Unilateral peripheral semicircular canal lesion and off-vertical axis rotation

The Spectrum of Vestibular Disorders Presenting With Acute Continuous Vertigo

Objective

To explore the composition of vestibular disorders presenting with the acute vestibular syndrome (AVS).

Methods

We performed a case analysis of 209 AVS patients between January 2016 and December 2020. These patients were grouped into different disorder categories according to the relevant diagnostic criteria.

Results

We classified the 209 patients into 14 disorder categories, including 110 cases of vestibular neuritis, 30 of idiopathic sudden sensorineural hearing loss with vertigo, 17 of the first attack of continuous vertigo with migraine, 15 of Ramsay Hunt syndrome, 11 of acute labyrinthitis secondary to chronic otitis media, 8 of vestibular schwannoma, 6 of posterior circulation infarction and/or ischemia, 3 of cerebellar abscess secondary to chronic otitis media, 3 of AVS caused by trauma or surgery, 2 of AVS with down-beating nystagmus, 1 of multiple sclerosis of the medulla oblongata, 1 of epidermoid cyst of the posterior cranial fossa, 1 of a probable acute otolithic lesion, and 1 of AVS without measurable vestibular dysfunction.

Conclusion

When a group of disorders present with AVS, characteristic clinical manifestations and imaging help with an accurate diagnosis.

Videoconference-Based Adapted Physical Exercise Training Is a Good and Safe Option for Seniors

Videoconference-based adapted physical exercise combines the benefits of supervised exercise training with staying at home, when conventional training is inaccessible. However, exercising with the use of a screen can be considered an optokinetic stimulation, and could therefore induce changes in sensory processing, affecting postural stability. The objectives of this study were to compare the effectiveness of the training delivered Face-to-Face and by Videoconferencing in improving physical capacities of older adults, and to evaluate the possible effects of the Videoconference mode on the processing of sensory information that could affect postural control. Twenty eight older adults underwent the supervised exercise program for sixteen weeks either Face-to-Face or by Videoconference. Muscular strength of knee and ankle flexors and extensors, maximum oxygen uptake, postural stability and horizontal rotational vestibulo-ocular reflex were evaluated before and after the training. Both modes of training similarly increased the VO₂ peak and strength of the motor muscles of lower limbs in all participants. The use of the Videoconference did not modify the vestibulo-ocular reflex in subjects or the importance of vision for postural control. Therefore, the Videoconference-based exercise training can be considered a safe and effective way to maintain good functional capacity in seniors.

Rotational testing

The natural stimulus for the semicircular canals is rotation of the head, which also might stimulate the otolith organs. Vestibular stimulation usually induces eye movements via the vestibulo-ocular reflex (VOR). The orientation of the subject with respect to the axis of rotation and the orientation of the axis of rotation with respect to gravity together determine which labyrinthine receptors are stimulated for particular motion trajectories. Rotational testing usually includes the measurement of eye movements via a video system but might use a subject's perception of motion. The most common types of rotational testing are whole-body computer-controlled sinusoidal or trapezoidal stimuli during earth-vertical axis rotation (EVAR), which stimulates primarily the horizontal semicircular canals bilaterally. Recently, manual impulsive rotations, known as head impulse testing (HIT), have been developed to assess individual horizontal semicircular canals. Most types of rotational stimuli are not used routinely in the clinical setting but may be used in selected research environments. This chapter will discuss clinically relevant rotational stimuli and several types of rotational testing that are used primarily in research settings.

Evaluation of the vestibulo-ocular reflex using sinusoidal off-vertical axis rotation in patients with canal paresis

OBJECTIVE

The vestibulo-ocular reflex (VOR) was studied to determine the utility of off-vertical axis rotation (OVAR) in evaluating vestibular function in patients with canal paresis (CP). Our goal was to determine whether there is any correlation between caloric responses and sinusoidal rotatory responses.

METHODS

Subjects were rotated in a sinusoidal pattern with eyes open in complete darkness. Frequencies of 0.4 and 0.8Hz with a maximum angular velocity of 60°/s at either earth-vertical axis rotation (EVAR) or OVAR were used.

