A special edition of Cognitive Neuropsychology (1988), 5(5), is dedicated to methodological issues related to single case and group studies in neuropsychology.
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.