Abstract
BackgroundThe results of transcranial direct current stimulation (tDCS) studies that seek to improve motor performance for people with neurological disorders, by targeting the primary motor cortex, have been inconsistent. One possible reason, among others, for this inconsistency, is that very little is known about the optimal protocols for enhancing motor performance in healthy individuals. The best way to optimize stimulation protocols for enhancing tDCS effects on motor performance by means of current intensity modulation has not yet been determined. We aimed to determine the effect of current intensity on motor performance using–for the first time–a montage optimized for maximal focal stimulation via anodal high-definition tDCS (HD-tDCS) on the right primary motor cortex in healthy subjects.MethodsSixty participants randomly received 20-min HD-tDCS at 1.5, 2 mA, or sham stimulation. Participants’ reaching performance with the left hand on a tablet was tested before, during, and immediately following stimulation, and retested after 24 h.ResultsIn the current montage of HD-tDCS, movement time did not differ between groups in each timepoint. However, only after HD-tDCS at 1.5 mA did movement time improve at posttest as compared to pretest. This reduction in movement time from pretest to posttest was significantly greater compared to HD-tDCS 2 mA. Following HD-tDCS at 1.5 mA and sham HD-tDCS, but not 2 mA, movement time improved at retest compared to pretest, and at posttest and retest compared to the movement time during stimulation. In HD-tDCS at 2 mA, the negligible reduction in movement time from the course of stimulation to posttest was significantly lower compared to sham HD-tDCS. Across all groups, reaction time improved in retest compared to pretest and to the reaction time during stimulation, and did not differ between groups in each timepoint.ConclusionsIt appears that 2 mA in this particular experimental setup inhibited the learning effects. These results suggest that excitatory effects induced by anodal stimulation do not hold for every stimulation intensity, information that should be taken into consideration when translating tDCS use from the realm of research into more optimal neurorehabilitation.Trial registration: Clinical Trials Gov, NCT04577768. Registered 6 October 2019 -Retrospectively registered, https://register.clinicaltrials.gov/prs/app/action/SelectProtocol?sid=S000A9B3&selectaction=Edit&uid=U0005AKF&ts=8&cx=buucf0.
Highlights
A major goal of clinical neuroscience is to develop effective, non-invasive methods for improving function via neuroplasticity modulation
It is commonly assumed that anodal stimulation leads to a subthreshold depolarization and increased cortical excitability whereas cathodal stimulation leads to hyperpolarization and decreased cortical excitability [for a review see [1, 2]]. transcranial direct current stimulation (tDCS) has been shown in some studies to be an effective means to improve motor performance in healthy subjects as well as patients suffering from neurological diseases such as stroke and Parkinson’s disease [for reviews see [3,4,5,6]]
The results of clinical studies, such as those aimed at improving motor recovery following stroke by targeting the primary motor cortex (M1), have been inconsistent [for reviews see [7, 8]], probably because, among other reasons, very little is known about the optimal protocols for enhancing motor ability in healthy individuals
Summary
A major goal of clinical neuroscience is to develop effective, non-invasive methods for improving function via neuroplasticity modulation. One non-invasive and painless stimulation method that has received increasing attention is transcranial direct current stimulation (tDCS). The results of clinical studies, such as those aimed at improving motor recovery following stroke by targeting the primary motor cortex (M1), have been inconsistent [for reviews see [7, 8]], probably because, among other reasons, very little is known about the optimal protocols for enhancing motor ability in healthy individuals. The results of transcranial direct current stimulation (tDCS) studies that seek to improve motor per‐ formance for people with neurological disorders, by targeting the primary motor cortex, have been inconsistent. We aimed to determine the effect of current intensity on motor performance using–for the first time–a montage opti‐ mized for maximal focal stimulation via anodal high-definition tDCS (HD-tDCS) on the right primary motor cortex in healthy subjects
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