Non-invasive brain current stimulation in neurorehabilitation.

Neurologic conditions such as stroke, traumatic brain injury, tumors, and degenerative diseases are often associated with dramatic impairments in brain function. Converging evidence in the fields of clinical neurorehabilitation and cognitive neuroscience has revealed that functional recovery after brain injury depends largely on compensatory plastic changes in remaining neural structures. Thus, while current treatments such as physical, occupational and speech therapy and pharmacological interventions have proven only modestly effective, there has been increasing interest in exploring potential therapies that can modulate the functioning of neural systems more directly. Noninvasive brain stimulation techniques such as transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS) and transorbital alternating current stimulation (ACS) are promising approaches that can potentially be employed to facilitate functional plasticity after brain injury, leading to enhanced recovery. This special issue of RNN is devoted to this topic. As will be illustrated by the contributions included here, noninvasive brain stimulation techniques have potentially important implications for rehabilitation of a variety of neurologic disorders. The modern era of noninvasive brain stimulation was initiated by the demonstration that a magnetic field could readily and painlessly traverse the scalp and skull to generate a weak, brief and localized current in the brain (Barker et al., 1987). Subsequent studies demonstrated that, depending on the stimulation parameters, this electrical current could either inhibit or enhance neural function in the stimulated area. Over the next few years, this exciting breakthrough was exploited in investigations that contributed substantially to the development of theories regarding the localization and time course of sensory and motor processing in the intact human brain. Beginning in the 1990 s, TMS was also used to address questions regarding the localization of “higher” cortical faculties such as language and reading. In the late 1990’s Nitsche, Paulus and colleagues demonstrated that low power (e.g., 1 mA) direct current applied to the skin could alter neuronal excitability. The effects induced by transcranial direct current stimulation (tDCS) were, at least in some instances, a function of electrode polarity; that is, anodal and cathodal stimulation produced different results. Like TMS, tDCS has been used to explore motor, sensory and cognitive functions. In this issue of RNN, Nitsche and Paulus review recent progress in the use of tDCS. This comprehensive overview discusses advances in our understanding of the neural mechanisms of stimulation, reviews important refinements that have made the administration of tDCS, and surveys the variety of basic and clinical applications that have been explored in the last several years with this versatile technology.