Contralesional motor deficits after unilateral stroke reflect hemisphere-specific control mechanisms

Abstract

We have proposed a model of motor lateralization, in which the left and right hemispheres are specialized for different aspects of motor control: the left hemisphere for predicting and accounting for limb dynamics and the right hemisphere for stabilizing limb position through impedance control mechanisms. Our previous studies, demonstrating different motor deficits in the ipsilesional arm of stroke patients with left or right hemisphere damage, provided a critical test of our model. However, motor deficits after stroke are most prominent on the contralesional side. Post-stroke rehabilitation has also, naturally, focused on improving contralesional arm impairment and function. Understanding whether contralesional motor deficits differ depending on the hemisphere of damage is, therefore, of vital importance for assessing the impact of brain damage on function and also for designing rehabilitation interventions specific to laterality of damage. We, therefore, asked whether motor deficits in the contralesional arm of unilateral stroke patients reflect hemisphere-dependent control mechanisms. Because our model of lateralization predicts that contralesional deficits will differ depending on the hemisphere of damage, this study also served as an essential assessment of our model. Stroke patients with mild to moderate hemiparesis in either the left or right arm because of contralateral stroke and healthy control subjects performed targeted multi-joint reaching movements in different directions. As predicted, our results indicated a double dissociation; although left hemisphere damage was associated with greater errors in trajectory curvature and movement direction, errors in movement extent were greatest after right hemisphere damage. Thus, our results provide the first demonstration of hemisphere specific motor control deficits in the contralesional arm of stroke patients. Our results also suggest that it is critical to consider the differential deficits induced by right or left hemisphere lesions to enhance post-stroke rehabilitation interventions.

Figure 1

Overlap images showing locations of lesions between groups with right and left hemisphere damage (colour scale of magenta to red shows increasing overlap). Lesions are confined to either left or right hemisphere.

Figure 2

Schematic of the experimental set-up. Subjects sat facing a mirror on which the start position and targets were projected using a high definition television, and they rested their arms in an air-sled system placed on a glass tabletop. FOB = Flock of Birds system.

Figure 3

(A) Comparison of hand-paths between representative left healthy (LHC) and right healthy control (RHC) subjects. (B–E) Comparison of hand-paths between patients with left (LHD) and right hemisphere damage (RHD) across severity of hemiparesis assessed using the Fugl-Meyer score. Each right and left hemisphere damage pair has similar Fugl-Meyer (FM) scores; Fugl-Meyer scores decrease from B to E, indicating increasing degree of hemiparesis.

Figure 4

(A) Mean duration and (B) mean peak tangential velocity for right and left arm of control subjects (LHC, RHC) (black) and the contralesional arm of patients with left hemisphere damage (LHD, grey) and right hemisphere damage (RHD, grey). (C) Final position error for the same groups for the three different targets (lateral, central, medial). Error bars indicate standard error of the mean. *Significant (P < 0.05) group differences.

Figure 5

(A) Mean distance error, (B) mean absolute initial direction error, (C) mean absolute direction error at end of movement, (D) mean hand path curvature and (E) mean variable direction error at peak velocity for the left healthy control (LHC), left hemisphere damage (LHD), right healthy control (RHC) and right hemisphere damage (RHD) groups. Error bars indicate standard error of the mean. *Significant (P < 0.05) group differences.

Figure 6

(A) Mean distance error, (B) mean absolute initial direction error, (C) mean hand path curvature and (D) mean variable direction error at peak velocity of mildly and moderately impaired patients with left hemisphere damage (LHD, black) and right hemisphere damage (RHD, grey). Error bars indicate standard error of the mean.