Modification of compensatory saccades after aVOR gain recovery

The recruitment of extra-vestibular mechanisms to assist a deficient angular vestibulo-ocular reflex (aVOR) during ipsilesional head rotations is well established and includes saccades of reduced latency that occur in the direction of the lesioned aVOR, termed compensatory saccades (CS). Less well known is the functional relevance of these unique saccades. Here we report a 42 y.o. male diagnosed with right unilateral vestibular hypofunction due to vestibular neuronitis who underwent a vestibular rehabilitation program including gaze stabilization exercises. After three weeks, he had a significant improvement in his ability to see clearly during head rotation. Our data show a reduction in the recruitment and magnitude of CS as well as improved peripheral aVOR gain (eye velocity/head velocity) and retinal eye velocity. Our data suggest an inverse, dynamic relationship between the recruitment of CS and the gain of the aVOR.     

Three-dimensional Analysis of Human Locomotion in Normal Subjects and Patients with Vestibular Deficiency

The purpose of this study was to clarify the role of the vestibular system in human locomotion. The subjects were nine healthy controls, nine patients with unilateral vestibular deficiency (UVD) and nine patients with bilateral vestibular deficiency (BVD). The UVD subjects were Ménière's disease patients who were being treated with administration of gentamicin into the tympanic cavity. BVD subjects were hearing-impaired individuals who showed no response to the ice-water caloric test. A total of 13 markers were attached to the head, trunk (C7), hip and foot in order to measure translational and rotational motions with the aid of a video image processing system. All subjects were instructed to restrict their stride length to 80 cm while walking on a treadmill and watching a visual target. However, walking speed varied depending on the ability of the subject to maintain body equilibrium. The results showed that walking speed and step frequency were significantly lower for the UVD and BVD groups than for the normal group. Analysis of head movements in the sagittal plane showed a counteracting motion between pitch rotations and vertical translation as previously reported. We also found head counteracting motions between yaw rotation and lateral translation in the horizontal plane. These mechanisms are thought to help stabilize the gaze during walking. When the head fixation point was calculated by projecting the naso-occipital axis line during walking, the head counteracting motion was found to assist the vestibulo-ocular reflex in stabilizing the gaze. In addition, normal subjects seemed to use head stabilization as a space strategy in order to minimize head yaw movement. In contrast, UVD and BVD subjects adopted head stabilization as a trunk strategy.     

Adaptations in horizontal head stabilization in response to altered vision and gaze during natural walking

The purpose of this study was to determine adaptations in head stability resulting from altered gaze control and vision during over-ground walking. Using over-ground walking permitted adaptations in walking velocity and cadence that are otherwise not possible during treadmill walking or walking-in-place. Gaze control and vision were manipulated by having 20 young adult subjects 1) walk naturally, 2) view a distant, earth-fixed target to enhance the vestibulo-ocular reflex (VOR), 3) view a head-fixed target to suppress the VOR, and 4) walk in darkness. Horizontal head and trunk angular velocities in space, walking velocity and cadence were measured. Root-mean-square head and trunk angular velocities were calculated and frequency analyses determined head-trunk movement patterns. Results demonstrated that when given the opportunity, subjects slowed down and decreased cadence in response to challenging tasks. Despite strongly reduced walking velocity and cadence, walking in darkness proved most challenging for head stabilization, indicating the importance of vision during this process. Viewing the earth-fixed target demonstrated the greatest head stability thereby, facilitating gaze stabilization. However, comparisons between the earth-fixed and head-fixed target conditions suggest a reciprocal relationship where gaze stability also facilitates head stability. This contribution of gaze stability to head stability is more important than vision alone as the head stabilization response was diminished during the VOR suppressed condition.     

