Measures of auditory evoked potentials during optokinetic stimulation

In this investigation auditory brainstem responses (ABR) elicited by click stimuli were recorded before, during, and after optokinetic stimulation in subjects that were (N = 10) or were not (N = 10) prone to visually induced motion sickness. The latency of Wave I, and the I-III and I-V interwave intervals were measured. A significant increase in the I-III interwave interval occurred only during optokinetic stimulation. Neither the Wave I latency nor the interwave interval differed with respect to subject groups and this factor did not interact with any other variables. These results suggest that optokinetic stimulation may alter neural activity in the region of the superior olivary complex, a structure known to be important in sound-source localization.

Gain and phase of cat vertical eye movements generated by sinusoidal pitch rotations with and without head tilt

Vertical EOGs were recorded in cats during sinusoidal head pitch from 0.01 to 4.0 Hz with peak velocities of 50 degrees X s-1. The purpose of the experiments was to determine whether dynamic response properties of the vertical vestibulo-ocular reflex (VOR) elicited by pitch with the animal lying on its side (on-side pitch) differ from those resulting from normal (upright) pitch. During on-side pitch (not changing head position with respect to gravity), the vertical VOR gain was 13.5% less than during upright pitch. Thus, the vertical VOR was more compensatory than during on-side pitch. Phase did not differ between the two conditions. The results indicate the importance of interactions between otolith and vertical canal stimulation for vertical eye movement control. The results imply that in micro-gravity, where head tilt does not lead to otolith stimulation as regards gravity, vertical head and eye movements may not be controlled appropriately, leading to vestibular-visual conflicts at the same time that horizontal eye movement controls are normal.

Effects of a visual-vestibular stimulus on the vestibulo-ocular reflex

Exposure before flight to an environment that provides altered visual-vestibular relationships similar to those seen in weightlessness might facilitate adaptation to spaceflight. Fifteen normal subjects were exposed for 0, 5, and 20 min to an abnormal visual-vestibular environment in which the vestibular sensation of lateral tilt was associated with the visual impression of lateral translation. Lateral eye movement elicited by similar tilt in darkness was measured before and at intervals after the exposure. Of the 15 subjects, 12 showed increased VOR amplitude following the exposure, but this increase appeared unrelated to the duration of exposure to the visual stimulus and may be an effect of the VOR measurement procedure. Most subjects showed little shift in the phase relationship between tilt and eye motion. Three subjects showed changes in VOR and five reported changes in perception of motion which appear clearly related to the visual-vestibular exposure.

The influence of gravitoinertial force level on oculomotor and perceptual responses to sudden stop stimulation

Our goal was to determine whether the vestibular response to vertical, z-axis body rotation in the dark is influenced by the magnitude of gravitoinertial force. We measured the nystagmus and the duration of illusory self-motion elicited in blindfolded subjects by cessation of such rotation during the free-fall, high, and terrestrial force phases of parabolic flight maneuvers. Both measures were significantly lower in zero G than in 1 G, and lower to a smaller extent in 1.8 G. The decreased intensity of nystagmus was due specifically to a decrease in the time constant of slow phase velocity decay with no decrement in peak velocity. This pattern of findings is consistent with the responses we had observed earlier to constant levels of Coriolis, cross-coupled stimulation during parabolic flight maneuvers both in terms of the mode of nystagmus suppression and the effect of G-level. Attenuation of the vestibular response to rotary acceleration in free-fall causes sensory-motor mismatches during natural head movements in orbital flight that may be important factors in the evocation of space motion sickness.

Human optokinetic afternystagmus. Is the fast component of OKAN decay due to smooth pursuit?

Eye movement after-effects subsequent to pursuit of a single LED target were studied in human subjects to test the hypothesis that constant velocity pursuit activates a velocity storage system in the neuronal pathway. The temporal characteristics of observed after-effects fall within those predicted from the Robinson model of eyeball mechanics, indicating that neuronal integration was not a factor.

