Neural correlates of modified state of consciousness induced by hypnosis using EEG

Background: Hypnotic state has been shown to be of clinical utility, however its neural mechanisms still remain unclear. This study investigated the underlying neural basis of hypnosis using resting state EEG connectivity measurements.
Method: Ten healthy subjects (7 females, mean age 24±3years) underwent high density EEG recordings in resting state and hypnotic state. The hypnotic instruction involved a 3-min induction procedure with muscle relaxation and eye fixation. The EEG data was preprocessed using connectivity approach, both hypothesis and data driven analysis. Classical frequency component analysis was performed for beta-2 (20.1-30Hz), beta-1 (12.1-20Hz), alpha (8.1-12Hz), theta (4.1-8Hz) and delta (1-4Hz) frequency bands. We then assessed connectivity between every pair of electrodes using weighted Phase Lag Index. Hypothesis-based, fronto-parietal and midline connectivity was computed for frontal, parietal and midline regions. Data-driven graph theory connectivity was carried out to measure brain connectivity network properties and altered hub regions.
Results: During hypnosis, increased spectral power were observed in delta frequency and decreases were noted in the beta and alpha frequency bands. From hypothesis based connectivity, we observed increased connectivity in interhemispheric frontal in the delta band. Decreased connectivity was measured in the beta and alpha frequency bands in midline (upper central with lower central), right frontal with ‘right parietal and upper central’. Graph theory measures showed differences between hypnotic state and resting state both at the global and local level. Through integrated nodal clustering coefficient, we found increased frontoparietal connectivity in the delta and theta bands, and decreased bilateral frontal and parietal connectivity in alpha and beta-2 frequency bands.
Conclusions: During hypnosis, we found increased connectivity in lower frequency range (i.e., delta) and decrease in higher frequency (i.e., beta) in frontal and parietal regions. These oscillations characterise modified state of consciousness induced by hypnosis. This possibly reflects states of efficient cognitive-processing and positive-emotional experiences.