The Interaction With Task-induced Activity is More Important Than Polarization: A tDCS Study (original) (raw)
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Is motor learning mediated by tDCS intensity?
Although tDCS has been shown to improve motor learning, previous studies reported rather small effects. Since physiological effects of tDCS depend on intensity, the present study evaluated this parameter in order to enhance the effect of tDCS on skill acquisition. The effect of different stimulation intensities of anodal tDCS (atDCS) was investigated in a double blind, sham controlled crossover design. In each condition, thirteen healthy subjects were instructed to perform a unimanual motor (sequence) learning task. Our results showed (1) a significant increase in the slope of the learning curve and (2) a significant improvement in motor performance at retention for 1.5mA atDCS as compared to sham tDCS. No significant differences were reported between 1mA atDCS and sham tDCS; and between 1.5mA atDCS and 1mA atDCS.
Task-specificity of unilateral anodal and dual-M1 tDCS effects on motor learning
Task-specific effects of transcranial direct current stimulation (tDCS) on motor learning were investigated in 30 healthy participants. In a sham-controlled, mixed design, participants trained on 3 different motor tasks (Purdue Pegboard Test, Visuomotor Grip Force Tracking Task and Visuomotor Wrist Rotation Speed Control Task) over 3 consecutive days while receiving either unilateral anodal over the right primary motor cortex (M1), dual-M1 or sham stimulation. Retention sessions were administered 7 and 28 days after the end of training. In the Purdue Pegboard Test, both anodal and dual-M1 stimulation reduced average completion time approximately equally, an improvement driven by online learning effects and maintained for about 1 week. The Visuomotor Grip Force Tracking Task and the Visuomotor Wrist Rotation Speed Control Task were associated with an advantage of dual-M1 tDCS in consolidation processes both between training sessions and when testing at long-term retention; both were maintained for at least 1 month. This study demonstrates that M1-tDCS enhances and sustains motor learning with different electrode montages. Stimulation-induced effects emerged at different learning phases across the tasks, which strongly suggests that the influence of tDCS on motor learning is dynamic with respect to the functional recruitment of the distributed motor system at the time of stimulation. Divergent findings regarding M1-tDCS effects on motor learning may partially be ascribed to task-specific consequences and the effects of offline consolidation.
PLOS ONE, 2015
Previous research suggests that anodal transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) modulates NMDA receptor dependent processes that mediate synaptic plasticity. Here we test this proposal by applying anodal versus sham tDCS while subjects practiced to flex the thumb as fast as possible (ballistic movements). Repetitive practice of this task has been shown to result in performance improvements that reflect use-dependent plasticity resulting from NMDA receptor mediated, long-term potentiation (LTP)-like processes. Using a double-blind within-subject cross-over design, subjects (n=14) participated either in an anodal or a sham tDCS session which were at least 3 months apart. Sham or anodal tDCS (1 mA) was applied for 20 min during motor practice and retention was tested 30 min, 24 hours and one week later. All subjects improved performance during each of the two sessions (p < 0.001) and learning gains were similar. Our main result is that long term retention performance (i.e. 1 week after practice) was significantly better when practice was performed with anodal tDCS than with sham tDCS (p < 0.001). This effect was large (Cohen's d=1.01) and all but one subject followed the group trend. Our data strongly suggest that anodal tDCS facilitates long-term memory formation reflecting use-dependent plasticity. Our results support the notion that anodal tDCS facilitates synaptic plasticity mediated by an LTP-like mechanism, which is in accordance with previous research.
Brain Stimulation, 2020
Background: A single session of anodal tDCS induces LTP-like plasticity which lasts for about 1 h, while repetition of stimulation within a time interval of 30 min results in late-phase effects lasting for at least 24 h with standard stimulation protocols. Objective: In this pilot study, we explored if the after-effects of a recently developed intensified single session stimulation protocol are relevantly prolonged in the motor cortex by repetition of this intervention. Methods: 16 healthy right-handed subjects participated in this study. The effects of an intensified (3 mA-20min) and a standard anodal tDCS protocol (1 mA-15min) with short (20 min) and long (3 h) repetition intervals were compared with the effects of respective single session tDCS protocols (3 mA-20min, 1 mA-15min, and Sham). Cortical excitability alterations were monitored by single-pulse TMS-elicited MEPs. Results: Compared to sham, both single session tDCS protocols (1 mA-15min, and 3 mA-20min) resulted in cortical excitability enhancements lasting for about 30 min after stimulation. The short repetition interval (20 min) resulted in a prolongation of after-effects for the standard protocol, which lasted for more than 24 h after stimulation. For the intensified protocol, the prolongation of after-effects was limited to 120 min after stimulation. The long repetition interval (3 h) resulted in no excitabilityenhancing after-effects for the intensified, and only minor excitability enhancement within the first 30 min after the intervention for the standard protocol. Conclusion: These results suggest a non-linearity of late-phase LTP-like plasticity induction, which was dependent not only on the interval of intervention repetition, but also on other protocol characteristics, including intensity, and duration of tDCS. Further studies in larger samples are needed to confirm these results.
