One second interval production task during post-rotatory sensation

Temporal intervals production of one second was found to be more variable during self-motion compared to no motion situations. Moreover, the temporal intervals production rhythm during self-motion deceleration decreased whereas it increased during self-motion acceleration, whatever the direction of motion. As somatosensory cues were not excluded in this previous experiment, we now examined whether the same temporal perturbation would occur without variable somatosensory information. In order to isolate the contribution of the vestibular system from that of the somatosensory system, the participants were required to perform a one second temporal interval production task (pressing a button each second) during the post-rotatory illusion following self-rotation. The intervals produced during the vestibular illusion were compared to those produced before the imposed rotation and during self-motion. The production regularity was affected as the temporal intervals were more variable with vestibular stimulation (real and illusory self-motion) than without. Furthermore, during post-rotatory illusion, the production rhythm decreased along the trial, as it was observed during self-motion deceleration. These findings suggest that vestibular stimulation (even vestibular illusion) impaired time estimation.     

Computer-aided techniques for the treatment and rehabilitation of patients suffering vertigo and balance disorders

A new computerized method is proposed for the correction and elimination of undesirable illusory sensations (dizziness), vestibulo-oculomotor (nystagmus), and vestibulo-postural (imbalance) reactions. The method allows to teach the subject about how to avoid generalization of afferent signals over the effector mechanisms in the central nervous system by developing a fixational reflex employing delayed biological feedback for the assessment of efforts being exerted (self-control of training results). Three variants of application of this technique were evaluated depending on the type of stimulation software intended to induce illusory and oculomotor reactions of a defined sensory modality (visual, vestibular or combined). The study involved 30 subjects divided into three groups. They had been taught using the visual (group 1), vestibular (group 2) or combined (group 3) methods. Each group was comprised of an approximately equal number of subjects with vestibulopathies of either peripheral or central origin. The study demonstrated that the proposed approach allows to invoke, with the use of a computer stimulation software, abnormal illusory and vestibulo-oculomotor responses and inhibit them by developing the adequate fixational reflex. Comparative analysis of the results obtained by teaching the patients with the help of the three different methods revealed the dependence of their effectiveness on the level of disturbances in the vestibular system. The visual method of correction proved to be especially efficacious for the patients with peripheral vestibulopathy, and the vestibular methods for the patients with central vestibulopathy. Patients with combined peripheral and central vestibulopathy required the choice of training modalities (either visual or vestibular) on an individual basis.     

Influence of visually induced self-motion on postural stability

CONCLUSION: Our results indicate that the illusion of self-motion is a significant factor leading to spatial disorientation. OBJECTIVE: Under normal circumstances, self-motion is perceived in response to motion of the head and body. However, under certain conditions, such as virtual reality environments, visually induced self-motion can be perceived even though the subject is not actually moving, a phenomenon known as "vection". The aim of this study was to examine the possible influence of illusory self-rotation (circular vection) on postural adjustments. MATERIAL AND METHODS: The subjects were 10 young females with no history of ocular or vestibular disease. Video-motion analysis was applied to measure postural movements during vertical optokinetic stimulation. RESULTS: For most subjects, movement of the visual surroundings induced head and body displacements in the same direction as that of the visual stimulus, regardless of the onset of self-motion perception. However, there was a significant increase in postural instability after the subjects began to perceive false self-motion in the opposite direction to that of the visual stimulus.     

Unilateral Vestibulotoxicity Due to Systemic Gentamicin Therapy

Systemic gentamicin can cause acute bilateral, simultaneous, symmetrical loss of vestibular function manifested by symptoms and signs of chronic vestibular insufficiency (ataxia and oscillopsia). We report 6 patients presenting with ataxia and oscillopsia, but without a history of vertigo, who had severe unilateral loss of vestibular function on caloric testing. The absence of vertigo in these patients could be explained by two possible mechanisms: either, the unilateral loss of vestibular function was subacute, occurring over several days so that compensation could occur, or bilateral vestibular loss occurred which was then followed by asymmetrical recovery of vestibular function. The second hypothesis is supported by the observation that vestibular hair cells can regenerate after aminoglycoside damage.     

