Vestibulo-ocular Responses Research Articles

Mechanism of Posterior Semicircular Canal Stimulation in Patients with Benign Paroxysmal Positional Vertigo

A quantitative study of the stimuli and vestibulo-ocular response associated with benign paroxysmal positional vertigo (BPPV) was made to test and further develop the canalithiasis theory of BPPV. The angular velocities of the head in the planes of the semicircular canals during the Dix-Hallpike test were measured in four healthy subjects using electromagnetic sensors to record the position of the head in a six degrees of freedom paradigm. Next, the nystagmus reactions in seven patients diagnosed with idiopathic BPPV were recorded with video-oculography. The characteristics of the vestibulo-ocular reflex (VOR) response were analyzed using three-dimensional vector techniques. The angular velocity of the head was primarily, but not exclusively, in the plane of the posterior semicircular canal (PSC) in question. Both the anterior and horizontal canals were also stimulated by a lesser degree. The duration of the motion stimulus in the PSC was <1.3 s with peak angular velocities of 150 deg/s. The eye response in BPPV patients began 4 s after the test and had a duration of 15-20 s. Peak slow-component eye velocities of about 42 deg/s were reached 3-5 s after onset of nystagmus. The motion of the eye, as predicted by the cupulolithiasis theory, is disconjugated and has torsional, vertical, and horizontal components. In the eye ipsilateral to the tested ear it is primarily torsional (0.80, 0.54, 0.16) and in the contralateral eye it is mainly vertical (0.57, 0.73, 0.08). These results suggest that particles, initially resting on the floor of the cupula dome in the PSC, are perturbed by the Dix-Hallpike test and disperse freely into the endolymph where they are propelled by gravity into the canal lumen. This creates abnormal pressure on the cupula and the specific VOR activation of the ipsilateral superior oblique and the contralateral inferior rectus muscles, whose force vectors are indistinguishable from the measured eye motion vectors. The estimated pressure exerted on the crista is approximately 10-2 dyn/cm2.

Central Versus Peripheral Origin of Vestibuloocular Reflex Recovery Following Semicircular Canal Plugging in Rhesus Monkeys

We have previously shown that there is a slowly progressing, frequency-specific recovery of the gain and phase of the horizontal vestibuloocular reflex (VOR) in rhesus monkeys following plugging of the lateral semicircular canals. The adapted VOR response exhibited both dynamic and spatial characteristics that were distinctly different from responses in intact animals. To discriminate between adaptation or recovery of central versus peripheral origin, we have tested the recovered vestibuloocular responses in three rhesus monkeys in which either one or both coplanar pairs of vertical semicircular canals had been plugged previously by occluding the remaining semicircular canals in a second plugging operation. We measured the spatial tuning of the VOR in two or three different mutually orthogonal planes in response to sinusoidal oscillations (1.1 Hz, +/-5 degrees, +/-35 degrees /s) over a period of 2-3 and 12-14 mo after each operation. Apart from a significant recovery of the torsional/vertical VOR following the first operation we found that these recovered responses were preserved following the second operation, whereas the responses from the newly operated semicircular canals disappeared acutely as expected. In the follow-up period of up to 3 mo after the second operation, responses from the last operated canals showed recovery in two of three animals, whereas the previously recovered responses persisted. The results suggest that VOR recovery following plugging may depend on a regained residual sensitivity of the plugged semicircular canals to angular head acceleration.

The three-dimensional vestibulo-ocular reflex during prolonged microgravity.

