Differential effects of anodal and dual tDCS on sensorimotor functions in chronic hemiparetic stroke patients

Background and purpose Previous tDCS studies in chronic stroke patients reported highly inconsistent effects on sensorimotor functions. Underlying reasons could be the selection of different kinematic parameters across studies and for different tDCS setups. We reasoned that tDCS may not simply induce global changes in a beneficial-adverse dichotomy, but rather that different sensorimotor kinematics are differentially affected. Furthermore, the often-postulated higher efficacy of bilateral-dual (bi-tDCS) over unilateral-anodal (ua-tDCS) could not yet be demonstrated consistently either. We investigated the effects of both setups on a wider range of kinematic parameters from standardized robotic tasks in patients with chronic stroke. Methods Twenty-four patients with arm hemiparesis received tDCS (20min, 1 mA) concurrent to kinematic assessments in a sham-controlled, cross-over and double-blind clinical trial. Performance was measured on four sensorimotor tasks (reaching, proprioception, cooperative and independent bimanual coordination) from which 30 parameters were extracted. On the group-level, the patterns of changes relative to sham were assessed using paired-samples t-tests and classified as (1) performance increases, (2) decreases and (3) non-significant differences. Correlations between parametric change scores were calculated for each task to assess effects on the individual-level. Results Both setups induced complex effect patterns with varying proportions of performance increases and decreases. On the group-level, more increases were induced in the reaching and coordination tasks while proprioception and bimanual cooperation were overall negatively affected. Bi-tDCS induced more performance increases and less decreases compared to ua-tDCS. Changes across parameters occurred more homogeneously under bi-tDCS than ua-tDCS, which induced a larger proportion of performance trade-offs. Conclusions Our data demonstrate profound tDCS effects on sensorimotor functions post-stroke, lending support for more pronounced and favorable effects of bi-tDCS compared to ua-tDCS. However, no uniformly beneficial pattern was identified. Instead, the modulations varied depending on the task and electrode setup, with increases in certain parameters occurring at the expense of decreases in others

Anodal transcranial direct current stimulation over S1 differentially modulates proprioceptive accuracy in young and old adults

Background: Proprioception is a prerequisite for successful motor control but declines throughout the lifespan. Brain stimulation techniques such as anodal transcranial direct current stimulation (a-tDCS) are capable of enhancing sensorimotor performance across different tasks and age groups. Despite such growing evidence for a restorative potential of tDCS, its impact on proprioceptive accuracy has not been studied in detail yet. Objective: This study investigated online effects of a-tDCS over S1 on proprioceptive accuracy in young (YA) and old healthy adults (OA). Methods: The effect of 15 min of a-tDCS vs. sham on proprioceptive accuracy was assessed in a cross-over, double blind experiment in both age groups. Performance changes were tested using an arm position matching task in a robotic environment. Electrical field (EF) strengths in the target area S1 and control areas were assessed based on individualized simulations. Results: a-tDCS elicited differential changes in proprioceptive accuracy and EF strengths in the two groups: while YA showed a slight improvement, OA exhibited a decrease in performance during a-tDCS. Stronger EF were induced in target S1 and control areas in the YA group. However, no relationship between EF strength and performance change was found. Conclusion: a-tDCS over S1 elicits opposing effects on proprioceptive accuracy as a function of age, a result that is important for future studies investigating the restorative potential of a-tDCS in healthy aging and in the rehabilitation of neurological diseases that occur at advanced age. Modeling approaches could help elucidate the relationship between tDCS protocols, brain structure and performance modulation

Effects of unilateral and bilateral tDCS over M1 on the kinematics of sensorimotor function in chronic stroke patients

The improvement of acute stroke treatment has increased survival, leaving more people in rehabilitative care. The recovery of function, however, typically stagnates after initial progress. Transcranial direct current stimulation (tDCS) has been suggested to facilitate recovery beyond this plateau. However, inconsistent results of tDCS effects were found in chronic patients, probably due to varying behavioural assessments with low spatial or temporal resolution, a selection bias introduced through group designs and small sample sizes. Here, we used high-resolution kinematic assessments to inspect a wide range of sensorimotor functions using established protocols for unilateral (utDCS) and bilateral tDCS (btDCS) in a cross-over, double-blind and sham-controlled clinical trial. 24 patients underwent all tDCS conditions (sessions separated by one week) concurrent to kinematic assessments. tDCS-induced effects were measured on 4 tasks. Kinematic parameters were extracted across these tasks and corrected for age and Fugl-Meyer score. The estimates were then compared between sham vs. utDCS and sham vs. btDCS, respectively, using paired-samples t-tests and classified as (1) non-significant changes, (2) significant increases or (3) decreases in performance. Class distributions across parameters were then compared between real and permuted data to account for falsely positive significance tests. utDCS induced significant increases in 7 and decreases in 18 parameters, btDCS increases in 11 and decreases in 15 parameters. The class distributions differed significantly between real and permuted data, indicating a profound tDCS effect. Our approach provides a new avenue for investigating tDCS effects across different sensorimotor domains after stroke. The results demonstrate the efficacy of tDCS to modulate sensorimotor function after stroke, but also indicate that the induced effects are complex and bidirectional