In the study, published in Revista Medica de Chile, researchers from the University of Chile, University of Michigan, Boston University and the University of California San Diego interviewed 654 21-year-old adults from Santiago, Chile about their marijuana use. Almost 70% reported some use in their lifetime, 46% in the past year and 27% in the past 30 days. Next, they tested the cohort using the International Shopping List Test (ISLT) from the Cogstate battery to measure verbal learning (ISLT acquisition) and memory (ISLT delayed recall).
“Given the relative commonality of marijuana use among the young adult population, it is important to understand possible consequences and associations of marijuana use,” wrote the study authors. “In particular, our intention was to test for neurocognitive associations of marijuana use within a Chilean young adult sample.”
While prior studies had found that marijuana use, including users at a young age, correlated with decreased cognitive performance, including psychomotor speed, sequencing ability, sustained attention and verbal memory, a recent meta-analysis concluded that these negative outcomes may be overstated. This study was designed to offer clarity to the issue.
Chosen by the researchers for its reliability across language translations and its validity for measuring verbal memory, the Cogstate ISLT assesses immediate and delayed recall of a 12-item list. Immediate recall is measured as the sum of 3 trials. Delayed recall is measured in one trial. Higher scores indicate better verbal learning and memory.
Study participants who used cannabis over the last year or past 30 days performed worse on the ISLT than those who had never used cannabis. Frequent use was also associated with worse performance. The researchers wrote, “Assessing varying levels of exposure, we found significant negative effects of marijuana use even for individuals who reported using “at least once” in the past year –relatively lenient criteria compared to other studies. As we altered the cut-off points to focus on more intense marijuana use in the exposed group, the magnitude of the effect increased.”
“We think this study is important because it shows that even recreational cannabis use is associated with some diminution of learning and memory,” said Dr. Paul Maruff, Chief Science Officer of Cogstate. “These data therefore show that trials designed to investigate the effects of drugs on cognition in young adults must take into account the potential effects of history and frequency of cannabis use. These data are also important because they show the Chilean version of the ISLT to be acceptable for use and sensitive to memory impairment in people in Chile.”