Final Report Summary - ENHANCING SUGGESTION (Enhancing hypnotic suggestibility using noninvasive brain stimulation: Cognitive and neural mechanisms)
An understudied capacity of the human brain is its ability to translate endogenous (self-generated) representations such as images or thoughts into vivid perceptual events that are experienced as though they were real, environmentally-produced events. This ability is evidenced by the placebo effect but is perhaps most clearly observed during hypnosis. Following particular hypnotic suggestions, highly suggestible individuals are capable of experiencing profound changes in affect, cognition, memory, and sensory processing with corresponding brain changes. The use of hypnosis to alter psychological functions has promise as both a therapeutic agent as well as an experimental tool.
One of the principal drawbacks in the therapeutic application and experimental use of hypnotic suggestion is that only a small subset of individuals displays high hypnotic suggestibility. Such highly suggestible individuals respond better to therapeutic interventions and are able to experience a diverse range of suggestions in a laboratory setting. That the efficacy of hypnosis is most pronounced in highly suggestible individuals presents a clear motivation for developing methods for increasing hypnotic suggestibility. Preliminary research shows that different methods can be used to enhance hypnotic suggestibility but these suffer from a number of important methodological limitations. The present project examined whether transcranial direct current stimulation (tDCS), a form of non-invasive, non-painful brain stimulation that involves the application of a low electrical current to the scalp to alter brain activity could be used to enhance hypnotic suggestibility.
The first two studies of the project investigated whether tDCS applied to left dorsolateral prefrontal cortex would enhance responsiveness to suggestions for superior attention in medium and highly suggestible individuals. The first study found that tDCS coupled with suggestion was associated with improved selective attention. The second study, in contrast, failed to observe a benefit of tDCS on response to a suggestion for superior sustained attention, although the suggestion itself was effective in enhancing performance. Cumulatively, these studies provide mixed evidence regarding the utility of tDCS for enhancing suggestion.
The third and fourth studies aimed to identify the neurochemical and cognitive correlates of hypnotic suggestibility. The third study used magnetic resonance spectroscopy (MRS), a technique that allows the reliable estimation of neurochemical concentrations in specific brain regions. Hypnotic suggestibility was negatively correlated with GABA levels in putamen, a region of the striatum, but not in control regions. Recent work suggests that dopamine suppresses GABA levels in striatum so this result provides indirect support for the hypothesis that hypnotic suggestibility is associated with elevated striatal dopamine levels, which is consistent with a number of previous studies. The fourth study tested the prediction that highly suggestible individuals have greater distortions in their sense of agency in cognitive tasks outside the context of hypnosis. Highly suggestible individuals were found to exhibit impaired metacognition of agency. These studies thus provide novel information on the neural and cognitive mechanisms underlying variability in hypnotic suggestibility.
Taken together, the studies of this project provide a number of insights regarding the neural and cognitive basis of hypnosis. Two studies suggest that the application of tDCS to prefrontal cortex does not produce a robust change in hypnotic suggestibility and thus is unlikely to represent a valuable method for modulating hypnotic responding. Two further studies suggest neurochemical and cognitive correlates of hypnotic suggestibility that may prove to be valuable in future research on the modification of hypnotic suggestibility.
One of the principal drawbacks in the therapeutic application and experimental use of hypnotic suggestion is that only a small subset of individuals displays high hypnotic suggestibility. Such highly suggestible individuals respond better to therapeutic interventions and are able to experience a diverse range of suggestions in a laboratory setting. That the efficacy of hypnosis is most pronounced in highly suggestible individuals presents a clear motivation for developing methods for increasing hypnotic suggestibility. Preliminary research shows that different methods can be used to enhance hypnotic suggestibility but these suffer from a number of important methodological limitations. The present project examined whether transcranial direct current stimulation (tDCS), a form of non-invasive, non-painful brain stimulation that involves the application of a low electrical current to the scalp to alter brain activity could be used to enhance hypnotic suggestibility.
The first two studies of the project investigated whether tDCS applied to left dorsolateral prefrontal cortex would enhance responsiveness to suggestions for superior attention in medium and highly suggestible individuals. The first study found that tDCS coupled with suggestion was associated with improved selective attention. The second study, in contrast, failed to observe a benefit of tDCS on response to a suggestion for superior sustained attention, although the suggestion itself was effective in enhancing performance. Cumulatively, these studies provide mixed evidence regarding the utility of tDCS for enhancing suggestion.
The third and fourth studies aimed to identify the neurochemical and cognitive correlates of hypnotic suggestibility. The third study used magnetic resonance spectroscopy (MRS), a technique that allows the reliable estimation of neurochemical concentrations in specific brain regions. Hypnotic suggestibility was negatively correlated with GABA levels in putamen, a region of the striatum, but not in control regions. Recent work suggests that dopamine suppresses GABA levels in striatum so this result provides indirect support for the hypothesis that hypnotic suggestibility is associated with elevated striatal dopamine levels, which is consistent with a number of previous studies. The fourth study tested the prediction that highly suggestible individuals have greater distortions in their sense of agency in cognitive tasks outside the context of hypnosis. Highly suggestible individuals were found to exhibit impaired metacognition of agency. These studies thus provide novel information on the neural and cognitive mechanisms underlying variability in hypnotic suggestibility.
Taken together, the studies of this project provide a number of insights regarding the neural and cognitive basis of hypnosis. Two studies suggest that the application of tDCS to prefrontal cortex does not produce a robust change in hypnotic suggestibility and thus is unlikely to represent a valuable method for modulating hypnotic responding. Two further studies suggest neurochemical and cognitive correlates of hypnotic suggestibility that may prove to be valuable in future research on the modification of hypnotic suggestibility.