It is now well-known that unilateral stroke not only results in weakness of the opposite half of the body, i.e., contralateral to the lesion or contralesional limb, but also significant motor deficits in the same half of the body, i.e., ipsilateral to the lesion or ipsilesional limb (1–4). Previous work suggests that deficits in the ipsilesional arm and hand varies with the severity of contralesional deficits, especially in the sub-acute and chronic phase after stroke (5–8). More interestingly, the unilateral motor deficits observed for contralesional and ipsilesional limbs seem to be hemisphere-specific and thus depend on side of stroke lesion (9–15). For predominantly right-handed cohorts, contralesional deficits appear to be more severe in those with right hemisphere damage (RHD), in whom the contralesional limb is non-dominant. For example, using clinical motor assessments of grip strength and hand dexterity, Harris and Eng (11) showed that contralesional motor impairments were less severe in chronic stroke survivors who suffered damage in the dominant (i.e., left) hemisphere (LHD) compared to those who suffered damage in the non-dominant (right) hemisphere (11, 15).
In contrast, considering ipsilesional motor deficits, the evidence is mixed concerning hemisphere-specific effects. For instance, some studies reported that individuals with LHD exhibited more severe ipsilesional arm and hand deficits compared to those with RHD (4, 15–17) while others have reported no difference in ipsilesional hand motor capacity between LHD and RHD (2). In acute stroke survivors, Nowak et al. demonstrated that deficits in grip force of the ipsilesional hand were significantly associated with clinical measures of function of the contralesional hand only in LHD (12). Contrary to this, de Paiva Silva et al. (14) found that compared to controls and LHD, the ipsilesional hand in chronic stroke survivors was significantly slower and less smooth in RHD especially when contralesional impairment was relatively more severe (UEFM < 34).
Taken together, there is converging evidence regarding the relationship between motor deficits of the contralesional and ipsilesional upper extremity, such that ipsilesional deficits are worse when contralesional impairment is greater (Figure 1A); however, it is uncertain whether the relationship between the two limbs depends on which hemisphere is damaged. In particular, motor deficits of the two limbs are most prominent for tasks that require dexterous motor control (e.g., grip force, tapping, tracking). For predominantly right-handed cohorts (as is the case in most studies), contralesional deficits appear to be more severe in those with RHD, in whom the contralesional limb is non-dominant; whereas ipsilesional deficits are more severe in those with LHD. An exception to this observation for those with RHD seems to be in the case when contralesional impairment is most severe (i.e., UEFM < 34) (14). Thus, one might predict that as contralesional impairment worsens, individuals with LHD would have proportionally worse ipsilesional deficits, but individuals with RHD (especially if say UEFM > 34) would not; see Figures 1B,C for two alternative hypotheses. To our knowledge, this prediction has not before been explicitly tested.