Summary: Study demonstrates the short-term effects of marijuana on human motivation.
Smoking the equivalent of a single ‘spliff’ of cannabis makes people less willing to work for money while ‘high’
The research, published in Psychopharmacology, is the first to reliably demonstrate the short-term effects of cannabis on motivation in humans. The researchers also tested motivation in people who were addicted to cannabis but not high during the test, and found that their motivation levels were no different to volunteers in the control group.
“Although cannabis is commonly thought to reduce motivation, this is the first time it has been reliably tested and quantified using an appropriate sample size and methodology,” says lead author Dr Will Lawn (UCL Clinical Psychopharmacology). “It has also been proposed that long-term cannabis users might also have problems with motivation even when they are not high. However, we compared people dependent on cannabis to similar controls, when neither group was intoxicated, and did not find a difference in motivation. This tentatively suggests that long-term cannabis use may not result in residual motivation problems when people stop using it. However, longitudinal research is needed to provide more conclusive evidence.”
57 volunteers were involved in the research, which consisted of two separate studies. The first involved 17 adult volunteers who all used cannabis occasionally. Through a balloon, they inhaled cannabis vapour on one occasion and cannabis-placebo vapour on separate occasion. Straight after, they completed a task designed to measure their motivation for earning money. This was a real-life task as the volunteers were given money they had earned at the end of the experiment.
In each trial of the task, volunteers could choose whether to complete low- or high-effort tasks to win varying sums of money. The low-effort option involved pressing the spacebar key with the little finger of their non-dominant hand 30 times in 7 seconds to win 50p. The high-effort option involved 100 space bar presses in 21 seconds, for rewards varying from 80p to £2.
“Repeatedly pressing keys with a single finger isn’t difficult but it takes a reasonable amount of effort, making it a useful test of motivation,” explains senior author Professor Val Curran (UCL Clinical Psychopharmacology). “We found that people on cannabis were significantly less likely to choose the high-effort option. On average, volunteers on placebo chose the high-effort option 50% of the time for a £2 reward, whereas volunteers on cannabis only chose the high-effort option 42% of the time.”
In the second study, 20 people addicted to cannabis were matched with 20 control participants who reported the same levels of non-cannabis drug use. Participants were not allowed to consume alcohol or drugs, other than tobacco or coffee, for 12 hours before the study. They were then asked to perform the same motivation task as participants in the first study. The results showed that cannabis-dependent volunteers were no less motivated than the control group. However, much more research is needed to fully understand the relationship between long-term cannabis use and possible amotivational deficits.
Funding: The first study was funded by Drug Science and the second study was funded by UCL and the Biotechnology and Biological Sciences Research Council.
Source: Harry Dayantis – UCL
Image Source: This NeuroscienceNews.com image is adapted from the UCL press release.
Original Research: Full open access research for “Acute and chronic effects of cannabinoids on effort-related decision-making and reward learning: an evaluation of the cannabis ‘amotivational’ hypotheses” by Will Lawn, Tom P Freeman, Rebecca A Pope, Alyssa Joye, Lisa Harvey, Chandni Hindocha, Claire Mokrysz, Abigail Moss, Matthew B Wall, Michael AP Bloomfield, Ravi K Das, Celia JA Morgan, David J Nutt, and H Valerie Curran in Psychopharmacology. Published online September 2 2016 doi:10.1007/s00213-016-4383-x
Acute and chronic effects of cannabinoids on effort-related decision-making and reward learning: an evaluation of the cannabis ‘amotivational’ hypotheses
Anecdotally, both acute and chronic cannabis use have been associated with apathy, amotivation, and other reward processing deficits. To date, empirical support for these effects is limited, and no previous studies have assessed both acute effects of Δ-9-tetrahydrocannabinol (THC) and cannabidiol (CBD), as well as associations with cannabis dependence.
The objectives of this study were (1) to examine acute effects of cannabis with CBD (Cann + CBD) and without CBD (Cann-CBD) on effort-related decision-making and (2) to examine associations between cannabis dependence, effort-related decision-making and reward learning.
In study 1, 17 participants each received three acute vaporized treatments, namely Cann-CBD (8 mg THC), Cann + CBD (8 mg THC + 10 mg CBD) and matched placebo, followed by a 50 % dose top-up 1.5 h later, and completed the Effort Expenditure for Rewards Task (EEfRT). In study 2, 20 cannabis-dependent participants were compared with 20 non-dependent, drug-using control participants on the EEfRT and the Probabilistic Reward Task (PRT) in a non-intoxicated state.
Cann-CBD reduced the likelihood of high-effort choices relative to placebo (p = 0.042) and increased sensitivity to expected value compared to both placebo (p = 0.014) and Cann + CBD (p = 0.006). The cannabis-dependent and control groups did not differ on the EEfRT. However, the cannabis-dependent group exhibited a weaker response bias than the control group on the PRT (p = 0.007).
Cannabis acutely induced a transient amotivational state and CBD influenced the effects of THC on expected value. In contrast, cannabis dependence was associated with preserved motivation alongside impaired reward learning, although confounding factors, including depression, cannot be disregarded. This is the first well powered, fully controlled study to objectively demonstrate the acute amotivational effects of THC.
“Acute and chronic effects of cannabinoids on effort-related decision-making and reward learning: an evaluation of the cannabis ‘amotivational’ hypotheses” by Will Lawn, Tom P Freeman, Rebecca A Pope, Alyssa Joye, Lisa Harvey, Chandni Hindocha, Claire Mokrysz, Abigail Moss, Matthew B Wall, Michael AP Bloomfield, Ravi K Das, Celia JA Morgan, David J Nutt, and H Valerie Curran in Psychopharmacology. Published online September 2 2016 doi:10.1007/s00213-016-4383-x