Synchronized oscillatory activity at alpha (8-12 Hz) and beta (13-30 Hz) frequencies plays a key role in motor control. Nevertheless, its exact functional significance has yet to be solved. Transcranial alternating current stimulation (tACS) allows the frequency-specific modulation of ongoing oscillatory activity. The goal of the present study was to investigate the effect of 10 and 20 Hz tACS over left primary motor cortex (M1) on motor functions and cortical excitability in healthy subjects. To this end, tACS was applied for 10 min. Sham stimulation served as control condition. Movement speed and accuracy of the right hand were assessed in 15 right-handed subjects before and after (0, 30 and 60 min) tACS of M1. Cortical silent period (CSP) and motor evoked potentials (MEPs) were determined as measures of M1 excitability. While 10 Hz tACS particularly increased movement variability, especially in tasks requiring internal pacing, 20 Hz tACS resulted in movement slowing. Behavioural effects occurred in distinct time windows. While 10 Hz effects developed over 30 min after stimulation, 20 Hz tACS effects were found immediately after stimulation. Following 10 Hz tACS these effects were significantly correlated with CSP duration, indicating interference with inhibitory pathways. The present findings suggest differential effects of stimulation frequency on motor behaviour and M1 excitability.
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