Potential Mechanisms of Sensory Augmentation Systems on Human Balance Control

Abstract

Numerous studies have demonstrated the real-time use of visual, vibrotactile, auditory, and multimodal sensory augmentation technologies for reducing postural sway during static tasks and improving balance during dynamic tasks. The mechanism by which sensory augmentation information is processed and used by the CNS is not well understood. The dominant hypothesis, which has not been supported by rigorous experimental evidence, posits that observed reductions in postural sway are due to sensory reweighting: feedback of body motion provides the CNS with a correlate to the inputs from its intact sensory channels (e.g., vision, proprioception), so individuals receiving sensory augmentation learn to increasingly depend on these intact systems. Other possible mechanisms for observed postural sway reductions include: cognition (processing of sensory augmentation information is solely cognitive with no selective adjustment of sensory weights by the CNS), "sixth" sense (CNS interprets sensory augmentation information as a new and distinct sensory channel), context-specific adaptation (new sensorimotor program is developed through repeated interaction with the device and accessible only when the device is used), and combined volitional and non-volitional responses. This critical review summarizes the reported sensory augmentation findings spanning postural control models, clinical rehabilitation, laboratory-based real-time usage, and neuroimaging to critically evaluate each of the aforementioned mechanistic theories. Cognition and sensory re-weighting are identified as two mechanisms supported by the existing literature.

Keywords: balance; balance prosthesis; biofeedback; sensory augmentation; sensory reweighting; sensory substitution.

Figure 1

Block diagram representation of a simple feedback control model of balance showing potential modes of action by which measures of body sway could be used to improve balance control via sensory augmentation effects on different subsystems or by direct activation (e.g., functional electrical stimulation). Natural sensory integration is represented by a weighted combination of proprioceptive (W_prop), visual (W_vis), vestibular (W_vest), and auditory (W_aud) orientation information. Corrective torque generation is represented by a “neural controller” with stiffness (K_p), damping (K_d), and time delay (τ) parameters. Corrective torque is applied at ankle joint level to an inverted pendulum representation of the body with moment of inertia (J), mass (m), center of mass height (h). “s” is the Laplace variable and “g” is acceleration due to gravity.

Figure 2

Left: Illustration of trunk-based IMU and vibrating actuators. Center: Bird's eye view of illustrative trunk sway data from a subject with cerebellar ataxia; top panel—real-time use without cues, bottom—real-time use with cues. Right: Pre-/per-/post-training computerized Dynamic Posturography SOT scores for two groups of older adults that performed balance training exercises 3x/week for 8 weeks in their homes either with or without (Control group) vibrotactile sensory augmentation (23).