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Phase‐locked responses to the vowel envelope vary in scalp‐recorded amplitude due to across‐frequency response interactions
Author(s) -
Easwar Vijayalakshmi,
Banyard Ashlee,
Aiken Steven J.,
Purcell David W.
Publication year - 2018
Publication title -
european journal of neuroscience
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.346
H-Index - 206
eISSN - 1460-9568
pISSN - 0953-816X
DOI - 10.1111/ejn.14161
Subject(s) - formant , vowel , scalp , amplitude , audiology , stimulus (psychology) , spectral envelope , acoustics , mathematics , cochlea , physics , speech recognition , psychology , medicine , computer science , anatomy , quantum mechanics , psychotherapist
Abstract Neural encoding of the envelope of sounds like vowels is essential to access temporal information useful for speech recognition. Subcortical responses to envelope periodicity of vowels can be assessed using scalp‐recorded envelope following responses ( EFR s); however, the amplitude of EFR s vary by vowel spectra and the causal relationship is not well understood. One cause for spectral dependency could be interactions between responses with different phases, initiated by multiple stimulus frequencies. Phase differences can arise from earlier initiation of processing high frequencies relative to low frequencies in the cochlea. This study investigated the presence of such phase interactions by measuring EFR s to two naturally spoken vowels (/ε/ and /u/), while delaying the envelope phase of the second formant band (F2+) relative to the first formant (F1) band in 45° increments. At 0° F2+ phase delay, EFR s elicited by the vowel /ε/ were lower in amplitude than the EFR s elicited by /u/. Using vector computations, we found that the lower amplitude of /ε/‐ EFR s was caused by linear superposition of F1‐ and F2+‐contributions with larger F1‐F2+ phase differences (166°) compared to /u/ (19°). While the variation in amplitude across F2+ phase delays could be modeled with two dominant EFR sources for both vowels, the degree of variation was dependent on F1 and F2+ EFR characteristics. Together, we demonstrate that (a) broadband sounds like vowels elicit independent responses from different stimulus frequencies that may be out‐of‐phase and affect scalp‐based measurements, and (b) delaying higher frequency formants can maximize EFR amplitudes for some vowels.