Field of view and base of support width influence postural responses to visual stimuli during quiet stance

The Untapped Potential of Virtual Reality in Rehabilitation of Balance and Gait in Neurological Disorders

Dynamic systems theory transformed our understanding of motor control by recognizing the continual interaction between the organism and the environment. Movement could no longer be visualized simply as a response to a pattern of stimuli or as a demonstration of prior intent; movement is context dependent and is continuously reshaped by the ongoing dynamics of the world around us. Virtual reality is one methodological variable that allows us to control and manipulate that environmental context. A large body of literature exists to support the impact of visual flow, visual conditions, and visual perception on the planning and execution of movement. In rehabilitative practice, however, this technology has been employed mostly as a tool for motivation and enjoyment of physical exercise. The opportunity to modulate motor behavior through the parameters of the virtual world is often ignored in practice. In this article we present the results of experiments from our laboratories and from others demonstrating that presenting particular characteristics of the virtual world through different sensory modalities will modify balance and locomotor behavior. We will discuss how movement in the virtual world opens a window into the motor planning processes and informs us about the relative weighting of visual and somatosensory signals. Finally, we discuss how these findings should influence future treatment design.

The Use of Frequency Analysis as a Complementary and Explanatory Element for Time Domain Analysis in Measurements of the Ability to Maintain Balance

Assessment of human balance is one of the most common diagnostic tests, both in medical applications and during sports training. Many new methods of measuring are introduced in these studies; however, the analysis of results is still carried out mainly based on the values determined in the time domain - the average COP speed or the ellipse field of the prediction. The aim of the current work is to present the possibilities for the practical application of frequency analyses in assessment of the ability to maintain body balance as a method supplementing standard analyses. As part of the study, measurements of the ability to maintain balance in sensory conflict conditions introduced in the form of an oscillating, three-dimensional, virtual scenery were carried out. 27 healthy volunteers (13 women and 14 men) took part in the study. The three-dimensional scenery, presented by means of the Oculus system, oscillated in the sagittal plane with frequencies equal to 0.7 Hz and 1.4 Hz. The frequency value during the measurement was constant or changed in the middle of the test. Measurements were conducted on the FDM Zebris platform. The results were analyzed using developed coefficients determined on the basis of the Short-time Fourier transform (STFT). The use of frequency-domain analyses confirmed that in the COP movement, one can observe a cyclical component corresponding to following the scenery, as well as the appearance of other cyclical components whose observation is important in terms of assessing the ability to maintain balance. It has been shown that the changes in the average COP speed that occur during the measurement can result from changes related to the movement of following the scenery as well as additional body movements indicating a greater or lesser loss of balance. It has been shown that there are differences in the COP movement provoked by the movement of the surrounding scenery, which depend on the parameters of the introduced disturbances - something that can only be observed in results obtained in the frequency domain. The conducted research shows that in measurements involving the ability to maintain one's balance conducted in sensory conflict conditions, standard time-domain analyses should be supplemented with other types of data analysis, e.g. frequency domain analyses.

Kinematic Validation of Postural Sway Measured by Biodex Biosway (Force Plate) and SWAY Balance (Accelerometer) Technology

Background

The Biodex Biosway® Balance System and SWAY Balance® Mobile smartphone application (SBMA) are portable instruments that assess balance function with force plate and accelerometer technology, respectively. The validity of these indirect clinical measures of postural sway merits investigation.

Purpose

The purpose of this study was to investigate the concurrent validity of standing postural sway measurements by using the portable Biosway and SBMA systems with kinematic measurements of the whole body Center of Mass (COM) derived from a gold-standard reference, a motion capture system.

Study Design

Cross-sectional; repeated measures.

Methods

Forty healthy young adults (21 female, 19 male) participated in this study. Participants performed 10 standing balance tasks that included combinations of standing on one or two legs, with eyes open or closed, on a firm surface or foam surface and voluntary rhythmic sway. Postural sway was measured simultaneously from SBMA, Biosway, and the motion capture system. The linear relationships between the measurements were analyzed.