Three-dimensional analysis of human locomotion in normal subjects and patients with vestibular deficiency

The purpose of this study was to clarify the role of the vestibular system in human locomotion. The subjects were nine healthy controls, nine patients with unilateral vestibular deficiency (UVD) and nine patients with bilateral vestibular deficiency (BVD). The UVD subjects were Ménière's disease patients who were being treated with administration of gentamicin into the tympanic cavity. BVD subjects were hearing-impaired individuals who showed no response to the ice-water caloric test. A total of 13 markers were attached to the head, trunk (C7), hip and foot in order to measure translational and rotational motions with the aid of a video image processing system. All subjects were instructed to restrict their stride length to approximately 80 cm while walking on a treadmill and watching a visual target. However, walking speed varied depending on the ability of the subject to maintain body equilibrium. The results showed that walking speed and step frequency were significantly lower for the UVD and BVD groups than for the normal group. Analysis of head movements in the sagittal plane showed a counteracting motion between pitch rotations and vertical translation as previously reported. We also found head counteracting motions between yaw rotation and lateral translation in the horizontal plane. These mechanisms are thought to help stabilize the gaze during walking. When the head fixation point was calculated by projecting the naso-occipital axis line during walking, the head counteracting motion was found to assist the vestibulo-ocular reflex in stabilizing the gaze. In addition, normal subjects seemed to use head stabilization as a space strategy in order to minimize head yaw movement. In contrast, UVD and BVD subjects adopted head stabilization as a trunk strategy.     

Acquired bilateral peripheral vestibular system impairment: rehabilitative options and potential outcomes

Acquired bilateral vestibular impairment can be a devastating disorder that is most frequently the result of aminoglycoside-induced toxicity. The presenting complaints are typically oscillopsia and gait and balance disturbances. These patients can be excellent candidates for vestibular rehabilitation therapy that focuses on facilitating maximal use of any remaining vestibular function, improving gaze and postural stability through the use of visual and somatosensory cues, and improving home and workplace safety. The prognosis for recovery is determined by the extent of the loss and the presence of other progressive disorders that may affect vision or somatosensation, coexisting illnesses, and the patient's compliance with the therapy program. Two cases are presented to illustrate the salient aspects of vestibular rehabilitation for patients with acquired bilateral vestibular system loss, including factors affecting patient progress and final outcome.     

Eye Movements Are Correctly Timed During Walking Despite Bilateral Vestibular Hypofunction

Individuals with bilateral vestibular hypofunction (BVH) often report symptoms of oscillopsia (the perception that the world is bouncing or unstable) during walking. Efference copy/proprioception contributes to locomotion gaze stability in animals, sometimes inhibiting the vestibulo-ocular reflex (VOR). Gaze stability requires both adequate eye velocity and appropriate timing of eye movements. It is unknown whether eye velocity (VOR gain), timing (phase), or both are impaired for individuals with BVH during walking. Identifying the specific mechanism of impaired gaze stability can better inform rehabilitation options. Gaze stability was measured for eight individuals with severe BVH and eight healthy age- and gender-matched controls while performing a gaze fixation task during treadmill walking. Frequency response functions (FRF) were calculated from pitch eye and head velocity. A one-way ANOVA was conducted to determine group differences for each frequency bin of the FRF. Pearson correlation coefficients were calculated to determine the relationship between the real and imaginary parts of the FRF and the Oscillopsia Visual Analog Scale (oVAS) scores. Individuals with BVH demonstrated significantly lower gains than healthy controls above 0.5 Hz, but their phase was ideally compensatory for frequencies below 3 Hz. Higher oVAS scores were correlated with lower gain. Individuals with BVH demonstrated ideal timing for vertical eye movements while walking despite slower than ideal eye velocity when compared to healthy controls. Rehabilitation interventions focusing on enhancing VOR gain during walking should be developed to take advantage of the intact timing reported here. Specifically, training VOR gain while walking may reduce oscillopsia severity and improve quality of life.     

Vestibular function after vestibular neuritis

Abstract

Objective: To measure horizontal semicircular canal function over days, weeks, and months after an acute attack of vestibular neuritis. Design: The video head impulse test (vHIT) was used to measure the eye movement response to small unpredictable passive head turns at intervals after the attack. Study sample: Two patients diagnosed with acute right unilateral vestibular neuritis. Results: There was full restoration of horizontal canal function in one patient (A) as shown by the return of the slow phase eye velocity response to unpredictable head turns, while in the other patient (B) there was little or no recovery of horizontal canal function. Instead this second patient generated covert saccades during head turns. Conclusion: Despite the objective evidence of their very different recovery patterns, both patients reported, at the final test, being happy and feeling well recovered, even though in one of the patients there was clear absence of horizontal canal function. The results indicate covert saccades seem a successful way of compensating for loss of horizontal canal function after unilateral vestibular neuritis. Factors other than recovery of the slow phase eye velocity are significant for patient recovery.     

