Effects of tDCS and tACS on operant tactile training: investigating individual differences in neuromodulation efficacy.

Tactile perceptual learning-the improvement of tactile discrimination abilities through experience or training-is fundamental to sensory rehabilitation and skill acquisition. Non-invasive brain stimulation techniques have emerged as promising tools to modulate somatosensory cortical excitability and plasticity, thereby enhancing tactile perceptual learning. Here we present two experiments in which we further examined the effectiveness of transcranial direct and alternating current stimulation on tactile operant learning in a grating orientation discrimination task (GOT). While in experiment 1 we compared the online and offline training effects of tDCS to sham stimulation and tDCS effects without training, in experiment 2, we contrasted the effects of tDCS and tACS on tactile training and examined whether training effects could be predicted by baseline oscillatory brain activity as measured with electroencephalography. In both experiments, participants improved their discrimination performance during training independent of group (experiment 1) or condition (experiment 2), even when controlling for pre-test thresholds. Offline effects of training were inconsistent between experiments. Irrespective of stimulation conditions (tDCS, tACS, sham), training effects could be predicted by baseline brain oscillatory patterns. Higher frontal theta-beta power ratio predicted stronger online training effects, particularly in the tDCS condition. Offline effects of tDCS and tACS combined with tactile training could be predicted by centro-parietal alpha power at pre-test. Interestingly, in experiment 2, participants revealed lower GOT thresholds during training in the tDCS than in the tACS condition. One might speculate that tACS interferes with task-related alpha desynchronization during training. This option requires further testing in future experiments.