RESULTS

Twenty-three control subjects and 21 patients with CP were investigated. Results showed that (1) the VOR gain difference between EVAR and OVAR in the CP patients was not significant at 0.4Hz and at 0.8Hz; (2) the gain during 0.4Hz EVAR was less in the bilateral CP patients compared to controls; and (3) the VOR gain of the affected side at 0.8Hz was significantly less during OVAR than during EVAR.

CONCLUSION

We concluded that the absence or reduction of caloric responses does not indicate the absence of vestibular function. In addition, separation analysis of the VOR gain of affected and intact sides is useful for evaluating laterality of otolith function in patients with CP.

Impaired smooth pursuit in schizophrenia results from prediction impairment only

BACKGROUND

Oculomotor abnormality is one of the endophenotypes in schizophrenia. The predictive component of smooth pursuit can be studied by comparing the gain, i.e., the ratio of smooth eye position to target position, during predictable (pure sinusoidal) and unpredictable (pseudorandom) target motions. The aim of this experiment was to study predictive and nonpredictive components of smooth pursuit in two groups of schizophrenia patients compared with control subjects.

METHODS

Fifty-one schizophrenia patients (40 nondeficit and 11 deficit) and 21 control subjects were studied. During a predictable task, subjects were asked to track a sinusoidal target (.4 Hz). For the unpredictable task, the pseudorandom target motion consisted of five superimposed sinusoidal waveforms (.1, .2, .4, .6, and .8 Hz). The smooth eye position (eye position without saccades), gain, and phase were calculated for each frequency in each participant and for both tasks.

RESULTS

The mean sinusoidal smooth eye position gain was significantly lower in patients than in control subjects with no significant difference between deficit and nondeficit patients. During the pseudorandom task, all groups had a similar gain at .4 Hz.

CONCLUSIONS

Our study reveals that patients have a normal nonpredictive component of smooth pursuit, regardless of their level of negative symptoms. In contrast, the predictive mechanisms involved in eye pursuit were impaired in schizophrenia patients. These results indicate that poor pursuit performance during smooth pursuit is primarily a consequence of a predictive problem and is not related to the ability to generate an accurate pursuit maintenance response.

Vestibulo-ocular reflex and motion sickness in figure skaters

In order to determine the effect of figure skating on the functional plasticity of the vestibular system, we quantified vestibulo-ocular reflex (VOR) and motion sickness (MS) intensity in 11 female figure skaters and 11 matched control subjects. Vestibular stimulation consisted of three cycles of sinusoidal rotation (0.025 Hz, +/-60 degrees /s) and two velocity steps of 60 degrees /s (acceleration 60 degrees /s(2)). Nauseogenic stimulation consisted of a constant velocity (60 degrees /s) off vertical axis rotation (OVAR) using a 15 degrees tilt angle. Subjective sickness symptoms were rated immediately after OVAR with the Pensacola diagnostic index. During sinusoidal stimulations, the skaters' VOR, as compared with that of the controls, demonstrates a gain that is 27% lower (0.44 +/- 0.12 vs. 0.58 +/- 0.10; P < 0.01) and a phase advance (10 +/- 12 degrees vs. -0.3 +/- 6.4 degrees ; P < 0.05). During velocity steps, the VOR gain is 32% lower among the skaters (0.52 +/- 0.14 vs. 0.71 +/- 0.12; P < 0.01), but there is no difference in time constant (10.8 +/- 1.8 s vs. 10.5 +/- 2.7 s; P = 0.78). Nauseogenic stimulation evokes significantly less MS in figure skaters than in control subjects (2.8 +/- 2.8 vs. 16.2 +/- 13.7; P < 0.01). Quantitative alterations in VOR parameters observed in figure skaters probably result from vestibular habituation induced by repeated unusual stimulations when practicing figure skating.

