Reduced postural differences between phobic postural vertigo patients and healthy subjects during a postural threat

Phobic postural vertigo is characterized by subjective imbalance and dizziness while standing or walking, despite normal values for clinical balance tests. Patients with phobic postural vertigo exhibit an increased high-frequency sway in posturographic tests. Their postural sway, however, becomes similar to the sway of healthy subjects during difficult balance tasks. Posturographic recordings of 30 s of quiet stance was compared to recordings of 30 s of quiet stance during a postural threat, which consisted of the knowledge of forthcoming vibratory calf muscle stimulation, in 37 consecutive patients with phobic postural vertigo and 24 healthy subjects. During quiet stance without the threat of forthcoming vibratory stimulation, patients with phobic postural vertigo exhibited a postural sway containing significantly more high-frequency sway than the healthy subjects. During the quiet stance with forthcoming vibratory stimulation, i.e., anticipation of a postural threat, the significant differences between groups disappeared for all variables except sagittal high-frequency sway. During postural threat, healthy subjects seemed to adopt a postural strategy that was similar to that exhibited by phobic postural vertigo patients. The lack of additional effects facing a postural threat among phobic postural vertigo patients may be due to an already maximized postural adaptation. Deviant postural reactions among patients with phobic postural vertigo may be considered as an avoidant postural response due to a constant fear of losing postural control.     

Visual dependence affects postural sway responses to continuous visual field motion in individuals with cerebral palsy

The current study aimed to explore the impact of visual dependence on sensorimotor coupling of postural sway and visual motion in adults and teens with spastic cerebral palsy (CP). We hypothesized that individuals with CP would exhibit greater magnitudes of sway than healthy individuals, and the presence of visual dependence (VD) would produce instability in the direction of visual motion. Participants stood in a virtual environment in which the visual scene remained static or continuously rotated 30 degree/second in pitch-up or pitch-down. Increased center of pressure and center of mass responses were observed in the direction of visual scene motion in those with CP. Those with VD exhibited reduced frequency responses in anterior-posterior direction than those who were visually independent. VD suggests deficient sensorimotor integration that could contribute to postural instability and reduced motor function. Individuals with CP who are visually dependent may benefit from more sensory focused rehabilitation strategies.

Abbreviations: AP, anterior-posterior; CP, cerebral palsy; COM, center of mass; COP, center of pressure; MDF, median frequency; ML, mediolateral; PD, pitch down (nose down) rotation; PU, pitch up (nose up) rotation; RFT, rod and frame test; RMS, root mean square; SLP, slope of the fitted line; TD, typical development; VD, visual dependence; VI, visual independence; VOR, vestibulo-ocular reflex; VPI, visual perceptual impairment     

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?C80 years) and 13 adults post-stroke (49?C70 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.     

Visually induced postural sway in anxiety disorders

Postural sensitivity to moving visual environments in patients with anxiety disorders was studied. We hypothesized that patients with anxiety disorders would have greater sway in response to a moving visual environment compared to healthy adults, especially if they have space and motion discomfort (SMD). Twenty-one patients with generalized anxiety without panic (NPA) and 38 patients with panic and agoraphobia (PAG) were compared to 22 healthy controls. SMD was evaluated in all subjects via questionnaire. Subjects stood on a force platform that was either fixed or rotating with the subject (i.e., sway referenced) during exposure to a sinusoidally moving visual surround. Center of pressure (COP) data were computed from force transducers in the platform as a measure of sway. Results showed that patients swayed significantly more in response to the moving visual scene compared to control subjects, with no differences between the NPA and PAG groups. SMD was a predictor of sway response in the patients: patients with high SMD swayed significantly more than both Controls and anxiety patients with low SMD. These results indicate that patients with anxiety disorders, particularly those with SMD, are more visually dependent for balance. This subgroup of patients may be amenable to treatment used for patients with balance disorders (i.e., vestibular rehabilitation) that focuses on sensory re-integration processes that address visual sensitivity.     

