Modulation of training by single-session transcranial direct current stimulation to the intact motor cortex enhances motor skill acquisition of the paretic hand

Abstract

Background and purpose: Mechanisms of skill learning are paramount components for stroke recovery. Recent noninvasive brain stimulation studies demonstrated that decreasing activity in the contralesional motor cortex might be beneficial, providing transient functional improvements after stroke. The more crucial question, however, is whether this intervention can also enhance the acquisition of complex motor tasks, yielding longer-lasting functional improvements. In the present study, we tested the capacity of cathodal transcranial direct current stimulation (tDCS) applied over the contralesional motor cortex during training to enhance the acquisition and retention of complex sequential finger movements of the paretic hand.

Method: Twelve well-recovered chronic patients with subcortical stroke attended 2 training sessions during which either cathodal tDCS or a sham intervention were applied to the contralesional motor cortex in a double-blind, crossover design. Two different motor sequences, matched for their degree of complexity, were tested in a counterbalanced order during as well as 90 minutes and 24 hours after the intervention. Potential underlying mechanisms were evaluated with transcranial magnetic stimulation.

Results: tDCS facilitated the acquisition of a new motor skill compared with sham stimulation (P=0.04) yielding better task retention results. A significant correlation was observed between the tDCS-induced improvement during training and the tDCS-induced changes of intracortical inhibition (R(2)=0.63).

Conclusions: These results indicate that tDCS is a promising tool to improve not only motor behavior, but also procedural learning. They further underline the potential of noninvasive brain stimulation as an adjuvant treatment for long-term recovery, at least in patients with mild functional impairment after stroke.

Figure 1

Experimental design. A, After baseline (BASE), patients attended a training composed of 5 blocks (B1–B5) combined with tDCS or sham (TRAIN-tDCS) 90 minutes and 24 hours after the effects were re-evaluated in a testing block followed by 4 blocks of practice (POST-90 and POST-24). VAS questionnaires were recorded before and after each session. Online and offline effects were analyzed (see “Methods”). The diagram illustrates the position of the tDCS electrodes with the cathode placed over the projection of cM1. B, In a different experiment, MEP and SICI were measured before and after tDCS in both hemispheres in a counterbalanced order. tDCS indicates transcranial direct current stimulation; VAS, visual analog scale; cM1, contralesional motor cortex; MEP, motor-evoked potential; SICI, short interval intracortical inhibition.

Figure 2

Follow-up sessions. A, The bar graph shows the number of correct sequences evaluated at Test-90 and Test-24; the percentages of improvement for tDCS compared with sham stimulation are included in the graph for both test blocks (*Scheffé post hoc P<0.05). B, Practice sessions performed at Post-90 minutes and Post-24 (late online learning) hours did not show further behavioral improvement (error bars=SEM). tDCS indicates transcranial direct current stimulation.

Figure 3

Early training session (TRAIN-tDCS). A, The number of correct sequences achieved in each block is displayed for cathodal tDCS and sham; the baselines blocks were not different between conditions (BASE). B, A significant improvement with tDCS compared with sham stimulation during the early training session was observed (early online learning). C, Relationship between tDCS-induced behavioral improvement and cortical excitability changes; the abscissa displays the online effects; the ordinate displays tDCS-induced decrease of SICI (Post0/baseline). tDCS indicates transcranial direct current stimulation; SICI, short interval intracortical inhibition.