The psychedelic state involves a breakdown in patterns of information flow in the brain. Using MEG recordings, researchers measured functional connectivity after administration of LSD, psilocybin, and low-dose ketamine, and compared them with the non-psychedelic drug tiagabine. All three psychedelics decreased directed functional connectivity (information flow) throughout the brain, as measured by Granger causality. For LSD, this decrease was coupled with an increase in undirected functional connectivity (correlation and coherence), a surprising opposite movement that highlights the importance of comparing multiple measures of functional connectivity in neuroimaging data.
LSD reduces the brain's automatic neural adaptation to familiar sounds and blunts the neural 'surprise' response to unexpected sounds. Using magnetoencephalography and dynamic causal modeling, the authors found that LSD modulates backward (top-down) connectivity within a fronto-temporal network and intrinsic connectivity in the primary auditory cortex. These patterns resemble those seen in schizophrenia, ketamine states, and reduced consciousness, suggesting that backward connectivity may reflect altered perceptual learning common to multiple altered states rather than a direct marker of conscious level. The findings challenge top-down hypotheses of consciousness and indicate that several altered states may share this biophysical mechanism.