Cerebellar Transcranial Direct Current Stimulation Applied over Multiple Days Does Not Enhance Motor Learning of a Complex Overhand Throwing Task in Young Adults

Abstract

Cerebellar transcranial direct current stimulation (tDCS) enhances motor skill and learning in relatively simple motor tasks, but it is unclear if c-tDCS can improve motor performance in complex motor tasks. The purpose of this study was to determine the influence of c-tDCS applied over multiple days on motor learning in a complex overhand throwing task. In a double-blind, randomized, between-subjects, SHAM-controlled, experimental design, 30 young adults were assigned to either a c-tDCS or a SHAM group. Participants completed three identical experiments on consecutive days that involved overhand throwing in a pre-test block, five practice blocks with concurrent c-tDCS, and a post-test block. Overhand throwing endpoint accuracy was quantified as the endpoint error. The first dorsal interosseous muscle motor evoked potential (MEP) amplitude elicited by transcranial magnetic stimulation was used to quantify primary motor cortex (M1) excitability modulations via c-tDCS. Endpoint error significantly decreased over the 3 days of practice, but the magnitude of decrease was not significantly different between the c-tDCS and SHAM group. Similarly, MEP amplitude slightly increased from the pre-tests to the post-tests, but these increases did not differ between groups. These results indicate that multi-day c-tDCS does not improve motor learning in an overhand throwing task or increase M1 excitability.

Keywords: motor skill; transcranial direct current stimulation; transcranial magnetic stimulation.

Figure 1

Schematic representation of the major components of the experimental protocol. (A) One of the 3 consecutive identical experimental sessions is depicted for illustration purposes. The experimental protocol comprised a pre-test block of overhand throws, a TMS testing paradigm testing the effects of c-tDCS on M1 excitability, 5 practice blocks of overhand throws performed concurrent with 20 min of c-tDCS or SHAM stimulation, and a post-test block of overhand throws; (B) the target and the quantification of endpoint error. The entire target area was 1.27 m in length, 1 m in width, the center of the target was 1.71 m from the floor, and the target circle had a diameter of 1 cm. An example data cloud of the endpoints of the ball is depicted for a block of 10 trials along with the x and y errors for a single trial shown that were used to calculate the trial’s endpoint error.

Figure 2

Endpoint error in the overhand throwing task averaged over all 7 daily trial blocks for the c-tDCS and SHAM groups. (A) Endpoint error declined across the test blocks for the 3 days of practice (p = 0.02), but the decline was similar for the c-tDCS and SHAM groups (p = 0.332); (B) endpoint error was similar for the two groups (p = 0.381) and across the 3 days (p = 0.507); (C) the online (p = 0.325), offline (p = 0.188), and total learning (p = 0.843) were similar for the c-tDCS and the SHAM groups.

Figure 3

MEP amplitude in the TMS pre-tests and post-tests for the 3 days in the c-tDCS and SHAM groups. MEP amplitude was significantly increased between the pre-test and post-test on all 3 days (Test main effect, p < 0.011), but the increase was not statistically significant between the c-tDCS and the SHAM groups (p = 0.677).

Figure 4

Associations between increases in MEP amplitude and increases in endpoint accuracy. (A–C) The absolute change (increase) in endpoint accuracy (decrease in endpoint error) was not associated with the absolute change (increase) in MEP amplitude for the participants in either group that displayed both increases in endpoint accuracy and MEP amplitude.