Ocular counterrolling as an indicator of vestibular otolith function

Atypical motor neuron disease with supranuclear vertical gaze palsy and slow saccades

In amyotrophic lateral sclerosis (ALS), eye movements are usually preserved even after the long-term use of respirators. The present study evaluated a 57-year-old male patient who showed clinical findings compatible with ALS but exhibited disorders of eye movements before he needed to be on an artificial respiration system. The patient had noted clumsiness and weakness in all extremities 5 years before presentation of abnormal eye movements. The results of electromyography and muscle biopsy were compatible with ALS. However, supranuclear vertical gaze palsy and slow saccades are seen. The present case might be representative of a distinct clinical entity, motor neuron disease with disorders of eye movement.

Vertical (Z-axis) acceleration alters the ocular response to linear acceleration in the rabbit

Whether ocular orientation to gravity is produced solely by linear acceleration in the horizontal plane of the head or depends on both horizontal and vertical components of the acceleration of gravity is controversial. Here, we compared orienting eye movements of rabbits during head tilt to those produced by centrifugation that generated centripetal acceleration along the naso-occipital (X-), bitemporal (Y-) and vertical (Z-) axes in a constant gravitational field. Sensitivities of ocular counter-pitch and vergence during pitch tilts were approximately 25 degrees /g and approximately 26 degrees /g, respectively, and of ocular counter-roll during roll tilts was approximately 20 degrees /g. During X-axis centripetal acceleration with 1 g of gravity along the Z-axis, pitch and vergence sensitivities were reduced to approximately 13 degrees /g and approximately 16 degrees /g. Similarly, Y-axis acceleration with 1g along the Z-axis reduced the roll sensitivity to approximately 16 degrees /g. Modulation of Z-axis centripetal acceleration caused sensitivities to drop by approximately 6 degrees /g in pitch, approximately 2 degrees /g in vergence, and approximately 5 degrees /g in roll. Thus, the constant 1g acceleration along the Z-axis reduced the sensitivity of ocular orientation to linear accelerations in the horizontal plane. Orienting responses were also modulated by varying the head Z-axis acceleration; the sensitivity of response to Z-axis acceleration was linearly related to the response to static tilt. Although the sign of the Z-axis modulation is opposite in the lateral-eyed rabbit from that in frontal-eyed species, these data provide evidence that the brain uses both the horizontal and the vertical components of acceleration from the otolith organs to determine the magnitude of ocular orientation in response to linear acceleration.

The coordination of binocular eye movements: vertical and torsional alignment

Precise binocular alignment of the visual axes is of utmost importance for good vision. The fact that so few of us ever experience diplopia is evidence of how well the oculomotor system performs this function in the face of changes due to development, disease and injury. The capacity of the oculomotor system to adapt to visual stimuli that mimic alignment deficits has been extensively explored in laboratory experiments. While the present paper reviews many of those studies, the primary focus is on issues involved in maintaining good vertical and torsional alignment in everyday viewing situations where the parsing of muscle forces may vary for the same horizontal and vertical eye positions due to changes in horizontal vergence and head posture.

Testing utricular function by means of on-axis rotation

CONCLUSIONS

Subjective visual vertical (SVV) estimation during on-axis rotation provides an efficient screening test of utricle function. The survey demonstrates that isolated disorders of peripheral utricular function can occur while SCC function appears normal.

OBJECTIVE

The present study aimed to investigate estimation of SVV during constant velocity yaw rotation (with the head held on-axis--to enhance any asymmetry between right and left utricular responses), as a useful screening test.

MATERIALS AND METHODS

In all, 230 patients were recruited from the dizziness clinic. For each patient, the SVV was estimated (a) while held stationary, and (b) during constant angular velocity (240 degrees/s), with the head centred on-axis. Bithermal caloric testing was also performed in 201 of the patients.

RESULTS

Of those patients with normal SVV results during stationary testing, 18.3% were pathological during rotation testing. In those cases with pathological SVV during stationary testing, a significantly greater deviation from the norm was observed during rotation (p<0.001). Of those patients with normal caloric responses, 44.4% showed pathological SVV estimates; this increased to 54.3% for cases with unilateral weakness, and 56.5% for unilateral loss. No clear correlation was found between reports of tilt illusion and pathological SVV, respectively, between rotatory vertigo and pathological caloric responses.

Dissociated Hysteresis of Static Ocular Counterroll in Humans

In stationary head roll positions, the eyes are cyclodivergent. We asked whether this phenomenon can be explained by a static hysteresis that differs between the eyes contra- (CE) and ipsilateral (IE) to head roll. Using a motorized turntable, healthy human subjects ( n = 8) were continuously rotated about the earth-horizontal naso-occipital axis. Starting from the upright position, a total of three full rotations at a constant velocity (2°/s) were completed (acceleration = 0.05°/s2, velocity plateau reached after 40 s). Subjects directed their gaze on a flashing laser dot straight ahead (switched on 20 ms every 2 s). Binocular three-dimensional eye movements were recorded with dual search coils that were modified (wires exiting inferiorly) to minimize torsional artifacts by the eyelids. A sinusoidal function with a first and second harmonic was fitted to torsional eye position as a function of torsional whole body position at constant turntable velocity. The amplitude and phase of the first harmonic differed significantly between the two eyes (paired t-test: P < 0.05): on average, counterroll amplitude of IE was larger [CE: 6.6 ± 1.6° (SD); IE: 8.1 ± 1.7°), whereas CE showed more position lag relative to the turntable (CE: 12.5 ± 10.7°; IE: 5.1 ± 8.7°). We conclude that cyclodivergence observed during static ocular counterroll is mainly a result of hysteresis that depends on whether eyes are contra- or ipsilateral to head roll. Static hysteresis also explains the phenomenon of residual torsion, i.e., an incomplete torsional return of the eyes when the first 360° whole body rotation was completed and subjects were back in upright position (extorsion of CE: 2.0 ± 0.10°; intorsion of IE: 1.4 ± 0.10°). A computer model that includes asymmetric backlash for each eye can explain dissociated torsional hysteresis during quasi-static binocular counterroll. We hypothesize that ocular torsional hysteresis is introduced at the level of the otolith pathways because the direction-dependent torsional position lag of the eyes is related to the head roll position and not the eye position.

