Infant-directed speech enhances temporal rhythmic structure in the envelope

Infant-directed speech (IDS) supports language learning via mechanisms that are still not well-understood. Here, we adopt a 'temporal sampling' perspective to investigate whether rhythmic enhancements in the temporal structure of IDS could support multi-timescale neuronal oscillatory sampling of the speech signal by the infant brain. We compare natural maternal speech directed to infants at the ages of 7-, 9-, 11and 19-months, to adult-directed speech (ADS). Speech temporal structure is analysed using a novel multi-timescale Spectral-Amplitude Modulation Phase Hierarchy (S-AMPH) model, which extracts the Stress-rate, Syllable-rate and Phoneme-rate modulations (i.e. temporal patterns). Compared to ADS, we find that IDS shows a 'stress-shifted' temporal profile. Stress-rate modulations dominate the modulation spectrum of IDS, whereas Syllable-rate modulations are dominant in ADS. Further, multi-timescale phasesynchronisation measures indicate that in IDS, Syllable-rate modulations are more synchronised to Stress-rate modulations and less synchronised to Phoneme-rate modulations. Thus, when speaking to infants, mothers pattern their syllables more regularly with prosodic stress, while allowing the phonemes within uttered syllables to vary more in timing. Accordingly, we conclude that the temporal structure of (Australian English) IDS is primarily stress-dominant, which could 'tune' the infant brain toward stress-based speech segmentation an adaptive strategy for boot-strapping early language learning.