Brain and Psychedelics

How Psychedelics Act on the Brain

Classic psychedelics (psilocybin, LSD, DMT, mescaline) primarily work by binding to serotonin 2A receptors (5-HT2A), which are densely concentrated in the cortex, the brain's outer layer responsible for higher-order thinking, perception, and integration of information. This binding appears to increase neural excitability and promote communication between brain regions that do not normally interact much, a phenomenon researchers describe as increased "entropy" or "complexity" in brain activity.

The Default Mode Network and Neuroplasticity

Two findings have captured particular attention. First, psychedelics tend to reduce the activity and internal coherence of the default mode network (DMN), a set of regions associated with self-referential thought and the narrative sense of identity. This disruption may correlate with the subjective experience of ego dissolution and the sense that habitual patterns of thinking have been loosened. Second, several psychedelic compounds appear to promote neuroplasticity, the growth of new dendritic connections between neurons. Animal studies have shown rapid increases in dendritic spine density after a single dose of psilocybin or DMT, and human imaging studies suggest that increased brain connectivity may persist for weeks after a session.

What Imaging Studies Reveal

Functional MRI and PET studies have provided increasingly detailed maps of how psychedelics alter brain dynamics. Under psychedelics, the brain appears to enter a more flexible, less hierarchically organized state. Communication between distant brain regions increases, while the dominance of well-worn neural pathways decreases. Some researchers describe this as a "reset" or "defragmentation" of brain networks, though such metaphors simplify a complex picture. The field is still working to connect these neural signatures to specific subjective experiences and therapeutic outcomes.