Transcranial direct current stimulation (tDCS) is a safe and non-invasive brain stimulation technique with the potential to improve upper limb function after stroke. Ipsilesional primary motor cortex (M1) excitability can be increased with anodal tDCS, contralesional M1 excitability can be decreased with cathodal tDCS or both anodal and cathodal tDCS can be used simultaneously on both cortices (bihemispheric). The impact of these different electrode arrangements on the efficacy of tDCS, and whether any of the changes are due to callosal connections between cortices, is unclear.
This study aimed to investigate the effect of tDCS electrode arrangement on motor sequence learning and upper limb function in chronic stroke survivors.
Patients and methods
21 stroke survivors (range 3–124 months post-stroke, 34–81 years of age) with upper limb impairment received 20 min of 1 mA tDCS (0.04 mA·cm−2) during performance of a motor sequence learning task which involved movement of a computer mouse with the paretic arm to circular targets on a monitor in a repeating pattern. Four tDCS conditions were studied in a repeated-measures design; (i) anodal to the ipsilesional M1, (ii) cathodal to the contralesional M1, (iii) bihemispheric and (iv) sham. Upper limb function was assessed before and after tDCS, using the Jebsen–Taylor hand function test (JTT). Changes in transcallosal inhibition (TCI) were assessed using transcranial magnetic stimulation (ipsilateral silent period duration).
There was no effect of tDCS condition on performance of the motor sequence learning task. Performance on the JTT improved significantly after unilateral tDCS (anodal or cathodal) compared to sham (p < 0.05), but not after bihemispheric (Fig. 1). There was no effect on TCI (p > 0.5), and no relationship between changes in TCI and upper limb function.
Unilateral, but not bihemispheric, tDCS improves upper limb function. The response to tDCS does not appear to be driven by changes in TCI. These results have implications for the use of tDCS for upper limb rehabilitation.