Identifying visual-vestibular contributions during target-directed locomotion.

The purpose of this experiment was to examine the potential interaction between visual and vestibular inputs as participants walked towards 1 of 3 targets located on a barrier 5m away. Visual and vestibular inputs were perturbed with displacing prisms and galvanic vestibular stimulation (GVS), respectively. For each target there were three vision conditions (no prisms, prisms left, and prisms right), and three GVS conditions (no GVS, anode left, and anode right). Participants were instructed to start with eyes closed, and to open the eyes at heel contact of the first step. GVS and target illumination were triggered by the first heel contact. This ensured that the upcoming visual condition and target were unknown and that both sensory perturbations occurred simultaneously. Lateral displacement was determined every 40 cm. Irrespective of target or direction, GVS or prism perturbation alone resulted in similar lateral deviations. When combined, the GVS and prism perturbations that had similar singular effects led to significantly larger deviations in the direction of the perturbations. The deviations were approximately equal to the sum of the single deviations indicating that the combined effects were additive. Conflicting GVS and prism perturbations led to significantly smaller deviations that were close to zero, indicating that opposite perturbations cancelled each other. These results show that when both visual and vestibular information remain important during task performance, the nervous system integrates the inputs equally.