Visual-vestibular interactions: the directional component of visual background movement

Legibility of a head-fixed display and visual suppression of the vestibulo-ocular reflex (VOR) were found to be superior when vestibular stimuli and optokinetic stimuli were of like direction (i.e. would produce the same directions of nystagmus) and inferior when they were opposite in direction. Velocities (relating to the head) of peripheral optokinetic stimuli ranging between -18 degrees/s and +180 degrees/s interacted effectively with vestibular stimuli to influence visibility of a head-fixed display. This indicates that peripheral optokinetic stimulation can influence visual suppression of the VOR at velocities that far surpass effective production of optokinetic nystagmus.     

Eye movements to yaw, pitch, and roll about vertical and horizontal axes: adaptation and motion sickness

BACKGROUND: In the search for parameters to predict motion sickness that can be measured in the laboratory, we performed a longitudinal investigation in aviators. Since the vestibular system is involved in the generation of motion sickness as well as eye movements, vestibulo-ocular reflex (VOR) parameters seemed relevant. We investigated three topics: 1) the effect of axis orientation and its orientation to gravity on the VOR; 2) changes in VOR parameters depending on flight experience; and 3) differences in VOR parameters in aircrew with high and low susceptibility to motion sickness. HYPOTHESIS: Nystagmus decay after angular velocity steps would be faster for non-susceptible and trained aviators. METHODS: We recorded eye movements evoked by angular on-axis velocity steps (+/- 90 degrees x S(-2), to and from 90 degrees x S(-1)) in yaw, pitch, and roll, about both the Earth vertical and Earth horizontal axes in 14 subjects with a low susceptibility to motion sickness. These data were compared with those of 10 subjects with a high susceptibility. RESULTS: Horizontal axis rotations are nauseogenic. We found that during (per) and post-condition, left- and rightward rotation responses were equal, and the orientation with respect to gravity did not alter the basic nystagmus decay, apart from a sinusoidal modulation. Moreover, pitch and roll rotations show equal nystagmus decays, significantly faster than for yaw; yaw and pitch peak velocities were equal and were larger than for roll. With regard to changes in VOR parameters depending on flight experience, we found that repeated vestibular stimulation reduced nystagmus decay as well as the otolith modulation. With respect to the changes in VOR parameters and motion sickness susceptibility, we found that subjects highly susceptible to motion sickness showed a slower decay of nystagmus with a larger peak velocity than less susceptible subjects. CONCLUSIONS: Group averages indicate a difference in eye movement parameters, only in yaw, depending on flight experience; and between subjects with low and high susceptibility to motion sickness. The involvement of the velocity storage mechanism as realized by an internal model is given as a plausible explanation.     

Relation between the asymmetry of optokinetic nystagmus and optovestibular and vestibulo-autonomic stability

Comparative quantitative analysis of physiological asymmetry of the optokinetic nystagmus produced predominantly by central optokinetic stimulation (field of vision = 30 degrees) or peripheral stimulation (field of vision = 110 degrees) reveals an objective correlation between the asymmetry coefficient of the central optokinetic nystagmus with respect to the angular velocity of its slow phase and susceptibility to motion sickness caused by Coriolis and pseudocoriolis acceleration, the nystagmus velocity being up to 45 deg/sec.     

Promethazine affects optokinetic but not vestibular responses in monkeys

BACKGROUND: Promethazine is used to treat motion sickness including Space Adaptation Syndrome, but there is incomplete information about how it affects vestibular and optokinetic responses. METHODS: Vestibular and optokinetic nystagmus, recorded with eye coils, were characterized in monkeys after administration of promethazine at dosages approximately equivalent to those used by humans in space. RESULTS: The initial increase of horizontal eye velocity during optokinetic nystagmus (OKN) was reduced after receiving the drug. Consequently, it took a longer time for eye velocity to rise to 60% of steady state value, the normal initial jump in eye velocity. Steady state OKN, maximum gains of optokinetic after-nystagmus (OKAN) and OKAN falling time constants were unaffected. The gains and time constants of the horizontal, vertical and roll angular vestibulo-ocular reflex (aVOR), the amplitude and velocity of saccades, and ocular counter-rolling (OCR), induced by off-vertical axis rotation (OVAR) were unaffected by promethazine. A two-component optokinetic model simulated the data simply by reducing the gain of the initial (rapid) component of OKN. A reduction in coupling between a non-linear element and the velocity storage integrator was required to simulate some vertical OKN data. CONCLUSIONS: Promethazine reduces the gain of the direct visual-oculomotor pathway in monkeys. It has little effect on saccades, the gain and time constant of the aVOR and the low frequency linear vestibulo-ocular reflex (IVOR), which orients the eyes during ocular counterrolling. The optokinetic deficit is consistent with reported reduction in ocular pursuit and VOR suppression after promethazine in humans.     

