Suppression Of Vestibulo-ocular Reflex Research Articles

Abnormalities of ocular motility in myotonic dystrophy.

Are the oculomotor disturbances in myotonic dystrophy (MD), i.e. reduced smooth pursuit (SP) gain and reduced saccadic peak velocity (PV), of muscular or central origin? To answer this question the following two approaches were used. (i) The performance of SP was compared with the patient's ability to suppress the vestibulo-ocular reflex (VOR) visually (VOR suppression; VOR-S). In the latter task the SP system is involved, but the eyes hardly move within the orbits. A parallel impairment of SP and VOR-S would indicate a central dysfunction. (ii) Peak saccadic velocity was compared between two saccades performed to and fro in rapid succession. The intention was to measure any myotonic effect which might build up after the first saccade and slow down the second saccade. We studied 15 MD patients and 15 age-matched controls. Stimuli for slow eye responses consisted of sinusoidal horizontal rotations of the SP target and/or the vestibular rotation chair at frequencies between 0.1 and 0.8 Hz. Saccades were analysed in terms of PV. accuracy, duration and latency, comparing centripetal versus centrifugal saccades at short and long intersaccadic intervals (ISI; 400 ms and 900 ms, respectively). The SP gain was reduced in patients compared with the controls, the effect being most pronounced (32% less) at the highest stimulus frequency. Whereas VOR was normal in the patients, VOR-S was clearly impaired (50% worse at 0.8 Hz). Despite normal saccadic accuracy, peak saccadic velocity was significantly lower in the patient group (23% less for saccades of 12 degrees amplitude), similarly for centrifugal and centripetal saccades; all these differences were independent of the ISI. Latency was normal with centrifugal saccades, but was considerably increased with centripetal saccades at short ISI (67% longer compared with controls). The observation of a parallel degradation of SP and VOR-S in the patients is interpreted in terms of a central deficit in the SP pathways. Thus, it appears that slow eye movements were not impaired by muscle dystrophy and myotonia to a considerable degree in our patients. The increase in saccadic latency for centripetal saccades at the short ISI also reflects a central deficit. However, the observed slowing of saccades might have a myopathic or neural origin; a distinction was not possible at present. A myotonic origin of the saccade slowing seems unlikely, because the effect was independent of the presaccadic activation of the relaxing (antagonistic) eye muscle.

Object motion perception during ego-motion: Patients with a complete loss of vestibular function vs. normals

Object motion perception was assessed in avestibular patients and normal controls. Two experiments were conducted, in which subjects were required to assess the motion of a visual stimulus with respect to earth. In the first experiment, we measured the velocity at which a briefly presented (200 ms) grating was perceived as earth fixed, while the subject maintained fixation on a visual target fixed relative to the body, during whole-body yaw rotation (VOR suppression). In this experimental setup, the influence of the semicircular canal signals on object motion perception was evaluated. The avestibular patients judged the grating to be stationary with respect to earth, when it was moving at the same velocity as their body, whereas for normal controls, the grating was perceived as stationary when it moved at a velocity slower than their body motion, but greater than zero. The difference between the two subject groups was significant, and showed the strong contribution of the vestibular system to object motion perception. Similarly, a measurement of the velocity at which a grating was perceived as stationary was obtained during smooth pursuit eye movements. In this experiment the contribution of the efference copy of the oculomotor signal and proprioceptive signals to object motion perception were assessed. As with the first experiment, the normal controls displayed a more veridical sense of object motion perception than the patients, although the difference was only just significant. We suggest that the difference could be an adaptive change in the patients perception of motion, which allows them to reduce the effects of oscillopsia.

Properties of superior vestibular nucleus flocculus target neurons in the squirrel monkey. II. Signal components revealed by reversible flocculus inactivation

