Individual differences in TMS sensitivity influence the efficacy of tDCS in facilitating sensorimotor adaptation

Interference underlies attenuation upon relearning in sensorimotor adaptation

Savings refers to the gain in performance upon relearning a task. In sensorimotor adaptation, savings is tested by having participants adapt to perturbed feedback and, following a washout block during which the system resets to baseline, presenting the same perturbation again. While savings has been observed with these tasks, we have shown that the contribution from implicit sensorimotor adaptation, a process that uses sensory prediction errors to recalibrate the sensorimotor map, is actually attenuated upon relearning (Avraham et al., 2021). In the present study, we test the hypothesis that this attenuation is due to interference arising from the washout block, and more generally, from experience with a different relationship between the movement and the feedback. In standard adaptation studies, removing the perturbation at the start of the washout block results in a salient error signal in the opposite direction to that observed during learning. As a starting point, we replicated the finding that implicit adaptation is attenuated following a washout period in which the feedback now signals a salient opposite error. When we eliminated visual feedback during washout, implicit adaptation was no longer attenuated upon relearning, consistent with the interference hypothesis. Next, we eliminated the salient error during washout by gradually decreasing the perturbation, creating a scenario in which the perceived errors fell within the range associated with motor noise. Nonetheless, attenuation was still prominent. Inspired by this observation, we tested participants with an extended experience with veridical feedback during an initial baseline phase and found that this was sufficient to cause robust attenuation of implicit adaptation during the first exposure to the perturbation. This effect was context-specific: It did not generalize to movements that were not associated with the interfering feedback. Taken together, these results show that the implicit sensorimotor adaptation system is highly sensitive to memory interference from a recent experience with a discrepant action-outcome contingency.

Four hours of normobaric hypoxia reduces Achilles tendon reflex inhibition

Acute exposure to hypoxia increases postural sway, but the underlying neurophysiological factors are unclear. Golgi tendon organs (GTOs), located within the musculotendinous junction (MTJ), provide inhibitory signals to plantar flexor muscles that are important for balance control; however, it is uncertain if GTO function is influenced by hypoxia. The aim of this study was to determine how normobaric hypoxia influences lower limb tendon-evoked inhibitory reflexes during upright stance. We hypothesized that tendon-evoked reflex area and duration would decrease during hypoxia, indicating less inhibition of postural muscles compared with normoxia. At baseline (BL; 0.21 fraction of inspired oxygen, FIO2) and at ∼2 (H2) and 4 (H4) h of normobaric hypoxia (0.11 FIO2) in a normobaric hypoxic chamber, 16 healthy participants received electrical musculotendinous stimulation (MTstim) to the MTJ of the left Achilles tendon. The MTstim was delivered as two sets of 50 stimuli while the participant stood on a force plate with their feet together. Tendon-evoked inhibitory reflexes were recorded from the surface electromyogram of the ipsilateral medial gastrocnemius, and center of pressure (CoP) variables were recorded from the force plate. Normobaric hypoxia increased CoP velocity (P ≤ 0.002) but not CoP standard deviation (P ≥ 0.12). Compared with BL, normobaric hypoxia reduced tendon-evoked inhibitory reflex area by 45% at H2 and 53% at H4 (P ≤ 0.002). In contrast, reflex duration was unchanged during hypoxia. The reduced inhibitory feedback from the GTO pathway could likely play a role in the increased postural sway observed during acute exposure to hypoxia.NEW & NOTEWORTHY The Ib pathway arising from the Golgi tendon organ provides inhibitory signals onto motor neuron pools that modifies force and, hence, postural control. Although hypoxia influences standing balance (increases sway), the underlying mechanisms have yet to be unraveled. Our study identified that tendon-evoked inhibition onto a plantar flexor motoneuron pool is reduced by acute exposure to normobaric hypoxia. This reduction of inhibition may contribute to the hypoxia-related increase in postural sway.

