[ARTICLE] Plasticity induced by non-invasive transcranial brain stimulation: A position paper – Full Text

Abstract

Several techniques and protocols of non-invasive transcranial brain stimulation (NIBS), including transcranial magnetic and electrical stimuli, have been developed in the past decades. Non-invasive transcranial brain stimulation may modulate cortical excitability outlasting the period of non-invasive transcranial brain stimulation itself from several minutes to more than one hour. Quite a few lines of evidence, including pharmacological, physiological and behavioral studies in humans and animals, suggest that the effects of non-invasive transcranial brain stimulation are produced through effects on synaptic plasticity. However, there is still a need for more direct and conclusive evidence. The fragility and variability of the effects are the major challenges that non-invasive transcranial brain stimulation currently faces. A variety of factors, including biological variation, measurement reproducibility and the neuronal state of the stimulated area, which can be affected by factors such as past and present physical activity, may influence the response to non-invasive transcranial brain stimulation. Work is ongoing to test whether the reliability and consistency of non-invasive transcranial brain stimulation can be improved by controlling or monitoring neuronal state and by optimizing the protocol and timing of stimulation.

1. Introduction

Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) are the most commonly used methods of non-invasive transcranial brain stimulation that has been abbreviated by previous authors as either as NIBS or NTBS. Here we use NIBS since it seems to be the most common term at the present time. When it was first introduced in 1985, TMS was employed primarily as a tool to investigate the integrity and function of the human corticospinal system (Barker et al., 1985). Single pulse stimulation was used to elicit motor evoked potentials (MEPs) that were easily evoked and measured in contralateral muscles (Rothwell et al., 1999). The robustness and repeatability of measures of conduction time, stimulation threshold and “hot spot” location allowed TMS to be developed into a standard tool in clinical neurophysiology.

As we review below, a number of NIBS protocols can lead to effects on brain excitability that outlast the period of stimulation. These may reflect basic synaptic mechanisms involving long-term potentiation (LTP)- or long-term depression (LTD)-like plasticity, and because of this there has been great interest in using the methods as therapeutic interventions in neurological and psychiatric diseases. Furthermore, recently they are more frequently applied to modify memory processes and to enhance cognitive function in healthy individuals. However, apart from success in treating some patients with depression (Lefaucheur et al., 2014; Padberg et al., 2002, 1999), there is little consensus that they have improved outcomes in a clinically meaningful fashion in any other conditions. The reason for this is probably linked to the reason why many other protocols failed to reach routine clinical neurophysiology: they are too variable both within and between individuals to make them practically useful in a health service setting (Goldsworthy et al., 2014; Hamada et al., 2013; Lopez-Alonso et al., 2014, 2015).

Below we review the evidence for the mechanisms underlying the “neuroplastic” effects of NIBS, and then consider the problems in reproducibility and offer some potential ways forward in research. […]

Continue —> Plasticity induced by non-invasive transcranial brain stimulation: A position paper – ScienceDirect

There are three major lines of evidence supporting NIBS produces effects…

Fig. 1. There are three major lines of evidence supporting NIBS produces effects through mechanisms of synaptic plasticity: (1) Drugs that modulate the function of critical receptors/channels for plasticity, e.g. Ca2+ channels and NMDA receptors, alter the effect of NIBS; (2) NIBS mainly changes I-waves rather than the D-wave in the epidural recording of descending volleys evoked by TMS, suggesting the effect of NIBS occurs trans-synaptically; and (3) NIBS interacts between protocols and with motor practice and cognitive learning processes, suggesting the effect of NIBS is involves in plasticity-related motor and psychological processes.

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