A clinical test of otolith function: static ocular counterroll with passive head tilt

When roll-tilted around the naso-occipital axis, humans exhibit compensatory torsional rotation of the eyes in the opposite direction owing to the torsional vestibulo-ocular reflex. In the static condition (sustained head roll), the utricles act as responsible sensors for 'static ocular counterroll'. Contributions of cervico-ocular reflexes remain unknown. To find an easy, clinically useful test of utricular function, we induced ocular counterroll in 10 healthy study participants (two men, mean age 27+/-2 years) under three stimulation conditions (active/passive head tilt and passive whole body tilt in roll plane), used three-dimensional video-oculography to measure it, and compared values. Active head-tilt-induced ocular counterroll varied most and was thus less reliable than passive head and body tilt-induced ocular counterroll. Utricular function can thus be tested simply by measuring passive head tilt with video-oculography.

Dynamic pitch rotation affects eye torsion

CONCLUSION

Most of the subjects studied had eye torsion responses to pitch, although the direction of torsion varied between subjects. Opposite responses may be the result of individual variation in anatomical or physiological vector orientations of hair cells in the anterior or posterior utricle or in the saccule.

OBJECTIVE

This study aimed to determine whether systematic changes in eye torsion occur when subjects are rotated in forward and backward pitch.

MATERIALS AND METHODS

Twenty-one normal subjects were seated in a dual axis human rotator, positioned so that the interaural axis was aligned with the axis of pitch rotation. Fixation LED suppressed vertical or horizontal eye movement. Recordings were carried out in darkness apart from the fixation LED, using a three-dimensional eye tracker based on CMOS image sensors. Subjects were twice tilted from upright to 90 degrees occiput down, then forward to 45 degrees face down.

RESULTS

Most subjects had eye torsion changes in response to pitch, with mean amplitudes of approximately 2 degrees to 90 degrees backward tilt and 1 degree to 45 degrees forward tilt. Ten subjects had clockwise torsion to backward pitch and counterclockwise to forward pitch; six subjects had the opposite responses. Statistical testing of the distributions of the regression slopes between these two groups were significant (p<0.001). Five subjects had unclear responses.

Skew Deviation Revisited

Skew deviation is a vertical misalignment of the eyes caused by damage to prenuclear vestibular input to ocular motor nuclei. The resultant vertical ocular deviation is relatively comitant in nature, and is usually seen in the context of brainstem or cerebellar injury from stroke, multiple sclerosis, or trauma. Skew deviation is usually accompanied by binocular torsion, torticollis, and a tilt in the subjective visual vertical. This constellation of findings has been termed the ocular tilt reaction. In the past two decades, a clinical localizing value for skew deviation has been assigned, and a cogent vestibular mechanism for comitant and incomitant variants of skew deviation has been proposed. Our understanding of skew deviation as a manifestation of central otolithic dysfunction in different planes of three-dimensional space is evolving. The similar spectrum of vertical ocular deviations arising in patients with congenital strabismus may further expand the nosology of skew deviation to include vergence abnormalities caused by the effects of early binocular visual imbalance on the developing visual system.

Subjective Visual Horizontal and Stabilometer Findings in Patients with Unilateral Severe Vestibular Dysfunction

The records of both stabilometer and subjective visual horizontal (SVH) testing can be used to evaluate vestibular compensation. When results of SVH testing have a strong significant correlation to those of stabilometer testing, clinicians can omit one of two tests from the test battery. To investigate the correlation, results of these two tests of patients with unilateral severe vestibular dysfunction were studied. Twenty-eight patients, 20 men and 8 women, with unilateral severe vestibular dysfunction were enrolled in this study. The correlation between these two tests was considered not significant according to the results. In conclusion, we cannot omit either stabilometer testing or SVH testing for the evaluation of patients with vestibular dysfunction.

Residual Torsion Following Ocular Counterroll

A recent study on static ocular counterroll suggested the existence of residual torsion (RT): when healthy subjects repositioned their head to the upright position after sustained static tilt, eye position differed from the original ocular torsion measured prior to the static head tilt. Our experiments aimed at further characterizing this phenomenon. Using a three-dimensional motorized turntable, healthy human subjects (n = 8) were rotated quasi-statically (0.05 deg/s2, 2 deg/s velocity plateau reached after 40 s) from the upright position about the naso-occipital axis. Three full whole-body rotations were completed while subjects fixed upon a blinking laser dot straight ahead in otherwise complete darkness. Three-dimensional eye movements were recorded with modified dual search coils (wires exiting inferiorly). Torsional position of the right eye at consecutive upright body positions was analyzed. The torsional eye position before the beginning of the chair rotation was defined as zero torsion. On average, the right eye was intorted by 1.3 degrees or extorted by 2.0 degrees after the first full chair rotation in the clockwise or counterclockwise direction, respectively. These torsional offset values of the right eye did not significantly change after the two subsequent full chair rotations. We conclude that RT observed after static ocular counterroll is the result of static hysteresis, that is, a position lag of the eye, which depends on the direction of head roll. The fact that residual torsion did not further increase after the first rotation cycle emphasizes that RT is a static rather than a dynamic phenomenon.

Dose- and duration-dependent effects of betahistine dihydrochloride treatment on histamine turnover in the cat

Drugs interacting with the histaminergic system are currently used for vertigo treatment and it was shown in animal models that structural analogues of histamine like betahistine improved the recovery process after vestibular lesion. This study was aimed at determining the possible dose and duration effects of betahistine treatment on histamine turnover in normal adult cats, as judged by the level of messenger RNA for histidine decarboxylase (enzyme synthesizing histamine) in the tuberomammillary nuclei. Experiments were conducted on betahistine-treated cats receiving daily doses of 2, 5, 10, or 50 mg/kg during 1 week, 3 weeks, 2 months, or 3 months. The 1-week, 3-week, and 2- and 3-month treatments correspond to the acute, compensatory, and sustained compensatory stages of vestibular compensation, respectively. The lowest dose (2 mg/kg) given the longest time (3 months) was close to the dosage for vestibular defective patients. Data from the experimental groups were compared to control, untreated cats and to placebo-treated animals. The results clearly show that betahistine dihydrochloride administered orally in the normal cat interferes with histamine turnover by increasing the basal expression level of histidine decarboxylase mRNA of neurons located in the tuberomammillary nuclei of the posterior hypothalamus. The effects were both dose- and time-dependent. In conclusion, compensation of both static and dynamic deficits is subtended by long-term adaptive mechanisms that could be facilitated pharmacologically using betahistine dihydrochloride.

