Behavioural Brain Research 241 (2013) 1–6 Contents lists available at SciVerse ScienceDirect Behavioural Brain Research j ourna l ho mepage: www.elsevier.com/locate/bbr Research report Effects of 10 Hz and 20 Hz transcranial alternating current stimulation (tACS) on motor functions and motor cortical excitability C. Wach a,b,1 , V. Krause a,1 , V. Moliadze c , W. Paulus d , A. Schnitzler a,b , B. Pollok a,∗ a Institute of Clinical Neuroscience and Medical Psychology, Medical Faculty, Heinrich-Heine-University, Universitaetsstr. 1, 40225 Duesseldorf, Germany b Department of Neurology, Medical Faculty, Duesseldorf University Hospital, Moorenstraße 5, 40225 Duesseldorf, Germany c Department of Child and Adolescent Psychiatry, Psychosomatics and Psychotherapy, Goethe-University, Theodor-Stern-Kai 7, House 7A, 60596 Frankfurt am Main, Germany d Department of Clinical Neurophysiology, Georg-August-University, Robert-Koch-Str. 40, 37075 Goettingen, Germany h i g h l i g h t s ◮ Synchronized oscillatory activity at alpha/beta band is central to motor control. ◮ Yet, its exact functional significance remains unclear. ◮ Oscillatory activity was modulated through tACS at 10 and 20 Hz. ◮ Differential effects of stimulation frequency on motor function were revealed. ◮ 10 Hz tACS increases movement variability, 20 Hz tACS slows movements. a r t i c l e i n f o Article history: Received 14 September 2012 Received in revised form 22 November 2012 Accepted 26 November 2012 Available online 3 December 2012 Key words: Transcranial alternating current stimulation (tACS) Motor control Primary motor cortex Motor cortical excitability a b s t r a c t 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. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Synchronized oscillatory activity represents a basic mechanism of functional communication within and between brain sites [1,2]. Dynamic functional interaction in a cerebello-thalamo-cortical network subserving motor control has been evidenced at alpha and beta frequencies [3–5]. While oscillatory activity at the beta range might mediate the control of more complex movements [6], ∗ Corresponding author. Tel.: +49 211 81 10767; fax: +49 211 81 13015. E-mail address: bettina.pollok@uni-duesseldorf.de (B. Pollok). 1 CW and VK share equal first authorship. functional interaction at the alpha range might represent a neuro- physiological marker of automatic motor control [7]. These results imply that different frequencies might subserve distinct functions for motor control. TACS offers the unique possibility to non-invasively modulate ongoing oscillatory activity in the brain by applying a weak alter- nating current [8]. It has been suggested that tACS leads to a rhythmic stimulation of underlying brain tissue which is assumed to be entrained at stimulation frequency [9,10]. Different stimula- tion frequencies ranging from theta and beta band stimulation [11] over ripple frequencies [140 and 250 Hz, [12]] up to 1, 2 and 5 kHz [13] have been reported to increase motor cortical excitability. However, inhibitory effects of 15 Hz tACS have been demonstrated as well [14]. Moreover, improved memory consolidation has been 0166-4328/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.bbr.2012.11.038