Binocular counterrolling in humans with unilateral labyrinthectomy and in normal controls

The Anatomical and Physiological Basis of Clinical Tests of Otolith Function. A Tribute to Yoshio Uchino

Otolithic receptors are stimulated by gravitoinertial force (GIF) acting on the otoconia resulting in deflections of the hair bundles of otolithic receptor hair cells. The GIF is the sum of gravitational force and the inertial force due to linear acceleration. The usual clinical and experimental tests of otolith function have used GIFs (roll tilts re gravity or linear accelerations) as test stimuli. However, the opposite polarization of receptors across each otolithic macula is puzzling since a GIF directed across the otolith macula will excite receptors on one side of the line of polarity reversal (LPR at the striola) and simultaneously act to silence receptors on the opposite side of the LPR. It would seem the two neural signals from the one otolith macula should cancel. In fact, Uchino showed that instead of canceling, the simultaneous stimulation of the oppositely polarized hair cells enhances the otolithic response to GIF—both in the saccular macula and the utricular macula. For the utricular system there is also commissural inhibitory interaction between the utricular maculae in each ear. The results are that the one GIF stimulus will cause direct excitation of utricular receptors in the activated sector in one ear as well as indirect excitation resulting from the disfacilitation of utricular receptors in the corresponding sector on the opposite labyrinth. There are effectively two complementary parallel otolithic afferent systems—the sustained system concerned with signaling low frequency GIF stimuli such as roll head tilts and the transient system which is activated by sound and vibration. Clinical tests of the sustained otolith system—such as ocular counterrolling to roll-tilt or tests using linear translation—do not show unilateral otolithic loss reliably, whereas tests of transient otolith function [vestibular evoked myogenic potentials (VEMPs) to brief sound and vibration stimuli] do show unilateral otolithic loss. The opposing sectors of the maculae also explain the results of galvanic vestibular stimulation (GVS) where bilateral mastoid galvanic stimulation causes ocular torsion position similar to the otolithic response to GIF. However, GVS stimulates canal afferents as well as otolithic afferents so the eye movement response is complex.

Sustained and Transient Vestibular Systems: A Physiological Basis for Interpreting Vestibular Function

Otolithic afferents with regular resting discharge respond to gravity or low-frequency linear accelerations, and we term these the static or sustained otolithic system. However, in the otolithic sense organs, there is anatomical differentiation across the maculae and corresponding physiological differentiation. A specialized band of receptors called the striola consists of mainly type I receptors whose hair bundles are weakly tethered to the overlying otolithic membrane. The afferent neurons, which form calyx synapses on type I striolar receptors, have irregular resting discharge and have low thresholds to high frequency (e.g., 500 Hz) bone-conducted vibration and air-conducted sound. High-frequency sound and vibration likely causes fluid displacement which deflects the weakly tethered hair bundles of the very fast type I receptors. Irregular vestibular afferents show phase locking, similar to cochlear afferents, up to stimulus frequencies of kilohertz. We term these irregular afferents the transient system signaling dynamic otolithic stimulation. A 500-Hz vibration preferentially activates the otolith irregular afferents, since regular afferents are not activated at intensities used in clinical testing, whereas irregular afferents have low thresholds. We show how this sustained and transient distinction applies at the vestibular nuclei. The two systems have differential responses to vibration and sound, to ototoxic antibiotics, to galvanic stimulation, and to natural linear acceleration, and such differential sensitivity allows probing of the two systems. A 500-Hz vibration that selectively activates irregular otolithic afferents results in stimulus-locked eye movements in animals and humans. The preparatory myogenic potentials for these eye movements are measured in the new clinical test of otolith function—ocular vestibular-evoked myogenic potentials. We suggest 500-Hz vibration may identify the contribution of the transient system to vestibular controlled responses, such as vestibulo-ocular, vestibulo-spinal, and vestibulo-sympathetic responses. The prospect of particular treatments targeting one or the other of the transient or sustained systems is now being realized in the clinic by the use of intratympanic gentamicin which preferentially attacks type I receptors. We suggest that it is valuable to view vestibular responses by this sustained-transient distinction.