Torsional optokinetic nystagmus after unilateral vestibular loss: asymmetry and compensation

The aim of this study was to analyse torsional optokinetic nystagmus (tOKN) in 17 patients with Menière's disease before and after (1 week, 1 month and 3 months) a curative unilateral vestibular neurotomy (UVN). The tOKN was investigated during optokinetic stimulations around the line of sight directed towards either the lesioned or the healthy side, at various constant angular velocities. Dynamic properties of tOKN and static ocular cyclotorsion were analysed using videonystagmography. Patients' performances were compared with those of 10 healthy subjects. The results indicate that, in the acute stage after UVN, patients exhibited drastic impairment of tOKN velocity that depended on the direction of stimulation: tOKN velocity increased for ipsilesional stimulations and decreased for contralesional stimulations. These changes were responsible for a dramatic tOKN asymmetry, with ipsilesional directional preponderance of torsional slow-phase eye velocity. The changes were associated with static ocular cyclotorsion towards the operated side. Despite progressive compensation of tOKN deficits over time, tOKN velocity still differed from that recorded preoperatively, and tOKN asymmetry remained uncompensated 3 months after UVN. A static ocular cyclotorsion remained up to 3 months after lesion. These results are the first description of tOKN deficits and recovery after unilateral vestibular loss. They show that vestibular cues contribute to gaze stabilization during optokinetic stimulation around the line of sight. They also strongly suggest that tOKN impairment could be part of the long-term asymmetrical functions reported after unilateral loss of vestibular functions.

Short latency responses in the averaged electro-oculogram elicited by vibrational impulse stimuli applied to the skull: could they reflect vestibulo-ocular reflex function?

OBJECTIVES

To investigate whether vibrational impulse stimuli applied to the skull can be used to evoke the vestibulo-ocular reflex (VOR) and detect vestibular lesions.

METHODS

Twenty four patients with unilateral vestibular loss (UVD), five with bilateral vestibular loss, two with ocular palsies, and 10 healthy subjects participated. Vibrations of the skull were induced with head taps and with a single period of 160 Hz tone burst on the inion, vertex, and the mastoids while the patients viewed a distant target. Several patients were also examined while viewing a near target, with eccentric gaze and in tilted postures. Responses were recorded by EOG.

RESULTS

Responses occurred between 5 ms and 20 ms and seemed to be compensatory to the second phase of the sine wave of vibration impulse and were greatly diminished/absent in patients with bilateral VD and ocular palsies. The patients with UVD had asymmetrical responses in the vertical EOG with stimuli applied on the inion and vertex, with enhancement of the response amplitude on the side of vestibular loss and/or diminution on the healthy side. The asymmetry ratios between the healthy subjects and patients with UVD, and among patients with UVD were statistically significant. Some gaze and positional influences could be demonstrated consistent with otolithic reflexes.

CONCLUSION

If the asymmetric responses to skull vibration in UVD result from passive oscillatory movements of the orbital tissues they may reflect the otolith mediated sustained skew torsion. Conversely, if generated by active eye movements, their likely origin is a phasic VOR.

Initiation and cancellation of the human heave linear vestibulo-ocular reflex after unilateral vestibular deafferentation

The effect of unilateral vestibular deafferentation (UVD) on the linear vestibulo-ocular reflex (LVOR) was studied in 11 humans an average of 52 months following surgical UVD. Controls consisted of seven healthy age-matched subjects. The LVOR was evoked by directionally random, transient whole body interaural (heave) translation with a peak acceleration of 0.5 g while subjects viewed earth-fixed (LVOR) and head-fixed (cancellation) targets 15, 25, and 200 cm distant. The magnitude of the LVOR slow phase was inversely proportional to target distance for both subject groups. Neither latency nor the magnitude of the LVOR significantly differed in the ipsi- vs contralesional directions (P>0.1) in UVD. When the target disappeared at heave onset, subjects with UVD had LVOR slow phase displacement 100 ms later that was 5% of ideal at 15 cm, 6% at 25 cm, and 16% at 200 cm. This was significantly less than corresponding control values of 41, 43, and 50%. During cancellation the LVOR magnitude 100 ms from heave onset was reduced at all target distances by an average of 40+/-4%, and the relative reduction did not significantly differ between controls and subjects with UVD (P>0.1). Cancellation latency did not vary significantly among target distances or subject groups. It is concluded that after UVD, the LVOR is bilaterally and symmetrically reduced but remains modulated by viewing distance and cancellation effort.

The effect of gravity on the stability of human eye orientation

OBJECTIVE

The stabilization of both the horizontal (H) and vertical (V) eye movements during voluntary fixation is believed to depend upon the visual feedback system in the upright position. However, ocular stability in the tilted position has been less well investigated. Therefore, in the present study, we examined the gaze stability of healthy human subjects in the three dimensions in the tilted position using a video image analysis system (VIAS).

METHODS

In 10 healthy human subjects, the eye movements were recorded after fixating the eye on a target in an upright position and also in the tilted position. The standard deviations of the eye movements in the three dimensions were calculated to evaluate the stability of the movements.

RESULTS

In the tilted position, there were no significant changes in the horizontal and vertical eye movements as compared those in the upright position. However, the standard deviation of the torsional (T) segment was significantly larger in the tilted position, compared to that in the upright position.

CONCLUSION

From these results, we speculate that, a combination of otolith and somatosensory inputs play a major role in maintaining the stability of eye movements.

Effect of side-down tilt on optokinetic nystagmus and optokinetic after-nystagmus in cats

Slow phase velocity (SPV) of optokinetic nystagmus (OKN) and after-nystagmus (OKAN) was examined in the cat in side-down tilts. In the upright position, the axis of SPV (direction of SPV vector) during OKN was always close to the stimulus axis. In side-down positions, yaw stimulation induced OKN with the vector deviating from the stimulus axis toward the pitch-axis, so as to make the SPV rotational plane be shifted toward the earth horizontal plane, while pitch-axis stimulation induced no change in vector direction. This yaw-to-pitch cross-coupling is qualitatively similar to that previously described in primates. SPV vector size during yaw stimulation is greatest when the stimulus axis is oriented vertically, and the vector size is smaller when the stimulus is in the gravity direction than when it is in the anti-gravity direction. The SPV vector for the rapid-rise response showed no clear change with head orientation, indicating that the direct optokinetic pathway has no contribution to the induction of cross-coupling. Cross-coupling was found also during OKAN. The SPV trajectories were fitted well using the velocity storage integrator model. SPV vector change of cat OKN/OKAN in head tilt could be fitted by changing gain elements in the model.

