Voluntarily controlled but not merely observed visual feedback affects postural sway

Postural sway during quiet standing reflects the interplay between destabilizing forces acting on the body and actions by the postural control system to prevent a loss of balance. Hence, balance impairments caused by altered sensory, motor, or central nervous function related to such factors as

Consistent motion simulation in Virtual Reality (VR) applications is challenging due to constraints on tracking technologies and locomotion or interaction paradigms that scale motion. In this study, we measured (a) the point of subjective stationarity (PSS) during active self-motion, (b) postural sway in the dark, and (c) visually-induced postural sway during quiet stance, both before and after adapting to different gains between physical self-motion and the motion portrayed in the virtual environment. Participants adapted to each of three adaptation gain levels in separate blocks: normal, reduced, and increased, in which observers' physical motion was scaled and displayed as the virtual motion. We measured PSS during active self-motion and postural sway during quiet stance in both left-right and front-back directions in separate sessions. We found that the PSS measured during self-motion did not vary with adaptation gain. However, postural sway elicited by visual perturbation was modulated after adapting to non-unity gains. We also measured baseline postural sway prior to adaptation and found that exposure to virtual motion under unity (normal) gain increased the postural variability along the left-right direction, when tested without visual feedback (in dark). Collectively these results suggest that while observers adapt to gain, active self-motion provides sufficient somatosensory feedback to counteract the shift in perceived motion. As a result, PSS remains consistent across all gain manipulations in our experiment setup. In contrast, postural responses during quiet stance did not recalibrate immediately after motion gain perturbation was removed suggesting that they operate independently of perceptual mechanisms.

This paper describes development of an activity based, multimodal balance rehabilitation training device. Various sensors can be used, including a force plate, inertial sensors, and depth sensing cameras, and various combinations of visual, auditory and tactile feedback can be configured depending on the rehabilitation task and activity. Tactile feedback is presented via a lightweight belt that is worn on the torso. Generally, visual feedback is only needed at the start of rehabilitation training (task orientation) while tactile feedback may be used to augment balance control. Tactile feedback can be configured as a cue that certain movement targets or limits have been reached or as an immediate indicator of the variance in postural sway. Tactile feedback allows the subject to naturally concentrate on the functional rehabilitation task, and is less reliant on visual or verbal cues.

The present study assessed the influence of visual feedback on stance stability and soleus H-reflex excitability. The centre of pressure (COP) displacement was measured in upright stance on a rigid surface (stable surface) and on a spinning top (unstable surface) while subjects either received "normal" visual feedback (without laser pointer = WLP) or pointed with a laser pointer on a target on the wall (LP). In order to verify that laser pointing influenced visual feedback, two additional experiments were conducted: (1) Subjects performed a finger reaction task which was thought to increase attention and cognitive demands without alteration of the visual feedback. (2) The effect of laser pointing on the wall was compared with pointing at a board, which was attached to the subjects themselves. In this case, the laser point could not serve as a reference for sway because the board moved in synchrony with the body. On stable and unstable surface, COP displacement was reduced in the LP compared to the WLP task (-17 cm +/- 6, P < 0.05; -14 cm +/- 6, P < 0.05). Conversely, H-reflexes were greater in the LP condition (stable: +20 microV +/- 30, not significant; unstable +115 microV +/- 40, P < 0.05). Stance stability and H-reflex modulation were negatively correlated (R(2) = -0.5; P < 0.001). The finger reaction task did neither influence COP displacement nor H-reflexes. Pointing at the body-fixed target did not alter COP displacement. These findings suggest that postural sway can be reduced by a handheld laser pointer targeting on an external reference point. It is argued that altered visual input was responsible for modulating the H-reflex.

We exposed standing men and women to motion relative to the illuminated environment in a moving room. During room motion, we measured the kinematics of standing body sway. Participants were instructed to discontinue immediately if they experienced any symptoms of motion sickness, however mild. For this reason, our analysis of body sway included only movement before the onset of motion sickness. We analyzed the spatial magnitude of postural sway in terms of the positional variability and mean velocity of the center of pressure. We analyzed the multifractality of postural sway in terms of the width of the multifractal spectrum and the degree of multiplicativity of center of pressure positions. Results revealed that postural sway differed between participants who later reported motion sickness and those who did not, replicating previous effects. In a novel effect, postural responses to motion of the illuminated environment differed between women and men. In addition, we identified statistically significant interactions that involved both Sex and motion sickness status. Effects were observed separately in the spatial magnitude and multifractality of sway. The results were consistent with the postural instability theory of motion sickness (Riccio and Stoffregen in Ecol Psychol 3:195-240, 1991) and suggest that Sex differences in motion sickness may be related to Sex differences in the control and stabilization of bodily activity.