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.

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.

Static and dynamic properties of vergence-induced reduction of ocular counter-roll in near vision

We have examined the characteristics of vergence-induced reduction of ocular counter-roll in near vision. Monkeys were trained to make convergent and divergent refixations with the head and body either upright or in various roll orientations. During near viewing requiring 17 degrees horizontal vergence, we found that static binocular torsion was suppressed by about 68% (averaged over both eyes, two monkeys and both near target locations). This result is in accordance with a previous study in which binocular torsion was quantified based on the displacement planes of eye positions in far and near viewing. Latency and duration of the change in torsional eye position depended (for each eye differently) on body roll and the depth plane of fixation. For instance, during convergent refixations in left-ear-down orientations, the latencies of the left eye were smaller and the durations were longer than those of the right eye. However, both eyes reached their final positions required to fixate the second visual target at roughly the same time. The different dynamics of the two eyes is explained by the fact that each eye rotated temporally when the eyes converged, a pattern named binocular extension of Listing's law. Coming from or aiming at a common torsional value (normal ocular counter-roll) in convergent or divergent refixations, the required torsion differs in the two eyes. The brain compensates for these differences by adjusting the dynamics of each eye's movement.

Gravity modulates Listing's plane orientation during both pursuit and saccades

Previous studies have shown that the spatial organization of all eye orientations during visually guided saccadic eye movements (Listing's plane) varies systematically as a function of static and dynamic head orientation in space. Here we tested if a similar organization also applies to the spatial orientation of eye positions during smooth pursuit eye movements. Specifically, we characterized the three-dimensional distribution of eye positions during horizontal and vertical pursuit (0.1 Hz, +/-15 degrees and 0.5 Hz, +/-8 degrees) at different eccentricities and elevations while rhesus monkeys were sitting upright or being statically tilted in different roll and pitch positions. We found that the spatial organization of eye positions during smooth pursuit depends on static orientation in space, similarly as during visually guided saccades and fixations. In support of recent modeling studies, these results are consistent with a role of gravity on defining the parameters of Listing's law.

Vertical misalignment in unilateral sixth nerve palsy

OBJECTIVE

To detect and determine the magnitude of vertical deviation in patients with unilateral sixth nerve palsy.

DESIGN

Prospective consecutive comparative case series.

PARTICIPANTS

Twenty patients with unilateral peripheral sixth nerve palsy, 7 patients with central palsy caused by brainstem lesions, and 10 normal subjects.

METHODS

Subjects were tested by the prism and cover test, Maddox rod and prism test, and magnetic search coil recordings in nine diagnostic eye positions. They were also tested during static lateral head tilt by the prism and cover, and Maddox rod and prism tests.

MAIN OUTCOME MEASURES

The magnitudes of horizontal and vertical deviations.

RESULTS

All patients had an abduction deficit and incomitant esodeviation that increased in the field of action of the paretic muscle, indicating sixth nerve palsy. Mean vertical deviations, for all positions of gaze in peripheral palsy were 0.3 +/- 0.8 prism diopters (PD) by prism and cover test, 1.3 +/- 1.6 PD by Maddox rod and prism test, and 2.0 +/- 1.4 PD by coil recordings. Mean vertical deviations in normal subjects were 0.0 +/- 0.0 PD by prism and cover test, 1.0 +/- 0.9 PD by Maddox rod and prism test, and 1.9 +/- 2.1 PD by coil recordings. Therefore, peripheral palsy did not cause abnormal vertical deviation. In central palsy, for all positions together mean vertical deviations were 0.9 +/- 1.3 PD by prism and cover test, 1.4 +/- 1.6 PD by Maddox rod and prism test, and 2.5 +/- 1.6 PD by coil recordings; they were not different from normal values. During static head roll, patients with peripheral palsy had a right hyperdeviation on right head tilt and a left hyperdeviation on left head tilt, regardless of the side of the palsy. In contrast, in central palsy, head tilt caused vertical strabismus that remained on the same side on head tilt to either side.

