A baby's smile is just the tonic to give mothers a natural high

An intriguing new study has shown that infant smiles light up the 'feel-good' parts of the brain that also respond to drugs and drink.

The findings shed light on the unique mother-baby bond and help explain why the sight of a smiling infant can be such a tonic for sleep-deprived and stressed parents.

A baby's smile can stimulate the 'feel-good' parts of the brain that also respond to drugs and drink

Dr Lane Strathearn, who carried out the study at Texas Children's Hospital, said baby smiles triggered parts of the brain that deal with sensations of reward and pleasure.

'These are the areas that have been activated in other experiments associated with the neurotransmitter dopamine,' he said.

'It may be that seeing your own baby's smiling face is like a natural high.'

The scientists put 28 first-time mothers whose babies were aged between five and ten months into a magnetic resonance imaging (MRI) brain scanner.

Then they asked the women to look at pictures of their own children and other babies.

In some of the pictures the babies were smiling or happy.

In others they were sad or had a neutral expression.

As the mothers gazed at the faces, the machine showed the flow of blood in their brains and revealed the regions that were the most active at any time.

When mothers saw their own children, areas of the brain associated with the reward brain chemical dopamine lit up.

The strength of the reaction depended on the expression of the babies, Dr Strathearn said. Smiling faces triggered the biggest reaction while neutral or sad-looking babies provoked the least response.

Crying babies made little difference to the reward centres of the brain. In fact, mothers reacted to their own crying babies in exactly the same way as a stranger's crying child.

The researchers, who report the findings in the American medical journal Pediatrics, say the findings could explain the strong bond between mothers and babies  -  and possibly why that bond is sometimes missing.

'Understanding how a mother responds uniquely to her own infant, when smiling or crying, may be the first step in understanding the neural basis of mother-infant attachment,' said Dr Strathearn.

'The relationship between mothers and infants is crucial for child development. For whatever reason, in some cases, that relationship doesn't develop normally.

'Neglect and abuse can result, with devastating effects on a child's development.'

Dopamine is a key chemical messenger in the body and is involved in sensations of 'reward'.

It is also important for learning, motivating, sleeping and controlling movement.

'It is linked to drugs, gambling and alcohol addictions.

People with Parkinson's disease suffer low levels of dopamine in vital regions of their brains.