We routinely have physical interactions with others, whether it be handing someone a glass of water or jointly moving a heavy object together. These sensorimotor interactions between humans typically rely on visual feedback and haptic feedback. Recent single-participant studies have highlighted that the unique noise and time delays of each sense must be considered to estimate the state, such as the position and velocity, of one's own movement. However, we know little about how visual feedback and haptic feedback are used to estimate the state of another person. Here, we tested how humans utilize visual feedback and haptic feedback to estimate the state of their partner during a collaborative sensorimotor task. Across two experiments, we show that visual feedback dominated haptic feedback during collaboration. Specifically, we found that visual feedback led to comparatively lower task-relevant movement variability, smoother collaborative movements, and faster trial completion times. We also developed an optimal feedback controller that considered the noise and time delays of both visual feedback and haptic feedback to estimate the state of a partner. This model was able to capture both lower task-relevant movement variability and smoother collaborative movements. Taken together, our empirical and modeling results support the idea that visual accuracy is more important than haptic speed to perform state estimation of a partner during collaboration.NEW & NOTEWORTHY Physical collaboration between two or more individuals involves both visual and haptic feedback. Here, we investigated how visual and haptic feedback is used to estimate the movements of a partner during a collaboration task. Our experimental and computational modeling results parsimoniously support the notion that greater visual accuracy is more important than faster yet noisier haptic feedback when estimating the state of a partner.

The purpose of this study was to determine the effect of visual feedback and verbal encouragement on concentric peak torque of the quadriceps and hamstrings muscles of males and females at an isokinetic velocity of 60 deg/s. All testing was performed on the Biodex B-2000 isokinetic dynamometer in three sessions, separated by 7 to 14 days. Fifteen males and 15 females (age = 25.4 ± 2.4 yrs, wt = 76.6 ± 16.5 kg, ht = 173.61 ± 9.5 cm) were tested under the following conditions: (a) visual feedback, (b) verbal encouragement, (c) combined visual feedback and verbal encouragement, and (d) no feedback (control). Testing order was counterbalanced to control for possible learning effects. Two separate 2 x 4 repeated measure analyses of variance (ANOVA) were used to examine the difference among test conditions. Examination of quadriceps force production revealed that subjects generated greater peak torque when visual feedback was provided than when verbal encouragement or no feedback were provided. Similarly, quadriceps force production was greater when combined visual feedback and verbal encouragement was provided than when verbal encouragement or no feedback were provided (p < 0.05). Examination of hamstrings force production revealed that subjects generated greater peak torque when combined visual feedback and verbal encouragement was provided than when verbal encouragement, and no feedback were provided. Additionally, hamstrings force production was greater when visual feedback was provided than when no feedback was provided (p < 0.05). These results indicate that the use of visual and combined visual and verbal feedback increased quadriceps and hamstrings force production when compared to the control condition where no feedback was provided.

Variability is an inherent and important feature of human movement. This variability has form exhibiting a chaotic structure. Visual feedback training using regular predictive visual target motions does not take into account this essential characteristic of the human movement, and may result in task specific learning and loss of visuo-motor adaptability. In this study, we asked how well healthy young adults can track visual target cues of varying degree of complexity during whole-body swaying in the Anterior-Posterior (AP) and Medio-Lateral (ML) direction. Participants were asked to track three visual target motions: a complex (Lorenz attractor), a noise (brown) and a periodic (sine) moving target while receiving online visual feedback about their performance. Postural sway, gaze and target motion were synchronously recorded and the degree of force-target and gaze-target coupling was quantified using spectral coherence and Cross-Approximate entropy. Analysis revealed that both force-target and gaze-target coupling was sensitive to the complexity of the visual stimuli motions. Postural sway showed a higher degree of coherence with the Lorenz attractor than the brown noise or sinusoidal stimulus motion. Similarly, gaze was more synchronous with the Lorenz attractor than the brown noise and sinusoidal stimulus motion. These results were similar regardless of whether tracking was performed in the AP or ML direction. Based on the theoretical model of optimal movement variability tracking of a complex signal may provide a better stimulus to improve visuo-motor adaptation and learning in postural control.

