Non-invasive brain stimulation as a tool to study cerebellar-M1 interactions in humans

Abstract

The recent development of non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) has allowed the non-invasive assessment of cerebellar function in humans. Early studies showed that cerebellar activity, as reflected in the excitability of the dentate-thalamo-cortical pathway, can be assessed with paired stimulation of the cerebellum and the primary motor cortex (M1) (cerebellar inhibition of motor cortex, CBI). Following this, many attempts have been made, using techniques such as repetitive TMS and transcranial electrical stimulation (TES), to modulate the activity of the cerebellum and the dentate-thalamo-cortical output, and measure their impact on M1 activity. The present article reviews literature concerned with the impact of non-invasive stimulation of cerebellum on M1 measures of excitability and “plasticity” in both healthy and clinical populations. The main conclusion from the 27 reviewed articles is that the effects of cerebellar “plasticity” protocols on M1 activity are generally inconsistent. Nevertheless, two measurements showed relatively reproducible effects in healthy individuals: reduced response of M1 to sensorimotor “plasticity” (paired-associative stimulation, PAS) and reduced CBI following repetitive TMS and TES. We discuss current challenges, such as the low power of reviewed studies, variability in stimulation parameters employed and lack of understanding of physiological mechanisms underlying CBI.

Cerebellar transcranial magnetic stimulation: The role of coil type from distinct manufacturers

“…In addition to the Deymed coil producing a robust CBI response at high intensities, this coil type also reliably elicited CBI at lower intensities tolerated by participants. This is important for the field of non-invasive brain stimulation since cerebellar function is increasingly investigated with neurostimulation techniques [32,33] and stimulation itself is commonly reported as uncomfortable. These findings, therefore, present an alternative and more comfortable option for future research designs involving both healthy and neurological patients.…”

Transcranial Alternating Current Stimulation (tACS) Mechanisms and Protocols

“…Following on animal models of cerebellar physiology (Ohyama et al, 2003 ; Ohmae and Medina, 2015 ; Giovannucci et al, 2017 ), there is a nascent implementation of cerebellar tACS in investigations of human motor function (Tremblay et al, 2016 ). Importantly, cerebellar neurophysiology excludes recurrent excitatory loops (Buzsáki, 2006 ; Rokni et al, 2008 , 2009 ; Duguid et al, 2015 ).…”

Targeting the Human Cerebellum with Transcranial Direct Current Stimulation to Modulate Behavior: a Meta-Analysis

“…In further support of the physiological data, computational modeling studies have confirmed that exogenous weak electric currents at an intensity of 2 mA can reach the outer layers of the cerebellar cortex [ 10 ]. The possibility to non-invasively target the human cerebellum with tDCS introduces new opportunities to study its role in motor and also non-motor functions [ 11 ].…”