Transcranial cerebellar direct current stimulation and transcutaneous spinal cord direct current stimulation as innovative tools for neuroscientists

Abstract

Two neuromodulatory techniques based on applying direct current (DC) non-invasively through the skin, transcranial cerebellar direct current stimulation (tDCS) and transcutaneous spinal DCS, can induce prolonged functional changes consistent with a direct influence on the human cerebellum and spinal cord. In this article we review the major experimental works on cerebellar tDCS and on spinal tDCS, and their preliminary clinical applications. Cerebellar tDCS modulates cerebellar motor cortical inhibition, gait adaptation, motor behaviour, and cognition (learning, language, memory, attention). Spinal tDCS influences the ascending and descending spinal pathways, and spinal reflex excitability. In the anaesthetised mouse, DC stimulation applied under the skin along the entire spinal cord may affect GABAergic and glutamatergic systems. Preliminary clinical studies in patients with cerebellar disorders, and in animals and patients with spinal cord injuries, have reported beneficial effects. Overall the available data show that cerebellar tDCS and spinal tDCS are two novel approaches for inducing prolonged functional changes and neuroplasticity in the human cerebellum and spinal cord, and both are new tools for experimental and clinical neuroscientists. Abbreviations CBI, cerebello-brain inhibition; cerebellar tDCS, transcranial cerebellar direct current stimulation; MEP, motor-evoked potential; PAS, paired associative stimulation; rTMS, repetitive transcranial magnetic stimulation; SCI, spinal cord injury; spinal tDCS, transcutaneous spinal direct current stimulation; TMS, transcranial magnetic stimulation.Alberto Priori is the head of the Clinical Center for Neurostimulation, Neurotechnology and Movement Disorders at the Fondazione IRCCS Ca' Granda Ospedale Maggiore di Milano and professor of neurology at the University of Milan Italy. He pioneered research on the effect of DC on the human motor cortex in the 1990s and more recently led his research group in studying how constant electric fields influence the human cerebellum and spinal cord. Matteo Ciocca is a neurologist and clinical research fellow. His research activity assesses the effects of spinal cord DC stimulation in healthy humans. Marta Parazzini is an engineer. Her research activity deals with the computational modeling of the interactions between electromagnetic field and biological system. She developed the first computational modeling studies specifically designed for cerebellar and spinal cord DC stimulation. Maurizio Vergari is a neurophysiology technician, who dealt with the technical aspects of cerebellar and spinal cord direct current stimulation since the first experiments. Roberta Ferrucci is a psychologist, postdoctoral fellow at the University of Milan. She began her research work by developing the technique for cerebellar DC stimulation and the cognitive and behavioural experimental protocols to assess its effects.

A technical guide to tDCS, and related non-invasive brain stimulation tools

“…The effect was specific because SEPs elicited by median nerve stimulation were not influenced by thoracic transcutaneous spinal DC stimulation. Several further studies replicated the observation that weak DC delivered over the spinal cord can influence different physiological variables of spinal cord function in humans and animals, both intact and injured (for a review see (Priori et al 2014). …”

“…Like tDCS, spinal DC often elicits short-lasting tingling sensations when stimulation begins and ends, and sometimes redness under the electrode. The effect of polarity on physiology is complex (Priori et al 2014). In general, in humans it can be assumed that thoracic transcutaneous DC under the anode electrode decreases responses involving spinal tract compartments below the stimulating electrode and increases segmental reflex responses, whereas the cathode electrode induces opposite effects.…”

“…The return electrode position over the right arm results in current prevailing at the thoracic level. Conversely, the return electrode position over Cz results also in supra-spinal effects at the bulbar level (see also Priori et al 2014 for a discussion). Hence, the type of montage should be selected according to the aim of the study.…”

“…Similar to the neuromodulation of any other CNS structure, the focality of ctDCS effects is presumably relatively limited, however recent studies aimed also for relatively focal stimulation. In this section we will describe the two most common electrode montages utilized in different laboratories (for a detailed review on the technique and cerebellar tDCS applications see Ferrucci and Priori 2014a; Grimaldi et al 2014; Priori et al 2014). …”

Transcranial direct current stimulation of SMA modulates anticipatory postural adjustments without affecting the primary movement

“…To our knowledge, of the few studies which reported significant effects of tDCS applied on SMA, two (Kwon & Kwon, 2013;Vollmann et al, 2013) With regards to the after-effects, many studies highlighted that the changes in excitability not only developed during DC stimulation, but lasted for several minutes or hours after its end. This effect was observed on several cortical and sub-cortical areas (Polania et al, 2012;Monte-Silva et al, 2013;Bolzoni et al, 2013a,b;Nonnekes et al, 2014;Priori et al, 2014) and also on the spinal cord (Cogiamanian et al, 2008;Ahmed, 2013;Priori et al, 2014). The after-effects are likely mediated by mechanisms of synaptic long-term potentiation and depression, which affect neuroplasticity (Nitsche et al, 2002;Ahmed, 2013;Monte-Silva et al, 2013).…”