CHANGES IN THE FORCE SHAVING PATTERN INDUCED BY MODIFICATIONS OF VISUAL FEEDBACK DURING FORCE PRODUCTION BY A SET OF FINGERS

We investigated force-sharing among three fingers which acted in paral lel and produced ramp profiles of total force from zero to the maximal voluntary force. The feedback to the subject was provided by a visual signal on the monitor and could correspond to the sum of forces of al l the fingers or to the sum of forces of two fingers, while the force of the third finger was added with a coefficient 2 or 0.5. If the subj ects did not know about the distorted feedback, they showed a template -sharing pattern within the whole range of total force values. This pa ttern did not depend on which finger force was multiplied and by which coefficient. If the subjects knew in advance how the feedback signal would be calculated, they tried to perform the task using either only the finger whose force was multiplied by 2 or two fingers when the for ce of the third one was multiplied by 0.5. Further into the trial, how ever, they were unable to track the ramp pattern using only one or two fingers and demonstrated a search activity that could continue until the end of the trial or lead eventually to a three-finger sharing patt ern similar to the template pattern used in cases of undistorted feedb ack. We conclude that the limited number of preferred sharing pattern within the studied task reflects an organization of the fingers into a structural unit (involving one, two, or all three fingers) by the cen tral nervous system. The availability of structural units defines the presence of stable solutions available for the system.