Heavy Lifetime Cannabis Use Linked to Lower Brain Activity During Memory Tasks

Using Human Connectome Project data, heavy lifetime cannabis users (>1,000 uses, n=88) showed significantly lower brain activation during a working memory task compared to nonusers, with a moderate effect size (Cohen d = -0.28). Affected regions included the anterior insula, medial prefrontal cortex, and dorsolateral prefrontal cortex. Recent cannabis use (detected by urine testing) was associated with lower performance on working memory and motor tasks, but these associations did not survive statistical correction. No other cognitive tasks (reward, emotion, language, motor, relational, theory of mind) showed associations with lifetime heavy use.

Cross-sectional study of 1,003 young adults (ages 22-36) from the Human Connectome Project. Functional MRI during seven cognitive tasks. Cannabis use history from structured clinical interviews. Recent use detected via urine toxicology on scan day. Linear mixed-effects regression with false discovery rate correction.

This is one of the largest neuroimaging studies to separate lifetime cannabis use history from recent use effects. The finding that working memory deficits persist even when controlling for recent use suggests heavy long-term cannabis use may leave lasting traces on brain function.

Working memory is foundational to daily cognitive functioning. If heavy cannabis use is associated with lasting reductions in working memory brain activation, this has implications for millions of regular users, particularly those who started young.

Cross-sectional design cannot determine whether reduced activation preceded or followed cannabis use. Self-reported lifetime use may be inaccurate. The heavy user group (n=88) was relatively small. Data collected 2012-2015 may not reflect current cannabis potency.