Looking for physical invariants in the mechanical response of a tactually scanned Braille dot

One human finger explored plastic Braille dots using a variety of velocity and force profiles. The fingertip friction forces were measured. Characteristics of the interaction were studied to explore the manifestation of the amplitude/duration interdependence of signals across velocity, normal force and dot height. Both amplitude, defined here as maximum tangential force, and duration, were seen to vary with velocity and normal force, however the integral of the tangential force over time was found to not have a strong dependence on either variable. When three consecutive dots of varying height were examined, the tangential force integral was not constant, but increased in proportion to height. We propose that the nervous system may use the tangential force integral as an invariant to recognise the same spatial asperity explored under different velocity and force conditions.