Hand-specific specialization of grip force control

<p>A large majority of human adults are right-handed. For most, this means a preference for using the right hand for most tasks, and a strong preference for using the right hand for difficult tasks. Most will extend this logic to bimanual tasks like uncorking a bottle of wine: right-handed individuals recruit the right hand for the ‘harder’ task.</p>

<p>This has been the dominant view in literature on handedness. The behavioral observations on hand use are extended to infer the characteristics of motor control exerted by the contralateral brain hemisphere on each hand. This <em>global dominance</em> view insists that the right hand is better at all tasks, and the left hand plays a supporting role.  </p>

<p>The <em>dynamic dominance </em>view is comparatively recent, and it claims that in right-handed individuals, both hands – and by extension, both hemispheres – 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 dynamic dominance theory is based on wrist movements, and it does not directly address holding and manipulating objects. Our goal was to test if dynamic 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 dynamic 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 so-called reaching circuit in the brain functioned differently in the two hemispheres, in accordance with the dynamic dominance view. Our work indicates that similar functional asymmetry may be apparent in the so-called 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>