Psilocybin shifted brain-wave patterns in 25 healthy volunteers in a crossover trial

The question

How does a single dose of psilocybin change electrical activity across the brain, and do those changes line up with what people report experiencing?

What they did

Twenty-five healthy adults (18 men, 7 women, average age 24.44 years) took part in a double-blind, randomized, placebo-controlled crossover study, meaning each person had two sessions: one with psilocybin (10-20 mg by mouth) and one with a placebo, without knowing which was which. Ten-minute resting EEG recordings, which measure the brain’s electrical rhythms through scalp sensors, were taken at the start and again when drug effects peaked. The researchers looked at the strength of different brain-wave frequencies and at connectivity, meaning how much separate brain regions acted in step with each other. They focused on the default-mode network (a set of regions active during inward-focused rest) and on frontal and parietal areas, and asked whether baseline EEG could predict the subjective experience.

What they found

Compared with placebo, psilocybin reduced the power of slower brain waves (theta and alpha) and increased the power of faster ones (beta, gamma1 and gamma2). Connectivity increased within the default-mode network and within a localized parietal network under psilocybin. Scores on the Altered States of Consciousness Questionnaire, a self-report measure of the psychedelic experience, correlated positively with these changes in EEG power and connectivity. The authors also report that baseline EEG features had predictive value for the subjective changes people reported.

The fine print

This was a small study of 25 young healthy volunteers, so the findings may not extend to people with mental health conditions or to older adults. The outcomes were brain recordings and same-day subjective ratings, not clinical improvement, so nothing here shows a treatment benefit. Blinding is hard to maintain with psilocybin, since effects are noticeable. Correlations do not establish that the EEG changes cause the experience, and the abstract does not give the size of the correlations or the predictive accuracy. This summary is based on the abstract alone.

What it means

Psilocybin produced a consistent shift in resting brain rhythms and in how brain regions synchronized, and these shifts tracked with how strongly people felt altered. The results add to the basic understanding of how the drug acts in the brain, but they do not yet show that EEG can guide or predict treatment outcomes.

Evidence grade: Moderate. A randomized, double-blind, placebo-controlled crossover trial, but with only 25 healthy participants and brain-activity measures rather than clinical outcomes.

This summary is based on the study’s published abstract.