Scientists say they have pinpointed differences in the way the brains of people with obsessive-compulsive disorder (OCD) work. OCD provokes recurrent irrational thoughts such as fears about contamination or accidents, and compulsion to follow fixed rituals.

Scans revealed less activity in a particular region of the brains of both OCD patients and their close relatives. The Cambridge University study appears in the journal Science.

The researchers said the discovery could potentially improve diagnosis of the condition.

It is estimated that between 2% and 3% of the population will suffer some kind of OCD at some point in their lives.

Although it tends to run in families, which suggest that genes may be involved, so far they have not been found, and at present diagnosis is based on an interview with a psychiatrist.

The Cambridge researchers used a type of scan called fMRI, which can display brain activity in real time, and the different parts of the brain in which it happens.

Volunteers, including some diagnosed with OCD, their immediate relatives, and some with no family history of the illness, were scanned.

To try to provoke an "OCD" response, they were asked to choose between pictures on two screens side by side, one of which had been randomly selected as the "target".

The pictures would alternate between left and right, and the volunteer was given occasional feedback as to whether the right target was being chosen.

The test was designed to stimulate "behavioural flexibility", one of the known problems in people with OCD.

In volunteers without OCD, or a family history of the condition, the test caused activity in a part of the brain called the orbitofrontal cortex.

However, in both the people with diagnosed OCD, and their relatives, the amount of activity was much lower.

Structural differences

While other studies have shown differences in the brain structure of people with OCD compared to people without the condition, this is thought to be the first time that differences in how they actually work have been recorded.

Dr Samuel Chamberlain, who led the study, said: "Impaired function in brain areas controlling flexible behaviour probably predisposes people to developing the compulsive rigid symptoms that are characteristic of OCD.

"This study shows that these brain changes run in families, and represent a candidate vulnerability factor."

He said that current method of diagnosing the condition could be supplemented by the scan.

Dr Rob Willson, a psychologist specialising in OCD, said that he was excited by the potential of fMRI, even though its cost made it impractical for diagnosing cases.

He said: "What it could do is help test the effectiveness of interventions for OCD, and the more we learn about which parts of the brain are being activated, the more chance we have of understanding the condition better."