Dexterous manipulation: differential sensitivity of manipulation and grasp forces to task requirements

How humans coordinate digit forces to perform dexterous manipulation is not well understood. This gap is due to the use of tasks devoid of dexterity requirements and/or the use of analytical techniques that cannot isolate the roles that digit forces play in preventing object slip and controlling object position and orientation (pose). In our recent work, we used a dexterous manipulation task and decomposed digit forces into F_G, the internal force that prevents object slip, and F_M, the force responsible for object pose control. Unlike F_G, F_M was modulated from object lift onset to hold, suggesting their different sensitivity to sensory feedback acquired during object lift. However, the extent to which F_G and F_M can be controlled independently remains to be determined. Importantly, how F_G and F_M change as a function of object property is mathematically indeterminate and therefore requires active modulation. To address this gap, we systematically changed either object mass or external torque. The F_M normal component responsible for object orientation control was modulated to changes in object torque but not mass. In contrast, F_G was distinctly modulated to changes in object mass and torque. These findings point to a differential sensitivity of F_G and F_M to task requirements and provide novel insights into the neural control of dexterous manipulation. Importantly, our results indicate that the proposed digit force decomposition has the potential to capture important differences in how sensory inputs are processed and integrated to simultaneously ensure grasp stability and dexterous object pose control.