The role of vestibular and somatosensory systems in intersegmental control of upright stance

Abstract

Upright stance was perturbed using sinusoidal platform rotations to see how vestibular and somatosensory information are used to control segment and intersegmental dynamics in subjects with bilateral vestibular loss (BVL) and healthy controls (C). Subjects stood with eyes closed on a rotating platform (+/-1.2 degrees) for frequencies ranging from 0.01-0.4 Hz in the presence and absence of light fingertip touch. Trunk movement relative to the platform of BVLs was higher than Cs at higher platform frequencies whereas leg movement relative to the platform was similar for both groups. With the addition of light touch, both groups showed similar trunk and leg segment movement relative to the platform. Trunk-leg coordination was in-phase for frequencies below 1 Hz and anti-phase above 1 Hz. Interestingly, BVLs showed evidence of a "legs-leading-trunk" relationship in the shift from in-phase to anti-phase around 1 Hz. Controls showed no preference for either segment to lead the coordinative shift from in- to anti-phase. The results suggest that the balance instability of BVL subjects stems from high variability of the trunk, rather than the legs. The high trunk variability may emerge from the "legs-leading" intersegmental relationship upon which BVLs rely. Because BVLs derive information about self-orientation primarily from the support surface when their eyes are closed, the legs initiate the shift to anti-phase trunk-leg coordination that is necessary for stable upright stance control. Higher trunk variability suggests that this strategy results in lower overall postural stability. Light touch substitutes for vestibular information, leading to lower trunk variability along with a trunk-leg phase shift similar to controls, without a preference for either segment to lead the shift. The results suggest that vestibulospinal control acts primarily to stabilize the trunk in space and to facilitate intersegmental dynamics.

Figures

Fig. 1

Sagittal view of a subject in a side-by-side stance on the force platform with their right index finger positioned above touch surface. Trunk and leg segment displacements were defined as the angular displacement with vertical. “o” designates placement of kinematic markers.

Fig. 2

Exemplar time series for a BVL subject (gray) and their age/gender matched control (black). A and C show trunk segment, leg segment, and platform trajectories for the no-touch condition for the 0.01 Hz (A) and 0.4 Hz (C) platform conditions while B and D show the light touch condition. The platform amplitude was ± 1.2 degrees in the A-P direction.

Fig. 3

Trunk and leg segment averages for gain (A&B), phase (C&D), and velocity variability (E&F) for BVL (gray) and control subjects (black). Error bars = standard error (all plots).

Fig. 4

Cophase (A), magnitude squared coherence (B), and complex coherence (C) between trunk and leg segments for BVL (gray) and control subjects (black). Shaded region in A corresponds to comparisons shown in C.