Affective-state biomarker

A measurable neural signal that tracks a clinically meaningful affective or symptom state — low mood, anxiety, anhedonia, obsessional urge — with enough specificity and temporal resolution to gate closed-loop stimulation. Candidate readouts include intracranial spectral power (for example limbic gamma or theta), inter-regional coherence and connectivity, or a decoded latent state from a dynamical model.

To be useful for control a biomarker must satisfy demanding criteria: it should be state-linked (moving with symptoms) rather than merely trait-linked, robust to daily non-stationarity and to stimulation artifact, and ideally causal, meaning that deliberately moving the biomarker moves the symptom. The central open problem of the field is that no single generalizable affective biomarker exists; validated examples so far are personalized, discovered per patient rather than shared across a population.