Orientation of Listing's plane during static tilt in young and older human subjects

Three-dimensional eye positions, when expressed as rotation vectors, are constrained to lie in a head-fixed Listing's plane. The offset and orientation of Listing's plane changes when the head is tilted. To assess the influence of age on this phenomenon, young (less than 30 years old) and older (>65 years old) human subjects were seated upright, pitched nose up and nose down, and rolled right ear down and left ear down. Listing's plane was computed from eye movements recorded using a dual scleral search coil while subjects scanned a complex visual scene. During pitch, Listing's plane counterpitched with respect to the head, while during roll, it translated in a manner consistent with "ocular counterrolling". There was no significant difference in this reorientation of Listing's plane between the young and older subjects. The only obvious difference between the two age groups was that the "thickness" of Listing's plane was greater in the older subjects. This suggests that aging has a small, but definite, influence on Listing's law.

Functional asymmetry estimated by measurements of otolith in fish

It is widely accepted that the incidence of space adaptation syndrome (SAS) is due to a mismatch of sensory information from various receptors to the central nervous system. We investigated the functional asymmetry of vestibular organ, which may caused sensory conflict in space, by measuring the weight difference of otolith between left and right side in goldfish and carp. In the goldfish utricular otolith, the maximum difference was 0.8 mg and the mean difference was 0.091 mg. The percentage of weight difference to the heavier otolith was calculated. The maximum difference was 20.57% and the mean was 3.035%. A difference exceeding 10% was found in only 2 goldfish. In the carp utricular otolith, the maximum percentage difference of weight was 24.8% and the mean was 3.491%. A difference exceeding 10% was found in only 3 carp. The maximum difference of saccular otolith was 11.8% with the mean of 6.92%, and that of lagenar otolith was 32% with the mean of 5.6% in goldfish. The close relationship of utricular otolith weight between both sides suggested that the otolith asymmetry might not be the main factor inducing SAS at least in goldfish and carp.

Gene expression in rat vestibular and reticular structures during and after space flight

Space flight produces profound changes of neuronal activity in the mammalian vestibular and reticular systems, affecting postural and motor functions. These changes are compensated over time by plastic alterations in the brain. Immediate early genes (IEGs) are useful indicators of both activity changes and neuronal plasticity. We studied the expression of two IEG protein products [Fos and Fos-related antigens (FRAs)] with different cell persistence times (hours and days, respectively) to identify brainstem vestibular and reticular structures involved in adaptation to microgravity and readaptation to 1 G (gravity) during the NASA Neurolab Mission (STS-90). IEG protein expression in flight animals was compared to that of ground controls using Fisher 344 rats killed 1 and 12 days after launch and 1 and 14 days after landing. An increase in the number of Fos-protein-positive cells in vestibular (especially medial and spinal) regions was observed 1 day after launch and 1 day after landing. Fos-positive cell numbers were no different from controls 12 days after launch or 14 days after landing. No G-related changes in IEG expression were observed in the lateral vestibular nucleus. The pattern of FRA protein expression was generally similar to that of Fos, except at 1 day after landing, when FRA-expressing cells were observed throughout the whole spinal vestibular nucleus, but only in the caudal part of the medial vestibular nucleus. Fos expression was found throughout the entire medial vestibular nucleus at this time. While both Fos and FRA expression patterns may reflect the increased G force experienced during take-off and landing, the Fos pattern may additionally reflect recent rebound episodes of rapid eye movement (REM) sleep following forced wakefulness, especially after landing. Pontine activity sources producing rhythmic discharges of vestibulo-oculomotor neurons during REM sleep could substitute for labyrinthine signals after exposure to microgravity, contributing to activity-related plastic changes leading to G readaptation. Reticular structures exhibited a contrasting pattern of changes in the numbers of Fos- and FRA-positive cells suggestive of a major influence from proprioceptive inputs, and plastic re-weighting of inputs after landing. Asymmetric induction of Fos and FRAs observed in some vestibular nuclei 1 day after landing suggests that activity asymmetries between bilateral otolith organs, their primary labyrinthine afferents, and vestibular nuclei may become unmasked during flight.

Comparison of vestibulo-ocular reflexes in earth-horizontal and earth-vertical axis rotations

Ten normal subjects and nine patients with peripheral labyrinthine lesions were subjected to sinusoidal rotation about an earth-horizontal (EHA) and earth-vertical axis (EVA) using a new device. In normal subjects the gain of the vestibulo-ocular reflex (VOR) in EHA rotation was significantly larger than that of the VOR in EVA rotation. This difference may be attributed to the interaction of the semicircular canals and otolithic organs. Unlike normal subjects, patients with unilateral lesions showed no differences in gain between EHA and EVA rotations toward either the intact or abnormal side. Patients with bilateral lesions also showed no differences in gain. These observations suggest that the interaction of the semicircular canals with the otolithic organs enhancing VOR gain does not occur if one of the otolithic organs is defective in either ear. This does not appear to recover once such a dysfunction develops.

Otolith function: basis for modern testing

The major challenge in developing a robust test of otolith function, particularly with regard to linear vestibulo-ocular reflex (LVOR) and perceptual measures, is to find a way in which graded lesions are reflected in graded response properties and abnormalities. The ability of the vestibulo-ocular reflex (VOR) to compensate and adapt to dysfunction and pathology presents formidable challenges for registering localizing clinical findings, whether in the angular vestibulo-ocular reflex (AVOR), the LVOR, or both. Based on a variety of considerations, various forms of eccentric rotation seem to provide the most convenient, and potentially the most useful, means to generate motion profiles from which otolith function can be directly assessed. Both translational and tilt responses can be recorded depending on the stimulus profile. The near-centric version is particularly enticing because of the ability to study one labyrinth at a time, much like calorics. In that case and in others in which the tilt-LVOR is prominent, measures of the perceived visual vertical are useful and by all accounts similar to ocular torsion. The latter does hold the important advantage of being an objective measure, requiring no intervention on the part of the patient. The translational-LVOR can be derived from eccentric rotation responses with the head displaced forward as well as backward, while viewing near targets in hopes of generating a large addition or subtraction (even inversion) of an otherwise AVOR-driven reflex. These considerations provide an impetus to pursue improved methods of quantifying otolith function in a clinical population. The sobering caveat is that the diagnosis of total unilateral vestibular loss presents little challenge either clinically or by classic testing (e.g., calorics), and yet most of our efforts in developing quantifiable measures of dysfunction over the years have yielded results that are modest and hardly compelling.

