Neurophysiology of Hypnosis
This is a review article summarising studies using neuroimaging and electrophysiological techniques to investigate changes in brain physiology and activity when people enter an hypnotic state.
Positive Emission Topography Studies.
Increases and reductions in blood flow to various regions of the brain in response to hypnotic procedures have been observed. There is a problem in knowing whether these changes are a result of a hypnotic state per se or whether they are attributable to a general response to the hypnotic procedure. Measuring these changes and correlating them with other measurements of specific subjects, hypnotic susceptibility has been used to filter the changes which can be directly attributable to the hypnotic state from those which are more likely to attribute to the hypnotic procedure in general, such as the alteration in brain function in response to the volume of the therapist voice. In one study, increase in brain activity was observed in precuneus and meso-frontal brain areas. Reduced activity was observed in lateral fronto-parietal regions usually active in attention demanding tasks. However, there are many divergent findings between different studies and it seems there is still not a clear consensus on what it happening.
Similar strategies comparing hypnotic susceptibility with hypnosis induced EEG patters show highly hypnotisable subjects have less phase synchronisation in frontal brain areas as well as increased activity in the left hemisphere and over anterior regions of cortex. Also, highly hypnotisable subjects have more labile fronto-parietal networks and are thereby more responsive to hypnotic induction.
Highly hypnotisable subjects generate more theta power then low subjects but some studies suggest this is more likely due to a side effect of relaxation.
How can hypnosis modulate pain perception?
Studies have shown hypnosis as an adjunct to conscious intravenous sedation provides better preoperative pain and anxiety relief. Several studies have suggested this is partially due to selective deactivation of the anterior cingulate cortex in response to hypnotic suggestion. The decreased activity being associated with decreasing unpleasantness of a painful stimulus.
Hypnosis as a substitute for Hysteria.
Highly hypnotisable subjects have been shown to be more responsive to suggestions of induced hysteria.