Optimizing the Effect of tDCS on Motor Sequence Learning in the Elderly

Aging, brain plasticity, and motor learning

Motor skill learning, the process of acquiring new motor skills, is critically important across the lifespan, from early development through adulthood and into older age, as well as in pathological conditions (i.e., rehabilitation). Extensive research has demonstrated that motor skill acquisition in young adults is accompanied by significant neuroplastic changes, including alterations in brain structure (gray and white matter), function (i.e., activity and connectivity), and neurochemistry (i.e., levels of neurotransmitters). In the aging population, motor performance typically declines, characterized by slower and less accurate movements. However, despite these age-related changes, older adults maintain the capacity for skill improvement through training. In this review, we explore the extent to which the aging brain retains the ability to adapt in response to motor learning, specifically whether skill acquisition is accompanied by neural changes. Furthermore, we discuss the associations between inter-individual variability in brain structure and function and the potential for future learning in older adults. Finally, we consider the use of non-invasive brain stimulation techniques aimed at optimizing motor learning in this population. Our review provides insights into the neurobiological underpinnings of motor learning in older adults and emphasizes strategies to enhance their motor skill acquisition.

Do Progressive Intensities of Transcranial Direct Current Stimulation with and Without 40 Hz Binaural Beats Change Pre-Frontal Cortex Hemodynamics? A Randomized Controlled Trial

Transcranial direct current stimulation (tDCS) is often reported to have positive effects on brain hemodynamics as well as cognitive performance. Binaural beats (BBs) have also shown the potential to improve cognitive performance. However, we could not find any studies assessing prefrontal hemodynamics using a combination of these techniques or assessing the effects on hemodynamic response at different intensity levels of tDCS (two and three mA). This study aimed to compare the immediate hemodynamic responses to tDCS at different intensities (two and three mA) with and without 40 Hz BBs. Sixty-eight healthy young individuals of both sexes were split into four groups: the tDCS 2 mA group; tDCS 3 mA group; tDCS 2 mA + BB group; and tDCS 3 mA + BB group. All groups received 20 min tDCS (F3-Fp2) alone or combined with BBs. The hemodynamic effect was assessed using a functional near-infrared intracranial spectroscope (fNIRS) positioned on the left supraorbital region (Fp1). The mean values of rates of oxygen saturation (SatO2) were recorded at baseline, during the intervention period, and post-stimulation. The oxygenated hemoglobin rates (HbO) were also extracted. No between-group differences were observed. The within-group analysis did not show significant differences in terms of the time×groups factor. However, the time factor showed significant within-group differences. No differences were found for the HbO rates. The present findings showed that two and three mA tDCS had effects on pre-frontal cortex SatO2; however, the use of additional BBs did not change the SatO2 levels compared to the use of tDCS alone.

Transcranial electrical stimulation for procedural learning and rehabilitation

Procedural learning is the acquisition of motor and non-motor skills through a gradual process that increases with practice. Impairments in procedural learning have been consistently demonstrated in neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. Considering that noninvasive brain stimulation modulates brain activity and boosts neuroplastic mechanisms, we reviewed the effects of coupling transcranial direct current stimulation (tDCS) with training methods for motor and non-motor procedural learning to explore tDCS potential use as a tool for enhancing implicit learning in healthy and clinical populations. The review covers tDCS effects over i. motor procedural learning, from basic to complex activities; ii. non-motor procedural learning; iii. procedural rehabilitation in several clinical populations. We conclude that targeting the primary motor cortex and prefrontal areas seems the most promising for motor and non-motor procedural learning, respectively. For procedural rehabilitation, the use of tDCS is yet at an early stage but some effectiveness has been reported for implicit motor and memory learning. Still, systematic comparisons of stimulation parameters and target areas are recommended for maximising the effectiveness of tDCS and its robustness for procedural rehabilitation.

Intensity-dependent effects of tDCS on motor learning are related to dopamine

Non-invasive brain stimulation techniques, such as transcranial direct current stimulation (tDCS), are popular methods for inducing neuroplastic changes to alter cognition and behaviour. One challenge for the field is to optimise stimulation protocols to maximise benefits. For this to happen, we need a better understanding of how stimulation modulates cortical functioning/behaviour. To date, there is increasing evidence for a dose-response relationship between tDCS and brain excitability, however how this relates to behaviour is not well understood. Even less is known about the neurochemical mechanisms which may drive the dose-response relationship between stimulation intensities and behaviour. Here, we examine the effect of three different tDCS stimulation intensities (1 mA, 2 mA, 4 mA anodal motor cortex tDCS) administered during the explicit learning of motor sequences. Further, to assess the role of dopamine in the dose-response relationship between tDCS intensities and behaviour, we examined how pharmacologically increasing dopamine availability, via 100 mg of levodopa, modulated the effect of stimulation on learning. In the absence of levodopa, we found that 4 mA tDCS improved and 1 mA tDCS impaired acquisition of motor sequences relative to sham stimulation. Conversely, levodopa reversed the beneficial effect of 4 mA tDCS. This effect of levodopa was no longer evident at the 48-h follow-up, consistent with previous work characterising the persistence of neuroplastic changes in the motor cortex resulting from combining levodopa with tDCS. These results provide the first direct evidence for a role of dopamine in the intensity-dependent effects of tDCS on behaviour.

