Transcranial direct current stimulation effects on reward processing in healthy subjects: a systematic review and meta-analysis with electrical field modeling

Background Transcranial direct current stimulation (tDCS) has been proposed to modulate neural circuits involved in reward processing. However, the optimal electrode placement and area of interest for maximum electrical field strength remain unclear. This pre-registered systematic review and meta-analysis aimed to synthesize current evidence on the modulation of reward processing by tDCS and to relate behavioral outcomes to modeled electric field distributions in healthy subjects. Methods We systematically analyzed studies combining tDCS with reward-processing tasks in healthy subjects. Effect sizes were computed for each behavioral outcome. In addition, the magnitude and normal component of the electric field were calculated. For the normal component, positive values indicate an inward current (entering the cortex), whereas negative values indicate an outward current (exiting the cortex). Finally, behavioral outcomes were correlated with the magnitude and normal component of the electric field, yielding the performance–electric field index (PEI). Results For the normal component of the electrical field we found positive correlations in the anterior parts of gyri in the fronto-to-parietal cortex, with a maximum PEI of 0.482 (p<.00001). Negative values were observed in the posterior banks of gyri in the same cortical region, with a minimum PEI of -0.477 (p<.00001). Only marginal associations were found for electric field strength in the right anterior prefrontal cortex (PEI=0.228; p=0.035). Conclusions Anodal stimulation over the right prefrontal cortex with a cathode over the right parietal cortex was associated with enhanced reward processing performance in healthy subjects.