Psilocybin brain imaging — what the fMRI studies actually show (explained clearly)

The most striking finding in the imaging studies is 'entropy': under psilocybin, the brain shows more random, diverse connectivity patterns than in normal consciousness. Regions that normally operate in isolated networks start communicating with each other. This is measured by looking at the correlation patterns between different brain regions over time (functional connectivity). The visualization is usually a connectome map showing connection strengths between regions — under psilocybin, you see more connections and more cross-network communication.

The default mode network (DMN) findings are the most therapeutically relevant: the DMN shows reduced activity and reduced internal coherence under psilocybin. In depression, the DMN is typically hyperactive (overconnected internally, stuck in ruminative loops). The psilocybin disruption of this pattern is correlated with antidepressant response — and the imaging shows this very clearly. What we can conclude: psilocybin changes brain connectivity in ways that correlate with therapeutic outcomes. What we can't conclude: that this is the sole or sufficient mechanism.

Important caveat: correlation vs. causation is always the issue with imaging studies. We see brain changes that correlate with therapeutic outcomes, but this doesn't tell us whether the brain changes caused the outcomes, were caused by them, or both resulted from a third factor. The field is careful about this distinction even when the press isn't.