Hypnotic induction decreases anterior default mode activity
Introduction
Our purpose in designing this study was to establish whether the induction of hypnosis produces a unique hypnotic state (Lynn, Kirsch, & Hallquist, 2008) and, if so, to identify its neural correlates. There have been several brain imaging studies on hypnosis, but these have not contributed consistent results, and the neurobiological correlates of the hypnotic induction per se have not been reliably identified (Oakley, 2008).
Inconsistency between findings might potentially be accounted for by methodological differences between the studies (Oakley, 2008). In many studies, the hypnotic induction has been confounded with the administration of specific hypnotic suggestions, so that brain activation following a hypnotic suggestion (i.e., a suggested change in experience given after the induction of hypnosis) is compared to activation without either the induction of hypnosis or the suggestion (Faymonville et al., 2000, Grond et al., 1995, Maquet et al., 1999). The experimental design adopted in these brain imaging experiments does not allow a clear distinction between differences in brain activation that might arise from the induction of hypnosis and those due to task-related suggestion.
A better strategy might be to hold suggestion and other task demands constant, so that the only difference would be the presence/absence of hypnotic induction. This strategy has been adopted in a few published studies (Egner et al., 2005, Rainville et al., 2002, Rainville et al., 1999). Even in these studies, however, the design was such that the effects of hypnosis on brain physiology per se could not be determined. For example, participants in the Egner et al. (2005) study engaged in a Stroop task, and those in the Rainville et al., 1999, Rainville et al., 2002 studies had their hand immersed in warm or painfully hot water. These studies reported modulation of activity in the anterior cingulate cortex due to hypnosis, but they included concomitant tasks (Stroop task/pain) which are also known to involve this area. The differences in brain activity in hypnotic and non-hypnotic conditions might, therefore, be task specific. In addition, in most studies the participants were aware of the purpose of the experiment and what was required of them, which might have influenced brain activity and produced changes that are not specific to hypnosis. Our alternative approach was to scan participants during rest periods following the induction procedure, while they were not performing any specific task and were unaware that assessment had begun. This method minimizes the confounding effects of task, demand characteristics, and performance expectations.
The pattern of spontaneous physiological brain activity that is normally detectable during normal resting state is referred to as the ‘default mode’ network of brain function (Raichle et al., 2001). Areas collectively activated during the default mode state involve a set of midline brain structures, including the anterior cingulate, ventral and dorsal medial prefrontal cortex, posterior cingulate and precuneus (Fox and Raichle, 2007, Mason et al., 2007, Raichle et al., 2001). Oakley and Halligan (2009) have suggested that a deviation from the normal default mode activity might provide a neural signature of hypnosis. In the present study, we examined whether any changes occurred to the standard pattern of brain activity during rest after a hypnotic induction.
A group of low suggestible participants (i.e. people who do not respond to hypnotic suggestions) were also included to see whether similar alterations to the pattern of spontaneous physiological brain activity occurs in both high and low suggestible people. Changes that are not specific to hypnosis should be found in both groups, whereas changes that are specific to hypnosis should be found only in people who are responsive to hypnosis.
In this study, periods of scanning while participants were resting were alternated with periods of passive and active viewing, which participants had been led to believe were the focus of the experiment. Brain activity during these conditions was recorded following a hypnotic induction and also without the induction of hypnosis. By comparing scans acquired during rest periods between hypnosis/non hypnosis runs within subjects in the first level analysis, this experimental procedure minimized the influence of demand characteristics, concurrent tasks, hypnotic suggestions, and performance expectations on brain activity. Instructions were worded in a way that participants had no awareness that scans collected during these resting periods would be used in data analyses. For the passive condition they had to perform a relatively undemanding passive visual perception task (e.g. look at a complex colour or greyscale pattern); for the active condition they had to perform a demanding active visual hallucination task (e.g. draining colour from the colour pattern or adding colour to the greyscale pattern). The aim of this fMRI study was to discover if alterations in the pattern of spontaneous physiological brain activity during rest occur in high and/or low suggestible participants once hypnosis is induced when compared to rest activity out of hypnosis.
Section snippets
Participants
Two hundred sixty three potential participants were screened for hypnotic suggestibility on a modified version of the Carleton University Responsiveness to Suggestion Scale (CURSS) (Comey and Kirsch, 1999, Spanos et al., 1983). The CURSS is a widely used group scale for assessing hypnotic suggestibility. It consists of a hypnotic induction followed by seven suggestions requesting ideomotor movements, movement inhibition, and alterations of perception and memory. Participants receive one point
Results
During data acquisition, one of the high suggestible participants had excessive movement and was, therefore, excluded from the analyses. This left the high suggestible group with 7 females and 3 males between the ages of 20 and 53 (mean 25.00, SD 10.32). The low suggestible group had 5 females and 2 males between the ages of 20 and 35 (mean 26.86, SD 6.54).
Discussion
This fMRI study of resting brain activity showed that brain activity decreased significantly in the anterior part of the ‘default mode’ network (prefrontal cortex) in high suggestible participants when hypnotized. No areas had significant increases in activation in this group when in hypnosis. Reduction of spontaneous brain activation in the dorsal and ventromedial prefrontal cortex was not observed in low suggestible people after the hypnotic induction, nor was any area of increased activation
Acknowledgments
The authors thank L. Nocetti, P. Nichelli, L. Foan and K. Roberts for their various contributions to this study. This study was partially funded by grants from MIUR to AV and from the BBC to IK and GM.
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