Thermal stimulation of the vestibular labyrinth during orbital flight

During the European Spacelab mission (SL1) in 1983, caloric testing was performed for the first time in long-term weightlessness. After 2 days into orbital flight an unequivocal caloric nystagmus was observed in both subjects tested which corresponded in both quality and intensity with that measured in one-g conditions on Earth. The subsequent D1 mission enabled the experiment to be repeated on further subjects and with improved measurement procedures. As with the SL1 findings, the observed caloric nystagmus response proved to be equivalent to that measured during baseline testing on Earth. Renewed consideration of peripheral and central mechanisms, which might be involved in the elicitation of the caloric response--both in one-g and zero-g environments--has led to the reopening of a number of associated issues. One important observation which has been addressed by various research groups concerns the influence of the labyrinth's orientation to the gravity vector on the caloric response. The present authors have examined a group of healthy subjects in various body positions in the sagittal plane. The interindividual variability in the response behavior was found to be high; indeed single cases were observed in which the nystagmus response did not invert from the supine to the prone positions. These findings are discussed together with earlier reports in the literature.

[Balloon method and water irrigation in thermal vestibular assessment. Electronystagmographic comparison of both methods]

In cases of large central perforation of the eardrum, caloric test of the vestibular organ cannot be carried out by irrigation of the external auditory canal with water but must be done with a balloon instilled in the external auditory canal perfused with water. Caloric responses were registered by using water irrigation and balloon irrigation in 40 healthy persons. Nystagmographic registrations were analysed. Frequency, slow phase velocity and amplitude of nystagmus were measured. Cumulative percentage curves of the four caloric responses are drawn to compare the distribution of data in balloon and water irrigation (Figures 2-5). In addition, mean values and standard deviations of data and of difference in excitability related to the total excitability are recorded in table 1. Statistics (Wilcoxon test for matched pairs) reveals no significant differences in nystagmographic pattern of caloric reactions between water and balloon stimulation. It is concluded that in the caloric test of the vestibular organ balloon irrigation is equivalent to water irrigation. Balloon irrigation is more comfortable for the patient and easier to handle.

Interobserver agreement in assessment of vestibulo-ocular responses

In 30 comatose head injured patients vestibulo-ocular responses were elicited by caloric stimulation. Inter- and intra-observer agreement of these responses was studied. The responses were shown in the form of a film to ten doctors who classified them into four categories: no reaction, tonic reaction, paradoxical nystagmus and nystagmus. The level of the coefficient Kappa (a chance-corrected measure of interobserver agreement) was 0.50; this is within the range of levels for most components of clinical examination reported in the literature.

The caloric vestibular test in weightlessness

Although the theoretical background for the caloric vestibular test was assumed to be known, recent studies in weightlessness have thrown doubts on the original theory of its mechanism. It is most likely that several mechanisms are involved in the generation of caloric nystagmus in addition to the convection current theory. Experiments with the caloric test in humans conducted during parabolic flight are described. These results are compared with the results of similar experiments performed in orbital missions.

General remarks on the role of the vestibular system in weightlessness

Different methods are described to experimentally achieve weightlessness. Since the function of the otolith system depends on the presence of contact forces opposing gravity, it is disabled in weightlessness and may send misleading positional information to the brain. Without the contributions of the otolith system it is difficult in space to distinguish self-motion from object motion. Furthermore, the disintegration of information from the neck position receptors from those of the otolith system can lead to additional illusory positional sensations. Since the function of the semicircular canal system in previous space flights was found to be essentially undisturbed, the vestibular experiments in the Spacelab-D1 missions concentrated more on the otolith system. The function of other orientational cues from the visual system, the semicircular canal system and proprioception can be studied in isolation from the otolith system in space. In the Spacelab-D1 mission, the space vestibular sled was used as a device for studying linear acceleration. The vestibular helmet "permitted" video and EOG recordings of all eye movements and provided caloric and optokinetic stimulation. Various factors contributing to static and dynamic forms of space sickness are identified.