Single-case, longitudinal studies of the three-dimensional vestibulo-ocular response (VOR) were conducted with two spaceflight subjects over a 180-day mission. For reference, a control study was performed in the laboratory with 13 healthy volunteers. Horizontal, vertical and torsional VOR was measured during active yaw, pitch and roll oscillations of the head, performed during visual fixation of real and imaginary targets. The control group was tested in the head-upright position, and in the gravity-neutral, onside and supine positions. Binocular eye movements were recorded throughout using videooculography, yielding eye position in Fick co-ordinates. Eye velocity was calculated using quaternion algebra. Head angular velocities were measured by a head-mounted rate sensor. Eye/head velocity gain and phase were evaluated for the horizontal, vertical and torsional VOR. The inclination of Listing's plane was also calculated for each test session. Control group gain for horizontal and vertical VOR was distributed closely around unity during real-target fixation, and reduced by 30-50% during imaginary-target trials. Phase was near zero throughout. During head pitch in the onside position, vertical VOR gain did not change significantly. Analysis of up/down asymmetry indicated that vertical VOR gain for downward head movement was significantly higher than for upward head movement. Average torsional VOR gain with real-target fixation was significantly higher than with imaginary-target fixation. No difference in phase was found. In contrast to vertical VOR gain, torsional VOR gain was significantly lower in the gravity-neutral supine position. Spaceflight subjects showed no notable modification of horizontal or vertical VOR gain or phase during real-target fixation over the course of the mission. However, the up/down asymmetry of vertical VOR gain was inverted in microgravity. Torsional VOR gain was clearly reduced in microgravity, with some recovery in the later phase. After landing, there was a dip in gain during the first 24 h, with subsequent recovery to near baseline over the 13-day period tested. Listing's plane appeared to remain stable throughout the mission. The findings reflect various functions of the otolith responses. The reduced torsional VOR gain in microgravity is attributed to the absence of the gravity-dependent, dynamic stimulation to the otoliths (primarily utricles). On the other hand, the reversal of vertical VOR up/down gain asymmetry in microgravity is attributed to the off-loading of the constant 1-g bias (primarily to the saccules) on Earth. The observed increase in torsional VOR gain from the 1st to the 6th month in microgravity demonstrates the existence of longer-term adaptive processes than have previously been considered. Likely factors are the adaptive reweighting of neck-proprioceptive afferents and/or enhancement of efference copy.

Convergent properties of vestibular-related brain stem neurons in the gerbil.

Three classes of vestibular-related neurons were found in and near the prepositus and medial vestibular nuclei of alert or decerebrate gerbils, those responding to: horizontal translational motion, horizontal head rotation, or both. Their distribution ratios were 1:2:2, respectively. Many cells responsive to translational motion exhibited spatiotemporal characteristics with both response gain and phase varying as a function of the stimulus vector angle. Rotationally sensitive neurons were distributed as Type I, II, or III responses (sensitive to ipsilateral, contralateral, or both directions, respectively) in the ratios of 4:6:1. Four tested factors shaped the response dynamics of the sampled neurons: canal-otolith convergence, oculomotor-related activity, rotational Type (I or II), and the phase of the maximum response. Type I nonconvergent cells displayed increasing gains with increasing rotational stimulus frequency (0.1-2.0 Hz, 60 degrees /s), whereas Type II neurons with convergent inputs had response gains that markedly decreased with increasing translational stimulus frequency (0.25-2.0 Hz, +/-0.1 g). Type I convergent and Type II nonconvergent neurons exhibited essentially flat gains across the stimulus frequency range. Oculomotor-related activity was noted in 30% of the cells across all functional types, appearing as burst/pause discharge patterns related to the fast phase of nystagmus during head rotation. Oculomotor-related activity was correlated with enhanced dynamic range compared with the same category that had no oculomotor-related response. Finally, responses that were in-phase with head velocity during rotation exhibited greater gains with stimulus frequency increments than neurons with out-of-phase responses. In contrast, for translational motion, neurons out of phase with head acceleration exhibited low-pass characteristics, whereas in-phase neurons did not. Data from decerebrate preparations revealed that although similar response types could be detected, the sampled cells generally had lower background discharge rates, on average one-third lower response gains, and convergent properties that differed from those found in the alert animals. On the basis of the dynamic response of identified cell types, we propose a pair of models in which inhibitory input from vestibular-related neurons converges on oculomotor neurons with excitatory inputs from the vestibular nuclei. Simple signal convergence and combinations of different types of vestibular labyrinth information can enrich the dynamic characteristics of the rotational and translational vestibuloocular responses.

