Cannabis might make people more prone to mistaken memories

New research has found that toxicity of tetrahydrocannabinol (THC), the psychoactive cannabis drug, makes people more predisposed to create false memories. The results have considerable legal consequences.

Young man smoking cannabis
THC toxicity, the psychoactive compound in cannabis, may make it possible to create false memories.

Lilian Kloft, Ph.D. — from Maastricht University’s Faculty of Psychology and Neuroscience in the Netherlands — and colleagues set out to explore the impact of cannabis use on memory formation.

Cannabis has a proven effect on memory. Earlier studies have shown that “acute and chronic cannabis use” impairs verbal memory, learning and attention.

The effect of cannabis on memory, the researchers note, is a particularly important topic that has drawn a great deal of interest, even from a legal perspective.

Convictions often rely on eyewitness testimony and memories but often memories are wrong.

Measurability” of memory refers to the fact that we are able to create memories of events that have not happened, modify specifics of past experiences, and even bring entirely false memories into the minds of someone else.

How does cannabis affect one’s susceptibility to such false memories, then? Kloft and the team studied the effects of THC poisoning on the memories of 64 healthy volunteers, to find out.

The authors published their findings in the Proceedings National Academy of Sciences journal.

Studying cannabis use and false memories

The authors performed a double-blind, randomized, placebo-controlled trial in which they tested the effects of THC poisoning on the susceptibility of false memory. They compared the results to those of a population that got placebo rather than the medication.

The team tested memories of the participants immediately after THC poisoning or placebo use and 1 week later, using associative word lists and two misunderstanding tasks.

The participants analyzed 15 lists of related words in the first task. The people who had received THC were more likely to say they accepted terms that they had not, in fact, been shown before relative to people who had received placebo.

The scientists used virtual reality in the misinformation tasks to build the impression of fighting at a train station and the theft of a handbag in a restaurant.

The participants were the eye-witnesses in the first case, and the victims in the second.

The participants had to answer questions regarding what had happened after seeing each scenario.

The researchers put in inaccurate information during these interviews by adding a second eyewitness ‘ testimony or by posing provocative and deceptive questions.

THC intoxication raises false reporting risk

Results from the team showed that the THC group was much more likely than the placebo group to create false memories.

They also had the same propensity to developing false memories when the tests were repeated 1 week after the THC overdose, when the participant was sober.

“Cannabis tends to increase false memory proneness, with declining association intensity between an occurrence and a test item as tested by different false memory paradigms,” the researchers conclude.

“Our results have consequences about how and when suspects and eye witnesses should be interviewed by police,” they say.

“In terms of interviewing witnesses, victims, or suspects after a crime occurs, this means that interviews while the person is still intoxicated should be reduced due to the high risk of false reporting,” Kloft and colleagues say.

“Our findings show that it would be safer for police officers and prosecutors to delay the interrogation of eyewitnesses and offenders under the influence of cannabis until they are sober,” says the study’s final author, Prof. Johannes Ramaekers.

“People under the influence of cannabis should actually be treated as a vulnerable group in a criminal investigation, comparable with children and the elderly.”

– Prof. Johannes Ramaekers

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