Role of optokinetic stimulation in realizing vestibulospinal reflexes

This paper presents data about the effect of optokinetic stimulation (OKS) on vestibulospinal reflexes and discusses mechanisms of interaction of the vestibular, optic and proprioceptive sensors during their combined stimulation. The vestibulospinal reflexes were investigated using a 2-minute step test and simultaneous OKS that was produced by a portable optokinetic drum mounted on the head of the test subject. During the tests optokinetic nystagmus was recorded and the angle of body rotation relative to the initial position was measured. It was found that during the step test the body turned along the OKS direction, i. e. towards the slow component of optokinetic nystagmus. During leftward OKS the angle of rotation was larger than during rightward OKS: 406.4 +/- 75.9 deg and 207.5 +/- 40.7 deg, respectively. During leftward OKS loss of equilibrium was recorded 4.5 times more often than during rightward OKS. It has been demonstrated that the capacity to track stimuli moving to the left is lower than that to pursue stimuli moving to the right. It has been shown that there is a correlation between the rate of the optokinetic nystagmus slow phase and the angle of body rotation during the step test. It has been concluded that optokinetic nystagmus can be used as an informative parameter when measuring statokinetic stability in response to multisensory stimulation.     

Pattern of vestibular reactions and sensory interactions in weightlessness (data of the experiment "Optokinesis")

This paper presents results of studying spontaneous and evoked optokinetic, opto-oculomotor and vestibulo-oculomotor reactions in microgravity. The examinations were performed during the 7-day Soviet-Indian and 237-day Salyut-7 missions. During an early stage of adaptation to microgravity the following changes occurred: enhanced spontaneous activity of the eyes, including spontaneous nystagmus in one of the crewmembers, decline of the efficiency of saccadic pursuit (disappearance of saccades in the course of pursuit of a moving stimulus, decrease of the saccadic amplitude, emergence of correction microsaccades in response to fixation events), and decrease of the threshold of the optokinetic nystagmus. The compensatory counter-rotation of the eyeballs during head side-to-side movements with open eyes in dark glasses did not disappear but slightly decreased, being slightly accompanied by low-amplitude nystagmus. When rocking the head with open eyes, one of the cosmonauts showed a complete eye destabilization and other cosmonauts displayed high-amplitude nystagmus. In response to head rotation with open eyes they developed clonic nystagmus. Vestibular stimulation in the form of active head movements helped to normalize the pattern of opto-ocular responses.     

Effects of gravitoinertial force variations on vertical gaze direction during oculomotor reflexes and visual fixation

Recordings of horizontal and vertical eye movement were obtained on eight subjects exposed to repeated patterns of vertical and horizontal optokinetic stimulation, visual fixation with a fixed or unseen target, and voluntary head oscillation in the high force and free-fall periods of parabolic flight. The downward shift of the beating field of vertical optokinetic nystagmus (OKN) observed in previous experiments was confirmed in the present study. The same directional shift was also noticed during optokinetic after-nystagmus (OKAN). Vertical direction of gaze clearly shifted downward during the decreased gravitoinertial force level when subjects were exposed to horizontal optokinetic stimulation, or when they attempted to track an unseen target in the dark with the head stationary or actively moved up and down. A vertical nystagmus with slow phases directed upward was observed during transition from high force level to free-fall when subjects were fixating their gaze on a stationary target. These findings are in agreement with those showing a general downward drive of the eyes on the first exposure to microgravity during orbital flight and an upward drive on the first day of return. Although this drive could be a consequence of a disorientation experienced by subjects undergoing parabolic flight or by astronauts, the phenomenon also supports the hypothesis of a tonic influence exerted by the otoliths on the postural and extra-ocular musculature in order to compensate for the downward pull by the gravitoinertial forces.     

