Social networking websites are causing alarming changes in the brains of young users, an eminent scientist has warned.

Sites such as Facebook, Twitter and Bebo are said to shorten attention spans, encourage instant gratification and make young people more self-centred.

The claims from neuroscientist Susan Greenfield will make disturbing reading for the millions whose social lives depend on logging on to their favourite websites each day.

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But they will strike a chord with parents and teachers who complain that many youngsters lack the ability to communicate or concentrate away from their screens.

More than 150million use Facebook to keep in touch with friends, share photographs and videos and post regular updates of their movements and thoughts.

A further six million have signed up to Twitter, the 'micro-blogging' service that lets users circulate text messages about themselves.

But while the sites are popular - and extremely profitable - a growing number of psychologists and neuroscientists believe they may be doing more harm than good.

Baroness Greenfield, an Oxford University neuroscientist and director of the Royal Institution, believes repeated exposure could effectively 'rewire' the brain.

Experts are concerned children's online social interactions can 'rewire' the brain

Computer games and fast-paced TV shows were also a factor, she said.

'We know how small babies need constant reassurance that they exist,' she told the Mail yesterday.

'My fear is that these technologies are infantilising the brain into the state of small children who are attracted by buzzing noises and bright lights, who have a small attention span and who live for the moment.'

Her comments echoed those she made during a House of Lords debate earlier this month. Then she argued that exposure to computer games, instant messaging, chat rooms and social networking sites could leave a generation with poor attention spans.

'I often wonder whether real conversation in real time may eventually give way to these sanitised and easier screen dialogues, in much the same way as killing, skinning and butchering an animal to eat has been replaced by the convenience of packages of meat on the supermarket shelf,' she said.

Lady Greenfield told the Lords a teacher of 30 years had told her she had noticed a sharp decline in the ability of her pupils to understand others.

'It is hard to see how living this way on a daily basis will not result in brains, or rather minds, different from those of previous generations,' she said.

She pointed out that autistic people, who usually find it hard to communicate, were particularly comfortable using computers.

'Of course, we do not know whether the current increase in autism is due more to increased awareness and diagnosis of autism, or whether it can - if there is a true increase - be in any way linked to an increased prevalence among people of spending time in screen relationships. Surely it is a point worth considering,' she added.

Psychologists have also argued that digital technology is changing the way we think. They point out that students no longer need to plan essays before starting to write - thanks to word processors they can edit as they go along. Satellite navigation systems have negated the need to decipher maps.

A study by the Broadcaster Audience Research Board found teenagers now spend seven-and-a-half hours a day in front of a screen.

Educational psychologist Jane Healy believes children should be kept away from computer games until they are seven. Most games only trigger the 'flight or fight' region of the brain, rather than the vital areas responsible for reasoning.

Sue Palmer, author of Toxic Childhood, said: 'We are seeing children's brain development damaged because they don't engage in the activity they have engaged in for millennia.

'I'm not against technology and computers. But before they start social networking, they need to learn to make real relationships with people.'

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