Relationship between postural control and motion sickness in healthy subjects

This study examined the relationship between reported susceptibility to motion sickness, anxious personality and postural control. Postural stability was assessed in 34 healthy subjects standing with eyes open, eyes closed and viewing a disorienting virtual reality display. These measures were repeated with vibration of the calf muscles to distort the somatosensory feedback from the legs. Susceptibility to motion sickness and anxious personality were evaluated by questionnaire. Greater postural instability was correlated with susceptibility to motion sickness. Motion sickness susceptibility correlated most strongly with increased sway when the visual and somatosensory feedback was absent or distorted. Anxiety was correlated with reported susceptibility to motion sickness but not with postural stability. These findings suggest that deficient perceptual-motor responses to disorienting conditions may contribute to motion sickness susceptibility.     

Posture and mental task performance when viewing a moving visual field

We investigated the characteristics of standing posture and performance of concurrent cognitive tasks in subjects confronted by whole field visual motion. Movements of the head and centre of pressure (COP) were recorded in 12 subjects who performed modified Brooks spatial and verbal tasks when in quiet stance viewing a chequerboard pattern, planar, visual field, moving with uniform velocity (25 degrees /s, 50 degrees /s and 76 degrees /s). Eight subjects were also tested seated to control for the effect of stance. Task load was monitored by heart rate and eye movements were recorded to ensure viewing compliance. Subjects rated their quotidian susceptibility to visual disorientation on a validated scale. In both lateral and antero-posterior directions there were small amplitude but significant increases in COP sway path length and standard deviations of both COP and head sway during exposure to visual motion in proportion to visual flow speed. Performing cognitive tasks during visual motion attenuated sway S.D. The effects on sway of task and visual flow were independent. Visual motion induced a slight tilt and turn of the head and body in the direction of flow together with slight neck flexion. Errors on both verbal and spatial tasks increased >250% during visual motion both when standing and when seated. Ratings of subjects' susceptibility to disorientation were un-related to either verbal or spatial task error rates. A current hypothesis is that the enhancement of sway by visual motion is destabilisation. We propose an alternative explanation that sway enhancement could be exploratory 'testing of the ground' movements to check for self motion. Hence decrease in sway magnitude during a cognitive task could be caused by a reduction in exploratory movement because attention is diverted from postural control to a secondary task. Mere passive viewing of a moving visual field may interfere with cognitive tasks possibly because the threat of disorientation by whole field motion diverts attentional resources.     

正常人不同年龄组平衡功能之间的比较

目的 观察分析不同年龄段平衡功能的不同表现,以及造成这种差异的主要因素.方法 将109例受试者按年龄划分为青少年组、中青年组和老年组.使用Swaystar检查装置记录7项平衡试验中躯体前后、左右晃动的角位移和晃动总面积.结果 (1)当视觉、本体觉或前庭觉传入信息受到干扰时,老年组平衡功能明显较差;单左腿站立时,老年组躯体晃动幅度明显大于另外两组(P＜0.05),然而单右腿站立时,三组之间无统计学差异;(2)青少年阶段的姿势调节主要依赖于视觉.结论 随着年龄的增加,本体觉和前庭觉减退会导致平衡功能的降低;而在青少年时期视觉在姿势调节过程中占主要地位.     

