The balance of interhemispheric inhibition becomes disrupted after a stroke (A). This leaves the healthy hemisphere in a position that it could exert too much of an unopposed/imbalanced inhibitory influence onto the lesional hemisphere and possibly interfere in the recovery process of the affected hemisphere. There are two possible ways to ameliorate this imbalance: either one upregulates the excitability in the affected (lesional) hemisphere (B) or one downregulates the excitability in the unaffected (normal) hemisphere (C).

A) This drawing shows the TDCS setup using a mobile, battery-operated direct current stimulator connected with two electrodes. One electrode (active) is positioned over C3 (corresponding to the precentral gyrus) and the reference electrode is positioned over the contralateral supraorbital region. If current flows from C3 to the supraorbital region, then the tissue underlying C3 is subjected to anodal (increase in excitability) stimulation. If current is reversed, then the tissue underlying C3 is subjected to cathodal (decrease in excitability) stimulation; B) Regional cerebral blood increases in the motor region underlying the electrode positioned over C3 after anodal stimulation. Regional cerebral blood was determined using a non-invasive arterial spin-labeling technique30.

This picture shows two patients with their representative corticospinal tract (CST) fibers that originate from the white matter underlying the precentral gyrus and travel through the internal capsule into the brainstem. The CST of the lesional hemispheres differs between both patients. Patient #1 shows a severely reduced number of fibers that don’t seem to originate from the dorsal part of the motor region (hand/arm region of the precentral gyrus), but still show a path through the posterior limb of the internal capsule into the brainstem while patient #2 has a very mild reduction in the number of CST fibers on the lesional hemisphere, but otherwise shows a similar origin and descent of the CST between the lesional and normal hemisphere. The improvement after TDCS stimulation was pronounced in the patient with intact pyramidal tract, but only minimal in the patient with the disrupted pyramidal tract.