Advanced feedback enhances sensorimotor adaptation

It is widely recognized that sensorimotor adaptation is facilitated when feedback is provided throughout the movement compared with when it is provided at the end of the movement. However, the source of this advantage is unclear: continuous feedback is more ecological, dynamic, and available earlier than endpoint feedback. Here, we assess the relative merits of these factors using a method that allows us to manipulate feedback timing independent of actual hand position. By manipulating the onset time of "endpoint" feedback, we found that adaptation was modulated in a non-monotonic manner, with the peak of the function occurring in advance of the hand reaching the target. Moreover, at this optimal time, learning was of similar magnitude as that observed with continuous feedback. By varying movement duration, we demonstrate that this optimal time occurs at a relatively fixed time after movement onset, an interval we hypothesize corresponds to when the comparison of the sensory prediction and feedback generates the strongest error signal.

Time dependent effects of cerebellar tDCS on cerebello-cortical connectivity networks in young adults

The cerebellum is involved in non-motor processing, supported by topographically distinct cerebellar activations and closed loop circuits between the cerebellum and the cortex. Disruptions to cerebellar function and network connectivity in aging or disease may negatively impact prefrontal function and processing. Cerebellar resources may be important for offloading cortical processing, providing crucial scaffolding for normative performance and function. Here, we used transcranial direct current stimulation (tDCS) to temporarily alter cerebellar function and subsequently investigated resting state network connectivity. This allows us to investigate network changes that may parallel what is seen in aging and clinical populations, providing additional insights into these key circuits. Critically, what happens to these circuits if the cerebellum is not functioning optimally remains relatively unknown. We employed a between-subjects design applying anodal (n=25), cathodal (n=25), or sham (n=24) stimulation to the cerebellum to examine the effect of stimulation on cerebello-cortical resting state connectivity in young adults. We predicted increased functional connectivity following cathodal stimulation and decreased functional connectivity following anodal stimulation. We found, anodal stimulation resulted in increased connectivity in both ipsilateral and contralateral regions of the cortex, perhaps indicative of a compensatory response to degraded cerebellar output. Additionally, a sliding window analysis also demonstrated a time dependent nature to the impacts of cerebellar tDCS on connectivity, particularly in cognitive region in the cortex. Assuming the difference in connectivity and network-behavior relationships here parallels what occurs in aging or disease, this may provide a mechanism whereby offloading of function to the cerebellum is negatively impacted, resulting in subsequent differences in prefrontal cortical activation patterns and performance deficits. These results might inform and update existing compensatory models of function to include the cerebellum as a vital structure needed for scaffolding.

Evolving techniques for reducing phantom limb pain

At least two million people in the United States of America live with lost limbs, and the number is expected to double by 2050, although the incidence of amputations is significantly greater in other parts of the world. Within days to weeks of the amputation, up to 90% of these individuals develop neuropathic pain, presenting as phantom limb pain (PLP). The pain level increases significantly within one year and remains chronic and severe for about 10%. Amputation-induced changes are considered to underlie the causation of PLP. Techniques applied to the central nervous system (CNS) and peripheral nervous system (PNS) are designed to reverse amputation-induced changes, thereby reducing/eliminating PLP. The primary treatment for PLP is the administration of pharmacological agents, some of which are considered but provide no more than short-term pain relief. Alternative techniques are also discussed, which provide only short-term pain relief. Changes induced by various cells and the factors they release are required to change neurons and their environment to reduce/eliminate PLP. It is concluded that novel techniques that utilize autologous platelet-rich plasma (PRP) may provide long-term PLP reduction/elimination.

Predicting individual differences in motor learning: a critical review

The ability to predict individual differences in motor learning has significant implications from both theoretical and applied perspectives. However, there is high variability in the methodological and analytical strategies employed as evidence for such predictions. Here, we critically examine the evidence for predictions of individual differences in motor learning by reviewing the literature from a 20-year period (2000-2020). Specifically, we examined four factors: (i) the predictor and predicted variables used, (ii) the strength of the prediction and associated sample size, (iii) the timescale over which the prediction was made, and (iv) the type of motor task used. Overall, the results highlight several issues that raise concerns about the quality of the evidence for such predictions. First, there was a large variation in both predictor and predicted variables, suggesting the presence of a large number of researcher degrees of freedom. Second, sample sizes tended to be small, and the strength of the correlation showed an inverse relation with sample size. Third, the timescale of most predictions was very short, mostly constrained to a single day. Last, most studies were largely restricted to two experimental paradigms - adaptation and sequence learning. Based on these issues, we highlight recommendations for future studies to improve the quality of evidence for predicting individual differences in motor learning.

