Human Tactile Discrimination of Pulse Shape Is Possible Without Pre-Adaptation.

Recent progress in the field of tactile coding suggests that discrimination of texture may involve the instantaneous kinematic analysis of frictional stick-slip movements. The idea has received support from analytical psychophysical experiments employing changes of pulsatile skin deflections in a well-adapted state - i.e. when changing repetitive pulsatile stimulation from one pulse shape to another. Here we investigated whether perception of pulse shapes is possible with single, isolated pulses. In other words, whether the well-described neuronal 'onset-responses' in central parts of the tactile pathway, which differ largely from 'adapted responses', carry information about pulse shape. We tested human participants on a pulse shape 2AFC discrimination paradigm: in each trial, 3 pulsatile skin indentations were presented at an interval of 0.5s. Either the first or the last pulse deviated in its shape, which the participants had to correctly identify. The task could not be solved by simply integrating tactile response across the 3 pulses. A majority of the participants (18 out of 30) yielded a performance of p(correct)>0.75, indicating that isolated pulse shapes reach perception. The performance was enhanced by presenting the same shape changes in the context of a preadapting series of pulses. Participants confronted with the 3-pulse test did not show a systematic preference for the kinematic parameter used to change the shape. We conclude that perceptual processes in principle have access to the kinematic shape of isolated pulsatile skin deflections. However, sensory adaption plays a crucial role for the quality and specificity of encoding kinematic pulse profiles.