YOUNG, casual marijuana smokers experience potentially harmful changes to their brains, with the drug altering regions of the mind related to motivation and emotion, researchers found.

The study,published yesterday in the Journal of Neuroscience, differs from many other research projects that are focused on chronic, heavy users of cannabis.

The collaborative effort of Northwestern University's medical school, Massachusetts General Hospital and Harvard Medical School showed a direct correlation between the number of times users smoked and abnormalities in the brain.

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"What we're seeing is changes in people 18 to 25 in core brain regions that you never, ever want to fool around with," said Dr. Hans Beiter, professor of psychiatry and behavioral sciences at Northwestern University.

The study identified changes to the nucleus accumbens and the nucleus amygdala, regions of the brain that are key to ­regulating emotion and motivation, in users who smoke between one and seven joints a week.