The largest study yet into the effects of cannabis on the brains of 18- to 36-year-olds has found that the drug can reduce brain function during cognitive tests. The findings will help people to make an informed decision about the pros and cons of using cannabis.
Archived version: https://web.archive.org/web/20250130042220/https://newatlas.com/brain/young-adult-cannabis-brain-function/
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I haven’t read the full study, but as the article says the effects lingered for 2-4 weeks, it sounds like the retesting after abstaining would eliminate confounding variables. Unless they somehow resolved the stress or ADHD at the same time.
Seems as though they didn’t retest, they only scanned people once. Scanning people is expensive and getting people to show back up for a second scan is difficult. The abstention periods are likely based on self-reports.