Maintenance of equilibrium is influenced by postural symmetry and deviations such as a forward head. An exercise program involving an effect such as self-joint mobilization was developed to address spinal curvature; however, its effect on postural stability is still unclear. The purpose of this study was to address to what extent the self-mobilization exercise program would influence postural sway while standing still. The Exercise group (n = 16) performed the self-mobilization while lying supine on a cylinder-shaped tube (98 cm length, 15 cm diameter), consisting of three warm-up positionings followed by seven motions. The Control group (n = 16) laid supine on a flat surface with their legs flexed. Before and immediately after respective interventions, the plantar center of pressure was quantified while subjects were standing with eyes open and closed. The results show the exercise group reduced the postural sway in the antero-posterior direction only during the eyes-closed condition. Neither exercise nor control groups showed a significant change in postural sway during the eyes-open condition. Our findings indicate the self-mobilization exercise program improved the participant’s stability when visual feedback was not reliable for postural control. The improved postural control in the antero-posterior direction may be accounted for by facilitated activation of planterflexors and dorsiflexors which are primary muscles in ankle strategy for postural control. Effects of this exercise program on postural stability appear to be direction dependent, which provide insight when healthcare professionals incorporate exercise programs for postural symmetry.

Tactile feedback is pivotal for grasping and manipulation in humans. Providing functionally effective sensory feedback to prostheses users is an open challenge. Past paradigms were mostly based on vibro- or electrotactile stimulations. However, the tactile sensitivity on the targeted body parts (usually the forearm) is greatly less than that of the hand/fingertips, restricting the amount of information that can be provided through this channel. Visual feedback is the most investigated technique in motor learning studies, where it showed positive effects in learning both simple and complex tasks; however, it was not exploited in prosthetics due to technological limitations. Here, we investigated if visual information provided in the form of augmented reality (AR) feedback can be integrated by able-bodied participants in their sensorimotor control of a pick-and-lift task while controlling a robotic hand. For this purpose, we provided visual continuous feedback related to grip force and hand closure to the participants. Each variable was mapped to the length of one of the two ellipse axes visualized on the screen of wearable single-eye display AR glasses. We observed changes in behavior when subtle (i.e., not announced to the participants) manipulation of the AR feedback was introduced, which indicated that the participants integrated the artificial feedback within the sensorimotor control of the task. These results demonstrate that it is possible to deliver effective information through AR feedback in a compact and wearable fashion. This feedback modality may be exploited for delivering sensory feedback to amputees in a clinical scenario.

We examined aging-related differences in the contribution of visuomotor correction to force fluctuations during index finger abduction via the analysis of two datasets from similar subjects. Study (1) Young (N = 27, 23 ± 8 years) and older adults (N = 14, 72 ± 9 years) underwent assessment of maximum voluntary contraction force (MVC) and force steadiness during constant-force (CF) index finger abduction (2.5, 30, 65% MVC). For each trial, visual feedback of the force (VIS) was provided for 8–10 s and removed for 8–10 s (NOVIS). Visual gain of the force feedback at 2.5% MVC was high; 12- and 26-fold greater than the 30 and 65% MVC targets. Mean force, standard deviation (SD) of force, and coefficient of variation (CV) of force was calculated for detrended (<0.5 Hz drift removed) VIS and NOVIS data segments. Study (2) A similar group of 14 older adults performed discrete, randomly-ordered VIS or NOVIS trials at low target forces (1–3% MVC) and high visual gain. Study (1) For young adults the CV of force was similar between VIS and NOVIS for the 2.5% (4.8 vs. 4.3%), 30% (3.2 vs. 3.2%) and 65% (3.5 vs. 4.2%) target forces. In contrast, for older adults the CV of force was greater for VIS than NOVIS for 2.5% MVC (6.6 vs. 4.2%, p < 0.001), but not for the 30% (2.4 vs. 2.4%) and 65% (3.1 vs. 3.3%) target forces. At 2.5% MVC, the increase in CV of force for VIS compared with NOVIS was significantly greater (age × visual condition p = 0.008) for older than young adults. Study (2) Similarly, for older adults performing discrete, randomly ordered trials the CV of force was greater for VIS than NOVIS (6.04 vs. 3.81%, p = 0.01). When visual force feedback was a dominant source of information at low forces, normalized force variability was ~58% greater for older adults, but only 11% greater for young adults. The significant effect of visual feedback for older adults was not dependent on the order of presentation of visual conditions. The results indicate that impaired processing of visuomotor information underlies the greater motor variability observed in older adults during lab-based isometric contractions of a hand muscle.