Eye movements during torso rotations in labyrinthine-defective subjects

The aim of this study was to examine whether the chronic loss of vestibular function modifies perceptual and oculomotor responses during torso rotations in darkness. Subjects (4 patients with complete vestibular loss and 7 healthy volunteers) were seated on a rotating chair. Stimuli consisted of sinusoidal chair rotations (+/-30 degrees, 0.1 Hz and 0.011 Hz). We used 2 conditions: space stationary head (neck stimulation) and space stationary head and shoulders (torso stimulation). Horizontal eye deviations and slow component of eye movements were analysed. The results showed that eye movements and perception of head motion in space during neck stimulation were similar to those during torso stimulation both in normal and labyrinthine-defective (LD) subjects. During low-frequency chair rotations (0.011 Hz) all subjects perceived illusory head or head and shoulder rotation in space (as if the lower part of the body was stationary relative to the room) and shifted their gaze in the direction of illusory head rotation. In these conditions there was no significant difference in eye movements between normal and LD subjects. During higher frequency chair rotations (0.1 Hz), LD subjects had significantly larger eye deviations as well as increases in the gain of the slow component of eye movements relative to normals. In these conditions patients mostly perceived illusory head or head and shoulder rotation in space while normal subjects mainly perceived the head as stationary in space. The results indicate that 1) neck and torso rotations can evoke similar ocular responses in LD subjects, 2) the chronic loss of vestibular function modifies the representation of axial body segment motion relative to space.     

Recovery of dynamic visual acuity in unilateral vestibular hypofunction

OBJECTIVE: To determine the effect of vestibular exercises on the recovery of visual acuity during head movement in patients with unilateral vestibular hypofunction.Study DESIGN: Prospective, randomized, double-blind study. SETTING: Ambulatory referral center. PATIENTS: Twenty-one patients with unilateral vestibular hypofunction, aged 20 to 86 years. INTERVENTION: One group (13 patients) performed vestibular exercises designed to enhance the vestibulo-ocular reflex, and the other group (8 patients) performed placebo exercises. The placebo group was switched to vestibular exercises after 4 weeks. OUTCOME MEASURES: Measurements of dynamic visual acuity (DVA) during predictable (DVA-predictable) and unpredictable (DVA-unpredictable) head movements by means of a computerized test and measurement of intensity of oscillopsia by means of a visual analog scale. RESULTS: As a group, patients who performed vestibular exercises showed a significant improvement in DVA-predictable (P<.001) and DVA-unpredictable (P<.001), while those performing placebo exercises did not (P =.07). On the basis of stepwise regression analysis, the leading factor contributing to improvement was vestibular exercises. This reached significance for DVA-predictable (P =.009) but not DVA-unpredictable (P =.11). Other factors examined included age, time from onset, initial DVA, oscillopsia, and duration of treatment. Changes in oscillopsia did not correlate with DVA-predictable or DVA-unpredictable. CONCLUSIONS: Use of vestibular exercises is the main factor involved in recovery of DVA-predictable and DVA-unpredictable in patients with unilateral vestibular hypofunction. Exercises may foster the use of centrally programmed eye movements that could substitute for the vestibulo-ocular reflex. The DVA-predictable would benefit more from this than would DVA-unpredictable.     

Room tilt illusion as a manifestation of peripheral vestibular disorders

Room tilt illusion is a subjective distortion of verticality with transient paradoxical rotation of the visual field, usually in the frontal plane. It might result from dysfunction of the vestibular pathways with subsequent contradictory vestibular, visual, and proprioceptive inputs and erroneous cortical integration. It has already been described in association with brain stem and cortical lesions, but reports of cases of peripheral origin are scarce. We report here 23 cases of room tilt illusion, all but 2 occurring in patients with either vestibular peripheral abnormalities or normal assessment findings. A review of the literature is presented, as well as a hypothesis addressing this phenomenon.     