Walking and Balance Outcomes Are Improved Following Brief Intensive Locomotor Skill Training but Are Not Augmented by Transcranial Direct Current Stimulation in Persons With Chronic Spinal Cord Injury

Motor training to improve walking and balance function is a common aspect of rehabilitation following motor-incomplete spinal cord injury (MISCI). Evidence suggests that moderate- to high-intensity exercise facilitates neuroplastic mechanisms that support motor skill acquisition and learning. Furthermore, enhancing corticospinal drive via transcranial direct current stimulation (tDCS) may augment the effects of motor training. In this pilot study, we investigated whether a brief moderate-intensity locomotor-related motor skill training (MST) circuit, with and without tDCS, improved walking and balance outcomes in persons with MISCI. In addition, we examined potential differences between within-day (online) and between-day (offline) effects of MST. Twenty-six adults with chronic MISCI, who had some walking ability, were enrolled in a 5-day double-blind, randomized study with a 3-day intervention period. Participants were assigned to an intensive locomotor MST circuit and concurrent application of either sham tDCS (MST+tDCS_sham) or active tDCS (MST+tDCS). The primary outcome was overground walking speed measured during the 10-meter walk test. Secondary outcomes included spatiotemporal gait characteristics (cadence and stride length), peak trailing limb angle (TLA), intralimb coordination (ACC), the Berg Balance Scale (BBS), and the Falls Efficacy Scale-International (FES-I) questionnaire. Analyses revealed a significant effect of the MST circuit, with improvements in walking speed, cadence, bilateral stride length, stronger limb TLA, weaker limb ACC, BBS, and FES-I observed in both the MST+tDCS_sham and MST+tDCS groups. No differences in outcomes were observed between groups. Between-day change accounted for a greater percentage of the overall change in walking outcomes. In persons with MISCI, brief intensive MST involving a circuit of ballistic, cyclic locomotor-related skill activities improved walking outcomes, and selected strength and balance outcomes; however, concurrent application of tDCS did not further enhance the effects of MST.

Inter-Individual Variability in tDCS Effects: A Narrative Review on the Contribution of Stable, Variable, and Contextual Factors

Due to its safety, portability, and cheapness, transcranial direct current stimulation (tDCS) use largely increased in research and clinical settings. Despite tDCS's wide application, previous works pointed out inconsistent and low replicable results, sometimes leading to extreme conclusions about tDCS's ineffectiveness in modulating behavioral performance across cognitive domains. Traditionally, this variability has been linked to significant differences in the stimulation protocols across studies, including stimulation parameters, target regions, and electrodes montage. Here, we reviewed and discussed evidence of heterogeneity emerging at the intra-study level, namely inter-individual differences that may influence the response to tDCS within each study. This source of variability has been largely neglected by literature, being results mainly analyzed at the group level. Previous research, however, highlighted that only a half-or less-of studies' participants could be classified as responders, being affected by tDCS in the expected direction. Stable and variable inter-individual differences, such as morphological and genetic features vs. hormonal/exogenous substance consumption, partially account for this heterogeneity. Moreover, variability comes from experiments' contextual elements, such as participants' engagement/baseline capacity and individual task difficulty. We concluded that increasing knowledge on inter-dividual differences rather than undermining tDCS effectiveness could enhance protocols' efficiency and reproducibility.

Acoustic stimulation increases implicit adaptation in sensorimotor adaptation

Sensorimotor adaptation is an important part of our ability to perform novel motor tasks (i.e., learning of motor skills). Efforts to improve adaptation in healthy and clinical patients using non-invasive brain stimulation methods have been hindered by inter-individual and intra-individual variability in brain susceptibility to stimulation. Here, we explore unpredictable loud acoustic stimulation as an alternative method of modulating brain excitability to improve sensorimotor adaptation. In two experiments, participants moved a cursor towards targets, and adapted to a 30º rotation of cursor feedback, either with or without unpredictable acoustic stimulation. Acoustic stimulation improved initial adaptation to sensory prediction errors in Study 1, and improved overnight retention of adaptation in Study 2. Unpredictable loud acoustic stimulation might thus be a potent method of modulating sensorimotor adaptation in healthy adults.

