This is an article worth starting with a disclaimer: at the present time, science does not have clear understanding of why some people can be hypnosized and what are the exact neurological mechanisms causing the state of hypnosis, and therefore this article will not answer the question posed in the title. All I can do here is to point to some interesting scientific data gathered over the recent years. These data provide some clues in regards to what the actual mechanism of hypnosis might be, but the subject definitely requires much more serious investigation.
What surprised me the most while researching this article is the scarcity of scientific data on the subject. There is a lot of psychological and semi-philosophical theories about what hypnosis is and how it works, but when it comes to solid experimental facts, the search of academic literature produces just a few useful hits.
I think that the study of hypnosis is one of those areas of science which most “serious” scientists shy away from. Because of its association with stage entertainment, the hypnosis was viewed with skepticism for very long time. The situation is similar with the studies of meditation, acupuncture or anything viewed as “alternative”, for that matter.
In addition to personal reputation issues for researchers with an interest in the subject, there is an issue of finance: not so many funding agencies are prepared to invest millions into such “grey areas” of science. The situation is changing, however. There was a number of interesting and informative recent research studies on hypnosis. The hope is that they are convincing enough to attract more researchers (and funds) into this field.
So what is hypnosis? A person in this trans-like state is far from being sleepy: quite opposite, he or she has heightened focus and concentration. Such state helps brain to control sensations and behavior, and this is why hypnosis can be used clinically in pain management and controlling phobias, anxiety and stress. That is, if you are hypnotizable. Many people, around one third of population, are not.
Brain signature of being hypnotized were first produced in 2012 using functional MRI. Areas of brain associated with executive control and attention were shown to be involved. More specifically, highly hypnotizable subjects demonstrate greater co-activation between components of the executive-control network (left dorsolateral prefrontal cortex) and the salience network (dorsal anterior cingulate cortex). In their brains, the two networks are activated in tandem. In people with low hypnotizability no such connectivity was observed.
This study points to the material basis of hyptotizability, but still doesn’t explain what happens in the brain of those who can be hypnotized.
In those who can be hypnotized the brain clearly works differently under hypnotic suggestion. Back in year 2000, well controlled experiments by Harvard scientists demonstrated that people under hypnosis can be made to see colors in black-and-white pictures or to not see them at all in brightly colored images. What put these experiments in a higher league is that researcher used fMRI to see which parts of brain get activated when hypnotized subjects were analyzing the colors. The color areas in both left and right hemispheres were activated when the subjects were asked to perceive colors.
The scientists concluded that hypnosis is indeed a distinct psychological state and certainly not a result of adopting a role. They emphasized, however, that experiments work only in “highly hypnotizable” subject, which is only around 8% of all people.
Another interesting observation from these experiments was the hemispheric differences between the hypnotized and non-hypnotized brain. When non-hypnotized subjects were asked to perceive colors from black-and-white picture, only right hemisphere was active. The left hemisphere became active only under hypnosis. This hemisphere deals with reason and logic, so it might need a bit of boost provided by hypnosis to dissociate itself from actual information received by senses.
Another study used positron-emission tomography (PET) to investigate cerebral blood flow during the hypnosis. The hypnotic state was associated with activation of several, mainly left-sided, cortical areas involving occipital, parietal, precentral, premotor, and ventrolateral prefrontal cortices, as well as some right-sided areas: occipital and anterior cingulate cortices. The pattern of activation shared many similarities with mental imagery, from which it differed by the relative deactivation of precuneus. Some scholars speculate that under hypnosis the subjects simply activate to a great extent the brain areas used in imagination, but without real perceptual changes.
Another functional MRI study demonstrates reduced activity in both anterior cingulate cortex and visual areas under hypnosis. The findings suggest that hypnosis affects cognitive control by modulating activity in specific brain areas, including early visual modules.
In several studies, hypnotizable subjects showed significantly more brain activity in the anterior cingulate gyrus compared to non-hypnotizable or weakly hypnotizable participants. The anterior cingulate gyrus responds to errors and evaluates emotional outcomes. Hypnotizable people also show much higher activity on the left side of the prefrontal cortex. Prefrontal cortex is associated with higher level cognitive processing and behavior.
Comparison of findings from different studies also points to rather contradictory results. Different areas of the brain appear to be activated in different experiments. This might be related to different experimental technique, both in terms equipment used and actual hypnotic approach adopted for the experiments. Future studies, hopefully, will provide a clearer picture of how hypnosis influences the brain.
What is clear from the existing studies is that hypnosis is really a specific psycholo