Group differences and individual variability in the frequency-following response to dynamic tonal glides as a function of age, hearing impairment, and sweep count
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2016-08-31
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Towson University. Department of Audiology, Speech-Language Pathology and Deaf Studies
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Abstract
Synchronous neural firing and accurate phase-locking support the encoding of time-varying acoustic features of speech critical for speech discrimination. Evidence suggests that phase-locking is disrupted in older and hearing-impaired adults, which may help account for the frequently-reported perceptual deficits in those populations not otherwise accounted for by peripheral hearing sensitivity. The frequency-following response (FFR) has previously been utilized to index subcortical encoding in various populations. However, normative data for the FFR has not been formally established to date, in part because the response may be elicited by a variety of stimuli, such as pure tones, tonal sweeps, and speech stimuli in a number of populations, such as in older individuals and individuals with hearing loss. As such, the aims of this study are two- fold. First, the study examined group differences in FFR quality between three listener groups: younger normal hearing (YNH) (N = 10, M = 28.1 years, range = 24-33), older normal hearing (ONH) (N = 10, M = 61.1 years, range = 51-66), and older hearing- impaired (OHI) (N = 10, M = 66.8 years, range = 54-78) adults as a function of sweep count. Second, individual response variability within each group was evaluated qualitatively by analyzing averaged time waveforms and corresponding spectrograms to begin documenting the range of responses which might be obtained in homogenous groups in which degree of hearing loss and age are controlled. Three thousand sweeps were collected in alternating polarity to rising tonal stimuli 120 ms in length spanning one-third, two-thirds, and one whole octave centered around 500 Hz. FFR waveforms were averaged in increasing increments of 100 consecutive sweeps and were quantitatively analyzed via cross correlation analysis. Results reveal that ONH and OHI adults require significantly more sweeps than YNH adults to achieve FFRs of similar quality, suggesting that older adults are more prone to desynchronization in temporal information encoding than younger individuals, independent of hearing status. However, further analysis of individual responses reveals that independent of hearing loss and age, each group included individuals who robustly encoded the stimuli, as well as individuals for which the FFR was indistinguishable from baseline biologic electroencephalographic activity.