Disgust and moral rigidity manipulation through non-invasive brain stimulation: a promising therapeutic tool for OCD patients?

Introduction: Previous studies supported the existence of a distinctive relation between deontological guilt and both disgust and obsessive-compulsive (OC) symptoms. Specifically, the experience of deontological guilt has been found to selectively activate the brain area of the insula, which is a well-known structure implicated in the emotional processing of disgust. Moreover, the insula seems to be more easily activated in persons with OCD than in others. The present study aimed to test the hypothesis that indirect inhibition of the insula via cathodal transcranial direct current stimulation (tDCS) would decrease disgust and moral rigidity. Methods: A randomized, sham-controlled, within-subject design was used. Thirty-six healthy individuals (18 women) underwent 15-min anodal, cathodal, and sham tDCS over T3 in three different days. Levels of trait anxiety, depression, disgust sensitivity, scrupulosity, and altruism as well as pre- and post-stimulation momentary emotional states were assessed. Subjects’ heart rate (HR) was recorded to derive measures of parasympathetic nervous system activity (Heart Rate Variability, HRV). After the first 10-min of tDCS stimulation, participants were asked to 1) rate how much a series of vignettes were morally wrong, 2) complete a series of 6-item words that could be completed with either a disgust-related word or neutral alternatives. Results: Compared to sham condition, a) anodal and cathodal stimulation of T3 respectively enhanced and decreased self-reported disgust, severity of moral judgements in the deontological domain, and HRV. Furthermore, b) a positive correlation emerged in the anodal condition between scores on the Obsessive-Compulsive Inventory-Revised (OCI-R) and self-reported disgust, between self-reported deontological guilt and the Fear-of-Sin (FoS) subscale of the Pennsylvania Inventory of Scrupolosity (PIOS), and between self-reported deontological guilt and the washing and obsessing subscales of the OCI-R; c) in the cathodal condition, disgust inversely correlated with the FoS and the washing and obsessing subscales of the OCI-R. Discussion: The most interesting results in light of future clinical implications are relative to the decrease in self-reported and physiological disgust following cathodal tDCS on T3, and the stronger effect of tDCS on disgust when we compare individuals with higher obsessive-compulsive tendencies to others. Conclusion: Further investigation is needed in order to clarify the brain correlates of deontological morality in healthy individuals and in OCD before moving to the next step, which may be to test the feasibility of these effects in the clinical setting.