Neural encoding of speech envelopes during development: A frequency-specific investigation

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Project Summary An important time-based cue for understanding speech in quiet and in the presence of noise is the periodicity in voicing at the fundamental frequency (f0). The ability of children to use f0-rate information for speech understand- ing depends on how well their auditory system extracts this information in each frequency region. The ability to extract such information could be influenced by the cochlear-place-specific development of neural pathways and the tendency for auditory processing to develop earlier in pathways innervating the cochlear base. f0 encod- ing can be evaluated using envelope following response (EFRs)—an electroencephalogram-based method that reflects neural activity phase-locked to the periodic f0 envelope. However, little is known about the frequency- specific development of f0 encoding because of the predominant use of broadband vowels to elicit EFRs. To this end, our goal is to systematically evaluate the frequency-specific developmental time course of f0 encoding. The proposed studies will use novel band-limited speech stimuli to investigate the developmental time course of f0 encoding initiated in different cochlear regions corresponding to low, mid and high frequencies. In addition to using a frequency-specific approach, f0 encoding will be assessed as a function of (i) f0 rate—a parameter that changes by talker age and gender, (ii) decreasing signal-to-noise ratio—a parameter that simulates challenging listening conditions especially for children, and (iii) behavioral relevance of the stimulus—a parameter that can disambiguate the impact of everyday listening experience. The two pediatric age groups of interest with immature perception of f0-rate envelope periodicity are, 0.3-1 years and 5-8 years, as they align with mature cochlear, and cochlear and brainstem function, respectively. For rigorous adult-child comparisons of f0 encoding in the presence of age-dependent ear and head sizes, the approach will entail: (i) stimulus calibration in each test ear to equalize stimulus level across all test ears, (ii) use of phase coherence—a normalized response measure independent of absolute response strength and latency—as the maturity metric, and (iii) between-group comparisons of stimulus- induced changes rather than raw EFR measures. Findings will reveal if f0 encoding at low frequencies is the last to achieve adult-like processing and if such a developmental trajectory is specific to speech. The proposed research has important implications for children with cochlear hearing loss who may experience frequency-specific abnormal f0 encoding as a function of hearing loss degree. The age-, frequency-, rate-, signal- to-noise ratio-, and stimulus-specific development of f0 encoding from the proposed work will advance our ability to detect and assess the nature and extent of abnormal f0 encoding due to cochlear hearing loss and/or signal processing in devices like hearing aids. The use of EFRs will enable assessment of f0 encoding in children too young to participate in behavioral hearing tests and in turn, offer the opportunity to inform changes in intervention earlier than behavioral hearing tests can.