Results

Significant correlations were found between Biosway and COM velocity for both progressively challenging single and double leg stances (τ_b = 0.3 to 0.5, p < 0.01 to <0.0001). SBMA scores and COM velocity were significantly correlated only for single leg stances (τ_b = −0.5 to −0.6, p < 0.0001). SBMA scores had near-maximal values with zero to near-zero variance in double leg stances, indicating a ceiling effect.

Conclusion

The force plate-based Biodex Biosway is valid for assessing standing postural sway for a wide range of test conditions and challenges to standing balance, whereas an accelerometer-based SWAY Balance smartphone application is valid for assessing postural sway in progressively challenging single leg stance but is not sensitive enough to detect lower-magnitude postural sway changes in progressively challenging double leg stances.

The essential role of optical flow in the peripheral visual field for stable quiet standing: Evidence from the use of a head-mounted display

It has long been thought that vision is the most essential factor in maintaining stable quiet standing compared to other sources (i.e., vestibular and somatosensory inputs) of information. Specifically, several vision studies on postural control have shown evidence for the importance of the visual system, particularly peripheral vision rather than central vision, and optical flow. Nevertheless, to date, no study has manipulated both visual field and optical flow concurrently. In the present study, we experimentally manipulated both the visual field (the central and peripheral visual fields) and the occurrence of optical flow during quiet standing, examining the effects of the visual field and optical flow on postural sway measured in terms of the center of pressure (CoP). Stationary random dot stimuli were presented exclusively in either the central or peripheral visual field, while the occurrence of optical flow was manipulated using a desktop (DTD) or a head-mounted (HMD) display. The optical flow that occurred while using the DTD was a function of the postural sway during quiet standing, while for the HMD, no optical flow occurred even when the body/head swayed during quiet standing. Our results show that the extent of postural sway (e.g., CoP area) was smaller when visual stimuli were presented in the peripheral visual field than that in the central visual field; this was the case while using the DTD alone, with no effects of the peripheral vision on the extent of postural sway while using the HMD. It is therefore suggested that the optical flow occurring in the peripheral visual field is essential for stable quiet standing.

Regularity of Center of Pressure Trajectories in Expert Gymnasts during Bipedal Closed-Eyes Quiet Standing

We compared postural control of expert gymnasts (G) to that of non-gymnasts (NG) during bipedal closed-eyes quiet standing using conventional and nonlinear dynamical measures of center of foot pressure (COP) trajectories. Earlier findings based on COP classical variables showed that gymnasts exhibited a better control of postural balance but only in demanding stances. We examined whether the effect of expertise in Gymnastic can be uncovered in less demanding stances, from the analysis of the dynamic patterns of COP trajectories. Three dependent variables were computed to describe the subject's postural behavior: the variability of COP displacements (A_CoP), the variability of the COP velocities (V_CoP) and the sample entropy of COP (SEn_CoP) to quantify COP regularity (i.e., predictability). Conventional analysis of COP trajectories showed that NG and G exhibited similar amount and control of postural sway, as indicated by similar A_CoP and V_CoP values observed in NG and G, respectively. These results suggest that the specialized balance training received by G may not transfer to less challenging balance conditions such as the bipedal eyes-closed stance condition used in the present experiment. Interestingly, nonlinear dynamical analysis of COP trajectories regarding COP regularity showed that G exhibited more irregular COP fluctuations relative to NG, as indicated by the higher SEn_CoP values observed for the G than for the NG. The present results showed that a finer-grained analysis of the dynamic patterns of the COP displacements is required to uncover an effect of gymnastic expertise on postural control in nondemanding postural stance. The present findings shed light on the surplus value in the nonlinear dynamical analysis of COP trajectories to gain further insight into the mechanisms involved in the control of bipedal posture.

