Hypnotic susceptibility, baseline attentional functioning, and the Stroop task

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Abstract

According to the theoretical framework relating hypnosis to attention, baseline attentional functioning in highly hypnotizable individuals should be more efficient than in low hypnotizable individuals. However, previous studies did not find differences in Stroop-like tasks in which the measure indicative of the Stroop interference effect was based on response latencies. This study was designed to determine whether subjects with different levels of hypnotic susceptibility show differences in baseline attentional functioning. To assess this hypothesis, high, medium, and low hypnotizable subjects performed a Stroop task designed to evaluate accuracy performance, before being subjected to hypnotic induction. Results showed that the Stroop interference effect was smaller in high hypnotizable subjects than in low hypnotizable subjects, whereas it was not different between high, and medium hypnotizable subjects. This outcome supports the notion that baseline attentional functioning is related to hypnotic susceptibility.

Introduction

The recent research on theoretical and experimental hypnosis underlines the study of cognitive and neurophysiological mechanisms related to hypnotic susceptibility as one of its major interests (see e.g., Raz & Shapiro, 2002; for a review). There is evidence indicating that the attentional system is involved in determining the responsiveness to the hypnotic induction. From an experimental point of view, this issue is studied by comparing the performance in attentional functioning between individuals who show different responsiveness to hypnosis, namely high, and low susceptible subjects.

In this framework, the neuropsychophysiological model (e.g., Crawford, 1994, Crawford and Gruzelier, 1992, Gruzelier, 1988, Gruzelier, 1998) emphasizes the stages in which the traditional hypnotic relaxation induction occurs. In the first stage, focused, and selective attention processes would be engaged to focus subject’s attention on hypnotist’s voice. A general decrease of attentional functioning is assumed to underlie the second and third stages of induction. This would lead to the suspension of critical evaluation and to the deep relaxation state that are peculiar to the hypnotic state. Thus, the neuropsychophysiological model posits that high hypnotizable (HH) subjects, compared to low hypnotizable (LH) subjects, should manifest (a) a higher performance in attentional functioning in the baseline state, and (b) a decrease of attentional functioning after the hypnotic induction. In other words, attentional processing by HH subjects compared to LH subjects should be more efficient in the baseline state and less efficient in the hypnotic state.

There are both neurophysiological and behavioral data supporting the decrease of attentional functioning after hypnotic induction. For example, a strong inhibition of the activity of the frontal cerebral regions has been shown (Kaiser, Barker, Haenschel, Baldeweg, & Gruzelier, 1997). This has been associated with the gradual loss by the subject of both the critical evaluation of reality and the awareness of motor volition typical of the hypnotic state. Later, two contemporary shifts of the general cerebral activation from left to right regions and from anterior to posterior areas take place, as supported by neuropsychological (Crawford and Allen, 1983, Gruzelier and Warren, 1993) and electrophysiological (Mészáros, Crawford, Szabó, Nagy-Kovács, & Résvész, 1989) evidence regarding measures of laterality (but see, for example, Jasiukaitis, Nouriani, Hugdahl, & Spiegel, 1997; for dissenting results). Given that frontal lobes are extensively involved in attentional networks (e.g., Posner & Raichle, 1997), these data strongly support the relevance of the attentional system for hypnotic susceptibility.

In addition, HH subjects during hypnosis score lower than LH subjects on a task measuring verbal fluency to letter-designated categories, whereas there is no difference in verbal fluency to semantically designated categories (Gruzelier and Warren, 1993, Kallio et al., 2001). The dissociation between letter fluency and semantic fluency is also seen in patients with lesions in the left dorsal lateral or medial frontal lobe. This suggests that an alteration of activity in these areas might affect processing when HH subjects are under hypnosis.

The clearest support to the notion that the attentional system of HH subjects functions less efficiently than that of LH subjects after hypnotic induction is provided by the performance on the Stroop task (Stroop, 1935; see MacLeod, 1991; for a review). In it, subjects are required to name the ink color of a colored word. In the case of incongruent color word (e.g., the word GREEN displayed in red ink), subjects usually make more errors and are slower with respect to both a congruent word (e.g., the word RED diplayed in red ink) and a control colored string (e.g., the string XXXX displayed in red ink). Attentional researchers refer to this phenomenon as the Stroop Interference Effect (SIE). The accounts of the SIE are based on the assumption that reading words is automatic and mandatory and that the processing of the word accesses the word’s meaning even if subjects are instructed to ignore it and attend only the word’s ink (but see Raz et al., 2002a, Raz et al., 2002b, Raz et al., 2003; for the modulation of the SIE with hypnotic suggestion). Basically, both the relevant (ink color) and irrelevant (meaning) information would be processed and would compete for the activation and emission of the response. In the case of an incongruent word, two competing responses are activated, thus generating errors and slowing of the performance. The Stroop task is considered a measure of the cognitive abilities that are fundamental for the selection of relevant information and the suppression of irrelevant information. Compared to LH subjects, HH subjects under hypnosis show an increase in the SIE as indexed by the number of errors (Kaiser et al., 1997, Nordby et al., 1999) and by the time difference in response latencies (Blum and Graef, 1971, Sheenan et al., 1988).

