Does cannabis use affect working memory through changes in hippocampal size? | Research

The takeaway

Using Human Connectome Project brain scans from 408 individuals, researchers found that more frequent cannabis use was associated with smaller hippocampal volumes, which in turn mediated the relationship between cannabis use and working memory impairment.

Readers interested in how cannabis may affect brain structure and what that means for memory, and researchers studying the neuroscience of cannabis effects.

Left hippocampal volume mediated memory deficit

What the researchers found

Higher frequency of cannabis use was associated with lower working memory scores and smaller bilateral hippocampal volumes. Mediation analysis showed that the association between cannabis use frequency and working memory impairment was statistically mediated by lower left hippocampal volume.

Why it matters

This study provides a potential brain mechanism linking cannabis use to cognitive deficits. If cannabis reduces hippocampal volume and the hippocampus supports working memory, this offers a neuroanatomical explanation for the cognitive effects reported by heavy users.

The numbers in context

234 cannabis-exposed + 174 unexposed individuals; bilateral hippocampal volume reductions with higher use frequency; left hippocampal volume mediated the use-memory relationship

How the study worked

Cross-sectional observational study using Human Connectome Project data from 234 cannabis-exposed and 174 unexposed individuals. T1-weighted MRI measured regional gray matter volumes. Working memory assessed via list-sorting task. Mediation analysis tested whether hippocampal volume explained the cannabis-cognition relationship.

What this study cannot tell us

Cross-sectional design cannot establish causation. Self-reported cannabis frequency. Human Connectome Project participants are generally healthy, limiting generalizability to heavy users. Unmeasured confounders possible.

How to read the evidence

Uses high-quality Human Connectome Project neuroimaging data with mediation analysis, but cross-sectional design and self-reported cannabis use.

When this study was published

Published in 2021; the authors note prospective longitudinal studies are needed to confirm causal direction.

The bigger picture

The hippocampus is rich in CB1 receptors, making it a prime target for THC effects. This mediation finding connects the dots between receptor biology, brain structure, and cognitive performance. However, the cross-sectional design cannot determine whether cannabis shrank the hippocampus or whether people with smaller hippocampi used more cannabis.

Questions still open

Does hippocampal volume recover with cannabis abstinence? Is there a threshold of cannabis use frequency below which hippocampal effects are negligible? Do these structural changes predict real-world functional impairments beyond task performance?

Common questions

Does cannabis shrink the hippocampus?

This study found that more frequent cannabis use was associated with smaller hippocampal volumes, but cannot prove cannabis caused the reduction. The hippocampus is rich in cannabinoid receptors, making it a plausible target for THC effects.

What does the hippocampus do for memory?

The hippocampus is crucial for working memory and forming new memories. This study found that lower left hippocampal volume statistically explained the link between cannabis use and poorer working memory performance.

Read the original research

Cannabis use-related working memory deficit mediated by lower left hippocampal volume.

Addiction biology, 26(4), e12984

View on PubMed ↗Publisher’s paper ↗

Citation

Paul, Subhadip; Bhattacharyya, Sagnik. (2021). Cannabis use-related working memory deficit mediated by lower left hippocampal volume.. Addiction biology, 26(4), e12984. https://doi.org/10.1111/adb.12984