The present study examined perception of individual finger forces during multi-finger force production tasks. In an ipsilateral force matching paradigm, 12 healthy subjects were instructed to produce a reference force pre-determined at 30% MVC of involved fingers (varying from 1 to 4 fingers, visual feedback of total force, 5 s), and then to reproduce only the index or little finger portion of the total reference force (i.e., a portion of the sum, no visual feedback, 4 s) after a brief relaxation period (visual feedback of all finger forces, 3 s). The absolute force that individual fingers produced was approximately 30% of single-finger maximal force across different multi-finger reference force production tasks. During subsequent force matching, the index finger matching force was not significantly different from its own reference force, independent of the number of simultaneously activated fingers. The little finger, in contrast, produced significantly greater matching forces when three (middle, ring, and little) or four (index, middle, ring and little) fingers, but not two (ring and little) fingers, were simultaneously activated. The results suggest that index finger forces are more accurately estimated than little finger forces during multi-finger force production. The disparity in perception of individual finger forces is likely due to the ability of the central nervous system to partition and direct descending motor commands to the index finger.
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