Eye movements during multi-axis whole-body rotations

Ocular torsion is related to perceived motion-induced position shifts

Ocular torsion (i.e., rotations of the eye about the line of sight) can be induced by visual rotational motion. It remains unclear whether and how such visually induced torsion is related to perception. By using the flash-grab effect, an illusory position shift of a briefly flashed stationary target superimposed on a rotating pattern, we examined the relationship between torsion and perception. In two experiments, 25 observers reported the perceived location of a flash while their three-dimensional eye movements were recorded. In Experiment 1, the flash coincided with a direction reversal of a large, centrally displayed, rotating grating. The grating triggered visually induced torsion in the direction of stimulus rotation. The magnitude of torsional eye rotation correlated with the illusory perceptual position shift. To test whether torsion caused the illusion, in Experiment 2, the flash was superimposed on two peripheral gratings rotating in opposite directions. Even though torsion was eliminated, the illusory position shift persisted. Despite the lack of a causal relationship, the torsion-perception correlations indicate a close link between both systems, either through similar visual-input processing or a boost of visual rotational signal strength via oculomotor feedback.

Contribution of intravestibular sensory conflict to motion sickness and dizziness in migraine disorders

Migraine is associated with enhanced motion sickness susceptibility and can cause episodic vertigo [vestibular migraine (VM)], but the mechanisms relating migraine to these vestibular symptoms remain uncertain. We tested the hypothesis that the central integration of rotational cues (from the semicircular canals) and gravitational cues (from the otolith organs) is abnormal in migraine patients. A postrotational tilt paradigm generated a conflict between canal cues (which indicate the head is rotating) and otolith cues (which indicate the head is tilted and stationary), and eye movements were measured to quantify two behaviors that are thought to minimize this conflict: suppression and reorientation of the central angular velocity signal, evidenced by attenuation ("dumping") of the vestibuloocular reflex and shifting of the rotational axis of the vestibuloocular reflex toward the earth vertical. We found that normal and migraine subjects, but not VM patients, displayed an inverse correlation between the extent of dumping and the size of the axis shift such that the net "conflict resolution" mediated through these two mechanisms approached an optimal value and that the residual sensory conflict in VM patients (but not migraine or normal subjects) correlated with motion sickness susceptibility. Our findings suggest that the brain normally controls the dynamic and spatial characteristics of central vestibular signals to minimize intravestibular sensory conflict and that this process is disrupted in VM, which may be responsible for the enhance motion intolerance and episodic vertigo that characterize this disorder.

Moving in a Moving World: A Review on Vestibular Motion Sickness

Motion sickness is a common disturbance occurring in healthy people as a physiological response to exposure to motion stimuli that are unexpected on the basis of previous experience. The motion can be either real, and therefore perceived by the vestibular system, or illusory, as in the case of visual illusion. A multitude of studies has been performed in the last decades, substantiating different nauseogenic stimuli, studying their specific characteristics, proposing unifying theories, and testing possible countermeasures. Several reviews focused on one of these aspects; however, the link between specific nauseogenic stimuli and the unifying theories and models is often not clearly detailed. Readers unfamiliar with the topic, but studying a condition that may involve motion sickness, can therefore have difficulties to understand why a specific stimulus will induce motion sickness. So far, this general audience struggles to take advantage of the solid basis provided by existing theories and models. This review focuses on vestibular-only motion sickness, listing the relevant motion stimuli, clarifying the sensory signals involved, and framing them in the context of the current theories.

[Diagnosis of otolith disorders in routine otolaryngology]

Nowadays, specific and reproducible side-separated neurotologic testing of all vestibular receptors is possible. Assessment of saccular function by measuring cervical vestibular-evoked myogenic potentials (cVEMP) is well established. Reliable screening tests exist for the isolated assessment of utricular function. These tests can be performed even in less well-equipped surroundings to generate information regarding a possible partial dysfunction of the otolith organs. Apart from assessing the functionality of the semicircular canals by caloric irrigation and the head impulse test, screening for otolith disorders should be part of a comprehensive modern vestibular examination. Neurofeedbacktraining is an effective therapy in the treatment of otholith disorders.

Sensory conflict compared in microgravity, artificial gravity, motion sickness, and vestibular disorders

Perceptual disturbances and motion sickness are often attributed to sensory conflict. We investigated several conditions: head movements in microgravity, periodic motions in 1-g, and locomotion with vestibular disorders. In every case, linear vectors such as linear and gravitational acceleration are crucial factors, as previously found for head movements in artificial gravity, and thus the importance of measuring linear vectors emerges as a common theme. By modeling the sensory conflict between the vestibular and somatosensory systems, we computed a measure of linear conflict known as the "Stretch Factor". We hypothesized that the motions with the greatest Stretch Factor would be the most provocative motions.

