Two experiments on the perception of slip at the fingertip

This paper describes the results of two experiments that investigate relative motion between a surface and the fingertip (slip) as part of a larger program of research on "fingertip haptics. " The primary intent of both experiments is to evaluate the importance of relative motion with respect to perceiving surface velocity. The perception of surface velocity is crucial to dexterous control and object recognition. The first experiment determines the just noticeable difference (JND) in the speed and direction of slip using the "method of adjustment" difference threshold technique over a range of 80 to 240 mm/sec on two different surface textures. 42 subjects participated the first experiment. We find slip speed perception to be highly dependent on surface texture, with Weber fractions between 0.04 and 0.25. Slip direction difference thresholds range between 3.6 and 11.7 degrees, also highly dependent on surface texture. The second experiment establishes the relative importance of proprioceptive feedback versus slip feedback in perceiving surface velocity. For both surface textures, we utilize an alternative forced choice experimental technique on 40 subjects. We find that slip feedback at the fingertip only subtly influences the perception of speed over proprioceptive feedback from the hand and arm for the two different base speeds studied (250 and 400 mm/sec).