The effect of habituation and plane of rotation on vestibular perceptual responses

A technique was applied to assess vestibular sensation without reference to external spatial, position cues. The stimuli were stopping responses to velocity-steps of 90 deg/s in the dark. Subjects indicated their perceived angular velocity by turning a flywheel connected to a tachogenerator. Two separate experiments were conducted. In one, subjects were rotated in yaw about an earth-vertical axis before and after prolonged rotational or visual (optokinetic) stimuli. In the second experiment, subjects were rotated in roll supine, with either the head ('roll centred') or the feet ('roll eccentric') on the axis of rotation. The two aims of the paper were to (i) examine the effect of repetitive vestibular and optokinetic stimulation on the time constant of decay of vestibular sensation in yaw; (ii) to compare vestibular sensation responses to rotation in roll both with and without the addition of a Z-axis centrifugal force. The pre-habituation sensation response in yaw decayed exponentially with a median time constant of 12.8 s. The duration of the sensation responses were significantly reduced following both prolonged vestibular and optokinetic stimulation. The reduction in vestibular responses following prolonged visual and vestibular stimuli, 1) is likely to occur in velocity storage mechanisms mediating ocular and perceptual responses, 2) may represent a mechanism for reducing the disorientating consequences of visual-vestibular conflict and 3) supports the use of optokinetic stimuli as a treatment for vestibular patients. The time constant of the sensation responses in roll was shorter and not significantly influenced by head position: 5.7 s in the head-centred position compared to 4.7 s in the eccentric head position. Therefore, perceptual as well as ocular responses to rotation in roll are determined primarily by cupula dynamics and not influenced by velocity storage.     

Vestibular adaptation to centrifugation does not transfer across planes of head rotation

Out-of-plane head movements performed during fast rotation produce non-compensatory nystagmus, sensations of illusory motion, and often motion sickness. Adaptation to this cross-coupled Coriolis stimulus has previously been demonstrated for head turns made in the yaw (transverse) plane of motion, during supine head-on-axis rotation. An open question, however, is if adaptation to head movements in one plane of motion transfers to head movements performed in a new, unpracticed plane of motion. Evidence of transfer would imply the brain builds up a generalized model of the vestibular sensory-motor system, instead of learning a variety of individual input/output relations separately. To investigate, over two days 9 subjects performed pitch head turns (sagittal plane) while rotating, before and after a series of yaw head turns while rotating. A Control Group of 10 subjects performed only the pitch movements. The vestibulo-ocular reflex (VOR) and sensations of illusory motion were recorded in the dark for all movements. Upon comparing the two groups we failed to find any evidence of transfer from the yaw plane to the pitch plane, suggesting that adaptation to cross-coupled stimuli is specific to the particular plane of head movement. The findings have applications for the use of centrifugation as a possible countermeasure for long duration spaceflight. Adapting astronauts to unconstrained head movements while rotating will likely require exposure to head movements in all planes and directions.     

Recovery of dynamic visual acuity in bilateral vestibular hypofunction

OBJECTIVE: To determine the effect of vestibular exercises on the recovery of visual acuity during head movement in patients with bilateral vestibular hypofunction (BVH). DESIGN: Prospective, randomized, double-blinded study. SETTING: Outpatient clinic, academic setting. PATIENTS: Thirteen patients with BVH, aged 47 to 73 years. INTERVENTION: One group (8 patients) performed vestibular exercises designed to enhance remaining vestibular function, and the other (5 patients) performed placebo exercises. MAIN OUTCOME MEASURES: Measurements of dynamic visual acuity (DVA) during predictable head movements using a computerized test; measurement of intensity of oscillopsia using a visual analog scale. RESULTS: As a group, patients who performed vestibular exercises showed a significant improvement in DVA (P = .001), whereas those performing placebo exercises did not (P = .07). Only type of exercise (ie, vestibular vs placebo) was significantly correlated with change in DVA. Other factors examined, including age, time from onset, initial DVA, and complaints of oscillopsia and disequilibrium, were not significantly correlated with change in DVA. Change in oscillopsia did not correlate with change in DVA. CONCLUSIONS: Use of vestibular exercises is the main factor involved in recovery of DVA in patients with BVH. We theorize that exercises may foster the use of centrally programmed eye movements that could substitute for the vestibulo-ocular reflex. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00411216.     

Optokinetic and vestibular interactions with smooth pursuit: psychophysical responses.

The effect was evaluated in normal subjects of the subjective perception of motion of a small visual target (VT) when combined with the effect of vestibular stimulation produced by different magnitudes of constant angular accelerations in the dark or the effect of optokinetic stimulation produced by different constant velocities of rotation. The visual target appeared to the subject to travel more slowly and for a shorter duration when it moved in the direction of the body's angular acceleration or against that of the optokinetic drum. The perceived error in motion was: (i) in the same direction as the subject's motion sensation produced by either of the two stimuli, and (ii) quantitatively related, although differently, to the magnitude of each of the two stimulus modalities; an heuristic model is proposed to account for these observations.     

