Cannabis use linked to increased brain effort in motor tasks, study finds

Regular cannabis users may need more brain power to perform simple motor tasks, according to a new study published in Human Brain Mapping.

Cannabis is the most popular recreational drug in the United States, with usage rapidly increasing, but the effects of regular cannabis use on the brain are not fully understood.

Previous studies have clearly demonstrated that cannabis can impair attention and memory, but its impact on functions such as motor control is less clear, with some studies suggesting an impairment and other studies suggesting no effect. The underlying brain mechanisms are even less well-understood.

Motor control is the ability to plan and execute movements, such as walking, typing, or playing an instrument. It involves the activity of nerve cells in the motor cortex, a part of the brain that sends signals to the muscles.

These nerve cells produce rhythmic patterns of electrical activity, called neural oscillations, that can be measured by a technique called magnetoencephalography (MEG), a non-invasive imaging technique used to measure the magnetic fields produced by neural activity in the brain. It offers high temporal resolution, enabling researchers to track brain activity in real time and understand how different areas of the brain communicate during various tasks and cognitive processes.

The research team, led by Thomas W. Ward from Boys Town National Research Hospital in Nebraska, set out to better understand motor function with cannabis usage.

They recruited 45 participants between 20 to 59 years old and used MEG to compare the neural oscillations of 18 regular cannabis users and 23 nonuser controls while they performed a motor sequencing task.

To qualify as cannabis users, participants had to consume cannabis no less than three times weekly for a minimum of three years. To qualify as nonusers, participants had to abstain from cannabis and any other illicit drugs, except for occasional past trials, and avoid any illicit drug use in the last 3 months.

The motor sequencing task required all the participants to place their hand on a button pad and tap a sequence of three numbers, after seeing a cue on a screen, with the corresponding fingers on their right hand. For instance, if “1” was visible on the screen, participants needed to tap their index finger, and if “2” was displayed they were required to tap their middle finger.

The researchers found that both groups performed the task equally well, with no differences in reaction time, accuracy, or movement duration.

However, they also found that the cannabis users had stronger neural oscillations in the beta frequency band (16–24 Hertz) in the primary motor cortex and several other movement-related brain regions.

These neural oscillations were increased during the execution phase of the task, but not during the planning phase. This suggests that the cannabis users needed more neural activity to perform the same task as the nonusers.

Ward and colleagues speculated that the stronger beta oscillations may reflect a compensatory mechanism that allows the cannabis users to maintain normal motor performance despite the potential impairments caused by cannabis. They also suggested that the beta oscillations may be influenced by the levels of a neurotransmitter called GABA, which is a chemical known to regulate the activity of the motor cortex and may be affected by cannabis.

The study authors concluded, “our results demonstrate that though regular cannabis users are able to perform the current motor sequencing task at the same level as nonuser controls, the two groups are quite different neurologically. These differences may reflect compensatory processing or be precursors of behavioral deficits that may emerge in the future. These compensatory mechanisms, though adequate for this relatively simple sequencing task, may break down in real-world situations where more complex motor control is needed.”

The study has some limitations, such as not controlling for the amount and type of cannabis consumed by the users, and using a relatively simple motor task that may not capture the complexity of real-world motor control.

The study, titled “Regular cannabis use alters the neural dynamics serving complex motor control”, was authored by Thomas W. Ward, Seth D. Springer, Mikki Schantell, Jason A. John, Lucy K. Horne, Anna T. Coutant, Hannah J. Okelberry, Madelyn P. Willett, Hallie J. Johnson, Abraham D. Killanin, Elizabeth Heinrichs-Graham, and Tony W. Wilson.