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It is precious 'me time' that many new mothers use to catch up on telephone calls and conversations.

But the next time your baby has a nap, watch what you say.

For at just three months old, babies can tell human voices from other sounds - and work out when someone is sad. Curiously, happy sounds just wash over them.

A British study shows that the brain areas devoted to processing speech develop much more quickly than was thought and could lead to new insights into conditions such as autism.

The research team from the Institute of Psychiatry at King’s College London started by getting 21 babies acclimatised to brain scanners.

Techniques used included giving their parents a CD to take home with the noise made by the machines.

The youngsters, who were aged from three to seven months, then had their brains scanned as various familiar noises were played while they slept.

The first experiment looked at whether the tots could tell the difference between human and non-human noises.

This revealed that coughing, sneezing, yawning, lapping water reminiscent of bath time and the squeaking of toys all activated a part of the brain known to process speech. But human sounds lit it up far more.

The researchers then checked whether the babies' brains reacted differently to happy, sad and neutral noises.

This time, a brain area linked to emotion sprung to life, with crying triggering it more than laughter or neutral sounds, the journal Current Biology reports.

Researcher Evelyne Mercure said it was rare to see such specialised brain regions so early in life.

Co-researcher Anna Blasi added: 'It is probably because the human voice is such an important social cue that the brain shows an early specialisation for its processing.

'This may represent the very first step in social interactions and language learning.'

(Read by Nelly Min. Nelly Min is a journalist at the China Daily Website.)

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(Agencies)