Sensitivity of Vowel-Evoked Envelope Following Responses to Spectra and Level of Preceding Phoneme Context.

Objective: Vowel-evoked envelope following responses (EFRs) could be a useful noninvasive tool for evaluating neural activity phase-locked to the fundamental frequency of voice (f0). Vowel-evoked EFRs are often elicited by vowels in consonant-vowel syllables or words. Considering neural activity is susceptible to temporal masking, EFR characteristics elicited by the same vowel may vary with the features of the preceding phoneme. To this end, the objective of the present study was to evaluate the influence of the spectral and level characteristics of the preceding phoneme context on vowel-evoked EFRs.

Design: EFRs were elicited by a male-spoken /i/ (stimulus; duration = 350 msec), modified to elicit two EFRs, one from the region of the first formant (F1) and one from the second and higher formants (F2+). The stimulus, presented at 65 dB SPL, was preceded by one of the four contexts: /∫/, /m/, /i/ or a silent gap of duration equal to that of the stimulus. The level of the context phonemes was either 50 or 80 dB SPL, 15 dB lower and higher than the level of the stimulus /i/. In a control condition, EFRs to the stimulus /i/ were elicited in isolation without any preceding phoneme contexts. The stimulus and the contexts were presented monaurally to a randomly chosen test ear in 21 young adults with normal hearing. EFRs were recorded using single-channel electroencephalogram between the vertex and the nape.

Results: A repeated measures analysis of variance indicated a significant three-way interaction between context type (/∫/, /i/, /m/, silent gap), level (50, 80 dB SPL), and EFR-eliciting formant (F1, F2+). Post hoc analyses indicated no influence of the preceding phoneme context on F1-elicited EFRs. Relative to a silent gap as the preceding context, F2+-elicited EFRs were attenuated by /∫/ and /m/ presented at 50 and 80 dB SPL, as well as by /i/ presented at 80 dB SPL. The average attenuation ranged from 14.9 to 27.9 nV. When the context phonemes were presented at matched levels of 50 or 80 dB SPL, F2+-elicited EFRs were most often attenuated when preceded by /∫/. At 80 dB SPL, relative to the silent preceding gap, the average attenuation was 15.7 nV, and at 50 dB SPL, relative to the preceding context phoneme /i/, the average attenuation was 17.2 nV.

Conclusion: EFRs elicited by the second and higher formants of /i/ are sensitive to the spectral and level characteristics of the preceding phoneme context. Such sensitivity, measured as an attenuation in the present study, may influence the comparison of EFRs elicited by the same vowel in different consonant-vowel syllables or words. However, the degree of attenuation with realistic context levels exceeded the minimum measurable change only 12% of the time. Although the impact of the preceding context is statistically significant, it is likely to be clinically insignificant a majority of the time.