Abstract
Anodal transcranial Direct Current Stimulation (tDCS) has been shown to be an effective non-invasive brain stimulation method for improving cognitive and motor functioning in patients with neurological deficits. tDCS over motor cortex (M1), for instance, facilitates motor learning in stroke patients. However, the literature on anodal tDCS effects on motor learning in healthy participants is inconclusive, and the effects of tDCS on visuo-motor integration are not well understood. In the present study we examined whether tDCS over the contralateral motor cortex enhances learning of grip-force output in a visually guided feedback task in young and neurologically healthy volunteers. Twenty minutes of 1 mA anodal tDCS were applied over the primary motor cortex (M1) contralateral to the dominant (right) hand, during the first half of a 40 min power-grip task. This task required the control of a visual signal by modulating the strength of the power-grip for six seconds per trial. Each participant completed a two-session sham-controlled crossover protocol. The stimulation conditions were counterbalanced across participants and the sessions were one week apart. Performance measures comprised time-on-target and target-deviation, and were calculated for the periods of stimulation (or sham) and during the afterphase respectively. Statistical analyses revealed significant performance improvements over the stimulation and the afterphase, but this learning effect was not modulated by tDCS condition. This suggests that the form of visuomotor learning taking place in the present task was not sensitive to neurostimulation. These null effects, together with similar reports for other types of motor tasks, lead to the proposition that tDCS facilitation of motor learning might be restricted to cases or situations where the motor system is challenged, such as motor deficits, advanced age, or very high task demand.
Highlights
Visually-guided movements are essential for everyday functioning
(1) Is there a general learning effect over the course of the experiment? (2) Does anodal transcranial Direct Current Stimulation (tDCS), applied over the contralateral motor cortex, modulate visuomotor performance and learning? (3) Does the performance modulation by tDCS occur during early-learning, defined as performance improvement during the first session, and/or during retention, defined as performance change between block six in Session 1 and block one in Session 2? In addition, we assessed whether tDCS might have differential effects on hard and easy trials
Overall the analysis showed that, while performance clearly improved across the two sessions, tDCS did not affect the learning associated with these performance changes in either phase of the experiment
Summary
Visually-guided movements are essential for everyday functioning. Such movements are based on visuomotor integration and range from simple reaching towards an object to complex abilities like driving. Visuomotor integration requires the complex interaction between the visual and the motor systems in order to use visual information to guide motor output and to use movement information to influence the processing of visual information. Stroke, Parkinson’s disease, traumatic brain injury and many other disorders can cause temporary or lasting motor function deficits, in which such visuomotor function and learning often become impaired [1]. Transcranial Direct Current Stimulation (tDCS) has been shown to facilitate motor learning, and to have positive therapeutic effects in patients with motor deficits [4,5]
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