The effect of anodal transcranial direct current stimulation on spatial motor skill learning in healthy and spinal cord injured humans

Abstract

Anodal transcranial direct current stimulation (tDCS) is an intervention which is thought to enhance motor learning in healthy and stroke-injured states, when applied adjunctively during skill learning. We set out to investigate whether anodal tDCS might enhance functional rehabilitation from incomplete tetraplegic SCI. To address current limitations in the measurement of task-dependent skill, a novel integrated skill training and measurement task, the Motor Skill Rehabilitation Task (MSRT) was designed and developed. Measures of performance from this task delivered the functional measure of spatial motor skill learning, Task Productivity Rate (TPR). TPR was analysed and validated as a univariate dependent outcome, which is of potential importance to the future development of clinical measures measuring goal-directed motor skills. The MSRT was included alongside conventional behavioural measures in a repeated-measures RCT pilot study, the first to investigate the effect of anodal tDCS on rehabilitation of motor skill from chronic spinal cord injury. Adjunctive application of anodal tDCS had a statistically significant benefit upon retention of skill in the incomplete spinal cord injured population, but only when the independent factor of sensory acuity was included in the analysis. Differences between the development of task-dependent skill and generic dexterity over time suggested that spatial skill development was subject to an interaction of short-term and lasting effects. A larger study in healthy persons further investigated these phenomena, also applying Transcranial Magnetic Stimulation (TMS)–evoked measurements to investigate intervention-dependent effects upon the excitability of projections between the primary motor cortex and muscles involved in the prehension task. The findings revealed that active tDCS did not enhance skill learning at 7 days beyond the training period, but did significantly alter the development of motor skill following a period of learning and subsequent skill consolidation which was associated with underlying perturbation of motor control strategy. Significant and divergent patterns of cortical plasticity were evoked in projections to muscles necessary for reaching and grasping. The main findings of this thesis do not support anodal tDCS as an effective adjunctive means of enhancing spatial motor skill in rehabilitation from incomplete tetraplegic SCI. If applied in patient populations, the clinical benefits of anodal tDCS may be contingent both on the nature of the sensorimotor deficit affecting upper limb function and the spatial demands of the behavioural task. The findings of this project serve to inform further research in relation to the effect of anodal tDCS on the brain and behavioural outcomes, the potential for efficacy in target patient groups and the sensitivity of outcome measures to spatial and temporal dimensions of practical motor skills.