Augmenting mirror visual feedback-induced performance improvements in older adults

Effects of transcranial direct current stimulation on motor learning in healthy elderly individuals: a systematic review and meta-analysis

BACKGROUND

Transcranial direct current stimulation (tDCS) is widely used in motor recovery. Nevertheless, whether tDCS improves motor learning in healthy older adults is still controversial. This review aims to investigate the effectiveness of tDCS on motor learning in healthy elderly individuals.

METHODS

The PubMed, Cochrane Library, Web of Science and Embase databases were initially searched from inception to December 5, 2022. The standard mean difference (SMD) with the corresponding 95% confidence intervals (CIs) were analysed via random-effect models.

RESULTS

Compared with the sham group, no significant effects were found regarding improvement in motor learning based on the speed or accuracy of the task and reaction time for the tDCS intervention group. After subgroup analysis, a significant effect was found for improved motor learning based on reaction time in the primary motor cortex (M1)-cerebellar group.

CONCLUSIONS

This review revealed that tDCS had no significant effect on improving the speed or accuracy of motor learning in healthy elderly adults. However, it has a significant effect on improving the motor learning ability based on the reaction time of the task (mainly referring to the tDCS stimulation position of M1 and cerebellar), although the results have obvious heterogeneity and uncertainty.

Effect of motor process-related priming via repeated transcranial magnetic stimulation on embodiment perception during mirror visual feedback: a pilot study

Introduction

Non-invasive brain stimulation has been combined with mirror visual feedback (MVF) as a priming strategy to enhance therapeutic efficacy. However, a superior combined effect is hindered by the lack of emphasis on MVF-relevant embodiment perception.

Objective

This study assessed the priming effect of repeated transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) and dorsolateral prefrontal cortex (dlPFC) on embodiment perception during MVF.

Methods

In the experiment, 15 healthy participants were required to complete tasks using their left hand while keeping their right hand static behind a mirror. They first received excitatory TMS over the left M1 or dlPFC, or sham-TMS in random order during three trial rounds and then performed three subsequent motor tasks and two task-oriented evaluations during MVF in each trial. Latency time (LT), number of embodiment occurrences, embodiment questionnaire (EQ) score, and time required to complete the task-oriented activities were recorded.

Results

The results showed that the LT of forearm rotation in the dlPFC-TMS round was shorter than that in the sham-TMS round, although a greater number of occurrences were obtained in both the M1-TMS and dlPFC-TMS rounds compared to the sham-TMS round within the three motor tasks, which suggested that TMS priming facilitated the elicitation of embodiment perception. The EQ results indicated strengthened embodiment perception after TMS priming, especially in the dlPFC-TMS round.

Conclusion

This study provides evidence that TMS priming over motor process-related regions, specifically the dlPFC, contributes to eliciting and intensifying embodiment perception during MVF, which benefited from a superior MVF paradigm for improving rehabilitation outcomes.

Clinical Trial Registration

Identifier ChiCTR2400089499 https://www.chictr.org.cn/showproj.html?proj=240385.

Age-related changes in motor cortex plasticity assessed with non-invasive brain stimulation: an update and new perspectives

It is commonly accepted that the brains capacity to change, known as plasticity, declines into old age. Recent studies have used a variety of non-invasive brain stimulation (NIBS) techniques to examine this age-related decline in plasticity in the primary motor cortex (M1), but the effects seem inconsistent and difficult to unravel. The purpose of this review is to provide an update on studies that have used different NIBS techniques to assess M1 plasticity with advancing age and offer some new perspective on NIBS strategies to boost plasticity in the ageing brain. We find that early studies show clear differences in M1 plasticity between young and older adults, but many recent studies with motor training show no decline in use-dependent M1 plasticity with age. For NIBS-induced plasticity in M1, some protocols show more convincing differences with advancing age than others. Therefore, our view from the NIBS literature is that it should not be automatically assumed that M1 plasticity declines with age. Instead, the effects of age are likely to depend on how M1 plasticity is measured, and the characteristics of the elderly population tested. We also suggest that NIBS performed concurrently with motor training is likely to be most effective at producing improvements in M1 plasticity and motor skill learning in older adults. Proposed NIBS techniques for future studies include combining multiple NIBS protocols in a co-stimulation approach, or NIBS strategies to modulate intracortical inhibitory mechanisms, in an effort to more effectively boost M1 plasticity and improve motor skill learning in older adults.

