Divergent effects of ketamine and psilocybin on EEG power spectral density in a mismatch negativity paradigm

Ketamine and psilocybin have demonstrated therapeutic potential for mental disorders, including major depressive disorder, yet they engage distinct mechanisms of action. Ketamine, a dissociative hallucinogen, acts by blocking N-methyl-D-aspartate receptors (NMDAR), whereas psilocybin primarily targets serotonin receptors. These divergent mechanisms are reflected in their electrophysiological biomarkers.

This study aimed to investigate the divergent effects of ketamine and psilocybin on different elements of the EEG frequency spectrum, focusing on aperiodic components as an indicator of excitation-inhibition balance in the neural circuitry.

We re-analyzed a previously acquired EEG dataset from healthy volunteers using a placebo-controlled within-subject crossover design (Schmidt et al., Neuropsychopharmacology 37(4):865-875 2012). Participants received either placebo or S-ketamine (N=19) and placebo or psilocybin (N=16) during an auditory roving paradigm. Spectral parameters including periodic and aperiodic were extracted and partial least squares analysis was employed.

Ketamine significantly altered the offset and slope of the EEG spectrum, suggesting a disruption in excitatory-inhibitory balance. While both drugs commonly reduced alpha power in similar regions, beta band activity was decreased exclusively under ketamine.

These findings highlight ketamine's unique effects on aperiodic EEG components, reinforcing its role as a neurochemical model of prodromal psychosis. Psilocybin's effects appear distinct, emphasizing its targeted influence on oscillatory activity.