Search

BackgroundSince a stroke can impair bimanual activities, enhancing bimanual cooperation through motor skill learning may improve neurorehabilitation. Therefore, robotics and neuromodulation with transcranial direct current stimulation (tDCS) are promising approaches. To date, tDCS has failed to enhance bimanual motor control after stroke possibly because it was not integrating the hypothesis that the undamaged hemisphere becomes the major poststroke hub for bimanual control.ObjectiveWe tested the following hypotheses: (I) In patients with chronic hemiparetic stroke training on a robotic device, anodal tDCS applied over the primary motor cortex of the undamaged hemisphere enhances bimanual motor skill learning compared to sham tDCS. (II) The severity of impairment correlates with the effect of tDCS on bimanual motor skill learning. (III) Bimanual motor skill learning is less efficient in patients than in healthy individuals (HI).MethodsA total of 17 patients with chronic hemiparetic stroke and 7 healthy individuals learned a complex bimanual cooperation skill on the REAplan® neurorehabilitation robot. The bimanual speed/accuracy trade-off (biSAT), bimanual coordination (biCo), and bimanual force (biFOP) scores were computed for each performance. In patients, real/sham tDCS was applied in a crossover, randomized, double-blind approach.ResultsCompared to sham, real tDCS did not enhance bimanual motor skill learning, retention, or generalization in patients, and no correlation with impairment was noted. The healthy individuals performed better than patients on bimanual motor skill learning, but generalization was similar in both groups.ConclusionA short motor skill learning session with a robotic device resulted in the retention and generalization of a complex skill involving bimanual cooperation. The tDCS strategy that would best enhance bimanual motor skill learning after stroke remains unknown.Clinical trial registrationhttps://clinicaltrials.gov/ct2/show/NCT02308852, identifier: NCT02308852.

Transcranial direct current stimulation (tDCS) has the potential to alter neuronal activity and modify behaviour. The enhancement of activity in one hemisphere relative to the other is assumed to benefit to the rehabilitation of hemineglect, a disorder of spatial attention consecutive to a unilateral lesion [1, 2]. Previous studies on neglect patients showed that anodal tDCS over the left or right posterior parietal cortex (PPC) reorient attention toward the left hemifield [3, 4, 5]. However, this mechanism of action has been mainly studied in visuospatial tasks, while visuomotor and representational aspects have received little consideration so far. In the present study, we investigated how tDCS modulates spatial attention during visuomotor behaviour and visual imagery in healthy humans. The visuomotor task required participants to cross out a maximum of targets randomly distributed over a digital tablet within thirty seconds. The digital tablet allowed us to track the participant's performance over time and compute the centre of gravidity of the crossed targets. The visual imagery task required participants to mentally compare the similarity of two previously seen geometrical shapes that could differ either on their left or right side. The anode was positioned over the left or right parietal cortex and the cathode was positioned over the controlateral frontal cortex. In the sham condition, the centre of gravity of the crossed targets revealed a leftward bias in half of the participants and a rightward bias in the other half. In the active condition, the attention of participants was shifted in the direction opposite to their natural bias in the sham condition, irrespective to the stimulated hemisphere. No effect was observed in the visual imagery task, possibly because this task relied on allocentric rather than egocentric spatial coordinates. The present findings demonstrate that activating the attention network through tDCS counteracts natural biases in visual search. Because the left-to-right or right-to-left current direction did not matter, we suggest that the net effect of tDCS is to re-balance activity within the whole network, which makes it an optimal technique to restore the full range of attention after a unilateral lesion.