By RONI CARYN RABIN
Published: November 22, 2010
Marijuana smoking often starts in adolescence — and the timing could not be worse, a new study suggests.
Young adults who started using the drug regularly in their early teens performed significantly worse on tests assessing brain function than did subjects who were at least 16 when they started, scientists reported last week.
The findings led researchers at McLean Hospital to surmise that the developing teenage brain may be particularly vulnerable to the ill effects of marijuana.
“We have to understand that the developing brain is not the same as the adult brain,” said Dr. Staci A. Gruber, the paper’s senior author and director of cognitive and clinical neuroimaging at McLean, a Harvard-affiliated hospital in Belmont, Mass.
The study, done in conjunction with brain scans, was small, consisting of 35 chronic marijuana smokers who were 22 years old on average. The subjects were asked to complete an assessment of executive function — the brain processes responsible for planning and abstract thinking, as well as understanding rules and inhibiting inappropriate actions. The test — in which participants were asked to sort cards with different shapes, numbers and colors — is a measure of cognitive flexibility.
At 15, Dr. Gruber said, the brain is still changing, and “the part that modulates executive function is the last part to develop.”