RESULTS

For head movements in microgravity, the Stretch Factor can explain why fast movements are more provocative than slow movements, and why pitch movements are more provocative than yaw movements. For off-vertical-axis rotation (OVAR) in 1-g, the Stretch Factor predicts that the most provocative frequency is higher than that for vertical linear oscillation (VLO). For example, the same sensor dynamics can predict a most provocative frequency around 0.2 Hz for VLO but 0.3 Hz for OVAR, solving a mystery of this experimentally observed discrepancy. Finally, we determined that certain sensory conflict perceptions reported by vestibular patients could be explained via mathematical simulation.

Differences between perception and eye movements during complex motions

During passive whole-body motion in the dark, the motion perceived by subjects may or may not be veridical. Either way, reflexive eye movements are typically compensatory for the perceived motion. However, studies are discovering that for certain motions, the perceived motion and eye movements are incompatible. The incompatibility has not been explained by basic differences in gain or time constants of decay. This paper uses three-dimensional modeling to investigate gondola centrifugation (with a tilting carriage) and off-vertical axis rotation. The first goal was to determine whether known differences between perceived motions and eye movements are true differences when all three-dimensional combinations of angular and linear components are considered. The second goal was to identify the likely areas of processing in which perceived motions match or differ from eye movements, whether in angular components, linear components and/or dynamics. The results were that perceived motions are more compatible with eye movements in three dimensions than the one-dimensional components indicate, and that they differ more in their linear than their angular components. In addition, while eye movements are consistent with linear filtering processes, perceived motion has dynamics that cannot be explained by basic differences in time constants, filtering, or standard GIF-resolution processes.

Visuomotor Velocity Transformations for Smooth Pursuit Eye Movements

Smooth pursuit eye movements are driven by retinal motion signals. These retinal motion signals are converted into motor commands that obey Listing's law (i.e., no accumulation of ocular torsion). The fact that smooth pursuit follows Listing's law is often taken as evidence that no explicit reference frame transformation between the retinal velocity input and the head-centered motor command is required. Such eye-position-dependent reference frame transformations between eye- and head-centered coordinates have been well-described for saccades to static targets. Here we suggest that such an eye (and head)-position-dependent reference frame transformation is also required for target motion (i.e., velocity) driving smooth pursuit eye movements. Therefore we tested smooth pursuit initiation under different three-dimensional eye positions and compared human performance to model simulations. We specifically tested if the ocular rotation axis changed with vertical eye position, if the misalignment of the spatial and retinal axes during oblique fixations was taken into account, and if ocular torsion (due to head roll) was compensated for. If no eye-position-dependent velocity transformation was used, the pursuit initiation should follow the retinal direction, independently of eye position; in contrast, a correct visuomotor velocity transformation would result in spatially correct pursuit initiation. Overall subjects accounted for all three components of the visuomotor velocity transformation, but we did observe differences in the compensatory gains between individual subjects. We concluded that the brain does perform a visuomotor velocity transformation but that this transformation was prone to noise and inaccuracies of the internal model.

Phase-linking and the perceived motion during off-vertical axis rotation

Human off-vertical axis rotation (OVAR) in the dark typically produces perceived motion about a cone, the amplitude of which changes as a function of frequency. This perception is commonly attributed to the fact that both the OVAR and the conical motion have a gravity vector that rotates about the subject. Little-known, however, is that this rotating-gravity explanation for perceived conical motion is inconsistent with basic observations about self-motion perception: (a) that the perceived vertical moves toward alignment with the gravito-inertial acceleration (GIA) and (b) that perceived translation arises from perceived linear acceleration, as derived from the portion of the GIA not associated with gravity. Mathematically proved in this article is the fact that during OVAR these properties imply mismatched phase of perceived tilt and translation, in contrast to the common perception of matched phases which correspond to conical motion with pivot at the bottom. This result demonstrates that an additional perceptual rule is required to explain perception in OVAR. This study investigates, both analytically and computationally, the phase relationship between tilt and translation at different stimulus rates-slow (45 degrees /s) and fast (180 degrees /s), and the three-dimensional shape of predicted perceived motion, under different sets of hypotheses about self-motion perception. We propose that for human motion perception, there is a phase-linking of tilt and translation movements to construct a perception of one's overall motion path. Alternative hypotheses to achieve the phase match were tested with three-dimensional computational models, comparing the output with published experimental reports. The best fit with experimental data was the hypothesis that the phase of perceived translation was linked to perceived tilt, while the perceived tilt was determined by the GIA. This hypothesis successfully predicted the bottom-pivot cone commonly reported and a reduced sense of tilt during fast OVAR. Similar considerations apply to the hilltop illusion often reported during horizontal linear oscillation. Known response properties of central neurons are consistent with this ability to phase-link translation with tilt. In addition, the competing "standard" model was mathematically proved to be unable to predict the bottom-pivot cone regardless of the values used for parameters in the model.

