Motor Lateralization

Our program of research is fundamentally translational. The information that we develop through research on basic motor control processes is used to develop models for understanding motor dysfunction in neurological diseases and stroke. In turn, stroke lesions are used to develop our understanding of the functional neuroanatomy of motor control processes. The best example fo this translational spectrum has been our research on motor lateralization.

Our research on motor lateralization has led to a model for understanding the motor control processes that give rise to handedness. According to the Dynamic Dominance Model, the left hemisphere (in right handers) is proficient for processes that predict the effects of body and environmental dynamics, while the right hemisphere is proficient at impedance control processes that can minimize potential errors when faced with unexpected mechanical conditions, and can achieve stable positions and postures. This model can be viewed as a motor component for the paradigm of brain lateralization that has been proposed by Rogers et al. (MacNeilage et al., 2009) that is based upon evidence from a wide range of behaviors across many vertebrate species. Rogers proposed a left-hemisphere specialization for well-established patterns of behavior performed in familiar environmental conditions, and a right hemisphere specialization for responding to unforeseen environmental events. The dynamic dominance hypothesis provides a framework for understanding the biology of motor lateralization that is consistent with Roger’s paradigm of brain lateralization.

Our laboratory has developed a model of motor lateralization based on fundamental principles of control theory that account for a range of experimental findings in different tasks and task conditions. The dynamic dominance hypothesis of motor lateralization proposes that the left hemsiphere (in right-handers) is specialized for processes that account for predictable dynamic conditions, in order to specify movements that are mechanically efficient, and have precise trajectories. In contrast, the right hemisphere (in right-handers) is specialized for impedance control mechanisms that ensure positional and velocity stabilization in the face of unpredictable mechanical events and conditions, and accuracy and stability of steady state postures. The former process assures mechanical efficiency and trajectory specificity under predictable conditions, while the latter imparts robustness under unpredictable conditions, as well as postural stability. Through studies in stroke patients with specific unilateral brain lesions, we have provided evidence that both processes contribute to control of each arm. However, the hemisphere contralateral to a given arm imparts the greatest influence to that arm’s performance. In terms of Roger’s hypothesis, the right hemisphere is specialized for a system that ensures stability and rapid online responses to unexpected stimuli in the internal and external environments, while the left hemisphere exploits predictive processes to assure trajectory precision and mechanical efficiency when conditions are consistent and predictable.

The dynamic dominance model is a bi-hemispheric model of motor control that attributes control of each arm to both hemispheres. This model predicts that damage to one hemisphere, due to stroke, should result in predictable and hemisphere dependent motor deficits in the less-effected arm of stroke patients. Our research, over the past decade, has not only confirmed these precise predictions of ipsilesional motor deficits in stroke patients, but has also shown hemisphere specific deficits in the contralesional arm, when the lesions do not produce paresis, or when the paresis is mild. In addition, we have detailed the functional neuroanatomy of predictive and impedance control by assessing motor function in focal lesioned stroke patients.