Directional acuity of whole-body perturbations during standing balance

Abstract

Little is known about whether deficits in motion perception contribute to balance impairment among individuals with sensorimotor deficits such as Parkinson’s disease (PD) and stroke. Deficits in directional acuity during behaviorally relevant conditions may be an important and understudied aspect of many sensorimotor impairments. Most tests of whole-body directional perception have been performed during seated tasks. However, seated tasks do not actively engage motor systems, which can affect proprioceptive signals in muscles, and therefore lead participants to rely on only cutaneous, vestibular, and visual information. These factors contribute to our ability to perceive the direction and magnitude of an impending fall and to appropriately activate muscles to restore balance when standing. Our first objective was to measure sensory thresholds of directional acuity, in a young adult population, as a method to quantify kinaesthetic perception of the body’s motion and direction in space during perturbations to standing balance. Our second objective was to validate methods for quantifying directional acuity thresholds in response to perturbations during standing by comparing two methods, parameter estimation by sequential testing (PEST) and the psychometric method. To avoid use of visual and auditory cues, subjects were blindfolded and wore headphones with a white noise masking stimulus. In each trial, two ramp and hold perturbations of identical magnitude (7.5 cm, 15 m/s, and 0.1 m/s2) were applied, with 3 seconds between perturbations to allow recovery of balance. The first perturbation was in the backward direction, causing forward displacement of the center of mass. The second perturbation was deviated by 0-30° to the right or left of the initial perturbation. After each pair of perturbations, subjects pressed a button to indicate whether they perceived the pair of perturbations to be in the same or different direction. We found that the directional acuity threshold of whole-body perturbations for a young adult population was 8.9 + 2.2°. Sensory thresholds identified using PEST, an adaptive algorithm, required fewer trials (approximately 31 trials vs. 100 trials) than those estimated from full psychometric curves but converged to thresholds, approximately 7.8 + 3.1°, with greater variability within the population. The directional acuity thresholds, measured in this study, are the first known quantification of whole-body directional perception during standing. Future work is aimed at determining thresholds in clinical populations, such as PD and stroke, and in the lateral and diagonal directions and to compare perceptual and motor responses.