How Psychedelics Change the Brain: The Default Mode Network

The Default Mode Network: Your Brain's Autopilot

The default mode network (DMN) is a set of interconnected brain regions that are most active when we are not focused on external tasks — during daydreaming, self-reflection, and mind-wandering. Key DMN structures include the medial prefrontal cortex, posterior cingulate cortex, and the angular gyrus.

The DMN is sometimes described as the brain's "narrator" — the network that creates our continuous sense of self, maintains our autobiographical memories, and generates the inner monologue that characterizes much of our waking experience.

What Happens to the DMN on Psychedelics?

Research pioneered by Robin Carhart-Harris at Imperial College London has revealed that psychedelics like psilocybin, LSD, and DMT produce a striking reduction in DMN activity.

Key Findings

Neuroimaging studies using fMRI and PET scans show:

30-50% reduction in DMN activity during peak psychedelic effects

Decreased connectivity within the DMN — the network becomes less organized

Increased connectivity between brain regions that normally don't communicate

The degree of DMN disruption correlates with the intensity of subjective effects

This pattern is sometimes described as the brain becoming "more entropic" — less constrained by habitual patterns and more open to novel connections.

The Entropic Brain Hypothesis

Carhart-Harris proposed the entropic brain hypothesis, which suggests that consciousness exists on a spectrum:

High entropy (psychedelic state) — More flexible, creative, less constrained thinking; reduced sense of self

Normal waking consciousness — Balanced entropy; functional self-model

Low entropy (rigid states) — Overly constrained thinking; characteristic of depression, OCD, addiction

Under this model, conditions like depression represent states of excessive order — the brain becomes trapped in rigid, repetitive patterns (rumination, negative self-talk). Psychedelics may help by temporarily increasing entropy, allowing the brain to break free from these patterns and establish new, healthier ones.

Beyond the DMN: Whole-Brain Changes

Psychedelics don't just affect the DMN. Whole-brain analyses reveal:

Increased Global Connectivity

During psychedelic states, brain regions that rarely communicate begin to interact. Visual cortex regions connect with auditory areas; emotional centers link with cognitive regions. This "hyperconnectivity" may underlie synesthesia and the novel perspectives reported by users.

Thalamic Gating

The thalamus normally acts as a filter, controlling what information reaches conscious awareness. Psychedelics appear to open this gate, allowing a flood of sensory and emotional information to reach higher brain areas. This may explain the heightened sensory experience and emotional intensity of psychedelic states.

Neuroplasticity

Studies show that psychedelics promote dendritic growth and synaptogenesis — the formation of new neural connections. Research from studies indexed on PsiHub indicates that a single dose of psilocybin can increase dendritic spine density by up to 10% within 24 hours.

Clinical Implications

Understanding these brain changes helps explain therapeutic outcomes:

Depression — DMN hyperactivity drives rumination; psychedelics break this cycle

Addiction — Rigid behavioral patterns may be disrupted by increased brain flexibility

End-of-life anxiety — Reduced ego-attachment may ease existential distress

PTSD — Enhanced emotional processing through reduced amygdala reactivity

The 5-HT2A Receptor Connection

All classic psychedelics share a common target: the serotonin 5-HT2A receptor. When activated, this receptor triggers intracellular signaling cascades that increase neural excitability and promote plasticity.

Explore detailed receptor binding data on PsiHub's substance profiles.

References

Carhart-Harris, R.L. et al. (2012). Neural correlates of the psychedelic state. PNAS.

Carhart-Harris, R.L. (2018). The entropic brain — revisited. Neuropharmacology.

Petri, G. et al. (2014). Homological scaffolds of brain functional networks. JRSI.

Browse neuroscience studies on PsiHub