1. Seven upward eye velocity flocculus target neurons (FTNs) and two flocculus projecting neurons (FPNs) were studied before and after ipsilateral flocculus inactivation by injection of muscimol in the alert squirrel monkey. An additional seven FTNs and seven FPNs recorded from the corresponding FTN and FPN areas were recorded after injection. Response properties of FTNs and FPNs were characterized by visual-vestibular interaction paradigms and were compared before and after flocculus inactivation. 2. In FTNs the mean firing rate increased within 2-5 min after muscimol injection in the flocculus and reached a plateau level in approximately 10-20 min. The average mean firing rate for seven FTNs increased from 117 to 174 spikes/s, a net increase of 57 spikes/s (49%). Accompanying the large increase of the mean firing rate, a spontaneous nystagmus in the darkness developed with the slow phase directed upward and contralateral. 3. The firing rate modulation during visual following of a sinusoidal optokinetic drum (0.5 Hz) decreased within 2-5 min after muscimol injection in the flocculus and reached a level of 0 in approximately 10-20 min for all FTNs. After that, some cells remained unmodulated for the period of recording; other cells gradually reversed their phase and developed a modulation out of phase with drum velocity. The depletion of the visual following eye velocity signal on superior vestibular nucleus (SVN) FTNs accompanied a small but consistent decrease of visual following eye velocity amplitude. The average maximum decrease of eye velocity was 26 +/- 9% (mean +/- SD). 4. After flocculus inactivation, even though the modulation response at 0.5 Hz during visual following was abolished, a slow-component eye velocity signal with the same on direction was revealed by a constant-velocity optokinetic stimulus. It is concluded that there are at least two kinds of eye velocity signals during the optokinetic response. These signals are combined at the FTNs and are subsequently relayed to the oculomotor neurons. The source of the fast component is the flocculus, and the source of the slow component is another, as yet unidentified brain structure. 5. The effect of flocculus inactivation on the modulation amplitude during the vestibuloocular reflex (VOR) in darkness was variable: two cells did not change, two cells decreased, and three cells increased their amplitude. The response phase tended to move toward a phase lead, but the change was small. The effect on VOR suppression was more prominent.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of superior vestibular nucleus flocculus target neurons in the squirrel monkey. I. General properties in comparison with flocculus projecting neurons

1. Using single-unit recording and microstimulation methods, a group of flocculus target neurons (FTNs) were identified in the superior vestibular nucleus (SVN) and were studied using visual-vestibular interaction paradigms in alert squirrel monkeys. The response properties of these FTNs were characterized and compared with those of flocculus projecting neurons (FPNs). 2. FTNs were monosynaptically inhibited by single-pulse flocculus stimulation. The mean inhibition latency was 1.0 +/- 0.57 (SD) ms (n = 40) and the mean inhibition period was 6.7 +/- 2.69 ms. FTNs were also monosynaptically activated by VIIIth nerve stimulation. The mean response latency was 1.10 +/- 0.25 ms (n = 12). This is about the same as that of the FPNs (1.14 +/- 0.16 ms, n = 17). 3. The most characteristic response property of the FTNs is their firing rate modulation during visual following eye movements induced by sinusoidal rotation of an optokinetic drum at 0.5 Hz. This modulation was mainly related to eye velocity and was therefore termed a visual following eye velocity signal. The average eye velocity gain for all FTNs is 0.79 spikes.s-1.deg-1.s-1. In contrast, the responses of FPNs were not modulated under the same conditions. 4. Even though FTNs are inhibited by the flocculus, they have a relatively higher mean firing rate (124 +/- 23 spikes/s, n = 45) than FPNs (66 +/- 28 spikes/s, n = 42). The underlying mechanism may be related to commissural facilitation of FTNs and commissural inhibition of FPNs. 5. Thirty FTNs were identified as upward eye velocity FTNs because their firing rate increased for upward eye velocity during a visual following eye movement. The mean eye velocity sensitivity was 1.09 spikes.s-1.deg-1.s-1. Most of these cells also modulated during vestibuloocular reflex (VOR) in the dark, with firing rate increasing for downward head velocity. During VOR suppression the firing rate either did not modulate or modulated in phase with head or drum velocity with a smaller amplitude in comparison with the response during visual following. For all cells (with 1 exception) the response during a visual following eye movement can be approximately predicted by a linear vectorial subtraction of the response during VOR suppression and the response during VOR in the dark [modulation response vector of FTNs during visual following of the optokinetic stimulus (OKR) approximately modulation response vector of FTNs during VOR suppression-modulation response vector of FTNs during VOR in the dark].(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebellar role in adaptation of the goldfish vestibuloocular reflex.

