Force variability during surface contact with bare finger or rigid probe

The current study evaluated subjects' ability to control finger force during contact with a smooth plate. They were instructed to maintain a constant "light" or "medium" force when statically contacting the surface or when moving the end effector (finger or rigid probe) back and forth at 20 or 222 mm/s. Subjects pressed harder when using the probe (cf. finger) and during static contact (cf. both motion conditions). Conversely, the coefficient of variation for force (standard deviation/mean) was smallest for static conditions and when the probe is used. Force variability proved to be relatively high in the current experimental conditions, which are typically used during texture discrimination. Despite such variability, psychophysical research has shown that texture discrimination is in fact excellent. This finding markedly contrasts with grasping tasks, where both force control and outcome measures are excellent. We consider a number of implications of these results.