Influence of visually induced self-motion on postural stability

Conclusion Our results indicate that the illusion of self-motion is a significant factor leading to spatial disorientation.

Objective Under normal circumstances, self-motion is perceived in response to motion of the head and body. However, under certain conditions, such as virtual reality environments, visually induced self-motion can be perceived even though the subject is not actually moving, a phenomenon known as “vection”. The aim of this study was to examine the possible influence of illusory self-rotation (circular vection) on postural adjustments.

Material and methods The subjects were 10 young females with no history of ocular or vestibular disease. Video-motion analysis was applied to measure postural movements during vertical optokinetic stimulation.

Results For most subjects, movement of the visual surroundings induced head and body displacements in the same direction as that of the visual stimulus, regardless of the onset of self-motion perception. However, there was a significant increase in postural instability after the subjects began to perceive false self-motion in the opposite direction to that of the visual stimulus.     

Vertical linear self-motion perception during visual and inertial motion: more than weighted summation of sensory inputs

We evaluated visual and vestibular contributions to vertical self motion perception by exposing subjects to various combinations of 0.2 Hz vertical linear oscillation and visual scene motion. The visual stimuli presented via a head-mounted display consisted of video recordings of the test chamber from the perspective of the subject seated in the oscillator. In the dark, subjects accurately reported the amplitude of vertical linear oscillation with only a slight tendency to underestimate it. In the absence of inertial motion, even low amplitude oscillatory visual motion induced the perception of vertical self-oscillation. When visual and vestibular stimulation were combined, self-motion perception persisted in the presence of large visual-vestibular discordances. A dynamic visual input with magnitude discrepancies tended to dominate the resulting apparent self-motion, but vestibular effects were also evident. With visual and vestibular stimulation either spatially or temporally out-of-phase with one another, the input that dominated depended on their amplitudes. High amplitude visual scene motion was almost completely dominant for the levels tested. These findings are inconsistent with self-motion perception being determined by simple weighted summation of visual and vestibular inputs and constitute evidence against sensory conflict models. They indicate that when the presented visual scene is an accurate representation of the physical test environment, it dominates over vestibular inputs in determining apparent spatial position relative to external space.     

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.     

Können optische Sinnestäuschungen die Körperhaltung beeinflussen?

Summary It is known that visual afferences take part in the regulation of body equilibrium. Experiments have been done to answer the question, if also optical illusions have an influence on posture. Diplopia has been produced by prisms, illusory movements by stroboscopic light effects or by a spiral disc. The influence of various optical conditions has been examined in healthy subjects and patients with a well known vestibular pathology.     

Effects of vestibular training on motion sickness, nystagmus, and subjective vertical

Pitch head-and-trunk movements during constant velocity rotation are a provocative vestibular stimulus that produces vertigo and nausea. When exposed to this stimulus repeatedly, motion sickness symptoms diminish as the subjects habituate. Acetylleucine is a drug that is used to treat acute vestibular vertigo. In this study, we wanted to ascertain whether this drug (a) lessened motion sickness or delayed habituation; (b) accelerated the recovery following habituation; and (c) whether changes in the subjective vertical accompanied habituation. Twenty subjects were administered acetylleucine or placebo in a double-blind study during a five-day vestibular training. Horizontal vestibulo-ocular reflex, optokinetic nystagmus, smooth pursuit, and subjective visual vertical were evaluated before, during, and up to two months after the vestibular training. Based on Graybiel's diagnostic criteria, motion sickness decreased steadily in each vestibular training session, and there was no difference between the scores in the acetylleucine and placebo groups. Post-rotatory nystagmus peak velocity and time constant also declined in both groups at the same rate. Thus, acetylleucine neither reduced the nausea associated with this provocative stimulus, nor hastened the acquisition or retention of vestibular habituation of motion sickness and nystagmus. There was no difference in optokinetic nystagmus and smooth pursuit between the acetylleucine and placebo groups. However, subjects showed larger error in the subjective visual vertical after habituation, which indicates that spatial orientation is also affected by vestibular training.     

