For the Harvard-Northwestern study, published in the April issue of The Journal of Neuroscience, the team scanned the brains of 40 young adults, most from Boston-area colleges. Half were nonusers; half reported smoking for one to six years and showed no signs of dependence. Besides the seven light smokers, nine used three to five days a week and four used, on average, daily. All smokers showed abnormalities in the shape, density and volume of the nucleus accumbens, which “is at the core of motivation, the core of pleasure and pain, and every decision that you make,” explained Dr. Hans Breiter, a co-author of the study and professor of psychiatry and behavioral sciences at Northwestern’s medical school.
Similar changes affected the amygdala, which is fundamental in processing emotions, memories and fear responses.
Evidence of long-term effects is also building. A study released in 2012 showed that teenagers who were found to be dependent on pot before age 18 and who continued using it into adulthood lost an average of eight I.Q. points by age 38. And last year at Northwestern, Dr. Breiter and colleagues also saw changes in the nucleus accumbens among adults in their early 20s who had smoked daily for three years but had stopped for at least two years.
They had impaired working memories as well. “Working memory is key for learning,” Dr. Breiter said. “If I were to design a substance that is bad for college students, it would be marijuana.”