Fragmentary control of vestibuloocular responses.

Intact pigeons (n = 64) were rotated in the dark in the horizontal plane at different orientations relative to the axis of rotation. The overall patterns of changes in nystagmus of the eyes arising as a result of displacement of the otolith membranes in several directions were analyzed. In ten pigeons, all changes in nystagmus (type 1 general patterns) could be explained in terms of the dynamics of peripheral neuron activity and non-specific (identical for all combinations of interacting inputs) central influences. In the remaining pigeons, part of the change in nystagmus (type 2 general patterns) was associated with central influences which were not identical for different combinations of interacting inputs. Repeated unusual combinations of vestibular stimuli and subsequent treatment with Nembutal transformed type 2 general patterns into type 1 general patterns. These data provide evidence for the fragmentary control of eye movements, whereby there is selective (fragmentary) modification of only some (individually specific) combinations of canal and otolith signals out of the whole set of vestibuloocular responses arising in response to stimulation of paired vestibular inputs; modification is mediated by changes in the sign of otolith influences on the canal components of these responses.

Vestibular compensation and substitution.

This is a very brief update on the major papers since August 1998. Unilateral vestibular loss causes oculomotor, postural and sensory symptoms, all of which would be appropriate responses in a healthy person to a strong maintained angular and linear acceleration stimulus directed towards the healthy side. Within hours or days these static symptoms (so called because they are present without any externally imposed vestibular stimulation) reduce, and their progressive disappearance is called 'vestibular compensation'. However, careful testing with natural vestibular stimuli shows that the dynamic vestibular response after unilateral vestibular loss to passively imposed vestibular stimuli does not recover; it is usually asymmetric and functionally ineffective. Major recent developments are: (1) the permanent asymmetrical and functionally ineffective dynamic rotational vestibulo-ocular reflex responses to passive natural vestibular stimulation after unilateral vestibular loss and canal blocks in human patients; (2) evidence for the substitution of other sensory input and responses during vestibular compensation; (3) perceptual testing using visual perception of a horizontal line to confirm permanent otolith dysfunction; (4) the clear and substantial differences in post-unilateral vestibular loss vestibulo-ocular reflex responses between passive and active head turning; and (5) new results in brainstem physiology explaining the disappearance of static symptoms.

Early components of the human vestibulo-ocular response to head rotation: latency and gain.

To characterize vestibulo-ocular reflex (VOR) properties in the time window in which contributions by other systems are minimal, eye movements during the first 50-100 ms after the start of transient angular head accelerations ( approximately 1000 degrees /s(2)) imposed by a torque helmet were analyzed in normal human subjects. Orientations of the head and both eyes were recorded with magnetic search coils (resolution, approximately 1 min arc; 1000 samples/s). Typically, the first response to a head perturbation was an anti-compensatory eye movement with zero latency, peak-velocity of several degrees per second, and peak excursion of several tenths of a degree. This was interpreted as a passive mechanical response to linear acceleration of the orbital tissues caused by eccentric rotation of the eye. The response was modeled as a damped oscillation (approximately 13 Hz) of the orbital contents, approaching a constant eye deviation for a sustained linear acceleration. The subsequent compensatory eye movements showed (like the head movements) a linear increase in velocity, which allowed estimates of latency and gain with linear regressions. After appropriate accounting for the preceding passive eye movements, average VOR latency (for pooled eyes, directions, and subjects) was calculated as 8.6 ms. Paired comparisons between the two eyes revealed that the latency for the eye contralateral to the direction of head rotation was, on average, 1.3 ms shorter than for the ipsilateral eye. This highly significant average inter-ocular difference was attributed to the additional internuclear abducens neuron in the pathway to the ipsilateral eye. Average acceleration gain (ratio between slopes of eye and head velocities) over the first 40-50 ms was approximately 1.1. Instantaneous velocity gain, calculated as Veye(t)/Vhead(t-latency), showed a gradual build-up converging toward unity (often after a slight overshoot). Instantaneous acceleration gain also converged toward unity but showed a much steeper build-up and larger oscillations. This behavior of acceleration and velocity gain could be accounted for by modeling the eye movements as the sum of the passive response to the linear acceleration and the active rotational VOR. Due to the latency and the anticompensatory component, gaze stabilization was never complete. The influence of visual targets was limited. The initial VOR was identical with a distant target (continuously visible or interrupted) and in complete darkness. A near visual target caused VOR gain to rise to a higher level, but the time after which the difference between far and near targets emerged varied between individuals.

