Manual asymmetries in grasp pre-shaping and transport-grasp coordination.

Few studies have directly compared the visuo-motor transformation of grasp pre-shaping or transport-grasp coordination of reach-to-grasp movements between the two hands. Our objective was to determine if there are manual asymmetries in right-handed adults as a foundation to investigate hemispheric specialization in individuals post-stroke. Twelve non-disabled right-handed adults performed rapid reach-to-grasp movements to cylinders of three sizes as vision of the arm and hand was partially occluded. We reasoned that the hand system (left or right) that is superior in anticipatory planning of aperture scaling and movement preparation would be more likely to exhibit early grasp pre-shaping under this experimental manipulation. Movement time, hand path, transport velocity, and aperture were derived from 3D electromagnetic sensor data. The visuo-motor transformation of object sizes into an action of aperture pre-shaping was quantified using the correlations between initial aperture velocity and object diameter, and peak aperture and object diameter. Coordination between hand transport and aperture grasping was quantified using the cross-correlation between transport velocity and aperture size. Peak aperture and object diameter were strongly correlated for both hands. However, early aperture velocity and object diameter were correlated only for left-hand movements. Cross-correlation analyses revealed a strong association between transport velocity and aperture only for right-hand movements. Together, these results suggest earlier anticipatory control for the left hand in the visuo-motor transformation of grasp pre-shaping and a stronger transport-grasp linkage for the right hand. Further, initial aperture velocity was a more sensitive measure of these manual asymmetries than peak aperture. Our findings compliment the specialization previously observed for pointing movements of the dominant and non-dominant hemispheric/limb system and the coordinated control of complex movements and visuo-spatial components, respectively.