Coherence

Coherence is a measure of functional coupling between brain regions — the degree to which their activity signals rise and fall together over time. When two regions are highly coherent, they oscillate in concert; when coherence is low, their signals vary more independently. The measure is used to infer how tightly different parts of the brain are coordinating, rather than simply noting that both are active.

Within this encyclopedia, coherence most often refers to the internal coherence of the default mode network (DMN): a set of midline structures — including the medial prefrontal cortex and posterior cingulate cortex — that are active during self-referential thought and the sense of a continuing self. Whether and how tightly these regions stay in step has become a central question in the neuroscience of altered states.

How it works · its role

Coherence is most commonly measured with electroencephalography (EEG), which records voltage oscillations across the scalp, or with functional MRI, which uses blood-oxygen-level changes as a proxy for neural activity. In EEG, coherence is computed between electrode pairs within specific frequency bands — theta, alpha, and gamma oscillations each reflect different functional modes. The fMRI equivalent, called functional connectivity, captures slower correlations in the BOLD signal.

Any change that reshapes the firing patterns of neurons within a network can alter coherence. Neurotransmitter tone, receptor activity, and neuromodulatory input all influence how synchronised a set of regions remains — which is why many psychoactive substances produce measurable coherence changes even when their primary targets are far from the regions being measured.

Relevance to substances & effects

Serotonergic psychedelics — psilocybin, LSD, DMT — consistently reduce internal coherence of the default mode network, particularly between the posterior cingulate cortex and other DMN hubs. This suppression co-occurs, in dose-dependent fashion, with reports of ego dissolution: the blurring or disappearance of the felt boundary between self and environment.

The correspondence is robust across multiple imaging studies. It does not, however, explain why a reduction in DMN coherence produces that shift in self-experience, or whether the network change drives the subjective effect, accompanies it, or is itself downstream of something else. A correlation between a signal and a report is not a mechanism.

Ketamine and other dissociatives disrupt coherence through different circuitry, altering thalamocortical synchrony rather than acting primarily at the DMN. Cannabis has been associated with coherence changes across several networks, though findings vary with dose and are less consistent. MDMA produces a different coherence signature — consistent with its distinct subjective profile and different primary receptor targets.