An overview of the method of brain stimulation known as tDCS is provided in this paper
Nitsche, M. A., Cohen, L. G., Wassermann, E. M., Priori, A., Lang, N., Antal, A., … Pascual-Leone, A. (2008). Transcranial direct current stimulation: State of the art 2008. Brain Stimulation, 1, 206–223.
A classic study in TMS research demonstrating how performance can sometimes be enhanced as well as impaired by brain stimulation
Walsh, V., Ellison, A., Battelli, L., & Cowey, A. (1998). Task-induced impairments and enhancements induced by magnetic stimulation of human area V5. Proceedings of the Royal Society of London B, 265, 537–543.