Perception of distance-to-obstacle through time-delayed tactile feedback

In previous vision-to-touch sensory substitution approaches, including most `electronic white canes´, typical approaches include mapping space-to-space, space-to-intensity, or space-to-frequency. To our knowledge, however, mapping space to time-delay has not been considered. Yet, because organisms must anticipate impending collisions with obstacles or anticipate being contacted by approaching objects, many organisms have developed computational short-cuts where distance-to-target is assumed to be proportional to a time-span. This short-cut often manifests itself in low-level sensorimotor behaviours and perceptual mechanisms. We studied whether untrained humans would spontaneously employ such a short-cut to estimate distance-to-obstacle in the absence of vision. The observers pressed a push button and a tactile pulse was delivered to the hand with a delay proportional to the distance to an obstacle detected by an optical range finder that they wore. The observers were not informed of the nature of the coding but could freely probe the obstacle while walking toward the target. Upon randomized presentation of obstacle distances, the observers quickly calibrated their judgement of distance-to-obstacle and were able to estimate this distance within a range of four meters for a proportionality factor corresponding to a velocity of one m/s.