Invited Symposium: Neural Bases of Hypnosis
Given that hypnotic susceptibility has been shown to be a stable individual differences marker with high test-retest reliability over 15 to 25 years, it is surprising that there exist few psychometric measures which correlate with susceptibility. Alternative approaches have emphasized psychophysiological measures, especially EEG activity. The most solid relationship between electrocortical activity, hypnosis, and hypnotic susceptibility exists in the EEG theta (4-8 Hz.) frequency range. Results from two separate studies in our lab as well as other labs have found high hypnotically susceptible individuals to show great EEG theta activity. Additionally, recent work from our lab has also shown differential fractal dimensionality between high and low hypnotically susceptible individuals. High hypnotically susceptible individuals show a pattern of EEG dimensionality more consistent with imagery processes whereas low susceptible individuals show a pattern more consistent with cognitive activity (e.g., mental math).
Hypnotic suggestions can produce the inability to monitor or govern basic motor and perceptual functions. Woody and Bowers propose a dissociated-control theory of hypnosis which further elucidates hypnotic phenomena in terms of contention scheduling processes, responsible for carrying out and even initiating basic motor and cognitive functions, and a supervisory attentional system, which is responsible for goals and intentions, as well as monitoring and indirectly modulating the contention scheduling processes. Woody and Bowers have demonstrated that this model is consistent with cognitive theory stating that the effects of hypnotic suggestion weaken the connections between the supervisory attentional system and the contention scheduling so that basic tasks may still be carried out while more complicated tasks are inhibited. They also note the similarities found with frontal lobe patients, indicating that perseverative and poor self-organized behaviors may be indicative of functional similarities. Given this background, one might conclude hypnosis modifies in some fashion executive function. One question yet to be ask is do high and low hypnotically susceptible individuals differ in their responses on tests designed to reflect executive function.
In this presentation we wish to follow-up our previous work in terms of three questions: first, can we use neural network classification algorithms to differentiate high and low hypnotically susceptible individuals; second, do high and low susceptible individuals differ in terms of patterns of EEG connections as described through measures of coherence; and third do high and low susceptible individuals have different styles that might be reflected in standard neuropsychological measures, especially those related to executive function.
Neural Network and Coherence Study
EEG data from 17 sites across the cortex were collected at initial baseline and at specific points following a standard hypnotic induction for 18 individuals (11 high, 7 low). Our results show that classifications employing multi-channel spectral theta values yielded high accuracies of 94% for cascade-correlation, 93% for backpropagation, and 72% for discriminant analysis. Classifications employing multi-channel fractal dimensionality values were less accurate showing 87% for backpropagation, 84% for cascade-correlation, and 79% for discriminant analysis.
Higher coherency magnitude was found at nine electrode pairs for the high-hypnotizable group, and at two electrode pairs for the low-hypnotizable group. Direction differences were found in nine electrode pairs with long-distance pairs showing an anterior to posterior direction for the high-hypnotizable group, and the reverse for the low-hypnotizable group. Time delay differences were found five electrode pairs, with all showing shorter propagation times for the high-hypnotizable group.
Overall, these results demonstrate that neural networks can be used to accurately classify hypnotic susceptibility and that the high and low susceptible individuals show differential cortical activity as reflected by coherence measures.
We administered four neuropsychological tests (WICS, Controlled Oral Word Association Test [or the FAS], Stroop, and the Towers of Hanoi) to high or low hypnotically suggestible individuals. These tests were selected to reflect a diversity along verbal and visual- spatial modalities to test the range of potential differences in frontal tasks which might reflect executive functioning.
Participants were chosen by their performance during a group hypnotic screening, however, participants were not informed of the selection criteria when contacted to for the study. Therefore, the present study was able to address executive function in hypnosis as an issue of individual differences. To examine the relationships between tests, indices from each were correlated. A significant positive correlation was found between the total number of moves on the Towers of Hanoi and the total correct moves on the Wisconsin Card Sort, r(15)=0.64, p<.008, perhaps reflecting similar visual spatial processing abilities that both tests measure. However, both tests show weaker, non-significant correlations to the FAS and Stroop, potentially suggesting those test's function as verbally mediated frontal tasks (Stroop reading time had a small positive correlation to FAS totals, however, it was also non-significant) .
Overall, high suggestible participants performed better than the low suggestible participants. The only significant finding was a faster performance for high suggestible participants with the WICS, t(14) = 2.21, p<.04, which would suggest a greater ability to detect relevant abstract information, shift cognitive set, and to decrease perseverative tendencies. However, while not significant, the high suggestible participants also performed better on both the FAS and Stroop, tests which should reflect verbally-mediated abilities. This may indicate that the high suggestible participants differed from the low suggestible participants on general intelligence as well. Therefore, the results would indicate that the high suggestive participants were more flexible in their ability to shift set, plan complex tasks, and perform effectively.
Discussion and Conclusion
Our current research focused on two levels of analysis, that of cortical functioning and that of executive function. In terms of the former, our research suggest that high and low susceptible individuals can be distinguished using neural networks in terms of their EEG. Further, as shown by coherence measures, high and low susceptible individuals also display different patterns of cortical activity. In terms of the later level of analysis, the use of neuropsychological measures presented a less certain pattern that clearly distinguished high and low susceptible individuals although high susceptible individuals did appear to be more flexible in their ability to shift set. Future research might test out this suggestion by having individuals perform these tasks during an hypnotic procedure. Additional research from other labs examining hypnosis and source memory indicate that hypnotized individuals have less detail in their recall and remember the ‘gist' of information rather than details. In regards to previous theory, this may indicate that easily hypnotizable people process information in a more fluid manner and that any differences in executive functions may be relative to those more primary cognitive abilities. Future research should continue to explore hypnosis as an execute function separate from general intelligence to better articulate the manner in which hypnotic processes can produce the inability to monitor or govern basic motor and perceptual functions.
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|Ray, W; Blai, A; Aikins, D; Coyle, J; Bjick, E; (1998). Understanding Hypnosis and Hypnotic Susceptibility from a Psychophysiological Perspective. 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/ray0556/index.html|
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