New research shows that people who start using marijuana at a young age and those who use the greatest amount of marijuana may be the most cognitively impaired.

 The research was presented at Neuroscience 2010, the annual meeting of the Society for Neuroscience, held in San Diego.

Marijuana users show deficits in the ability to switch behavioral responses according to the context of a situation, also known as cognitive flexibility. The new study, directed by Staci Gruber, PhD, at McLean Hospital and Harvard Medical School, compared people’s performance on the Wisconsin Card Sorting Task, a test of cognitive flexibility. During the task, people are shown four shown cards that differ in color, symbol, and value. Based on the rules they glean from these displayed cards, they must then sort a deck of cards. The participants are not told what the rules are — only whether their sorting attempt is correct or incorrect. During the test, the researchers change the rules without warning, and participants must adjust accordingly. How a participant responds is a strong indicator of cognitive flexibility.

The researchers found that habitual marijuana users made repeated errors despite feedback that they were wrong. Marijuana users also had more difficulty maintaining a set of rules, suggesting an inability to maintain focus. Those participants who began using marijuana before the age of 16 and those who used the most marijuana showed the greatest impairment.

“Our results provide further evidence that marijuana use has a direct effect on executive function, and that both age of onset and magnitude of marijuana use can significantly influence cognitive processing,” said Gruber. “Given the prevalence of marijuana use in the United States, these findings underscore the importance of establishing effective strategies to decrease marijuana use, especially in younger populations,” she said.

Research was supported by the National Institute on Drug Abuse.

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