[Determination of visual acuity using optokinetic nystagmus. A newly developed instrument based on Günther's principle]

The two most important methods used for objective determination of visual acuity are those of Ohm-von Romberg and Günther. A newly developed instrument based on Günther's principle is described which combines the advantages of both methods. Using a Leitz projector and a specially designed accessory unit, a constantly moving screen of dots measuring 64 X 64 cm is projected. The setting was standardized on the basis of 358 eyes with visual acuities ranging from finger counting to 1.0. The correlation between the measured distance (from screen to patient) at which an optokinetic nystagmus was just induced and visual acuity was r = 0.98 over the entire visual acuity range. The visual acuity to be expected can be read off from the diagrams or a table, with a scatter range of +/- 1 sigma, +/- 2 sigma, or +/- 3 sigma. The relevant literature is discussed. Replication of the accessory unit described can be recommended.

The role of saccular afferents in vertical optokinetic nystagmus in primates. A study in relation to optokinetic nystagmus in microgravity

After bilateral sacculectomies, the squirrel monkey's vertical optokinetic nystagmus showed a gain increase of slow-phase down-nystagmus and a gain decrease of the slow-phase up-nystagmus. The nystagmus beating field of the former shifted upward but that of the latter did not change clearly. These results are in general agreement with the findings in man during the recent shuttle flight and indicate the functional linkage of saccular afferents to the vertical optokinetic nystagmus.

Testing the vestibular-ocular reflexes: abnormalities of the otolith contribution in patients with neuro-otological disease

Conventional vestibular rotation testing with the head centered on the axis stimulates the semicircular canals evoking compensatory eye movements. If the head is placed forwards of the axis in an eccentric position the otoliths are also stimulated by a tangential linear acceleration acting laterally to the skull. In normal subjects the additional otolithic stimulus evokes compensatory eye movements with a higher gain than with head centred, particularly for high frequency (greater than 0.1 Hz) stimuli. The responses with head centred and eccentric in various patients with known/suspected neuro-otological abnormalities have been compared. Patients with vestibular neurinectomies who have asymmetrical head centred responses showed greater asymmetry with head eccentric at higher stimulus frequencies. Some patients with cerebellar lesions showed abnormally enhanced or depressed and asymmetrical responses with head eccentric in comparison with head centred responses, which could be normal. The enhancing effects could be specific to low frequency stimuli. All patients who showed abnormal responses with head eccentric also had positional nystagmus provoked by the gravity acceleration vector when the head was tilted laterally. The direction of the positional nystagmus with respect to the gravity vector was not necessarily the same as the direction of the effect on eye movements of lateral acceleration during eccentric oscillation. Patients with benign paroxysmal vertigo or chronic linear vertigo in whom otolithic abnormalities are suspected were not found to have abnormal responses with head eccentric. We conclude that this method of testing may be useful in elucidating pathophysiology but is not a decisive clinical test for the presence of disordered otolith function.

Nystagmus episodes related to sea sickness

In order to test the possible presence of nystagmus during episodes of sea sickness, we performed electronystagmography (ENG) on nine subjects before and during sailing on a 35-ton vessel in a somewhat heavy sea. In addition, the accelerations of the x, y and z axes of the ship were also monitored and simultaneously recorded. Nystagmus was not detected in pre-sailing recordings, while all of the subjects demonstrated clear positional nystagmus during sailing. The nystagmus was episodic, direction-fixed and its intensity was greater in sea-sick than in non-sick subjects. Our findings suggest that the nystagmus recorded is labyrinthine in origin and stems from temporary peripheral vestibular dysfunction elicited by sea sickness.

[Effects of visual field stabilization on the behavior of automobile drivers: visuo-vestibular model trials]

Postural changes of body equilibrium on a moving force plate were examined for sinusoidal movements, saccades and frequency whilst different stimuli were applied under various visual conditions. If a dynamic stimulus was used, the stabilization of vision shows a distinct visual-vestibular conflict which provokes postural disequilibrium. This is the cause of motion sickness during car driving. This disequilibrium is also the cause of the motor insecurity experienced during walking and reading at the same time.

Slow eye movements induced by apparent target motion in monkey

Slow eye movements were observed while the monkey fixated on a subject-stationary, small target light in front of a moving optokinetic drum in an attempt to suppress optokinetic nystagmus (OKN). These slow eye movements of low amplitude were directed opposite to the moving optokinetic stimulus and, hence, were not identical to slow phases of incompletely suppressed OKN. It is assumed, based on comparable findings in humans, that these slow eye movements are induced by a perceived target motion, i.e. by the perception of an apparent motion of the subject-stationary fixation light opposite to the actual motion of the optokinetic drum.