TDCS effects on pointing task learning in young and old adults

Skill increase in motor performance can be defined as explicitly measuring task success but also via more implicit measures of movement kinematics. Even though these measures are often related, there is evidence that they represent distinct concepts of learning. In the present study, the effect of multiple tDCS-sessions on both explicit and implicit measures of learning are investigated in a pointing task in 30 young adults (YA) between 27.07 ± 3.8 years and 30 old adults (OA) between 67.97 years ± 5.3 years. We hypothesized, that OA would show slower explicit skill learning indicated by higher movement times/lower accuracy and slower implicit learning indicated by higher spatial variability but profit more from anodal tDCS compared with YA. We found age-related differences in movement time but not in accuracy or spatial variability. TDCS did not skill learning facilitate learning neither in explicit nor implicit parameters. However, contrary to our hypotheses, we found tDCS-associated higher accuracy only in YA but not in spatial variability. Taken together, our data shows limited overlapping of tDCS effects in explicit and implicit skill parameters. Furthermore, it supports the assumption that tDCS is capable of producing a performance-enhancing brain state at least for explicit skill acquisition.

The Effects of Priming Intermittent Theta Burst Stimulation on Movement-Related and Mirror Visual Feedback-Induced Sensorimotor Desynchronization

The potential benefits of priming intermittent theta burst stimulation (iTBS) with continuous theta burst stimulation (cTBS) have not been examined in regard to sensorimotor oscillatory activities recorded in electroencephalography (EEG). The objective of this study was to investigate the modulatory effect of priming iTBS (cTBS followed by iTBS) delivered to the motor cortex on movement-related and mirror visual feedback (MVF)-induced sensorimotor event-related desynchronization (ERD), compared with iTBS alone, on healthy adults. Twenty participants were randomly allocated into Group 1: priming iTBS—cTBS followed by iTBS, and Group 2: non-priming iTBS—sham cTBS followed by iTBS. The stimulation was delivered to the right primary motor cortex daily for 4 consecutive days. EEG was measured before and after 4 sessions of stimulation. Movement-related ERD was evaluated during left-index finger tapping and MVF-induced sensorimotor ERD was evaluated by comparing the difference between right-index finger tapping with and without MVF. After stimulation, both protocols increased movement-related ERD and MVF-induced sensorimotor ERD in high mu and low beta bands, indicated by significant time effects. A significant interaction effect favoring Group 1 in enhancing movement-related ERD was observed in the high mu band [F_(1,18) = 4.47, p = 0.049], compared with Group 2. Our experiment suggests that among healthy adults priming iTBS with cTBS delivered to the motor cortex yields similar effects with iTBS alone on enhancing ERD induced by MVF-based observation, while movement-related ERD was more enhanced in the priming iTBS condition, specifically in the high mu band.

Reversal of Visual Feedback Modulates Somatosensory Plasticity

Reversed visual feedback during unimanual training increases transfer of skills to the opposite untrained hand and modulates plasticity in motor areas of the brain. However, it is unclear if unimanual training with reversed visual feedback also affects somatosensory areas. Here we manipulated visual input during unimanual training using left-right optical reversing spectacles and tested whether unimanual training with reversed vision modulates somatosensory cortical excitability to facilitate motor performance. Thirty participants practiced a unimanual ball-rotation task using the right hand with either left-right reversed vision (incongruent visual and somatosensory feedback) or direct vision (congruent feedback) of the moving hand. We estimated cortical excitability in primary somatosensory cortex (S1) before and after unimanual training by measuring somatosensory evoked potentials (SEPs). This was done by electrically stimulating the median nerve in the wrist while participants rested, and recording potentials over both hemispheres using electroencephalography. Performance of the ball-rotation task improved for both the right (trained) and left (untrained) hand after training across both direct and reversed vision conditions. Participants with direct vision of the right hand during training showed SEPs amplitudes increased bilaterally. In contrast, participants in the reversed visual condition showed attenuated SEPs following training. The results suggest that cortical suppression of S1 activity supports skilled motor performance after unimanual training with reversed vision, presumably by sensory gating of afferent signals from the movement. This finding provides insight into the mechanisms by which visual input interacts with the sensorimotor system and induces neuroplastic changes in S1 to support skilled motor performance.