Effect of altered blood circulation on human nystagmic reactions

Optokinetic and vestibulo-optokinetic nystagmus in response to optokinetic and combined vestibulo-optokinetic stimulation applied during head-down tilt was investigated. Tilt-induced circulation changes produced a modifying effect on nystagmic reactions which included a decrease of optokinetic and vestibulo-optokinetic nystagmus and its subsequent recovery after return to the horizontal position. The absolute parameters of vestibulo-optokinetic nystagmus changed depending on the direction of head movements in the sagittal plane relative to the long axis of the body. Some aspects of the relationships between sensory and nystagmic components of vestibular reactions during "conflict" stimulation of the vestibular and visual sensory systems are discussed.     

Dynamics of motor components of optokinetic nystagmus during exposure of the vestibular apparatus to angular accelerations]

The effect of angular accelerations on the optokinetic nystagmus was studied on 25 test subjects using a special vestibulo-optokinetic trainer rotated according to five programs and electrooculography. Angular accelerations were found to induce indirectly changes in the amplitude and time of the optokinetic nystagmus via the vestibular apparatus. Optimization of parameters of the optokinetic nystagmus was reflected in the criterion of positive difference + delta between angular velocities of the slow component of the nystagmus and optokinetic stimuli. It is postulated that the criterion + delta is a necessary factor in the organization of optimal time relations between optokinetic stimuli and central processes that are responsible for the final result--sensory image of movement.     

Some effects of sleep loss on vestibular responses

This study assessed ocular nystagmus and motion experiences of men exposed to both simple (angular acceleration) and complex (Coriolis) vestibular stimulation during approximately 55 h of sleep loss. Control and sleep-deprived groups each comprised 10 young men. Angular accelerations and Coriolis stimulation (30 degrees head movements during CW rotation) were accomplished in darkness in an enclosed Still-Werner rotating device. Nystagmus and motion experiences (turning, "diving," and "climbing") were recorded throughout each session. Tests were given at 0900 and 1300 on each of 3 successive days. Subjects ingested 10-mg of d-amphetamine at 1200 on Day 3. During simple stimulation, the sleep-deprived group showed regular declines across sessions in slow phase and duration measures of nystagmus but fast phase ocular frequency and measures of experienced turning resisted declines until the final predrug session; subjective response latencies increased with sleep loss. Declines during rotation for ocular output and measures of perceived displacement during rightward head tilts ("climbing" sensation) were obtained for the sleep deprived, but both nystagmus and sensations were unaffected by return (leftward) movements of the head ("diving" sensation). d-Amphetamine had no consistent effect on responses of control subjects, but significantly increased nystagmus and elevated (but not significantly) measures of turning experiences for the sleep deprived.     

Effects of gravitoinertial force variations on optokinetic nystagmus and on perception of visual stimulus orientation.

Recordings of horizontal and vertical eye movement were obtained with subjects exposed to vertical, horizontal, and oblique optokinetic stimulation during parabolic flight. When the optokinetic stimulation was vertical, the upward slow phase eye velocity increased during transition from high force level to free-fall, and decreased during transition from free-fall to high force level. During optokinetic stimulation in the horizontal and oblique plane, the gravitoinertial forces of parabolic flight induced changes in the velocity of the vertical component of the eye movements, and, therefore, changes in the plane of the eye movements. Some subjects also perceived modifications in the apparent orientation of the visual motion. These findings are in agreement with previous observations on the presence of a vertical nystagmus induced by changes in plane vertical acceleration. They also suggest a close interaction of reflexive eye movements induced by graviceptor inputs and visual inputs for visual stabilization during variations of gravitoinertial force level.     

