For the first time the use of any quantity of marijuana has been correlated with observable physical changes in brain structure. The research was conducted by Dr. Jodi Gilman, Dr. Anne Blood, and Dr. Hans Breiter, of Northwestern University, Massachusetts General Hospital, and Harvard Medical School. The study published April 16, 2014, in The Journal of Neuroscience.
The researchers used magnetic resonance imaging to compare the brains of 20 marijuana users and 20 people who did not use marijuana. The participants were between 18 and 25 years of age. The marijuana users showed an increase in the size of the nucleus accumbens and an alteration in the shape of this part of the brain. The size, shape, and density of the amygdala were also different in marijuana users.
The nucleus accumbens has been linked to pleasure, reward processing, fear, reinforcement learning, addiction, and laughter. The amygdala has been linked to emotional reactions, decision making, anxiety, coping with stress, and memory. None of the participants involved were considered to be addicted to marijuana.
A higher frequency of use of marijuana was related to a higher level of alteration of the size, shape, and density of the amygdala and nucleus accumbens in marijuana users. Previous research has demonstrated physical changes in brain structures associated with what is considered heavy marijuana use. This is the first research that shows physical changes in brain structures with small quantities of marijuana.
One should note that no correlation between frequency of use of marijuana and the amount of marijuana consumed was made with changes in motivation, altered attention spans, learning, and memory impairments in those participants who used marijuana. The researchers do not present any evidence that discontinuing the use of marijuana produces a return to normal shape, size, and density in the amygdala and nucleus accumbens. The long term effects of infrequent marijuana use in young people are still an incomplete story after almost 100 years of research.