Motion sickness induced by otolith stimulation is correlated with otolith-induced eye movements

This article addresses the relationships between motion sickness (MS) and three-dimensional (3D) ocular responses during otolith stimulation. A group of 19 healthy subjects was tested for motion sickness during a 16 min otolith stimulation induced by off-vertical axis rotation (OVAR) (constant velocity 60 degrees /s, frequency 0.16 Hz). For each subject, the MS induced during the session was quantified, and based on this quantification, the subjects were divided into two groups of less susceptible (MS-), and more susceptible (MS+) subjects. The angular eye velocity induced by the otolith stimulation was analyzed in order to identify a possible correlation between susceptibility to MS and 3D eye velocity. The main results show that: (1) MS significantly correlates in a multiple regression with several components of the horizontal vestibular eye movements i.e. positively with the velocity modulation (P<0.01) and bias (P<0.05) of the otolith ocular reflex and negatively with the time constant of the vestibulo-ocular reflex (P<0.01) and (2) the length of the resultant 3D eye velocity vector is significantly larger in the MS+ as compared with the MS- group. Based on these results we suggest that the CNS, including the velocity storage mechanism, reconstructs an eye velocity vector modulated by head position whose length might predict MS occurrence during OVAR.

Are otolithic inputs interpreted better in figure skaters?

The aim of this study was to investigate whether figure skaters, as individuals who experience intense vestibular stimulation, presented modification of the otolith-ocular reflex. The reflexes of 12 figure skaters were assessed using off vertical axis rotation (OVAR). Horizontal otolith-ocular reflex during OVAR is characterized by two parameters: the eye velocity horizontal modulation, assumed to compensate for perceived lateral linear translation, and the bias, assumed to compensate for the perceived rotation. We observed that skaters presented smaller amplitude of modulation and truly compensatory bias compared with control participants. Thus, the otolithic signal during OVAR seems to be interpreted more as rotation and less as translation or inclination in figure skaters.

Total sleep deprivation can increase vestibulo-ocular responses

The effect of sleep deprivation on the vestibular function is largely unknown. Some studies have found that postural balance or vestibular reflexes are decreased in sleep-deprived subjects while others found no change. The aim of this study was to evaluate the effect of sleep deprivation on the vestibulo-ocular reflex (VOR). Horizontal eye movements were recorded in healthy subjects during earth vertical axis rotation in darkness once after an ordinary night sleep and once after 26-29 h of sleep deprivation. In the first experiment (n = 8), for which rotation was a 60 degrees s(-1) velocity step, sleep deprivation induced a significant increase in VOR gain. In the second experiment (n = 12), for which rotation was sinusoidal (0.2 Hz +/- 25 degrees s(-1)), sleep deprivation induced no significant modification in VOR gain. The difference between the two studies was the abrupt onset of the step stimulation in comparison with the sinusoidal rotation. Because of its unexpected onset and the potential threat to postural balance, the step stimulation may activate the system specialized in reorienting attention towards salient or behaviourally relevant events. This system includes the right temporoparietal cortex, an area also involved in VOR control. A number of studies have found that sleep deprivation alters the activity of this cortical area during attentional tasks. It is therefore our hypothesis that the difference between the effects of these two vestibular stimulations results from a sleep deprivation-induced modulation of the right temporoparietal cortex.

Spatial coding capacity of central otolith neurons

This review focuses on recent approaches to unravel the capacity of otolith-related brainstem neurons for coding head orientations. In the first section, the spatiotemporal features of central vestibular neurons in response to natural otolithic stimulation are reviewed. Experiments that reveal convergent inputs from bilateral vestibular end organs bear important implications on the processing of spatiotemporal signals and integration of head orientational signals within central otolith neurons. Another section covers the maturation profile of central otolith neurons in the recognition of spatial information. Postnatal changes in the distribution pattern of neuronal subpopulations that subserve the horizontal and vertical otolith systems are highlighted. Lastly, the expression pattern of glutamate receptor subunits and neurotrophin receptors in otolith-related neurons within the vestibular nuclear complex are reviewed in relation to the potential roles of these receptors in the development of vestibular function.

Total sleep deprivation effect on disengagement of spatial attention as assessed by saccadic eye movements

OBJECTIVE

Previous research has shown that total sleep deprivation (TSD) of short duration (one night) affects performance to some cognitive tasks subserved by a fronto-parietal network. The aim of our study was to assess the effects of TSD on visuo-spatial attention, which is a cognitive task involving this network. Specifically, the disengagement of spatial attention was investigated with gap and overlap paradigms of saccadic eye movements.