Visual Information and Support Surface for Postural Control in Visual Search Task

When standing on a reduced support surface, people increase their reliance on visual information to control posture. This assertion was tested in the current study. The effects of imposed motion and support surface on postural control during visual search were investigated. Twelve participants (aged 21 ± 1.8 years; six men and six women) stood on a reduced support surface (45% base of support). In a room that moved back and forth along the anteroposterior axis, participants performed visual search for a given letter in an article. Postural sway variability and head-room coupling were measured. The results of head-room coupling, but not postural sway, supported the assertion that people increase reliance on visual information when standing on a reduced support surface. Whether standing on a whole or reduced surface, people stabilized their posture to perform the visual search tasks. Compared to a fixed target, searching on a hand-held target showed greater head-room coupling when standing on a reduced surface.

Assessment of Visual Reliance in Balance Control: An Inexpensive Extension of the Static Posturography

Ability of humans to maintain balance in an upright stance and during movement activities is one of the most natural skills affecting everyday life. This ability progressively deteriorates with increasing age, and balance impairment, often aggravated by age-related diseases, can result in falls that adversely impact the quality of life. Falls represent serious problems of health concern associated with aging. Many investigators, involved in different science disciplines such as medicine, engineering, psychology, and sport, have been attracted by a research of the human upright stance. In a clinical practice, stabilometry based on the force plate is the most widely available procedure used to evaluate the balance. In this paper, we have proposed a low-cost extension of the conventional stabilometry by the multimedia technology that allows identifying potentially disturbing effects of visual sensory information. Due to the proposed extension, a stabilometric assessment in terms of line integral of center of pressure (COP) during moving scene stimuli shows higher discrimination power between young healthy and elderly subjects with supposed stronger visual reliance.

Modulation of visually evoked postural responses by contextual visual, haptic and auditory information: a 'virtual reality check'

Externally generated visual motion signals can cause the illusion of self-motion in space (vection) and corresponding visually evoked postural responses (VEPR). These VEPRs are not simple responses to optokinetic stimulation, but are modulated by the configuration of the environment. The aim of this paper is to explore what factors modulate VEPRs in a high quality virtual reality (VR) environment where real and virtual foreground objects served as static visual, auditory and haptic reference points. Data from four experiments on visually evoked postural responses show that: 1) visually evoked postural sway in the lateral direction is modulated by the presence of static anchor points that can be haptic, visual and auditory reference signals; 2) real objects and their matching virtual reality representations as visual anchors have different effects on postural sway; 3) visual motion in the anterior-posterior plane induces robust postural responses that are not modulated by the presence of reference signals or the reality of objects that can serve as visual anchors in the scene. We conclude that automatic postural responses for laterally moving visual stimuli are strongly influenced by the configuration and interpretation of the environment and draw on multisensory representations. Different postural responses were observed for real and virtual visual reference objects. On the basis that automatic visually evoked postural responses in high fidelity virtual environments should mimic those seen in real situations we propose to use the observed effect as a robust objective test for presence and fidelity in VR.

Stance width influences frontal plane balance responses to centripetal accelerations

Whenever the body is moving in a curvilinear path, inertial torques resulting from centripetal accelerations act on the body and must be counteracted to maintain stability. We tested the hypothesis that healthy subjects orient their center of mass in the position where gravitational torques offset the inertial torques due to centripetal accelerations. Ten healthy subjects stood on a platform that rotated in a circle at either a slow or fast speed, eyes open or closed, and in narrow or wide stance. Upper body, lower body, and center of mass (CoM) tilt with respect to vertical were measured and averaged across a 40 second time period of constant velocity. Body tilt was compared to the gravito-inertial acceleration (GIA) angle with respect to vertical. In all moving conditions, the upper body, lower body, and CoM tilted inward. However, this inward tilt did not reach the predicted GIA angle (CoM tilt was ~78% and 39% toward the predicted GIA angle in narrow and wide stance, respectively). Ratios of body tilt to GIA angle were minimally influenced by visual availability and magnitude of centripetal acceleration; but were largely influenced by stance width whereby narrow stance inward tilt was greater than wide stance. These results further highlight the important influence of the base of support on balance control strategies and enhance our understanding of how the balance control system compensates for inertial torques generated from centripetal accelerations.

