Transcranial direct current stimulation (tDCS) is a promising technique to treat a wide range of neurological conditions including stroke. The pathological processes following stroke may provide an exemplary system to investigate how tDCS promotes neuronal plasticity and functional recovery. Changes in synaptic function after stroke, such as reduced excitability, formation of aberrant connections, and deregulated plastic modifications, have been postulated to impede recovery from stroke. However, if tDCS could counteract these negative changes by influencing the system’s neurophysiology, it would contribute to the formation of functionally meaningful connections and the maintenance of existing pathways. This paper is aimed at providing a review of underlying mechanisms of tDCS and its application to stroke. In addition, to maximize the effectiveness of tDCS in stroke rehabilitation, future research needs to determine the optimal stimulation protocols and parameters. We discuss how stimulation parameters could be optimized based on electrophysiological activity. In particular, we propose that cortical synchrony may represent a biomarker of tDCS efficacy to indicate communication between affected areas. Understanding the mechanisms by which tDCS affects the neural substrate after stroke and finding ways to optimize tDCS for each patient are key to effective rehabilitation approaches. 1. Introduction Poststroke consequences including sensorimotor and cognitive impairments impose a stressful situation and a great burden to the victims, their families, and the society. Indeed, stroke is one of the leading causes of adult disability in the western world [1]. Among extensive efforts devoted to the search for more effective rehabilitation therapies of stroke, the idea of using electricity can be traced back almost a century ago (as noted by Priori [2]). After diminished interest due to mixed results, recent studies with promising results regained the interest in the application of mild electrical currents to the brain as a potential therapy for neurological disorders [2]. Research by Priori [2, 3] and Nitsche and colleagues [4–6] led to the development of a technique consisting of the application of weak electrical currents through the scalp, which is now called transcranial direct current stimulation (tDCS). Recent findings suggest that tDCS may be beneficial in a wide range of disorders such as epilepsy [7, 8], Parkinson’s disease [9–11], chronic pain [12–14], depression [15], drug cravings [16], pain conditions such as fibromyalgia [17–19], and traumatic
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