Dyslexia is one of the most common learning disorders, affecting nearly one in 20 people.

People with dyslexia often have great difficulty learning to read, write and spell.

In the past, the established view has been that dyslexia is the result of a problem in the highly developed language centres of the brain found in the cerebral cortex.

But the new research, by psychologists from Sheffield University, indicates that the problem is linked to abnormalities in one of the most primitive areas of the brain, the cerebellum.

Situated at the base of the brain, where the spinal cord is attached, the cerebellum controls motor functions and is common to humans and lower animals.

Brain scanning

The researchers, led by Professor Roderick Nicolson, compared the brain activity of six normal and six dyslexic adults using a scanning technique called positron emission tomography (PET).

Volunteers were monitored as they performed a pre-learned sequence of right hand finger movements, and as they learned a new sequence of finger movements, also with their right hand.

During the tasks, activation of the cerebellum in the dyslexic individuals was only about 10% of that in the other volunteers.

When learning the new sequence, the dyslexics' brains were significantly less active in the area of the right cerebellum. Both the right cerebellar cortex and part of the cerebral cortex were less active when they performed the pre-learned sequences.

Writing in The Lancet medical journal, the researchers said: "Our findings support the hypothesis that a substantial proportion of dyslexic children have a cerebellar deficit that adversely affects the learning of new skills and the performance of automatic, over-learned skills."

Evidence has emerged recently that as well as having language problems, dyslexic children often face difficulties with rapid information processing, co-ordination, balance and fluency.

The researchers said other parts of the brain may also be involved, but the findings indicated "strongly" that the cerebellum was a key structure affected in dyslexia.