Invariance of vestibulo-ocular reflex gain to head impulses in pitch at different initial eye-in-orbit elevations: Implications for Alexander's law

Abstract Conclusions: These findings are in line with previous data on the horizontal vestibulo-ocular reflex (VOR) from this laboratory and suggest that eye position signals do not modulate natural vestibular responses. Hence, the Alexander's law (AL) phenomenon cannot be interpreted simply as a consequence of vestibular or oculomotor nuclei activity modulation with desired gaze. Background: AL states that the intensity of the spontaneous nystagmus of a patient with a unilateral vestibular lesion grows with increasing gaze in the direction of the fast phase. Some of the mechanisms proposed to account for the gaze effects assume a direct modification of the normal VOR by eye position signals. We tested the validity of these assumptions and investigated the effects of gaze direction on the normal vertical human VOR in the behaviorally relevant high frequency range. Methods: Head and eye movements were recorded with the search coil method during passive head impulses in pitch, while subjects were asked to hold gaze at various elevation angles in 8° steps within ± 16° from the straight ahead reference position. Results: Upward and downward head rotations produced VOR gains of similar magnitude. Furthermore, the gain remained unaffected by eye-in-orbit position for both upward and downward head impulses.     

The effect of betahistine on vestibular habituation: comparison of rotatory and sway habituation training

This study was designed to investigate the effect of histaminergic agonists and antagonists on the acquisition of vestibular habituation. The experimental animals, pigeons, were subjected to unilateral rotatory and sway habituation training sessions. The habituation of postural reflexes and post-rotatory head nystagmus was assessed. Vestibular habituation in the control group was achieved by adopting the kinetic reflex posture after approximately 9 training sessions, and after 10 and 14 training sessions, respectively for 50% reduction of the total number of beats (TNB) and the duration of post-rotatory head nystagmus. In the sway adaptation test control pigeons needed nearly 15 training sessions while pigeons receiving betahistine adapted after approximately 8 sessions. Administration of histamine and, most notably, betahistine accelerated the process, while both H1 and H2 antagonists (clemastine, cimetidine) tended to retard it, indicating a less significant contribution of H2 receptors. The cholinergic agent physostigmine strongly retarded habituation while the anticholinergic agent scopolamine markedly accelerated it. In addition the adrenomimetic agent ephedrine also accelerated habituation while the adrenolytic agent droperidol retarded reduction of nystagmus beats. The results indicate that histaminergic receptors play a significant role in the vestibular habituation mechanism but are intricately involved with other types of receptors. Betahistine is clearly the agent of choice for attenuating vestibular effects.     

Changes in postural control in healthy elderly subjects are related to vibration sensation, vision and vestibular asymmetry.

The aim of this study was to analyze the composition of sway in adults and "healthy" elderly people and to evaluate the influence of vibration sensation and asymmetric vestibular function on the sway pattern. Ten adults with a mean age of 37.5 years and 40 healthy senior citizens with a mean age of 74.6 years living independently in the community were studied. Vibration-induced body sway was measured on a force platform. The sway was analyzed and separated into its high and low frequency components above and below 0.1 Hz, respectively. Additionally the elderly subjects were observed for the occurrence of spontaneous gaze and head shake-induced nystagmus using infrared charge-coupled device cameras and the vibration perception in the lower limbs was tested with a tuning fork. Vibration perception was the major determinant for postural control in the elderly subjects. Postural control among the elderly subjects with intact vibration perception in their lower limbs was very similar to that of the adults. The elderly subjects with impaired vibration sensation had increased high frequency sway compared to adults and the elderly subjects with intact sensation. Regardless of the strong influence of vibration sensation on postural control, asymmetric vestibular function might also be a contributing factor to postural instability in the elderly. Age per se had little effect on the outcome of the tests except that the elderly subjects had diminished ability to use visual cues to reduce postural sway. We concluded that sensory status in the lower limbs is of utmost importance for postural control in the elderly. Rehabilitation programs for senior citizens should therefore include exercises to preserve recognition of body motion by the lower limbs. Exercises to facilitate vestibular compensation could be useful for elderly people with vestibular dysfunction.     