Reorientation to vertical modulated by combined support surface tilt and virtual visual flow in healthy elders and adults with stroke

We explored how changes in visual attention impacted postural motor performance in healthy elders and adults post-stroke within a virtual reality environment, including when vestibular information was not perceptible. Visual dependence in 13 healthy (50-80 years) and 13 adults post-stroke (49-70 years) was assessed with a rod-and-frame task. Three degree support surface dorsiflexion tilts at 30°/s were combined with 30° and 45°/s continuous pitch rotations of the visual environment. The support surface remained tilted for 30 s followed by a 0.1°/s return to neutral during continued visual field rotation. Body displacement and ankle muscle responses were recorded, and wavelet transforms calculated. Muscle frequencies and kinematic measures were examined with functional principal component analysis, and weights compared through mixed model repeated measures ANOVA. Both populations exhibited increased backward sway with pitch upward visual field motion; adults post-stroke produced significantly larger muscle responses. Lateral sway was most regulated when visual flow velocity matched platform velocity. Visual flow summed with direction of support surface instability and visually dependent individuals produced more controlled lateral sway when viewing a dynamic visual field. Provoking postural instability within a dynamic visual flow field could serve as a training tool for postural stabilizing actions, particularly when visual dependence is exhibited.

Incorporating virtual reality graphics with brain imaging for assessment of sport-related concussions

There is a growing concern that traditional neuropsychological (NP) testing tools are not sensitive to detecting residual brain dysfunctions in subjects suffering from mild traumatic brain injuries (MTBI). Moreover, most MTBI patients are asymptomatic based on anatomical brain imaging (CT, MRI), neurological examinations and patients' subjective reports within 10 days post-injury. Our ongoing research has documented that residual balance and visual-kinesthetic dysfunctions along with its underlying alterations of neural substrates may be detected in "asymptomatic subjects" by means of Virtual Reality (VR) graphics incorporated with brain imaging (EEG) techniques.

Effect of narrowing the base of support on the gait, gaze and quiet eye of elite ballet dancers and controls

We determined the gaze and stepping behaviours of elite ballet dancers and controls as they walked normally and along progressively narrower 3-m lines (l0.0, 2.5 cm). The ballet dancers delayed the first step and then stepped more quickly through the approach area and onto the lines, which they exited more slowly than the controls, which stepped immediately but then slowed their gait to navigate the line, which they exited faster. Contrary to predictions, the ballet group did not step more precisely, perhaps due to the unique anatomical requirements of ballet dance and/or due to releasing the degrees of freedom under their feet as they fixated ahead more than the controls. The ballet group used significantly fewer fixations of longer duration, and their final quiet eye (QE) duration prior to stepping on the line was significantly longer (2,353.39 ms) than the controls (1,327.64 ms). The control group favoured a proximal gaze strategy allocating 73.33% of their QE fixations to the line/off the line and 26.66% to the exit/visual straight ahead (VSA), while the ballet group favoured a 'look-ahead' strategy allocating 55.49% of their QE fixations to the exit/VSA and 44.51% on the line/off the line. The results are discussed in the light of the development of expertise and the enhanced role of fixations and visual attention when more tasks become more constrained.

