The Influence of Sensation Level on Speech-Evoked Envelope Following Responses

Abstract

Objectives: To evaluate sensation level (SL)–dependent characteristics of envelope following responses (EFRs) elicited by band-limited speech dominant in low, mid, and high frequencies. Design: In 21 young normal hearing adults, EFRs were elicited by 8 male-spoken speech stimuli—the first formant, and second and higher formants of /u/, /a/ and /i/, and modulated fricatives, /∫/ and /s/. Stimulus SL was computed from behaviorally measured thresholds. Results: At 30 dB SL, the amplitude and phase coherence of fricative-elicited EFRs were ~1.5 to 2 times higher than all vowel-elicited EFRs, whereas fewer and smaller differences were found among vowel-elicited EFRs. For all stimuli, EFR amplitude and phase coherence increased by roughly 50% for every 10 dB increase in SL between ~0 and 50 dB. Conclusions: Stimulus and frequency dependency in EFRs exist despite accounting for differences in audibility of speech sounds. The growth rate of EFR characteristics with SL is independent of stimulus and its frequency.

The Influence of Male- and Female-Spoken Vowel Acoustics on Envelope-Following Responses

“…Formant frequencies and their bandwidths have a direct effect on the order and number of harmonics that get amplified; harmonics are higher in amplitude when they fall within a formant peak, and the number of harmonics that are amplified due to the formant depends on the formant bandwidth. In general, consistent with the effects of level on EFRs, 14,21,22 voices with stronger harmonics, especially the higher order harmonics, facilitate EFR amplitudes. 1,8,17,29 In summary, the effects of vowel acoustics on EFRs are better known for some parameters like f0 and are relatively less well-known or consistent for parameters like formant characteristics, even among studies that include only male talkers.…”

“…12,19,20 Studies that have used different f0 in different formant regions also provide supporting evidence for this positive correlation between EFR amplitude and formant frequency. 21 However, the degree to which EFRs elicited by higher formants are larger than lower formants is likely underestimated in these studies because the benefit of higher frequency formants is offset by the decrement related to higher f0. 6,7 Relative to formant frequency, the effect of formant level is more difficult to discern.…”

“…6,7 Relative to formant frequency, the effect of formant level is more difficult to discern. Although stimulus intensity is known to have a positive relationship with EFR amplitude, [21][22][23] formant level has shown a negative relationship with EFR amplitude. 15 One possible explanation is that formant level is nega-tively correlated with formant frequency, and possibly, formant frequency has a stronger influence on EFR amplitude relative to level.…”

Characteristics of Speech-Evoked Envelope Following Responses in Infancy

“…In infants who received the higher stimulus level, the overall level boost would likely have provided the most benefit to the EFRs elicited by high frequency stimuli. Because level improves EFR characteristics in general (Easwar et al, 2015c(Easwar et al, , 2021, the possibility of higher stimulus levels for higher frequency stimuli could have reduced adult-infant differences for fricative stimuli relative to lower-frequency vowel stimuli.…”

“…While such discrepancies may indicate developmental changes, it is also possible that some of the differences are due to the range of stimulus levels (or sensation levels) used. The range of stimulus levels used matters because of the non-linear rate of growth in EFR characteristics, irrespective of stimulus or its frequency (Easwar et al, 2021). Nonetheless, comparisons with studies in adults need to be interpreted with caution as they mostly use within-subject designs.…”