Image caption Scanning babies' brains in an fMRI scan shed new light on how early they hear emotion in voices

Babies can respond to emotions in a human voice at just three months old - earlier than previously thought - researchers believe.

The results of scans on 21 sleeping babies suggested the brain responded to the type of sounds being played.

Researchers said it had previously been unclear when human brains developed the ability to process voices and emotions.

Scientists hope to learn the difference between the way autistic and non-autisitc brains develop as a result.

They used functional magnetic-resonance imaging (fMRI) to record how babies responded to tapes playing "emotional sounds", such as laughing and crying, as well as background noises, like water or toys.

A part of the brain - the temporal cortex - was activated when human voices were played - the same region as in adults.

'Advance in understanding'

The limbic brain region responded strongly to negative or sad sounds, but did not differentiate between neutral and happy sounds.

Professor Declan Murphy, from King's College London, said: "This discovery fundamentally advances our understanding of infant development."

Dr Evelyne Mercure, of University College London, said: "It is a rare demonstration that specialised areas exist in the brain very early in development."

The researchers hope the study will allow them to compare what happens in autistic and non-autistic brains.

Professor Murphy told the BBC: "By identifying when brain systems come on stream, we can understand where it goes wrong."

The researchers are also scanning the brains of babies thought to be at risk of developing autism, such as those who have a sibling with the condition.

They are investigating when differences can be detected.