METHODS

Ten healthy young male subjects performed the two tasks the morning after a normal night and after a TSD night. The study was conducted using a balanced, crossover design.

RESULTS

TSD significantly increased the gap effect (difference of latency between overlap and gap).

CONCLUSIONS

This result can be interpreted as an impaired disengagement of attention after TSD. As the peak velocity, which is an indicator of alertness, was not altered by TSD, the impairment in the disengagement of spatial attention does not result from a decrease in alertness.

SIGNIFICANCE

This study shows that saccadic eye movements enable studying alertness and disengagement of spatial attention simultaneously. The idea that specific brain areas are affected by TSD is confirmed by our results.

Are eye movement abnormalities indicators of genetic vulnerability to schizophrenia?

Fifty to eighty-five percent of schizophrenic patients are impaired on ocular pursuit paradigms. However, results regarding the relatives are more discordant. The aim of this study was to investigate whether eye movement disorders could be a vulnerability marker of schizophrenia.

METHOD

Twenty-one schizophrenic patients (DSM-IV), 31 first-degree relatives of those patients without schizophrenic spectrum disorders, and two groups of healthy controls matched by age and sex were included. Three oculomotor tasks (smooth pursuit, reflexive saccades and antisaccades) were used.

RESULTS

Patients had a lower averaged gain (P= 0.035) during smooth pursuit than controls, made less correct visually guided saccades (P< 0.001) and more antisaccades errors (P= 0.002) than controls. In contrast, none of the comparison between the relatives and their controls was significant.

CONCLUSION

Schizophrenic patients were impaired on smooth pursuit and antisaccade paradigms. None of these impairments was, however, observed in their first-degree relatives. Our results suggest that the eye movement parameters tested could not be considered as vulnerability markers for schizophrenia.

Early visual evoked potentials are modulated by eye position in humans induced by whole body rotations

Background

To reach and grasp an object in space on the basis of its image cast on the retina requires different coordinate transformations that take into account gaze and limb positioning. Eye position in the orbit influences the image's conversion from retinotopic (eye-centered) coordinates to an egocentric frame necessary for guiding action. Neuroimaging studies have revealed eye position-dependent activity in extrastriate visual, parietal and frontal areas that is along the visuo-motor pathway. At the earliest vision stage, the role of the primary visual area (V1) in this process remains unclear. We used an experimental design based on pattern-onset visual evoked potentials (VEP) recordings to study the effect of eye position on V1 activity in humans.

Results

We showed that the amplitude of the initial C1 component of VEP, acknowledged to originate in V1, was modulated by the eye position. We also established that putative spontaneous small saccades related to eccentric fixation, as well as retinal disparity cannot explain the effects of changing C1 amplitude of VEP in the present study.

Conclusions

The present modulation of the early component of VEP suggests an eye position-dependent activity of the human primary visual area. Our findings also evidence that cortical processes combine information about the position of the stimulus on the retinae with information about the location of the eyes in their orbit as early as the stage of primary visual area.

Residual effects of hypnotics on disengagement of spatial attention

In an earlier study, we postulated that the residual effects of hypnotics could induce a spatial attention disengagement deficit independent of any decrease in alertness. To test this hypothesis, we compared the residual effects of zolpidem, zopiclone and flunitrazepam in two ocular saccade tests, gap and overlap. In the gap paradigm, the lateral target is illuminated 200 ms after the extinguishing of the central target. In the overlap paradigm, the central target stayed on when the lateral target was illuminated. Zopiclone increases latency in the overlap, but not in the gap test, which appears to be specific to a deficit of disengagement of spatial attention. Zopiclone impairs the saccadic precision in gap, but not in overlap, which may be interpreted as an impairment of visuospatial memory. The effects of zolpidem are limited to visuospatial impairment. The effects of flunitrazepam are massive and probably the results of a decrease in alertness.

