Invited Symposium: Neural Bases of Hypnosis


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A Working Model of the Neurophysiology of Hypnotic Relaxation

Contact Person: John H. Gruzelier (j.gruzelier@ic.ac.uk)


Neuropsychophysiological evidence is summarised concerning a top-down working model of the traditional hypnotic relaxation, which has evolved out of empirical and theoretical considerations from experiments carried out in the Charing Cross Laboratory of Cognitive Neuroscience. Hypnosis was defined operationally on the basis of the traditional relaxation procedure, and throughout there were behavioural challenges to assess depth of hypnosis and at the end questions about memory for the induction, a post hypnotic suggestion and subjective ratings. An integration of the results a decade ago (Gruzelier, 1988, 1990) was conceptualised around a three-stage model of the induction procedure:-

Stage I: The initial instructions of fixating on a small object and listening to the hypnostist's voice was posited to involve an attentional network including thalamocortical systems and parietofrontal connections with engagement of a left anterior focussed attention control system. This underpins the focussed, selective attention inherent in fixation and listening to the hypnotist's voice, processes which together require left hemispheric frontotemporal processing.

Stage II: This first stage is then replaced by eye closure, suggestions of fatigue at continued fixation, and tiredness together with deep relaxation. This sets in motion frontolimbic inhibitory processes underpinning the suspension of reality testing and critical evaluation, and the handing over of executive and planning functions to the hypnotist; the "letting go" component of the hypnotic induction.

Stage III: The third stage involves instructions of relaxed, passive imagery leading to a redistribution of functional activity and an augmentation of posterior cortical activity, particularly of the right hemisphere in high susceptibles. Simplifying the verbal content of the induction message may also facilitate right hemispheric processing as does emphasising past experience and emotion. In contrast the low susceptible fails to show engagement of left frontal attentional control mechanisms, or if there is focal attentional engagement fails to undergo the inhibitory, letting go process.

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Electrodermal Orienting, Habituation, Sensitisation and Tonic Reactivity.We first investigated the basic attentional process of orienting, which represents focussing of attention, and the process of habituation with stimulus repetition, which allows attention to be redirected, both of which involve modulation by the limbic system, in particular the amygdala and hippocampus (e.g., Pribram and McGuinness, 1975). We used a standardised tone orienting and habituation paradigm with tones interspersed with the hypnotic induction (Gruzelier and Brow, 1985). Subjects were first monitored to provide a baseline measure and to equate groups for individual differences in rate of habituation. Three control sessions all separated by four weeks consisted of a baseline tone series and either a story read by the hypnotist, or relaxing listening to a story for a period equivalent in length to the hypnotic induction prior to the introduction of the tones which were presented without any accompanying verbal message. The hypnosis condition was distinguished from the three control conditions through a higher incidence of both nonresponding and non-habituation, a bimodal distribution. High susceptibles showed a reduction in orienting and/or faster habituation with hypnosis, whereas low susceptibles showed retarded habituation with hypnosis. The facilitation of habituation with hypnosis was replicated in an experiment designed to compare hypnosis with simulating hypnosis in medium/high hypnotisables (Gruzelier et al, 1988). As before rate of habituation was faster with hypnosis in the susceptible subjects. In simulators habituation was slower.

Recordings from intracranial electrodes in epileptic patients have disclosed the importance of changes during hypnosis in limbic structures including the amygdala and hippocampus (Crasilneck et al, 1956; De Benedittis and Sironi, 1986, 1988). The amygdala has been shown to exert mainly excitatory influences on electrodermal orienting activity whereas the inhibitory action of the hippocampus facilitates the habituation of the orienting response with stimulus repetition (Gruzelier &Venables, 1972; Pribram & McGuinness, 1975). Neuroanatomically the influence of hypnosis on electrodermal orienting and habituation was compatible with the evidence of De Benedittis and Sironi (1988) of functional inhibition of the amygdala and activation of the hippocampus by hypnosis.

