The control of upper body (UB) orientation relative to the pelvis in the frontal plane was characterized by analyzing responses to external perturbations consisting of continuous pelvis tilts (eyes open [EO] and eyes closed [EC]) and visual surround tilts (EO) at various amplitudes. Lateral sway of the lower body was prevented on all tests. UB sway was analyzed by calculating impulse-response functions (IRFs) and frequency-response functions (FRFs) from 0.023 to 10.3 Hz for pelvis tilt tests and FRFs from 0.041 to 1.5 Hz for visual tests. For pelvis tilt tests, differences between FRFs were limited to frequencies<3 Hz and were dependent on stimulus amplitude. IRFs were nearly identical across all pelvis tilt tests for the first 0.2 s, but showed amplitude-dependent changes in their time course at longer time lags. The availability of visual orientation cues (EO vs. EC) had only a small effect on the UB sway during pelvis tilt tests. This small effect of vision was consistent with the small UB sway evoked on visual tilt tests. Experimental results were interpreted using a feedback model of UB orientation control that included time-delayed sensory integration, short-latency reflexive mechanisms, and intrinsic biomechanical properties of the UB. Variation in model parameters indicated that subjects shifted toward reliance on vestibular information and away from proprioceptive information as pelvis tilt amplitudes increased. For visual tilt stimuli, model parameters indicated that subjects shifted toward reliance on vestibular and proprioceptive information and away from visual information as the stimulus amplitude increased.

We investigated the influence of feedback conditions on the effectiveness of a balance prosthesis. The balance prosthesis used an array of 12 tactile vibrators (tactors) placed on the anterior and posterior surfaces of the torso to provide body orientation feedback related to several different combinations of angular position and velocity of body sway in the sagittal plane. Control tests were performed with no tactor activation. Body sway was evoked in subjects with normal sensory function by rotating the support surface upon which subjects stood with eyes closed. Body sway was analyzed by computing root-mean-square sway measures and by a frequency-response function analysis that characterized the amplitude (gain) and timing (phase) of body sway over a frequency range of 0.017 to 2.2 Hz. Root-mean-square sway measures showed a reduction of surface stimulus evoked body sway for most vibrotactile feedback settings compared to control conditions. However, frequencyresponse function analysis showed that the sway reduction was due primarily to a reduction in sway below about 0.5 Hz, whereas there was actually an enhancement of sway above 0.6 Hz. Finally, we created a postural model that accounted for the experimental results and gave insight into how vibrotactile information was incorporated into the postural control system.

To quantify the contribution of sensory information to multisegmental frontal plane balance control in humans, we developed a feedback control model to account for experimental data. Subjects stood with feet close together on a surface that rotated according to a pseudorandom waveform at three different amplitudes. Experimental frequency-response functions and impulse-response functions were measured to characterize lower body (LB) and upper body (UB) motion evoked during surface rotations while subjects stood with eyes open or closed. The model assumed that corrective torques in LB and UB segments were generated with no time delay from intrinsic musculoskeletal mechanisms and with time delay from sensory feedback mechanisms. It was found that subjects' LB control was primarily based on sensory feedback. Changes in the LB control mechanisms across stimulus amplitude were consistent with the hypothesis that sensory reweighting contributed to amplitude-dependent changes in balance responses whereby subjects decreased reliance on proprioceptive cues that oriented the LB toward the surface and increased reliance on vestibular/visual cues that oriented the LB upright toward earth vertical as stimulus amplitude increased in both eyes open and closed conditions. Sensory reweighting in the LB control system also accounted for most of the amplitude-dependent changes observed in UB responses. In contrast to the LB system, sensory reweighting was not a dominant mechanism of UB control, and UB control was more influenced by intrinsic musculoskeletal mechanisms. The proposed model refines our understanding of sensorimotor integration during balance control by including multisegmental motion and explaining how intersegmental interactions influence frontal plane balance responses.

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.