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What is the science behind hypnosis?

This question originally appeared on Quora. Answer by Colin Gerber, Neuroscientist.

There has been, and still is, a lot of controversy about the validity of hypnosis. However, more and more research has been coming out in support of hypnosis existing and actually affecting brain function to an extent.

Who Does It Affect?

People are often broken into two groups when doing hypnosis research: High hypnotizable people and non-hypnotizable people. As most people have heard, hypnosis does not work on everybody. But why is that? Well research shows that high hypnotizable people actually have structural differences in their brains.

A 2004 study conducted by researchers at the University of Virginia revealed that subjects prone to be hypnotized actually had structural differences within their brains. In the study, highly hypnotizable subjects, on average, sported a 31.8 percent larger rostrum, a part of the brain involved in the allocation of attention and transfer of information between prefrontal cortices.

It would seem that becoming hypnotized has had at least something to do with the rostrum. So, people that are more susceptible to hypnotism have small differences in brain structure. That does not necessarily mean that hypnotism actually affects the brain.

Does hypnotism affect brain activity?

There have been several studies that have shown affects on brain activity after hypnotism. Mendelsohn et al. studied this through giving people posthypnotic amnesia (PHA). This basically means that the hypnotist told people to forget a certain thing after they are hypnotized until they hear a recall trigger word, which lets them remember it again.

In the study they took a group of people that were susceptible to PHA and a group that was not and had them both watch a movie. A week later they tested their recall on the movie after receiving PHA. The group what was susceptible to PHA scored much lower on the test than the non-PHA group. They also found that functional magnetic resonance imaging (fMRI) showed high levels of activity in areas responsible for visualizing scenes (the occipital lobes) and for analyzing verbally presented scenarios (the left temporal lobe). In stark contrast, when people in the PHA group performed the recognition task and failed to remember the content of the movie, fMRI showed little or no activity in these areas.

What is especially interesting about this study is after the PHA was canceled with the recall trigger the PHA group preformed just as well as the non-PHA group and their fMRI's showed similar results. Another fMRI study looked at hypnosis as well and found supporting reports here Scans Show How Hypnosis Affects Brain Activity.

There is science that backs up hypnosis and it seems that people who are more susceptible to hypnosis have enlarged rostrums. There is still a lot of work going on to figure out all of the details of hypnosis.

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