Electrocortical Event-Related Attentional Components and Frontal Inhibition. Alterations in attentional processing with the induction of hypnosis have been demonstrated with electrocortical procedures, and like electrodermal rates of habituation these changed from baseline to hypnosis in opposite directions for high and low susceptibles (Gruzelier, 1996). Cortical evoked potentials were measured to infrequent tones mixed with frequent tones in a standard P300 paradigm with particular interest in a negative going (N2a) attentional component. The difference between the wave to the infrequent target or deviant when subtracted from the frequent non-target or standard belongs to the class of phenomena termed MisMatch Negativity (MMN). This is thought to involve a preattentive sensory specific process generated in auditory cortex (superior temporal gyrus). Aside from bilateral temporal maxima it has a predominant single maximum over the frontal cortex suggestive of frontal involvement (Naatanen, 1992). Baseline measures were first recorded and these tones were repeated following the hypnotic induction, as in the electrodermal studies, and repeated a second time following an extended induction. High susceptibles showed a large magnitude MMN at baseline and a progressive reduction in MMN with each stage of the induction in keeping with frontal inhibition. By the later stage of the induction MMN was negligible in both the lateral frontal placements. Importantly opposite changes were manifested by the low susceptible group. Whereas at baseline their difference wave was absent, there was a progressive increase in MMN through the experiment, until in the last condition the magnitude of the difference wave was on a par with the results in high susceptibles at baseline, suggesting an increasing enhancement of attentional processing.

In summary, there was consistency between the electrocortical MMN and electrodermal measures in depicting opposite changes from baseline to hypnosis in susceptible and unsusceptible subjects. Congruent opposing effects on attention have also been found recently with a computerised vigilance task. High susceptibles showed an increase in omission errors and greater variability in RTs from baseline to hypnosis, while low susceptibles showed a reduction in errors and RT variance (Kallai et al,1998). Together these studies show that whereas susceptible subjects evinced inhibitory influences on attention with hypnosis, unsusceptible subjects improved attentional performance as the induction progressed.

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Fronto-Limbic Supervisory Attentional System.We went on to examine evidence of frontal inhibition in the context of a model of a supervisory attentional system which involves the frontal lobes and limbic system (Posner & Peterson, 1990; Shallice & Burgess, 1991). This system monitors ongoing activity and modulates behaviour in response to novelty, as in orienting, and when environmental stimuli convey conflicting information. Electrophysiological evidence has shown that following an erroneous response in a reaction time task there is a large negative going error detection wave at about 100msecs, referred to as error-related negativity, which is not elicited following correct responses (Falkenstein et al, 1990; Gehring et al, 1993). This negative wave is followed by a positive error evaluation wave which varies with a range of task-related factors and may represent context updating, error evaluation and adjustment of response strategies (Falkenstein et al, 1995). Utilising Stroop-like tasks with hypnosis susceptible subjects showed an increase in errors on incongruent trials - the Stroop interference effect - but no change in errors on congruent trials, whereas the performance of low susceptibles remained constant (Kaiser et al, 1997). Reaction times were not influenced by hypnosis in either group. Therefore in susceptibles there was a failure to inhibit the automatic response, in keeping with an inhibition of frontal attentional control. However, the large negative going error detection waves that were elicited were non-significantly larger in the susceptible than low susceptible group, and these waves were unaltered by hypnosis. In other words an error detection system which operates at an early and possibly pre-conscious stage of processing was not compromised by hypnosis; to wit the unconscious hidden observer. In contrast the positive going error evaluation wave following the error detection wave was reduced in amplitude with hypnosis in susceptible subjects. This was in keeping with inhibition of a frontal error evaluation process and was compatible with the behavioural data showing a higher error rate on incongruous trials in the medium/high susceptibles with hypnosis.

The error detection wave has been localised to a midline anterior cingulate generator (Dehaene et al, 1994), a promising candidate for involvement in hypnosis. The anterior cingulate performs executive functions which have been subdivided into affective and cognitive components (Devinsky et al, 1995). The cognitive executive component is involved in response selection in advance of any movement and in cognitively demanding information processing such as Stroop interference, localised by blood flow imaging and lesion studies to the anterior cingulate. The affective executive functions are involved in regulation of autonomic and endocrine functions, assessment of motivational context and significance of sensory stimuli and emotional valence. These are mediated through extensive connections with the amygdala and periaqueductal grey and autonomic brainstem nuclei. Our results have indicated that the monitoring of motor performance carried out by the cognitive executive component remained intact, for the error detection wave and RTs were unchanged by hypnosis. Rather the affect system involving connections with the rostral limbic system including the amygdala would appear to be unresponsive with hypnosis, as shown by the absence of the error evaluation wave and apparently motivational influences on performance. This interpretation is also in keeping with the reduced electrodermal orienting activity reflecting a reduction in excitatory modulatory influences of the amygdala. Dissociation between cognitive and affective anterior cingulate executive systems would explain the increase in the Stroop interference effect with hypnosis.