CONCLUSIONS

Small vertical deviations in sixth nerve palsy are consistent with normal hyperphorias that become manifest in the presence of esotropia. In peripheral sixth nerve palsy, static head roll to either side induces hyperdeviation in the eye on the side of the head tilt. Hyperdeviation of the same eye induced by head tilt to either direction implicates a brainstem lesion as the cause of paretic abduction. Quantitative study of sixth nerve palsy demonstrates that if a vertical deviation falls within the normal range of hyperphoria, multiple cranial nerve palsy or skew deviation may not be responsible. Conversely, vertical deviation > 5 PD indicates skew deviation or peripheral nerve palsy in addition to abduction palsy.

Ocular counterrolling differs in dynamic and static stimulation

In the past, the majority of ocular counterrolling (OCR) studies were performed with subjects tilted and held statically. Studies in our laboratory have focused on dynamic rotation below the threshold of the semicircular canals. The present study compares OCR in both static and dynamic modes. Ten normal subjects, mean age 50.9 years (SD 16.2 years), underwent rotation about their naso-occipital axis to 90 degrees to the right and left, at a constant velocity of 3 degrees/s and an acceleration of 0.2 degree/s2. Subsequently, they were tilted at the same acceleration and velocity to 30 degrees, 60 degrees, 90 degrees, 60 degrees, 30 degrees and 0 degree to both sides and held in each position for 1 min. The results showed that OCR varied substantially in the two protocols. The most dramatic difference was disconjugacy in the static mode, with the two eyes differing by as much as 4 degrees, in contrast to the generally conjugate OCR in the dynamic mode. Amplitudes also tended to differ, some subjects having greater and others lesser OCR in one mode vis-à-vis the other. Possible explanations for these differences may be found in the work of Hudspeth and colleagues, who found that mechanical deflection of the bullfrog saccula resulted in gradated responses in the underlying hair cells. Further, hair cells in the process of active bending led to different responses than those in a fixed position. Possibly in humans, too, the otoconia do not maintain a fixed relation to the underlying hair cells. Additionally, this study confirms our earlier finding of independent control in the two eyes.

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.

Primate translational vestibuloocular reflexes. IV. Changes after unilateral labyrinthectomy

The effects of unilateral labyrinthectomy on the properties of the translational vestibuloocular reflexes (trVORs) were investigated in rhesus monkeys trained to fixate near targets. Translational motion stimuli consisted of either steady-state lateral and fore-aft sinusoidal oscillations or short-lasting transient displacements. During small-amplitude, steady-state sinusoidal lateral oscillations, a small decrease in the horizontal trVOR sensitivity and its dependence on viewing distance was observed during the first week after labyrinthectomy. These deficits gradually recovered over time. In addition, the vertical response component increased, causing a tilt of the eye velocity vector toward the lesioned side. During large, transient lateral displacements, the deficits were larger and longer lasting. Responses after labyrinthectomy were asymmetric, with eye velocity during movements toward the side of the lesion being more compromised. The most profound effect of the lesions was observed during fore-aft motion. Whereas responses were kinematically appropriate for fixation away from the side of the lesion (e.g., to the left after right labyrinthectomy), horizontal responses were anticompensatory during fixation at targets located ipsilateral to the side of the lesion (e.g., for targets to the right after right labyrinthectomy). This deficit showed little recovery during the 3-mo post-labyrinthectomy testing period. These results suggest that inputs from both labyrinths are important for the proper function of the trVORs, although the details of how bilateral signals are processed and integrated remain unknown.