Perspectives for the comprehensive examination of semicircular canal and otolith function

A review is presented on the three-dimensional aspects of the vestibulo-oculomotor system and the current functional tests for unilateral examination of the individual receptors in the vestibular labyrinth. In the presentation, attention is directed towards the recently developed vestibular tests, which promise a more comprehensive examination of labyrinth function. More explicitly, unilateral tests for the utricle, saccule and the individual semicircular canals are discussed. Caloric irrigation and rotatory testing are widely used as tests for the integrity of the (horizontal) semicircular canals. Little useful diagnosis is made however on the vertical canals, not to mention the otolith organs. A promising approach to the examination of individual semicircular canal function has been described. This involves the perception of self-rotation in each of the planes of the semicircular canals. The patient/subject is rotated by an arbitrary amount on a standard Barany chair and then required to return the chair to its original position, by joystick control of the chair velocity. In order to test the vertical canals, the head of the subject/patient is positioned so that the plane of each canal lies in the plane of rotation. A promising unilateral test of saccular function involves the use of vestibular evoked myogenic potentials. Here it has been demonstrated that the saccules can be activated using brief, high-intensity acoustic clicks. The myogenic potential is measured using surface electrodes over the sternocleidomastoid muscles. Initial data from patients has indicated that the test is specific for unilateral saccule disorders. The unilateral test of utricle function is based on the eccentric displacement profile. Thus, eccentric displacement of the head to 3.5 cm during constant velocity rotation about the earth-vertical axis generates an adequate unilateral stimulation of the otolith organ, without involving the semicircular canals. This paradigm has also proved efficient in localizing peripheral otolith dysfunction by means of SVV estimation. This represents a novel test of otolith function that can be easily integrated into routine clinical testing. In contrast to the otolith-ocular response, the subjective visual vertical also reflects the processing of otolithic information in the higher brain centres (thalamus, vestibular cortex). Exploitation of the two complementary approaches therefore provides useful information for both experimental and clinical scientists. Of direct interest is the finding that testing with the subject rotating on-centre is sufficient to localize peripheral otolith dysfunction by means of SVV estimation. This represents a novel test of otolith function that can be easily integrated into routine clinical testing. In addition to caloric testing, which has remained the classical unilateral test of vestibular function, the newly developed tests should improve the differential diagnosis of vestibular disorders.

Orientation of the eyes to gravitoinertial acceleration

Orientation of the eyes to gravitoinertial acceleration, i.e., to the sum of gravity and the linear accelerations acting on the head and body, is a basic property of the linear vestibulo-ocular reflex to support vision. Present in a wide range of species from the lateral-eyed rabbit to frontal-eyed monkeys and humans, the eyes deviate in pitch, roll and yaw in response to pitch, roll and yaw head movements. The eyes also converge in response to naso-occipital linear acceleration. This paper provides examples of ocular orientation generated by static tilt and off-vertical axis rotation in three dimensions and demonstrates specifically how vergence would support vision in the rabbit.

Three-dimensional eye position during static roll and pitch in humans

We investigated how three-dimensional (3D) eye position is influenced by static head position relative to gravity, a reflex probably mediated by the otolith organs. In monkeys, the torsional component of eye position is modulated by gravity, but little data is available in humans. Subjects were held in different head/body tilts in roll and pitch for 35 s while we measured 3D eye position with scleral coils, and we used methods that reduced torsion artifacts produced by the eyelids pressing on the contact lens and exit wire. 3D eye positions were described by planar fits to the data (Listing's plane), and changes in these planes showed how torsion varied with head position. Similar to findings in monkeys, the eyes counterrolled during roll tilts independent of horizontal and vertical eye position, reaching a maximum torsion of 4.9 degrees. Counterroll was not proportional to the shear force on the macula of the utricles: gain (torsion/sine of the head roll angle) decreased by 50% from near upright to ear down. During pitch forward, torsion increased when subjects looked right, and decreased when they looked left. However, the maximum change of torsion was only -0.06 degrees per degree of horizontal eye position, which is less than reported in monkey. Also in contrast to monkey, we found little change in torsion when subjects were pitched backwards.

Adaptation of torsional eye alignment in relation to head roll

The coordination of head tilt, ocular counter-roll and vertical vergence is maintained by adaptive mechanisms; the desired outcome being clear single vision. A disruption or imbalance in otolith-ocular pathways may result in diplopia which stimulates these adaptive processes. In the present experiment, dove prisms were used to create cyclodisparities that varied with head tilt about a naso-occipital axis (roll). A stimulus for incyclovergence was presented with the head rolled 45 degrees to one side and a stimulus for an excyclovergence was presented with the head rolled 45 degrees to the other side. At the end of 1 h of training, all subjects demonstrated a change in open-loop cyclovergence that would help to correct for the cyclodisparities experienced during the closed-loop training period. The change appeared to be a simple gain change in the ocular counter-roll of one or both eyes.

Vertical divergence and counterroll eye movements evoked by whole-body position steps about the roll axis of the head in humans

In healthy human subjects, a head tilt about its roll axis evokes a dynamic counterroll that is mediated by both semicircular canal and otolith stimulation, and a static counterroll that is mediated by otolith stimulation only. The vertical ocular divergence associated with the static counterroll too is otolith-mediated. A previous study has shown that, in humans, there is also a vertical divergence during dynamic head roll, but this report was not conclusive on whether this response was mediated by the semicircular canals only or whether the otoliths made a significant contribution. To clarify this issue, we applied torsional whole-body position steps (amplitude 10 degrees, peak acceleration of 90 degrees /s(2), duration 650 ms) about the earth-vertical (supine body position) and earth-horizontal (upright body position) axis to healthy human subjects who were monocularly fixating a straight-ahead target. Eye movements were recorded binocularly with dual search coils in three dimensions. The dynamic parameters were determined 120 ms after the beginning of the turntable movement, i.e., before the first fast phase of nystagmus. The static parameters were measured 4 s after the beginning of the turntable movement. The dynamic gain of the counterroll was larger in upright (average gain: 0.48 +/- 0.10 SD) than in supine (0.36 +/- 0.10) position. The static gain of the counterroll in the upright position (0.21 +/- 0.06) was smaller than the dynamic gain. Divergent eye movements (intorting eye hypertropic) evoked during the dynamic phase were not significantly different between supine (average vergence velocity: 0.87 +/- 0.51 degrees /s) and upright (0.84 +/- 0.64 degrees /s) positions. The static vertical divergence in upright position was 0.32 +/- 0.14 degrees. The results indicate that the dynamic vertical divergence in contrast to the dynamic ocular counterroll is not enhanced by otolith input. These results can be explained through the different patterns of connectivity between semicircular canals and utricles to the eye muscles. Alternatively, we hypothesize that the small dynamic vertical divergence represents the remaining vertical error necessary to drive an adaptive control mechanism that normally maintains a vertical eye alignment.

