The new study suggests marijuana use could lead to cognitive laziness.
Lead study author Mason Silveira, of the Department of Psychology at Columbia, and colleagues report their findings in the Journal of Psychiatry and Neuroscience.
According to the National Institute on Drug Abuse (NIDA), marijuana is the most commonly used illicit drug in the United States; a 2014 survey found more than 22 million Americans reported using the drug in the past month.
However, as of June 2016, 25 U.S. states and Washington, D.C., have legalized marijuana for medical or recreational use, and more states are expected to follow suit. This highlights the need to gain a better understanding of the risks and benefits marijuana use may pose.
A sense of euphoria, increased appetite, heightened sensory perception, hallucinations and delusions, and fear and anxiety are some of the well-known effects of marijuana use. Such effects are primarily down to the two main active compounds - known as cannabinoids - in the drug: tetrahydrocannabinol (THC) and cannabidiol (CBD).
There is a common belief that marijuana use can also affect a person's motivation, making them lazy, though there has been little scientific evidence to support this association.
Silveira and colleagues note that previous studies have suggested the activation of cannabinoid receptors - which occurs through marijuana use - can affect decision-making related to physical effort, making a person physically lazy.
It is unclear, however, whether decision-making related to cognitive effort is affected in the same way. Silveira and colleagues wanted to find out.
THC led rats to opt for easier cognitive task
The researchers trained 29 male rats to complete a behavioral experiment, whereby the animals had to decide whether they wanted to complete an easy or difficult cognitive task in order to receive a reward; the more difficult challenge produced a higher reward.
The easy task required the rats to react to a light that flashed for 1 second, which earned them one sugar pellet. In the difficult task, the rats had to react to a light that flashed for only 0.2 seconds, which gave them two sugar pellets.
The rats completed the experiment under normal conditions, as well as after being given THC and CBD independently and combined.
As expected, under normal conditions, the rats opted for the more difficult task, earning them a higher reward.
However, after being given THC, the rats opted for the easier task, even though it earned them fewer treats.
"Perhaps unsurprisingly, we found that when we gave THC to these rats, they basically became cognitively lazy. What's interesting, however, is that their ability to do the difficult challenge was unaffected by THC. The rats could still do the task - they just didn't want to."
CBD failed to counteract THC's negative effects
After being given CBD alone, the team identified no difference in rats' task choices.
Following administration of THC and CBD combined, however, the team was surprised by the results. Previous research had suggested that CBD can counteract the negative effects of THC, but this was not the case in the current study - both cannabinoids combined led the rats to opt for the easy task.
The team explains the findings further in the video below:
The researchers say their findings demonstrate the negative effect marijuana use may have on cognitive effort; put simply, the results show that using marijuana could make a person cognitively lazy, which could reduce their chances of successful life outcomes.
"Although a chronic dosing experiment would be required to assess this association directly, we hypothesize that associations between THC and poorer life outcomes may be due to a drug-induced decrease in willingness to allocate cognitive effort, rather than impairments in fundamental cognitive abilities per se," they add.
The authors conclude that further research is warranted to determine the underlying mechanisms by which THC alters decision-making. Such information could lead to ways to block the negative effects of THC, while maintaining the medical benefits, such as pain relief.