Delta rhythm and voice familiarity organize neural entrainment in the infant brain

Delta frequency band rhythms (0.5-3.5 Hz) are characteristic of biological communication across species. Neural entrainment to this rhythm is believed to facilitate the generation of temporal predictions within the brain. While adult entrainment is well-understood, the mechanisms during early development, especially concerning stimulus rhythmicity, frequency specificity, and voice familiarity, remain less explored.

To investigate this, researchers utilized high-density EEG to examine neural responses in thirty six-month-olds listening to vocally rhythmic syllables. These were spoken by either the infants' mother or a stranger at delta (2 Hz) or theta (4 Hz) rates, alongside a non-rhythmic control.

The study revealed a broad frontotemporal entrainment response to auditory temporal regularity. Crucially, delta-band entrainment was found to be frequency-specific across frontal and temporal brain regions. Selectivity for the right-frontal cortex increased as the infants became more engaged with the rhythmic stimulus, suggesting this area actively refines a temporal model of the input rather than simply responding passively.

In contrast, no significant theta-band specificity was observed, indicating a preference for slower delta rhythms in this age group.

Additionally, the study highlighted the influence of voice familiarity on neural entrainment. The left temporal cortex showed a selective response to the mother's voice, while the right frontal cluster demonstrated a preference for the stranger's voice. This finding implies a complex interplay between familiarity-based processing within mid-temporal language-related regions and novelty-sensitive mechanisms in frontal areas.

The researchers suggest this reflects the ongoing updating of internal predictive models as the infants interact with their environment, rather than mere passive sensory registration.

Overall, the co-occurrence of frequency-specific delta entrainment and experience-dependent hemispheric asymmetries illustrates the development of a neural functional network for hierarchical temporal predictions. This network is shaped by early auditory experiences and is already established by six months of age, highlighting the rapid and dynamic processes at work in infant brain development.