London, Nov 11 (IANS): Researchers at Cambridge University in Britain have claimed that contrary to the previously accepted view, dopamine dysfunction is not the main cause of attention deficit hyperactivity disorder.

A new study at the British university questions whether attention deficit hyperactivity disorder (ADHD) is the result of fundamental abnormalities in dopamine transmission, and claims that the main cause of the disorder may lie, instead, in structural differences in the grey matter in the brain.

The study, published in Brain magazine, could significantly improve our understanding of ADHD.

The study found that administering methylphenidate (more commonly known as Ritalin) to healthy adult volunteers as well as those who exhibit symptoms of ADHD as adults, led to similar increases of the chemical dopamine in their brain.

Both groups also had equivalent level of improvements as a result of the drug when tested on their ability to concentrate and pay attention.

In both groups, volunteers were given either a dose of Ritalin or a placebo pill.

Researchers then analysed the results of tasks done by the volunteers which tested their ability to concentrate and pay attention over a period of time.

Patients suffering from ADHD, who had significant loss of grey matter in the brain, as measured by magnetic resonance imaging, were found to be less attentive as compared to healthy individuals.

"We feel these results are extremely important since they show that people who have poor concentration improve with methylphenidate (Ritalin) treatment whether they have a diagnosis of adult ADHD or not," lead author of the study professor Barbara Sahakian said.

"These new findings demonstrate that poor performers, including healthy volunteers, were helped by the treatment and this improvement was related to increases in dopamine in the brain," she said.

"These findings question the previously accepted view that major abnormalities in dopamine function are the main cause of ADHD in adult patients. While the results show that Ritalin has a 'therapeutic' effect to improve performance, it does not appear to be related to fundamental underlying impairments in the dopamine system in ADHD," co-author of the study, professor Trevor Robbins said.