Using low levels of stochastic vestibular stimulation to improve locomotor stability

Abstract

Low levels of bipolar binaural white noise based imperceptible stochastic electrical stimulation to the vestibular system (stochastic vestibular stimulation, SVS) have been shown to improve stability during balance tasks in normal, healthy subjects by facilitating enhanced information transfer using stochastic resonance (SR) principles. We hypothesize that detection of time-critical sub-threshold sensory signals using low levels of bipolar binaural SVS based on SR principles will help improve stability of walking during support surface perturbations. In the current study 13 healthy subjects were exposed to short continuous support surface perturbations for 60 s while walking on a treadmill and simultaneously viewing perceptually matched linear optic flow. Low levels of bipolar binaural white noise based SVS were applied to the vestibular organs. Multiple trials of the treadmill locomotion test were performed with stimulation current levels varying in the range of 0-1500 μA, randomized across trials. The results show that subjects significantly improved their walking stability during support surface perturbations at stimulation levels with peak amplitude predominantly in the range of 100-500 μA consistent with the SR phenomenon. Additionally, objective perceptual motion thresholds were measured separately as estimates of internal noise while subjects sat on a chair with their eyes closed and received 1 Hz bipolar binaural sinusoidal electrical stimuli. The optimal improvement in walking stability was achieved on average with peak stimulation amplitudes of approximately 35% of perceptual motion threshold. This study shows the effectiveness of using low imperceptible levels of SVS to improve dynamic stability during walking on a laterally oscillating treadmill via the SR phenomenon.

Keywords: balance control; body motion threshold; perceptual motion threshold; stochastic electrical stimulation; stochastic resonance; vestibular stimulation.

FIGURE 1

An exemplar subject performing the walking task on a treadmill mounted on a six degree of freedom motion base that provided continuous, sinusoidal, lateral motion of the support surface. Subjects were simultaneously viewing perceptually matched linear optic flow while walking on the treadmill.

FIGURE 2

Ratio data of all the parameters [Tax, Tay, Taz, Trv, Tpv, Tyv, Gait cycle time variability (GCV)] during the stimulus period to that during the baseline period (plotted on the primary y-axis on the left) and the cost function value (plotted on the secondary y-axis on the right) at different peak stimulus amplitudes for a typical subject. Note that for this subject, locomotor balance performance improved at the peak stimulus amplitude of 500 μA.

FIGURE 3

The mean (± 1 SEM) values across all subjects (n = 13) for all the seven parameters of interest during the two periods (baseline and stimulus) of the control and optimal trials.

FIGURE 4

Individual values for the cost function value calculated as the sum of ratios for the stimulus period to the baseline period across the seven parameters for the control and optimal trials as well as the individual values for the parameters GCV, Tay, and Taz that showed significant interactions between the factors of Period and Trial (n = 13) during the two periods (baseline and stimulus) of the control and optimal trials. Please note that the subject in Figure 2 is subject#10 in this figure.