Hand-specific specialization of grip force control

<p>A large majority of human adults are right-handed; right handers prefer using the right hand for most tasks, and strongly prefer using the right hand for difficult tasks. This thinking extends to bimanual tasks like uncorking a bottle of wine: right-handers will use the right hand for the ‘harder’ task of pulling out the cork.</p>

<p>Such behavioral observations on hand use are codified in the <em>global dominance</em> view which insists that the right hand is better at all tasks, and the left hand plays a supporting role.  </p>

<p>The alternative <em>complimentary dominance </em>view is recent, and it claims that in right-handed individuals, both hands – and by extension, both brain hemispheres that control the contralateral hand – are specialized for different types of movements and control. The left hemisphere is better at predictions, it specializes in feedforward control and produces better coordinated movements with the right hand. In contrast, the right hemisphere is better at impedance control, and it specializes in stabilizing the left hand. The left hand of right-handers is not just a support; rather, the left hand is better than the right hand at certain tasks. This is one reason why right-handers will use the left hand to hold the wine bottle while uncorking it.</p>

<p>All the supporting evidence for the complimentary dominance theory is based on wrist movements, and it does not directly address holding and manipulating objects. Our goal was to test if complimentary dominance extends to tool use. We tested this by having healthy young participants hold two objects connected by a spring and perform asymmetric bimanual tasks resembling bread slicing. Participants moved one object while holding the other object static; the spring stretched and disturbed both objects. The pressing grip forces of each hand and the movements of each object were measured. Both hands performed the movement and the stabilization tasks in different conditions.</p>

<p>Our findings supported the complimentary dominance theory. Compared to the left hand, the right hand produced more accurate object movements accompanied by feedforward modulations in grip force that coupled more strongly with the predicted loads on the moving object. In contrast, compared to the right hand, the left hand stabilized the object’s position better by exerting a higher grip force on the object.</p>

<p>The results have implications for how movement control is organized in the brain. Previous work on wrist motions indicated that the reaching circuit in the brain functioned differently in the two hemispheres, in accordance with complimentary dominance. Our work suggests that there may be similar functional asymmetry in the grasping circuit as well, and furthermore, in the interactions between the reaching and the grasping circuits. These are important open questions generated by our findings. On a functional level, our results provide a lens to evaluate dexterity loss with aging and pathology.</p>