Time series analysis of postural responses to combined visual pitch and support surface tilt

The purpose of using time-series analyses is to provide interpretation of information on curves or functions, such as dynamic, biomechanical data. We evaluated the application of one method of time-series analysis for assessing changes in postural responses when exposed to a continuously rotating visual field combined with a tilted support surface. Functional Principal Component Analysis (fPCA) was applied to center of mass (CoM) trajectories collected from 22 young adults (20-39 y.o.) on a fixed surface or following a 3 degree (30°/s) dorsiflexion tilt of the support surface combined with continuous upward or downward pitch rotation of the visual field at 30 and 45°/s. The usefulness of this analytical tool is that each curve is treated as a distinct observation by itself, allowing for traditional PCA to be applied to the analysis of curves. Results of the fPCA highlighted 5 distinct time periods in the CoM curves that explained 91% of the variability in the data. These periods in which the young adults altered their CoM in response to visual field motion would not have been identified if we had relied on the onset and offset of the transient disturbance to distinguish responses. Young adults significantly displaced their CoM in response to visual motion over both the period of support surface tilt and while the support surface returned to a neutral position. Our results indicate that fPCA is a viable method when applied to the small but complex changes that emerge in postural data and might allow for a better understanding of time dependent processes occurring with pathology and intervention.

Influence of moving visual environment on sit-to-stand kinematics in children and adults

The effect of visual field motion on the sit-to-stand kinematics of adults and children was investigated. Children (8 to12 years of age) and adults (21 to 49 years of age) were seated in a virtual environment that rotated in the pitch and roll directions. Participants stood up either (1) concurrent with onset of visual motion or (2) after an immersion period in the moving visual environment, and (3) without visual input. Angular velocities of the head with respect to the trunk, and trunk with respect to the environment, w ere calculated as was head andtrunk center of mass. Both adults and children reduced head and trunk angular velocity after immersion in the moving visual environment. Unlike adults, children demonstrated significant differences in displacement of the head center of mass during the immersion and concurrent trials when compared to trials without visual input. Results suggest a time-dependent effect of vision on sit-to-stand kinematics in adults, whereas children are influenced by the immediate presence or absence of vision.

Postural and spatial orientation driven by virtual reality

Orientation in space is a perceptual variable intimately related to postural orientation that relies on visual and vestibular signals to correctly identify our position relative to vertical. We have combined a virtual environment with motion of a posture platform to produce visual-vestibular conditions that allow us to explore how motion of the visual environment may affect perception of vertical and, consequently, affect postural stabilizing responses. In order to involve a higher level perceptual process, we needed to create a visual environment that was immersive. We did this by developing visual scenes that possess contextual information using color, texture, and 3-dimensional structures. Update latency of the visual scene was close to physiological latencies of the vestibulo-ocular reflex. Using this system we found that even when healthy young adults stand and walk on a stable support surface, they are unable to ignore wide field of view visual motion and they adapt their postural orientation to the parameters of the visual motion. Balance training within our environment elicited measurable rehabilitation outcomes. Thus we believe that virtual environments can serve as a clinical tool for evaluation and training of movement in situations that closely reflect conditions found in the physical world.

Influence of visual scene velocity on segmental kinematics during stance

We investigated how the velocity of anterior-posterior movement of a visual surround affected segmental kinematics during stance. Ten healthy young adults were exposed to sinusoidal oscillation of an immersive virtual scene at five peak velocities ranging from 1.2 to 188 cm/s at each of four frequencies: 0.05, 0.1, 0.2 and 0.55 Hz. Root mean square (RMS) values of head, trunk, thigh and shank angular displacements were calculated. RMS values of head-neck, hip, knee and ankle joint angles were also calculated. RMS values of head, trunk, thigh and shank displacements exhibited significant increases at a scene velocity of 188 cm/s when compared with lower scene velocities. RMS values of hip, knee and ankle joint angles exhibited significant increases at scene velocities of 125 and 188 cm/s when compared with lower scene velocities. These results suggest that visual cues continued to drive postural adjustments even during high velocity movement of the virtual scene. Significant increases in the RMS values of the lower-limb joint angles suggest that as visually-induced postural instability increased, the body was primarily controlled as a multi-segmental structure instead of a single-link inverted pendulum, with the knee playing a key role in postural stabilization.

