Weed or Wheel! fMRI, Behavioural, and Toxicological Investigations of How Cannabis Smoking Affects Skills Necessary for Driving

Battistella, Giovanni; Fornari, Eleonora; Thomas, Aurélien; Mall, Jean‐Frédéric; Chtioui, Haïthem; Appenzeller, Monique; Annoni, Jean‐Marie; Favrat, Bernard; Maeder, Philippe; Giroud, Christian · 2013 · OpenAlex-citations

DOI: 10.1371/journal.pone.0052545

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Summary

This study investigates the acute effects of cannabis smoking on cognitive functions and psychomotor skills essential for safe driving. While the prevalence of cannabis use among drivers is high, the specific neural mechanisms by which cannabis impairs driving ability remain poorly understood, and legal thresholds for impairment are controversial. The researchers aimed to map how cannabis alters brain networks involved in a visuo-motor tracking task and to correlate these neural changes with behavioral performance and toxicological data. The study employed a double-blind, placebo-controlled, cross-over design involving 31 healthy male occasional cannabis smokers. Participants underwent functional magnetic resonance imaging (fMRI) while performing a pursuit tracking task, which required them to keep a moving target centered using a joystick. This task was designed to simulate the cognitive demands of driving. The experimental protocol included smoking either a high-potency cannabis joint (11% THC) or a placebo, with fMRI scans conducted before and after smoking. Psychomotor skills were further assessed using a Critical Tracking Task (CTT) outside the scanner. Toxicological analyses measured whole-blood concentrations of THC and its metabolites, while subjective effects were recorded via questionnaires. The results demonstrated that cannabis smoking significantly impaired psychomotor skills, as evidenced by decreased performance in both the CTT and the fMRI tracking task. Neuroimaging revealed that cannabis altered the activity of specific brain networks. There was a relative decrease in Blood Oxygen Level Dependent (BOLD) response in the anterior insula, dorsomedial thalamus, and striatum, suggesting a disruption in the saliency detection network. Additionally, reduced activity in the right superior parietal cortex and dorsolateral prefrontal cortex indicated impaired executive control. Conversely, cannabis increased activity in the rostral anterior cingulate cortex and ventromedial prefrontal cortices, reflecting a shift toward self-oriented mental activity. These neural changes correlated with subjective feelings of confusion rather than with blood THC levels. Notably, significant impairments persisted even when blood THC concentrations were low (median 9.3 ng/ml during fMRI), well below levels often considered legally significant. The findings suggest that cannabis impairs driving ability by disrupting the brain networks responsible for detecting salient environmental cues and executing cognitive control, while simultaneously increasing internal, self-focused processing. This leads to insufficient allocation of resources to task-oriented performance. Because these impairments occur at low blood concentrations and correlate with subjective confusion rather than specific toxicological thresholds, the study supports the adoption of zero-tolerance policies for driving under the influence of cannabis. The results highlight the limitations of using blood THC levels alone to determine fitness to drive, emphasizing the need for stricter regulatory approaches to ensure road safety.

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• cannabis