Saccadic and smooth-pursuit eye movements in deficit and non-deficit schizophrenia

We have analyzed eye movement performances in schizophrenics showing primary negative or deficit symptoms (n=16) and non-deficit schizophrenics (n=55), and compared them with those of controls (n=34) in order to study the relationships between negative symptoms and eye movement abnormalities. Patients were subtyped into deficit and non-deficit subgroups using the Schedule for the Deficit Syndrome. Three oculomotor paradigms were used: smooth pursuit, a reflexive saccade paradigm and an antisaccadic task. The smooth pursuit gain was significantly decreased (and the rate of catch-up saccades increased) in schizophrenics as compared with controls, but no difference was observed between patient groups. In the reflexive saccade paradigm, no difference was found between controls and patients, except for latency in deficit patients. In the antisaccade paradigm, the number of errors and the latency of successful antisaccades were significantly increased in schizophrenics as compared with controls. The latency of successful antisaccades in both directions was significantly increased in deficit patients as compared with non-deficit patients. The latency of rightward successful antisaccades was significantly increased as compared with the latency of leftward antisaccades in deficit patients only. However, when patients were classified into negative and non-negative groups using the PANSS, no difference was found in the antisaccade paradigm. Smooth pursuit impairment does not seem to depend on the primary enduring negative symptoms.In deficit schizophrenics, the abnormalities observed in the antisaccadic task are consistent with prefrontal dysfunction, and may suggest parietal lobe dysfunction as well.

Motion sickness occurrence does not correlate with nystagmus characteristics

The purpose of this study was to test the hypothesis whereby eye movements as such may be an important factor in the development of motion sickness (MS). The horizontal eye movements of 27 subjects were measured during earth vertical axis rotation (EVAR) and during off vertical axis rotation (OVAR). Two groups were set up, one including subjects who suffered severe MS during the test, and the other including subjects with no MS symptoms. We found no differences in nystagmus parameters (EVAR: gain and time constant; OVAR: eye velocity modulation and eye position modulation) between the MS and the non-MS groups. We can conclude that eye movements are not involved in the development of MS.

PET study of the human foveal fixation system

Positron emission tomography (PET) was used to investigate the functional anatomy of the foveal fixation system in 10 subjects scanned under three different conditions: at rest (REST), during the fixation of a central point (FIX), and while fixating the same foveal target during the presentation of peripheral visual distractors (DIS). Compared with the REST condition, both FIX and DIS tasks activated a common set of cortical areas. First, in addition to the involvement of the occipital visual cortex, both the frontal eye field (FEF) and the intraparietal sulcus (IPS) were bilaterally activated. Right frontal activation was also found in the dorsolateral prefrontal cortex, the inferior part of the precentral gyrus, and the inferior frontal gyrus. These results suggest that both FEF and IPS may constitute the main cortical regions subserving bilaterally the foveal fixation system in humans. The remaining right frontal activations may be considered as part of the anterior attentional network, supporting a role for the right frontal lobe in the allocation of the attentional mechanisms. Compared with the FIX condition, the DIS task also revealed the perceptual and cognitive processes related to the presence of peripheral visual distractors during foveal fixation. In addition to a bilateral activation of the V5/MT motion-sensitive area, a right FEF-IPS network was activated which may correspond to the engagement of the visuospatial attention. Finally, normalized regional cerebral blood flow (NrCBF) decreases were also observed during both DIS and FIX condition performance. Such NrCBF decreases were centered in the superior and middle temporal gyri, the prefrontal cortex, and the precuneus and the posterior retrosplenial part of the cingulate gyrus.

Low intensity galvanic vestibulo-ocular reflex in normal subjects

An electrical stimulation in man applied between the two mastoids could facilitate the distinction between labyrinthine and retrolabyrinthine lesions by stimulating directly the primary vestibular afferences. However, for this test to be really effective in current medical practice, the results obtained in normal subjects must be symmetrical and reproducible one day to another. The ocular responses induced by a constant electrical stimulation of 2.5 mA, applied between the two mastoids for 30 s (electrically evoked vestibulo-ocular reflex [EVOR]), in one direction and the other, were quantified in ten healthy subjects. Each subject was studied in two different sessions separated by 1 week. Horizontal eye movements were recorded in darkness by an infrared light reflection eye-tracking system. Slow-phase velocity and nystagmus frequency were about 20% higher when the cathode was on the right mastoid than when it was on the left mastoid. This directional preponderance (DP) displayed large individual differences between the two sessions. The reproducibility of the reflectivity (mean of right and left EVOR) was high (r about 0.8). The weak reproducibility of the DP makes the EVOR at weak intensity inadequate to evaluate unilateral vestibular hypofunction. On the other hand, because of the high reproducibility of reflectivity, the EVOR should be effective in detecting bilateral vestibular hypofunction. Moreover, because of the weak intensity of stimulation, no local anaesthesia is needed so the manoeuvre is easy to repeat in case of chronic diseases.

