The complementary dominance hypothesis: a model for remediating the 'good' hand in stroke survivors

The complementary dominance hypothesis is a novel model of motor lateralization substantiated by decades of research examining interlimb differences in the control of upper extremity movements in neurotypical adults and hemisphere-specific motor deficits in stroke survivors. In contrast to earlier ideas that attribute handedness to the specialization of one hemisphere, our model proposes complementary motor control specializations in each hemisphere. The dominant hemisphere mediates optimal control of limb dynamics as required for smooth and efficient movements, whereas the non-dominant hemisphere mediates impedance control, important for countering unexpected mechanical conditions and achieving steady-state limb positions. Importantly, this model proposes that each hemisphere contributes its specialization to both arms (though with greater influence from either arm's contralateral hemisphere) and thus predicts that lesions to one hemisphere should produce hemisphere-specific motor deficits in not only the contralesional arm, but also the ipsilesional arm of stroke survivors - a powerful prediction now supported by a growing body of evidence. Such ipsilesional arm motor deficits vary with contralesional arm impairment, and thus individuals with little to no functional use of the contralesional arm experience both the greatest impairments in the ipsilesional arm, as well as the greatest reliance on it to serve as the main or sole manipulator for activities of daily living. Accordingly, we have proposed and tested a novel intervention that reduces hemisphere-specific ipsilesional arm deficits and thereby improves functional independence in stroke survivors with severe contralesional impairment. image Abstract figure legend Summary of the complementary dominance hypothesis of motor lateralization and its application in remediating functional motor deficits of the ipsilesional arm in stroke survivors. The complementary dominance model of motor lateralization highlights distinct yet complementary functional contributions of each hemisphere to movement control (top panel). The dominant (left) hemisphere specializes in optimal control of limb dynamics and is therefore advantaged for well-established behavioural patterns under predictable circumstances. The non-dominant (right) hemisphere specializes in impedance control, which regulates steady-state limb position and responses to unexpected stimuli in the environment. Unilateral stroke results in hemisphere-specific motor deficits in both the contralesional and ipsilesional arms that are functionally limiting and consistent with the complementary dominance hypothesis. Left hemisphere stroke leads to deficits in initial direction accuracy, resulting in curved hand trajectories (middle panel, right). By contrast, right hemisphere stroke impairs final position accuracy, as demonstrated by larger deviations from the target location (middle panel, left). Based on these observations, we are now testing a novel training intervention to remediate functional motor deficits of the ipsilesional arm for chronic, severely impaired stroke survivors. The intervention includes hemisphere-specific virtual-reality training (bottom panel, top row) and real-world dexterity training (bottom panel, lower row), which we expect to translate to improved functional independence. image