The Neural Processing of Speech Temporal Modulations During Early Phonological Development

Purpose: Infants gradually attune to the consonants of their native language throughout the first year of life. This study investigated the role of auditory temporal processing in this early perceptual attunement. Method: We used electrophysiological measures to investigate the processing of temporal modulations, the amplitude modulation (AM) and frequency modulation (FM) in speech. Cortical auditory evoked potentials (CAEPs) were measured for native (voiced /b/ and unvoiced unaspirated /p/) and nonnative (unvoiced aspirated English /p/) consonants in French-learning 6-month-old and 10-month-old infants. We also included a group of young adult listeners to serve as a reference group for qualitative comparisons with infants, as there were no prior studies on the effects of temporal degradation on adults' CAEPs. The three consonants were presented under three conditions where AM and FM were differentially degraded or preserved: (a) original FM and AM preserved (no stimulus degradation), (b) fast AM (degrading FM and preserving the original AM), and (c) slow AM only (degrading FM and faster AM fluctuations). Results: Temporal degradation affected CAEPs differently across infant age groups, suggesting ongoing development of auditory processing mechanisms for FM and AM cues. Six-month-olds exhibited heightened sensitivity to the reduction of fast AM cues, whereas 10-month-olds were more sensitive to FM degradation. Conclusions: Age significantly shapes cortical auditory responses to speech temporal cues. Six-month-olds' neural responses were primarily influenced by fast AM reduction (> 8 Hz), whereas 10-month-olds' responses were more susceptible to FM degradation. Adults did not show either pattern, indicating that the perceptual weighting of temporal modulations in speech continues to develop beyond infancy. Supplemental Material: https://doi.org/10.23641/asha.32030250