1. The time course of eye velocity responses elicited by head velocity steps was compared in normal, adapted, and cerebellectomized goldfish. Vestibuloocular reflex (VOR) adaptation was induced by combined visual and vestibular stimulation that altered the ratio of eye to head velocity (VOR gain) toward values either higher or lower than the control amplitude. The velocity step consisted of alternating periods of head rotation at a constant velocity of 16 degrees/s zero-to-peak around the vertical axis. 2. The VOR produced by head velocity steps consisted of an early acceleration-related component, the dynamic response, separated from a sustained period of constant velocity, the plateau, by a sag that occurred around 125-150 ms. Latency of the VOR averaged 18 ms for the adducting eye and 20 ms for abducting eye independent of the initial VOR gain. Adapted dynamic VOR responses diverged from the control records at the earliest detectable latency after both high and low VOR gain training. This result demonstrates modification in the shortest latency brain stem VOR pathway, presumably, the three-neuron reflex arc. 3. After acute cerebellectomy the adapted dynamic response was unaltered for approximately 50 ms in the low-gain and 70 ms in the high-gain VOR states. Not less than 30% of the altered velocity was retained throughout the remaining dynamic and sustained component. These results demonstrate that the vestibulocerebellum is not necessary for the maintenance of the earliest adapted eye velocity. Hence brain stem pathways are sufficient for the expression of the modified VOR. 4. Purkinje cells identified by simple and complex spikes were recorded extracellularly in the area of the vestibulocerebellum, where electrical stimulation produced conjugate ipsiversive horizontal eye movements. Independent eye and head velocity sensitivities were determined in response to visual world motion and VOR suppression, respectively. The two signals either added, canceled, or were both present in Purkinje cells throughout the range of eye velocity induced by vertical axis visual-vestibular stimulation. 5. Latency of Purkinje cell discharge to either a vestibular or visual velocity step exhibited means of 43 and 70 ms, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Anatomical segregation of different adaptative processes within the vestibulocerebellum of the cat.

Bilateral surgical lesions of the flocculus or the nodulo-uvular lobes were performed in the cat. Effects of these lesions on optokinetic and optokinetic afternystagmus OKAN), vestibulo-ocular reflex (VOR), visual suppression, and adaptation and habituation of VOR were studied using an identical experimental protocol. After flocculectomy, all these functions were impaired, except for habituation. Long-term postoperative recordings only revealed a recovery of the suppression of VOR, suggesting a limited contribution of the flocculus to this function. After nodulo-uvulectomy, only habituation and OKAN were modified. When the lesion was restricted to part of the uvula, OKAN duration was decreased. For other lesions involving the uvula together with the nodulus and/or the lobules VII-VIII,OKAN duration was increased. Habituation was lost after destruction of the nodulo-uvular lobes. When this latter structure was damaged, the retention component of habituation was selectively impaired, sparing the acquisition. Additional lesions outside the vestibulocerebellum appeared necessary to suppress the two components. Comparison of results obtained after flocculectomy and after nodulouvulectomy confirms and extends to non-primate species the concept of a "differential control" of adaptation and habituation by distinct vestibulocerebellar structures.

Preview control of gaze saccades: efficacy of prediction modulates eye-head interaction during human gaze saccades.

Healthy human subjects made orienting saccades towards visual target stimuli, either with the head fixed or during intended time optimal head movements. Four experimental paradigms were used to study the influence of target predictability on eye-head coordination. They represented different sequences of horizontal target steps, that were varied in amplitude, direction and frequency. In some subjects midflight perturbations of the active head movements were applied to examine the intrasaccadic vestibulo-ocular reflex (VOR). In coordinated gaze saccades, latencies and dynamics of the eye saccade and the additional head trajectory demonstrated specific task-related changes with respect to the head fixed condition. Highly predictable target steps result in the relatively earlier onset of the head movement and an increase of the intrasaccadic head contribution to the overall gaze displacement. Differences in the level of VOR suppression became significant when gaze amplitudes exceeded 60 degrees. Consequently, an effective speed up of large gaze saccades was found with increased target predictability. We concluded, that eye-head coordination during human gaze saccades underlies high level preview control mechanisms. A parametric modulation of the intrasaccadic VOR maintains gaze accuracy, although the actual contribution of the more flexible head motor system varied, depending on gaze amplitude and prediction. The efficacy of preview control depends on interaction of these factors.

Eye Movements in a Familial Vestibulocerebellar Disorder

Eye movement abnormalities consisting of poor or absent smooth pursuit and vestibulo-ocular reflex suppression, gaze-paretic and rebound nystagmus, slow build-up of optokinetic nystagmus, mildly hyperactive vestibulo-ocular reflex, and a high incidence of strabismus were inherited in an autosomal dominant fashion in 10 members of a non-consanguineous English caucasian family. The onset was in early childhood, but was not congenital. In 7 cases there was no tremor, dizziness, consistent ataxia, or other cerebellar signs that are often associated with these ocular motor deficits, and apart from strabismus, patients were asymptomatic. Magnetic resonance imaging of the propositus was normal. After childhood there appears to be no progression, with the oldest affected member being 40 years. Two members had been prone to falling in childhood, and one admitted to dizziness when tired. This condition, which is probably benign, has not been previously described and may represent a very mild variant of episodic ataxia or a new vestibulocerebellar syndrome.