Left Anterior Inhibition. There is evidence that anterior inhibition may be laterally asymmetrical and biased towards the left hemisphere in hypnosis. This was disclosed by measuring right and left hemisphere processing times with a haptic object sorting task in two studies (Gruzelier et al, 1984). High susceptibles showed an increase in processing times with hypnosis specific to the right hand while low susceptibles showed no increase in processing. The increase in the processing times of the right hand (indexing the left hemisphere) correlated positively with the hypnotic susceptibility score. Left-sided inhibition of somatosensory functions was further replicated (Cikurel & Gruzelier, 1990) by examining haptic processing with an active-alert induction of Banyai & Hilgard (1976). Comparing induction procedures, the active-alert induction shared with the conventional procedure the slowing of left hemispheric processing but the active-alert induction was solely responsible for the improvement in right hemispheric processing.

Lateralised anterior inhibitory functions were further examined with neuropsychological tasks (Gruzelier & Warren, 1993). These included word and category fluency which are both left hemisphere tasks:- word fluency left frontal and category fluency left temporal. Design fluency was included to index right anterior processing. With hypnosis a patterned deficit was found in high susceptibles. Amongst the fluency tasks impairment was specific to the left prefrontal task, a result since replicated (Kallai et al 1998). This supported neurophysiological changes because task demands were similar.

Anterior disconnection with hypnosis in susceptible subjects was recently disclosed by Kaiser with EEG coherence. In high susceptibles with hypnosis there was a reduction in connectivity within the left prefrontal region - specifically between left lateral (FP1 and F7) and medial (F3 and FTC1) placements - whereas the opposite effect, namely an increase in connectivity, was found in low susceptibles. In baseline there was also a highly significant difference between susceptibility groups in the direction of greater left anterior connectivity in high than low susceptibles. Importantly the coherence result did not generalise to all bands but was restricted to high alpha activity which relates to cognitive as distinct from connative processing.

In summary there was further evidence of a selectivity of neurophysiological action of hypnosis shown through examination of anterior inhibitory influences:- 1) the dissociation between error detection and error evaluation waves; 2) the left lateralised influences on haptic processing and the improvement in right-sided processing that was specific to the active-alert induction; 3) the specificity within the left hemisphere for the effects on verbal fluency which were restricted to letter and not semantic designated categories; 4) the localisation of the changes in EEG coherence to within the left frontal lobe; 5) the restriction of the EEG coherence changes to the high alpha band.

These factors serve to introduce a note of caution in attempts to interpret changes in brain blood flow and metabolism in hypnosis which do not permit such fine grained interpretation, nor do they discriminate between facilitatory and inhibitory functional systems.

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Focal versus distributed influences of hypnosis have also been demonstrated in experiments investigating right hemispheric involvement in hypnosis. Firstly, in our original experiment on electrodermal orienting and habituation processes (Gruzelier and Brow, 1985) bilateral recording disclosed an asymmetry in the amplitude of orienting responses favouring the right hemisphere in hypnosis in high susceptibles whereas there was no reliable asymmetry in low susceptibles. This was in contrast to the baseline session where predominantly there was an asymmetry favouring the left hemisphere in high susceptibles and again no reliable asymmetry in low susceptibles.

An enhancement of right posterior functions was found in an experiment involving the divided visual field presentation of flashes requiring brightness judgements analysed with signal detection methodology (McCormack and Gruzelier, 1993). Perceptual sensitivity was enhanced with hypnosis compared with control sessions before and after hypnosis. In high susceptibles this was specific to the right hemisphere whereas in medium susceptibles this was bilateral and was of lower magnitude. An improvement in perceptual sensitivity according to signal detection theory is indicative of an increase in signal to noise ratio as will occur with a reduction in central levels of arousal. Analysis of cognitive bias showed that judgements were more conservative for the right hemisphere than the left for the group as a whole (p<0.05, providing no evidence for the possibility that the influence of hypnosis may be due to a shift in attitude, such as an adoption of a lax response criterion, leaving perceptual sensitivity unaffected. Thus the results which disclosed an enhancement of right posterior processing with hypnosis, showed that only in high susceptibles was this strictly lateralised. In medium susceptibles it was more widely distributed to include bilateral processing enhancement (and was of lesser magnitude). A similar conclusion was reached with the haptic sorting task (Cikurel and Gruzelier, 1989).