Influence of virtual reality on postural stability during movements of quiet stance

INTRODUCTION

Balance problems during virtual reality (VR) have been mentioned in the literature but seldom investigated despite the increased use of VR systems as a training or rehabilitation tool. We examined the influence of VR on body sway under different stance conditions.

METHODS

Seventeen young subjects performed four tasks (standing with feet close together or tandem stance on firm and foam surfaces for 60s) under three visual conditions: eyes open without VR, eyes closed, or while viewing a virtual reality scene which moved with body movements. Angular velocity transducers mounted on the shoulder provided measures of body sway in the roll and pitch plane.

RESULTS

VR caused increased pitch and roll angles and angular velocities compared to EO. The effects of VR were, for the most part, indistinguishable from eyes closed conditions. Use of a foam surface increased sway compared to a firm surface under eyes closed and VR conditions.

CONCLUSION

During the movements of quiet stance, VR causes an increase in postural sway in amplitude similar to that caused by closing the eyes. This increased sway was present irrespective of stance surface, but was greatest on foam.

Effects of roll visual motion on online control of arm movement: reaching within a dynamic virtual environment

Reaching toward a visual target involves the transformation of visual information into appropriate motor commands. Complex movements often occur either while we are moving or when objects in the world move around us, thus changing the spatial relationship between our hand and the space in which we plan to reach. This study investigated whether rotation of a wide field-of-view immersive scene produced by a virtual environment affected online visuomotor control during a double-step reaching task. A total of 20 seated healthy subjects reached for a visual target that remained stationary in space or unpredictably shifted to a second position (either to the right or left of its initial position) with different inter-stimulus intervals. Eleven subjects completed two experiments which were similar except for the duration of the target's appearance. The final target was either visible throughout the entire trial or only for a period of 200 ms. Movements were performed under two visual field conditions: the virtual scene was matched to the subject's head motion or rolled about the line of sight counterclockwise at 130 degrees/s. Nine additional subjects completed a third experiment in which the direction of the rolling scene was manipulated (i.e., clockwise and counterclockwise). Our results showed that while all subjects were able to modify their hand trajectory in response to the target shift with both visual scenes, some of the double-step movements contained a pause prior to modifying trajectory direction. Furthermore, our findings indicated that both the timing and kinematic adjustments of the reach were affected by roll motion of the scene. Both planning and execution of the reach were affected by roll motion. Changes in proportion of trajectory types, and significantly longer pauses that occurred during the reach in the presence of roll motion suggest that background roll motion mainly interfered with the ability to update the visuomotor response to the target displacement. Furthermore, the reaching movement was affected differentially by the direction of roll motion. Subjects demonstrated a stronger effect of visual motion on movements taking place in the direction of visual roll (e.g., leftward movements during counterclockwise roll). Further investigation of the hand path revealed significant changes during roll motion for both the area and shape of the 95% tolerance ellipses that were constructed from the hand position following the main movement termination. These changes corresponded with a hand drift that would suggest that subjects were relying more on proprioceptive information to estimate the arm position in space during roll motion of the visual field. We conclude that both the spatial and temporal kinematics of the reach movement were affected by the motion of the visual field, suggesting interference with the ability to simultaneously process two consecutive stimuli.

Postural strategies in normal subjects and in patients with instability due to central nervous system diseases after sudden changes in the visual flow

CONCLUSION

The results suggest that after a sudden change in the visual input, postural adaptation is impaired in patients with instability and central nervous system disorders (CNSD).

OBJECTIVE

The aim of this study was the assessment of postural adaptation, as a transient phenomenon, when sudden changes in visual perception occur in normal subjects and in patients with instability due to different CNSD.