Direction-specific postural instability in subjects with Parkinson's disease

The purpose of this study was to determine whether and why subjects with Parkinson's disease (PD) have greater instability in response to specific directions of perturbations than do age-matched control subjects and how instability is affected by stance width. This study compared postural responses to 8 directions of surface translations in PD subjects and age-matched control subjects while standing in a narrow and wide stance. PD subjects were tested in their practical OFF state. A postural stability margin was quantified as the difference between peak center of pressure (CoP) and peak center of mass (CoM) displacement in response to surface translations. The control subjects maintained a consistent stability margin across directions of translations and for both narrow and wide stance by modifying rate of rise of CoP responses. PD subjects had smaller than normal postural stability margins in all directions, but, especially for backwards sway in both stance widths and for lateral sway in narrow stance width. The reduced stability margin in PD subjects was due to a slower rise and smaller peak of CoP in the PD subjects than in control subjects. Lateral postural stability was compromised in PD subjects by lack of trunk flexibility and backwards postural stability was compromised by lack of knee flexion, resulting in excessive displacements of the body CoM. Stability margins in PD subjects were related to their response on the pull test in the Unified Parkinson's Disease Rating Scale. Thus, PD patients have directionally specific postural instability due to an ineffective stiffening response and inability to modify their postural responses for changing postural demands related to direction of perturbation and initial stance posture. These results suggest that the basal ganglia, in addition to regulating muscle tone and energizing postural muscle activation, also are critical for adapting postural response patterns for specific biomechanical conditions.     

Open-loop and closed-loop control of posture: Stabilogram-diffusion analysis of center-of-pressure trajectories among people with stroke

Many people with stroke (PwS) demonstrate reduced balance and increased postural sway afterwards, which may ultimately lead to falls and injury. In this study, we aimed to better understand postural sway behavior and the mechanisms of balance control by examining balance in upright standing among PwS using methods from statistical mechanics i.e., the Stabilogram diffusion analysis (SDA). Center-of-pressure displacements while standing still were measured in 25 PwS and 11 healthy subjects. The traditional postural sway parameters were measured, and the SDA was used to characterize balance control in eyes-open and eyes-closed conditions. We found that PwS demonstrated significantly greater postural sway in the mediolateral and anterior–posterior directions and significantly higher SDA short-term diffusion coefficients and critical displacement in both eyes-open and eyes-closed conditions. There was also a significant group-by-condition interaction, whereas PwS demonstrated more sway in the eyes-closed condition. The SDA analysis revealed unstable behavior during short-term intervals, interpreted as larger distance of sway until closed-loop control took place. This significant group-by-condition interaction suggests that PwS have a significantly greater reliance on visual input compared with healthy subjects.     

Postural threat differentially affects the feedforward and feedback components of the vestibular‐evoked balance response

Circumstances may render the consequence of falling quite severe, thus maximising the motivation to control postural sway. This commonly occurs when exposed to height and may result from the interaction of many factors, including fear, arousal, sensory information and perception. Here, we examined human vestibular‐evoked balance responses during exposure to a highly threatening postural context. Nine subjects stood with eyes closed on a narrow walkway elevated 3.85 m above ground level. This evoked an altered psycho‐physiological state, demonstrated by a twofold increase in skin conductance. Balance responses were then evoked by galvanic vestibular stimulation. The sway response, which comprised a whole‐body lean in the direction of the edge of the walkway, was significantly and substantially attenuated after ~800 ms. This demonstrates that a strong reason to modify the balance control strategy was created and subjects were highly motivated to minimise sway. Despite this, the initial response remained unchanged. This suggests little effect on the feedforward settings of the nervous system responsible for coupling pure vestibular input to functional motor output. The much stronger, later effect can be attributed to an integration of balance‐relevant sensory feedback once the body was in motion. These results demonstrate that the feedforward and feedback components of a vestibular‐evoked balance response are differently affected by postural threat. Although a fear of falling has previously been linked with instability and even falling itself, our findings suggest that this relationship is not attributable to changes in the feedforward vestibular control of balance.     

Postural inflexibility in parkinsonian subjects.

