Interlimb Coordination and Neural Substrates of Complementary Bilateral Arm Movements in Young and Old Individuals

Abstract

Understanding deficits in bilateral arm coordination with aging is essential given that everyday activities rely primarily on bilateral movements. A model of motor lateralization, based on unilateral movements, suggests that the left hemisphere is specialized for predicting task dynamics and the right hemisphere is specialized for impedance control. These arm-hemisphere specializations have been suggested to provide the mechanism for complementary bimanual coordination of the right (dominant) arm for reaching, and the left (non-dominant) arm for stabilizing. Aging has been shown to reduce lateralization of both cognitive and motor tasks. The overall objective of this thesis was to investigate the effect of hemispheric specializations during bilateral coordination in young and old non-disabled individuals. We hypothesized that these arm-hemisphere specializations would be expressed during complementary bilateral tasks in young adults and reduced in older adults. To test this hypothesis, young and old right-hand dominant individuals, completed a bilateral coordination paradigm in which one arm maintained its spatial position and the other arm performed a center-out reaching task with a spring affixed between the arms (conditions: right reach - left stabilize, right stabilize - left reach). Movement performance of reaching and stabilizing was compared across arms and age-group using both kinematic and dynamic variables. To identify the neural substrates underlying complementary bilateral coordination, effective connectivity of a bilateral motor network was investigated using fMRI. Subjects completed a modified version of the bilateral behavioral tasks and performed each task unimanually. In young adults, the right hand showed better reaching performance (less deviation from linearity) and the left hand stabilized better (less displacement). Movement performance of older adults illustrated a) reduced absolute performance; b) preserved asymmetry of reaching; and c) greater asymmetry of stabilizing, indicating an increase in motor lateralization. Finally, effective connectivity analyses illustrated a) greater intrahemispheric modulation for right reach - left stabilize bilateral task compared to greater interhemispheric modulation for the left reach - right stabilize bilateral task; b) reciprocal positive coupling between bilateral SMA for the right reach and left stabilize unilateral tasks; c) greater role for positive modulation from S1 to M1 for left vs. right stabilize tasks.