低氧对视动刺激诱发的运动病的影响

为探讨中度低氧对视动刺激诱发运动病的影响及其特点和规律,２０岁１８￣２５岁的健康男性,按照“拉丁方”排列和“自身对照”法,进行了单纯视动刺激和低氧视动刺激试验。主要观察和记录了运动病症状及视动性水平眼震（ＯＫＨＮ）。结果显示：（１）低氧视动刺激（ＨＯＳ）的运动病症状得分显著高于单纯视动刺激（ＯＳ）时的得分（Ｐ〈０．００１）;（２）低氧视动刺激时的视动性水平眼震慢相速度（ＳＰＶ）显著小于单纯视动刺激     

Vestibulo-ocular responses in man to +Gz hypergravity

The influence of high +Gz gravito-inertial force on the vestibular system in man was investigated in a 4-m centrifuge with a freely swinging gondola. The Gz profile was: acceleration +0.2 Gz/s, +3 Gz sustained for 3 min, deceleration -0.2 Gz/s. The subject was exposed to this profile under two conditions in randomized order: facing forward and facing backward. Under these conditions, the effective angular velocity in the plane of the vertical semicircular canals is opposed. Adding the slow phase velocity responses from these conditions yields the Gz effect only; subtracting yields the angular velocity effect only. Vertical vestibular nystagmus was analysed in five subjects. Results indicate that +3 Gz induced a subject-dependent vertical nystagmus with slow phase downwards. The average amplitude of this nystagmus reached a maximum of 27 degrees/s at 16 s from G onset, and was 11 degrees/s after 3 min of sustained +3 Gz. The vestibular stimulation by +Gz could result in false subjective perception of attitude, and play a major role in spatial disorientation in flight.     

Optokinetic drum tilt hastens the onset of vection-induced motion sickness

BACKGROUND: Under optokinetic drum conditions, a stationary participant views the patterned interior of a rotating drum. Quickly, most participants perceive illusory self-rotation in the direction opposite to the drum's true rotation (vection). It has been documented that up to 60% of participants experience motion sickness-like symptoms under optokinetic conditions perhaps because of conflicting sensory information from the visual and vestibular systems. METHODS: Keeping rotation speed constant (10 RPM), drum tilt relative to the axis of rotation was systematically manipulated (0 degrees, 5 degrees, 10 degrees), producing a wobble effect. Overall well-being and eight motion sickness symptoms were assessed every 2 min using subjective scales. RESULTS: Participants reported 1) a complex type of circular vection that included a "wobble" or "sway" component and 2) a quicker onset of motion sickness-like symptoms as tilt increased. CONCLUSION: In a tilted drum, the vestibular system correctly indicates that the participant is stationary while the visual system indicates a complex type of self-rotation. This type of sensory conflict is more severe than what takes place under typical optokinetic drum conditions (no tilt). Results suggest that as visual/vestibular sensory conflict increases, so does the speed at which motion sickness symptoms occur.     

Vestibular nystagmus as a result of the interactions between semicircular canals and the otolithic membrane subsystems of the vestibular system]

Untreated and treated (unilateral section of utricular and saccular branches of the vestibular nerve) pigeons Columba livia were rotated in the dark in the horizontal plane, the head being in a different position relative to the axis of rotation. The range of angular acceleration was 7-19 deg/c2 and the peak value of centrifugal acceleration was 0.5 g. The neck and eye nystagmus was recorded. It was found that: 1) the result of canal-otolith interaction was not directly related to the pattern of changes of otolith afferentation but was determined by the ratio of otolith afferent signals in the right and left labyrinths and, consequently, by the ratio of patterns of activities in the CNS paired structures that receive otolith afferentation; 2) the same result of interaction (enhancement or attenuation of vestibular responses) can be achieved through both increase or decrease of otolith afferentation; 3) if joint stimulation of semicircular canals and otolith organs induces asymmetry of neuronal activities of paired brain structures that perceive otolith afferentation, then, irrespective of the mechanism of origin of this asymmetry, it is followed by changes of oppositely directed nystagmus of different sign (increase for one nystagmus and decrease for the other). It is concluded that the symmetry of reactions recorded in response to isolated stimulation of semicircular canals (otolith organs) cannot be considered as a reliable criterion of functional symmetry of semicircular canals (otolith organs).     