Transcranial Direct Current Stimulation Effect on Virtual Hand Illusion

Virtual reality (VR) is effectively used to evoke the mirror illusion, and transcranial direct current stimulation (tDCS) synergistically facilitates this illusion. This study investigated whether a mirror virtual hand illusion (MVHI) induced by an immersive, first-person-perspective, virtual mirror system could be modulated by tDCS of the primary motor cortex. Fourteen healthy adults (average age 21.86 years ±0.47, seven men and seven women) participated in this study, and they experienced VR with and without tDCS-the tDCS and sham conditions, each of which takes ∼30 minutes-on separate days to allow the washout of the tDCS effect. While experiencing VR, the movements of the virtual left hand reflected the flexion and extension of the real right hand. Subsequently, electroencephalogram was recorded, the magnitude of the proprioceptive shift was measured, and the participants provided responses to a questionnaire regarding hand ownership. A significant difference in the proprioceptive shift was observed between the tDCS and sham conditions. In addition, there was significant suppression of the mu power in Pz, and augmentation of the beta power in the Pz, P4, O1, and O2 channels. The difference in proprioceptive deviation between the two conditions showed significant negative correlation with mu suppression over the left frontal lobe in the tDCS condition. Finally, the question "I felt that the virtual hand was my own hand" received a significantly higher score under the tDCS condition. In short, applying tDCS over the motor cortex facilitates the MVHI by activating the attentional network over the parietal and frontal lobes such that the MVHI induces more proprioceptive drift, which suggests that the combination of VR and tDCS can enhance the immersive effect in VR. This result provides better support for the use of the MVHI paradigm in combination with tDCS for recovery from illnesses such as stroke.

Enhancing mirror visual feedback with intermittent theta burst stimulation in healthy adults

BACKGROUND

Excitatory brain stimulation, in the form of intermittent theta burst stimulation (iTBS), combined with mirror visual feedback (MVF), is hypothesized to promote neuroplasticity and motor performance.

OBJECTIVE

This study aimed to investigate the combined effects of iTBS with mirror training (MT) on the MVF-induced sensorimotor event-related desynchronization (ERD) and the non-dominant hand motor performance in healthy adults.

METHODS

Eighteen healthy right-handed subjects were randomly assigned to one of three groups (Group 1: iTBS plus MT, Group 2: iTBS plus sham MT, or Group 3: sham iTBS plus MT). For participants in Groups 1 and 3, motor training was performed for 15 minutes for the right hand over four consecutive days, with MVF superimposing on their inactive left hand behind a mirror. Participants in Group 2 received the same right-hand motor training, but the mirror was covered without MVF. iTBS or sham iTBS was applied daily over the right primary motor cortex prior to the training. Electroencephalography at pre/post-training was recorded while participants performed right-hand movement under mirror and direct view. Motor performance was assessed at baseline and post-training.

RESULTS

Baseline comparisons demonstrated that a shift in sensorimotor ERD towards the right hemisphere was induced by MVF, in mu-1 (8-10 Hz) (p = 0.002), mu-2 (10-12 Hz) (p = 0.004) and beta-1 (12-16 Hz) (p = 0.049) bands. After the training, participants in Group 1 showed a stronger MVF-induced sensorimotor ERD in mu-1 (p = 0.017) and mu-2 (p = 0.009) bands than those in Group 3. No significant between-group difference in motor outcomes was observed.

CONCLUSIONS

iTBS appears to prime subjects' brain to be more receptive to MVF.