Does the world rock when the eyes roll?

When the head is inclined sideways, the eyes are counter-rotated with respect to the head (ocular-counterroll, OCR). In man, the gain of OCR in static body tilts is limited to about 10% of the angle of roll tilt, which suggests that its function is vestigial. However, it is still unclear how the residual OCR is related to the perceived orientation of visual stimuli. Wade and Curthoys (1997) claim that the brain does not “take into account” the OCR, so that the eye position directly interferes with perception of visual orientation. Alternately, it has been argued that OCR is partly compensated by an extraretinal eye-position information such as, e.g., an efference copy ( Haustein, 1992 ; Haustein & Mittelstaedt, 1990 ). The two experiments reported in this study are targeted towards critically examining this inter-relation between OCR and perceived visual orientation. The latter was assessed via the subjective visual vertical, SVV, which is determined when a subject judges the orientation of an indicator (e.g., a short line segment) as apparently vertical. The OCR was measured by using a video-oculographic system. In Experiment 1, a human centrifuge was used to test the effect of an increase of the gravito-inertial force (GIF) on SVV and OCR. Experiment 2 was inspired by the fact that OCR can also be elicited during “barbecue rotation”. Again, it was the aim to compare OCR and SVV in different body positions, such as pure roll and barbecue rotated tilts. The present study provides convincing experimental evidence that SVV is widely uninfluenced by the course of OCR. Increasing the GIF in Experiment 1 had a divergent effect on SVV and OCR; the gain of OCR increases whereas the SVV changed differently, at obtuse tilt angles even in the opposite direction. OCR and SVV were again found to dissociate in Experiment 2, which emphasizes the fact that the SVV and OCR are not controlled by the same neural mechanism, but rather use different spatial reference information.

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 (OCR(S)) and dynamic vestibulo-ocular reflex gains during head impulses (VOR(HI)) 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 degrees 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 OCR(S) gain reduction and binocular cyclorotation (CRb). The main feature that distinguished chronic VN-patients from chronic VD-patients was the normalization of the torsional VOR(HI) gain to the affected side, whereas the VOR(HI) 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 OCR(S) gains, ii) a less pronounced CRb toward the affected side, and iii) a normal torsional VOR(HI) gain toward the affected side. Since the ipsilesional torsional VOR(HI) 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.

Assessing otolith function by the subjective visual vertical

The effects of peripheral vestibular diseases on the subjective visual vertical (SVV) are resumed and provide the basis for some insights into the otolith pathophysiology. With a normal range of 0 +/- 2 deg (when measured in an upright body position), the SVV was shifted by 11 +/- 6 deg toward the ipsilateral ear in 40 patients following an acute unilateral vestibular deafferentiation (UVD), but in the opposite direction in 9 of 52 patients after stapes surgery. These opposite effects suggest a push-pull mechanism of the pairs of otolith organs with respect to the SVV. The dissociation between the SVV and the perception of body position indicates influences by unconscious reflexive mechanisms such as ocular cyclotorsion on the SVV. In chronic UVD patients, lateral shifts of the subjects during constant angular velocity rotation into various eccentric positions (+/- 16 cm) revealed a shift of the "center of graviception" close to the remaining intact contralateral inner ear. To date, this seems to be the most consistent test for clinical identification of a chronic compensated unilateral loss of otolith function. The findings regarding asymmetries in otolithic sensitivity to medially and laterally directed roll-tilts remain controversial, probably mainly because of influences of extravestibular cues.

Clinical testing of otolith function

The subjective visual horizontal and vestibular-evoked myogenic potentials are simple, robust, and reproducible tests of otolith dysfunction that can prove clinically useful diagnostic information in patients with vertigo and other balance disorders. While they appear to have high specificity for unilateral otolith dysfunction, further clinical research will be required to establish their sensitivity.

Physiological Principles of Vestibular Function on Earth and in Space

Physiological mechanisms underlying vestibular function have important implications for our ability to understand, predict, and modify balance processes during and after spaceflight. The microgravity environment of space provides many unique opportunities for studying the effects of changes in gravitoinertial force on structure and function of the vestibular system. Investigations of basic vestibular physiology and of changes in reflexes occurring as a consequence of exposure to microgravity have important implications for diagnosis and treatment of vestibular disorders in human beings. This report reviews physiological principles underlying control of vestibular processes on earth and in space. Information is presented from a functional perspective with emphasis on signals arising from labyrinthine receptors. Changes induced by microgravity in linear acceleration detected by the vestibulo-ocular reflexes. Alterations of the functional requirements for postural control in space are described. Areas of direct correlation between studies of vestibular reflexes in microgravity and vestibular disorders in human beings are discussed. (Otolaryngol Head Neck Surg 1998;118:S5-S15.)

Otolith and semicircular canal contributions to the human binocular response to roll oscillation

Three normal human subjects were oscillated about their naso-occipital axis in a supine position at 0.4 Hz and 0.1 Hz, both in darkness and in the light with a structured fixation target. The same subjects were oscillated in roll about an upright position, at the same frequencies, in darkness; and also about axes directed 20 degrees and 40 degrees to the left and to the right of the midsagittal plane, at 0.4 Hz, in darkness. Three-dimensional binocular eye movements were recorded using video-oculography. All stimuli induced a predominantly torsional nystagmus with small disconjugate head-vertical (skew) and conjugate head-horizontal components. For roll oscillation, the torsional slow phase velocity gain was higher in the light and generally increased with the stimulation frequency. In darkness, only one subject had significantly higher torsional gains in the upright compared to the supine position (12% difference), suggesting that the otolith contribution to the roll response is minimal at the frequencies tested. The slow phase velocity gain of the skew increased with fixation in two subjects. but decreased in the third subject; these changes were related to changes in eye vergence. In the upright position, with oblique axes of rotation, the head-vertical eye movements were asymmetrical, with the outermost eye executing a larger amplitude movement. The disconjugate head-vertical eye movements observed can be explained by the pattern of vertical semicircular canal stimulation and their connections to the extraocular muscles. In humans, skewing of the eyes may compensate for the eccentricity of the foveae which lie in the temporal retina and undergo vertical translations in opposite directions when the eyes tort.

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.

