Perception and modulation of pain in waking and hypnosis: functional significance of phase-ordered gamma oscillations.

Somatosensory event-related phase-ordered gamma oscillations (40-Hz) to electric painful standard stimuli under an odd-ball paradigm were analyzed in 13 high, 13 medium, and 12 low hypnotizable subjects during waking, hypnosis, and post-hypnosis conditions. During these conditions, subjects received a suggestion of Focused Analgesia to produce an obstructive hallucination of stimulus perception; a No-Analgesia treatment served as a control. After hypnosis, a post-hypnotic suggestion was given to draw waking subjects into a deep hypnosis with opened eyes. High hypnotizables, compared to medium and low ones, experienced significant pain and distress reductions for Focused Analgesia during hypnosis and, to a greater extent, during post-hypnosis condition. Correlational analysis of EEG sweeps of each individual revealed brief intervals of phase ordering of gamma patterns, preceding and following stimulus onset, lasting approximately six periods. High and medium hypnotizable subjects showed significant reductions in phase-ordered gamma patterns for Focused Analgesia during hypnosis and post-hypnosis conditions; this effect was found, however, more pronounced in high hypnotizable subjects. Phase-ordered gamma scores over central scalp site predicted subject pain ratings across Waking-Pain and Waking-Analgesia conditions, while phase-ordered gamma scores over frontal scalp site predicted pain ratings during post-hypnosis analgesia condition. During waking conditions, this relationship was present in high, low and medium hypnotizable subjects and was independent of stimulus intensity measures. This relationship was unchanged by hypnosis induction in the low hypnotizable subjects, but not present in the high and medium ones during hypnosis, suggesting that hypnosis interferes with phase-ordered gamma and pain relationship.