Deviation of the subjective vertical in long-standing unilateral vestibular loss

We evaluated changes in the subjectively perceived gravitational vertical as an index of imbalance in the function of the right and left otolith organs. In addition to normal subjects (n = 25), we measured patients with a longstanding (mean 4.5 year +/- 3.2 SD; range 0.5-11.5 years) unilateral vestibular loss after surgery for acoustic neuroma (n = 32), patients with partial unilateral vestibular loss (n = 7) and patients with bilateral vestibular hyporeflexia (n = 8). Normal subjects could accurately align a vertical luminous bar to the gravitational vertical in an otherwise completely dark room (mean setting -0.14 degree +/- 1.11 SD). Patients with left-sided (complete; n = 13) or right-sided (complete; n = 19 and partial; n = 7) unilateral vestibular loss made mean angular settings at 2.55 degrees +/- 1.57 (SD) leftward and 2.22 degrees (+/-1.96 SD) rightward, respectively. These means differed highly significantly from the normal mean (p < 0.00001). In the time interval investigated (0.5-11.5 years) the magnitude of the tilt angle showed no correlation with the time elapsed since the operation. The mean setting by patients with clinically bilateral vestibular loss (-1.17 degrees +/- 1.96 SD; n = 8) did not significantly differ from the control group. The systematic tilts of the subjective vertical in patients with a unilateral vestibular impairment were correlated with their imbalance in canal-ocular reflexes, as reflected by drift during head-oscillation at 2 Hz (r2 = 0.44) and asymmetries in VOR-gain for head-steps (r2 = 0.48-0.67). These correlations were largely determined, by the signs of the asymmetries; correlation between the absolute values of the VOR gain asymmetries and subjective vertical angles proved to be virtually absent. We conclude that the setting of the subjective vertical is a very sensitive tool in detecting a left-right imbalance in otolith function, and that small but significant deviations towards the defective side may persist for many years (probably permanently) after unilateral lesions of the labyrinth or the vestibular nerve.

Determination of ocular torsion by means of automatic pattern recognition

A new, automatic method for determination of human ocular torsion (OT) was developed based on the tracking of iris patterns in digitized video images. Instead of quantifying OT by means of cross-correlation of circular iris samples, a procedure commonly applied, this new method automatically selects and recovers a set of 36 significant patterns in the iris by the technique of template matching as described by In den Haak et al. Each relocated landmark results in a single estimate of the torsion angle. A robust algorithm estimates OT from this total set of individually determined torsion angles, thereby largely correcting for errors which may arise due to misjudgement of the rotation center. The new method reproduced OT in a prepared set of images of an artificial eye with an accuracy of 0.1 degree. In a sample of 256 images of human eyes, a practical reliability of 0.25 degrees was achieved. To illustrate the method's usefulness, an experiment is described in which ocular torsion was measured during two dynamic conditions of whole-body roll, namely during sinusoidally pendular motion about either an earth horizontal or earth vertical axis (that is "with" and "without" otolith stimulation, respectively).

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.

Otolith-ocular testing in human subjects

Assessment of the otolith-ocular reflex of human subjects involves linear acceleration and/or changes in the orientation of the head with respect to gravity. Several such stimuli are currently under investigation regarding their applicability to the evaluation of patients with dizziness and balance disorders. Discussed in this paper are off-vertical axis rotation, eccentric rotation, pitch and roll rotation, and linear acceleration. For each of these stimuli, basic principles, normative human data, and patient data are described. Although none of these methods are currently established for clinical use, each of them, especially off-vertical axis rotation and linear acceleration, have the potential for developing into a clinically useful method for assessing otolith function in man.

Vestibular-neck interaction of human ocular counterroll

Compensatory rolling of the eye about the line of sight (ocular counterroll, OCR) was studied in four normal human subjects during 10 min of lateral tilt (20 degrees to the left/right) of body, head and trunk. OCR was objectively measured at 1-min intervals by determining the rotation of a circular sample of iris pattern using a videocamera and digital image processing. Body and head tilt caused symmetrical OCR (range 2.4 to 9.5 degrees) to either side, whereas trunk tilt induced no significant change in torsional eye position. No adaptation in OCR was found during the time spent in tilted orientation in neither paradigm though unsystematic torsional fluctuation was a regular finding in all subjects. These findings are taken as evidence that only the non-adapting otolith system, as opposed to the proprioceptive system of the neck, contributes to the generation and maintenance of OCR.