Changes in Postural Control in Healthy Elderly Subjects are Related to Vibration Sensation,Vision and Vestibular Asymmetry

The aim of this study was to analyze the composition of sway in adults and "healthy" elderly people and to evaluate the influence of vibration sensation and asymmetric vestibular function on the sway pattern. Ten adults with a mean age of 37.5 years and 40 healthy senior citizens with a mean age of 74.6 years living independently in the community were studied. Vibration-induced body sway was measured on a force platform. The sway was analyzed and separated into its high and low frequency components above and below 0.1 Hz, respectively. Additionally the elderly subjects were observed for the occurrence of spontaneous gaze and head shake-induced nystagmus using infrared charge-coupled device cameras and the vibration perception in the lower limbs was tested with a tuning fork. Vibration perception was the major determinant for postural control in the elderly subjects. Postural control among the elderly subjects with intact vibration perception in their lower limbs was very similar to that of the adults. The elderly subjects with impaired vibration sensation had increased high frequency sway compared to adults and the elderly subjects with intact sensation. Regardless of the strong influence of vibration sensation on postural control, asymmetric vestibular function might also be a contributing factor to postural instability in the elderly. Age per se had little effect on the outcome of the tests except that the elderly subjects had diminished ability to use visual cues to reduce postural sway. We concluded that sensory status in the lower limbs is of utmost importance for postural control in the elderly. Rehabilitation programs for senior citizens should therefore include exercises to preserve recognition of body motion by the lower limbs. Exercises to facilitate vestibular compensation could be useful for elderly people with vestibular dysfunction.     

Clinical assessment of postural stability following vestibular nerve section

Little is known about the recovery of postural control in patients following acute vestibular loss. This paper reports on the results of moving-platform posturography to assess the recovery of postural stability in 24 patients following vestibular nerve section. Posturography was abnormal prior to surgery in 17% of the patients. Seven days following surgery, 44% of the patients demonstrated vestibular deficit patterns, while 56% of the patients had normal posturography. Posturography was normal in all patients 1 month following surgery. Abnormal posturography was subsequently noted in 21% of the patients between 3 and 20 months following surgery. Each of the patients with abnormal preoperative posturography subsequently demonstrated abnormal late postoperative posturography. These results suggest that perioperative posturography may be useful in the evaluation and counseling of patients considering ablative vestibular surgery.     

Objective measures of physical activity in patients with chronic unilateral vestibular hypofunction,and its relationship to handicap,anxiety and postural stability

Objective

Dizziness is one of the most common symptoms in the general population. Patients with dizziness experience balance problems and anxiety, which can lead to decreased physical activity levels and participation in their daily activities. Moreover, recovery of vestibular function from vestibular injury requires physical activity. Although there are reports that decreased physical activity is associated with handicap, anxiety, postural instability and reduced recovery of vestibular function in patients with chronic dizziness, these data were collected by self-report questionnaires. Therefore, the objective data of physical activity and the relationships between physical activity, handicap, anxiety and postural stability in patients with chronic dizziness are not clear. The purpose of this research was to objectively measure the physical activity of patients with chronic dizziness in daily living as well as handicap, anxiety and postural stability compared to healthy adults. Additionally, we aimed to investigate the relationships between physical activity, handicap, anxiety and postural stability in patients with chronic dizziness.

Video head impulse test in bilateral vestibulopathy

IntroductionBilateral vestibulopathy is a rare chronic condition with multiple etiologies. Bilateral vestibulopathy is characterized mainly by unsteadiness when walking or standing, which worsens in darkness, as well as oscillopsia. The degree of handicap caused by bilateral vestibulopathy is variable and remains controversial.ObjectivesTo determine the value of the video Head Impulse Test in quantifying vestibular deficit and to establish its impact on the quality of life.MethodsTwenty patients (mean age, 41.9 years; range 14–80 years) fulfilling the recent Barany criteria of bilateral vestibulopathy, responded to the Situational Vertigo Questionnaire and underwent vestibular examination including fixation, positional tests, oculomotor test battery and video head impulse test.ResultsThe relation between each of the video head impulse test parameters and the scores from the questionnaire were statistically analyzed. We observed that patients with covert saccades on the video head impulse test were more likely to have a better quality of life than those with both covert and overt saccades, regardless of the vestibulo-ocular reflex gain in each semicircular canal. The presence of covert saccades was found to be associated with an improved quality of life regardless of the severity of vestibule ocular reflex-deficit. Our conclusion was that vestibule ocular reflex gain, measured by video head impulse test, does not quantify the severity of affection of quality of life in patients with bilateral vestibulopathy.ConclusionCovert saccades are strategies aiming at minimizing the blurring of vision during head movement, that is an adaptive mechanism that improves quality of life. Therefore, we recommend that video head impulse test should be a part of the routine diagnostic workup of bilateral vestibulopathy.     

