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

Abstract

The delta frequency band (∼0.5−3.5 Hz) represents a phylogenetically conserved tempo of biological communication, and neural entrainment to this timescale would be a key mechanism through which the brain might generate temporal predictions. While entrainment at different levels has been characterized in adults, the mechanisms of auditory temporal prediction remain poorly studied across developmental trajectories, particularly regarding the roles of stimulus rhythmicity, frequency specificity, and voice familiarity. Using high-density EEG, we examined neural entrainment in thirty 6-month-old infants exposed to vocally rhythmic syllables produced by their mother or a stranger at delta (2 Hz) or theta (4 Hz) rates, as well as a non-rhythmic control. Auditory temporal regularity elicited a broad frontotemporal entrainment response. Critically, delta-band entrainment was frequency-specific across frontal and temporal clusters, with right-frontal selectivity progressively strengthening throughout the entrainment period as expected if this region actively refines a temporal model of the input rather than passively resonating to it; no significant theta-band specificity was observed. Entrainment was further shaped by voice familiarity: the left temporal cortex entrained selectively to the mother’s voice, whereas the right frontal cluster responded selectively to the stranger’s voice. This is consistent with an implementation in which familiarity-based processing within mid-temporal language-related regions interfaces with novelty-sensitive frontal mechanisms, and reflects the ongoing updating of internal predictive models rather than passive sensory registration. The co-occurrence of frequency-specific delta entrainment and experience-dependent hemispheric asymmetries reveals a neural functional network for hierarchical temporal predictions, shaped by early auditory experience and already in place at 6 months. Significance Statement Across many species, including humans, communication unfolds at a slow rhythm of 0.5–3.5 cycles per second: the delta rhythm around 2.7 Hz. The developing brain already detects rhythmic regularities, but whether and how early experience shapes this remains unknown. Recording brain activity in 6-month-old infants listening to systematic and experimentally manipulated rhythmic syllables from their mother or a stranger, we show that neural tracking is selectively tuned to the delta rhythm, that this tuning sharpens over the session in the right frontal cortex, and that the maternal voice specifically engages the left temporal cortex, relevant for speech. Already at 6 months, the brain continuously updates its internal models based on experience to predict the temporal structure of the world.