Clinical testing of the statolith system in patients with Menière's disease

The statolith ocular reflex induced by whole body roll was assessed in 10 patients with Meniere's disease who had undergone a standard vestibular test battery. Normal eye torsion was noted in 4 patients despite the presence of severe symptoms, and a caloric hyporeflexia in one patient. Eye torsion was diminished in one direction of body roll in 4 patients and in both directions of body roll in 2 patients. An abnormal asymmetry of eye torsion (> 16%) was found in the case of body roll towards the normal ear irrespective of whether a caloric asymmetry was found. An abnormal asymmetry of the statolith ocular reflex seems to indicate the side with the functional hearing loss. The results suggest that Meniere's disease might involve pathology of the statolith system. However, the absence of abnormal eye torsion does not imply normal vestibular function.

Perception of direction of visual motion. II. Influence of linear body acceleration

We investigated whether linear whole-body acceleration along the interaural y-axis influenced the concurrent perception of visual motion direction as has been shown for angular accelerations. A sled running on air bearings along a 7.5-m track was used to accelerate 18 subjects at two different linear accelerations. These young, healthy volunteers, aged 25.50 +/- 7.38 years, used a joystick to indicate whether or not they perceived visual motion to the left within a random-dot kinematogram continuously presented on a monitor moving with them. The percentage of coherently leftward moving pixels presented for a 640-ms period during acceleration was adjusted according to a Modified Binary Search (MOBS) procedure. Six conditions were tested, two acceleration levels of 1 and 2 m/s2 to both left and right with, at the higher acceleration, two different times of visual motion presentation. Conditions were sequenced by means of a 6 x 6 Latin square balanced for order and carry over. A MANOVA did not show any statistically significant effects either for the independent variables acceleration, velocity, and direction of motion of the sled or for their interactions. The results obtained are in clear contrast to those obtained under rotatory stimulation. We conclude that the otolithic contribution to vestibular-visual motion processing is negligible.

Adaptation of vertical eye alignment in relation to head tilt

Binocular visual feedback is used to continually calibrate binocular eye alignment so that the retinal images of the two eyes remain in correspondence. Past experiments have shown that vertical eye alignment (measured as vertical phoria) can be altered by training to disparities that vary as a function of orbital eye position. The present experiments demonstrate that vertical eye alignment can also be trained to differ with head position when eye position (with respect to the orbit) is held constant. Changes in head position were about either an earth-vertical or earth-horizontal axis to distinguish otolith-ocular related adaptation from cervical-ocular related adaptation. Changes in head position were implemented by either by rotating the whole body (WB) or by rotating the head with the body stationary (HO). Following training, adaptation of eye alignment was observed in all cases of rotation about an earth-horizontal axis and for HO pitch rotations about an earth-vertical axis. The results illustrate the ability of the oculomotor system to compensate for imbalances in otolith-ocular pathways.

Perceived body position and the visual horizontal

This contribution examines the relation between the subjective visual vertical, the subjective visual horizontal, and the perceived body position of human subjects. Firmly fixed on a tiltable chair with head and torso restrained, 11 healthy subjects were rolled sideways and indicated their subjective horizontal body position. In these positions the subjects were also asked to adjust a luminous line alternately to the vertical and to the horizontal. The adjustments of the subjective horizontal body position cluster around a mean of 96.3 degrees with a remarkably broad range (SD: 19.7 degrees). In the subjective horizontal body position, the luminous line does not appear horizontal when in line with one's own spinal axis. It is set further down by 27.4 degrees on average and, therefore, perpendicular to the subjective visual vertical. This finding supports the idea that the judgement of the own body position and the judgement of the orientation of a seen object respective to gravity are based on different references. Contradictory to other investigations [23,24], is the empirical fact that the individual subjects were not able to adjust the horizontal body position with the reported accuracy (range of mean adjustments 77.5 to 117.6 degrees).

The subjective visual horizontal for different body tilts in the roll plane: characterization of normal subjects

In order to establish a method for estimation of the perceptual horizontal as a test of otolith function in diagnosis of atypical vertigo, in a first study we have standardized a test procedure and characterized a body of normal material consisting of 72 healthy subjects, 24 of them examined with tests followed by retests. The perceptual visual horizontal in darkness was estimated in the upright body position and at body tilts of 10, 20, and 30 degrees to the right and to the left by means of a narrow luminous bar. The deviation of the perceptual horizontal relative to the gravitational horizontal is expressed as a function of body tilt. In the upright body position, 95% had a perceptual horizontal within the range of +/- 2.5 degrees. In the tilted positions, there was a tendency to set the light bar tilted oppositely with respect to the body tilt. The results suggest that roll tilt to the right and to the left is sensed by two independent functional units. Furthermore, the results imply that some other factor might be of importance and that the perceptual horizontal in the upright position and tilt perception are complementary in reflecting vestibular function. Differences between individuals were great in comparison with intraindividual variability and the test-retest variability. The results are discussed against the background of the extensive literature.

Off-axis rotation as a test of otolith function

This experiment was performed to determine whether linear acceleration causes a change in the vestibulo-linear reflex (VOR) that could be exploited as a research and/or clinical test of otolith function. The effect of otolith stimulation on the VOR was studied in the horizontal and vertical planes in 10 cats. The VOR was induced by pulses of angular acceleration (velocity ramps). During some of the test runs, a centripetal linear acceleration stimulus was also applied to stimulate the otolith organs. Using the stimuli combination as tested in this project, the authors found that otolith stimulation affected the vertical VOR but not the horizontal VOR. The vertical VOR was asymmetric. While the methods used in this study are useful and the equipment requirements make these techniques generally impractical for clinical testing.

New diagnostic tests for the function of utricles, saccules and somatic graviceptors

Clinical tests that allow us to discriminate between utricular, saccular and somatic effects on gravity perception and control are desirable but wanting. A new test battery is presented which combines four experimental paradigms based on the subjective horizontal body position (SHP), namely, a test on a tiltable board and on a sled centrifuge under varied leg position, with two paradigms based on the subjective visual vertical (SVV). It is shown by a combination of experiments and deductions, that, why, and how these tests can separate the effects of otoliths from those of somatic graviceptors, the effects of the utricles from those of the saccules, and the effects of the constituents of the somatic graviception from each other. The present study demonstrates the capabilities of the tests as well as their limitations.

Ocular torsion during microgravity on a space mission in 1992

On a space mission in 1992 we investigated the effect of pure neck receptor stimulation on eye roll position in space. To do this, we used the flash afterimage method. We found that eye rotations in static tilted head positions are absent in weightlessness. This suggests that in microgravity the neck position receptors do not contribute to a measurable extent to static OCR.

