DIFFERENTIAL-EFFECTS OF LOAD STIFFNESS ON MATCHING PINCH FORCE, FINGER SPAN, AND EFFORT

Disparities in load stiffness were used to differentiate and character ize pinch-force, finger-span, and pinch-effort matching in two experim ents. All subjects squeezed a spring-loaded manipulandum in each hand using three-finger pinch. Subjects in the first experiment were instru cted explicitly to match one of the three continua. Subjects matching force or span were told to attend carefully to sensations from the han d or arm and to ignore differences in the effort required to make the sensations equal. They had to achieve and hold a particular target for ce with the reference hand and then match force or span with the oppos ite hand, usually against a spring with a different stiffness. These s ubjects were given as much time as necessary to make their matches and were told which hand was serving as the reference in each trial. Effo rt-matching subjects were told to ignore peripheral sensations and to match effort or motor commands. These subjects were not told which han d was the reference and were given only 1 s to make a match, so they m ade matches by rapidly squeezing both manipulanda simultaneously and, presumably, with the same voluntary motor command. The matching behavi ors of the three groups were clearly distinguishable and were consiste nt with instructions. Results were similar whether different subjects were assigned different instructions or the same subjects performed al l three match types. In a second experiment, naive subjects were given purposely ambiguous instructions without reference to a specific cont inuum and had no time or accuracy constraints. Subjects produced the s ame three sensorimotor behaviors obtained with explicit instructions, showing that the different behaviors were not artifacts of strict prot ocols. Taken together, the results show that force, span, and effort a re distinct sensorimotor continua that can be judged reliably.