Mini-review: Transcranial Alternating Current Stimulation and the Cerebellum.

Oscillatory activity in the cerebellum and linked networks is an important aspect of neuronal processing and functional implementation of behavior. So far, it was challenging to quantify and study cerebellar oscillatory signatures in human neuroscience due to the constraints of non-invasive cerebellar electrophysiological recording and interventional techniques. The emerging cerebellar transcranial alternating current stimulation technique (CB-tACS) is a promising tool, which may partially overcome this challenge and provides an exciting non-invasive opportunity to better understand cerebellar physiology.Several studies have successfully demonstrated that CB-tACS can modulate the cerebellar outflow and cerebellum-linked behavior. In the present narrative review, we summarize current studies employing the CB-tACS approach and discuss open research questions. Hereby, we aim to provide an overview on this emerging electrophysiological technique and strive to promote future research in the field. CB-tACS will contribute in the further deciphering of cerebellar oscillatory signatures and its role for motor, cognitive, or affective functions. In long term, CB-tACS could develop into a therapeutic tool for retuning disturbed oscillatory activity in cerebellar networks underlying brain disorders.

Neurotechnologies as tools for cognitive rehabilitation in stroke patients.

INTRODUCTION: Cognitive impairments are one of the most common remaining symptoms after a stroke. The use of neurotechnologies to enhance cognitive performance is a rapidly emerging field with encouraging results. AREAS COVERED: Here, the authors empirically review the respective literature and critically discuss the technologies that are currently most often used for cognitive enhancement in stroke patients, which are computerized cognitive training, virtual reality, noninvasive brain stimulation and brain-computer interfaces. The authors describe their advantages/disadvantages and the challenges and limitations to overcome. EXPERT OPINION: Although the current results are promising, more research is needed to be able to make conclusive statements and translate these approaches successfully in daily clinical life. Multidiscipline collaborations could aid to improve current neurotechnologies and provide guidelines for future implementations.

Non-invasive brain stimulation to enhance cognitive rehabilitation after stroke.

Stroke is a main cause for long-term disability. Stroke symptoms cover various domains, e.g., motor, sensory, language, or other cognitive functions. In clinical practice and rehabilitation research, especially motor impairment attracts much attention. However, also cognitive impairments are common after stroke, effecting approximately two-thirds of stroke patients in the acute phase. Although 30% of stroke patients spontaneously recover from their cognitive impairments, large amounts of patients remain cognitively impaired. These patients have more problems reintegrating in personal and professional life. To date, cognitive rehabilitation strategies are not yet satisfactory. One promising strategy is combining non-invasive brain stimulation (NIBS) with cognitive training. In the current review, we will discuss the relevance of cognitive impairment after stroke and innovative interventional strategies to improve cognition, such as NIBS. Furthermore, we will address the potential of using cognitive training to enhance recovery in other behavioural domains, such as the motor domain.

Cerebellar transcranial alternating current stimulation in the gamma range applied during the acquisition of a novel motor skill.

The development of novel strategies to augment motor training success is of great interest for healthy persons and neurological patients. A promising approach is the combination of training with transcranial electric stimulation. However, limited reproducibility and varying effect sizes make further protocol optimization necessary. We tested the effects of a novel cerebellar transcranial alternating current stimulation protocol (tACS) on motor skill learning. Furthermore, we studied underlying mechanisms by means of transcranial magnetic stimulation and analysis of fMRI-based resting-state connectivity. N = 15 young, healthy participants were recruited. 50 Hz tACS was applied to the left cerebellum in a double-blind, sham-controlled, cross-over design concurrently to the acquisition of a novel motor skill. Potential underlying mechanisms were assessed by studying short intracortical inhibition at rest (SICIrest) and in the premovement phase (SICImove), intracortical facilitation at rest (ICFrest), and seed-based resting-state fMRI-based functional connectivity (FC) in a hypothesis-driven motor learning network. Active stimulation did not enhance skill acquisition or retention. Minor effects on striato-parietal FC were present. Linear mixed effects modelling identified SICImove modulation and baseline task performance as the most influential determining factors for predicting training success. Accounting for the identified factors may allow to stratify participants for future training-based interventions.

The Effects of Stimulator, Waveform, and Current Direction on Intracortical Inhibition and Facilitation: A TMS Comparison Study.

Background: Cortical function is dependent on the balance between excitatory and inhibitory influences. In the human motor cortex, surrogates of these interactions can be measured in vivo, non-invasively with double-pulse transcranial magnetic stimulation (TMS). To compare results from data acquired with different available setups and bring data together, it is inevitable to determine whether different TMS setups lead to comparable or differential results. Objective: We assessed and compared short intracortical inhibition (SICI) and intracortical facilitation (ICF) testing four different experimental conditions. Methods: SICI and ICF were studied with different stimulators (Magstim BiStim2 or MagVenture MagPro X100), waveforms (monophasic or biphasic), current directions (anterior-posterior or posterior-anterior) at interstimulus intervals (ISIs) of 1, 3, 10, 15 ms. Results: We were not able to detect differences for SICI and ICF, when comparing the tested conditions, except for 3 ms SICI in which the anterior-posterior current direction led to stronger modulation. Correlation analysis suggested comparability for 3 ms SICI for the Magstim monophasic posterior-anterior condition with both tested MagVenture conditions. Conclusions: 3 ms SICI data sets obtained with two different, commonly used stimulators (Magstim BiStim2 or MagVenture MagPro X100) with conventionally used stimulation parameters are largely comparable. This may allow the combination of data sets in an open science view.