Memory on Trial
Do cannabis users make unreliable witnesses?
Phoebe Harkin | | Quick Read
Washington State University (WSU) researchers have found a way around restrictive US federal legislation that limits cannabis studies to low-potency (<10 percent) plants: have participants buy their products in a state where recreational cannabis is legal and watch them use it in their own homes (1). The Zoom study is now one of few to investigate the effects of cannabis concentrates containing more than 10 percent THC.
The trial saw 80 participants divided into four groups. Two used cannabis flower with more than 20 percent THC – one containing CBD, one without. Another group vaped cannabis concentrates with more than 60 percent THC that included CBD. A fourth control group remained sober. The participants were observed smoking (or not) over Zoom before they were set a series of cognitive tests.
Interestingly, the researchers found no impact on the users’ performance on decision-making tests (including risk perception and confidence in knowledge) relative to the control group, but did find some memory impairments related to free recall, source memory, and false memories. There was also an unexpected finding: people who vaped concentrates with more than 60 percent THC performed comparably to those who smoked cannabis flower, despite its significantly lower potency.
We spoke to lead author and Associate Professor in the Department of Psychology at WSU, Carrie Cuttler, to find out more
Do you think the <10 percent THC ruling is limiting cannabis research and understanding?
Yes. It is a major limitation because the vast majority of cannabis flower sold in dispensaries exceeds 20 percent THC. Also, cannabis concentrates are becoming very popular among consumers and these can exceed 90 percent THC. THC is the primary psychoactive constituent in cannabis and is believed to be responsible for most of its intoxicating and impairing effects. Studying cannabis with relatively low levels of THC would therefore make it difficult to detect some of the potentially beneficial and detrimental effects of the drug.
This restriction is in the process of being lifted, though. The DEA was recently sued and, in response, they are opening up the cannabis supply chain for researchers to include producers other than NIDA. This is all still in the works and, to my knowledge, no one has yet received cannabis from a producer other than NIDA for research in the US.
Were you surprised to find no significant differences between the cannabis-using and sober groups for prospective and temporal order memory?
A little. I was not terribly surprised to find that high-potency cannabis did not have a significant effect on our prospective or temporal order memory tests. The acute effects of cannabis on memory, particularly verbal free recall, have been fairly well established, but are not reliably detected in every study. This is the first study ever to attempt to examine the acute effects of cannabis on either of these domains of memory and there are a number of reasons why we may not have detected significant effects on these tests.
First, it’s possible that high-potency cannabis does not affect these domains of memory. Second, it’s possible that our experienced cannabis user participants had become tolerant to the acute effects of high-potency cannabis on these domains of memory. Third, it’s possible that high-potency cannabis has small effects on these domains of memory and that we didn’t have enough power (perhaps didn’t test enough participants) to detect them. Fourth, it’s possible that our tests were not sensitive enough to detect these effects. This may especially be the case for the prospective memory tests we used.
There is some indication that sober chronic cannabis users perform worse than non-users on time-based prospective memory tests (which involve remembering to do something at a specific time or after a specific amount of time has elapsed), but not event-based prospective memory tests (which involve remembering to do something when a specific event occurs). We only used event-based prospective memory tests in our study. We plan on assessing time-based prospective memory in a subsequent study to determine whether high-potency cannabis has detrimental effects on those tests more specifically.
Some studies claim CBD may have a protective effect on memory; how do they fit in with your findings?
Only a couple of studies have revealed evidence that CBD might have a protective effect on memory and one of the authors of one of those studies was not able to replicate their own effects. There is a contradictory line of evidence with more support that suggests that CBD can potentiate the effects of THC. My study found no significant differences between the cognitive test performance of participants who smoked a joint containing high concentrations of THC (> 20 percent) but no CBD and those who smoked a joint containing equally high concentrations of THC (> 20 percent) with some CBD (> .70 percent). As such, I don’t believe that CBD protects against the memory-impairing effects of THC but it is also of course possible that the levels of CBD used by my participants were not high enough to produce protective effects or that CBD needs to be administered repeatedly over several days to produce such protection.
Is there a reason why all three cannabis-using groups did poorly on a false memory test?
Several other studies have also shown that participants under the influence of cannabis are more prone to “intrusions” on memory tests. In other words, they are more likely to say they previously heard or saw a word that was not actually presented to them. Another study using high-potency cannabis conducted outside the US also found a similar pattern of detrimental effects on false memory. This could be due to effects of cannabis on inhibition and/or lowering the threshold for accepting information as old rather than new. Additional research will be needed to probe the precise mechanisms producing these effects and/or brain regions underlying them.
What's next for your research?
We’re currently using this same novel Zoom methodology to study the chronic and acute effects of cannabis on symptoms of attention deficit/hyperactivity disorder (ADHD) and executive functioning in those with ADHD. We’re doing this study because many people with ADHD use cannabis and say that it helps them. Previous research has indicated that medical cannabis users may be spared the detrimental effects of cannabis on cognition and we’re curious as to whether cannabis has beneficial or detrimental effects on symptoms of ADHD and ADHD-related executive dysfunction.
- Washington State University (2021). Available at: bit.ly/3yR6t53.