In order to identify the types of postural deficits seen in parkinsonian patients with postural instability, we compared the performance of parkinsonian subjects with young and old control subjects in 3 aspects of postural control: (1) the use of sensory information for postural orientation, (2) the coordination of postural movement patterns in response to surface displacements, and (3) the flexible modification of postural response patterns to changes in support conditions. Parkinsonian subjects had very small sway, even under altered sensory conditions. Postural response latencies to displacements were also normal. Postural instability was associated with abnormal patterns of postural responses including excessive antagonist activity and inflexibility in adapting to changing support conditions. Some parkinsonian subjects appeared to have difficulty sequencing motor programs for postural correction. The parkinsonian subjects appeared stiffer since the rate-of-change of sway in response to displacements was reduced. Levodopa improved postural coordination but not the flexible adaptation to changing support conditions.     

A Biofeedback Study of Postural Sway

An experiment was performed to control postural sway under auditory and visual feedback signals using a new apparatus. The subjects of 52 healthy high school girls were divided into 13 small groups. Each small group had four subjects who were assigned to one of the following four groups; 1) visual feedback group, 2) auditory feedback group, 3) auditory and visual feedback group, and 4) control group. Comparisons were made as to the duration of the green lamp being on, which indicated sway stayed around the initial central gravity. The results showed that auditory and visual feedback groups had a significant increase in the duration as compared to other three groups. With regard to the changes in the areas of postural sway, the largest increase was seen in the auditory and visual group. While the three feedback groups showed downward curves, the control group showed an upward one. Thus it is suggested that postural sway can be voluntarily controlled by a combination of an auditory feedback procedure and a visual feedback procedure, but not by either of them independently.     

A biofeedback study of postural sway.

An experiment was performed to control postural sway under auditory and visual feedback signals using a new apparatus. The subjects of 52 healthy high school girls were divided into 13 small groups. Each small group had four subjects who were assigned to one of the following four groups; 1) visual feedback group, 2) auditory feedback group, 3) auditory and visual feedback group, and 4) control group. Comparisons were made as to the duration of the green lamp being on, which indicated sway stayed around the initial central gravity. The results showed that auditory and visual feedback groups had a significant increase in the duration as compared to other three groups. With regard to the changes in the areas of postural sway, the largest increase was seen in the auditory and visual group. While the three feedback groups showed downward curves, the control group showed an upward one. Thus it is suggested that postural sway can be voluntarily controlled by a combination of an auditury feedback procedure and a visual feedback procedure, but not be either of them independently.     

Kinematic properties of voluntary postural sway in patients with unilateral primary hemispheric lesions

The temporal and spatial characteristics of rapid voluntary body sway following unilateral cerebrovascular accident were assessed in single and combined planes of motion during a visually cued non-choice reaction time movement. Three distinct directions of intentional body sway were assessed: forward, toward the paretic, and toward the non-paretic lower extremity. Fluctuations in the center of foot pressure served as the basis for calculating body displacement and velocity by combining frontal and sagittal movement in anX-Y coordinate system (a bi-planar analysis). A linear regression analysis was used on individual force output channels to reduce the sway motion into separate component parts. The velocity profile in each direction of intended sway was calculated from the slope of the regression line (a single-plane analysis). Bi-planar velocity analysis showed that hemiplegics swayed as fast as normal subjects in any sway direction. In contrast, single plane analysis revealed a decrease in the velocity of hemiplegic sway when weight shifts were directed toward the paretic extremity. Response velocity in the non-paretic direction was similar to a comparison group of normal individuals. The apparent contradiction between single and bi-planar analyses was attributed to hemiplegic sway in extraneous planes of motion surrounding the intended sway path. During the course of body sway, hemiplegics showed a statistically larger range of extraneous movement away from the plane of intended movement. The directional deficits in sway velocity and excursion are compared to previously reported sway pathology associated with lesions of the human central nervous system.     

