MANUAL DISCRIMINATION OF FORCE USING ACTIVE FINGER MOTION

In these experiments, two plates were grasped between the thumb and forefinger and squeezed together along a linear track. An electromechanical system presented a constant resistance force during the squeeze up to a predetermined location on the track, whereupon the force effectively went to infinity (simulating a wall) or to zero (simulating a cliff). The task of the subject was to discriminate between two alternative levels of the constant resistance force (a reference level and a reference-plus-increment level). Results of these experiments indicate a just noticeable difference of roughly 7% of the reference force using a one-interval paradigm with trial-by-trial feedback over the ranges 2.5 less-than-or-equal-to F(o) less-than-or-equal-to 10.0 newtons, 5 less-than-or-equal-to D less-than-or-equal-to 30 mm, 45 less-than-or-equal-to S less-than-or-equal-to 125 mm, and 25 less-than-or-equal-to V less-than-or-equal-to 160 mm/sec, where F(o) is the reference force, D is the distance squeezed, S is the initial finger-span, and V is the mean velocity of the squeeze. These results, based on tests with 5 subjects, are consistent with a wide range of previous results, some of which are associated with other body surfaces and muscle systems and many of which were obtained with different psychophysical methods.