A new brain imaging study has found that people who regularly use cannabis show reduced spontaneous activity in a key motor control region of the brain. While their actual task performance did not differ significantly from non-users, the weakened brain activity was linked to more severe cannabis use disorder symptoms and faster reaction times during a cognitive task. The research was published in the Journal of Psychopharmacology.
Cannabis, commonly known as marijuana, is a psychoactive substance derived from the Cannabis plant. It is widely used both recreationally and medicinally and has become increasingly accessible due to changing laws around the world. Cannabis contains compounds like tetrahydrocannabinol (THC) that interact with the brain’s endocannabinoid system, producing a range of effects, including relaxation, altered perception, and changes in attention or motor coordination.
While some people experience therapeutic benefits from cannabis, regular use has been associated with cognitive and neurological changes. The researchers behind the current study wanted to understand how cannabis affects brain activity involved in motor control, an area that has been less studied than attention or memory.
Past studies have shown that frequent cannabis use can alter brain function during tasks that involve memory, attention, or decision-making. But there has been relatively little research into how cannabis might influence the brain systems that control voluntary movement. This gap is important because motor control relies on precise coordination of brain activity, and disruptions in these systems could have broader implications for functioning in daily life. The researchers designed a study to explore whether regular cannabis use changes patterns of brain activity in motor regions during a commonly used test of cognitive control, and whether these changes are linked to how often and how heavily people use cannabis.
The study involved 67 adults from the Omaha, Nebraska area. Thirty-four of the participants reported using cannabis at least twice a week for the past six months, while 33 reported no recent cannabis use. Both groups were matched on age, gender, and other demographic factors. All participants completed a structured interview about their substance use history and filled out standardized questionnaires, including one that assessed symptoms of cannabis use disorder. To measure brain activity, the researchers used magnetoencephalography (MEG), a non-invasive technique that detects magnetic fields produced by neural activity with high temporal precision.
During the MEG session, participants completed a version of the Eriksen flanker task, which involves identifying the direction of a central arrow while ignoring surrounding “flanker” arrows that can either match or conflict with the target. This task is widely used to measure attention and cognitive control, but it also requires participants to make rapid motor responses using their hands, allowing researchers to assess activity in motor-related brain areas. The researchers focused on oscillatory activity in the beta and gamma frequency bands—patterns of brain waves known to play roles in motor planning and execution.
The researchers found that all participants, regardless of cannabis use, showed typical patterns of motor-related brain activity. Specifically, there was a drop in beta activity (known as beta desynchronization) and a burst of gamma activity (gamma synchronization) around the time participants responded with a hand movement. These responses were strongest in the left primary motor cortex, a brain region that controls movement on the right side of the body. Importantly, the task successfully activated the expected motor networks in both cannabis users and non-users, indicating that the basic functioning of these systems remained intact.
However, when the researchers looked at spontaneous brain activity—measured during the rest period just before each task trial—they found a significant difference between groups. Cannabis users showed markedly lower gamma activity in the left primary motor cortex compared to non-users. This difference in spontaneous activity was not seen in the beta band, suggesting the effect was specific to gamma oscillations. The more severe a participant’s cannabis use disorder symptoms were, the lower their spontaneous gamma activity in this brain region.
The study also found that spontaneous gamma activity was related to task performance. Across all participants, those with lower spontaneous gamma power tended to respond more quickly during the flanker task. This relationship was especially pronounced among cannabis users. Further analysis suggested that spontaneous gamma activity may partly explain the link between cannabis use disorder symptoms and reaction time, pointing to a potential pathway by which changes in brain function could be tied to behavioral differences.
Interestingly, while spontaneous brain activity differed between groups, there were no significant differences in how the two groups performed on the flanker task overall. Both groups showed slower responses during trials with conflicting arrows, a common pattern known as the flanker effect. They also had similar levels of accuracy. This suggests that the changes in brain activity among cannabis users did not lead to detectable differences in how well they performed the task. The authors suggest that participants who regularly use cannabis may be compensating for underlying changes in brain activity to maintain normal performance.
The findings highlight the complex relationship between regular cannabis use, brain function, and behavior. The observed reduction in spontaneous gamma activity in the motor cortex aligns with previous studies showing similar decreases in other brain regions among cannabis users, including areas involved in sensory processing. The gamma frequency range is thought to support coordinated activity across brain circuits, and its suppression could reflect broader changes in neural communication.
One possibility is that cannabis disrupts the functioning of inhibitory brain cells that help generate gamma rhythms. These cells rely on the neurotransmitter GABA, and some evidence suggests that THC may interfere with GABAergic signaling.
The researchers note several limitations to their study. While participants were asked not to use cannabis on the day of testing, the exact timing of their last use was not measured. Differences in cannabis potency, method of consumption, and duration of use could also influence brain activity, but these factors were not fully controlled. In addition, the study used a relatively simple cognitive task, which may not have been challenging enough to reveal subtle differences in performance. Future research using more demanding tasks, larger samples, and direct measures of brain chemistry could help clarify the observed effects and their underlying causes.
Even with these limitations, the study adds to growing evidence that regular cannabis use is linked to changes in brain function. While these changes did not impair performance in this study, the suppression of spontaneous gamma activity could reflect a broader pattern of neural alterations associated with heavy cannabis use.
The study, “Regular cannabis use modulates gamma activity in brain regions serving motor control,” was authored by Lauren K. Webert, Mikki Schantell, Jason A. John, Anna T. Coutant, Hannah J Okelberry, Lucy K. Horne, Megan E. Sandal, Amirsalar Mansouri, and Tony W. Wilson.