Developing Proprioceptive Countermeasures to Mitigate Postural and Locomotor Control Deficits After Long-Duration Spaceflight

Astronauts experience post-flight disturbances in postural and locomotor control due to sensorimotor adaptations during spaceflight. These alterations may have adverse consequences if a rapid egress is required after landing. Although current exercise protocols can effectively mitigate cardiovascular and muscular deconditioning, the benefits to post-flight sensorimotor dysfunction are limited. Furthermore, some exercise capabilities like treadmill running are currently not feasible on exploration spaceflight vehicles. Thus, new in-flight operational countermeasures are needed to mitigate postural and locomotor control deficits after exploration missions. Data from spaceflight and from analog studies collectively suggest that body unloading decreases the utilization of proprioceptive input, and this adaptation strongly contributes to balance dysfunction after spaceflight. For example, on return to Earth, an astronaut's vestibular input may be compromised by adaptation to microgravity, but their proprioceptive input is compromised by body unloading. Since proprioceptive and tactile input are important for maintaining postural control, keeping these systems tuned to respond to upright balance challenges during flight may improve functional task performance after flight through dynamic reweighting of sensory input. Novel approaches are needed to compensate for the challenges of balance training in microgravity and must be tested in a body unloading environment such as head down bed rest. Here, we review insights from the literature and provide observations from our laboratory that could inform the development of an in-flight proprioceptive countermeasure.

Brain stimulation by tDCS as treatment option in Autism Spectrum Disorder-A systematic literature review

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social communication and interaction as well as stereotypical and repetitive behavior. Transcranial direct current stimulation (tDCS) has been proposed as a new intervention method in ASD with the potential to improve cognitive, motor and social communication abilities by targeting specific underlying neuronal alterations. Here, we report results of a systematic literature review on tDCS effects on EEG and behavioral outcomes, and discuss tDCS as treatment option for ASD. PsychInfo, PubMed, ScienceDirect, Web of Science, https://clinicaltrials.gov and the German Clinical Trials Register (Deutsches Register Klinischer Studien) were searched systematically for randomized, sham-controlled clinical trials of tDCS in individuals with ASD, and information regarding study designs and relevant results was extracted. Six eligible studies were identified. The dorsolateral prefrontal cortex (DLPFC) was targeted in four trials, with core ASD symptoms and working memory as outcome measures. One study targeted the primary motor cortex (M1) with motor skills as outcome, and one study targeted the temporoparietal junction (TPJ) with social communication skills as outcome measure. Comparison of the implemented study designs showed high methodological variability between studies regarding stimulation parameters, trial design and outcome measures. Study results indicate initial support for improved cognitive and social communication skills in ASD following tDCS stimulation. However, systematic and comparison studies on the best combination of stimulation intensity, duration, location as well as task related stimulation are necessary, before results can be translated into routine clinical application.

Reexposure to a sensorimotor perturbation produces opposite effects on explicit and implicit learning processes

The motor system demonstrates an exquisite ability to adapt to changes in the environment and to quickly reset when these changes prove transient. If similar environmental changes are encountered in the future, learning may be faster, a phenomenon known as savings. In studies of sensorimotor learning, a central component of savings is attributed to the explicit recall of the task structure and appropriate compensatory strategies. Whether implicit adaptation also contributes to savings remains subject to debate. We tackled this question by measuring, in parallel, explicit and implicit adaptive responses in a visuomotor rotation task, employing a protocol that typically elicits savings. While the initial rate of learning was faster in the second exposure to the perturbation, an analysis decomposing the 2 processes showed the benefit to be solely associated with explicit re-aiming. Surprisingly, we found a significant decrease after relearning in aftereffect magnitudes during no-feedback trials, a direct measure of implicit adaptation. In a second experiment, we isolated implicit adaptation using clamped visual feedback, a method known to eliminate the contribution of explicit learning processes. Consistent with the results of the first experiment, participants exhibited a marked reduction in the adaptation function, as well as an attenuated aftereffect when relearning from the clamped feedback. Motivated by these results, we reanalyzed data from prior studies and observed a consistent, yet unappreciated pattern of attenuation of implicit adaptation during relearning. These results indicate that explicit and implicit sensorimotor processes exhibit opposite effects upon relearning: Explicit learning shows savings, while implicit adaptation becomes attenuated.