Multisensory fusion: simultaneous re-weighting of vision and touch for the control of human posture

We examined the generally held belief that the postural control system is able to re-weight its available sensory inputs in order to optimize stance control in altered sensory environments. Our view is that previous accounts of sensory re-weighting provide only indirect evidence, which is subject to alternative explanations. The present results provide strong evidence for sensory re-weighting as the primary mechanism for changes observed in postural sway between conditions. Subjects were presented with small-amplitude, oscillatory visual and somatosensory stimuli at 0.20 and 0.28 Hz, respectively, in five conditions that manipulated the amplitudes of stimulus motion. Gain calculated in each trial with respect to each of the two stimuli was found to change systematically as stimulus motion amplitudes changed across condition. The observed pattern of gain rules out a constant-weight, linear account of posture and is consistent with the re-weighting hypothesis. Parameter fits of a third-order, linear stochastic model to postural sway trajectories in each condition showed that changes in gain across condition were primarily due to changes in coupling coefficients rather than changes in parameters that characterize the stability of the postural system. Visual gain was found to depend upon visual motion amplitude and touch gain was found to depend upon touch motion amplitude, indicating intra-modality dependencies. Visual gain also depended upon touch motion amplitude, indicating an inter-modality dependence. To our knowledge, simultaneous re-weighting of more than one sensory input has never been rigorously demonstrated. These techniques may be able to resolve the source of balance control deficits across populations with far more certainty than currently possible.     

Postural control and automaticity in dyslexic children: the relationship between visual information and body sway

Difficulty with literacy acquisition is only one of the symptoms of developmental dyslexia. Dyslexic children also show poor motor coordination and postural control. Those problems could be associated with automaticity, i.e., difficulty in performing a task without dispending a fair amount of conscious efforts. If this is the case, dyslexic children would show difficulties in using “unperceived” sensory cues to control body sway. Therefore, the aim of the study was to examine postural control performance and the coupling between visual information and body sway in dyslexic children. Ten dyslexic children and 10 non-dyslexic children stood upright inside a moving room that remained stationary or oscillated back and forward at frequencies of 0.2 or 0.5 Hz. Body sway magnitude and the relationship between the room's movement and body sway were examined. The results indicated that dyslexic children oscillated more than non-dyslexic children in both stationary and oscillating conditions. Visual manipulation induced body sway in all children but the coupling between visual information and body sway was weaker and more variable in dyslexic children. Based upon these results, we can suggest that dyslexic children use visual information to postural control with the same underlying processes as non-dyslexic children; however, dyslexic children show poorer performance and more variability while relating visual information and motor action even in a task that does not require an active cognitive and conscious motor involvement, which may be a further evidence of automaticity problem.     

Support stability influences postural responses to muscle vibration in humans

We studied the effect of support stability on postural responses to the vibration of Achilles tendons and of neck dorsal muscles in healthy humans. For this purpose we compared postural responses on a rigid floor and on 6 cm high rocking supports (see-saws) of different curvatures (different radii: 30, 60 and 120 cm). The subject stood with eyes closed, the centre of the feet coincided with the centre of the see-saw. We recorded platform tilt, horizontal displacements of the upper body, ankle joint angle and activity of ankle joint muscles. On the rocking platform subjects maintained balance in a sagittal direction by making see-saw rotations placing the support under the body's centre of gravity. Equilibrium maintenance requires that the torque in the ankle joint increases during forward body displacements, as on the rigid floor, and be accompanied by a plantar flexion (not by a dorsiflexion) in the ankle joint. The directional dependence of vibration-induced reactions on the see-saw was the same (relative to space) as on the rigid floor: backward body displacement during Achilles tendon vibration and forward body displacement during neck muscle vibration. A decrease of support stability (with a decrease of the radius from 120 to 30 cm) diminished significantly the effect of Achilles tendon vibration and to a lesser extent the effect of neck muscle vibration. In contrast, the increase of platform stability by hand contact with a stable external object gave rise to prominent body sway in response to Achilles tendon vibration. Neck muscle vibration on the movable support provoked a quick initial forward body sway. This initial quick response was absent during vibration of the Achilles tendons. We conclude that postural responses to muscle vibration reflect the participation of different muscles in posture control and depend on the support properties. Support instability changes the role of proprioceptive information and the state of the system of equilibrium maintenance.     