Individuals with an incomplete spinal cord injury (iSCI) are highly susceptible to falls during walking or standing. Our objective was to evaluate a therapeutic tool for standing balance that combined functional electrical stimulation, applied bilaterally to the plantarflexors and dorsiflexors, with visual feedback balance training (FES+VFBT). Five adults with iSCI completed 12 FES+VFBT sessions over 4 weeks. During the training sessions, participants completed each of the four balance exercises twice. Visual feedback of the center-of-pressure (COP) location was provided as participants completed the balance exercises and received FES to assist with performance of the exercises. A closed-loop FES system was used in which the COP was continually monitored and the level of electrical current administered was automatically adjusted. Balance abilities were assessed pre- and post- training using clinical balance scales (i.e., Berg Balance Scale, Mini-Balance Evaluation Systems Test, and Activities-specific Balance Confidence Scale) and biomechanical assessments (i.e., postural sway measures and limits of stability test during standing). User acceptability was explored through semi-structured interviews. Improvements were seen for four of the five participants on at least one of the clinical scales following completion of the training intervention. All participants showed greater maximal COP excursion area during the limits of stability test after the training intervention, whereas only one participant demonstrated a reduction in postural sway. Specific components of FES+VFBT, including the ability to safely practice challenging balance exercises, were deemed important by the participants. These results suggest that FES+VFBT has potential as an intervention for standing balance after iSCI.

This study aimed to determine the validity and reliability of a new device called MOTI for measuring balance by comparing its performance that with of the gold-standard force platform. The study involved collecting data from both devices in dual- and single-leg standing positions with eyes open and closed and using statistical measures to compare their performance. The results showed that MOTI can accurately measure balance during dual-leg standing tasks but has poor to moderate performance during single-leg standing tasks. However, it could detect small changes in postural sway caused by a reduced base of support and/or visual feedback. The study also found that the test-retest reliability was poor to moderate for both devices. These findings suggest that MOTI has potential as a reliable tool formeasuring balance during certain tasks, but further research is needed to improve its performance during single-leg standing. This study provides valuable insights into the validity and reliability of MOTI for measuring balance and highlights the need for further investigation.

Background Type 2 Diabetes (T2D) is associated with a higher magnitude of static postural sway. This investigation compared three statistical methods to explore the role of sensory modalities contributions to posture in T2D. Research design and methods Two groups were evaluated in this study ( n = 20); T2D group - 10 participants with T2D (age 54.6 ± 11.09 years), comparison group - 10 age/sex matched healthy participants (age 53.18 ± 9.89 years). Postural sway data was collected using the modified Clinical Test of Sensory Integration in Balance (mCTSIB), consisting of four 20-s trials on a balance plate with manipulations of vision and support surface to target the contributions of proprioceptive, visual, and vestibular senses. Scores were assessed by group wise analysis of path length, group wise analysis of percentile rank, and distribution of percentile rank. Results The two-way ANOVA used for the group wise analysis of path length and percentile rank showed significant differences between groups scores ( p &lt; .05), but no significant interactions between group and condition. The frequency distribution of percentile rank of the T2D group revealed unimodal distributions for all conditions except for vestibular, which was found to have the highest and lowest percentile ranks of any condition. Conclusion The results show that the individualized normative analysis revealed aspects of individual impairments that would have otherwise been missed using a group-wise method. Though limited, our findings also suggest that impairments to the vestibular system may be more pronounced but less frequent compared to proprioceptive and visual impairments.

It is widely assumed that healthy aging includes a decline in the stability of standing body sway. Certainly, the spatial magnitude of postural sway increases with age. However, the interpretation of this effect as a decline in the ability to stabilize posture rests, in part, on assumptions about the nature and definition of stability in stance. In this article, I review data on the control of standing posture in healthy older adults. I focus on a growing list of studies that demonstrate the retention, among healthy older adults, of the ability functionally to modulate postural sway in support of “suprapostural” activities. I address laboratory research, but also field studies carried out in a setting that dramatically challenges the control of stance: life on ships at sea. I argue that it may be possible, and certainly will be useful, to address directly the functional control of stance in older adults.