SUBJECTS AND METHODS

Horizontal optokinetic stimulation (40 s and suddenly stopped) was performed in 16 patients with CNSD, and also in 22 normal subjects. Measurements were made of the body center of pressure area (COP) and the body sway velocity (SV) during 10 s before and after the stop and labeling the COP trajectory.

RESULTS

Values of COP and SV (Wilcoxon signed rank test, p=0.979 and 0.496, respectively) in normal subjects did not show any significant change before and after the stop. In 15 of the 16 assessed patients with instability associated with CNSD an increase of the COP and SV values (Wilcoxon signed test, p=0.001 and 0.004, respectively) was observed in the 10 s after the visual stop. COP labeling showed 'roaming' of the COP spatial evolution approaching the limits of stability.

Aging and selective sensorimotor strategies in the regulation of upright balance

Background

The maintenance of upright equilibrium is essentially a sensorimotor integration task. The central nervous system (CNS) has to generate appropriate and complex motor responses based on the selective and rapid integration of sensory information from multiple sources. Since each sensory system has its own coordinate framework, specific time delay and reliability, sensory conflicts may arise and represent situations in which the CNS has to recalibrate the weight attributed to each particular sensory input. The resolution of sensory conflicts may represent a particular challenge for older adults given the age-related decline in the integrity of many postural regulating systems, including musculoskeletal and sensory systems, as well as neural processing and conduction of information. The effects of aging and adaptation (by repeated exposures) on the capability of the CNS to select pertinent sensory information and resolve sensory conflicts were thus investigated with virtual reality (VR) in the present study.

Methods

Healthy young and older adults maintained quiet stance while immersed in a virtual environment (VE) for 1 hour during which transient visual and/or surface perturbations were randomly presented. Visual perturbations were induced by sudden pitch or roll plane tilts of the VE viewed through a helmet-mounted display, and combined with or without surface perturbations presented in a direction that was either identical or opposite to the visual perturbations.

Results

Results showed a profound influence of aging on postural adjustments measured by electromyographic (EMG) responses and displacements of the center of pressure (COP) and body's center of mass (COM) in the recovery of upright stance, especially in the presence of sensory conflicts. Older adults relied more on vision as compared to young adults. Aging affects the interaction of the somatosensory and visual systems on the control of equilibrium during standing and the ability of CNS to resolve sensory conflicts. However, even with a one-hour immersion in VE and exposure to sensory conflicts, it is possible for the CNS to recalibrate and adapt to the changes, while improving balance capability in older adults.

Conclusion

Preventive and rehabilitation programs targeting postural control in older adults should take into account the possible impairment of sensory organization or sensorimotor integration and include VE training under conditions of sensory conflicts.

Visual motion combined with base of support width reveals variable field dependency in healthy young adults

We previously reported responses to induced postural instability in young healthy individuals viewing visual motion with a narrow (25 degrees in both directions) and wide (90 degrees and 55 degrees in the horizontal and vertical directions) field of view (FOV) as they stood on different sized blocks. Visual motion was achieved using an immersive virtual environment that moved realistically with head motion (natural motion) and translated sinusoidally at 0.1 Hz in the fore-aft direction (augmented motion). We observed that a subset of the subjects (steppers) could not maintain continuous stance on the smallest block when the virtual environment was in motion. We completed a posteriori analyses on the postural responses of the steppers and non-steppers that may inform us about the mechanisms underlying these differences in stability. We found that when viewing augmented motion with a wide FOV, there was a greater effect on the head and whole body center of mass and ankle angle root mean square (RMS) values of the steppers than of the non-steppers. FFT analyses revealed greater power at the frequency of the visual stimulus in the steppers compared to the non-steppers. Whole body COM time lags relative to the augmented visual scene revealed that the time-delay between the scene and the COM was significantly increased in the steppers. The increased responsiveness to visual information suggests a greater visual field-dependency of the steppers and suggests that the thresholds for shifting from a reliance on visual information to somatosensory information can differ even within a healthy population.