Biomedical subjects
Publications and source records attributed to D Straumann.
Our detailed understanding of the physiology and anatomy of the ocular motor system allows an accurate differential diagnosis of pathological eye movement patterns. This review covers important clinical studies and studies in basic research relevant for the neurologist published during the past year.
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.
When a human subject is oscillated about the nasooccipital axis and fixes upon targets along the horizontal head-fixed meridian, angular eye velocity includes a vertical component that increases with the horizontal eccentricity of the line-of-sight. This vertical eye movement component is necessary to prevent retinal slip. We asked whether fixation on a near head-fixed target during the same torsional vestibular stimulation would lead to differences of vertical eye movements between the right and the left eye, as the directions of the two lines-of-sight are not parallel during convergence. Healthy human subjects (n = 6) were oscillated (0.3 Hz, +/-30 degrees) about the nasooccipital axis on a three-dimensional motor-driven turntable. Binocular movements were recorded using the dual search coil technique. A head-fixed laser dot was presented 1.4 m (far head-fixed target) or 0.25 m (near head-fixed target) in front of the right eye. We found highly significant (P < 0.01) correlations (R binocular = 0.8, monocular = 0.59) between the convergence angle and the difference of the vertical eye velocity between the two eyes. The slope of the fitted linear regression between the two parameters (s = 0.45) was close to the theoretical slope necessary to prevent vertical retinal slippage (predicted s = 0.5). Covering the left eye did not significantly change the slope (s = 0.52). In addition, there was a marked gain reduction (approximately 35%) of the torsional vestibuloocular reflex (VOR) between viewing the far and the near targets, confirming earlier results by others. There was no difference in torsional gain reduction between the two eyes. Lenses of +3 dpt positioned in front of both eyes to decrease the amount of accommodation did not further change the gain of the torsional VOR. In conclusion, ocular convergence on a near head-fixed target during torsional vestibular stimulation leads to deviations in vertical angular velocity between the two eyes necessary to prevent vertical double vision. The vertical deviation velocity is mainly linked to the amount of convergence, since it also occurs during monocular viewing of the near head-fixed target. This suggests that convergence during vestibular stimulation automatically leads to an alignment of binocular rotation axes with the visual axes independent of retinal slip.
Saccade-related burst neurons in the paramedian pontine reticular formation (PPRF) of the head-restrained monkey provide a phasic velocity signal to extraocular motoneurons for the generation of rapid eye movements. In the superior colliculus (SC), which directly projects to the PPRF, the motor command for conjugate saccades with the head restrained in a roll position is represented in a reference frame in between oculocentric and space-fixed coordinates with a clear bias toward gravity. Here we studied the preferred direction of premotor burst neurons in the PPRF during static head roll to characterize their frame of reference with respect to head and eye position. In 59 neurons (short-lead, burst-tonic, and long-lead burst neurons), we found that the preferred direction of eye displacement of these neurons changed, relative to head-fixed landmarks, in the horizontal-vertical plane during static head roll. For the short-lead burst neurons and the burst-tonic group, the change was about one-fourth of the amount of ocular counterroll (OCR) and significantly different from a head-centered representation. In the long-lead burst neurons, the rotation of the preferred direction showed a larger trend of about one-half of OCR. During microelectrical stimulation of the PPRF (9 sites in 2 monkeys), the elicited eye movements rotated with about one-half the amount of OCR. In a simple pulley model of the oculomotor plant, the noncraniocentric reference frame of the PPRF output neurons could be reproduced for recently measured pulley positions, if the pulleys were assumed to rotate as a function of OCR with a gain of 0.5. We conclude that the saccadic displacement signal is transformed from a representation in the SC with a clear bias to gravity to a representation in the PPRF that is closely craniocentric, but rotates with OCR, consistent with current concepts of the oculomotor plant.
PURPOSE: To establish an objective Hess screen test that allows a simultaneous and binocular analysis of all three axes of eye rotation. METHODS: In orthotropic and strabismic human subjects, both eyes were recorded with dual scleral search coils in a three-field magnetic system. Before mounting the search coil annuli on the eyes, the voltage offsets of each channel and the relative magnitudes of the three magnetic fields were determined. For calibration, subjects were only required to fix monocularly on a single reference target. During fixation of targets on the Hess screen by the uncovered eye, the three-dimensional eye position of both the occluded and the viewing eye was simultaneously measured. RESULTS: For clinical interpretation, an easy to understand graphical description of the three-dimensional Hess screen test was developed. Positions of orthotropic and strabismic eyes tended to follow Listing's law, which in both eyes allowed the determination of the primary position, that is, the position of gaze from which pure horizontal and pure vertical movements do not lead to an ocular rotation about the line-of-sight. To a first approximation, the location of primary position is a result of the summation of the individual rotation axes of the six extraocular muscles and thus can be used to infer which muscle is paretic. CONCLUSIONS: The three-dimensional Hess screen test with binocular dual search coils in a three-field magnetic system is an objective method to assess the ocular alignment in three dimensions with high precision. From these recordings, the clinician can relate deviations of primary position to specific eye muscle palsies.