More controversial is whether at the baseline state, that is, out of hypnosis, HH subjects perform better than LH subjects in attentional tasks. Here, we will briefly review the works that investigated baseline attentional functioning, with specific attention to those studies, where Stroop-like tasks were employed. Kallio and et al. (2001) found no difference between HH and LH subjects in four neuropsychological tests assumed to assess frontal functions (Stroop task, verbal fluency tasks, reaction time tasks, and vigilance task). In the Stroop task that they developed, subjects were required to name aloud, on a handout, the color of control colored strings (Xes-strings) and incongruent words. The author measured the time required to name colors in the two conditions and found that HH and LH subjects performed very similarly in the baseline state. Similar results were also reported by Aikins and Ray (2001) that presented HH and LH subjects with different neuropsychological tests of executive functioning (word fluency to letter, Stroop test, Towers of Hanoi, Wisconsin Card Sorting test) in the baseline state. The authors found that HH subjects were better than LH subjects in the task-shifting ability only, as measured by the Wisconsing Card Sorting Task (Heaton, 1981). The Stroop task that they used was administered by a computer program that generated screens of 50 incongruent words each. The response time of five screens were recorded. No control or congruent conditions were provided, thus the performances referred to the incongruent condition latencies only. Again and consistently with Kallio et al.’s study (2001), no difference was found between HH and LH subjects in the baseline state.

Stroop-like tasks were also employed in two works by Dixon and collaborators (Dixon et al., 1990, Dixon and Laurence, 1992). In their first work, the authors employed a Stroop task introduced by Cheesman and Merikle (1986) in which visual words were presented with (a) different congruent-trial probability (the ratio between congruent and incongruent stimuli was either 25 or 75%), and (b) perceptually degraded letters that delayed the time necessary to recognize words. The degraded words were presented in black ink color while a colored rectangular appeared around the stimuli. Participants were requested to name the ink of the rectangular border around the words. HH subject had a larger SIE as measured by the difference in the reaction time of congruent and incongruent stimuli than both LH subjects and medium hypnotizable (MH) subjects. Such a finding was described as supporting the notion that HH subjects process words more automatically than LH subjects and was confirmed in a later study (Dixon & Laurence, 1992). However, given that the SIE was more pronounced in HH than in LH individuals, this finding seems to contradict the idea that baseline attentional functioning is more efficient in HH subjects.

To sum up, while there is evidence that a transient decrease of attentional functioning is related to high hypnotizability, it is still not clear whether HH subjects have a more efficient functioning of the attentional system at the baseline state. Recall that the neuropsychophysiological model (e.g., Crawford, 1994, Crawford and Gruzelier, 1992, Gruzelier, 1988, Gruzelier, 1998) predicts that HH subjects, compared to LH subjects, should manifest a higher performance in attentional functioning at baseline, which allows them to focus attention on instructions during hypnotic induction. We believe that among attentional mechanisms, the ability to select, and focus on the relevant information and to suppress irrelevant information is fundamental for complying with the instructions given during hypnotic induction. Therefore, it seems surprising that previous studies on the Stroop task did not find empirical support of the expected more efficient performance of HH subjects. A possible reason might rely on the kind of Stroop task used and on the measures that have been considered as indicative of the SIE. Even if very different Stroop-like tasks were so far used, all of them had focused on the time needed to perform the task, and there is evidence that other measures are, in some cases, more sensitive than reaction times for discriminating different populations. More specifically, accuracy performance as measured by the number of errors in the Stroop task have suggested, in some works, discriminating between frontal damaged patients, and normal control subjects, while reaction times have not (Swick and Jovanovic, 2002, Vendrell et al., 1995).

The main purpose of the present work was to investigate whether high susceptibility to hypnosis is related to a more efficient performance on the Stroop task in the baseline state. We decided to design a Stroop task along the following lines. In our view, the Stroop task proposed by Kallio et al. (2001) is the most suitable for studying the selection/suppression ability during a given period of time. In it, handouts with neutral (control) or incongruent stimuli were given, and the time required to name colors in the two conditions was recorded. In this Stroop task, the selection/suppression ability must be maintained over time, until all the stimuli present in the handout are named. This situation strictly mimics the ability to focus on hypnotic instructions, which is supposed to be more efficient in HH subjects. For the reasons exposed above, instead of recording the time needed to name in the two conditions, we preferred to give a settled period of time for naming neutral, and incongruent handouts. The measures considered as indicative of the SIE were the number of correct namings in the two conditions and the errors (see Section 2 for a detailed description).

Section snippets

Subjects

Participants were volunteers among 100 students of the University of Modena and Reggio Emilia who decided to attend, for extra credit, an introductory course on hypnosis. Thirty-four subjects (13 male and 21 female), who were unaware of the experimental aims, took part in the experimental session. Written informed consent was obtained from all participants to investigate the relation between hypnosis and cognitive performance.

The Italian version of the Stanford Hypnotic Susceptibility Scales

Results

Table 1 presents the mean scores and the standard deviations for the SHSS-A and the Stroop task as a function of the 3 groups (i.e., LH, MH, and HH group of subjects).

An analysis of variance (ANOVA) considering Group (high, medium, and low suggestible subjects) as a between-subjects factor and the SIE score as a dependent variable, was significant, F2,32 = 8.15, p = .0015. Planned comparisons revealed that the SIE value of LH subjects was significantly lower than that of both MH (p = .033) and HH (p = 

Discussion

The present work was designed to test the relationships between hypnotic susceptibility and the attentional system. A high level of hypnotic susceptibility is characterized by high concentration on hypnotic instructions and by a pronounced loss of awareness after the hypnotic induction. According to the neuropsychophysiological model (e.g., Crawford, 1994, Crawford and Gruzelier, 1992, Gruzelier, 1988, Gruzelier, 1998), these characteristics, which determine the level of hypnotic

Acknowledgments

The study was supported by grants from MIUR. We thank two anonymous reviewers for their very helpful comments on an earlier draft of this work.

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