Primary motor cortex excitability as a marker of plasticity in a stimulation protocol combining action observation and kinesthetic illusion of movement

Action observation combined with proprioceptive stimulation able to induce a kinesthetic illusion of movement (AO-KI) was shown to elicit a plastic increase in primary motor cortex (M1) excitability, with promising applications in rehabilitative interventions. Nevertheless, the known individual variability in response to combined stimulation protocols limits its application. The aim of this study was to examine whether a relationship exists between changes in M1 excitability during AO-KI and the long-lasting changes in M1 induced by AO-KI. Fifteen volunteers received a conditioning protocol consisting in watching a video showing a thumb-opposition movement and a simultaneous proprioceptive stimulation that evoked an illusory kinesthetic experience of their thumbs closing. M1 excitability was evaluated by means of single-pulse transcranial magnetic stimulation before, DURING the conditioning protocol, and up to 60 min AFTER it was administered. M1 excitability significantly increased during AO-KI with respect to a rest condition. Furthermore, AO-KI induced a long-lasting increase in M1 excitability up to 60 min after administration. Finally, a significant positive correlation appeared between M1 excitability changes during and after AO-KI; that is, participants who were more responsive during AO-KI showed greater motor cortical activity changes after it. These findings suggest that M1 response during AO-KI can be considered a neurophysiological marker of individual responsiveness to the combined stimulation since it was predictive of its efficacy in inducing long-lasting M1 increase excitability. This information would allow knowing in advance whether an individual will be a responder to AO-KI.

No effect of anodal tDCS on motor cortical excitability and no evidence for responders in a large double-blind placebo-controlled trial

BACKGROUND

Transcranial direct current stimulation (tDCS) has emerged as a non-invasive brain stimulation technique. Most studies show that anodal tDCS increases cortical excitability. However, this effect has been found to be highly variable.

OBJECTIVE

To test the effect of anodal tDCS on cortical excitability and the interaction effect of two participant-specific factors that may explain individual differences in sensitivity to anodal tDCS: the Brain Derived Neurotrophic Factor Val66Met polymorphism (BDNF genotype) and the latency difference between anterior-posterior and lateromedial TMS pulses (APLM latency).

METHODS

In 62 healthy participants, cortical excitability over the left motor cortex was measured before and after anodal tDCS at 2 mA for 20 min in a pre-registered, double-blind, randomized, placebo-controlled trial with repeated measures.

RESULTS

We did not find a main effect of anodal tDCS, nor an interaction effect of the participant-specific predictors. Moreover, further analyses did not provide evidence for the existence of responders and non-responders.

CONCLUSION

This study indicates that anodal tDCS at 2 mA for 20 min may not reliably affect cortical excitability.

Lack of cerebellar tDCS effects on learning of a complex whole body dynamic balance task in middle-aged (50-65 years) adults

Background

Cerebellar transcranial direct current stimulation (tDCS) is widely considered as a promising non-invasive tool to foster motor performance and learning in health and disease. The results of previous studies, however, are inconsistent. Our group failed to provide evidence for an effect of cerebellar tDCS on learning of a complex whole body dynamic balance task in young and healthy participants. Ceiling effects in the young study population are one possible explanation for the negative findings.

Methods

In the present study, we therefore tested 40 middle-aged healthy participants between the ages of 50 to 65 years. Participants received either anodal or sham cerebellar tDCS using a double-blinded study design while performing a balance task on a Lafayette Instrument 16,030 stability platform®. Mean platform angle and mean balance time were assessed as outcome measures.

Results

Significant learning effects were found in all participants. Balancing performance and learning rate was significantly less in the group of middle-aged adults compared to our previous group of young adults. No significant effects of cerebellar tDCS were observed.

Conclusions

Our findings are in line with other studies that have failed to prove robust effects of cerebellar tDCS on motor learning. The present findings, however, do not exclude cerebellar tDCS effects. tDCS effects may be more prominent after repeated stimulation, using other stimulus parameters, in patient populations, or in other motor learning tasks.

