Effects of Amplification on Neural Phase Locking, Amplitude, and Latency to a Speech Syllable

Objective: Older adults often have trouble adjusting to hearing aids when they start wearing them for the first time. Probe microphone measurements verify appropriate levels of amplification up to the tympanic membrane. Little is known, however, about the effects of amplification on auditory-evoked responses to speech stimuli during initial hearing aid use. The present study assesses the effects of amplification on neural encoding of a speech signal in older adults using hearing aids for the first time. It was hypothesized that amplification results in improved stimulus encoding (higher amplitudes, improved phase locking, and earlier latencies), with greater effects for the regions of the signal that are less audible. Design: Thirty-seven adults, aged 60 to 85 years with mild to severe sensorineural hearing loss and no prior hearing aid use, were bilaterally fit with Widex Dream 440 receiver-in-the-ear hearing aids. Probe microphone measures were used to adjust the gain of the hearing aids and verify the fitting. Unaided and aided frequency-following responses and cortical auditory-evoked potentials to the stimulus /ga/ were recorded in sound field over the course of 2 days for three conditions: 65 dB SPL and 80 dB SPL in quiet, and 80 dB SPL in six-talker babble (+10 signal to noise ratio). Results: Responses from midbrain were analyzed in the time regions corresponding to the consonant transition (18 to 68 ms) and the steady state vowel (68 to 170 ms). Generally, amplification increased phase locking and amplitude and decreased latency for the region and presentation conditions that had lower stimulus amplitudes—the transition region and 65 dB SPL level. Responses from cortex showed decreased latency for P1, but an unexpected decrease in N1 amplitude. Previous studies have demonstrated an exaggerated cortical representation of speech in older adults compared to younger adults, possibly because of an increase in neural resources necessary to encode the signal. Therefore, a decrease in N1 amplitude with amplification and with increased presentation level may suggest that amplification decreases the neural resources necessary for cortical encoding. Conclusion: Increased phase locking and amplitude and decreased latency in midbrain suggest that amplification may improve neural representation of the speech signal in new hearing aid users. The improvement with amplification was also found in cortex, and, in particular, decreased P1 latencies and lower N1 amplitudes may indicate greater neural efficiency. Further investigations will evaluate changes in subcortical and cortical responses during the first 6 months of hearing aid use. ACKNOWLEDGMENTS: We thank Francis Kuk, Widex USA, for feedback on the project, and Widex USA, Inc. for providing hearing aids and participant funds and providing KEMAR measurements. We also thank Dr. Richard Wilson for providing the six-talker babble used in the noise presentation condition. We also acknowledge Lauren Evans, Arielle Abrams, Alyson Schapiro, Andrea Kaplanges, Alanna Schloss, and other lab members for their help with data collection and analysis. S.A. designed the experiment; K.J., C.F., A.P., and S.A. collected and analyzed the data; and K.J., C.F., A.P., and S.A. wrote the article. We have no conflict of interest to report. This study was funded by University of Maryland’s Department of Hearing and Speech Sciences, the Hearing Health Foundation, NIH-NIDCD Grant T32DC000046, and Widex USA, Inc. who provided hearing aids for the duration of the study and contributed to subject compensation. Address for correspondence: Samira Anderson, Department of Hearing and Speech Sciences, 0100 Lefrak Hall, 7251 Preinkert Drive, College Park, MD 20742, USA. E-mail: sander22@umd.edu Received May 14, 2016; accepted November 5, 2017. Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved.

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