Ocular Counter-rolling during Active Head Tilting in Humans

Oculomotor stimulation without visual input has no impact on postural control

It has been well established that eye movements have an impact on balance, and it has been hypothesized that extraocular oculomotor signals could play a significant role in this effect. Unfortunately, this hypothesis could not be confirmed as the previous methodology did not allow for the independent assessment of the differential effects of visual and oculomotor stimulation. The objective of the present study is to examine the impact of motor movements of the eyes without visual stimulation on balance. Static postural control, a prerequisite for balance, was assessed using a force platform in 20 participants. They were asked either to remain still without moving or to make movements of the tongue or eyes at a rate of two cycles per second. Movements were monitored using electrophysiological recordings. Each of the conditions was performed with eyes open and with eyes closed. Significant changes in postural control were observed due to eye movements when the eyes were open, but no significant differences were found between the conditions when the eyes were closed. The results confirm that the visual system provides important spatial cues for balance, allowing the body to be better positioned in space, and reject the possibility that extraocular signals are directly involved in postural stability.

Optokinetic stimulation induces vertical vergence, possibly through a non-visual pathway

Vertical vergence is generally associated with one of three mechanisms: vestibular activation during a head tilt, induced by vertical visual disparity, or as a by-product of ocular torsion. However, vertical vergence can also be induced by seemingly unrelated visual conditions, such as optokinetic rotations. This study aims to investigate the effect of vision on this latter form of vertical vergence. Eight subjects (4m/4f) viewed a visual scene in head erect position in two different viewing conditions (monocular and binocular). The scene, containing white lines angled at 45° against a black background, was projected at an eye-screen distance of 2 m, and rotated 28° at an acceleration of 56°/s2. Eye movements were recorded using a Chronos Eye-Tracker, and eye occlusions were carried out by placing an infrared-translucent cover in front of the left eye during monocular viewing. Results revealed vergence amplitudes during binocular viewing to be significantly lower than those seen for monocular conditions (p = 0.003), while torsion remained unaffected. This indicates that vertical vergence to optokinetic stimulation, though visually induced, is visually suppressed during binocular viewing. Considering that vertical vergence is generally viewed as a vestibular signal, the findings may reflect a visually induced activation of a vestibular pathway.

The effects of meclizine on motion sickness revisited

AIMS

Antihistamines make up the first line of treatments against motion-sickness. Still, their efficacy and specific mechanism have come into question. The aim of this study was to investigate the effect of meclizine on motion-sensitivity.

METHODS

This study was carried out as a triple-blinded randomized trial involving 12 healthy subjects who were exposed to (i) vestibular (VES), (ii) visual (VIS) and (iii) visual-vestibular (VIS+VES) stimulations in the roll plane. Subjects were divided into 2 groups by stratified randomization, receiving either meclizine or a placebo. Stimulations were carried out before, and after, drug administration, presented at 2 intensity levels of 14 and 28°/s² . Eye movements were tracked, and torsional slow-phase velocities, amplitudes and nystagmus beats were retrieved. Subjects initially graded for their motion-sickness susceptibility.

RESULTS

Susceptibility had no effect on intervention outcome. Despite large variations, repeated ANOVAS showed that meclizine led to a relative increase in torsional velocity compared to placebo during vestibular stimulation for both intensities: 2.36 (7.65) from -0.01 (4.17) during low intensities, and 2.61 (6.67) from -3.49 (4.76) during high. The visual-vestibular stimuli yielded a decrease during low acceleration, -0.40 (3.87) from 3.75 (5.62), but increased during high, 3.88 (6.51) from -3.88 (8.55).

CONCLUSIONS

Meclizine had an inhibitory effect on eye movement reflexes for low accelerations during VIS+VES trials. This indicates that meclizine may not primarily work through sensory-specific mechanisms, but rather on a more central level. Practically, meclizine shows promise in targeting motion-sickness evoked by everyday activities, but its use may be counterproductive in high-acceleration environments.

