Functional MRI (fMRI) does not watch nerve cells fire. It tracks signals linked to blood flow and blood oxygen, which stand in indirectly for activity (Carhart-Harris et al., 2012). “Functional connectivity” describes how closely the signals from two regions rise and fall together, which is how studies talk about networks such as the default mode network.
Psychedelic studies use fMRI to see which brain networks change while a drug is active. A 2023 study of 20 volunteers combined EEG and fMRI during DMT and found global connectivity rose, in a pattern that correlated with a map of 5-HT2A receptor density (Timmermann et al., 2023). A 2026 mega-analysis of 11 fMRI datasets found that some popular claims, such as networks “dissolving”, were overstated (Girn et al., 2026).
There is a catch specific to these drugs: psychedelics also act on blood vessels, so a change in the blood signal is not always a change in neurons. Scans show what a brain does during an experience. They cannot by themselves show lasting change or benefit.
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