Babies' brains track delta rhythms and distinguish mother's voice by six months.

Humans speak at slow rhythms, but scientists do not fully understand how baby brains learn to predict these speech timing patterns. Researchers tested whether six-month-old infants predict rhythms better when they hear familiar voices. They played rhythmic speech syllables to babies at two different speeds, using either the baby’s mother or a stranger, and measured their brainwaves. Neural entrainment happens when brain activity syncs its timing to match an outside rhythm. The baby brains tuned in specifically to the slow delta speech speed, not the faster rate. Over time, the right front part of the brain grew even better at tracking this slow rhythm. Furthermore, voice familiarity changed how the brain responded. The left side of the brain synced specifically to the mother’s voice, while the right front area synced to the stranger. This shows that six-month-old brains actively update predictions about speech instead of just passively hearing sounds.

Why it matters

This shows that by six months, an infant's brain is already building predictive models of speech rhythms, tuned to familiar voices. Understanding this early neural specialisation could help spot when language development is on track or at risk. It also suggests that exposure to maternal speech drives specific brain changes.

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