Acquisition of chopstick-operation skills with the non-dominant hand and concomitant changes in brain activity

Despite their common use as eating utensils in East Asia, chopsticks require complex fine motor-skills for adequate operation and are thus most frequently used with the dominant hand; however, the effect of training time on the proficiency of using chopsticks with the non-dominant hand, as well as the brain activity underlying changes in skill, remain unclear. This study characterised the effect of time spent training in chopstick operation with the non-dominant hand on chopstick-use proficiency and the related brain activity to obtain data that may help individuals who are obliged to change handedness due to neurological disease to learn to use their non-dominant hand in performing daily activities. Thirty-two healthy right-handed students were randomly allocated to training (n = 16) or control (n = 16) groups; the former received 6 weeks of training in chopstick use with their non-dominant (left) hand, and the latter received none. After training, significant improvements in the execution speed and smoothness of upper extremity joints were observed in the training group. Moreover, left dorsolateral prefrontal cortex activity significantly decreased, and bilateral premotor cortex activity significantly increased across training. These results indicated that 6 weeks of chopstick training with the non-dominant hand effectively improved chopstick operation.

TRANSCRANIAL DIRECT-CURRENT STIMULATION IN COMBINATION WITH EXERCISE: A SYSTEMATIC REVIEW

ABSTRACT Introduction Transcranial direct-current stimulation (tDCS) is a noninvasive technique that allows the modulation of cortical excitability and can produce changes in neuronal plasticity. The application of tDCS has recently been associated with physical activity. Objectives To verify the effect of Transcranial Direct-Current Stimulation (tDCS) in combination with physical exercise, characterizing methodological aspects of the technique. Methods In the database search, studies with animals, other neuromodulation techniques and opinion and review articles were excluded. Publications up to 2016 were selected and the methodological quality of the articles was verified through the PEDro scale. Results The majority of studies (86%) used tDCS on the motor cortex area, with anodal current and the allocation of monocephalic electrodes (46.5%). The prevalent current intensity was 2mA (72%), with duration of 20min (55.8%). The profile of the research participants was predominantly of subjects aged up to 60 years (72.1%). The outcomes were favorable for the use of anodal tDCS in combination with physical exercise. Conclusion Transcranial Direct-Current Stimulation is a promising technique when used in combination with aerobic and anaerobic exercises; however, it is necessary to investigate concurrent exercise. Level of Evidence II; Therapeutic Studies Investigating the Results of Treatment (systematic review of Level II studies or Level I studies with inconsistent results).

Impaired Motor Skill Acquisition Using Mirror Visual Feedback Improved by Transcranial Direct Current Stimulation (tDCS) in Patients With Parkinson's Disease

Recent non-invasive brain stimulation techniques in combination with motor training can enhance neuroplasticity and learning. It is reasonable to assume that such neuroplasticity-based interventions constitute a useful rehabilitative tool for patients with Parkinson's Disease (PD). Regarding motor skill training, many kinds of tasks that do not involve real motor movements have been applied to PD patients. The purpose of this study is to elucidate whether motor skill training using mirror visual feedback (MVF) is useful to patients with PD in order to improve untrained hand performance dependent on the time course of training; and whether MVF combined with anodal transcranial direct current stimulation (tDCS) over primary motor cortex (M1) causes an additional effect based on increased motor cortical excitability. Eighteen right-handed patients with PD in the off-medication state and 10 age-matched healthy subjects (HS) performed four sessions of right-hand ball rotation using MVF (intervention) on two separate days, 1 week apart (day 1 and day 2). HS subjects received only sham stimulation. The intervention included four sessions of motor-skill training using MVF for 20 min comprised of four sets of training for 30 s each. PD patients were randomly divided into two intervention groups without or with anodal tDCS over the right M1 contralateral to the untrained hand. As the behavior evaluation, the number of ball rotations of the left hand was counted before (pre) and immediately after (post) intervention on both days (pre day 1, post day 1, pre day 2, and post day 2). Motor evoked potential (MEP), input-output function, and cortical silent period were recorded to evaluate the motor cortical excitatory and inhibitory system in M1 pre day 1 and post day 2. The number of ball rotations of the left hand and the facilitation of MEP by intervention were significantly impaired in patients with PD compared to HS. In contrast, if anodal tDCS was applied to right M1 of patients with PD, the number of ball rotations in accordance with I-O function at 150% intensity was significantly increased after day 1 and retained until day 2. This finding may help provide a new strategy for neurorehabilitation improving task-specific motor memory without real motor movements in PD.