Cross-coupling between horizontal and vertical eye movements during optokinetic nystagmus and optokinetic afternystagmus elicited in microgravity

The occurrence of horizontal jerks with larger amplitudes than on Earth was observed during vertical optokinetic nystagmus in astronauts tested throughout a 7-day spaceflight. During early exposure to microgravity, a horizontal spontaneous-like nystagmus was recorded in darkness following both vertical and horizontal optokinetic stimulation. In addition, the time constant of vertical OKAN with slow phase up was larger than on the ground. These effects disappeared on flight day 2. Then, the horizontal and vertical OKAN time constants decreased, and gradually returned to the preflight values, as previously observed with the gain of the vestibulo-ocular reflex. The early changes in microgravity are in agreement with those obtained on Earth in monkeys and humans during static tilt relative to gravity. Our findings suggest that the absence of otolithic input in microgravity may have an effect on the optokinetic system which could be mediated by the velocity storage mechanism.

Linear acceleration perception in the roll plane before and after unilateral vestibular neurectomy

The ability of 33 patients to perceive the direction, relative to the body long axis, of a linear acceleration vector acting in the coronal plane, roll-tilt perception, was studied at various times, before and from 1 week to 6 months after unilateral, selective vestibular neurectomy for Meniere's disease, acoustic neuroma or intractable paroxysmal vertigo. The results of these patients were compared with the results of 31 normal subjects and two control patients who had both vestibular nerves surgically sectioned. Rotating on a fixed-chair centrifuge in an otherwise darkened room, each observer was required to indicate his perception of the direction of the resultant gravito-inertial vector by setting a small, motor-driven, illuminated bar, attached to the chair but rotatable in the fronto-parallel plane, to the perceived gravitational horizontal. Normal subjects accurately align the bar with respect to the gravito-inertial resultant vector which, in the dark, they assume to be the gravitational vertical. This percept has been called the oculogravic illusion. Accurate roll-tilt perception is due to vestibular (probably mainly otolithic) sensory information since patients with bilateral vestibular neurectomies do not perceive the resultant vector accurately. Whereas normal subjects perceive resultant vectors directed to the right and to the left equally accurately, roll-tilt perception was invariably asymmetrical one week after unilateral vestibular neurectomy. Even at rest there was an asymmetry in the baseline settings, so that patients set the bar down on the side of the operated ear, in order for it to appear gravitationally horizontal: if a patient had a right vestibular nerve section then he set the bar clockwise (from the patient's view) below the true gravitational horizontal. With increasing gravito-inertial resultant angles there was an increasing asymmetry of roll-tilt perception due both to decreased sensitivity to roll-tilt stimuli directed towards the operated ear and to transiently increased sensitivity to roll-tilt stimuli directed towards the intact ear. A progressive decrease in both perceptual asymmetries followed, rapidly in the first 3 weeks, more slowly in the next 6 months.(ABSTRACT TRUNCATED AT 400 WORDS)

An analysis of ocular counter-rolling measured with search coils

Ocular counter-rolling (OCR) was studied by using a scleral search coil magnetic system in normal subjects and in pathological cases. Normal ocular counter-rolling was 2.7 degrees-7 degrees when the head was tilted 10 to 30 degrees. In most cases of benign paroxysmal positional vertigo, the OCR to the ipsilateral side was reduced, while that to the contralateral side was normal or only slightly reduced. Soon after unilateral labyrinthectomy, the OCR to the ipsilateral side was reduced or was 0, whereas OCR to the contralateral side was normal or slightly reduced. Some 3-5 years after the operation, however, the OCR seemed to depend on the compensation achieved. In cases of acoustic neurinoma, OCR to both sides was reduced, that to the ipsilateral side being more strongly impaired than the OCR to the contralateral side.

