The Selective Influence of Rhythmic Cortical versus Cerebellar Transcranial Stimulation on Human Physiological Tremor

Abstract

The influence of central neuronal oscillators on human physiological tremor is controversial. To address this, transcranial alternating current stimulation (TACS) was delivered at peak tremor frequency to 12 healthy volunteers in a 2 ϫ 2 crossover study. Two sites were stimulated [contralateral primary motor cortex (M1), vs ipsilateral cerebellum] while participants performed two types of tasks designed to probe the different manifestations of physiological tremor of the hand-kinetic and postural tremor. Tremor was measured by accelerometry. Cortical coherence with the accelerometry signal was also calculated in the absence of stimulation. The phase synchronization index, a measure of the phase entrainment of tremor, was calculated between stimulation and tremor waveforms. The amplitude modulation of tremor was similarly assessed. There was significant phase entrainment that was dependent both on tremor type and site of stimulation: M1 stimulation gave rise to phase entrainment of postural, but not kinetic, tremor, whereas cerebellar stimulation increased entrainment in both cases. There was no effect on tremor amplitude. Tremor accelerometry was shown to be coherent with the cortical EEG recorded during postural, but not kinetic, tremor. TACS modulates physiological tremor, and its effects are dependent both on tremor type and stimulation site. Accordingly, central oscillators play a significant role in two of the major manifestations of tremor in health.

Reversing motor adaptation deficits in the ageing brain using non‐invasive stimulation

“…Current density modelling suggests that this montage with the reference on the shoulder provides maximal current flow within the cerebellar hemispheres (Parazzini et al 2014;Rahman et al 2014). Moreover, electrode montages with reference on the shoulder have been used successfully in a few stimulation studies (Joundi et al 2012;Brittain et al 2013;Mehta et al 2014Mehta et al , 2015. Finally, in a recent study using a similar montage, we found that TDCS over the cerebellum affected saccadic adaptation in a polarity-dependent manner (Panouillères et al 2015).…”

“…; Mehta et al . , ). Finally, in a recent study using a similar montage, we found that TDCS over the cerebellum affected saccadic adaptation in a polarity‐dependent manner (Panouillères et al .…”

“…For cerebellar TDCS, we used an extra-cephalic reference electrode to avoid the confound arising from placing the cathodal electrode on the participant's head which would influence the activity of the brain beneath. The cathodal electrode was placed over the trapezius muscle as this montage was successfully used to stimulate the cerebellum and cerebral cortex in several published studies (Joundi et al 2012;Brittain et al 2013;Mehta et al 2014Mehta et al , 2015Panouillères et al 2015). In both stimulation conditions, anodal stimulation was delivered at 2 mA (Iyer et al 2005;Ferrucci et al 2008;Galea et al 2011) with the stimulation intensity gradually ramped on and off over a 10 s period.…”

Guiding transcranial brain stimulation by EEG/MEG to interact with ongoing brain activity and associated functions: A position paper

“…In another example using motor cortex TACS to reduce tremor in Parkinson’s disease, actigraphy from the tremulous limb was adopted as a proxy for central neural activity (Brittain et al, 2013). The approach involved an initial open-loop followed by a closed-loop intervention that afforded several key advantages: First, knowledge of phase information for both TACS waveform and target oscillations (made possible by the proxy measure of central activity) permitted a direct measure of entrainment (Mehta et al, 2014, 2015, Brittain et al, 2015). Second, the phase-precession associated with open-loop stimulation permitted the construction of a stimulus response profile, in order to identify the most effective stimulation parameters for tremor suppression.…”

“…In addition, in several studies entrainment has been reported to be weak (Mehta et al, 2014, 2015, Brittain et al, 2015). Since open-loop protocols rely on assumptions about underlying mechanisms (e.g.…”