By Katherine Hunt

WASHINGTON (Reuters) - Trying to choose between a cookie or a carrot at snack time?

A newly identified "hot spot" on the brain may be the key to impulsive behavior, involved in the critical choice between instant gratification and delayed reward, researchers said on Thursday.

In a study reported in the journal Science, researchers examined the nucleus accumbens, located at the base of the forebrain. The region responds to natural rewards such as food and sex, as well as drugs such as amphetamines and cocaine, through their effects on the neurotransmitter chemical dopamine.

"We have shown that damage or dysfunction of the nucleus accumbens can cause, without a doubt, impulsive choice," said study author Rudolf Cardinal, of the Department of Experimental Psychology in Britain's University of Cambridge.

The findings have implications for the treatment and understanding of drug addiction, attention deficit/hyperactivity disorder (ADHD) and anti-social behavior, all of which are linked to the impulse for instant gratification.

The researchers trained rats to choose between a small immediate reward and a larger, delayed reward. In tests after their nucleus accumbens were damaged, the researchers found the rats tended to choose the instant gratification of the small reward, rather than the larger reward delivered after a delay.

"We already knew that there was a correlation between abnormalities in the nucleus accumbens and impulsive behavior," Cardinal said. "Now we have clear evidence that such abnormalities can cause this behavior."

Lesions on two other regions of the brain that send information to the nucleus accumbens, the anterior cingulate cortex and medial prefrontal cortex, did not cause the rats to act impulsively, the study found.

The findings "suggest the nucleus accumbens core, at least for impulsive choice, is a critical region," Cardinal said.

The research also sheds new light on drugs like Ritalin, used to treat ADHD by affecting the dopamine systems in the brain.

"Our research suggests that (Ritalin's) actions in the nucleus accumbens may responsible for its beneficial affects on impulsive behavior," he said.

A SMOKER'S CHOICE

Like most discoveries, the study has triggered more questions.

"I think it's an interesting question to ask, why were the animals with nucleus accumbens lesions impulsive?" Cardinal said. "Were they impulsive because they were less sure that in fact, delayed reward was coming?"

The cigarette smoker's choice is a good analogy, he said, and further research is needed to determine what is going on, he said.

The choice is cigarettes now or avoiding lung cancer later, he said. So does the smoker really believe that cigarettes cause lung cancer? Or maybe the smoker believes cigarettes cause cancer but cares less about long-term health than the instant buzz.

"That's one question about the underlying psychology," Cardinal said. "Do you make your impulsive choices in the full knowledge of what you are getting yourself into? Or do you lack insight into the consequences?"