Is the use of vestibular information weighted differently across the initiation of walking?

The purpose of this experiment was to examine vestibular contributions at specific times during the initiation of walking in human subjects. Subjects began walking forward at the sound of an auditory tone, with vision present or occluded. Galvanic vestibular stimulation (GVS) was delivered with the anode electrode on the right or left side at either: (1) onset of the anticipatory postural adjustment (APA), (2) toe-off of the first swing limb (TO) or (3) heel contact of the first swing limb (HC). Ground reaction forces and kinematic data were collected. Upper body (roll angles from head, trunk and pelvis) and lower body (foot placement) data were analysed to determine whether the timing and magnitude of the response to GVS, and therefore the level of vestibular contribution, was modulated at different points during the initiation of gait. With vision present and occluded, the magnitude of the lower body response varied depending on the event in the gait cycle at which the stimulation was delivered. These novel results demonstrate evidence that vestibular weighting during gait initiation is dependent upon the specific gait initiation events. Upper body roll also exhibited magnitude differences between events. However, these changes are proposed to occur due to the transition from a stationary position into a dynamic state, prompting the increased weighting of vestibular information. With vision present no significant changes were seen in the segment roll response. The observations suggest a distinction in vestibular regulation of upper body roll versus foot placement for successful completion of the gait initiation task. Changes in upper body roll are influenced by the dynamic nature of the task, whereas foot placement changes are modulated based on the event during gait initiation at which GVS is delivered.