Does transcranial direct current stimulation of the primary motor cortex improve implicit motor sequence learning in Parkinson's disease?

Implicit motor sequence learning (IMSL) is a cognitive function that is known to be associated with impaired motor function in Parkinson's disease (PD). We previously reported positive effects of transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) on IMSL in 11 individuals with PD with mild cognitive impairments (MCI), with the largest effects occurring during reacquisition. In the present study, we included 35 individuals with PD, with (n = 15) and without MCI (n = 20), and 35 age- and sex-matched controls without PD, with (n = 13) and without MCI (n = 22). We used mixed-effects models to analyze anodal M1 tDCS effects on acquisition (during tDCS), short-term (five minutes post-tDCS) and long-term reacquisition (one-week post-tDCS) of general and sequence-specific learning skills, as measured by the serial reaction time task. At long-term reacquisition, anodal tDCS resulted in smaller general learning effects compared to sham, only in the PD group, p = .018, possibly due to floor effects. Anodal tDCS facilitated the acquisition of sequence-specific learning (M = 54.26 ms) compared to sham (M = 38.98 ms), p = .003, regardless of group (PD/controls). Further analyses revealed that this positive effect was the largest in the PD-MCI group (anodal: M = 69.07 ms; sham: M = 24.33 ms), p < .001. Although the observed effect did not exceed the stimulation period, this single-session tDCS study confirms the potential of tDCS to enhance IMSL, with the largest effects observed in patients with lower cognitive status. These findings add to the body of evidence that anodal tDCS can beneficially modulate the abnormal basal ganglia network activity that occurs in PD.

Effect of transcranial direct current stimulation on learning in older adults with and without Parkinson's disease: A systematic review with meta-analysis

Older adults with and without Parkinson's disease show impaired retention after training of motor or cognitive skills. This systematic review with meta-analysis aims to investigate whether adding transcranial direct current stimulation (tDCS) to motor or cognitive training versus placebo boosts motor sequence and working memory training. The effects of interest were estimated between three time points, i.e. pre-training, post-training and follow-up. This review was conducted according to the PRISMA guidelines (PROSPERO: CRD42022348885). Electronic databases were searched from conception to March 2023. Following initial screening, 24 studies were eligible for inclusion in the qualitative synthesis and 20 could be included in the meta-analysis, of which 5 studies concerned motor sequence learning (total n = 186) and 15 working memory training (total n = 650). Results were pooled using an inverse variance random effects meta-analysis. The findings showed no statistically significant additional effects of tDCS over placebo on motor sequence learning outcomes. However, there was a strong trend showing that tDCS boosted working memory training, although methodological limitations and some heterogeneity were also apparent. In conclusion, the present findings do not support wide implementation of tDCS as an add-on to motor sequence training at the moment, but the promising results on cognitive training warrant further investigations.

Dual-hemisphere anodal transcranial direct current stimulation improves bilateral motor synergies

Transcranial direct current stimulation (tDCS) is one of the non-invasive brain stimulation techniques that can improve motor functions. As bimanual motor actions require high motor cortical activations between hemispheres, applying bilateral anodal stimulation on left and right sides of primary motor cortex (M1) can improve for improvements in bimanual motor tasks. This study investigated which bilateral tDCS protocol effectively improves bimanual hand-grip force control capabilities in healthy young adults. We used three different bilateral tDCS protocols: (a) dual-anodal stimulation on the M1 of bilateral hemispheres (Bi-AA), (b) anodal-cathodal stimulation on the M1 of dominant and nondominant hemispheres (Bi-AC), and (c) sham stimulation (Sham). The results indicated that applying the Bi-AA significantly improved bilateral motor synergies estimated by uncontrolled manifold analysis relative to Sham. However, these differences were not observed in the comparison between Bi-AA and Bi-AC as well as between Bi-AC and Sham. These findings suggest that facilitating motor cortical activations between both hemispheres may be an additional option for advancing interlimb motor coordination patterns.

Microstructural and functional plasticity following repeated brain stimulation during cognitive training in older adults

The combination of repeated behavioral training with transcranial direct current stimulation (tDCS) holds promise to exert beneficial effects on brain function beyond the trained task. However, little is known about the underlying mechanisms. We performed a monocenter, single-blind randomized, placebo-controlled trial comparing cognitive training to concurrent anodal tDCS (target intervention) with cognitive training to concurrent sham tDCS (control intervention), registered at ClinicalTrial.gov (Identifier NCT03838211). The primary outcome (performance in trained task) and secondary behavioral outcomes (performance on transfer tasks) were reported elsewhere. Here, underlying mechanisms were addressed by pre-specified analyses of multimodal magnetic resonance imaging before and after a three-week executive function training with prefrontal anodal tDCS in 48 older adults. Results demonstrate that training combined with active tDCS modulated prefrontal white matter microstructure which predicted individual transfer task performance gain. Training-plus-tDCS also resulted in microstructural grey matter alterations at the stimulation site, and increased prefrontal functional connectivity. We provide insight into the mechanisms underlying neuromodulatory interventions, suggesting tDCS-induced changes in fiber organization and myelin formation, glia-related and synaptic processes in the target region, and synchronization within targeted functional networks. These findings advance the mechanistic understanding of neural tDCS effects, thereby contributing to more targeted neural network modulation in future experimental and translation tDCS applications.