Guidance of visual direction by topographical vibrotactile cues on the torso.

Vibration on localised areas of skin can be used to signal spatial orientation, multi-directional motion and also to guide arm and hand movements. This study investigated the possibility that vibration at loci on the skin might also be used to cue gaze direction. Eight subjects made eye or (head + eye) gaze saccades in the dark cued by vibration stimulation at discrete loci spaced on a horizontal contour across the chest. Saccade and gaze amplitudes, latencies, and directions were analysed. In the first experiment, performed without training, subjects could only use vibration cues to direct their gaze in cardinal directions and gross quadrature. There was a high variability in the relationship between locus on the trunk and gaze direction in space, both within and between subjects. Saccade latencies ranged from 377 to 433 ms and were related to the loci of vibration; the further from the body midline the quicker the response. Since the association of skin loci with gaze direction did not appear intuitive a sub-group of four subjects were retested after intensive training with feedback until they attained criterion on midline identical with 0 degrees and 15 cm (to right/left of midline) identical with 45 degrees gaze shifts right and left. Training gave a moderate improvement in directional specificity of gaze to a particular locus on the skin. Gaze direction was linearly rescaled with respect to skin loci but variability and saccade latencies remained high. The uncertainty in the relationship between vibration locus and gaze direction and the prolonged latencies of responses indicate circuitous neuronal processing. There appears to be no pre-existing stimulus-response compatibility mapping between loci on the skin and gaze direction. Vibrotactile cues on the skin of the trunk only serve a gross indication of visual direction in space.