In rhesus monkeys, the dynamic properties of the torsional vestibulo-ocular reflex (VOR) are modified by otolith input: compared with torsional oscillations about an earth-vertical axis (canal-only stimulation), the phase lead observed at frequencies below 0.1 Hz is cancelled when the animals are rotated about an earth-horizontal axis (canal-and-otolith stimulation); the gains of the torsional VOR, however, are nearly identical in both conditions. To test whether or not canal-otolith interaction in humans is similar to that in rhesus monkeys, we examined ten healthy human subjects on a three-axis servo-controlled motor-driven turntable. The subjects were oscillated in upright or supine position in complete darkness over a similarly wide range of frequencies (0.05-1.0 Hz) with peak velocities <40 degrees/s. Eye movements were recorded using the three-dimensional search coil technique. Compared with the torsional vestibulo-ocular gains during canal-stimulation only (earth-vertical axis), the gains obtained during combined canal-otolith-stimulation (earth-horizontal axis) were significantly higher throughout the entire frequency range (P<0.05). The gain increased by 0.100+/-0.074 (SD), independent of frequency. During the earth-horizontal axis stimulation, the phase remained always around zero, which is in contrast to the canal-stimulation only, during which one finds an increasing phase lead as frequency decreases. We conclude that, in healthy humans as in rhesus monkeys, the phase lead from the canal signals at low frequencies is effectively cancelled by the otolith input. In contrast to rhesus monkeys, however, otolith signals in healthy humans increase the gain of the torsional VOR at frequencies from 0.05 to 1.0 Hz. This normal database is crucial for the interpretation of results obtained in patients with vestibular disorders.
We report on 3 siblings (2 females and 1 male) with chronic progressive external ophthalmoplegia (CPEO), compatible with inherited mitochondrial cytopathy. The younger of the two sisters died at the age of 37 due to progressive respiratory failure. The older one presented with a status epilepticus at the age of 39 and was treated with valproate. Five months after the start of treatment, she developed fulminant liver failure and died. The brother has suffered from CPEO since early childhood but has had so far no other symptoms of a mitochondrial disease. A muscle biopsy from the younger sister revealed ragged-red fibers and decreased activities of complex I and IV of the respiratory chain but no pathogenic mutations in the mitochondrial tRNA genes or in several locations in the coding region of the mitochondrial genome. In the older sister's liver (obtained post-mortem), mitochondrial DNA was fragmented and could not be investigated. The clinical presentation and the biochemical findings suggest that all 3 siblings suffered from a mitochondrial cytopathy. Since mitochondrial cytopathies and valproate-induced fulminant liver failure are both rare events, an association between them is likely. Mitochondrial diseases should therefore be considered as a risk factor for valproate-induced liver failure and be excluded before treatment with valproate.
One of the signs of the cerebellar ocular motor syndrome is the inability to maintain horizontal and vertical fixation. Typically, in the presence of cerebellar atrophy, the eyes show horizontal gaze-evoked and vertical downbeat nystagmus. We investigated whether or not the cerebellar ocular motor syndrome also includes a torsional drift and, specifically, if it is independent from the drift in the horizontal-vertical plane. The existence of such a torsional drift would suggest that the cerebellum is critically involved in maintaining the eyes in Listing's plane. Eighteen patients with cerebellar atrophy (diagnosis confirmed by magnetic resonance imaging) were tested and compared with a group of normal subjects. Three-dimensional eye movements (horizontal, vertical, and torsional) during attempted fixations of targets at different horizontal and vertical eccentricities were recorded by dual search coils in a three-field magnetic frame. The overall ocular drift was composed of an upward drift that increased during lateral gaze, a horizontal centripetal drift that appeared during lateral gaze, and a torsional drift that depended on horizontal eye position. The vertical drift consisted of two subcomponents: a vertical gaze-evoked drift and a constant vertical velocity bias. The increase of upward drift velocity with eccentric horizontal gaze was caused by an increase of the vertical velocity bias; this component did not comply with Listing's law. The horizontal-eye-position-dependent torsional drift was intorsional in abduction and extorsional in adduction, which led to an additional violation of Listing's law. The existence of torsional drift that is eye-position-dependent suggests that the cerebellum is critically involved in the implementation of Listing's law, perhaps by mapping a tonic torsional signal that depends on the direction of the line of sight. The magnitude of this signal might reflect the difference in torsional eye position between the torsional resting position determined by the mechanics of the eye plant and the torsional position required by Listing's law.
