Consensus: "Can tDCS and TMS enhance motor learning and memory formation?"

Noninvasive brain stimulation has developed as a promising tool for cognitive neuroscientists. Transcranial magnetic (TMS) and direct current (tDCS) stimulation allow researchers to purposefully enhance or decrease excitability in focal areas of the brain. The purpose of this paper is to review information on the use of TMS and tDCS as research tools to facilitate motor memory formation, motor performance and motor learning in healthy volunteers. Studies implemented so far have mostly focused on the ability of TMS and tDCS to elicit relatively short lasting motor improvements and the mechanisms underlying these changes have been only partially investigated. Despite limitations including the scarcity of data, work that has been already accomplished raises the exciting hypothesis that currently available noninvasive transcranial stimulation techniques could modulate motor learning and memory formation in healthy humans and potentially in patients with neurological and psychiatric disorders.

Consensus: Can transcranial direct current stimulation and transcranial magnetic stimulation enhance motor learning and memory formation?

Friedhelm Christoph Hummel

Noninvasive brain stimulation has developed as a promising tool for cognitive neuroscientists. Transcranial magnetic (TMS) and direct current (tDCS) stimulation allow researchers to purposefully enhance or decrease excitability in focal areas of the brain. The purpose of this article is to review information on the use of TMS and tDCS as research tools to facilitate motor memory formation, motor performance, and motor learning in healthy volunteers. Studies implemented so far have mostly focused on the ability of TMS and tDCS to elicit relatively short-lasting motor improvements and the mechanisms underlying these changes have been only partially investigated. Despite limitations, including the scarcity of data, work that has been already accomplished raises the exciting hypothesis that currently available noninvasive transcranial stimulation techniques could modulate motor learning and memory formation in healthy humans and potentially in patients with neurologic and psychiatric disorders.

2008

Low intensity transcranial electric stimulation: Safety, ethical, legal regulatory and application guidelines

Friedhelm Christoph Hummel

Low intensity transcranial electrical stimulation (TES) in humans, encompassing transcranial direct current (tDCS), transcutaneous spinal Direct Current Stimulation (tsDCS), transcranial alternating current (tACS), and transcranial random noise (tRNS) stimulation or their combinations, appears to be safe. No serious adverse events (SAEs) have been reported so far in over 18,000 sessions administered to healthy subjects, neurological and psychiatric patients, as summarized here. Moderate adverse events (AEs), as defined by the necessity to intervene, are rare, and include skin burns with tDCS due to suboptimal electrode-skin contact. Very rarely mania or hypomania was induced in patients with depression (11 documented cases), yet a causal relationship is difficult to prove because of the low incidence rate and limited numbers of subjects in controlled trials. Mild AEs (MAEs) include headache and fatigue following stimulation as well as prickling and burning sensations occurring during tDCS at peak-to-baseline intensities of 1-2mA and during tACS at higher peak-to-peak intensities above 2mA. The prevalence of published AEs is different in studies specifically assessing AEs vs. those not assessing them, being higher in the former. AEs are frequently reported by individuals receiving placebo stimulation. The profile of AEs in terms of frequency, magnitude and type is comparable in healthy and clinical populations, and this is also the case for more vulnerable populations, such as children, elderly persons, or pregnant women. Combined interventions (e.g., co-application of drugs, electrophysiological measurements, neuroimaging) were not associated with further safety issues. Safety is established for low-intensity 'conventional' TES defined as

2017

Does one size fit all? - Towards the optimization and personalization of non-invasive brain stimulation paradigms to enhance motor learning

Laurijn Rianne Draaisma

The acquisition and re-acquisition of motor skills is an important aspect of daily life and in the recovery after a stroke. Non-invasive brain stimulation (NIBS) is a technique that is used to improve motor learning and enhance motor recovery in stroke survivors. Although the current results are promising, the outcomes are heterogeneous with responders and non-responders. This thesis aimed to investigate multiple, novel NIBS strategies to enhance stimulation efficacy and to develop approaches for protocol personalization. These alternative strategies include targeting other areas of the motor network than the primary motor cortex (M1), targeting the brain as a network with the use of multifocal stimulation, and using a variety of stimulation techniques (TMS, tACS, tDCS) to study underlying mechanisms. Study 1 explored the methodological implications of TMS for measuring inhibitory and excitatory neurotransmissions. With the use of TMS, short intracortical inhibition (SICI) and intracortical facilitation (ICF) were measured in healthy young adults. SICI and ICF were studied with different stimulators, waveforms, and current directions using a set of interstimulus intervals. Our findings indicated high comparability among the different stimulation paradigms, except for SICI at 3 ms, which enables data sharing across units and facilitates the conduction of multi-center studies.Study 2 measured the effect of 50 Hz tACS applied to the cerebellum on a novel motor learning task in healthy young adults. Targeting the cerebellum with NIBS is a relatively new field of research with open questions that need to be addressed. Therefore, we explored for the first time the effect of CB-tACS on a motor learning task. Our results did not show an improvement of learning with 50 Hz CB-tACS. We argue that this might have been related to the task and/or the selected stimulation frequency. Therefore, this stimulation paradigm requires further optimization to be effective for motor learning.Study 3 compared multifocal sequential stimulation to monofocal tDCS on motor learning in stroke patients. The multifocal paradigm consisted of an orchestrated M1 â CB stimulation setup, the monofocal paradigm targeted the M1. Our results indicated a significant effect of multifocal M1-CB stimulation, mainly driven by CB-tDCS during the early phase of learning. Moreover, baseline performance and neurophysiology were related to stimulation responsiveness that could potentially lead to biomarkers to predict stimulation efficacy in stroke patients.Study 4 measured the effect of personalized bifocal theta tACS applied to the FPN on motor learning in healthy older adults. Sequence learning has a cognitive component related to working memory (WM) capacity, a process mediated by the FPN. We hypothesized that targeting the FPN might be beneficial for motor learning. tACS to the FPN improved performance, when WM load was high, but not when WM-load was low. Therefore, we conclude that the FPN is a promising new target to enhance motor learning which might be most beneficial for individuals with decreased WM capacity due to age or stroke. In conclusion, this thesis demonstrates that targeting alternative motor network areas, and multifocal stimulation is promising. These results expand on the current knowledge of NIBS and identified open questions that require further examination but could ultimately lead towards enhanced efficacy and the personalization of study protocols.

EPFL2022

Does one size fit all? - Towards the optimization and personalization of non-invasive brain stimulation paradigms to enhance motor learning

Laurijn Rianne Draaisma

EPFL2022