P 154. State dependent effects of transcranial alternating current stimulation of the motor system: What you think matters

Abstract

Introduction Recent evidences suggested that imperceptible transcranial alternating current stimulation (tACS) resonates in a frequency-specific manner with the endogenous cortical oscillatory activity. Such a phenomenon causes behavioral consequences on perceptual, motor and cognitive tasks. In the motor system, 20 Hz tACS in beta range, coincident with the idling rhythm of the motor cortex at rest, increased corticospinal output. Objectives To assess how tACS modulates corticospinal excitability in a differential frequency dependent fashion during motor imagery and rest, we delivered transcranial oscillatory frequencies on the primary motor cortex ranging from theta to gamma band. Thus, we aimed to verify whether facilitatory tACS effects persisted during motor imagery, a cognitive task which desynchronizes the rolandic 20 Hz rhythm of quiescent motor areas, thereby becoming theoretically less susceptible to resonance effects of beta stimulation. Materials and methods Eighteen fully healthy right-handed volunteers (8 females, 10 males; mean age 32.2 ± 7.05 years) underwent fourteen different randomized conditions. Both for motor imagery and for resting condition, a “basal 1” session (without tACS), tACS on the left motor cortex at 5 Hz ( θ band), 10 Hz ( α band), 20 Hz ( β band), 40 Hz ( γ band), as well as 20 Hz on the right parietal cortex (as a control for unspecific effects on cortical excitability) and a “basal 2” session (again without tACS), were run. Each session of stimulation lasted 1.5–2 min. TMS was applied over the sponge electrode used for tACS overlying the left M1. Corticospinal excitability changes during stimulation at different frequencies were indexed by motor evoked potentials (MEPs) through navigated Transcranial Magnetic Stimulation (TMS) of the primary motor cortex. MEPs were recorded from the right First Dorsal Interosseus. For motor imagery tasks, subjects were requested to visually imagine a thumb-index finger pinch grip with their right hand. Each TMS pulse was delivered 1–2 s after the initiation of the motor imagery task as well as for rest condition. Results A General Estimation Equation (factors motor imagery and rest x frequency conditions) showed that the maximal increase of corticospinal excitability took place when tACS was applied at 5 Hz and an additional slighter effect of 10 Hz tACS with subjects engaged in a motor imagery task. On the other hand, tACS at 20 Hz confirmed the maximal increase of corticospinal excitability with subjects at rest. Conclusions On one hand results confirmed the frequency-dependence effects of tACS. On the other hand a state-dependent effect of tACS emerged. The MEPs increase in theta range stimulation during motor imagery may reflect reinforcement of working memory processes required to mentally elaborate and “execute” the task. We infer that tACS induces an entrainment effect by dragging the endogenous oscillatory activity to the one induced by stimulation. This indicates that human brain motor processes might be driven and promoted by application of external sinusoidal electrical forces. Download high-res image (87KB) Download full-size image