Effect of visual feedback on the performance of the star excursion balance test

INTRODUCTION

Visual feedback is an effective method to enhance postural and balance control in clinical and sports training. The aim of this study was to explore the effect of real-time visual feedback provided by a video camera on the performance of a dynamic balance test, which is the star excursion balance test in healthy subjects.

METHODS

We compared the performance of the star excursion balance test using the maximum reach distance in 20 healthy participants (10 male and 10 female, 26.8 ± 3.7 years) under two conditions: without feedback and whilst they viewed their movements in real-time on a screen in front of them via a video camera.

RESULTS

The results showed that real-time visual feedback had a significant effect on maximum reach distance of the star excursion balance test in the posterolateral direction (P < 0.001). There was a non-significant increase in the maximum reach distance in the anterior and posteromedial directions.

CONCLUSION

The result indicates that the real-time visual feedback appears to be an effective means for improving the performance of the star excursion balance test in the posterolateral direction, and may be a promising tool for clinical rehabilitation and athlete training to enhance dynamic postural control.

Motor Learning Improvement Remains 3 Months After a Multisession Anodal tDCS Intervention in an Aging Population

Healthy aging is associated with decline of motor function that can generate serious consequences on the quality of life and safety. Our studies aim to explore the 3-month effects of a 5-day multisession anodal transcranial direct current stimulation (a-tDCS) protocol applied over the primary motor cortex (M1) during motor sequence learning in elderly. The present sham-controlled aging study investigated whether tDCS-induced motor improvements previously observed 1 day after the intervention persist beyond 3 months. A total of 37 cognitively-intact aging participants performed five consecutive daily 20-min sessions of the serial-reaction time task (SRTT) concomitant with either anodal (n = 18) or sham (n = 19) tDCS over M1. All participants performed the Purdue Pegboard Test and transcranial magnetic stimulation (TMS) measures of cortical excitability were collected before, 1 day after and 3 months after the intervention. The last follow-up session also included the execution of the trained SRTT. The main findings are the demonstration of durable effects of a 5-day anodal tDCS intervention at the trained skill, while the active intervention did not differ from the sham intervention at both the untrained task and on measures of M1-disinhibition. Thus, the current article revealed for the first time the durability of functional effects of a-tDCS combined with motor training after only 5 days of intervention in an aging population. This finding provides evidence that the latter treatment alternative is effective in achieving our primary motor rehabilitation goal, that is to allow durable motor training effects in an aging population.

Targeted ballet program mitigates ataxia and improves balance in females with mild-to-moderate multiple sclerosis

Background

Multiple sclerosis (MS) is a disease of the central nervous system that causes ataxia and deficits in balance. Exercise-based therapies have been identified as integral to the recovery of motor function in MS, but few studies have investigated non-traditional movement interventions. We examined a targeted ballet program (TBP) designed to mitigate ataxia and improve balance in females with mild-to-moderate relapsing-remitting MS.

Methods and findings

Twelve females with mild-to-moderate disability due to MS were assessed for study eligibility for the study. Ten participants met the inclusion criteria. Two were lost to unrelated health complications. Eight participants completed the TBP. The TBP met twice a week for 60 minutes for 16 weeks. Assessments included (a) the International Cooperative Ataxia Rating Scale (ICARS), (b) the Mini-Balance Evaluations Systems Test (Mini-BESTest), (c) smoothness of movement during a five-meter walk, and (d) balance in a step to stand task before and after the TBP. There were no TBP-related adverse events. Single-tailed paired samples t-tests and Wilcoxon tests were conducted. Improvements were observed in ICARS (p = 7.11E-05), Mini-BESTest (p = 0.001), smoothness of movement in the left (p = 0.027) and right (p = 0.028) sides of the body, and balance in a step-to-stand task in the back (p = 0.025) direction. Results yielded 42% and 58% improvements in the mean Mini-BESTest and ICARS scores, respectively.