Value of otoneurological tests in the staging of multiple sclerosis

We looked for correlations between otoneurological findings and neurological signs in multiple sclerosis (MS). 41 patients were tested for: spontaneous and caloric nystagmus, horizontal pursuit eye tracking, full-field optokinetic nystagmus and eye movements. The findings were compared with the course of the disease (progressive or remitting), age at onset of MS and disability, in order to test the predictive value of this type of test. The correlation between otoneurological abnormalities and lesion site is very high for brainstem lesions. Subclinical brainstem lesions were detected in 66% of patients with optic neuritis only and in 42% of those with spinal cord involvement only. Eye movement disorders are more frequent (p less than 0.1) in disease of long duration. The vestibular examination was altered in 100% of patients with clinically definite and probable MS, in 91% of those with possible and in 80% of those with suspected MS. No specific otoneurological pattern was identified. No significant correlation with age at onset or with disability was found.

Eye movements and vestibulo-ocular reflex in the blind

To assess the effect of chronic deprivation of visual feedback, 21 blind patients underwent clinical and electronystagmographical examination. Patients with congenital blindness were characterized by spontaneous eye movements, inability to consciously move the eyes and absence of the vestibulo-ocular reflex (VOR), whereas eye movement abnormalities were practically absent in those with blindness acquired late in life. Active visual experience, at least in early life, seems to be crucial for the development of eye movements and VOR adjustment.

Pseudo-internuclear ophthalmoplegia with downshoot in myasthenia gravis

Internuclear ophthalmoplegia is characterized by an adduction deficit on lateral gaze with dissociated nystagmus of the abducting eye. It is seen with lesions of the medial longitudinal fasciculus. In myasthenia gravis, extraocular muscle weakness can cause the same oculomotor pattern, which has been referred to as pseudo-internuclear ophthalmoplegia. We report the additional finding of downshoot in the adducting eye in two patients with pseudo-internuclear ophthalmoplegia and positive Tensilon tests.

Vertical and horizontal visual whole-field motion differently affect the metabolic activity of the rat medial terminal nucleus

The metabolic activity of the medial terminal nucleus (MTN) of the Accessory Optic System was studied by means of the [14C]2-deoxyglucose (2-DG) method in Long-Evans rats exposed to moving and stationary visual stimuli. In particular we explored the rate of local cerebral glucose utilization (LCGU) and the spatial distribution of 2-DG uptake within MTN related to visual stimuli capable of triggering optokinetic nystagmus. It was found that increases in MTN metabolism accompanied the retinal slip signals evoked by whole-field visual patterns moving in the vertical as well as in the horizontal direction. At the same level of luminous flux neither the same but stationary pattern, nor constant, diffuse illumination were able to elicit comparable changes in MTN metabolic rates. The effects of vertical and horizontal motions differed, however, from each other. In binocular testing LCGU rates resulted significantly higher after vertically moving patterns and upon the same stimulus condition the spatial distribution of 2-DG matched very closely the spatial distribution of the retinal afferents and the cellular density within MTN, in sharp contrast with the diffuse spreading out of the label across the nucleus following horizontal motion. In monocular testings only the vertically moving patterns were able to increase LCGU rates significantly and then in contralateral MTN alone. However, comparison between the levels of glucose consumption measured in binocular and in monocular vision also showed the involvement of the uncrossed retinal path in relaying the retinal slip signals to MTN. No difference in LCGU and in spatial distribution of the label were finally observed in relation to the upward or to the downward direction of the moving pattern.

Influence of head or trunk oscillations on visually induced self-motion perception in humans

The influence of concurrent head or trunk movements on optokinetically induced self-motion perception has been investigated by measuring circularvection (CV) latencies. Active head as well as passive trunk oscillation cause an increase in circularvection latency of about 50% as compared to latencies with the head and trunk stationary during full field optokinetic stimulation. The results point towards an intersensory inhibition of self-motion perception.