Sensorimotor interactions between pairs of fingers in bimanual and unimanual manipulative tasks

The tuning of fingertip forces to the physical properties of objects during manipulation may be controlled partly by digit-specific mechanisms using local afferent information and partly by controllers that support interdigital coordination and use sensory information from more than one digit. In the present study we addressed digital interactions when humans used the tips of two fingers to restrain a pair of horizontally oriented plates from moving when subjected to tangential force loads in the distal direction. Subjects used the right and left index fingers in a bimanual grasp, and the right index and middle fingers in an unimanual grasp. The plates were loaded at unpredictable times by identical force profiles consisting of a force increase of up to a 3-N force plateau. The plates were concurrently loaded in 85% of the trials and each plate was loaded separately in 7% of the trials. For each plate, we measured its movement and the normal and tangential forces applied by the finger to restrain it. When a finger was loaded, the subject automatically responded by a normal force increase to a level that remained fairly constant during the subsequent load plateau. The initial part of this finger grasp response was affected by simultaneous loading of its partner finger; the magnitude of the response was boosted with a bimanual grasp, whereas the onset latency tended to be shorter with a unimanual grasp. Responses also occurred at a non-loaded finger during both bimanual and unimanual grasps, but these responses were weaker than those evoked when the same finger was loaded. In the bimanual grasp, they were largely characterized by a brief force pulse whose onset was delayed by some 15 ms compared with the response onset of the loaded finger, i.e., there was no sustained response. In the unimanual grasp, the onset of the response coincided in time with that of the accompanying (loaded) finger, and the dynamic response was stronger and prolonged, with more than one force rate peak. There was also a significant static response present. We conclude that during unimanual as well as bimanual reactive restrain tasks there are interactions between digits engaged in terms of neural control that facilitate the response of a digit when an accompanying digit is simultaneously loaded. However, digit-specific afferent inputs are necessary for eliciting the full-size reactive grasp responses required to successfully restrain the manipulandum.