Conclusions

This study adds to current research by providing support for a TBP intervention targeting ataxia and balance in MS. The TBP was well tolerated, improved balance, and mitigated ataxia. Clinical improvements were larger than those of previous studies on physical rehabilitation in MS with similar outcome measures.

Trial registration

ISRCTN ISRCTN67916624.

Multisession anodal transcranial direct current stimulation induces motor cortex plasticity enhancement and motor learning generalization in an aging population

OBJECTIVES

The present aging study investigated the impact of a multisession anodal-tDCS protocol applied over the primary motor cortex (M1) during motor sequence learning on generalization of motor learning and plasticity-dependent measures of cortical excitability.

METHODS

A total of 32 cognitively-intact aging participants performed five consecutive daily 20-min sessions of the serial-reaction time task (SRTT) concomitant with either anodal (n = 16) or sham (n = 16) tDCS over M1. Before and after the intervention, all participants performed the Purdue Pegboard Test (PPT) and Transcranial Magnetic Stimulation (TMS) measures of cortical excitability were collected.

RESULTS

Relative to sham, participants assigned to the anodal-tDCS intervention revealed significantly greater performance gains on both the trained SRTT and the untrained PPT as well as a greater disinhibition of long-interval cortical inhibition (LICI). Generalization effects of anodal-tDCS significantly correlated with LICI disinhibition.

CONCLUSION

Anodal-tDCS facilitates motor learning generalisation in an aging population through intracortical disinhibition effects.

SIGNIFICANCE

The current findings demonstrate the potential clinical utility of a multisession anodal-tDCS over M1 protocol as an adjuvant to motor training in alleviating age-associated motor function decline. This study also reveals the pertinence of implementing brain stimulation techniques to modulate age-associated intracortical inhibition changes in order to facilitate motor function gains.

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

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 <4mA, up to 60min duration per day. Animal studies and modeling evidence indicate that brain injury could occur at predicted current densities in the brain of 6.3-13A/m2 that are over an order of magnitude above those produced by tDCS in humans. Using AC stimulation fewer AEs were reported compared to DC. In specific paradigms with amplitudes of up to 10mA, frequencies in the kHz range appear to be safe. In this paper we provide structured interviews and recommend their use in future controlled studies, in particular when trying to extend the parameters applied. We also discuss recent regulatory issues, reporting practices and ethical issues. These recommendations achieved consensus in a meeting, which took place in Göttingen, Germany, on September 6-7, 2016 and were refined thereafter by email correspondence.

Neural Correlates of Mirror Visual Feedback-Induced Performance Improvements: A Resting-State fMRI Study

Mirror visual feedback (MVF) is a promising approach to enhance motor performance without training in healthy adults as well as in patients with focal brain lesions. There is preliminary evidence that a functional modulation within and between primary motor cortices as assessed with transcranial magnetic stimulation (TMS) might be one candidate mechanism mediating the observed behavioral effects. Recently, studies using task-based functional magnetic resonance imaging (fMRI) have indicated that MVF-induced functional changes might not be restricted to the primary motor cortex (M1) but also include higher order regions responsible for perceptual-motor coordination and visual attention. However, aside from these instantaneous task-induced brain changes, little is known about learning-related neuroplasticity induced by MVF. Thus, in the present study, we assessed MVF-induced functional network plasticity with resting-state fMRI (rs-fMRI). We performed rs-fMRI of 35 right-handed, healthy adults before and after performing a complex ball-rotation task. The primary outcome measure was the performance improvement of the untrained left hand (LH) before and after right hand (RH) training with MVF (mirror group [MG], n = 17) or without MVF (control group [CG], n = 18). Behaviorally, the MG showed superior performance improvements of the untrained LH. In resting-state functional connectivity (rs-FC), an interaction analysis between groups showed changes in left visual cortex (V1, V2) revealing an increase of centrality in the MG. Within group comparisons showed further functional alterations in bilateral primary sensorimotor cortex (SM1), left V4 and left anterior intraparietal sulcus (aIP) in the MG, only. Importantly, a correlation analysis revealed a linear positive relationship between MVF-induced improvements of the untrained LH and functional alterations in left SM1. Our results suggest that MVF-induced performance improvements are associated with functional learning-related brain plasticity and have identified additional target regions for non-invasive brain stimulation techniques, a finding of potential interest for neurorehabilitation.

