–103. [Article] [PubMed]×
Billings, C. J., Tremblay, K. L., Souza, P. E., Binns, M. A. (2007). Effects of hearing aid amplification and stimulus intensity on cortical auditory evoked potentials. Audiology and Neurotology, 12, 234–246. [Article] [PubMed]
Billings, C. J., Tremblay, K. L., Souza, P. E., Binns, M. A. (2007). Effects of hearing aid amplification and stimulus intensity on cortical auditory evoked potentials. Audiology and Neurotology, 12, 234–246. [Article] [PubMed]×
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May 2007
Volume 12, Issue 7