Transcranial direct current stimulation: functional lateralization in healthy subjects with a virtual lesion?

The excitability of the human primary motor cortex can be increased or decreased following anodal and cathodal transcranial direct current stimulation (tDCS), respectively (Nitsche, 2001 and 2003; Priori, 2003). Studies in animals and humans show that primary motor cortices are strongly connected by transcallosal fibers and inhibit each other. This suggests that modification of excitability of the one motor cortex should effect the excitability in the homologous contralateral cortex in an opposite manner. This can be quantified by the lateralization index, which compares the differential employment of bilateral…