PERCEPTION AT THE ONSET OF TOUCH
The onset of tactile exploration induces rich force dynamics and there is increasing evidence that these dynamics contribute to the tactile perception of textures and materials. However, little is known about the peripheral and central neural codes mediating the perception of these transient cues. We aimed to investigating the afferents’responses to the onset of active exploration on flat materials.
In a forced-choice task, we asked human participants to discriminate by touch alone glass plates from transparent polymethyl methacrylate (PMMA) plastic plates. While the surfaces were flat and did not exhibit geometric features beyond a few tens of nanometres, the materials differed by their molecular structures. They produced similar coefficients of friction and thermal effects were controlled.
Most participants performed well above chance and participants with dry fingers discriminated the materials especially well. Current models of tactile surface perception appeal to surface topography and cannot explain our results. A correlation analysis between detailed measurements of the interfacial forces and discrimination performance suggested that the perceptual task depended on the transitory contact phase leading to full slip. This result demonstrates that differences in interfacial mechanics between the finger and a material can be sensed by touch and that the evanescent mechanics that take place before the onset of steady slip have perceptual value.