Role of digit placement control on sensorimotor transformations for dexterous manipulation

Dexterous manipulation relies on the ability to modulate grasp forces to variable digit position. However, the sensorimotor mechanisms underlying such critical ability are not well understood. The present study addressed whether digit force-to-position modulation relies entirely on feedback of digit placement and force, or on the integration of such feedback with motor commands responsible for digit positioning. In two experiments, we asked 25 subjects to estimate the index fingertip position relative to the thumb (Perception test), or grasp and lift an object with an asymmetrical mass distribution while preventing object roll (Action test). Both tests were performed after subjects' digits were placed actively or passively at different distances (Active and Passive condition, respectively) and without visual feedback. As motor commands for digit positioning would be integrated with position and force feedback in the Active condition, we hypothesized this condition to be characterized by greater accuracy of digit position estimation and digit force-to-position modulation. Surprisingly, discrimination of digit position and force-to-position modulation were statistically indistinguishable in the Active and Passive conditions. However, the Active condition of the Action test was characterized by larger peak load force rate and shorter load phase duration (p < 0.05 and 0.001, respectively). We conclude that voluntary commands for digit positioning are not essential for accurate digit force-to-position modulation. Thus, digit force-to-position modulation can be implemented by integrating sensory feedback of digit position and voluntary commands of digit force production following contact.

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