Adaptation to rotating artificial gravity environments

A series of pioneering experiments on adaptation to rotating artificial gravity environments was conducted in the 1960s. The results of these experiments led to the general belief that humans with normal vestibular function would not be able to adapt to rotating environments with angular velocities above 3 or 4 rpm. By contrast, our recent work has shown that sensory-motor adaptation to 10 rpm can be achieved relatively easily and quickly if subjects make the same movement repeatedly. This repetition allows the nervous system to gauge how the Coriolis forces generated by movements in a rotating reference frame are deflecting movement paths and endpoints and to institute corrective adaptations. Independent mechanisms appear to underlie restoration of straight movement paths and of accurate movement endpoints. Control of head movements involves adaptation of vestibulo-collic and vestibulo-spinal mechanisms as well as adaptation to motor control of the head as an inertial mass. The vestibular adaptation has a long time constant and the motor adaptation a short one. Surprisingly, Coriolis forces generated by natural turning and reaching movements in our normal environment are typically larger than those elicited in rotating artificial gravity environments. They are not recognized as such because self-generated Coriolis forces during voluntary trunk rotation are perceptually transparent. After adaptation to a rotating environment is complete, the Coriolis forces generated by movements within it also become transparent and are not felt although they are still present.     

Computerized dynamic visual acuity with volitional head movement in patients with vestibular dysfunction

OBJECTIVES: Patients with uncompensated vestibular dysfunction frequently report blurred vision during head movement, a symptom termed oscillopsia. One way to measure the functional deficit associated with an impaired vestibulo-ocular reflex is by comparing visual acuity from a baseline condition in which there is no head movement to visual acuity obtained during a dynamic condition with head movement. A previously described test incorporated a treadmill upon which patients walked during assessment of visual acuity. The objective of the current investigation was to evaluate an alternative method of assessing dynamic visual acuity that uses volitional head movement instead of walking on a treadmill. METHODS: Fifteen participants with normal vestibular function and 16 participants with impaired vestibular function were enrolled. All participants performed the visual acuity task under baseline conditions with no movement and also under dynamic conditions that included 1) walking on a treadmill and 2) volitionally moving their head in the vertical plane. RESULTS: No difference in performance was observed between the treadmill task and the volitional head movement task. Participants with impaired vestibular function performed more poorly under the dynamic conditions than did participants with normal vestibular function. CONCLUSIONS: The results suggest that the volitional head movement paradigm may be useful in identification of patients with functional deficits of the vestibulo-ocular reflex.     

Preserved vestibular evoked myogenic potentials (VEMP) in some patients with walking-induced oscillopsia due to bilateral vestibulopathy

Bilateral vestibulopathy, i.e. decreased peripheral vestibular function affecting both ears, is characterized by unsteadiness of gait, particularly in darkness and by motion-induced oscillopsia. We have recently seen a few patients with severely impaired semicircular canal function albeit with rather normal vestibular evoked myogenic potentials (VEMP) suggesting normal saccular function. The five young patients, mean age 27 years (range 15-45), 4 males and 1 female, had severely impaired balance in darkness and they all reported walking-induced vertical oscillopsia. Hence, these patients with incomplete vestibular lesions had symptoms that were indistinguishable from the typical patient with bilateral vestibulopathy. Further, the findings in these patients suggest that saccular function probably contributes little to prevent walking-induced vertical oscillopsia.     

Sensorimotor aspects of high-speed artificial gravity: I. Sensory conflict in vestibular adaptation

Short-radius centrifugation offers a promising and affordable countermeasure to the adverse effects of prolonged weightlessness. However, head movements made in a fast rotating environment elicit Coriolis effects, which seriously compromise sensory and motor processes. We found that participants can adapt to these Coriolis effects when exposed intermittently to high rotation rates and, at the same time, can maintain their perceptual-motor coordination in stationary environments. In this paper, we explore the role of inter-sensory conflict in this adaptation process. Different measures (vertical nystagmus, illusory body tilt, motion sickness) react differently to visual-vestibular conflict and adapt differently. In particular, proprioceptive-vestibular conflict sufficed to adapt subjective parameters and the time constant of nystagmus decay, while retinal slip was required for VOR gain adaptation. A simple correlation between the strength of intersensory conflict and the efficacy of adaptation fails to explain the data. Implications of these findings, which differ from existing data for low rotation rates, are discussed.