Characteristics of Speech-Evoked Envelope Following Responses in Infancy.
Vijayalakshmi Easwar, Susan Scollie, Michael Lasarev and 3 others
PMID 34251887WHAT IT FOUND
Speech-evoked brainstem responses in infants look adult-like only when stimulus levels are boosted by their smaller ear canals.
When levels are corrected to match adults, low-frequency responses remain immature. Do not assume infant responses are mature just because they appear normal at standard settings.
Key findings
01Adults had significantly larger response amplitudes and signal-to-noise ratios than infants for low-frequency vowel stimuli, but this difference disappeared for many stimuli when infants received higher stimulus levels.
02Infants' responses became more adult-like (higher amplitude and signal-to-noise ratio) when stimulus levels were increased, particularly for mid-to-high frequency stimuli.
03Most response characteristics did not change significantly with age during the first year of life, except for specific mid-frequency stimuli.
STILL TO COME
How it was doneWhat they foundWhat it means for SLPs
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What it does not show
Infants were not randomized to level groups; the level-matched group was tested first. The study only included typically developing infants with normal hearing, so it does not tell us how these level effects apply to infants with hearing loss. The 'higher-level' group did not have their in-ear levels controlled, so the exact level difference varied by individual ear canal size.
Declared interests
The authors declared no potential conflicts of interest. The study was funded by Canadian Institutes of Health Research, Natural Sciences and Engineering Research Council of Canada, and NIH National Center for Advancing Translational Sciences.
The easy way to misread this
Do not assume that infant EFRs are fully mature just because they look like adult responses at standard test levels. The apparent maturity may be due to the level boost provided by the infant's smaller ear canal, which can mask underlying neural immaturity, particularly for low-frequency speech sounds.