Characterization of electrotactile stimulation intensity to exploit the funneling illusion

Exploiting the funneling illusion is a promising approach to increase the spatial resolution of tactile sensory feedback, by eliciting a phantom sensation located midway two stimuli on the human skin. To date, this illusion has been tested with vibrotactile stimulation only, although it could be used also with the well-established alternative of electrotactile stimulation. To ensure a seamless feedback delivery combining electrotactile stimulation and funneling illusion, a characterization of the key stimulation parameters is required. Our work aims to characterize the variation of the perceived intensity generated by two electrodes simultaneously activated, depending on the variation of their individual intensities. We asked 6 healthy volunteers to complete a two-alternative forced choice task in three conditions (75%, 50% and 25%) corresponding to different phantom locations. We found that when a two-pad stimulation is delivered, the cumulative intensity of the two pads should be increased to elicit a phantom sensation with the same intensity as a single pad. Specifically, this increment depends on the phantom location, with a peak at the midpoint (i.e., a cumulative activation of 110% and 113,27% for phantom sensations close to the real pads, and 124.6% for the middle point). These findings represent a first step toward the characterization of the funneling illusion with electrotactile stimulation and could be exploited to enhance haptic feedback in fields such as human movement augmentation, prostheses and teleoperation.