Timmen L. Cermak MD December 2, 2022
KEY POINTS
- Motivation is a subjective experience and therefore extremely difficult to measure objectively.
- Motivational syndrome has long been seen as a sign of cannabis addiction, but has only recently been measured.
- Liking and wanting are two different forces. Drug use changes the brain in ways that stimulate wanting the drug.
Until recently, I ignored the idea of a cannabis-induced amotivational syndrome. Of course, I was familiar with the stereotypical view of potheads couch-locked into immobility, but this could have a variety of causes other than amotivational syndrome. Besides, I could not imagine how motivation could be measured objectively.
Then Meghan Martz[1], at the University of Michigan, published research that changed my mind. Martz used a delayed monetary reward protocol, which means people were given a simple computer task that promised cash rewards at the end of the test—a low monetary reward for poor performance and a higher reward for better performance. While watching the computer screen and pressing a button whenever a stimulus appeared, and before any money was received, Martz used functional magnetic imaging (fMRI) to measure activity in a small part of the brain called the nucleus accumbens, the reward center. She tested individuals three times, at ages 20, 22, and 24. She also recorded their report of marijuana use at each age.
Her data showed that, while everyone at age 20 had the same level of reward center activation in anticipation of the cash reward, those who most increased their cannabis use over the next four years showed progressively less activation at ages 22 and 24. Cannabis users no longer viewed cash with as much anticipation of the reward. Martz concluded that the effects of long-term cannabis use results in a general blunting of reward response. While it could be argued cannabis produces enlightenment and freedom from materialistic desires, a deeper look at nucleus accumbens functioning points in other directions.