Trial registration

Not applicable.

Lateralized effects of post-learning transcranial direct current stimulation on motor memory consolidation in older adults: An fMRI investigation

Previous research has consistently demonstrated that older adults have difficulties transforming recently learned movements into robust, long-lasting memories (i.e., motor memory consolidation). One potential avenue to enhance consolidation in older individuals is the administration of transcranial direct current stimulation (tDCS) to task-relevant brain regions after initial learning. Although this approach has shown promise, the underlying cerebral correlates have yet to be revealed. Moreover, it is unknown whether the effects of tDCS are lateralized, an open question with implications for rehabilitative approaches following predominantly unilateral neurological injuries. In this research, healthy older adults completed a sequential motor task before and 6 h after receiving anodal or sham stimulation to right or left primary motor cortex (M1) while functional magnetic resonance images were acquired. Unexpectedly, anodal stimulation to right M1 following left-hand sequence learning significantly hindered consolidation as compared to a sham control, whereas no differences were observed with left M1 stimulation following right-hand learning. Impaired performance following right M1 stimulation was paralleled by sustained engagement of regions known to be critical for early learning stages, including the caudate nucleus and the premotor and parietal cortices. Thus, post-learning tDCS in older adults not only exerts heterogenous effects across the two hemispheres but can also disrupt ongoing memory processing.

Baseline sensorimotor GABA levels shape neuroplastic processes induced by motor learning in older adults

Previous research in young adults has demonstrated that both motor learning and transcranial direct current stimulation (tDCS) trigger decreases in the levels of gamma-aminobutyric acid (GABA) in the sensorimotor cortex, and these decreases are linked to greater learning. Less is known about the role of GABA in motor learning in healthy older adults, a knowledge gap that is surprising given the established aging-related reductions in sensorimotor GABA. Here, we examined the effects of motor learning and subsequent tDCS on sensorimotor GABA levels and resting-state functional connectivity in the brains of healthy older participants. Thirty-six older men and women completed a motor sequence learning task before receiving anodal or sham tDCS to the sensorimotor cortex. GABA-edited magnetic resonance spectroscopy of the sensorimotor cortex and resting-state (RS) functional magnetic resonance imaging data were acquired before and after learning/stimulation. At the group level, neither learning nor anodal tDCS significantly modulated GABA levels or RS connectivity among task-relevant regions. However, changes in GABA levels from the baseline to post-learning session were significantly related to motor learning magnitude, age, and baseline GABA. Moreover, the change in functional connectivity between task-relevant regions, including bilateral motor cortices, was correlated with baseline GABA levels. These data collectively indicate that motor learning-related decreases in sensorimotor GABA levels and increases in functional connectivity are limited to those older adults with higher baseline GABA levels and who learn the most. Post-learning tDCS exerted no influence on GABA levels, functional connectivity or the relationships among these variables in older adults.

Anodal Transcranial Direct Current Stimulation over the Vertex Enhances Leg Motor Cortex Excitability Bilaterally

In many studies, anodal transcranial Direct Current Stimulation (tDCS) is applied near the vertex to simultaneously facilitate leg motor cortex (M1) of both hemispheres and enhance recovery of gait and balance in neurological disorders. However, its effect on the excitability of leg M1 in either hemisphere is not well known. In this double-blind sham-controlled study, corticospinal excitability changes induced in leg M1 of both hemispheres by anodal (2 mA for 20 minutes) or sham tDCS (for 20 min) over the vertex were evaluated. Peak amplitudes of Transcranial Magnetic Stimulation (TMS) induced motor evoked potentials (MEPs) were measured over the contralateral Tibialis Anterior (TA) muscle before and up to 40 min after tDCS in 11 normal participants. Analysis of data from all participants found significant overall increase in the excitability of leg M1 after tDCS. However, in individual subjects there was variability in observed effects. In 4 participants, 20 min of tDCS increased mean MEPs of TAs on both sides; in 4 participants there was increased mean MEP only on one side and in 3 subjects there was no change. It’s not known if the benefits of tDCS in improving gait and balance are dependent on excitability changes induced in one or both leg M1; such information may be useful to predict treatment outcomes.