Improvements in trunk sway observed for stance and gait tasks during recovery from an acute unilateral peripheral vestibular deficit

OBJECTIVES: Our aim was to track improvements in postural control during recovery from an acute unilateral peripheral vestibular deficit (UVL), presumably due to vestibular neuritis, and to determine if recovery rates were different for stance and gait tasks. Postural control was quantified using simple measurements of trunk sway: amplitudes of trunk sway angle and angular velocity, in the roll and pitch directions as well as task duration, were examined for a battery of stance and gait tasks. These measures were collected at the onset of the deficit and then 3 weeks and 3 months later. STUDY DESIGN: A repeated-measures design was used for UVL subjects and age-matched healthy controls. Stance tasks involved standing on 1 or 2 legs with eyes open or closed. Gait tasks consisted of tandem gait, walking normally with eyes closed, or with the head rotating or head pitching, walking up and down stairs and walking over a series of low barriers. Stance and tandem gait tasks were repeated using a foam support surface instead of a normal floor. PATIENTS: Twenty-eight patients with acute UVL were examined. MAIN OUTCOME MEASURES: The range of trunk sway angular displacement and angular velocity in the pitch and roll directions was measured for each task in addition to task duration. The measures were compared with those of normal subjects. RESULTS: The amplitudes of pitch trunk sway for 2-legged stance tasks with eyes closed underwent the greatest reduction 3 weeks after UVL onset. At 3 months, trunk sway was almost normal for all 2-legged stance tasks. One-legged stance tasks with eyes open showed a similar but slower improvement. Stance time without a fall showed a very rapid improvement for 1-legged tasks but was still shorter than that of normal subjects at 3 months. Trunk sway for the simple gait tasks was within normal range at 3 months; however, task duration was still longer than normal. More complex gait tasks, such as walking 8 tandem steps on foam or walking up and down stairs, showed no improvement in trunk roll sway at 3 months. A mix of variables from mainly gait tasks best identified a balance deficit due to UVL, with complex gait tasks becoming more important for identification purposes as compensation progressed. The accuracy of UVL identification with durations alone was 75% of the accuracy with combined trunk sway and duration measures. CONCLUSIONS: These data suggest that recovery of normal trunk control during the compensation process for unilateral vestibular hypofunction is more rapid for stance tasks than gait tasks. Even at 3 months, trunk sway for complex gait tasks was not normal. Thus, trunk sway for gait tasks provides a better insight into remaining deficits in balance control of vestibular-loss patients than the sway of stance tasks.     

Receptor function of the semicircular canals. Part 2: pathophysiology, diseases, clinical findings and treatment aspects

Perturbation of semicircular canal function may result in a pathological angular vestibulo-ocular reflex (aVOR). The resulting impairment in gaze stabilization is perceived as "vertigo" or "dizziness" and may occur following receptor function impairment of all three semicircular canals. The head impulse test reveals hidden (covert-catchup) or visible (overt-catchup) saccades in disturbances of semicircular function. Most peripheral vestibular disorders can be treated conservatively. There are surgical treatment options for some diseases, such as intractable benign paroxysmal positional vertigo and superior semicircular canal dehiscence. Vestibular training promotes central reorganization of the VOR. Impaired semicircular receptor function, in particular bilateral vestibulopathy, may affect spatial orientation and cognitive processes. Balance prostheses could serve as a replacement for receptors in the future.     

Effect of low body temperature during unilateral labyrinthectomy on vestibular compensation in the guinea pig

