New Insight into Neural Mechanisms of Hypnosis

by Jim Schnabel

August 14, 2009

In the eerie mental state known as hypnosis, people’s experiences seem to disconnect from their senses and their actions from their willpower, as their minds instead come under the control of the hypnotist’s suggestions. So powerful is this effect that before the advent of chemical anesthetics it was used widely to block surgical pain, and it is still used in applications from psychotherapy to weight control.

Yet in recent decades, few people have studied hypnosis, says researcher Yann Cojan of the University of Geneva, so “it is very difficult to have a clear hypothesis on its neural underpinnings.” Most recent research in this small field, he notes, has examined hypnosis’s effects on pain.

In the June 25 issue of Neuron, however, Cojan and colleagues report on a functional magnetic resonance imaging (fMRI) experiment designed to shed some light on hypnosis’s effects on the human motor system. The researchers recruited eighteen subjects with strong susceptibility to hypnosis, confirmed with standard screening methods. They then put each subjects into an fMRI machine and had them perform a series of “go/no go” trials similar to those used to study inhibitory processes in the brain.

In each trial, a grayscale image of a left or a right hand, appearing on a computer screen, cued a subject to prepare to use that hand. After a brief but variable interval, the hand image would turn green or red—green being the go signal to press a button using the indicated hand, red being the no go signal to inhibit any motion with that hand. Twelve of the subjects did half of the trials while under hypnosis, with the suggestion that their left hand was “paralyzed.” A control group of six subjects did the trials under instructions to pretend the hand was paralyzed.

In key brain areas, the average fMRI pattern seen when hypnotic paralysis prevented subjects from responding to the go signal showed important differences from the patterns seen during ordinary, willed, no go inhibition or simulated paralysis, Cojan and his colleagues found.

Together with behavioral data on reaction times and other measures, the fMRI data suggests that even in the hypnotic state the brain was able to prepare as usual for motor actions, because the corresponding premotor and motor cortex regions seemed to activate normally, he says. But after the go command, the motor cortex seemed to become less connected to these motor intention circuits and more connected to a few other hotspots in the brain, especially in the visual cortex and in a parietal region known as the precuneus.

Previous research has suggested that the precuneus helps to mediate the imagination of oneself experiencing something and appears also to be involved in the sense of “agency”—the feeling that one is causing something to happen. To Cojan, the area’s apparent increase in activity and in motor-cortex connectivity during hypnotic paralysis hints that the precuneus is key in a neural handover process in which control of specific motor functions is passed from the ordinary “planning” circuits of the premotor cortex to an internalized visual representation of the hypnotist’s suggestion.

To Tobias Egner, a neuroscientist at Duke University who has worked in this area, the results overall suggest that “the effects of hypnosis do not simply reflect an exaggerated form of the typical neural processing underlying a particular task, but rather derive from some more radical reconfiguration of normal brain processes.”

“It’s an interesting paper, and I think it’s a step in the right direction,” says Amir Raz, a neuroscientist at McGill University in Montreal who also has used hypnosis experiments in his research. “I would contextualize it as an example of how attention can control action.”

In keeping with other research, the fMRI data reported by Cojan and colleagues suggest that key attention circuits in the prefrontal cortex were more active when subjects were under hypnosis, regardless of whether they were seeing the go or no go signal. Cojan says that these data are “likely to reflect more general attentional and motivational factors associated with enhanced focusing and monitoring during hypnosis.”

One improvement Raz suggests in the experimental protocol is to replace hypnotic suggestion with “post hypnotic” suggestion, which takes effect after the subject has returned to a more normal mental state. “When you’re imaging somebody who’s in the hypnotic state, it’s difficult to tease apart exactly what is the influence of the suggestion versus what is the influence of the entire [hypnotic] state,” he says.

But Raz adds that he’s pleased that this research is being conducted at all. “I’m really happy to see that there’s a growing openness in some of the best journals to the incorporation of hypnosis into cutting-edge scientific investigation.”