Investigating Transcranial Alternating Current Stimulation (tACS) Preconditioning Effects on Resting and Active Motor Threshold

Previous studies have looked at the potential of using transcranial direct current stimulation (tDCS) to prime the brain in order to lower the resting motor threshold. The resting motor threshold (RMT) is the intensity at which repeated transcranial magnetic stimulation (rTMS) is applied for treatments. The RMT is the lowest intensity at which the participant's muscle responds with the motor evoked potential peak-to-peak amplitude greater than 50μV for 5 of 10 trials when the hand is relaxed. The active motor threshold (AMT) is the lowest intensity that produces MEP with peak-to-peak amplitude greater than 100 μV for 5 of 10 trials while the participant maintains 10-30% of maximal voluntary contraction. The RMT and AMT is different for each person and the high intensity TMS pulses can be difficult for some of the patients in the current Alzheimer's rTMS treatment study to tolerate. Therefore, if the treatment can still be applied with lower intensities, it would be more tolerable for some people. The limited research on both tDCS and transcranial alternating current stimulation (tACS) in priming the brain for TMS are not enough to determine if they are effective in priming the brain. The studies that include tDCS have small sample size and inconclusive results. The studies with tACS are using very high frequencies of 140 Hz to 250 Hz. The objective of this research study is to determine if priming the brain with tACS at 40 Hz or tRNS for 10 minutes is able to reduce the resting and active motor threshold. Therefore, for people with higher RMT and AMT, priming with tACS or tRNS could be used to lower intensities while stimulating the brain for treatments.