Do psychedelics make brain networks talk to each other more?
Psilocybin made many normally separate brain networks fluctuate together more, whereas MDMA, with similarly intense subjective effects, changed almost none of these links.
Source
The effects of psilocybin and MDMA on between-network resting state functional connectivity in healthy volunteers
Study at a glance
- Design
- Human experiment — New analysis of two within-subject placebo-controlled fMRI studies (IV psilocybin; oral MDMA), comparing between-network coupling of 13 template resting-state networks under drug vs placebo.
- N
- 15 volunteers in the psilocybin study; 13 analysed in the MDMA study after excluding 12 of the original 25 (low effect intensity or head motion).
- Population
- Healthy adult volunteers with previous experience of the drug
- Outcome
- Between-resting-state-network functional connectivity (regression coupling strength) under drug versus placebo
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The authors re-analysed resting-state fMRI from two earlier studies in which healthy volunteers were scanned once on a drug and once on placebo: 15 people given intravenous psilocybin and 13 given oral MDMA. They took thirteen standard brain networks (visual, auditory, sensorimotor, default mode, frontoparietal, attention and others) from a published template, extracted each network's activity time course, and measured how strongly every pair of networks was coupled. Drug and placebo coupling were compared pair by pair with correction for multiple comparisons.
What they found
Under psilocybin, dozens of network pairs showed stronger coupling, including links between the default mode network and visual or attention networks, and between higher-order frontoparietal networks. Coupling fell only between the three visual networks and the sensorimotor network. Under MDMA just one pair changed (an anterior default-mode/executive-control link, which also rose under psilocybin), even though both drugs were rated as similarly intense. The authors interpret this as a 'flattening' of the brain's attractor landscape, letting it move more freely between states.
The limits
What it doesn't show
The two drugs came from separate studies using different scanners, routes of administration and participants, so the authors stress this is not a formal head-to-head comparison. Head motion increased under both drugs and correlated with the coupling change for several psilocybin pairs, including all the visual-sensorimotor decreases, so those results may partly reflect movement. Samples were small, mostly male in the psilocybin study, and made up of experienced drug users. The attractor-landscape and consciousness interpretations are theoretical and were not directly tested.
Key terms
- Resting-state network (RSN)
- A set of brain regions whose activity rises and falls together while a person rests, such as the default mode network.
- Between-network functional connectivity
- How strongly the activity of one whole network correlates over time with the activity of another.
- Default mode network (DMN)
- Regions more active during introspection and mind-wandering, usually anti-correlated with externally focused attention networks.
- 5-HT2A receptor
- The serotonin receptor that classic psychedelics like psilocybin strongly stimulate; MDMA acts on it only weakly.
- Dual regression
- A method that uses group network maps to extract each person's time course for each network.
- False discovery rate (FDR)
- A correction that limits the expected proportion of false positives when many comparisons are tested.
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Quiz yourself
What was the overall effect of psilocybin on coupling between resting-state networks?
Common questions
Why include MDMA at all?
MDMA is also serotonergic and feels intense but is not a classic psychedelic, so it helps show which network changes are specific to psilocybin's deeper alteration of consciousness.
Could head movement explain the results?
Partly. People moved more on both drugs, and for some psilocybin network pairs the connectivity change tracked the extra motion, so those findings need caution even after motion was modelled.
Why were 12 MDMA participants dropped?
Eleven rated the drug's effects as weak, and removing them made subjective intensity comparable to psilocybin; one more was dropped for excessive head movement.
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