Getting high on cannabis makes people vulnerable to ‘false memories

People who are high on cannabis are more likely to form false memories, in which they wrongly “remember” information that they never actually learned or recall snippets of an event that never happened, new research suggests.

False memories can arise spontaneously when people draw faulty inferences from their actual experiences. For instance, you might remember your co-worker being at the big meeting last Monday because everyone else attended when, in reality, he was out sick. In other cases, external sources supply the misleading information that fuels false memories, whether in the form of leading questions, faulty personal accounts from other people or misinformed media coverage.

Everyone occasionally crafts false memories, even when sober. But now, a study published Feb. 10 in the journal Proceedings of the National Academy of Sciences  suggests that cannabis use may raise the risk of forging false memories — a point that could prove critical in court.

“The law has recognized that certain witnesses are vulnerable, so you need to take extra care” when questioning them, said co-author Elizabeth Loftus, a distinguished professor of psychological science and law at the University of California Irvine School of Law. Children and people with mental disabilities are considered “vulnerable” witnesses, for example. “Maybe cannabis-intoxicated witnesses should join that club,” Loftus said.

Two experts told Live Science that, while the effects of cannabis on memory should be taken seriously in court, more research is needed to determine when and how police should question intoxicated witnesses to obtain more reliable testimonies.

Spontaneous confusion

As cannabis use becomes more commonplace and widely accepted around the world, understanding how the drug affects memory will become critical to the way officials handle criminal cases, said lead author Lilian Kloft, a graduate student in the Department of Neuropsychology and Psychopharmacology at Maastricht University in the Netherlands.

“Cannabis is the most widely used drug worldwide, after legal substances such as alcohol and nicotine,” Kloft told Live Science in an email. “There is a need to find out how this affects [witnesses’] memory, their reports, so that in turn evidence-based policies can be shaped.”

With this goal in mind, Kloft and her colleagues recruited 64 volunteers in the Netherlands to inhale a dose of vaporized cannabis and have their memory tested. The team designed the experiment to examine two kinds of false memories: those that arise somewhat spontaneously and those that external sources introduce.

Related: 25 odd facts about marijuana

To test spontaneous false memories, the team turned to a well-known experiment known as the Deese-Roediger-McDermott (DRM) Task. In that experiment, volunteers memorize a list of related words — such as “tired,” “pillow,” “bed” and “snore” — and then get tested on their recognition of those words. The catch is that, during the testing round, learned words get mixed in with new words that the volunteers weren’t asked to memorize. In one experiment, the volunteers memorized a word list while high, and in another, they memorized a different list while sober.

The new words ranged from totally unrelated to highly related to the words on the original list. Typically, people wrongly remember highly related words despite not having seen them before.

Indeed, this was the case when the volunteers were tested immediately after the memorization round, whether they were high or sober. While intoxicated, however, the participants were more likely to flag somewhat related and totally unrelated words as belonging to the original list. For example, when high, people might have mistakenly said  the word “tomato” was in their original word list even if it was a sleep-themed list.

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