Neural Plasticity
Transcranial direct current stimulation (tDCS) is a noninvasive technique that modulates the excitability of neurons within the motor cortex (M1). Although the aftereffects of anodal tDCS on modulating cortical excitability have been described, there is limited data describing the outcomes of different tDCS intensities on intracortical circuits. To further elucidate the mechanisms underlying the aftereffects of M1 excitability following anodal tDCS, we used transcranial magnetic stimulation (TMS) to examine the effect of different intensities on cortical excitability and short-interval intracortical inhibition (SICI). Using a randomized, counterbalanced, crossover design, with a one-week wash-out period, 14 participants (6 females and 8 males, 22-45 years) were exposed to 10 minutes of anodal tDCS at 0.8, 1.0, and 1.2 mA. TMS was used to measure M1 excitability and SICI of the contralateral wrist extensor muscle at baseline, immediately after and 15 and 30 minutes following cessation of anodal tDCS. Cortical excitability increased, whilst SICI was reduced at all time points following anodal tDCS. Interestingly, there were no differences between the three intensities of anodal tDCS on modulating cortical excitability or SICI. These results suggest that the aftereffect of anodal tDCS on facilitating cortical excitability is due to the modulation of synaptic mechanisms associated with long-term potentiation and is not influenced by different tDCS intensities.
Effects of tDCS on motor learning and memory formation: a consensus and critical position paper
2016
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 and retention of motor skills and adaptation. A majority of reports focused on the application of tDCS with the anode placed over the primary motor cortex (M1) during motor skill acquisition, while some evaluated tDCS applied over the cerebellum during adaptation of existing motor skills. Work in multiple laboratories is under way 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 are moderate (up to d= 0.5) (Hashemirad, ...
Brain Stimulation
Background: Transcranial direct current stimulation (tDCS) is used to induce neuroplasticity in the human brain. Within certain limits of stimulation duration, anodal tDCS (a-tDCS) over the primary motor cortex induces long term potentiation-(LTP) like plasticity. A reversal of the direction of plasticity has however been described with prolonged a-tDCS protocols. Objective: We aimed to systematically investigate the intervention duration threshold for reversal of a-tDCS-induced effects on corticospinal excitability (CSE) and to determine the probable mechanisms involved in these changes. Methods: Fifteen healthy participants received a-tDCS of 1 mA for five different durations in pseudorandom session order. Transcranial magnetic stimulation (TMS) was delivered over the left M1, and motor evoked potentials (MEPs) of a contralateral hand muscle were recorded before, immediately and 30 min following intervention to measure CSE changes. Short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), and long interval facilitation (LIF) were assessed via paired-pulse TMS protocols. Results: A-tDCS significantly increased CSE as expected at stimulation durations of 22 and 24 min. However, this effect of a-tDCS on CSE decreased and even reversed when stimulation duration increased to 26, 28, and 30 min. Respective alterations of ICF, LIF, and SICI indicate the involvement of glutamatergic, and GABAergic systems in these effects. Conclusions: These results confirm a duration threshold for reversal of the excitability-enhancing effect of a-tDCS with stimulation durations ! 26 min. Counter-regulatory mechanisms are discussed as a mechanistic foundation for these effects, which might prevent excessive brain activation.
Cortex, 2019
Transcranial direct current stimulation (tDCS) non-invasively induces polaritydependent excitability alterations in the human motor cortex lasting for more than an hour after stimulation. Clinical applications with encouraging results have been reported in several pilot studies, but the optimal stimulation protocols remain to be determined. This is also important because the efficacy and directionality of tDCS effects follow non-linear rules regarding neuroplastic effects for the stimulation parameters duration and intensity. In this study, we systemically explored the association between tDCS, these parameters and induced after-effects on motor cortex excitability. Cathodal tDCS was applied in four different intensities (sham, 1, 2 and 3mA) and three durations (15, 20 and 30mins) in 16 young healthy subjects and the after-effects were monitored with TMS-induced motor evoked potentials (MEP) until the next day evening after stimulation. The results of the repeated measures ANOVA conducted to disentangle the effects of tDCS intensity and duration show a main effect of intensity in which 1 mA and 3 mA stimulation induced a reduction of MEP amplitudes, but 2 mA resulted in excitability enhancement. The results of a secondary ANOVA conducted to compare if active stimulation effects differ from those of sham stimulation revealed a significant main effect of tDCS condition in which 1mA-15min, 1mA-30min and 3mA-20 min cathodal tDCS induced LTD-like plasticity, while LTP-like plasticity was observed after 2mA-20min stimulation. Our study thus provides further insights on the dependency of tDCS-induced neuroplasticity from the stimulation parameters, and therefore delivers crucial information for future applications.
Task-concurrent anodal tDCS modulates bilateral plasticity in the human suprahyoid motor cortex
Frontiers in human neuroscience, 2015
Transcranial direct current stimulation (tDCS) is a non-invasive method to modulate cortical excitability in humans. Here, we examined the effects of anodal tDCS on suprahyoid motor evoked potentials (MEP) when applied over the hemisphere with stronger and weaker suprahyoid/submental projections, respectively, while study participants performed a swallowing task. Thirty healthy volunteers were invited to two experimental sessions and randomly assigned to one of two different groups. While in the first group stimulation was targeted over the hemisphere with stronger suprahyoid projections, the second group received stimulation over the weaker suprahyoid projections. tDCS was applied either as anodal or sham stimulation in a random cross-over design. Suprahyoid MEPs were assessed immediately before intervention, as well as 5, 30, 60, and 90 min after discontinuation of stimulation from both the stimulated and non-stimulated contralateral hemisphere. We found that anodal tDCS (a-tDCS) ...