Vestibular Eye Movements Are Heavily Impacted by Visual Motion and Are Sensitive to Changes in Visual Intensities

Purpose

Eye movement evaluation constitutes the basis of diagnosis in dizzy patients. Through evaluating ocular torsion and vertical skewing during balance provoking stimulation, the aim of this study was to investigate the impact of vision on a typical vestibular eye movement response.

Methods

Twelve healthy subjects (six young, six old) were exposed to (1) vestibular (VES), (2) visual (VIS), and (3) visual-vestibular (VIS+VES) stimulation. These were carried out as whole-body roll (VES), optokinetic rolling of visual scenes (VIS), and a combination of both (VIS+VES). Visual scenes were presented at two intensity levels. Eye movement velocities were used to evaluate the relative impact of visual and vestibular stimulation.

Results

Torsional velocities were lowest for VIS regardless of age. Velocities for the old group did not differ between VES and VIS+VES, whereas those for the young group were higher for VIS+VES. Regardless of age, amplified visual intensity resulted in an increased torsion-skewing ratio, seen as more degrees of torsion per degrees of skewing. The contributions of VIS and VES in proportion to VIS+VES were calculated as 0.18 (0.08) and 0.74 (0.14), respectively.

Conclusions

Our findings demonstrate that vertical skewing is physiologically seen in combination with ocular torsion as a response to visual stimulation, with young subjects exhibiting a more dynamic response. The torsion-skewing ratio was sensitive to small changes in visual intensities, which may prove useful when evaluating visual motion sensitivity. The visual contribution to the vestibular eye movement response highlights the clinical importance of visual examinations when evaluating dizzy patients.

Static Ocular Counterroll: Video-based Analysis After Minimizing the False-Torsion Factors

PURPOSE

To determine the validity and usefulness of a newly developed measurement method of static ocular counterrolling (s-OCR) that eliminates false-torsion factors and to test the Jampel hypothesis that s-OCR does not exist.

METHODS

A lightweight measurement device, consisting of a video camera, a coaxial light source, and a laser pointer projecting a fixation target on the wall, was fixed to a subject's head by means of a mouthpiece. In 11 healthy adults (mean age: 30 +/- 15 years), digital images of the right eye were captured while the subject kept his head tilted at a randomly selected angle ranging from 0 degrees to 50 degrees . By a frame-by-frame analysis of movements of the corneal light reflex and the iris patterns, OCR was evaluated.

RESULTS

Torsional eye movement in the opposite direction to head tilt was found in all subjects. The amount of torsion continuously increased until the head-tilt angle reached 40 degrees. The average (+/- SD) amplitude of a fitted sine curve was 7.6 +/- 3.2 degrees (range: 4.3 degrees-10.3 degrees), and the individual amplitude was significantly larger than the test-retest repeatability of the measurement (+/-1.7 degrees).

CONCLUSIONS

The measurement method used in this study provided good test-retest repeatability and ease of application. The characteristics of torsional eye movements that we observed after minimizing the false-torsion factors agree with previous reports supporting the existence of s-OCR.