Rapid eye movements include saccades and quick phases of nystagmus and may have components around all three axes of ocular rotation: horizontal, vertical, and torsional. In this study, we recorded horizontal, vertical, and torsional eye movements in normal subjects with their heads upright and stationary. We asked how the eyes are brought back to Listing's plane after they are displaced from it. We found that torsional offsets, induced with a rotating optokinetic disk oriented perpendicular to the subject's straight ahead, were corrected during both horizontal and vertical voluntary saccades. Thus three-dimensional errors are synchronously reduced during saccades. The speed of the torsional correction was much faster than could be accounted for by passive mechanical forces. During vertical saccades, the peak torsional velocity decreased and the time of peak torsional velocity was delayed, as the amplitude of vertical saccades increased. In contrast, there was no consistent reduction of torsional velocity or change in time of peak torsional velocity with an increase in the amplitude of horizontal saccades. These findings suggest that 1) the correction of stimulus-induced torsion is neurally commanded and 2) there is cross-coupling between the torsional and vertical but not between the torsional and horizontal saccade generating systems. This latter dichotomy may reflect the fact that vertical and torsional rapid eye movements are generated by common premotor circuits located in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF). When horizontal or vertical saccade duration was relatively short, the torsional offset was not completely corrected during the primary saccade, indicating that although the saccade itself is three-dimensional, saccade duration is determined by the error in the horizontal or the vertical, but not by the error in the torsional component.
PURPOSE: To determine whether volitional effort on the part of the subject can influence the results of the Bielschowsky Head Tilt Test. PATIENTS AND METHODS: Bielschowsky Head Tilt testing was performed in five normal subjects. Vertical amplitudes were measured with prism alternate cover testing when the head tilt was voluntary (volitionally maintained in a tilted position by the subject), forced (restrained in the tilted position by the examiner while the patient actively resisted) and active (restrained in a tilted position by the examiner with the subject actively trying to increase the tilt). Three-dimensional scleral search coil recordings were performed in three additional normal subjects using the same paradigm to determine the effect of volition on the torsional positions of the eyes. RESULTS: No vertical deviation of the eyes was detectable with prism alternate cover testing in any position of tilt, regardless of whether the tilt was voluntary, forced, or active. Volitional attempts to tilt the head were preceded by a transient ipsiversive torsional movement of the measured eye, which was quickly followed by a normal ocular counterroll. Following completion of the counterroll, the position of the eyes was constant for any position of head tilt, regardless of whether the tilt was forced, active, or voluntary. CONCLUSION: Anticipatory torsional movements of the eyes are evoked by an attempted volitional head movement in the roll plane and its associated innervation to the cervical musculature. However, these volitional movements do not alter the final torsional position of the eyes, which is a function of the degree of head tilt and the normal ocular counterroll. These anticipatory torsional movements do not influence the results of the Bielschowsky Head Tilt Test clinically by prism alternate cover testing.
OBJECTIVE: To analyze the nystagmus evoked by hyperventilation in patients with unilateral vestibular schwannoma and to use this information to predict the effects of hyperventilation on individual ampullary nerves. METHODS: Three-dimensional scleral search coil eye movement recording techniques were used to record the magnitude and time course of eye movements in six patients with unilateral vestibular schwannoma and hyperventilation-induced nystagmus. The presenting complaints in five of these patients were vertigo or dysequilibrium. RESULTS: The eye movement response to hyperventilation was a "recovery" nystagmus with slow-phase components corresponding to excitation of the affected vestibular nerve. Projection of the eye velocity vector into the plane of the semicircular canals revealed that fibers arising from the ampulla of the horizontal canal were most affected by hyperventilation with lesser activation of fibers to the superior canal and smaller, more variable responses from posterior canal fibers. CONCLUSIONS: The three-dimensional characteristics of the nystagmus evoked by hyperventilation in patients with vestibular schwannoma provide insight into the vestibular end organs affected by the tumor and the mechanism responsible for the nystagmus. This finding indicates that hyperventilation resulted in a transient increase in activity from these partially demyelinated axons.
