There are many motor strip studies showing that anodal stimulation at C3 or C4, with a contralateral orbit reference, enhances fine motor control, while cathodal stimulation impairs it. A motor-strip study of stroke patients by Lindenberg, et al, (2010), found that by placing the anode over the motor-strip lesion and the cathode over the contralateral strip, function increased by 21% and lasted for one week.
There are several studies on the treatment of pain. Most of them found that anodal stimulation of the motor strip, exactly as the enhancement studies with a contralateral supraorbital placement of the cathode, is ideal for reducing pain on the opposite side of the body. Here, the A1/C1 placement would suppress pain in the right side and vice-versa for A2/C2. It seems counterintuitive that boosting the motor strip would reduce pain or could the reference electrode over the prefrontal lobe be involved
Siever, D. (2013). Transcranial DC Stimulation. Neuroconnections, Spring Issue, 33-40. http://www.mindalive.com/1_0/article%2011.pdf.