Characteristics of Speech-Evoked Envelope Following Responses in Infancy

Abstract

Envelope following responses (EFRs) may be a useful tool for evaluating the audibility of speech sounds in infants. The present study aimed to evaluate the characteristics of speech-evoked EFRs in infants with normal hearing, relative to adults, and identify age-dependent changes in EFR characteristics during infancy. In 42 infants and 21 young adults, EFRs were elicited by the first (F1) and the second and higher formants (F2+) of the vowels /u/, /a/, and /i/, dominant in low and mid frequencies, respectively, and by amplitude-modulated fricatives /s/ and /∫/, dominant in high frequencies. In a subset of 20 infants, the in-ear stimulus level was adjusted to match that of an average adult ear (65 dB sound pressure level [SPL]). We found that (a) adult–infant differences in EFR amplitude, signal-to-noise ratio, and intertrial phase coherence were larger and spread across the frequency range when in-ear stimulus level was adjusted in infants, (b) adult–infant differences in EFR characteristics were the largest for low-frequency stimuli, (c) infants demonstrated adult-like phase coherence when they received a higher (i.e., unadjusted) stimulus level, and (d) EFR phase coherence and signal-to-noise ratio changed with age in the first year of life for a few F2+ vowel stimuli in a level-specific manner. Together, our findings reveal that development-related changes in EFRs during infancy likely vary by stimulus frequency, with low-frequency stimuli demonstrating the largest adult–infant differences. Consistent with previous research, our findings emphasize the significant role of stimulus level calibration methods while investigating developmental trends in EFRs.

Performance of Statistical Indicators in the Objective Detection of Speech-Evoked Envelope Following Responses

“…The current work builds on the findings of Easwar et al (2020a,b; 2021) in two important ways. First, sensitivity analyses are extended to an infant population and two additional statistical indicators are assessed.…”

“…Various characteristics of speech-evoked EFRs, as well as response detection, differ significantly in infant and adult populations (Easwar et al 2021; Savio et al 2001; Van Dyke et al 2017). Both amplitude, and to a lesser extent phase coherence, may be lower in an infant population (Easwar et al 2021). It is possible that decreased detectability is due to spectral magnitude differences rather than decreased phase locking in the infant population (Easwar et al 2021; Lins et al 1996; Van Dyke et al 2017).…”

“…The researchers found that after approximately 30 minutes of testing, the statistical indicators that incorporated phase-information explicitly had comparable performance as those that did not. In a consequent study, Easwar et al (2021) determined that the differences in EFR characteristics in adults and infants are the largest for low-frequency stimuli and that developmental-related changes in EFRs vary by carrier frequency. Infants were found to have adult-like phase coherence when ear canal level was uncorrected but not when ear-canal level differences were accounted for.…”

“…Both amplitude, and to a lesser extent phase coherence, may be lower in an infant population (Easwar et al 2021). It is possible that decreased detectability is due to spectral magnitude differences rather than decreased phase locking in the infant population (Easwar et al 2021; Lins et al 1996; Van Dyke et al 2017). If this is the case, it is expected that statistical indicators that heavily rely on phase information will perform more accurately for speech-evoked EFR detection in infants than amplitude-based indicators.…”

“…The age of the individual can also significantly impact speech-evoked EFR detection. Various characteristics of speech-evoked EFRs, as well as response detection, differ significantly in infant and adult populations (Easwar et al 2021; Savio et al 2001; Van Dyke et al 2017). Both amplitude, and to a lesser extent phase coherence, may be lower in an infant population (Easwar et al 2021).…”