The rotation axis for each of the six extraocular muscles was determined in four eyes from three perfused rhesus monkeys. Measurements of the locations of muscle insertions and origins were made in the stereotaxic reference frame with the x-y plane horizontal and the x-z plane sagittal. The computed rotation axes of the horizontal recti were close to being in the x-z plane at an angle of about 15 degrees to the z axis. The rotation axes of the vertical recti and the obliques were close to being in the x-y plane at an angle of about 30 degrees to the y axis. In five alert rhesus monkeys, we simultaneously recorded extraocular motoneuron activity and eye position in three dimensions (3D). The activity of 51 motoneuron axons was obtained from the oculomotor (n=34), trochlear (n=11), and abducens nerve (n=6) during spontaneous eye movements. To extend the torsional range of eye position, the animals were also put in different static roll positions, which induced ocular counterroll without dynamic vestibular stimulation. Periods of 100 ms during fixation or slow eye movements (<10 degrees/s) were chosen for analysis. For each motoneuron, a multiple linear regression was performed between firing frequency and 3D eye position, expressed as a rotation vector, in both stereotaxic and Listing's reference frame. The direction with the highest correlation coefficient (average R=0.94+/-0.07 SD) was taken as the on-direction. Each unit's activity could be unequivocally attributed to one particular muscle. On-directions for each motoneuron were confined to a well-defined cone in 3D. Average on-directions of motoneurons differed significantly from the corresponding anatomically determined muscle rotation axes expressed in the stereotaxic reference frame (range of deviations: 11.9 degrees to 29.0 degrees). This difference was most pronounced for the vertical recti and oblique muscles. The muscle rotation axes of the vertical rectus pair and the oblique muscle pair form an angle of 58.3 degrees, whereas the corresponding angle for paired motoneuron on-directions was 105.6 degrees. On-directions of motoneurons were better aligned with the on-directions of semicircular canal afferents (range of deviation: 9.4-18.9 degrees) or with the anatomically determined sensitivity vectors of the semicircular canals (range of deviation: 3.9-15.9 degrees) than with the anatomically determined muscle rotation axes, but significant differences remain to be explained. The on-directions of motoneurons were arranged symmetrically to Listing's plane, in the sense that the torsional components for antagonistically paired muscles were almost equal, but of opposite sign. Thus, the torsional components of motoneuron on-directions cancel when eye movements are confined to Listing's plane. This arrangement simplifies the neuronal transformations for conjugate head-fixed voluntary eye movements, while the approximate alignment with the semicircular canal reference frame is optimal for generating compensatory eye movements.
If horizontal saccades or smooth-pursuit eye movements are made with the line-of-sight at different elevations, the three-dimensional (3D) angular rotation axis of the globe tilts by half the vertical eye eccentricity. This phenomenon is named "half-angle rule" and is a consequence of Listing's law. It was recently found that the ocular rotation axis during the horizontal vestibulo-ocular reflex (VOR) on a turntable also tilts in the direction of the line-of-sight by about a quarter of the eye's vertical eccentricity. This is surprising, since, in a "perfect" VOR, the angular rotation axis of the eye should be independent from the position of the eye to fully compensate for the 3D angular head rotation. We asked whether this quarter-angle strategy is a general property of the VOR or whether the 3D kinematics of ocular movements evoked by vestibular stimulation would be less eye-position dependent at higher stimulus frequencies. Nine healthy subjects were exposed to horizontal head impulses (peak velocity approximately 250 degrees /s). The line-of-sight was systematically changed along the vertical meridian of a tangent screen. Three-dimensional eye and head movements were monitored with dual search coils. The 3D orientation of the angular eye-in-head rotation axis was determined by calculating the average angular velocity vectors of the initial 10 degrees displacements. Then, the difference between the tilt angles of the ocular rotation axis during upward and downward viewing was determined and divided by the difference of vertical eccentricity ("tilt angle coefficient"). Control experiments included horizontal saccades, smooth-pursuit eye movements, and eye movements evoked by slow, passive head rotations at the same vertical eye eccentricities. On average, the ocular rotation axis during horizontal head-impulse testing at different elevations of the line-of-sight was closely aligned with the rotation axis of the head (tilt angle coefficient of pooled abducting and adducting eye movements: 0.11+/-0.17 SD). Values for slow head impulses, however, exceeded somewhat the quarter angle (0.33+/-0.12), while smooth-pursuit movements (0. 50+/-0.09) and saccades (0.44+/-0.11) were closest to the half angle. These results demonstrate that the 3D orientation of the ocular rotation axis during rapid head thrusts is relatively independent of the direction of the line-of-sight and that ocular rotations elicited by head impulses are kinematically different from saccades, despite similar movement dynamics.
