Biofeedback improves postural control recovery from multi-axis discrete perturbations

Abstract

Background: Multi-axis vibrotactile feedback has been shown to significantly reduce the root-mean-square (RMS) sway, elliptical fits to sway trajectory area, and the time spent outside of the no feedback zone in individuals with vestibular deficits during continuous multidirectional support surface perturbations. The purpose of this study was to examine the effect of multidirectional vibrotactile biofeedback on postural stability during discrete multidirectional support surface perturbations.

Methods: The vibrotactile biofeedback device mapped tilt estimates onto the torso using a 3-row by 16-column tactor array. The number of columns displayed was varied to determine the effect of spatial resolution upon subject response. Torso kinematics and center of pressure data were measured in six subjects with vestibular deficits. Transient and steady state postural responses with and without feedback were characterized in response to eight perturbation directions. Four feedback conditions in addition to the tactors off (no feedback) configuration were evaluated. Postural response data captured by both a force plate and an inertial measurement unit worn on the torso were partitioned into three distinct phases: ballistic, recovery, and steady state.

Results: The results suggest that feedback has minimal effects during the ballistic phase (body's outbound trajectory in response to the perturbation), and the greatest effects during the recovery (return toward baseline) and steady state (post-recovery) phases. Specifically, feedback significantly decreases the time required for the body tilt to return to baseline values and significantly increases the velocity of the body's return to baseline values. Furthermore, feedback significantly decreases root mean square roll and pitch sway and significantly increases the amount of time spent in the no feedback zone. All four feedback conditions produced comparable performance improvements. Incidences of delayed and uncontrolled responses were significantly reduced with feedback while erroneous (sham) feedback resulted in poorer performance when compared with the no feedback condition.

Conclusions: The results show that among the displays evaluated in this study, no one tactor column configuration was optimal for standing tasks involving discrete surface perturbations. Feedback produced larger effects on body tilt versus center of pressure parameters. Furthermore, the subjects' performance worsened when erroneous feedback was provided, suggesting that vibrotactile stimulation applied to the torso is actively processed and acted upon rather than being responsible for simply triggering a stiffening response.

Figure 1

Multidirectional vibrotactile feedback system.

Figure 2

Vibrotactile display and perturbation directions. Arrows show the direction of platform motion for the eight discrete perturbations. Subjects were presented vibrotactile feedback using four columns (circles), eight columns (circles and diamonds) or sixteen columns (circles, diamonds, and stars) of tactors.

Figure 3

Illustration of experimental set-up.

Figure 4

Sample data from a 225° perturbation. (a) Magnitude of R COP and (b) Phi are shown as functions of time, indicating peak values (R_max, Phi_max), peak times (T_cop, T_phi), recovery time (T_rec), and time to peak recovery velocity (T_vel). DZ indicates the degree of tilt within which no tactors are activated. The three stages of the trajectory are marked as I, II, and III for panels (a) and (b). (c) X and Y components of COP are shown in a bird’s-eye view with the peak time (black filled circle) and angle (dashed line), as well as the platform motion (blue arrow). (d) A/P and M/L components of tilt are shown in a bird’s-eye view with the peak time (red circle) and angle (dashed line), T_vel (yellow filled circle), and T_rec (green filled circle).

Figure 5

Variation of parameters (see text for detailed description) across display configurations. Error bars indicate standard error of the mean. The four configurations with the display on (4, 4-I, 8, 16) show few significant differences amongst themselves, but are each significantly different from the display OFF case for T_phi, T_vel, T_rec, V_max, pct0, and RMS phi.

Figure 6

Peak COP and tilt excursions for non-cardinal perturbation directions. Mean values are shown for each subject with the display off (open symbol) and on (filled symbol). Green lines indicate 95% confidence intervals of the mean angles (A_cop and A_phi) for each direction and black lines mark the directions of platform motion.