Hypnosis involves an amplification of focused attention either towards or away from an internal or external event (e.g., Hilgard, 1965, 1986; Krippner & Bindler, 1974). Since the nineteenth century hypnotically responsive persons commonly report profound physical relaxation (for exceptions, see Bânyai & Hilgard, 1976) and alterations in perception following a hypnotic induction. In this physically relaxed state, they report their experiences as being more involuntary and effortless (e.g., Bowers, 1982-83), yet, somewhat paradoxically, at the same time more intense and involving than in a nonhypnotic condition. Such paradoxical reports suggest a dissociation between awareness of attentional effort (perceived workload) and perceptual awarenesses. If we view hypnotizable persons as active and 'creative problem-solving agents' (Lynn & Sivec, 1992) who can draw upon their abilities (including absorption, imagery, giving up of reality testing and focused and sustained attention) during hypnosis, then the paradox is eliminated. Contrary to common conceptions in the clinical and experimental literature, recent EEG and cerebral metabolism research supports the view that hypnosis may take cognitive effort that demands further allocations of attention and disattention (Crawford, 1994a,b; Crawford & Gruzelier, 1992; Hilgard, 1986).
EEG DIFFERENCES BETWEEN LOW AND HIGHLY HYPNOTIZABLE PERSONS
In studies of EEG brain wave activity, a robust finding is that theta power (3 -7 Hz), hypothesized to be associated with focused attention (e.g., Schacter, 1977), is positively related to hypnotic susceptibility (e.g., Akpinar, Ulett & Itil, 1971;
Crawford, 1990; Galbraith, London, Leibovitz, Cooper & Hart, 1970; Graffin, Ray & Lundy, 1995; Sabourin, Cutcomb, Crawford & Pribram, 1990; Tebecis, Provins, Farnbach & Pentony, 1975; Ulett, Akpinar & Itil, 1972a,b; for review, see Crawford & Gruzelier, 1992). In a nonhypnotic state, highly hypnotizable persons (referred to as 'highs') are likely to generate more theta power than the low hypnotizable persons ('lows'). This is supportive of behavioral research that finds highs have greater extremely focused and sustained attentional abilities, as measured by the Tellegen Absorption Scale (e.g., Crawford, Brown & Moon, 1993; Tellegen & Atkinson 1974; for review, see Roche & McConkey 1990) or the Differential Attentional Processes Inventory (Crawford, Brown & Moon, 1993), and by performance measures involving attentional processing. Highs have shown superior performance on attentional tasks such as visual search (Wallace & Patterson, 1984), gestalt closure (Crawford, 1981; Wallace, 1990), reversible figures and visual illusions (e.g., Crawford, Brown & Moon, 1993, Wallace, 1986, 1988) and other attentional tasks (for review, see Crawford, 1994b).
As individuals enter into hypnosis, EEG theta power often increases, sometimes in both lows and highs. Highs continue to generate more theta than lows in various brain regions (e.g., Crawford, 1990; Graffin, Ray & Lundy, 1995; Sabourin et al., 1990). Sabourin et al. (1990) noted theta power increases in both hemispheres of frontal, central and occipital regions during hypnotic induction and a subsequent series of standardized hypnotic suggestions provided by the Stanford Hypnotic Susceptibility Scale, Form C (Weitzenhoffer & Hilgard, 1962). Graffin, Ray and Lundy (1995) reported that during an induction theta power increased in the posterior areas, while during a subsequent passive hypnotic condition theta decreased for highs. Within hypnosis, Crawford (1990) found highly hypnotizable persons generated significantly more high theta (5.5-7.5 Hz) than did lows at frontal, temporal, parietal and occipital regions. Highs showed asymmetrical EEG high theta power shifts, particularly in the temporal region, during cold pressor pain when focusing on pain (left hemisphere dominant) or experiencing hypnotic analgesia (right hemisphere dominant), suggesting differential involvement of possibly the hippocampal system from which theta may be generated, particularly during vigilant conditions (e.g., Crowne, Konow, Drake & Pribram, 1972; Michel, Lehmann, Henggeler & Brandeis, 1992).
