Almost 20% of the population has moderate to severe numerical disabilities, and more lose their numerical competence when tragic events such as a stroke or degenerative diseases arise. A study by Kadosh (2010) tested anodal stimulation at P4 with the cathode at P3 and showed an increase in mathematical abilities. While the opposite placement lowered the same skills.
An impressive note from the study was that these math abilities were still present almost 6 months after the initial training, this sort of specificity and longevity makes tDCS a potential tool to help people with the loss of numerical abilities or troubles developing numerical abilities.
Siever, D. (2013). Transcranial DC Stimulation. Neuroconnections, Spring Issue, 33-40. http://www.mindalive.com/1_0/article%2011.pdf.