Examining Speech Perception in People with Normal Hearing Could Help Improve Cochlear Implants in the Future

March, 2012 — One of our 2012 AHRF grant recipients, Jong Ho Won, PhD, of the University of Washington, Seattle, is interested in how people with normal hearing perceive speech. He says that a better understanding of how the brain processes sounds in people without any hearing loss could help guide future improvements in cochlear implants.

Sound is composed of many distinct temporal and frequency information. Figure 1(A) shows the waveform for a vowel “/A/” from a word “ABA”. This sound can be broken into two components: temporal envelope plotted in Figure 1(B), and temporal fine structure plotted in Figure 1(C). In Figure 1(A), the temporal envelope is plotted as a red line. As shown in Figure1 (B) and (C), temporal envelope is a slowly changing component over time, whereas temporal fine structure is a fast changing component. A major problem with current cochlear implants is that they do a poor job of translating the temporal fine structure of sound. This makes it difficult for implant users to follow conversations in some places, like in a busy restaurant. Music also loses much of the detail for cochlear implant users, because musical pitch information is encoded in the temporal fine structure.

Figure 1. (A) Time-domain display of a spoken word /A/ of “ABA”. (B) The envelope component of this speech is shown. (C) The temporal fine structure of this speech is shown.

Dr. Won’s research focuses on measuring and describing how normal listeners encode acoustic cues. He studies listeners’ perception abilities as well as the brain response (called frequency following response). To do this, he will measure the brain responses from individual listeners with normal hearing as they are presented with various sounds containing only temporal envelope, or only temporal fine structure, or both (“natural sound”). He will measure the brain response from electrodes placed on his subjects’ head vertex, forehead and earlobes. Dr. Won will then look at how the neural response correlates to the different sound conditions.

“The frequency following responses measured from normal listeners should closely reflect the properties of sound,” says Dr. Won. “By studying this in normal listeners, and really understanding exactly what is at work in sound perception, we should be able to provide guidance to cochlear implant engineers on what they need to add to future devices to better represent the finer details of sound.”Dr. Won’s research is timely important because there is increasing awareness of the importance of cortical processes involved when using a cochlear implant. “I am very grateful to American Hearing Research Foundation for supporting my research. I hope my efforts may contribute to restore hearing and improve the quality of life of those who suffer from hearing loss”, says Dr. Won.