THC, Confusion and Visual Tracking: What an fMRI Study Actually Linked

A controlled fMRI study linked greater feelings of confusion after THC with worse visual tracking. The findings help explain one laboratory performance effect, but they do not make either a brain image or a self-rating a stand-alone test of driving impairment.

What the volunteers actually did

Battistella and colleagues recruited 31 healthy male volunteers who used cannabis occasionally; 23 were included in the analysis. In crossover sessions, participants smoked cannabis or placebo and performed psychomotor tasks, including a visual-tracking task inside an MRI scanner.

During active tracking, participants tried to keep a cursor aligned with a moving target. Passive viewing and rest provided comparison conditions. The investigators also collected subjective ratings and measured cannabinoids in whole blood.

Tracking performance and confusion moved together

Following cannabis, the analyzed group spent less time tracking correctly and showed a larger gap between target and cursor than after placebo. Greater reported confusion correlated with both poorer tracking measures.

The imaging analyses identified changes in regions involved in externally directed control and in internally directed processing. These results were interpreted as altered allocation of attention during the task. That interpretation concerns task-related brain activity, not a direct reading of what any person was thinking.

Why a concentration cannot replace the experiment

Scanning was timed after the rapid initial distribution period following smoking. The study found that subjective confusion related more closely to the studied performance changes than did measured blood THC. This does not validate confusion ratings as a replacement for toxicology or establish that a particular blood concentration is harmless.

The evidence came from a small, selected group of young men performing constrained laboratory tasks. Actual driving involves additional demands, and the exclusions between recruitment and analysis matter when judging generalizability. Associations between brain signals, ratings and performance do not identify the cause of an unrelated person’s behavior.

A useful forensic application is to ask what independent evidence describes the person’s function at the relevant time. The broader distinction between a THC concentration and evidence of impairment remains necessary even when sophisticated imaging supplies a plausible mechanism.

Primary research

Weed or Wheel! fMRI, Behavioural, and Toxicological Investigations of How Cannabis Smoking Affects Skills Necessary for Driving. DOI: 10.1371/journal.pone.0052545.