| Summary: | Background: In a masked prime choice reaction task, presentation of a compatible prime increases the reaction time to the following imperative stimulus if the interval between mask and prime is around 80–250 ms. This is thought to be due to automatic suppression of the motor plan evoked by the prime, which delays reaction to the imperative stimulus. Oscillatory activity in motor networks around the beta frequency range of 20 Hz is important in suppression of movement. Transcranial alternating current at 20 Hz may be able to drive oscillations in the beta range. Objective/hypothesis: To investigate whether transcranial alternating current stimulation (tACS) at 20 Hz would increase automatic inhibition in a masked prime task. As a control we used 10 Hz tACS. Methods: Stimulation was delivered at alpha (10 Hz) and beta (20 Hz) frequency over the supplementary motor area and the primary motor cortex (simultaneous tACS of SMA-M1), which are part of the BG-cortical motor loop, during the execution of the subliminal masked prime left/right choice reaction task. We measured the effects on reaction times. Corticospinal excitability was assessed by measuring the amplitude of motor evoked potentials (MEPs) evoked in the first dorsal interosseous muscle by transcranial magnetic stimulation (TMS) over M1. Results: The 10 and 20-Hz tACS over SMA-M1 had different effects on automatic inhibition. The 20 Hz tACS increased the duration of automatic inhibition whereas it was decreased by 10 Hz tACS. Neurophysiologically, 20 Hz tACS reduced the amplitude of MEPs evoked from M1, whereas there was no change after 10 Hz tACS. Conclusion: Automatic mechanisms of motor inhibition can be modulated by tACS over motor areas of cortex. tACS may be a useful additional tool to investigate the causal links between endogenous brain oscillations and specific cognitive processes.
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