Investigating potential interactions between envelope following responses elicited simultaneously by different vowel formants

Characteristics of Speech-Evoked Envelope Following Responses in Infancy

“…The first stimulus factor that differentiates vowels from fricative stimuli in the present study is the number of EFRs simultaneously elicited—all vowel stimuli were designed to elicit two EFRs, whereas the fricative stimuli were designed to only elicit one EFR at a time. The simultaneous elicitation of two EFRs by vowel stimuli does not significantly influence EFR amplitudes in adults ( Easwar et al., 2019 ); however, the impact has not been measured in infants for vowel stimuli. In a tone-evoked EFR study ( Hatton & Stapells, 2011 ), 6- to 38-week-old infants demonstrated a significant reduction in EFR amplitude when four tones were presented simultaneously compared to when they were presented individually.…”

“…Vowels were modified to carry two f 0 : the natural f 0 in the region of the second formant (F2+) and a lowered f 0 in the region of the lower frequency first formant (F1; Easwar et al., 2015b ). The rationale for such a modification was to improve frequency and place specificity of responses in comparison to those evoked with vowel stimuli with a single f 0 ( Easwar et al., 2019 ). The average f 0 in F1 was lower than the original f 0 in F2+ by ∼8.5 Hz.…”

“…Envelope following responses (EFRs) may be a useful objective tool to evaluate the audibility of speech sounds in children who are unable to participate in behavioral hearing tests ( Easwar et al., 2015b , 2015c , 2020a ). Scalp-recorded EFRs reflect neural activity phase-locked to the stimulus envelope and have been elicited by a variety of speech stimuli, including naturally spoken vowels ( Aiken & Picton, 2006 ; Choi et al., 2013 ), synthesized vowels ( Anderson et al., 2015 ; Skoe et al., 2015 ), high- or low-pass filtered vowels ( Easwar et al., 2015b ; Vanheusden et al., 2019 ), individual vowel formants ( Easwar et al., 2015a , 2019 ; Laroche et al., 2013 ), and modified fricatives ( Easwar et al., 2015b , 2020b ). The rationale for using certain speech stimuli, such as individual vowel formants and modified fricatives, has been to assess audibility, using EFRs, at a wide range of frequencies with reasonable specificity ( Easwar et al., 2015c ).…”

The influence of phoneme contexts on adaptation in vowel‐evoked envelope following responses

“…The vowel /i/ was modified to elicit two EFRs simultaneously, one from the low‐frequency first formant (F1) and one from the second and higher formants (F2+). As done in previous studies for improved frequency specificity of vowel stimuli (Easwar, Purcell, et al, 2015a; Easwar et al, 2019), differentiation of the two formant bands was maintained to examine the presence and nature of adaptation in EFRs elicited by formants dominant in different spectral regions (Figure 1). The vowel was modified using the following steps to enable eliciting two EFRs simultaneously: (i) The average f 0 of the vowel was reduced by 8.57 Hz in Praat, (ii) the F1 was obtained by low‐pass filtering the lowered‐ f 0 vowel at 1140 Hz, (iii) the F2+ was obtained by high‐pass filtering the original f 0 vowel at 1250 Hz and (iv) the F1 and F2+ were combined without changes in their relative levels.…”

“…The time course remains uncertain since prior studies either reported single‐trial data that are susceptible to changes in noise or they averaged over consecutive stimuli leading to a loss of resolution in terms of stimulus repetition order/number. To seek clarity in both these aspects, in the present study, we (i) modified vowel stimuli to elicit independent EFRs from first (F1) and second and higher‐frequency formants (F2+; Easwar, Purcell, et al, 2015a; Easwar et al, 2019) and (ii) used a vertical averaging approach that would maintain the vowel repetition order during averaging and reduce the impact of noise while evaluating the adaptation time course (e.g. Prado‐Gutierrez et al, 2015).…”