Cannabis triggers false memories?
When neuropsychologist Lilian Kloft came across a 2015 study showing a link between cannabis use and false memories, she became interested in the legal implications of these findings. The study found that heavy cannabis users were more likely to create false memories (for example, memories of events that never happened or distorted memories of events that did happen) than a control group, even if they were not under the influence of the drug at the time.
"These false memories can make it difficult for people to prove their testimony in a criminal case," says Kloft, a doctoral student in psychopharmacology and forensic psychology at Maastricht University in the Netherlands. The growing popularity of cannabis around the world therefore raises not only the question of how the drug affects memory, but also how law enforcement officials should conduct interviews with suspects, victims and witnesses who may be under the influence of the drug or who use it regularly.
In order to further investigate the link between cannabis use and false memory formation, Kloft and her colleagues recruited 64 volunteers who underwent a series of experiments. The participants, who were occasional cannabis users, were given vaporizer containing either cannabis or a placebo and then told them to take a deep breath and hold it for 10 seconds. Then the researchers tested them using three different tasks designed to elicit false memories.
In the first task, the team asked volunteers to memorize lists of words and then select those words from lists that included other words. As expected, participants from both groups misremembered some of the incorrect words. But while the sober participants mostly misremembered words that were related to the words on the original lists, the drugged participants also selected less related and completely unrelated terms.
In the other two tasks, the researchers attempted to test whether they could elicit false memories by providing participants with false information. Kloft and her colleagues hoped these tests would better simulate real-world experiences than a mere list of words, so they designed two virtual reality scenarios involving common crimes. In the first "eyewitness scenario," participants observed a fight on a train platform, after which a virtual second witness narrated the incident, but with several errors, including false memories of a police dog that was not part of the event. In the scenario called "the perpetrator," the participants entered a crowded bar and were instructed to commit the crime themselves and steal a purse.
Researchers have observed a number of effects associated with cannabis use when intoxicated subjects interact in these virtual environments. According to Kloft, some participants laughed uncontrollably and conversed with the virtual characters, while others were paranoid and required help when stealing purses. "One participant even ran away so fast that he ripped out the entire VR device and it fell to the ground," she says. When the researchers subsequently interviewed participants using a combination of leading and non-leading questions, those who were under the influence of the drug showed higher rates of false memories for both the eyewitness and perpetrator scenarios compared to controls.
To find out the long-term effects of cannabis, the researchers convened the experiment participants a week later and tested them again on word lists, this time with several different fictitious words. They also re-interviewed the experimental subjects about the VR scenarios using a combination of old and new questions. As before, they found lower memory accuracy on a word association test for the subjects who were under the influence compared to the sober participants. For the virtual reality scenarios, there were no statistically significant differences between the groups, which Kloft said may indicate memory decline over time in all participants.
Neuropsychopharmacologist Manoj Doss, a doctoral student at Johns Hopkins University who was not involved in the study, used word association and other tasks in his own research to show that tetrahydrocannabinol (THC), the main psychoactive ingredient in cannabis, increases the amount of false memories when participants try to recall information they have previously learned. Doss says the study by Kloft and her colleagues is novel not only because it uses virtual reality, but also because it shows that both real-world scenarios and a word association task can trigger false memories.
For tests conducted a week later, however, Doss notes that it is difficult to determine whether the researchers observed actual false memories because participants could remember both accurate and erroneous information encountered in the original experiment. In a follow-up test, "people could answer yes to things where they shouldn't have said yes, just because they saw them in the first test," Doss says. He hypothesizes that increasing the number of items tested, as well as a separate analysis of the new and previously used word tests and interview questions, might reveal a higher incidence of false memories in the delayed test among participants who had used cannabis.
Giovanni Marsicano, a neurobiologist at the University of Bordeaux in France who was not involved in the research, says the new results are consistent with findings he made in mice: animals injected with THC were more likely than controls to associate unrelated stimuli, itself a type of false memory. His work also showed that a cannabinoid receptor known as CB1, which is very abundant in the hippocampus and prefrontal cortex, likely plays a key role in making these coincidental associations. One of the main functions of this receptor is to reduce the release of neurotransmitters. Marsicano hypothesizes that when the CB1 receptor is activated, neural signaling is inhibited in such a way that the brain is unable to adequately separate correct information from incorrect information.
Roger Pertwee, a pharmacologist at the University of Aberdeen in the UK who was not involved in the research, says the Dutch team's results are not surprising given what is known about how cannabinoids they affect memory. Unlike endogenous cannabinoids that selectively activate CB1 receptors and others do not, the compounds in cannabis activate all CB1 receptors at once. This mass activation may also somehow contribute to the creation of false memories, explains Pertwee, who works with GW Pharmaceuticals, which makes prescription drugs derived from cannabis.
Kloft says that in the future, she would like to look at how people perceive the memories they form while under the influence of the drug to see if they believe those memories.
Study co-author Elizabeth Loftus, a cognitive psychologist and human memory expert at the University of California, Irvine, says the team's study should get people thinking about the procedures used when it comes to interviewing witnesses under the influence of drugs. "The law recognizes that there are witnesses who need special care and attention during interrogation: young children, people with intellectual disabilities, and sometimes the elderly fall into that category," Loftus says. "Couldn't people who are under the influence of drugs be another example of witnesses whose questioning needs to be conducted with special care?"
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