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.
Methods to measure eye rotations in 3D have developed to a stage where routine clinical application is realistic. Besides the equipment, it requires a basic understanding of 3-dimensional geometry for calibration and interpretation. Relevant parameters are orientation, displacement and thickness of Listing's plane for spontaneous or goal-directed eye movements, and counterrolling or nystagmus with a roll component for vestibular function. The method with the highest temporal and spatial resolution is the magnetic search coil technique. Video-based systems are still slow and cannot be used to characterize saccades. Often, the task of reconstructing the 3-dimensional eye position from a 2-dimensional image of the eye is underestimated. Search coil measurements have shown no firm correlation between the orientation of Listing's plane and "classical" landmarks like stereotaxic head position, emphasizing that Listing's plane is functionally, and not anatomically, determined.
Most of the various hypotheses on the pathomechanism of the slight ocular upward drift in normal mammals and on the prominent downbeat nystagmus following cerebellar lesions assume an inherent vertical asymmetry in the central vestibulo-ocular pathways. In this paper we propose that this vertical asymmetry is simply based on the anatomical orientation of the six semicircular canals in the head which is right-left symmetrical but lacks symmetry in the cranio-caudal direction. Presuming that each semicircular canal elicits eye movements in a direction roughly in its anatomical plane, vectorial addition of the tonic resting activity of all six canals leads to a cancellation of horizontal and torsional eye movement components but leaves an important vertical (slow phase) upward component. This peripheral vestibular bias is centrally cancelled by floccular and parafloccular inhibitory pathways which are related to the smooth pursuit system, but becomes disinhibited in the presence of posterior cerebellar lesions.
PURPOSE: Dissociated vertical deviation (DVD) has eluded explanation for more than a century. The purpose of this study has been to elucidate the etiology and mechanism of DVD. METHODS: Eye movement recordings of six young adults with DVD were made with dual-coil scleral search coils under various conditions of fixation, illumination, and head tilt. Horizontal, vertical, and torsional eye movements were recorded for both eyes simultaneously. Analyses of the simultaneous vertical and torsional movements occurring during the DVD response were used to separate and identify the component vergence and version eye movements involved. RESULTS: Typically, both horizontal and cyclovertical latent nystagmus developed upon occlusion of either eye. A cycloversion then occurred, with the fixing eye intorting and tending to depress, the covered eye extorting and elevating. Simultaneously, upward versions occurred for the maintenance of fixation, consisting variously of saccades and smooth eye movements, leading to further elevation of the eye behind the cover. The cyclovertical component of the latent nystagmus became partially damped as the DVD developed. CONCLUSIONS: In patients with an early-onset defect of binocular function, the occlusion of one eye, or even concentration on fixing with one eye, produces unbalanced input to the vestibular system. This results in latent nystagmus, sometimes seen only with magnification. The cyclovertical component of the latent nystagmus, when present, is similar to normal vestibular nystagmus induced by dynamic head tilting about an oblique axis. Such vestibular nystagmus characteristically produces a hyperdeviation of the eyes. In the case of cyclovertical latent nystagmus, the analogous hyperdeviation will persist unless corrected by a vertical vergence. A normal, oblique-muscle-mediated, cycloversion/vertical vergence is called into play. This occurs in the proper direction to correct the hyperdeviation, but it occurs in an exaggerated form in the absence of binocular vision, probably as a learned response. The cycloversion/vertical vergence helps damp the cyclovertical nystagmus (a cyclovertical "nystagmus block-age" phenomenon), aiding vision in the fixing eye. But this mechanism also produces unavoidable and undesirable elevation and extortion of the fellow eye, which we call DVD.
The direction of spontaneous nystagmus was recorded in three dimensions with scleral dual search coils in three patients after vestibular neurectomy and in seven patients with vestibular neuritis. The rotation vectors of the spontaneous nystagmus clustered along the sensitivity vector of the lateral semicircular canal (SCC). The direction of the spontaneous nystagmus after resection of the whole eighth nerve was not different from that after resection of only the superior branch of the vestibular nerve. Deviations from this direction were observed only after resection of the inferior vestibular nerve and in one patient with vestibular neuritis. The absence of nystagmus components in direction of the vertical SCC reflects an anisotropy of oculomotor efferents of the vestibulo-ocular reflex are rather than a lesion limited to the lateral SCC afferents. Therefore, the three-dimensional analysis of spontaneous nystagmus does not permit accurate localization of a peripheral vestibular lesion.