Optic flow dominates visual scene polarity in causing adaptive modification of locomotor trajectory

Locomotion and posture are influenced and controlled by vestibular, visual and somatosensory information. Optic flow and scene polarity are two characteristics of a visual scene that have been identified as being critical in how they affect perceived body orientation and self motion. The goal of this study was to determine the role of optic flow and visual scene polarity on adaptive modification in locomotor trajectory. An object is said to have visual polarity, or to be "visually polarized", when it contains an identifiable principal axis with one end distinct from the other. Two computer-generated virtual reality scenes were shown to subjects during 20 min of treadmill walking. One scene was a highly polarized scene, while the other was composed of objects displayed in a non-polarized fashion. Both virtual scenes depicted constant rate self motion equivalent to walking counterclockwise around the perimeter of a room. Subjects performed Stepping Tests blindfolded before and after scene exposure to assess adaptive changes in locomotor trajectory. Subjects showed a significant difference in heading direction, between pre- and post-adaptation Stepping Tests, when exposed to either scene during treadmill walking. However, there was no significant difference in the subjects' heading direction between the two visual scene polarity conditions. Therefore, it was inferred from these data that optic flow has a greater role than visual polarity in influencing adaptive locomotor function.     

Postural instability precedes motion sickness

We evaluated the hypothesis that postural instability precedes the onset of motion sickness. Subjects standing in a "moving room" were exposed to nearly global oscillating optical flow. In the experimental condition, the optical oscillations were a complex sum-of-sines between 0.1 and 0.3 Hz, with an excursion of 1.8 cm. This optical motion was of such low frequency and magnitude that it was sometimes not noticed by subjects. However, in two experiments, exposure to the moving room produced significant increases in scores on a standard motion sickness questionnaire. In addition, approximately half of subjects reported motion sickness. Analysis of postural motion during exposure to the moving room revealed increases in postural sway before the onset of subjective motion sickness symptoms. This confirms a key prediction of the postural instability theory of motion sickness.     

Effects of deep brain stimulation and levodopa on postural sway in Parkinson's disease

OBJECTIVE: To quantify postural sway in subjects with Parkinson's disease and elderly controls, and determine the effects of Parkinson's disease, deep brain stimulation, levodopa, and their interactions on postural control during quiet stance. METHODS: Centre of foot pressure (CoP) displacement under each foot was measured during three 60 s trials of quiet stance with eyes open in 11 controls and six patients with Parkinson's disease. Subjects with Parkinson's disease were tested in four treatment conditions: off both deep brain stimulation and levodopa (off condition); on deep brain stimulation; on levodopa; and on both deep brain stimulation and levodopa. The variables extracted from CoP included: root mean square distance (rms), mean velocity, 95% power frequency (f(95%)), area of the 95% confidence ellipse (ellipse area), direction of its major axis (mdir), and postural asymmetry between the feet. RESULTS: rms and area of postural sway were larger than normal in subjects with Parkinson's disease in the off condition, increased further with levodopa, and significantly decreased with deep brain stimulation. Mean velocity and f(95%) were also larger than normal but were restored to normal by all treatments, especially by deep brain stimulation. The combined effect of deep brain stimulation and levodopa resulted in a postural sway that was an average of the effect of each treatment individually. Levodopa increased sway more in the mediolateral than in the anterior-posterior direction. Subjects with Parkinson's disease had asymmetrical mean velocity and f(95%) between the feet, and this asymmetry increased with levodopa but decreased with deep brain stimulation. The f(95%) of the CoP correlated with tremor, posture, and gait subcomponents of the unified Parkinson's disease rating scale. CONCLUSIONS: Subjects with Parkinson's disease have abnormal postural sway in stance. Treatment with levodopa increases postural sway abnormalities, whereas treatment with deep brain stimulation improves postural sway. Quantitative evaluation of static posturography may be a useful adjunct to clinical measures in patients with Parkinson's disease.