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

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

Testing otolith function

Otolithic signals contribute to; (1) perception of orientation and linear motion, (2) generate compensatory eye movements in response to linear acceleration of the head and (3) participate in the co-ordination of movement and balance. Tests of these functions shown to be useful in identifying clinical disorders have been reviewed: (1) Evaluation of orientation to gravity, as estimated by adjustment of the visual vertical, indicates deranged otolith function at a peripheral or central level and the sensitivity of this test can be enhanced by performing estimates during centrifugation on a motorised turntable. Estimation of thresholds of self motion on a parallel swing identifies global reduction or unilateral loss of peripheral function, with central disorders awaiting study. (2) Otolith ocular reflexes to linear head translation can be used to demonstrate overall integrity of peripheral function and reveal central abnormalities. Counter-rolling responses to head roll-tilt and measurements of cyclodeviation of the eyes demonstrate functional asymmetries, with some lateralising value, particularly in central lesions. Global function and asymmetries may also be evaluated by 'head eccentric' rotational testing, which adds a tangential linear acceleration to the angular stimulus. The linear acceleration enhances the canal response by adding an otolith component. (3) Latency and amplitude of surface electro-myography (EMG) responses in the limbs to sudden falls, which can be recorded with the subject suspended on a hinged bed, indicate gross peripheral abnormality of function and can lateralize disorders of CNS motor pathways. It is concluded that some tests of otolith function can be of use in indicating global loss of peripheral otolith function, others are capable of lateralizing a marked loss of function and all have the potential to give information about central disorders. They all have to be interpreted within the clinical context and, unfortunately, none have yet been shown to be sensitive to partial, particularly unilateral, dysfunction.

A compact equipment package for vestibular experiments during spaceflight

A compact measurement and stimulus equipment package for vestibular testing is described. The package is designed on a modular concept so that a customised version can be assembled for each experimental situation. Although primarily conceived for space-related research, the equipment has also been introduced successfully into the clinical diagnostic procedure. An essential function of the equipment is the recording and evaluation of eye movements. This is performed by a video-based measurement system which permits evaluation of horizontal, vertical and torsional components of eye movement. Objective testing of the vestibulo-ocular reflex in all three orthogonal planes is therefore possible. Furthermore evaluation of the otolithic function in weightlessness is made feasible by the possibility of measuring dynamic ocular counterrolling. Some applications of the equipment are described.

Measuring three dimensions of eye movement in dynamic situations by means of videooculography

A primary function of the vestibular system is the stabilisation of the eye during head movement. Consequently, evaluation of reflex eye movements represents an essential means to both clinical diagnosis and researching of the vestibular function. Movements in the eye can be resolved into three orthogonal components, i.e. horizontal, vertical and torsional. As an improvement on most current techniques, which provide only measurement of the horizontal and vertical components, videooculography (VOG) facilitates non-invasive measurement of all three of the defined components. To date, only the scleral coil technique, which involves the semi-invasive placement of coil rings onto the bulbi, yields a continuous measure of eye torsion. Employment of suitable solid-state devices permit the integration of a compact, high resolution video recording system. In the basic configuration, eye movements can be observed and simultaneously recorded for later analysis or documentation. The video images of the eye are obtained by means of a miniaturised CCD video sensor mounted on a light-occluding mask. Image processing of the acquired video images determines horizontal and vertical coordinates of eye position online. Ocular torsion, as reflected by the rotation of the natural iris, is measured for each video frame. The VOG algorithm has been implemented on a PC based workstation, which permits online observation, recording and evaluation of eye movements. In addition, the technique has found clinical application as a portable eye-movement observation and recording system, allowing bedside examination and recording of transient symptoms. Preliminary results from various studies, including the objective evaluation of positional nystagmus (BPPN), are presented.

The human torsional vestibulo-ocular reflex during rotation about an earth-vertical axis

Using the magnetic search coil technique, we have measured the gain and time constant (Tvor) of the torsional vestibulo-ocular reflex (VOR) in 4 subjects who were rotated about an earth-vertical axis with their necks extended and faces supine. Following a 1-min period of rotation in darkness at 50 degrees/s, the post-rotational response to a velocity off-step had a group mean gain of 0.43 and Tvor of 3.7 s. Following a 1-min period of rotation in the light at 50 degrees/s, the post-rotational response in darkness had a group mean gain of 0.29 and Tvor of 4.1 s. Following rotation in darkness with the neck flexed and head prone, the post-rotational response, measured in two subjects, had a mean gain of 0.39 and Tvor of 5.7 s. Similar results were obtained with 100 degrees/s stimuli. In all subjects, the gain and Tvor of the torsional VOR were smaller than corresponding values for their horizontal VOR; these smaller values can be related to the different visual demands made of the torsional VOR.