Modulation of movement-associated cortical activation by transcranial direct current stimulation

Transcranial direct current stimulation (tDCS) is currently attracting increasing interest as a tool for neurorehabilitation. However, local and distant effects of tDCS on motor-related cortical activation patterns remain poorly defined, limiting the rationale for its use. Here we describe the results of a functional magnetic resonance imaging (MRI) experiment designed to characterize local and distant effects on cortical motor activity following excitatory anodal stimulation and inhibitory cathodal stimulation. Fifteen right-handed subjects performed a visually cued serial reaction time task with their right hand in a 3-T MRI scanner both before and after 10 min of 1-mA tDCS applied to the left primary motor cortex (M1). Relative to sham stimulation, anodal tDCS led to short-lived activation increases in the M1 and the supplementary motor area (SMA) within the stimulated hemisphere. The increase in activation in the SMA with anodal stimulation was found also when directly comparing anodal with cathodal stimulation. Relative to sham stimulation, cathodal tDCS led to an increase in activation in the contralateral M1 and dorsal premotor cortex (PMd), as well as an increase in functional connectivity between these areas and the stimulated left M1. These increases were also found when directly comparing cathodal with anodal stimulation. Significant within-session linear decreases in activation occurred in all scan sessions. The after-effects of anodal tDCS arose primarily from a change in the slope of these decreases. In addition, following sham stimulation compared with baseline, a between-session decrease in task-related activity was found. The effects of cathodal tDCS arose primarily from a reduction of this normal decrease.