Individual differences in hypnotic suggestibility

Differences in suggestibility have been described for centuries, and must be explained by state and non-state theorists alike. State theorists generally account for these differences in terms of abilities or predispositions to enter an altered state of consciousness, whereas non-state theorists attribute such differences to socio-cognitive variables (e.g. beliefs, expectancies, cognitive strategies or imaginative abilities). Finding baseline individual differences in physiological response as a function of suggestibility has the potential of illuminating the trait-like character of hypnotic responsiveness. For example, Graffin et al. (1995) found indications of greater theta power in the frontal cortex during base-line in highly suggestible versus participants with a low level of suggestibility (see also Sabourin et al. 1990). Whereas EEG differences surfaced during baseline, no such differ-ences were apparent during hypnosis: the theta power of participants with both high and low levels of suggestibility increased in the posterior areas of the cortex, whereas alpha increased over all areas measured.

The baseline findings are consistent with both state and non-state theories. According to a non-state, social psychological model, subjects who have a low suggestibility are not necessarily lacking in the ability to respond to suggestions, but are instead more resistant or anxiously focused on the task than highly suggestible participants before hypnosis. In short, participants with high and low suggestibility are different subject populations that in turn exhibit different sets toward the task. In addition, non-state theorists have posited individual differences in the capacity for role involvement (Sarbin 1950) including fantasy proneness (Lynn and Rhue 1988), which may underlie differences in hypnotic responsive-ness. Kirsch (1991, p. 461) proposed ‘a personal factor that is not entirely determined by expectancy’ underlying individual differences in hypnotic suggestibility.

Studies have revealed differences in theta activity during hypnosis as a function of sug-gestibility across a wide range of sites (Graffin 1995; Ray et al. 1997; De Pascalis et al.

1998). These studies prompted Killeen and Nash (2003) to comment that theta activity

‘… seems to be one of the rare individual differences that correlates with hypnotizability’

(p. 214). However, questions can be raised about the meaning of increased theta in highly suggestible subjects. As Graffin et al. stated, ‘Because our overall understanding of EEG theta activity is limited, it is difficult to state clearly whether the increase in theta activity seen during the actual hypnotic induction was related to depth of hypnotic trance or the accompanying relaxation and absorption, or was more related to the process of cogni-tively focusing on the instructions verbally presented’ (p. 128). Indeed, Williams and Gruzelier (2001) found effects only for a narrow sub-band within theta which they inter-pret as relaxation rather than concentration. Accordingly, it is problematic to use an index of brain activity to distinguish between state and non-state accounts of hypnosis.

A similar issue crops up in studies of hemispheric asymmetries. Horton and Crawford (2004) concluded that highly suggestible participants often display greater EEG hemi-spheric asymmetries and hemihemi-spheric specificity for tasks than subjects with low sug-gestibility (see also De Pascalis and Palumbo 1986). The authors interpreted these findings as evidence for differences in interhemispheric communication, cognitive flexibility or

both. Although Horton and Crawford (2004) may be correct, a definitive interpretation is highly problematic insofar as frontal asymmetries are associated with a hodge-podge of variables (see Cacioppo 2004) including socio-economic status (Tomarken et al. 2004), basal cortisol levels (Kalin et al. 1998), immune function (e.g. Kang et al. 1991), self-report measures of affect and personality (e.g. Tomarken and Davidson 1994), shyness and social anxiety (Schmidt 1999), and memory for sad narratives (Nitschke et al. 2004). In any case, many of these variables are correlated with responses to suggestions. As Kirsch and Lynn (1995) noted, there ought to be correlations between brain activity and the performance of suggested activities, just as there should be brain activity correlates of the successful performance of any cognitive task. Accordingly, it is not altogether clear what hemispheric differences mean in terms of support for state versus non-state theories.

EEG asymmetry is not itself a mechanism, but rather a marker of underlying neural processes. The fact that responses to hypnotic suggestions are associated or correlated with EEG asymmetries or increased theta in highly suggestible individuals does not war-rant the conclusion that hypnosis causes the EEG patterns. Other variables that poten-tially co-vary with hypnotic responses and differences in suggestibility (e.g. expectancies, motivation, fantasy proneness, etc.; see Braffman and Kirsch 1995) may be responsible for the differences in asymmetries observed. As Hasewgawa and Jamieson (2002) have pointed out: (1) the functional significance of differences in neurophysiology are not clearly established; (2) the measurements only provide a limited picture of the brain’s functional organization; and (3) the extent to which local markers contribute to hypnotic experience is uncertain (p. 113).

The lack of consistency in findings is another problem in interpreting the physiological underpinnings of individual differences in suggestibility (Perlini and Spanos 1991;

Crawford and Gruzelier 1992; Ray 1997; Gruzelier 1998; De Pascalis 1999; Rainville et al.

1999b; Williams and Gruzelier 2001). For instance, Crawford (1990) found that asymme-tries of high theta were observed in highly suggestible subjects when attending to (left greater than right) and ignoring pain (right greater than left), whereas no such asymme-tries or differences between conditions were observed in participants with low sug-gestibility. DePascalis and Perrone (1996), in contrast to Crawford (1990), used a between-group design (waking, hypnosis with no analgesia and hypnosis with analgesia) to examine hemispheric asymmetries. Although the researchers did not assess theta in the temporal region, where Crawford identified hemispheric shifts, DePascalis and Perrone (1996) observed no differences in theta across multiple brain sites (mid-frontal, central and posterior regions). Whereas Graffin et al. (1995) found no individual differ-ences in theta activity during hypnosis, Sabourin et al. (1990) found increases in theta during hypnosis for highly suggestible subjects, but not for those with a low suggestibility.

