Brain mechanisms of hypnotic paralysis

THE term 'hypnosis' was coined by the Scottish physician James Braid in his 1853 book Neurypnology. Braid defined hypnosis as "a peculiar condition of the nervous system, induced by a fixed and abstracted attention of the mental and visual eye". He argued that it was a form of "nervous sleep", and tried to distinguish his theory from that of the mesmerists, who believed that the effects of hypnosis were mediated by a vital force, or animal magnetism.

Because of mesmerism, and its association with stage entertainment and charlatanry, hypnosis was regarded with skepticism for much of its history. In recent years, though, it has come under the scrutiny of cognitive neuroscientists, and is now thought of as an altered state of consciousness - sometimes referred to as being trance-like - which is associated with increased suggestibility, enhanced imagery and reduced reality testing. We know that hypnosis can profoundly affect the mind and behaviour, so that thought processes and perceptions can be easily manipulated, but the underlying neural mechanisms are poorly understood.

According to a new study of the neural mechanisms of hypnosis-induced paralysis, Braid's definition was remarkably accurate. The study, published in the journal Neuron, demonstrates that hypnosis does indeed lead to increased activity in areas of the brain involved in attention, as well as in other areas involved in mental imagery and self-awareness. It can therefore exert control over bodily movements by enhancing mental representations of the self (or self-imagery) and focusing attention on them. 

Yann Cojan and his colleagues of the University of Geneva's Center for Neuroscience used functional neuroimaging to test whether the hypnotic suggestion of paralysis would activate motor inhibitory processes and, if so, whether these processes correspond to the processes responsible for such inhibition under non-hypnotic conditions. For the new study, they recruited 18 healthy volunteers, and asked them to perform a "go-no go" task while their brains were scanned by functional magnetic resonance imaging (fMRI).

The participants were first required to fixate on a cross which was shown for half a second. This was followed by a grayscale picture of either a left or a right hand; this was a cue shown to prepare them for an upcoming action. After an interval of 1-5 seconds, the hand changed colour: if it turned green, they had to respond, as quickly as possible, by pressing a button with the corresponding hand; if it turned red, they were to withhold the prepared movement and do nothing. This task was performed while the participants were either under hypnosis, and told that their left hand was paralyzed, or in a normal state. In a third condition (the control), 6 of the participants performed the task whilst feigning paralysis (acting "as if" they were unable to move the fingers of the left hand).

In the normal state, the participants carried out this task very accurately, responding correctly in the "go" and "no-go" conditions with both hands in more than 97% of the trials. Under hypnosis, they performed the task just as accurately with the right hand, but made no movements with their left hands. These behavioural data indicate that the hypnotic suggestion of paralysis had been successful, and that hypnosis did not affect performance of the task with the unaffected hand.

Because of the way in which the experiment was designed, the fMRI data allowed the researchers to test two hypotheses. First, they could test whether hypnotic suggestion of paralysis suppressed the intention to move, by analyzing brain activity during the preparatory interval, or whether it inhibited the movements themselves. Second, they could determine whether hypnotic paralysis involves the same inhibitory neural mechanisms as voluntary suppression of movement, by comparing the brain activity measured during the "go" and "no go" conditions under hypnosis and in the control trials in which participants feigned paralysis.

Preparing to perform a movement leads to activity in the motor cortex which is associated with planning to execute the necessary commands. (Planning to move one's right hand causes activation of the left motor cortex, and vice versa.) If hypnotic paralysis suppresses the intention to move, then one would expect this early motor cortical activity to be absent in all of the trials performed under hypnosis. But when the researchers examined the activity of the motor cortex during the interval after the preparation cue, they found no difference between the normal and the hypnotized states.

There was, however, a difference in the brain activity associated with actually executing the hand movements: during those trials involving movements of the right arm, activity in the left motor cortex was observed in participants in both the normal and hypnotized states. By contrast, in trials involving moving the left hand (the "left go" trials), right motor cortical was observed in participants in the normal state, but not those who were hypnotized. This is, of course, to be expected - the hypnotized participants did not actually move their left hands in any of the trials, so there was no corresponding brain activity.

But there should be a distinct activation pattern associated with hypnotic paralysis, so the researchers compared the global brain activity in the three experimental conditions. This showed that during the "left go" trials in hypnotized participants,  but not those in the normal state or controls, there was increased activity in areas of the prefrontal and parietal cortices, which are involved in executive control and attention, respectively. There was also increased activity in an area of the brain called the precuneus, which is known to be involved in mental imagery, and especially in representations of the self. In the controls, feigning paralysis of the left hand led to increased activity in the right inferior frontal gyrus, which is known to be involved in motor inhibition. However, hypnotic paralysis did not cause increased activity in this area, and so it seems not to occur as a result of inhibition of motor planning.

These findings suggest that hypnosis induces the control of movements by means of internal representations, which can be generated because of enhanced self imagery. Because of the enhanced self-monitoring and increased attention and suggestibility induced by hypnosis, these internal representations take control of the left hand, and prevent it from moving. This occurs because hypnotic paralysis causes a reconfiguration of activity in the centres of executive function, leading to changes in the functional connectivity between the premotor and motor cortices, such that there is reduced coupling between the areas involved in planning movements and those which execute them. This model provides a new framework for future studies of the neurobiological bases of hypnosis.

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Cojan, Y. et al (2009). The Brain under Self-Control: Modulation of Inhibitory and Monitoring Cortical Networks during Hypnotic Paralysis. Neuron 62: 862-875. DOI: 10.1016/j.neuron.2009.05.021.