Abstract
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation method to modulate the local field potential in neural tissue and consequently, cortical excitability. As tDCS is relatively portable, affordable, and accessible, the applications of tDCS to probe brain–behavior connections have rapidly increased in the last 10 years. One of the most promising applications is the use of tDCS to modulate excitability in the motor cortex after stroke and promote motor recovery. However, the results of clinical studies implementing tDCS to modulate motor excitability have been highly variable, with some studies demonstrating that as many as 50% or more of patients fail to show a response to stimulation. Much effort has therefore been dedicated to understand the sources of variability affecting tDCS efficacy. Possible suspects include the placement of the electrodes, task parameters during stimulation, dosing (current amplitude, duration of stimulation, frequency of stimulation), individual states (e.g., anxiety, motivation, attention), and more. In this review, we first briefly review potential sources of variability specific to stroke motor recovery following tDCS. We then examine how the anatomical variability in tDCS placement [e.g., neural target(s) and montages employed] may alter the neuromodulatory effects that tDCS exerts on the post-stroke motor system.
Highlights
Stroke is a neurological deficit induced by the interruption of the blood flow to the brain due to either a vessel occlusion or less frequently an intracerebral hemorrhage [1]
In addition to the parameters mentioned here that may contribute to variability, it is possible that other parameters that have not yet been explored may affect the efficacy of Transcranial direct current stimulation (tDCS)
Studies that compare the effects of tDCS to another stimulation condition, rather than to a sham or true baseline condition, may introduce further confounds when interpreting the effects of tDCS
Summary
Stroke is a neurological deficit induced by the interruption of the blood flow to the brain due to either a vessel occlusion or less frequently an intracerebral hemorrhage [1]. Neural plasticity processes and cortical reorganization inducing differential recruitment of different brain regions may be dependent of the size and location of the stroke [67] Another source of stroke-specific variability is the nature of the stroke (i.e., ischemic or hemorrhagic), with some studies mixing both populations together [18, 65, 66, 68]. This could be a potential confound as the deficits and brain plasticity processes associated with cortical/subcortical localization or ischemic or hemorrhagic stroke [69,70,71] could be different. These results suggest that the choice of and nature of a motor task is of crucial importance to maximize tDCS effects
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