Spectral power, but not phase connectivity, decodes the subanaesthetic ketamine state from surface EEG: the role of between-subject transferability

Abstract

Subanaesthetic ketamine alters the content of consciousness while leaving responsiveness intact. We asked whether this state can be decoded from single eyes-closed EEG epochs, and how spectral power and phase-based connectivity compare when used as features. Re-analysing openly available 62-channel EEG from ten participants (awake versus subanaesthetic ketamine), we trained classifiers under leave-one-subject-out cross-validation to discriminate the two conditions from log band power, weighted phase-lag index (wPLI) connectivity, and their concatenation. Band power decoded the ketamine state above chance (balanced accuracy 0.71), whereas wPLI connectivity computed on the same epochs was at chance (0.47), and combining the feature sets did not improve on power alone. The dissociation held across three classifier families and across spatial montages, and was not explained by the dimensionality of the connectivity feature space. To identify its source, we decomposed the per-feature drug effect into components shared across subjects and subject-specific. The ketamine effect on connectivity was large within individuals but largely subject-specific, and therefore not transferable to held-out subjects (shared fraction 0.05), whereas the spectral effect was substantially shared across subjects (0.53); both feature classes carried comparable individual identity, so the asymmetry reflects transferability rather than fingerprint-likeness. The spectral signature was also recoverable from a sparse electrode subset, as a five-channel lateral montage performed as well as the full array. Undirected phase connectivity thus fails to decode subanaesthetic ketamine not through insufficient spatial sampling, but because the connectivity drug effect is subject-specific in form.