By Michael Kahn

LONDON, Feb 26 (Reuters) - Scientists have identified the part of the brain that may hold the key to why some cocaine users become addicts while others just take the drug socially, researchers said on Tuesday.

Brain scans of cocaine users while they performed simple computer tasks showed changes in the part of the brain responsible for controlling behaviour and making appropriate decisions, they said.

This could explain why some people find it easier to quit than others and may shed light on long-term addiction, said Hugh Garavan, a cognitive neuroscientist at Trinity College Dublin who presented his research to a meeting of the Royal Society in London.

"Most people who try to quit drugs relapse," Garavan said in a telephone interview. "It might have to do with how intact these brain regions are."

Cocaine, initially used in patent medicines, beverages and tonics around the turn of the 20th Century, is a drug that in powdered form can be snorted or dissolved in water and injected. Its derivative crack cocaine is even more powerful.

An estimated 1 to 3 percent of adults in developed countries use the drug, which has been linked to a number of medical, psychological and social problems including crime, violence and the spread of diseases like AIDS and hepatitis, according to the World Health Organisation.

Garavan and colleagues used MRI scans to show that cocaine users had reduced neural activity marked by reduced blood flow to the part of the brain involved in things like problem solving, decision making and controlling behaviour. Some people were administered cocaine in the experiments.

"This research helps us move away from thinking of drug dependence as a moral weakness and allows us to see it as more of a medical condition."

It was unclear whether the changes were due to the drug itself or whether some kind of natural mechanism in the brain triggers the change, Garavan said.

But better understanding the brain's response to cocaine could eventually help predict people most at risk of developing an addiction and lead to better treatments, he added.

"One would hope this research would guide the development of new treatments including the development of pharmacological solutions to addiction," Garavan said. (Editing by Will Dunham and Matthew Jones)

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