Additionally, Sabourin et al. (1990) observed that theta increased during hypnosis across both hemispheres of the frontal, central, and occipital regions, but Graffin et al. observed theta increases only in the posterior regions of the brain. Horton and Crawford (2004) contend that research that encompasses functional magnetic resonance imaging (fMRI) studies (e.g. Crawford et al. 1998, Crawford et al. 1998, 2000), PET studies (Rainville et al.

1997; Maquet et al. 1999; Wik et al. 1999; Faymonville et al. 2000), regional blood flow

studies (Crawford et al. 1993) and electrophysiological research (e.g. Spiegel et al. 1989;

De Pascalis and Perrone 1996; Crawford et al. 1998a, b, d) points to a singular conclusion regarding hypnotic analgesia: ‘highly hypnotizable individuals have more physiological flexibility involving an active inhibitory process of supervisory executive control by the anterior frontal cortex …’ (pp. 137–138).

Horton and Crawford’s (2004) conclusion might be correct. However, it bears no rele-vance for the state–non-state issue, as all mind–body monists agree that there should be neurophysiological correlates of hypnotic suggestibility and of responses to particular suggestions (Kirsch and Lynn 1995), including the suggestion to experience an altered state (i.e. a hypnotic induction; see Wagstaff 1998). However, a key impediment to evalu-ating mechanisms proposed to account for the effects of hypnosis or particular sugges-tions is that the findings from which they are derived are, virtually without exception, based on comparisons between exceptionally good and exceptionally poor hypnotic sub-jects. We suspect that the ‘extreme group’ strategy that is typically used in the literature may be as limited in the domain of hypnosis as attempts to understand gifted children by comparing them with profoundly retarded children, or by comparing schizophrenic patients with the best adjusted, most highly functioning individuals in the population. To understand the neurophysiology of hypnosis, it is important to assess subjects with a medium level of suggestibility, as well as those with high and low suggestibility.

Even though we (Kirsch and Lynn 1995) raised the issue regarding difficulties in inter-preting comparisons between individuals who test at the extremes of suggestibility more than a decade ago, it has only recently been revived by psychophysiologically oriented researchers. More specifically, Horton and Crawford (2004) stated ‘it is crucial that future research into the neurophysiological correlates of hypnotizability include the highly hypnotizable virtuosos as well as moderate and low hypnotizable individuals’ (p. 142).

The inclusion of participants with a medium level of suggestibility is essential for at least three reasons tied to an ‘extreme group’ design. According to Preacher et al. (2005), extreme group designs have numerous disadvantages, including: (1) artificially increas-ing the variance, often resultincreas-ing in spuriously inflated effect sizes; (2) assumincreas-ing linearity when it may be absent (e.g. middle suggestible and highly suggestible subjects may show similar brain wave patterns); and (3) rendering interpretations of group differences ambiguous (e.g. are the obtained effects due to the high group being especially sug-gestible, the low group being especially non-suggestible or some mixture of both possi-bilities?). Accordingly, the failure to include participants with medium suggestibility could create the superficial appearance of a distinctly different hypnotic state when such a state is more apparent than ‘real’.

Subjects of low and high suggestibility may differ in ways that extend well beyond their responsiveness to hypnotic suggestions. Willingham and Dunn (2003) cautioned about conflating the process of interest in psychophysiological studies with some other vari-able. For example, if subjects with low suggestibility feel frustrated when thinking of the suggested situation, the activation (or lack of it) observed may be due to frustration.

Accordingly, the alterations in consciousness that people with low suggestibility experi-ence during hypnosis may have nothing to do with the hypnotic induction, the specific

suggestion (which they might not even attempt to experience) or with suggestibility per se, but may instead reflect performance-based concern, negative emotions (e.g. bore-dom) and lack of motivation. It follows that rather than reflecting a presence of an altered state or trance in the highly suggestible participants, any differences between per-sons with low and high hypnotizability may be attributable to the inability of those with low suggestibility to respond to basic task demands and attendant frustration. These dif-ferent cognitive and emotional correlates of hypnosis among the difdif-ferent sets of subjects surely have at least some corresponding neuropsychological correlates. As Meehl (1978) noted, a good deal of research in psychology suffers from an unwarranted assumption of the ‘ceteris parabis’ (all things being equal) clause, i.e. the assumption that one variable being manipulated by the experimenter is the only variable that is changing. This problematic assumption may apply to this body of research as well.

Horton et al. (2004) recently discovered differences in brain structure between partici-pants with high and low suggestibility, as revealed by structural MRI. More specifically, highly suggestible subjects, capable of eliminating pain perception in a cold-pressor task during hypnotic analgesia, had a significantly larger rostrum (a corpus callosum area) than participants with low suggestibility. Whereas these results could be taken to imply that highly suggestible subjects have unusually effective frontal attentional systems that account for their ability to experience hypnotic analgesia, it is difficult to interpret the findings in the absence of subjects with a medium level of suggestibilty. Specifically, it may be the case that the brains of those with low suggestibility rather than highly suggestible people are atypical.