Aging effects on auditory and vestibular responses: A longitudinal study

Purpose: To identify age-related changes in the auditory and vestibular systems in normal older subjects followed serially over time. Methods: Pure tone hearing levels, speech reception threshold, speech discrimination scores, and vestibulo-ocular responses were measured in 57 normal older subjects (mean age, 82) on 5 yearly examinations. Results: There was a significant ( P < .001) increase in pure tone threshold averages (about 1 dB per year) and speech reception threshold averages (about 2 dB per year), and a significant ( P < .001) decrease in discrimination scores (about 2% per year) over the 5 years of follow-up. There was a significant ( P < .05) amplitude-dependent decrease in gain and increase in phase lead of the vestibulo-ocular reflex and a significant ( P < .01) decrease in gain of visual-vestibular responses at low frequency sinusoidal stimulation over the 5 examinations. Years 1 to 5 differences in individual subjects were significantly ( P < .01) correlated across auditory and vestibular tests, but not between auditory and vestibular tests ( P > .01). Conclusion: This is the first longitudinal study to show age-related decreases in auditory and vestibular responses in normal older subjects. Because the age-related changes in the auditory system were not correlated with age-related changes in the vestibular system, the 2 systems may age at different rates in the same individual. These changes likely result from degeneration in both peripheral and central structures, although age-related changes in the central nervous system probably are more important from a functional point of view.

Short-latency primate vestibuloocular responses during translation.

Short-lasting, transient head displacements and near target fixation were used to measure the latency and early response gain of vestibularly evoked eye movements during lateral and fore-aft translations in rhesus monkeys. The latency of the horizontal eye movements elicited during lateral motion was 11.9 +/- 5.4 ms. Viewing distance-dependent behavior was seen as early as the beginning of the response profile. For fore-aft motion, latencies were different for forward and backward displacements. Latency averaged 7.1 +/- 9.3 ms during forward motion (same for both eyes) and 12.5 +/- 6.3 ms for the adducting eye (e.g., left eye during right fixation) during backward motion. Latencies during backward motion were significantly longer for the abducting eye (18.9 +/- 9.8 ms). Initial acceleration gains of the two eyes were generally larger than unity but asymmetric. Specifically, gains were consistently larger for abducting than adducting eye movements. The large initial acceleration gains tended to compensate for the response latencies such that the early eye movement response approached, albeit consistently incompletely, that required for maintaining visual acuity during the movement. These short-latency vestibuloocular responses could complement the visually generated optic flow responses that have been shown to exhibit much longer latencies.

Vestibular function in children with the CHARGE association.

Histopathological examinations and computed tomographic scans of the temporal bone in patients with the CHARGE association (a malformative syndrome that includes coloboma, heart disease, choanal atresia, retarded development, genital hypoplasia, and ear anomalies, including hypoplasia of the external ear and hearing loss) have shown an absence of semicircular canals and a Mondini form of cochlear dysplasia. Until recently, no information was available concerning a possible loss of vestibular function, which could be a factor in retarded posturomotor development. To our knowledge, this is the first report of otolith tests done on patients with the CHARGE association. To test residual vestibular function in patients with the CHARGE association. In 7 patients with the CHARGE association, we made electro-oculographic recordings of vestibulo-ocular responses to earth-vertical and off-vertical axis rotations to evaluate the function of the canal and the otolith-vestibular systems. None of the 7 patients had semicircular canals in the computed tomographic scan, and none had canal vestibulo-ocular responses to earth-vertical axis rotation, but all had normal otolith vestibulo-ocular responses to the off-vertical axis rotation test. These results support the hypothesis of a residual functional otolith organ in the hypoplastic posterior labyrinth of children with the CHARGE association. The severe delays in psychomotor development presented by these children are more likely a consequence of multiple factors: canal vestibular deficit, visual impairment, and environmental conditions (long hospital stays and breathing and feeding problems). The remaining sensitivity of the otolith system to gravity and linear acceleration forces in these children could be exploited in early education programs to improve their posturomotor development.

