Social networking websites are causing alarming changes in the brains of young users, a famous scientist has warned.Sites such as Facebook, Twitter and Bebo are said to shorten attention spans  , encourage instant satisfaction and make children more self-centered. Warnings from neuroscientist   Susan Greenfield will disturb the millions whose social lives depend on surfing their favorite websites each day.But they will strike a chord   with parents and teachers who complain that many teenagers lack the ability to communicate or concentrate away from their screens. More than 150 million 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 exchange 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 Tarot, an Oxford University neuroscientist believes repeated exposure could rewire the brain.Computer games and fast-paced TV shows were also a factor, she said."My fear is that these technologies are weakening the brain to the state of small children who are attracted by buzzing noises and bright lights, who have a small attention span and live for the moment." "I often wonder whether real conversation in real time may eventually give way to these easier and faster screen dialogues, in much the same way as killing and butchering an animal to eat has been replaced by the convenience of packages on the supermarket shelf," she said. Psychologists have also argued that digital technology is changing the way we think.They point out that students no longer need to plan their term papers before starting to write--thanks to word processors they can edit as they go along. A study by the Broadcaster Audience 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 excite the "flight or fight" areas of the brain, rather than the areas responsible 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 thousands of years.I'm not against technology and computers.But before they start social networking, they need to learn to make real relationships with people." According to the passage, social networking websites might    _   .
make young users more selfish lengthen young users' attention span encourage young users constant satisfaction help young users communicate better with their families