The effect of high-definition transcranial direct current stimulation to the pre-supplementary motor area on habit: A randomized cross-over experiment using reinforcement learning in healthy subjects

Abstract Background Compulsivity is a transdiagnostic marker in various psychiatric disorders and is postulated to be mediated by a shift from goal-directed to habitual behavior. Pre-supplementary motor area (pre-SMA) functioning has been linked to habit learning. We examined the montage and polarity-specific effects of high-definition transcranial direct current stimulation (HD-tDCS) on pre-SMA in modulating habitual learning in healthy subjects. We hypothesized that anodal stimulation would lead to more goal-directed responses in a reinforcement learning task and vice versa with cathodal stimulation. Materials and methods 28 consenting healthy volunteers were administered anodal, cathodal, and sham HD-tDCS targeting pre-SMA while they performed a computer-delivered sequential reinforcement learning task using a single-blind, randomized cross-over design. Logistic regression was performed to assess the effects of stimulation on habit-driven/model-free (MF) vs. goal-directed/model-based (MB) responses in task performance. A hybrid Bayesian model-fitting technique was also used to generate MB and MF parameters. Results The participants showed a hybrid pattern of learning. Cathodal stimulation reduced stay behavior irrespective of previous reward or previous transition [B= -0.165, p = 0.018]. Further, cathodal stimulation was significantly associated with the MF parameter (0.805, p = 0.009), while no association between anodal or sham stimulation with any of the outcomes was found. Conclusion Our study provides preliminary evidence that habitual and goal-directed learning could be modulated by transcranial direct current stimulation applied to the pre-SMA. Further exploration is needed to understand the underpinnings of habits and their role in disorders and to translate tDCS as a treatment modality for disorders of compulsivity. The study was registered with the Clinical Trials Registry- India (CTRI/2021/04/032753) on 12 th April 2021