Vestibulo-Ocular Responses During Static Head Roll and Three-Dimensional Head Impulses After Vestibular Neuritis

This study aimed to investigate whether unilateral vestibular neuritis (VN) causes the same deficits of ocular counter-roll during static head roll (OCRS) and dynamic vestibulo-ocular reflex gains during head impulses (VORHI) as unilateral vestibular deafferentation (VD). Ten patients with acute and 14 patients with chronic vestibular paralysis after VN were examined. The testing battery included fundus photography of both eyes with the head upright (binocular cyclorotation) and dual search coil recordings in a three-field magnetic frame. With one dual search coil on the right eye and the other on the forehead, the following stimuli were given: i) Halmagyi-Curthoys head impulses about the vertical, horizontal and torsional axes. ii) Static roll positions of the head up to 20° right- and left-ear-down by movement of the neck. The comparison group consisted of 19 healthy subjects. Compared with the VD-patients, as reported in the literature, acute VN-patients showed the same pattern of OCRS gain reduction and binocular cyclorotation (CRb). The main feature that distinguished chronic VN-patients from chronic VD-patients was the normalization of the torsional VORHI gain to the affected side, whereas the VORHI gains in the horizontal and vertical directions did not show recovery (as in the patients with chronic VD). Chronic VN-patients differed from acute VN-patients by: i) symmetrical OCRS gains, ii) a less pronounced CRb toward the affected side, and iii) a normal torsional VORHI gain toward the affected side. Since the ipsilesional torsional VORHI gain did not recover in VD-patients, the normalization of this gain in our VN-patients can only be explained by a (partial) recovery of otolith function on the side of the lesion after the neuritis.

Vestibulo-oculomotor research and measurement technology for the space station era

Recent improvements in measurement techniques and mathematical representations for eye, head and body movement have enhanced our appreciation of the complexity of spatial orientation and locomotion in three-dimensional space. The shortcomings of present measurement techniques, and their solution with emerging technologies are described. The prolonged microgravity conditions on the space station provide a unique opportunity to investigate these three-dimensional aspects of the vestibular and oculomotor systems, and in particular, the role of the otolith afferences. While the canal–ocular responses and their central pathways are reasonably well understood, the community has only recently become aware of the variety of functions fulfilled by otolith-mediated information, i.e., translational otolith–ocular reflex, inertial processing, gravitational reference, vergence control. Recent results, largely from experiments performed on the Mir Station, where the emphasis was on the otolith contribution to the vestibulo-ocular response mechanisms, are reviewed.

Dynamic characteristics and adaptability of mouse vestibulo-ocular and optokinetic response eye movements and the role of the flocculo-olivary system revealed by chemical lesions.

