Drinking alcohol dulls the brain's ability to detect threats, U.S. researchers said on Tuesday in a study that helps explain why people who are drunk cannot tell when the guy at the end of the bar is angling for a fight.

They said the study is the first to show how alcohol affects the human brain as it responds to threats.

"You see this all of the time. People get into confrontations when they are intoxicated that they probably wouldn't get into when they are sober," said Jodi Gilman of the National Institutes on Alcohol Abuse and Alcoholism, whose study appears in the Journal of Neuroscience.

Gilman studied 12 people who were given intravenous infusions of alcohol and then monitored their brain activity using functional magnetic resonance imaging while they looked at pictures of frightened and neutral faces.

Her team did the same study on these people when they were given a simple saline infusion as a placebo. As expected, when people were given the placebo, their brains responded to the fearful faces.

"Our brains respond more to fearful stimuli," Gilman said in a telephone interview. "They signal to us that we are in threatening situations."

When these same people were given infusions of alcohol, however, this response was dulled, suggesting that while intoxicated, "our brain can't distinguish between the threatening and non-threatening stimuli," said Gilman.

She said this impaired appreciation for threats could lead to a host of risky situations, including drunk driving. And it also explains why alcohol is sometimes called a social lubricant.

"People have used alcohol for years to become euphoric and to decrease anxiety. Alcohol has been used in particular to increase sociability. How alcohol acts on the brain to produce these effects has not been well understood or studied," Gilman said.

Her study found that alcohol increases activity in a reward center of the brain known as the striatum. And they found a link between the level of activation in this region and how intoxicated people said they were feeling, which could help account for the addictive properties of alcohol.

"This is important because we think we can develop potential treatments for alcoholism," Gilman said.

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