Feeling multiple edges: The tactile perception of short ultrasonic square reductions of the finger-surface friction

This study investigates human perception of tactile feedback using ultrasonic lubrication, in situation where feedback is provided using short frictional cues of varying duration and sharpness. In a first experiment, we asked participants to discriminate the transition time and duration of short square ultrasonic reductions of friction. They proved very sensitive to discriminate millisecond differences in these two parameters with the average psychophysical thresholds being 2.4 ms for discriminating duration and 2.06 ms for transition time. A second experiment focused on participant's perception of square friction reductions of variable transition time and duration and we found that for durations of the stimulation larger than 90 ms, participants often perceived 3 or 4 edges when only two stimulations were presented while they consistently felt 2 edges for signals shorter than 50 ms. These results confirm the sensitivity of touch to transient frictional cues on smooth surfaces and raises the question of how such cues are processed by the neural mechanisms mediating the perception of friction. Moreover, the knowledge of how potentially ambiguous frictional cues are resolved is central to the implementation of tactile patterns on friction-based displays with haptic feedback as well as to the definition of unambiguous core frictional blocks.