Effects of tDCS on motor learning and memory formation: A consensus and critical position paper

Motor skills are required for activities of daily living. Transcranial direct current stimulation (tDCS) applied in association with motor skill learning has been investigated as a tool for enhancing training effects in health and disease. Here, we review the published literature investigating whether tDCS can facilitate the acquisition, retention or adaptation of motor skills. Work in multiple laboratories is underway to develop a mechanistic understanding of tDCS effects on different forms of learning and to optimize stimulation protocols. Efforts are required to improve reproducibility and standardization. Overall, reproducibility remains to be fully tested, effect sizes with present techniques vary over a wide range, and the basis of observed inter-individual variability in tDCS effects is incompletely understood. It is recommended that future studies explicitly state in the Methods the exploratory (hypothesis-generating) or hypothesis-driven (confirmatory) nature of the experimental designs. General research practices could be improved with prospective pre-registration of hypothesis-based investigations, more emphasis on the detailed description of methods (including all pertinent details to enable future modeling of induced current and experimental replication), and use of post-publication open data repositories. A checklist is proposed for reporting tDCS investigations in a way that can improve efforts to assess reproducibility.

Mirror Visual Feedback to Improve Bradykinesia in Parkinson's Disease

Mirror visual feedback (MVF) therapy has been applied to improve upper limb function in stroke. When combined with motor training, MVF improves the performance of the trained and untrained hand by enhancing the excitability of both primary motor cortices (M1s). Bradykinesia is a typical feature of Parkinson's disease (PD), characterized by slowness in the execution of movement. This condition is often asymmetrical and possibly supported by a volitional hypoactivation of M1. MVF therapy could tentatively treat bradykinesia since the untrained hand, which benefits from the exercise, is generally more severely impaired in undertaking sequential movements. Aim of the study was to evaluate whether MVF therapy may improve bradykinesia of the more affected hand in PD patients. Twelve PD patients and twelve healthy controls performed for 10 minutes a finger sequence, receiving MVF of the more affected/nondominant hand. Before and after MVF training, participants performed a finger sequence at their spontaneous pace with both hands. M1 excitability was assessed in the trained and untrained hemispheres by means of transcranial magnetic stimulation. Movement speed increased after MVF training in either hand of both groups. MVF therapy enhanced cortical excitability of M1s in both groups. Our preliminary data support the use of MVF therapy to improve bradykinesia in PD patients.

Age-related deficits in voluntary control over saccadic eye movements: consideration of electrical brain stimulation as a therapeutic strategy

Sudden changes in our visual environment trigger reflexive eye movements, so automatically they often go unnoticed. Consequently, voluntary control over reflexive eye movements entails considerable effort. In relation to frontal-lobe deterioration, adult aging adversely impacts voluntary saccadic eye movement control in particular, which compromises effective performance of daily activities. Here, we review the nature of age-related changes in saccadic control, focusing primarily on the antisaccade task because of its assessment of 2 key age-sensitive control functions: reflexive saccade inhibition and voluntary saccade generation. With an ultimate view toward facilitating development of therapeutic strategies, we systematically review the neuroanatomy underpinning voluntary control over saccadic eye movements and natural mechanisms that kick in to compensate for age-related declines. We then explore the potential of noninvasive electrical brain stimulation to counteract aging deficits. Based on evidence that anodal transcranial direct current stimulation can confer a range of benefits specifically relevant to aging brains, we put forward this neuromodulation technique as a therapeutic strategy for improving voluntary saccadic eye movement control in older adults.

