A novel method using EEG to characterize the cortical processes involved in active and passive touch

We present a novel method to compare brain responses to identical tactile stimuli in active and passive touch. Using electroencephalography (EEG) to record steady-state evoked brain potentials (SS-EPs), our goal was to characterize the cortical activity related to the tactile exploration of a textured surface. For this purpose, we used a novel tactile display, which is able to produce tactile texture experiences using ultrasonic stationary waves to transiently modulate tangential friction between the finger and the display. Because the change in friction depends on the amplitude of the ultrasonic vibrations, modulation of ultrasonic vibration amplitude was used to periodically modulate friction at a frequency of 11 Hz, producing a tactile percept resembling that of a square-wave grating. After recording the exact motion of the fingertip and interfacial forces used for each trial while one participant freely explored the surface (active touch condition), the same motion and normal force was reproduced by a high-precision robotic device with force feedback (passive touch condition). Frequency analysis of the recorded EEG signals showed that, for both active and passive touch conditions, the interactions between the fingertip and the plate elicited a robust SS-EP at 11 Hz, corresponding to the frequency of friction modulation, maximal over the parietal region contralateral to the stimulated finger. Our results suggest that the cortical activity related to active and passive touch can be characterized in humans by combining the recording of SS-EPs with an ultrasonic device generating a periodic tactile experience whose frequency is independent of finger movements.