The dynamic characteristics of reflex eye movements were measured in two strains of chronically prepared mice by using an infrared television camera system. The horizontal vestibulo-ocular reflex (HVOR) and horizontal optokinetic response (HOKR) were induced by sinusoidal oscillations of a turntable, in darkness, by 10 degrees (peak to peak) at 0.11-0.50 Hz and of a checked-pattern screen, in light, by 5-20 degreesat 0.11-0.17 Hz, respectively. The gains and phases of the HVOR and HOKR of the C57BL/6 mice were nearly equivalent to those of rabbits and rats, whereas the 129/Sv mice exhibited very low gains in the HVOR and moderate phase lags in the HOKR, suggesting an inherent sensory-motor anomaly. Adaptability of the HOKR was examined in C57BL/6 mice by sustained screen oscillation. When the screen was oscillated by 10 degrees at 0.17 Hz, which induced sufficient retinal slips, the gain of the HOKR increased by 0.08 in 1 h on average, whereas the stimuli that induced relatively small or no retinal slips affected the gain very little. Lesions of the flocculi induced by local applications of 0. 1% ibotenic acid and lesions of the inferior olivary nuclei induced by i.p. injection of 3-acetylpyridine in C57BL/6 mice little affected the dynamic characteristics of the HVOR and HOKR, but abolished the adaptation of the HOKR. These results indicate that the olivo-floccular system plays an essential role in the adaptive control of the ocular reflex in mice, as suggested in other animal species. The data presented provide the basis for analyzing the reflex eye movements of genetically engineered mice.

Asymmetric otolith vestibulo-ocular responses in children with idiopathic scoliosis.

A suggested cause of idiopathic scoliosis (IS) in children is a disequilibrium in the vestibulospinal control of trunk muscles. We sought a correlation between otolith vestibular dysfunction and IS. A recently developed test for evaluation of otolith vestibular function (off-vertical axis rotation, OVAR) was applied to 30 children with IS, 12 control subjects, and 3 with congenital scoliosis as a result of spinal deformities. Of the patients with IS, 67% had significantly greater values of directional preponderance on the OVAR test (a measure of otolith system imbalance) compared with control subjects. Patients with congenital scoliosis showed normal responses on the OVAR test. No correlation was found between the direction of the preponderance and the side of the spine imbalance, or between the directional preponderance and the curve magnitude. The rate of progression of the scoliosis was not significantly correlated with the amplitude of the directional preponderance. These results support the hypothesis that central otolith vestibular system disorders lead to a vestibulospinal system imbalance, and may be a factor in the cause of IS.

Development of the vestibulo-ocular responses in toddlers learning to walk

Development of the vestibulo-ocular responses in toddlers learning to walk

Long-term deficits in otolith, canal and optokinetic ocular reflexes of pigmented rats after unilateral vestibular nerve section

Static and dynamic otolith, horizontal vestibular and optokinetic ocular reflexes were investigated in pigmented rats 1-6 and more months after unilateral vestibular nerve (UVN) section. Evoked responses were compared with published data from control rats studied under identical conditions. Static lateral tilt of UVN rats in the light evoked a vertical deviation in static eye position that was as large as in controls. In darkness, the evoked responses in UVN rats 6 months after the lesion were consistently smaller than in controls. Linear horizontal acceleration in darkness evoked vertical and torsional response components in UVN rats that were parallel-shifted towards lower gains and larger phase lags. Off-vertical axis rotation on a platform provoked responses that differed markedly from those recorded in intact rats with respect to the bias velocity component. These results suggest a permanent deficiency in the static and dynamic otolith-ocular reflex performance of UVN rats. Ocular responses to horizontal table velocity steps in darkness exhibited a direction-specific asymmetry in UVN rats. Step responses evoked by acceleration towards the intact side were larger in gain and longer in duration than responses evoked by acceleration towards the operated side. When compared with control data, responses to either side were reduced in UVN rats and the velocity store mechanism was barely activated by velocity steps towards the operated side. Responses evoked by horizontal optokinetic stimulation with constant pattern velocities were below control values in either direction. Slow-phase eye velocity saturated at much lower values than in intact rats, particularly during pattern motion towards the intact side. The duration of the optokinetic afternystagmus was asymmetrically reduced with respect to control data. Practically identical reductions in duration were found for vestibulo-ocular responses in the opposite directions. Behaving animals exhibited no obvious impairment in their spontaneous locomotory or exploratory activities. However, each UVN rat was impaired, even 2 years after the lesion, in its postural reaction to being lifted by the tail in the air. This observation suggests the presence of a permanent deficit in static and dynamic otolith-spinal reflexes that may be substituted on the ground by proprioceptive inputs.

