Theta-gamma transcranial alternating current stimulation and complex visuomotor training in healthy young adults

Theta-gamma (TG) transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique that can improve performance of simple motor tasks when applied over primary motor cortex (M1). However, the effects of TG tACS on performance of more complex motor tasks remain to be investigated. Forty healthy young (mean age 25.8 ± 5.1 years) individuals were recruited to receive either 20 mins of TG (75 Hz gamma burst nested at the peak of a 6 Hz theta wave) or sham tACS over M1 during training of a sequential visual isometric pinch force task (SVIPT). Skill was quantified as a function of speed and accuracy and was used to index changes in performance. Transcranial magnetic stimulation was used to assess corticomotor excitability and intracortical inhibition before and after the intervention. We found that skill increased during training (P < 0.001), but there was no difference between interventions (P = 0.729). Furthermore, training increased corticomotor excitability (P < 0.001), but did not modulate intracortical inhibition (P = 0.328), and this was not different between interventions (P > 0.196). Overall, our results suggest that the TG tACS applied over M1 has limited impact on neurophysiological and behavioral effects of complex visuomotor training in young healthy adults.