A memory is not a simple recall of a truth. Memory is more of a restructuring of what one may think is the truth. Its formation is dependent on context, social expectations, and subjective wishes; an especially complex system of thought that can lead to the formation of false memories. It is difficult enough to remember things correctly — it’s especially so when you throw in a little weed.
Marijuana and its main psychoactive ingredient THC have been found to impair memory before. Animal studies suggest regular marijuana use harms long-term memory skills. But a new study raises the stakes, linking using weed to false memories.
In a new study, researchers specifically examine the link between marijuana use and the formation of false memories.
Across three experiments, they found that marijuana consistently increases a person’s susceptibility to false memories — which they point out is especially problematic in the context of a crime, and cannabis-intoxicated witnesses and suspects.
The study was published Monday in the journal Proceedings of the National Academy of Sciences.
Seeing does not mean believing
There are two kinds of false memories: They are either spontaneous — a result of internal cognitive processes — or suggestion-based — occurring because of external suggestions. The study looked at the effects of marijuana on the ability to form both. It turns out that elevated false memories are the norm when THC affects memory retrieval.
The most important takeaway from the study is that cannabis exerted a general impact on memory by increasing various types of recollective errors, the researchers say. There is a debate over how different types of false memories relate to each other, but the current study shows that intoxicated individuals may be at higher risk of forming “all kinds of memory errors, which can be perilous in investigative interview settings.”
First author Lilian Kloft, a psychopharmacology researcher at Maastricht University, tells Inverse that the study can’t explain why using marijuana increased susceptibility to false memories. But other research suggests some of the effects on memory are mediated by activation of cannabinoid receptors in the hippocampus — a major memory center in the brain.
Since this study found that people can become more vulnerable to suggestion when high, Kloft wants to investigate whether being high also makes people more likely to admit that they committed a crime they did not actually commit.
“The next step for us is to investigate the effects of cannabis in a ‘false confession’ paradigm,” Kloft says. “False confessions are a major contributing cause to wrongful convictions and we cannot exclude that drug influence can magnify vulnerability for making a false confession.”
Vaping and false memories
The study includes 64 occasional marijuana users. On different test days, each subject inhaled the vapor of a single dose of marijuana, or a placebo.
The team used a machine called the Volcano Vaporizer to administer marijuana. The vapor created by the machine was used to fill up a balloon, and then the participants inhaled one full balloon per dose. Rather than try gulp all that weed in at once, the participants were able to take their time and inhale multiple times. It usually took around five minutes for a person to finish their balloon, Kloft says. As of now, there is no evidence that other methods of consumption would change the effects witnessed, she says.
Across three experiments, the participants’ memories were tested immediately after inhalation, and then again one week later.
In the first experiment, participants were asked to look at a list of words, then recall the words. When shown a list with some new words, cannabis-intoxicated individuals were more likely to say they had seen the new words on the first list. This test evaluated “spontaneous” false memory formation.
During the other two experiments, the participants witnessed a fight or perpetrated a theft in virtual reality. Afterward, they were exposed to misinformation about what they saw, either through suggestive, leading questioning from an interviewer, or through a misleading testimony given by a “witness.” The participants were then asked what they remembered about what they saw.
Overall, marijuana appeared to amplify the intoxicated participant’s susceptibility to “suggested” false memories. In the eye-witness scenario, the stoned participants showed the highest false memory rates in response to both leading questions and neutral questions. When it came to the perpetrator scenario, all of the intoxicated participants had an overall higher tendency to respond “yes” to questions.
When all groups were tested one week later, they performed poorly. This, the researchers say, is less likely due to THC, and more to do with the fact that memory decays over time. Past research shows that people are more prone to be influenced by misinformation if time has passed between an event and the request to recall memory, they say.
The study doesn't get at why being stoned makes it easier for false memories to take root — but it does have important implications, especially for criminal justice.
If witnesses and suspects are stoned, whatever questioning that needs to happen should take place as soon as the person has sobered up, the study suggests. Ideally, the questioning would occur between false memory susceptibility and expected memory decay.
But if a person is under the influence when an event happens, they may still display a "yes" bias toward new information later, the researchers say. This means cannabis-intoxicated people may need to be treated as a “vulnerable” group in the context of crimes, similar to a child or the elderly, they say. If so, that could dramatically change both how we investigate and prosecute crimes.
Abstract: With the growing global acceptance of cannabis and its widespread use by eyewitnesses and suspects in legal cases, understanding the popular drug’s ramifications for memory is a pressing need. In a double-blind, randomized, placebo-controlled trial, we examined the acute and delayed effects of Δ9-tetrahydrocannabinol (THC) intoxication on susceptibility to false memory in 64 healthy volunteers. Memory was tested immediately (encoding and retrieval under drug influence) and 1 wk later (retrieval sober). We used three different methods (associative word lists and two misinformation tasks using virtual reality). Across all methods, we found evidence for enhanced false-memory effects in intoxicated participants. Specifically, intoxicated participants showed higher false recognition in the associative word-list task both at immediate and delayed test than controls. This yes bias became increasingly strong with decreasing levels of association between studied and test items. In a misinformation task, intoxicated participants were more susceptible to false-memory creation using a virtual-reality eyewitness scenario and virtual-reality perpetrator scenario. False-memory effects were mostly restricted to the acute-intoxication phase. Cannabis seems to increase false-memory proneness, with decreasing strength of association between an event and a test item, as assessed by different false-memory paradigms. Our findings have implications for how and when the police should interview suspects and eyewitnesses.