Effects of galvanic vestibular stimulation during human walking

Abstract

1. To identify vestibular influences on human walking, galvanic vestibular stimulation was applied to normal adult subjects as they walked to a previously seen target. A transmastoidal step stimulus commenced as subjects started walking. With the eyes shut, the galvanic stimulus caused large turns towards the side with the anodal current. 2. Ability to perceive the trajectory of gait without visual cues was measured by guiding blindfolded subjects from one arbitrary point to another, either walking or seated in a wheelchair. On reaching a destination position and removing the blindfold, subjects pointed to indicate the starting position. Subjects made considerable errors in estimating the trajectory, but were equally accurate whether in the wheelchair or walking. 3. To determine the effects of vestibular stimulation on the perception of trajectory, the galvanic stimulus was applied to blindfolded subjects as they were guided from one point to another in the wheelchair. The vestibular stimulus produced an illusory shift in the trajectory travelled. This shift was towards the side with the cathode, i.e. in the opposite direction to the turn produced by the stimulus during walking. 4. We conclude that galvanic vestibular stimulation during walking causes subjects to turn from their planned trajectory. In part, this altered course may compensate for an altered perception of trajectory produced by the stimulus. However, altered perception of the vertical or the base of support, or direct vestibulo-fugal influences on the leg muscles could contribute to the changes in gait.

Figure 1. Experimental set-up

A, Experiment 1. Subjects stood and viewed a target 4 m ahead, and after shutting the eyes, walked straight ahead towards the target. A step current of different intensities and polarities was passed between the mastoid processes, commencing as the subject began to walk. Trajectory of gait was measured using a camera positioned above and behind the subject. B, Experiment 2. Subjects sat in a wheelchair, blindfolded and wearing earmuffs. The experimenter pushed them at walking pace from a start to an end position along a curved trajectory. As they started to move, a step current was passed between the mastoid processes. On reaching the end point, subjects removed the blindfold and indicated the perceived start point of the trajectory.

Figure 2. Walking trajectories

Results are shown for two different subjects (A and B). Subjects walked from the starting line attempting to reach the target circle. Without a vestibular stimulus, subjects walked straight to the target (shaded footsteps). With an anode-right stimulus, subjects initially veered to the right (open footsteps) and with an anode-left stimulus, they veered to the left (black footsteps). Both subjects turned during the first 4 steps. After that, subject A continued to walk away from the target whereas subject B turned back towards the target.

Figure 3. Turn during individual steps

Stride length and trajectory are shown for individual steps for the 1 mA galvanic stimulus (mean ±s.e.m., 25 cm grid). The distance and direction of each foot relative to the previous position and direction of that foot are plotted for steps 1–8. Subjects start with the right foot (step 1) and therefore it is the length of only half a stride. With no vestibular stimulus the steps are uniform (top panel). The vestibular stimulus does not affect the first step, but in the following 3 steps it changes the direction of the step towards the anodal side, with turn greater for the cadence of 52 steps min⁻¹ (bottom left panel) than for 104 steps min⁻¹ (bottom right panel). During the following steps (5 and 6) there is some turn back towards the target before straightening in the last steps. Asterisks indicate a significant turn compared with control steps (P < 0.05).

Figure 4. Perceptions of trajectories

Subjects were guided from a start position within the shaded region (S) to an end position (offset to the common point E). From position E, they indicated the filled points (P) as the perceived start positions. Subjects made considerable errors in direction and distance in estimating the start positions, but were equally accurate when they walked (A) and when they were pushed in the wheelchair (B). Galvanic vestibular stimulation shifted the perceived starts to the right (from the subject's view) with the anode right (C), and to the left with the anode left (D). The vestibular stimuli caused a greater variance in the estimate of the start directions.

Figure 5. Mean perception of trajectory

Trajectories are normalized to a common start (•) and end (○) position. The mean perceived start positions (± 2 s.e.m. of direction and distance) are indicated by the shaded regions for walking (W) and wheelchair (O) without a vestibular stimulus, and for the wheelchair for the stimulus with the anode right (+R) and the anode left (+L). In all situations, subjects underestimated the distance travelled. The vestibular stimuli caused significant deviations in the perception of trajectory. Note that the smaller s.e.m. for walking arises because of a greater number of trials (56 vs. 42) rather than a smaller variance.