Ground based eccentric chair experiments

Two subjects were rotated eccentrically in a manner described before. In contrast to a normal control group settings of a luminous line to the subjective vertical were almost unrelated to the gravitoinertial summation vector of gravity and centrifugal forces about four weeks before and totally so shortly after space flight. Only after four days post flight had passed a noticeable relation to the gravitoinertial vector re-established itself in the one subject which actually flew. The correspondence became normal six days after flight. Since there were no clinical abnormalities evident in the subjects, it is suggested, that both subjects suppressed their vestibular information presumably as an effect of vestibular deconditioning training before flight. In addition as a consequence of the flight experience one subject continued to ignore it several days after the flight.

Orientation of human optokinetic nystagmus to gravity: a model-based approach

Optokinetic nystagmus (OKN) was induced by having subjects watch a moving display in a binocular, head-fixed apparatus. The display was composed of 3.3 degrees stripes moving at 35 degrees/s for 45 s. It subtended 88 degrees horizontally by 72 degrees vertically of the central visual field and could be oriented to rotate about axes that were upright or tilted 45 degrees or 90 degrees. The head was held upright or was tilted 45 degrees left or right on the body during stimulation. Head-horizontal (yaw axis) and head-vertical (pitch axis) components of OKN were recorded with electro-oculography (EOG). Slow phase velocity vectors were determined and compared with the axis of stimulation and the spatial vertical (gravity axis). With the head upright, the axis of eye rotation during yaw axis OKN was coincident with the stimulus axis and the spatial vertical. With the head tilted, a significant vertical component of eye velocity appeared during yaw axis stimulation. As a result the axis of eye rotation shifted from the stimulus axis toward the spatial vertical. Vertical components developed within 1-2 s of stimulus onset and persisted until the end of stimulation. In the six subjects there was a mean shift of the axis of eye rotation during yaw axis stimulation of approximately 18 degrees with the head tilted 45 degrees on the body. Oblique optokinetic stimulation with the head upright was associated with a mean shift of the axis of eye rotation toward the spatial vertical of 9.2 degrees. When the head was tilted and the same oblique stimulation was given, the axis of eye rotation rotated to the other side of the spatial vertical by 5.4 degrees. This counterrotation of the axis of eye rotation is similar to the "Müller (E) effect," in which the perception of the upright is counterrotated to the opposite side of the spatial vertical when subjects are tilted in darkness. The data were simulated by a model of OKN with a "direct" and "indirect" pathway. It was assumed that the direct visual pathway is oriented in a body, not a spatial frame of reference. Despite the short optokinetic after-nystagmus time constants, strong horizontal to vertical cross-coupling could be produced if the horizontal and vertical time constants were in proper ratio and there were no suppression of nystagmus in directions orthogonal to the stimulus direction. The model demonstrates that the spatial orientation of OKN can be achieved by restructuring the system matrix of velocity storage. We conclude that an important function of velocity storage is to orient slow-phase velocity toward the spatial vertical during movement in a terrestrial environment.

Effects of spaceflight on ocular counterrolling and the spatial orientation of the vestibular system

We recorded the horizontal (yaw), vertical (pitch), and torsional (roll) eye movements of two rhesus monkeys with scleral search coils before and after the COSMOS Biosatellite 2229 Flight. The aim was to determine effects of adaptation to microgravity on the vestibulo-ocular reflex (VOR). The animals flew for 11 days. The first postflight tests were 22 h and 55 h after landing, and testing extended for 11 days after reentry. There were four significant effects of spaceflight on functions related to spatial orientation: (1) Compensatory ocular counterrolling (OCR) was reduced by about 70% for static and dynamic head tilts with regard to gravity. The reduction in OCR persisted in the two animals throughout postflight testing. (2) The gain of the torsional component of the angular VOR (roll VOR) was decreased by 15% and 50% in the two animals over the same period. (3) An up-down asymmetry of nystagmus, present in the two monkeys before flight was reduced after exposure to microgravity. (4) The spatial orientation of velocity storage was shifted in the one monkey that could be tested soon after flight. Before flight, the yaw axis eigenvector of optokinetic afternystagmus was close to gravity when the animal was upright or tilted. After flight, the yaw orientation vector was shifted toward the body yaw axis. By 7 days after recovery, it had reverted to a gravitational orientation. We postulate that spaceflight causes changes in the vestibular system which reflect adaptation of spatial orientation from a gravitational to a body frame of reference. These changes are likely to play a role in the postural, locomotor, and gaze instability demonstrated on reentry after spaceflight.

Three-dimensional aspects of caloric nystagmus in humans: II. Caloric-induced torsional deviation

The oculomotor response to caloric stimulation was recorded using video-oculography (VOG), yielding measures of eye rotation about all three orthogonal axes. During an ongoing caloric nystagmus response, a slow torsional deviation of the eye in the direction of the slow phase of the torsional component of nystagmus was observed. The effect appeared to be systematically influenced by the gravitoinertial force employed during a centrifuge study (see Part I, this issue), and was observed in all tested subjects. Furthermore, in subsequent laboratory tests warm and cold irrigations elicited opposite responses, as did stimulation to right and left ears. Testing in the supine and prone positions, however, did not lead to a reversal of direction, as is the case with the horizontal, vertical and torsional nystagmus components as such. The rate of torsional deviation was of the order of 0.05 degrees/s, well below the physiological transduction range of the cupular organs. Testing with a torsional optokinetic stimulus produced only the expected OK nystagmus without any slow deviation. This would argue against a central vestibular integratory, or oculomotor origin for the phenomenon. It is proposed that this caloric-induced, torsional deviation is of peripheral otolithic origin, most likely elicited by way of direct thermal stimulation to the hair cells of the utricular maculae.

Oculovestibular interactions under microgravity

On a space mission in March 1992 a set of experiments were performed aimed at clarifying the interaction between visual, proprioceptive and vestibular inputs to the equilibrium system. Using the VESTA goggle facility from the European Space Agency we investigated the effect of pure neck receptor stimulation on eye position as measured by the flash afterimage method and on perception of a head-fixed luminous line in space. Space vestibular adaptation processes were measured by rotating pattern perception during prescribed head movements. It was found that static ocular counterrotation does not occur under microgravity conditions. This result suggests that the neck receptors apparently do not contribute to a measurable extent. The subjective orientation of a vertical line was perceived correctly inflight. Obviously neck receptors on the perception level can fully substitute for the ineffective equilibrium organs of the inner ear within less than 4 days. The rotating pattern perception during different head motion patterns is not influenced by the absence of a gravity reference.