Transcranial Electrical Stimulation of the sensorimotor system: intensity effects of beta tACS

The present methodological study aimed to investigate the effects of tACS intensity (ranged 0.25-2 mA) influence of beta (20 Hz) stimulation of primary motor cortex (M1), and to test the hypothesis of these effects to be non-monotonic. We collected data from 20 healthy subjects (mean age 23.55±4.16, 10 females), with no history of mental and neurological diseases or traumas, all right-handed, with no drug, alcohol or caffeine use in the days preceding the experiment. The results are confusing, as we have observed not only absence of any difference between all the beta intensity conditions into M1 excitation, but also no difference between all beta conditions and neither alpha or sham controls nor baselines. On the other hand, our data reveal a cumulative effect of prolonged stimulation and sensitivity to this effect based on an individual motor threshold (MT). According to the correlation findings, MT-dependent tACS effects are more effective when delivered at 1500 and 2000 mA. Besides the limitation of the study, it seems that tACS induces non-linear effects. Nevertheless, our findings require further closer inspection, in order to draw a distinction between factors that cause such findings.