Turning to central and temporal regions, an electrophysiological study was performed measuring evoked potentials to tone probes presented simultaneously with the hypnotic induction and with a story read by the hypnotist. Event-related potentials in both conditions were referred to a baseline condition giving three conditions in all (Jutai et al, 1993). Comparing the story with the hypnosis condition there was a reversal of asymmetry in the N100 attentional component to favour the right hemisphere in susceptible subjects with hypnosis at the temporal location which did not extend to the lateral central (C3/4) placements.

In summary, the asymmetries in electrodermal orienting responses and the N100 potential indicate a shift in the balance of temporo-limbic processes to favour the right hemisphere in high susceptibles with hypnosis, as did the measure of visual sensory sensitivity indexing primary sensory cortex. Medium susceptibles shown bilateral involvement.

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This series of experiments has shown a number of reproducible changes in brain function which distinguished medium to high susceptibles after instructions of hypnosis from both their baseline state and from low susceptibles. The attempts were modest in scope and must be confined to the traditional hypnotic relaxation induction. Continuing support was provided for associations between hypnosis and :- 1) activation of anterior fronto-limbic inhibitory processes, 2) anterior inhibition or disconnection, either lateralised to left hemispheric regions or bilateral depending on the processes examined, 3) involvement of right temporoposterior processing, 4) evidence of superior attentional abilities in high susceptibles, 5) evidence of poor attentional abilities in unsusceptible subjects with progressive improvement through the induction, and 6) no evidence of right hemisphericity in the baseline state in susceptible subjects.

Across electrodermal, electrocortical and behavioural domains susceptible subjects evinced inhibitory influences on attention with hypnosis whereas unsusceptible subjects improved attentional performance as the induction progressed. Evidence was also found for bilateral alterations of function in medium susceptibles in situations where changes in high susceptibles were lateralised, suggesting more diffuse or distributed changes in medium susceptibles and more focal changes in high susceptibles. Together these results indicate the importance of stratifying groups into low, medium and high susceptibles.

In our neuropsychological translation of the traditional hypnotic induction, hypnosis was initiated by engaging anterior executive control systems. Aside from alterations in cortical functions along anterior-posterior and lateral axes, these will orchestrate top down changes influencing thalamic and brain stem mechanisms. Currently there is renewed interest in the electrophysiology of thalamocortical mechanisms in perceptual binding, conscious perception and altered states of awareness (Llinas & Pare, 1991; Singer, 1993; Baldeweg et al, 1998). Llinas et al (1994) have proposed that consciousness is a noncontinuous event determined by simultaneity of activity in specific thalmocortical nuclei which provide the content of experience, and the nonspecific diffuse thalamic projection system which provides the context and alertness. In this regard the anterior and posterior cingulate appear of particular promise in the top down control of thalamic activity relevant to hypnosis. Devinsky et al (1995) have remarked "One of the unique features of anterior cingulate cortex circuitry is its diverse thalamic afferents and consequent ability to sample inputs from more thalamic nuclei than any other cortical region. The ability to sample from a wide range of thalamic inputs may be crucial for its contributions to motor response selection functions" (p 280). The same could be said for connative functions and the limbic thalamus (Bentivoglio et al, 1993). Hypnosis research would benefit from examining the interplay between cortical and thalamo-cortical systems, for which the methodology of fast frequency EEG transients holds much promise.

In Llinas's model dreaming is regarded as a state of hyperattentiveness to intrinsic activity without the registration of sensory input, a state with an obvious affinity with hypnosis. This serves to acknowledge that the dream analogy remains appealing for aspects of the hypnotic experience. Consider Fuster's (1995) description of cognitive features of dreaming which include the altered sense of time and absence of temporality, the lack of guiding reality and critical judgement, the anchoring in personal experience, affective colouring, dissociation from sensory input and context. " The fragmented networks activated in the dream seem to lack the associative links to a time frame, anchored as they are in the present, without time tags and references." This could equally be a description of the hypnotic state as high susceptibles experience it.



This paper represents a shortened version of a report:- Gruzelier, J.H. A working model of the neurophysiology of hypnosis: A review of evidence. Contemporary Hypnosis, 15,3-21, 1998.

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Gruzelier, J; (1998). A Working Model of the Neurophysiology of Hypnotic Relaxation. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Invited Symposium. Available at URL http://www.mcmaster.ca/inabis98/woody/gruzelier0814/index.html
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