The findings, published in the science magazine Nature, point to subtle abnormalities in the part of the brain associated with detecting visual motion as being one source of the disorder.

Gavin Reid, co-ordinator of the Centre for Specific Learning Difficulties at Moray House Institute in Edinburgh, said: “For some time now the evidence has indicated that dyslexia was phonological and involved an inability to distinguish sounds. But this suggests that there is some kind of visual dyslexia as well. Logically, if this research continues, then you should be able to detect dyslexia before reading begins.”

If there are different sub-types of dyslexia, treatment could be tailored to individuals in a more specific way, Mr Reid added.

The research team that produced the report is based at the National Institute for Health in Bethesda outside Washington and is led by Guinevere Eden, a British scientist. Dr Eden says: “What we have shown is that the problems associated with dyslexia exist in a number of functions. There is a deficit that is affecting all areas of the brain. This affects about two-thirds of dyslexics.”

Failure to detect visual motion may be part of a wider problem in the brain, Dr Eden believes.

Previous research has concentrated on problems of cognition but not on the fundamental neurological reasons behind dyslexia. By using magnetic resonance imaging to detect blood flow, Dr Eden studied the activity of the part of the brain associated with visual motion and compared the response to a series of moving dots in dyslexics with that of a control group.

Blood flow was detected in the brains of the control group but not in those of dyslexics. “By using scans we can keep looking at parts of the brain for various responses,” Dr Eden states. “The process is entirely safe allowing us to make early diagnosis.”

There appears to be a link between inability to process visual and auditory information in dyslexics. Imbalance in hearing can lead to incorrect repetition of a word and hence a failure to recognise it when reading. In the past critics have asserted that dyslexia is diagnosed when middle-class parents become frustrated at children’s poor educational performance. “Excuses” are then provided by sympathetic psychologists, it is claimed.

Tony Booth, senior lecturer in education at the Open University, remains to be persuaded by the research. “It does not end the search for an explanation why children are underperforming. It does not make the problem go away. There are other aspects of your brain that are different from other people,” Mr Booth said.

Dr Eden maintains that the brain patterns of dyslexics reveal clear differences and her results conclusively refute any notion that dyslexia does not exist.