Asymmetric otolith vestibulo-ocular responses in children with idiopathic scoliosis

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

Short latency vestibular evoked potentials (VsEPs) to linear acceleration impulses in rats

In this study, short latency (t < 12.7 ms) vestibular evoked potentials (VsEPs) in response to linear acceleration impulses were recorded in 37 rats. A new technique (based on a solenoid) was used for generating linear force impulses that were delivered to the animal's head. The impulse had a maximal peak acceleration of 12 g. During the impulse, the displacement was 50 microns (at 4 g) and the rise time was 1.0 ms. A stimulation rate of 2/s was usually used. The VsEPs (averaged responses to 128 stimulations, digital filter: 300-1500 Hz) were recorded with electrodes on pinna and vertex, and were composed of 4-6 clear waves with mean amplitudes (for a 4 g stimulus) of 1-5 microV. The VsEPs were resistant to white noise masking, and were significantly suppressed (P < 0.05) following bilateral application of a saturated KCl solution to the inner ear, showing that contributions of the auditory and somatosensory systems are negligible. The latency of the response decreased as a power law function of stimulus magnitude, and the amplitude of the first wave increased as a sigmoid function of stimulus magnitude. VsEP responses were still present at the lowest intensities attainable (0.06-0.4 g) and reached saturation at 9 g. The amplitude of the later components was reduced when stimulus rate was elevated to 20/s. These results suggest that VsEPs in response to linear accelerations are similar in their nature to VsEPs in response to angular acceleration impulses that were previously recorded. These VsEPs to linear accelerations are most likely initiated in the otolith organs.

Unilateral vestibular neuritis with otolithic signs and off-vertical axis rotation

Off-vertical axis rotation (OVAR) at constant velocity is a dynamic otolith stimulus that induces horizontal and vertical eye movement responses. To determine the value of this examination as a test for unilateral otolithic hypofunction, we compared the OVAR responses of patients suffering from acute vestibular neuritis (VN) without any sign of otolith affection, with those of patients suffering from acute VN with otolithic signs. The horizontal eye movement bias component shows directional preponderance (DP) significantly higher in patients with otolithic signs than in patients not presenting them. However, as bias DP also reflects the imbalance between right and left horizontal canals activity, this greater bias DP could be explained by the more severe canals impairment-evaluated by caloric test-found in patients with otolithic signs. No significant difference can be shown on horizontal modulation. The DP of vertical modulation is significantly higher in patients presenting otolithic signs than in patients not presenting them: in the case of otolithic signs, the responses are smaller during rotations toward the affected side. Therefore, this variable could be used as an indication of unilateral otolithic hypofunction.

Ocular counterrolling in response to asymmetric radial acceleration

The fact that the labyrinths are positioned at approx. 3.5 cm from the head-centric axis leads to the generation of systematic radial acceleratory, or centrifugal, forces during physiological head rotations. Under physiological conditions, the forces exerted on the right and left maculae are generally assumed to be equal and opposite. In the present test, however, the subject was displaced eccentrically during constant angular velocity so that one labyrinth was aligned concentrically with the rotatory axis, whilst the other was exposed to the radial acceleration component generated by the chair rotation. In comparison with previous testing based on subjective setting of the visual vertical, three-dimensional eye movements were recorded in order to obtain an objective measure of the otolith-ocular response mechanisms in the brainstem. The results obtained from normal subjects demonstrate a predominant ocular counterroll (OCR) response to this asymmetric, or unilateral application of linear acceleration. The direction of the OCR response proved to be independent of direction of rotation. The findings demonstrate a symmetrical response magnitude for stimulation of the left and right maculae. In comparison with the OCR response to head-tilt, which can be described as critically damped, the time course of the present response appeared to be underdamped.