P215 Driving human motor cortical oscillations leads to behaviourally relevant changes in local GABA(A) Inhibition: A tACS-TMS study

Abstract

Objectives Beta and gamma oscillations are the dominant oscillatory activity in the human motor cortex (M1). However, their physiological basis and precise functional significance remain poorly understood. To this end, we employed Transcranial Magnetic Stimulation (TMS) to examine the physiological basis and behavioural relevance of driving beta and gamma oscillatory activity in the human M1 using transcranial alternating current stimulation (tACS). Methods 20 healthy volunteers participated in four experimental sessions separated by at least 1 week. In Session 1, all subjects had a MEG scan in order to determine individual beta and gamma peak frequency for subsequent tACS sessions. In Sessions 2–4, subjects received tACS at beta frequency (∼20 Hz), gamma frequency (75 Hz) or sham stimulation. tACS was applied for 20 min over the left M1 and the contralateral orbit at individualised intensity. TMS measures (a single MEP, SICI and ICF) were performed at rest and during movement preparation. Results Gamma tACS led to a significant, duration-dependent decrease in local resting-state GABA(A) inhibition (p = 0.002), as quantified by SICI. The magnitude of this effect was positively correlated with the magnitude of GABA(A) decrease during movement preparation (p = 0.003), when gamma activity in motor circuitry is known to increase. In addition, gamma tACS-induced change in GABA(A) inhibition was closely related to performance in a motor learning task, such that subjects who demonstrated a greater increase in GABA(A) inhibition also showed faster short-term learning (p Discussion and conclusions Our findings show that gamma tACS is capable of modulating GABA(A) inhibition in M1 in a behaviourally relevant manner and suggest that tACS may have similar physiological effects to endogenously driven local oscillatory activity. Significance This study contributes to our understanding of the neurophysiological basis of motor rhythms and their behavioural relevance. Moreover, the ability to modulate local interneuronal circuits by tACS in a behaviourally relevant manner provides a basis for tACS as a putative therapeutic intervention.