# Binanual Movements and Chronic Stroke Rehabilitation: Looking Back and Looking Forward
## 4 Looking Forward  
What is on the horizon for bimanual movement interventions and chronic stroke rehabilitation? Rehabilitation interventions should aim for maximum recovery of function through motor learning improvements on the hemiplegic side [22]. Applying non-invasive brain stimulation (NIBS) in addition to motor training may be an attractive treatment protocol for improving bimanual coordination function post-stroke. [PERSON] and [PERSON] [93] suggested potential effects of transcranial direct current stimulation (tDCS), one of the NIBS protocols, on bimanual motor skills in healthy individuals. Two tDCS stimulations consist of anodal tDCS that may potentially increase cortical excitability and cathodal tDCS that may potentially suppress cortical excitability. Specific tDCS protocols for facilitating bimanual motor function improvements involved (a) anodal tDCS on the primary motor cortex (M1) of the ipsilesional hemisphere and cathodal tDCS on M1 of the contralesional hemisphere and (b) anodal tDCS on M1 of bimanual hemispheres [94; 95; 96]. Theoretically, these tDCS protocols are expected to be effective for re-balancing brain activations between affected and unaffected hemispheres, contributing to functional improvements in bimanual actions (e.g., bimanual typing performance and Perdue pegboard test). In testing chronic stroke patients, many prior studies reported transient and sustained treatment effects of tDCS protocols on unilateral paretic arm functions [22; 97; 98], whereas potential tDCS effects on bimanual motor functions are still insufficient. A limited number of studies revealed that bihemispheric tDCS in addition to conventional physical therapy improved interlimb coordinative skills in patients with stroke [99; 100]. Beyond the interhemispheric competition model emphasizing the balanced excitatory and inhibitory activations between hemispheres post-stroke via tDCS [20; 101], a recent approach proposed the bimodal balance-recovery model integrating both vicariation and interhemispheric competition approaches [75]. Intriguingly, this model posited that the vicariation model, assuming the important role of the unaffected hemisphere for functional recovery of the paretic limbs, may be beneficial for stroke patients with lower structure reserve (e.g., more severe and wide ranges of brain lesion), whereas the interhemispheric competition model may be effective for stroke patients with higher structure reserve (e.g., more recovered brain regions). Based on this model, applying cathodal tDCS suppressing the contralesional hemisphere may decrease treatment effects on motor recovery of patients with severe brain damages and less recovered brain functions (e.g., acute and subacute phases). In fact, several meta-analytic findings evidenced that tDCS protocols including cathodal tDCS on the contralesional hemisphere revealed overall significant positive effects on motor recovery, whereas this protocol failed to show functional improvements in the paretic arms of the acute and subacute patients with stroke [102; 103]. These findings support a proposition that bihemispheric tDCS protocols should be individualized based on either the severity or the recovery state of affected brain regions. For example, applying anodal tDCS on the primary motor cortex of bimanual hemispheres may be more effective for improving bimanual motor functions in patients with acute and subacute patients [104]. One caveat about tDCS protocols concerns the general brain assumptions necessary to ensure individual treatment benefits. That is, tDCS stimulation effects that are dose controlled according to electrode size, location placement, and stimulus intensity will minimize the trial-and-error effect frequently seen with so many stroke-rehabilitation protocols [105]. Establishing accepted procedures in administering tDCS should lead to individualized dose-controlled treatments [98].