Effects of transcranial direct current stimulation (tDCS) at different cortical targets on cognition in obsessive compulsive disorder (OCD)

BackgroundTranscranial direct current stimulation (tDCS) holds promise as a treatment for obsessive compulsive disorder (OCD). Patients with OCD show impairment in specific aspects of executive function of relevance to wellbeing, manifesting as motor impulsivity and cognitive inflexibility. In a recently published feasibility study, tDCS produced a positive clinical effect on OCD symptoms (Fineberg et al 2023). We report a secondary analysis of neurocognitive data collected in this study. Our aim is to investigate whether tDCS additionally changes markers of motor impulsivity and cognitive flexibility. MethodsIn this randomised, double-blind, sham-controlled, cross-over, multicentre feasibility study, adults with DSM-5 OCD received three courses of clinic-based tDCS, targeting the bilateral supplementary motor area (SMA), left lateral orbitofrontal cortex (L-OFC) and sham, randomly allocated and delivered in counterbalanced order. Each tDCS arm comprised four 20-minute 2 mA stimulations, delivered over two consecutive days, separated by a ‘washout’ period of at least four weeks. Cognitive assessments were conducted before and 2 hours after the first stimulation in each arm, using the Stop Signal (SST) test for motor-impulsivity and the Intra/Extra-Dimensional-Set-Shifting-Task (IED) for cognitive inflexibility. We used a paired samples T-test to compare the baseline and post-stimulation scores for each treatment arm. ResultsNineteen adults were recruited. tDCS of the lateral OFC and SMA significantly improved cognitive inflexibility (total errors at the extra-dimensional shift stage of the ID/ED) (p=0.038 and p=0.030, respectively), while sham treatment did not (paired sample t-test, pre-post effect size, baseline vs. 2-hours after stimulation). No significant effect of tDCS was found for motor impulsivity (stop-signal reaction time, SSRT) in any of the three arms. ConclusionsNotwithstanding the small sample size, stimulating the OFC and SMA with a single administration of tDCS acutely reduced cognitive inflexibility but not motor impulsivity in patients with OCD. tDCS may act to improve OCD via cognitive mechanisms. A definitive randomised, controlled trial of tDCS targeting both the OFC and SMA, including cognitive markers, is indicated.Keywords: obsessive compulsive disorder; Cognitive inflexibility; Motor impulsivity; Transcranial Direct Current Stimulation; tDCS