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

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

Pica behavior induced by body rotation in mice

OBJECTIVES

To study whether rotational stimulus induced pica and whether the vestibular apparatus was necessary for obtaining rotation-induced pica in mice.

METHODS

Pica behavior in mice was investigated following 60 min of rotation once daily at 70 rpm (15 s on with 5 s off) for 3 consecutive days. After evaluating vestibular function and histology of vestibular epithelia, we examined rotation-induced kaolin intake, so-called pica, in sham-lesioned and chemically labyrinthectomized mice.

RESULTS

The labyrinthectomized mice exhibited loss of the contact righting and swimming capability while the destruction of hair cells of vestibular epithelia was observed. Moreover, mice subjected to rotation, but not labyrinthectomized mice, showed a significant increase in kaolin intake at the last 2 rotation sessions and the first postrotation session.

CONCLUSIONS

The findings indicated that a functioning vestibular system is necessary for rotation-evoking pica in mice and thus pica can be a behavioral index of motion sickness in mice.

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.

Head-shaking nystagmus depends on gravity

In acute unilateral peripheral vestibular deficit, horizontal spontaneous nystagmus (SN) increases when patients lie on their affected ear. This phenomenon indicates an ipsilesional reduction of otolith function that normally suppresses asymmetric semicircular canal signals. We asked whether head-shaking nystagmus (HSN) in patients with chronic unilateral vestibular deficit following vestibular neuritis is influenced by gravity in the same way as SN in acute patients. Using a three-dimensional (3-D) turntable, patients (N = 7) were placed in different whole-body positions along the roll plane and oscillated (1 Hz, +/-10 degrees ) about their head-fixed vertical axis. Eye movements were recorded with 3-D magnetic search coils. HSN was modulated by gravity: When patients lay on their affected ear, slow-phase eye velocity significantly increased upon head shaking and consisted of a horizontal drift toward the affected ear (average: 1.2 degrees /s +/-0.5 SD), which was added to the gravity-independent and directionally nonspecific SN. In conclusion, HSN in patients with chronic unilateral peripheral vestibular deficit is best elicited when they are lying on their affected ear. This suggests a gravity-dependent mechanism similar to the one observed for SN in acute patients, i.e., an asymmetric suppression of vestibular nystagmus by the unilaterally impaired otolith organs.

Human 3-D aVOR with and without otolith stimulation

We describe in detail the frequency response of the human three-dimensional angular vestibulo-ocular response (3-D aVOR) over a frequency range of 0.05-1 Hz. Gain and phase of the human aVOR were determined for passive head rotations in the dark, with the rotation axis either aligned with or perpendicular to the direction of gravity (earth-vertical or earth-horizontal). In the latter case, the oscillations dynamically stimulated both the otolith organs and the semi-circular canals. We conducted experiments in pitch and yaw, and compared the results with previously-published roll data. Regardless of the axis of rotation and the orientation of the subject, the gain in aVOR increased with frequency to about 0.3 Hz, and was approximately constant from 0.3 to 1 Hz. The aVOR gain during pitch and yaw rotations was larger than during roll rotations. Otolith and canal cues combined differently depending upon the axis of rotation: for torsional and pitch rotations, aVOR gain was higher with otolith input; for yaw rotations the aVOR was not affected by otolith stimulation. There was a phase lead in all three dimensions for frequencies below 0.3 Hz when only the canals were stimulated. For roll and pitch rotations this phase lead vanished with dynamic otolith stimulation. In contrast, the horizontal phase showed no improvement with additional otolith input during yaw rotations. The lack of a significant otolith contribution to the yaw aVOR was observed when subjects were supine, prone or lying on their sides. Our results confirm studies with less-natural stimuli (off-vertical axis rotation) that the otoliths contribute a head-rotation signal to the aVOR. However, the magnitude of the contribution depends on the axis of rotation, with the gain in otolith-canal cross-coupling being smallest for yaw axis rotations. This could be because, in humans, typical yaw head movements will stimulate the otoliths to a much lesser extent then typical pitch and roll head movements.

Neural control of three-dimensional eye and head movements

Although the eyes and head can potentially rotate about any three-dimensional axis during orienting gaze shifts, behavioral recordings have shown that certain lawful strategies--such as Listing's law and Donders' law--determine which axis is used for a particular sensory input. Here, we review recent advances in understanding the neuromuscular mechanisms for these laws, the neural mechanisms that control three-dimensional head posture, and the neural mechanisms that coordinate three-dimensional eye orientation with head motion. Finally, we consider how the brain copes with the perceptual consequences of these motor acts.