Organizational principles of velocity storage in three dimensions. The effect of gravity on cross-coupling of optokinetic after-nystagmus

1. Optokinetic nystagmus was elicited in alert monkeys by movement of the visual surround in their horizontal or yaw plane, and optokinetic after-nystagmus was recorded in darkness. The animals were upright or were statically tilted at various angles from the upright. While upright, the OKAN was horizontal, and there were no vertical or roll components. When animals were tilted to either side or forward or back, vertical or roll components appeared both in the primary and secondary OKAN. Such components were also observed during nystagmus and after-nystagmus induced by electrical stimulation of the nucleus of the optic tract. The characteristics of the cross-coupled components indicated that they were mediated through the velocity storage mechanism in the vestibulo-ocular reflex. 2. A principle was inferred that explained the appearance of cross-coupled vertical or roll components in the primary and secondary OKAN: With the animal in a tilted position, a vector of eye velocity during OKN along the body-vertical axis was converted during primary OKAN toward a spatial-vertical axis with the same sense by a right hand rule. Thus, slow phase velocity along a vector toward the top of the animal's head during horizontal OKN rotated so that it tended to be directed spatially upward during primary OKAN. The reverse was true for OKN with a velocity vector directed toward the animal's feet. It rotated during primary OKAN so that it tended to be aligned with the direction of gravity. The vector of velocity during secondary OKAN was opposite to the direction of the vector during primary OKAN and was approximately aligned with spatial vertical. 3. OKN and OKAN were elicited about the animal's pitch and roll axes while they were upright and statically tilted at various angles away from the spatial vertical. There was a graded increase in the strength of vertical and roll OKN and OKAN and in the falling time constant of OKAN as the animals were tilted so that the axis of pitch or roll eye movements was moved toward alignment with the spatial vertical. Thus, velocity storage during pure vertical and roll nystagmus was similar to that during yaw OKN and OKAN in tilted positions: it was maximal along the pitch and roll axes when these axes were aligned with gravity. 4. The data indicate that the gravitational field is of fundamental importance in imposing a spatial reference onto the velocity storage integrator.(ABSTRACT TRUNCATED AT 400 WORDS)

Binocular counterrolling during sustained body tilt in normal humans and in a patient with unilateral vestibular nerve section

Two normal persons and a patient with unilateral vestibular nerve section were held motionless for ten minutes in the upright position, at 60 degrees tilt right ear down, and at 60 degrees tilt left ear down. In addition, one normal subject was held for ten minutes at each of 30, 60, and 90 degrees tilt left ear down. Photographs were taken of both eyes every ten seconds. Measurements of ocular counterrolling during these trials revealed torsional eye movements in all positions, including the upright, even though the head and body were stationary. Variations in torsion in the upright position ranged up to 2.75 degrees. At the tilt positions, variations ranged up to 4 degrees. Disconjugate movements were seen in all subjects in all positions. There were no significant differences in measurements of ocular counterrolling during static tilt between the normal subjects and the patient with the vestibular nerve section, in contrast to measures obtained during slow velocity dynamic testing.

Tullio phenomenon with torsion of the eyes and subjective tilt of the visual surround

A 44-year-old male patient had an acoustic trauma three years previously, after which he suffered from vertigo and tilting of the environment to the right when uttering the vowels u or e. At such times, a tonic eye torsion to the left, which lasted throughout the utterance, was observed under Frenzel's glasses along with head tilt to the left. The phenomenon could be elicited experimentally by right-ear stimulation with low-frequency noise (mean frequency, 125 Hz; 90 dB), as well as by constant pressure. The patient also reported observing the phenomenon with loud noises, nose blowing, obstruction of his right external meatus with the finger, and with altitude pressure changes in a car. This suggests that the phenomenon is elicited via the eardrum and the ossicular chain. Since lateral head tilt and counterrolling were tonic and without nystagmus, it is unlikely that one of the semicircular canals is involved as in usual Tullio cases. Rather, the otoliths may play a role in pathogenesis. Possible causative mechanisms are discussed along with the relevant literature.