2211 Dynamic characteristics and adaptation of horizontal vestibulo-ocular reflex and optokinetic response in mice

2211 Dynamic characteristics and adaptation of horizontal vestibulo-ocular reflex and optokinetic response in mice

“Torso Rotation” Experiments; 1: Adaptation to Motion Sickness does not Correlate with Changes in VOR Gain

Following a 30-minute exposure to an unusual motor strategy called "Torso Rotation" (TR), the signs and symptoms of motion sickness appear along with perceptual illusions during movement, gaze and postural instability, and a significantly reduced vestibulo-ocular response (VOR) gain. With repeated exposure to TR, the motion sickness disappears and gaze instability seems to be reduced. Is this apparent improvement in gaze stability associated with a reduction of the transient change in VOR gain? Motion sickness (subjective questionnaire) and VOR gain (passive step rotations in darkness) were measured before and repeatedly after TR on 7 consecutive days. Despite a complete loss of symptoms in 3 to 4 days, the transient, daily change in VOR gain remained unaffected. Furthermore, there was no increase in the use of compensatory saccades. It is concluded that adaptation to TR-induced motion sickness is not the result of a change in VOR's sensitivity to TR.

“Torso Rotation” Experiments; 2: Gaze Stability During Voluntary Head Movements Improves with Adaptation to Motion Sickness

Following a 30-minute exposure to an unusual motor strategy called "Torso Rotation" (TR), the signs and symptoms of motion sickness appear along with perceptual illusions during movement, gaze and postural instability, and a significantly reduced vestibulo-ocular response (VOR) gain. With repeated exposure to TR, motion sickness symptoms disappear and gaze instability seems reduced, but without any concomitant change in VOR gain. Is the reduction in gaze instability a perceptual illusion or a real, measurable phenomenon? Velocity gain (eye velocity/ head velocity) was evaluated during voluntary head shaking in the light over the frequency range 0.3 to 3.0 Hz. A significant improvement was seen after 3 days of testing (P < 0.01). Furthermore, the time course of improvement in gaze stability was correlated with the loss of motion sickness symptoms reported in the previous study (1). We suggest that adaptation to motion sickness could be related to an overall change in sensori-motor strategy, perhaps including a de-emphasis of a vestibular reference.

Visual-Vestibular Interaction During Off-Vertical Axis Rotation1

The aim of this study was to further define the eye movement response to combined visual and vestibular stimulation, especially during linear acceleration. Subjects included 15 asymptomatic healthy individuals (8 females and 7 males) between the ages of 20 and 31 years. Vestibular stimulation consisted of earth-vertical axis rotation and off-vertical axis rotation (OVAR). Visual stimuli consisted of projected vertical stripes that were rotated for optokinetic trials and stationary for visual augmentation trials. A small laser target (0.5 mW, 0.5 degree arc) that rotated with the subject was used for fixation trials. Eye movements were measured with electro-oculography. Results showed that visual-vestibular interaction during sinusoidal rotation was not affected by a 15 degree off-vertical tilt. Constant velocity OVAR induced a continuous nystagmus whose slow component velocity contained a nonzero baseline, that is, a bias, and a periodic fluctuation at the rotation frequency, that is, a modulation component. The modulation component during visual fixation was reduced as compared with that seen during rotation in the dark, but was not absent. Constant velocity OVAR in the presence of earth-fixed stripes induced a consistent sinusoidal modulation. Our results suggest that visual-vestibular interaction for otolith stimulation differs from visual-vestibular interaction for semicircular canal stimulation. The modulation component of the response to OVAR appears to be modified by visual stimulation to a lesser extent than other vestibular-induced eye movements and thus may reflect a more "direct" vestibulo-ocular response. The bias component of the response to OVAR can be substantially influenced by vision and thus may depend upon more "indirect" pathways.