05.2020 | THE SCIENTIST 17ANDRZEJ KRAUZE
Hazy
Recollections
When Lilian Kloft stumbled across a 2015
study showing a connection between can-
nabis use and susceptibility to false mem-
ories, she found herself wondering about
the legal implications of the results. The
study had discovered that heavy users of
cannabis were more likely than controls
to form false memories—recollections of
events that never occurred, for example, or
warped memories of events that did—even
when they were not at the moment “high.”
This kind of false remembering can
pose difficulties for people gathering reli-
able testimony in the event of a crime, says
Kloft, a PhD student in psychopharmacol-
ogy and forensic psychology at Maastricht
University in the Netherlands. Conse-
quently, the growing acceptance of can-
nabis worldwide raises questions not only
about how the drug affects memory, but
also about how law enforcement officials
should conduct interviews with suspects,
victims, and witnesses who may be under
the influence or regular users of the drug.
In order to further investigate the con-
nection between cannabis and false mem-
ory formation, Kloft and collaborators
recruited 64 volunteers for a series of exper-
iments. Participants, who were occasional
cannabis users, were given a vaporizer con-
taining either cannabis or a hemp placebo
and then told to inhale deeply and hold
their breath for 10 seconds. After that, the
researchers tested them in three different
tasks designed to induce false memories.
In the first task, the team asked the
volunteers to memorize lists of words,
and then to pick out those words from test
lists that also included dummy words. As
expected, both the sober and the intoxi-
cated participants falsely remembered
some of the dummy words. But while the
sober participants mostly falsely remem-
bered words that were strongly associ-
ated with words on the original lists, the
intoxicated participants also selected less-
related and completely unrelated terms.
In the next two tasks, the research-
ers wanted to see if they could induce falsememories by providing misinformation to
the participants. Hoping to imbue these tests
with more real-world relevance than a list of
words, Kloft and colleagues designed two
immersive virtual reality scenarios involv-
ing common crimes. In the first, the “eyewit-
ness scenario,” participants observed a fight
on a train platform, after which a virtual co-
witness recounted the incident but with sev-
eral errors, including falsely recalling a police
dog that wasn’t part of the altercation. In the
“perpetrator scenario,” participants entered a
crowded bar and were instructed to commit a
crime themselves—to steal a purse.The researchers observed a range of
effects associated with cannabis as the
intoxicated subjects interacted in these
virtual environments. Some participants
laughed and talked to the virtual charac-
ters in the scenarios, Kloft reports, while
others became paranoid and required
assistance in stealing the purse. “One per-
son even ran away so quickly that they
ripped out the whole VR setup and it fell
to the ground,” she says. When research-
ers interviewed the participants afterward
using a combination of leading and non-
leading questions, those who were intoxi-