Effects of caloric vestibular stimulation on head and trunk movements during walking

The effects of vestibular stimulation on head and trunk movements were investigated during human walking (4.0 km/h). Vestibular stimulation was produced by irrigating an external auditory meatus with 4 °C ice water for 10 s. Using a 3-D motion analysis system, the linear (medial/lateral and vertical) translations and angular (yaw, pitch and roll) rotations were determined at the head, thorax, pelvis, knee, and foot. After caloric stimulation, waking trajectory deviated toward the stimulated side during dizziness. In addition, the amplitude of medial/lateral (M/L) linear translation and yaw rotation were significantly increased by caloric stimulation, especially at the head and thorax, whereas changes in vertical translation and pitch and roll rotations were not significant. The compensatory coordination (i.e., the yaw rotation to oppose the M/L linear translation) of the head was precisely maintained both before and after caloric stimulations, but it was decreased at the thorax and pelvis after stimulation. Our results suggested that vestibular sensory information, probably via the horizontal semicircular canals, contributes predominantly to the regulation of dynamic head and trunk movements in the M/L direction.     

Coordination of the head and eyes in pursuit of predictable and random target motion.

Subjects were required to use their head and eyes in pursuit of visual targets which moved randomly or sinusoidally in the horizontal plane. All subjects disliked moving their heads to pursue the random motion, apparently because the motion broke fixation which resulted in a predominance of the vestibulo-ocular compensatory reflex over the smooth pursuit reflex. As a consequence gaze (head plus eye movement) was at times in the opposite direction to the motion of the target. In steady state pursuit of sinusoidal targets, eye movement consisted of a combination of pursuit and vestibulo-ocular reflex eye movements. At frequencies below 0.8 HZ, the vestibular reflex was used at times of minimum target velocity to stabilize fixation whereas during maximum target velocity the head movement was slowed and the smooth pursuit reflex predominated. At 1 HZ and over, there was a failure to suppress the compensatory vestibulo-ocular reflex; however, the saccades of vestibular nystagmus were used to "catch up" the target. There was a preference not to use the head in predictable pursuit.     

Comparative assessment of vestibular, optokinetic, and optovestibular stimulation in the development of experimental motion sickness

The contribution of vestibular, optokinetic, and optovestibular stimulation to experimental motion sickness was evaluated in 29 volunteer subjects. Vestibular stimulation (Coriolis effect) was found to induce the most significant vestibular-autonomic disorders. Optokinetic stimulation (pseudo-Coriolis effect) and optovestibular stimulation could provoke such disorders only in susceptible subjects. In quantitative terms, optokinetic and optovestibular stimulation were less effective than vestibular Coriolis stress. Nystagmic reactions of susceptible subjects to the three types of stimulation differed significantly from those of tolerant subjects. This may be important from the theoretical point of view because susceptibility to motion sickness and responses to vestibular and optokinetic stimulation may be universal and associated with the general CNS mechanism, i.e. inhibition mechanism. The identified correlation between the duration of postoptokinetic illusion and motion sickness susceptibility may be used to differentiate susceptible and tolerant subjects.     

Motion sickness induced by optokinetic drums

Motion sickness is not only elicited by certain kinds of self-motion, but also by motion of a visual scene. In case of the latter, optokinetic drums are often used and a visual-vestibular conflict is assumed to cause the sickness. When the rotation axis is Earth vertical however, different studies show different results. Here, we propose that visual-vestibular conflicts per se do not cause sickness whereas subjective vertical mismatch theory can reconcile the disparate findings. The theory attributes the nausea induced by horizontal optokinetic stimulation to the subjects self-inducing pseudo-Coriolis by head movement. This highlights the shortcomings of an optokinetic apparatus--that is non-rigid or inaccurately oriented--and the importance of constraining the subject's behavior.     

The effect of weightlessness on vestibular function

During and after space missions crew members showed significant vestibular changes. The cosmonaut-researcher of the orbital complex Soyuz T-7--Salyut-7 developed inhibition of compensatory and enhancement of nystagmic eye movements in response to active head movements in the frontal plane. During an acute stage of readaptation crew members of the orbital complex Soyuz T-4--Salyut-6 displayed attenuation of the oculomotor reaction to optokinetic stimulation. One of them exhibited overt asymmetry of this oculomotor reaction. This cosmonaut also developed distinct spontaneous nystagmus in the dark and floating eye movements when he tried to fix his gaze at a stationary object. These findings give evidence that space flight may induce vestibular asymmetry and disorders of mechanisms of opto-vestibular interaction.