The so-called '40-Hz band' is a high frequency, low amplitude EEG rhythm centered around 40 Hz that has been found to be a covariate of focused arousal (e.g., Sheer, 1976). It appears to be from localized cortical neurons that receive thalamic afferents (Steriade, Gloor, Llonas, Lopes da Silva & Mesulam, 1990) and has 'been taken to be indicative of a mechanism linking or temporally coordinating the distributed cortical representation of stimuli' (Barlow, 1993, p. 165). Akpiner, Ulett and Ital (1971) reported more 40-50 Hz-activity during nonhypnotic rest and reaction time tasks in highs than lows. De Pascalis and Penna (1990) found highs showed greater right-hemispheric 40-Hz production during hypnosis, while lows showed reduced activity in both hemispheres. In line with the hypothesis that highs become more deeply involved in their emotional states, highs, but not lows, showed greater 40-Hz density at both left and right parieto-occipito-temporal cortex junctions during emotional states compared to rest in both nonhypnotic (De Pascalis, Marucci, Penna & Pessa, 1987) and hypnotic (De Pascalis, Marucci & Penna, 1989) conditions. Using mean magnitude 40-Hz, Crawford, Clarke and Kitner-Triolo (1996) did not find differences between lows and highs during self-generated happy and sad emotions. Interestingly, Schnyer and Allen (1995) found highs who experienced recognition amnesia generated significantly more 40-Hz power in preinduction but not postinduction conditions than highs not experiencing recognition amnesia or lows.
High hypnotizable persons have a greater disposition for more sustained attention and deeper involvement. In addition, they appear to have greater cognitive flexibility, the ability to shift from one strategy to another and from one alternative state of consciousness to another (e.g., Crawford, 1989; Crawford & Allen, 1983; Crawford & Gruzelier, 1992). Similarly, at a neurophysiological level, highs often demonstrate greater EEG hemispheric specificity in hypnotic and nonhypnotic conditions.
MacLeod-Morgan and Lack (1982) noted highs shifted in EEG alpha power hemispheric dominance when performing analytical and nonanalytical tasks while lows did not. Greater hemispheric specificity in certain EEG frequency bands, in nonhypnosis and hypnosis conditions, among highs has been noted elsewhere (e.g., Crawford, 1989; Crawford, Clarke & Kitner-Triolo, 1996; De Pascalis & Palumbo, 1986; Meszaros & Banyai, 1978; Meszaros, Crawford, Szabo, Nagy-Kovacs & Revesz, 1989; Sabourin et al., 1990).
Hypnosis facilitates access to and involvement in emotional material and for this reason is often seen as a facilitator of hypnotherapy. Quite relevant to hypnotherapy, highs generally report more intense affect when viewing violent films (Crowson, Conroy & Chester, 1991) and experiencing positive and negative emotions (Crawford, 1989; Crawford, Clarke & Kitner-Triolo, 1996; Crawford, Kapelis & Harrison, 1995) during nonhypnotic conditions. During hypnosis, possibly due to greater focused attention and decreased generalized reality orientation, highs report enhanced intensity and vividness of emotionally laden imagery (e.g., Crawford, Clarke & Kitner-Triolo, 1996). This may help explain why hypnoprojective and abreactive techniques (e.g., Brown & Fromm, 1986; Watkins, 1993), often utilized in therapy to elicit, titrate and metabolize traumatic material, can be useful for some patients. Furthermore, it may help us understand why desensitization techniques are often facilitated by hypnosis.
At a neurophysiological level, when presented with emotional stimuli (Crawford, Kapelis & Harrison, 1995), or asked to generate emotional memories (Crawford, Clarke & Kitner-Triolo, 1996), highs show, respectively, greater visual field and
EEG hemispheric differences in both hypnotic and nonhypnotic conditions. Highs were significantly faster than lows in recognizing angry and happy affect in the discrimination of faces presented to the left or right visual field (Crawford, Kapelis & Harrison, 1995). For highs only, angry faces were identified faster when presented to the right (left visual field) than left (right visual field) hemispheres, while lows showed no significant asymmetries. During self-generated happy and sad emotions in hypnosis and nonhypnosis conditions, in comparison to lows, highs showed significantly greater hemispheric asymmetries (greater right than left) in the parietal region, in high theta, high alpha and beta activity between 16 and 25 Hz, all frequency bands that are associated with sustained attentional processing (Crawford, Clarke & Kitner-Triolo, 1996). Taken together, these two studies suggest that highs have more focused and sustained attention. Greater right parietal activity, as indicated by faster reaction times and more EEG activity, is suggestive of greater emotional arousal (e.g., Heller, 1993) and/or sustained attention among the highs.
Our work suggests that highly hypnotizable persons have more effective and flexible frontal attentional and inhibitory systems (Crawford 1994a,b; Crawford, Brown & Moon, 1993; Crawford & Gruzelier, 1992; Gruzelier & Warren, 1993). Consistent with the above discussed research showing a relationship between hypnotizability and sustained attentional processing, an intriguing neurochemical study by Spiegel and King (1992) suggests that frontal lobe activation is related to hypnotizability. In 26 male psychiatric inpatients and 7 normal male controls, levels of the dopamine metabolite homovanillic acid were assessed in the cerebrospinal fluid. While preliminary in nature, the results suggested that dopamine activity, possibly involving the frontal lobes, was necessary for hypnotic concentration.