OBJECTIVEs: Vestibular compensation, the recovery that follows unilateral vestibular deafferentation (UVD), is a model for central nervous system plasticity. Recovery from the static symptoms of UVD may involve temperature-dependent processes that modulate the immediate effects of UVD and/or the capability of the central nervous system to undergo adaptive plasticity. In this study we investigated changes in oculomotor and postural vestibular symptoms resulting from low body temperature during UVD. MATERIAL AND METHODS: To study the effect of low temperatures at the time of UVD on vestibular compensation, we compared the rate of compensation and peak values for postural [roll head tilt (RHT) and yaw head tilt (YHT)] and oculomotor [spontaneous nystagmus (SN)] symptoms in three groups of guinea pigs. Animals in Group 1 (n = 6) were maintained at 38 degrees C throughout unilateral labyrinthectomy (UL). Animals in Group 2 (n = 6) were not temperature-controlled and animals in Group 3 (n = 4) were cooled with ice to 25 degrees C throughout UL. RESULTS: Cooled animals showed significantly higher rates of SN upon recovery from anaesthesia and took a significantly longer time to compensate. Cooled animals were also slower to compensate for postural symptoms (RHT and YHT), with 2 animals showing no compensation for RHT 52 h after UL. CONCLUSION: Hypothermia (25 degrees C) during UVD surgery exacerbates postural and oculomotor symptoms following UL and significantly slows recovery.     

Static and dynamic vestibulo-cervico-ocular responses after prolonged exposure to microgravity

The vestibular function was investigated in 13 Russian crew members of the ISS missions on days 1(2), 4(5), and 8(9) after prolonged exposure to microgravity (126 to 195 days). The static torsional otolith-cervico-ocular reflex was studied, as well as the dynamic vestibulo-cervico-ocular responses, vestibular reactivity, and spontaneous oculomotor activity using videooculography (VOG) and electrooculography (EOG) for simultaneous recording of eye movements. On days 1-2 of return to the gravity (R+1-2), the cosmonauts were found to increase the spontaneous oculomotor activity (floating eye movements, both typical and atypical forms of spontaneous nystagmus, square wave jerks, gaze nystagmus) with the head held in the vertical position. The otolith function during static head inclinations to the right or left shoulder at 30 degrees was suppressed as determined by the inversion or absence, or reduction by half of the amplitude of torsional compensatory eye counter-rolling and the vestibular reactivity during head yaw movements at 0.125 Hz was increased as revealed by a lowered threshold and an increased intensity of vestibular nystagmus. The pattern, depth, dynamics, and velocity of the vestibular function recovery varied with individual participants in the investigation. However, the suppressed otolith functioning in the period of readaptation to the normal gravity was, as a rule, accompanied by an exaggerated vestibular reactivity.     

Over-stimulation of the vestibular system and body balance

The purpose of this study was to examine whether an over-stimulation of the vestibular system, induced by thousands of time saccadic head stimulations, affects the vestibular sensitivity, and consequently if such a phenomenon could contribute to the deterioration of postural stability observed after a long distance running exercise. Eighteen athletic subjects performed a 20.5 km over ground race with an average speed of 15 km x h(-1), corresponding roughly to 7,500 strides shocks with associated saccadic accelerations transmitted to the head. A preliminary validation of the exercise protocol was realized to confirm the effect of the sustained exercise on body balance by recording standard postural parameters. A visually perceived eye level (VPEL) task was used to indirectly assess otolithic sensitivity motionless or undergoing low centrifugation conditions, before and after exercise. Results obtained from body balance analysis confirmed a decreased postural stability illustrated by increased postural oscillations after the 20.5 km run. Under low centrifugation conditions, results showed a lowering of the VPEL with the increase of the gravito-inertial acceleration in accordance with the literature. However, no significant change in the VPEL after a sustained running exercise was observed. In conclusion, the vestibular sensitivity at the otolithic level does not seem to be altered by an intensive running exercise and then failed to play a key role in the post-exercise deterioration of postural stability.     

Effect of head orientation on human postural stability following unilateral vestibular ablation.

Effective interpretation of vestibular inputs to postural control requires that orientation of head on body is known. Postural stability might deteriorate when vestibular information and neck information are not properly coupled, as might occur with vestibular pathology. Postural sway was assessed in unilateral vestibulopathic patients before and acutely, 1, 4, and 18+ months after unilateral vestibular ablation (UVA) as well as in normal subjects. Postural equilibrium with eyes closed was quantified as scaled pk-pk sway during 20 s trials in which the support surface was modulated proportionally with sway. Subjects were tested with the head upright and facing forward, turned 45 degrees right, and 45 degrees left. Equilibrium was uninfluenced by head orientation in normal subjects. In contrast, patients after UVA showed both a general reduction in stability and a right/left head orientation-dependent asymmetry. These abnormalities adaptively recovered with time. It is concluded that vestibular inputs to postural control are interpreted within a sensory-motor context of head-on-body orientation.