Mirror Visual Feedback-Induced Performance Improvement and the Influence of Hand Dominance

Mirror visual feedback (MVF) is a promising technique in clinical settings that can be used to augment performance of an untrained limb. Several studies with healthy volunteers and patients using transcranial magnetic stimulation (TMS) or functional magnetic resonance imaging (fMRI) indicate that functional alterations within primary motor cortex (M1) might be one candidate mechanism that could explain MVF-induced changes in behavior. Until now, most studies have used MVF to improve performance of the non-dominant hand (NDH). The question remains if the behavioral effect of MVF differs according to hand dominance. Here, we conducted a study with two groups of young, healthy right-handed volunteers who performed a complex ball-rotation task while receiving MVF of the dominant (n = 16, group 1, MVF_DH) or NDH (n = 16, group 2, MVF_NDH). We found no significant differences in baseline performance of the untrained hand between groups before MVF was applied. Furthermore, there was no significant difference in the amount of performance improvement between MVF_DH and MVF_NDH indicating that the outcome of MVF seems not to be influenced by hand dominance. Thus our findings might have important implications in neurorehabilitation suggesting that patients suffering from unilateral motor impairments might benefit from MVF regardless of the dominance of the affected limb.

Does transcranial direct current stimulation enhance cognitive and motor functions in the ageing brain? A systematic review and meta- analysis

The use of transcranial direct current stimulation (tDCS) to enhance cognitive and motor functions has enjoyed a massive increase in popularity. Modifying neuroplasticity via non-invasive cortical stimulation has enormous potential to slow or even reverse declines in functions associated with ageing. The current meta-analysis evaluated the effects of tDCS on cognitive and motor performance in healthy older adults. Of the 81 studies identified, 25 qualified for inclusion. A random effects model meta-analysis revealed a significant overall standardized mean difference equal to 0.53 (SE=0.09; medium heterogeneity: I(2)=57.08%; and high fail-safe: N=448). Five analyses on moderator variables indicated significant tDCS beneficial effects: (a) on both cognitive and motor task performances, (b) across a wide-range of cognitive tasks, (c) on specific brain areas, (d) stimulation offline (before) or online (during) the cognitive and motor tasks. Although the meta-analysis revealed robust support for enhancing both cognitive and motor performance, we outline a number of caveats on the use of tDCS.

The Influence of Mirror-Visual Feedback on Training-Induced Motor Performance Gains in the Untrained Hand

The well-documented observation of bilateral performance gains following unilateral motor training, a phenomenon known as cross-limb transfer, has important implications for rehabilitation. It has recently been shown that provision of a mirror image of the active hand during unilateral motor training has the capacity to enhance the efficacy of this phenomenon when compared to training without augmented visual feedback (i.e., watching the passive hand), possibly via action observation effects [1]. The current experiment was designed to confirm whether mirror-visual feedback (MVF) during motor training can indeed elicit greater performance gains in the untrained hand compared to more standard visual feedback (i.e., watching the active hand). Furthermore, discussing the mechanisms underlying any such MVF-induced behavioural effects, we suggest that action observation and the cross-activation hypothesis may both play important roles in eliciting cross-limb transfer. Eighty participants practiced a fast-as-possible two-ball rotation task with their dominant hand. During training, three different groups were provided with concurrent visual feedback of the active hand, inactive hand or a mirror image of the active hand with a fourth control group receiving no training. Pre- and post-training performance was measured in both hands. MVF did not increase the extent of training-induced performance changes in the untrained hand following unilateral training above and beyond those observed for other types of feedback. The data are consistent with the notion that cross-limb transfer, when combined with MVF, is mediated by cross-activation with action observation playing a less unique role than previously suggested. Further research is needed to replicate the current and previous studies to determine the clinical relevance and potential benefits of MVF for cases that, due to the severity of impairment, rely on unilateral training programmes of the unaffected limb to drive changes in the contralateral affected limb.