Neural Bases of Hypnosis

Pierre Rainville
pierre-rainville@uiowa.edu

Dear Jean-Roch,

In the series of psychophysical and PET studies reported we used a tonic heat pain test consisting in the immersion of the hand in 45-47.5 Celsius water for 1min. The temperature of the water was adjusted individually to produce pain ratings between 40 and 60 (on a 0-100 scale). In the control stimulation conditions we used warm water (35degree). The main analyses of PET data is based on the subtraction method in which the control condition (warm) is subtracted from the target condition (pain). This procedure is performed in order to extract changes in rCBF that are specifically associated with the processes that distinguish these two conditions (pain), by cancelling out the effect of processes called upon in both conditions (position of the hand and body, mechanical contact of the hand in water, evaluation of the stimulus, ...). This allows to obtain brain volumes of pain-related activation that can be compared across different conditions, namely high vs low pain affect, and high vs low pain sensation.

The psychophysical studies were performed in groups of 20-24 subjects. Susceptibility ranged from low to high on the Stanford scale-A. The magnitude of pain modulation was correlated to susceptibility scores. In the PET experiments, we selected subjects from the psychophysical studies based on their ability to modulate pain consistently in response to the suggestions. In the PET study on pain affect modulation, we scanned 8 subjects showing the highest susceptibily scores (ranged from moderate to high), the greatest pain unpleasantness modulation, and the smallest changes in the perception of pain intensity. In the Pain sensation modulation experiment, 10 subjects were scanned that showed reliable changes in both pain intensity and unpleasantness ratings. These subjects presented a wider range of hypnotic susceptibility (from low to high). This allows the possibility of looking at the moderating effect of hypnotic susceptibility on hypnosis- and suggestions-related activity. Preliminary analyses suggest that the frontal activation associated with suggestions is positively correlated to hypnotic susceptibility. This correlation appears to be stronger in the right frontal lobe.

In regard to your question on pooling results across pain conditions, I am not exactly sure of what you mean. Let me just say that the pain-related activation analyses are performed by comparing pain-related activation in HIGH (this is pain-with-suggestion minus warm stimulation w/o suggestions) to pain-related activation in LOW suggestion conditons. The hypnotic induction-related changes reported are based on analyses pooling the warm and the pain stimulation conditions (all hypnosis warm and pain w/o pain modulation suggestions minus all control awake warm and pain stimulation). The general suggestion-related changes are based on analyses pooling the HIGH and the LOW suggestion conditions (all suggestions w/ pain stimulation minus pain stimulation w/o suggestion for pain modulation). Although there was stronger activation in the HIGH suggestion condition, the difference did not reach significance suggesting that both suggestion conditions contributed to the stong frontal increase in rCBF.


>The increase in occipital activation is often linked to imagery as you mention.  Did your induction procedures include specific imagery? Similarly what suggestions did you use to differentiate affect and sensory conditions?

We did NOT include instructions for visual imagery in our induction procedure (based on the induction procedure accompanying the Stanford scale), but some subjects did report spontaneous visual imagery. However, I think that a more basic process is at work here that might explain the occipital increase in rCBF. The subtraction analysis tells us that occipital rCBF is higher in the hypnotic state than in the awake control condition. In the upcoming paper in press in the Journal of Cognitive Neuroscience, we argue that this increase is in fact reflecting a release from an active suppression of activity in the awake condition. This might facilitate internally generated imaginative processes and reinterpretation of sensory messages.

The narrative of the suggestions for pain affect and pain sensation modulation will be included in a paper describing the psychophysical studies and submitted to Pain. In summary, the instruction for low pain affect suggested that "although you will still feel the burning and pricking sensation when you put your hand in the water, this sensation will be surprisingly more agreable than you might have expected, ... as you put your hand in the water and you feel the stimulus, a sensation of comfort and well-being will just sweep through your hand, will spread to your arm, to your whole body and all through your experience." In the Low Sensation condition, instructions suggested that "although you can feel warmth on your hand, the sensation will be much less than you expected, and has the warnth increase you can turn down the dial of your sensation, ... , hand may  become numb and you may not feel anything in your hand altogether." Suggestions for high pain affect and high pain sensation were based on the suggestions for low with the valence reversed.

I hope that these answers and comments are helpful,
Thank you for your interest,

Pierre