Transcranial direct current stimulation of primary motor cortex reduces thermal pain.

abstractTranscranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique with growing potential for pain modulation. However, previous studies have often been limited by small sample sizes, limited outcome reliability, and blinding issues. Pre-registered randomized trials are needed to establish efficacy and reproducibility. Here, we report results from a pre-registered, triple-blind, randomized within-subject crossover trial (N = 50), investigating the effects of left motor cortex (M1) tDCS on experimentally evoked thermal pain in healthy adults. Participants underwent individualized pain calibration, then received anodal, cathodal, and sham M1-tDCS in counterbalanced order on separate visits 4 or more days apart. We assessed post-tDCS thermal pain, potential blinding, side effects, and reaction time during pain rating. Anodal tDCS significantly reduced pain compared with cathodal and sham stimulation ( P < 0.001, Cohen's d = 0.93 and 0.85, respectively; P < 0.001). Cathodal tDCS increased pain relative to sham (Cohen's d = 0.34, P < 0.01). These effects remained stable throughout the 35-minute post-tDCS pain testing period, with no evidence of decay over time. Blinding assessments confirmed that participants were unable to reliably identify the sham session. Anodal tDCS also modestly increased reaction time in the left (ipsilateral) hand ( P < 0.05, Cohen's d = 0.16), consistent with interhemispheric inhibition, which was uncorrelated with analgesia ( P > 0.10). This study provides strong evidence for reliable and polarity-dependent modulation of experimental pain by M1-tDCS.