Gruzelier and Brow (1985) found highs showed fewer orienting responses and increased habituation to relevant auditory clicks during hypnosis, suggesting increased activity in frontal inhibitory action (Gruzelier, 1990). Gruzelier and his colleagues (Gruzelier, 1990; Gruzelier, 1999; Gruzelier & Warren, 1993; for review, see Crawford & Gruzelier, 1992) proposed that during the hypnotic induction there is an engagement of the left frontal attentional system and then a significant decrease of left frontal involvement with a shift to other regions of the brain, dependent upon the hypnotic task involved. Our hypnotic analgesia work reviewed below also strongly implicates the active involvement of the frontal inhibitory processing system.
CEREBRAL METABOLISM DIFFERENCES BETWEEN LOW AND HIGHLY HYPNOTIZABLE PERSONS
Only recently have we been able to begin to explore cortical and subcortical processes during hypnosis with neuroimaging techniques such as regional cerebral blood flow (rCBF), positron emission tomography (PET), single photon emission computer tomography (SPECT) and functional Magnetic Resonance Imaging (fMRI).
Consistently, regional cerebral metabolism studies [unlike EEG studies reviewed above] have reported no waking differences between low and highly hypnotizable persons. A robust finding has been that highs show increases in cerebral metabolism in certain brain regions during hypnosis (for reviews, see Crawford, 1994a,b, 1996; Crawford & Gruzelier, 1992). This has been found in normally healthy (Crawford, Gur, Skolnick, Gur & Benson, 1993; De Benedittis & Longostreui, 1988; Meyer, Diehl, Ulrich & Meinig, 1989) and psychiatric (Walter, 1992; Halama, 1989, 1990) populations. Given that increased blood flow and metabolism may be associated with increased mental effort (Frith, 1991), these data suggest hypnosis may involve enhanced cognitive effort.
Among healthy individuals, De Benedittis and Longostreui (1988) found highs but not lows showed increases in brain metabolism during hypnosis. Using the xenon inhalation method, Crawford, Gur et al. (1993) found substantial increases in rCBF during hypnosis (rest; ischemic pain with and without suggested analgesia) in highs but not lows. During rest while reviewing past memories of a trip taken, fCBF enhancements in the anterior, parietal, temporal and temporo-posterior regions ranged from 13 to 28%, with the largest being in the bilateral temporal area in highs (unpublished data). Among hypnotically responsive individuals, Meyer et al. (1989) found global increases of rCBF in both hemispheres during hypnotically suggested arm levitation. An additional activation of the temporal centers was observed during acoustic attention. Under hypnotically narrowed consciousness focus, there was 'an unexplained deactivation of inferior temporal areas' (p. 48). Discussed in greater detail below, Crawford, Gur et al. (1993) found further rCBF enhancements of orbito-frontal and somatosensory regions during hypnotic analgesia among highs only.
Within a psychiatric population (16 neurotic, 1 epileptic) using SPECT, Halama (1989) reported a global blood flow increase during hypnosis, with those more deeply hypnotizable showing greater CBF increases than the less hypnotically responsive. During hypnosis 'a cortical ''frontalization,'' takes place particularly in the right hemisphere and in higher areas (7 cm above the meato-orbito-level) more than in the deeper ones (4 cm above the meato-orbital-level)' (p. 19). Frontal region increases included the gyrus frontal, medial and inferior, as well as the superior and precentral gyrus regions. These are suggestive of greater involvement of the frontal attentional system during hypnosis. By contrast, there was a significant decrease in brain metabolism in the left hemisphere in the gyrus temporalis and inferior region, as well as in Brodmann areas (BA) 39 and 40.
Hypnotic instructions (i.e., inductions and suggestions) trigger a process that alters brain functional organization, a process that is moderated by hypnotic susceptibility level. No longer can we hypothesize hypnosis to be a right-hemisphere task, a commonly espoused theory popular since the 1970s (e.g.,
Graham, 1977; MacLeod-Morgan, 1982). The studies reviewed here suggest that hypnosis is much more dynamic, activating differentially regions in either the left or right hemispheres, or both hemispheres dependent upon the attentional, perceptual and cognitive processes involved. Since pain management is perhaps the most dramatic and clinically useful application of hypnosis, the neurophysiological evidence for hypnotic analgesia effects are examined in greater detail in the following section.
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