Predictive Mechanisms of Head-Eye Coordination and Vestibulo-Ocular Reflex Suppression in Humans

Head and eye movements of human subjects have been recorded during head-free pursuit in the horizontal plane of a target executing sinusoidal motion at a frequency of 0.26 to 0.78 Hz and a peak velocity of +/- 96 degrees/s. The target was not presented continuously but was exposed for brief durations of 120 to 320 ms as it passed through the centre of the visual field at peak velocity. This technique allowed the timing of each response to be assessed in relation to the onset of target appearance. During the first 3 to 4 target presentations, there was a progressive buildup of both head velocity and the smooth component of gaze velocity, while, simultaneously, the responses became more phase-advanced with respect to target onset. In the steady state, similar temporal response trajectories were observed for head and gaze velocity, which were initiated approximately 500 ms prior to target onset, rose to a peak that increased with the duration of target exposure, and then decayed with a time constant of 0.5 to 1 s. Whenever the target failed to appear as expected, the gaze and head velocity trajectories continued to be made, indicating that predictive suppression of the vestibulo-ocular reflex (VOR) was taking place in darkness. In a further experiment, subjects attempted to suppress the VOR during whole body oscillation at 0.2 or 0.4 Hz on a turntable by fixating a head-fixed target that appeared for 10 to 160 ms at the time of peak head velocity. Again, VOR suppression was initiated prior to target appearance in the same manner as for natural head movements, and when the target suddenly disappeared but rotation continued, predictive VOR suppression was observed in darkness. The similarity of these predictive effects to those obtained previously for head-fixed pursuit provides further support for the hypothesis that both pursuit and visual suppression of the VOR are controlled primarily by identical visual feedback mechanisms.

Discharge patterns in nucleus prepositus hypoglossi and adjacent medial vestibular nucleus during horizontal eye movement in behaving macaques.

1. Monkeys were trained to perform a variety of horizontal eye tracking tasks designed to reveal possible eye movement and vestibular sensitivities of neurons in the medulla. To test eye movement sensitivity, we required stationary monkeys to track a small spot that moved horizontally. To test vestibular sensitivity, we rotated the monkeys about a vertical axis and required them to fixate a target rotating with them to suppress the vestibuloocular reflex (VOR). 2. All of the 100 units described in our study were recorded from regions of the medulla that were prominently labeled after injections of horseradish peroxidase into the abducens nucleus. These regions include the nucleus prepositus hypoglossi (NPH), the medial vestibular nucleus (MVN), and their common border (the "marginal zone"). We report here the activities of three different types of neurons recorded in these regions. 3. Two types responded only during eye movements per se. Their firing rates increased with eye position; 86% had ipsilateral "on" directions. Almost three quarters (73%) of these medullary neurons exhibited a burst-tonic discharge pattern that is qualitatively similar to that of abducens motoneurons. There were, however, quantitative differences in that these medullary burst-position neurons were less sensitive to eye position than were abducens motoneurons and often did not pause completely for saccades in the off direction. The burst of medullary burst position neurons preceded the saccade by an average of 7.6 +/- 1.7 (SD) ms and, on average, lasted the duration of the saccade. The number of spikes in the burst was well correlated with saccade size. The second type of eye movement neuron displayed either no discernible burst or an inconsistent one for on-direction saccades and will be referred to as medullary position neurons. Neither the burst-position nor the position neurons responded when the animals suppressed the VOR; hence, they displayed no vestibular sensitivity. 4. The third type of neuron was sensitive to both eye movement and vestibular stimulation. These neurons increased their firing rates during horizontal head rotation and smooth pursuit eye movements in the same direction; most (76%) preferred ipsilateral head and eye movements. Their firing rates were approximately in phase with eye velocity during sinusoidal smooth pursuit and with head velocity during VOR suppression; on average, their eye velocity sensitivity was 50% greater than their vestibular sensitivity. Sixty percent of these eye/head velocity cells were also sensitive to eye position. 5. The NPH/MVN region contains many neurons that could provide an eye position signal to abducens neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of prediction in head-free pursuit and vestibuloocular reflex suppression.

Annals of the New York Academy of SciencesVolume 656, Issue 1 p. 687-694 The Role of Prediction in Head-Free Pursuit and Vestibuloocular Reflex Suppression G. R. BARNES, G. R. BARNES MRC Human Movement & Balance Unit National Hospital Queen Square London WC1N 3BG, EnglandSearch for more papers by this authorM. A. GREALY, M. A. GREALY MRC Human Movement & Balance Unit National Hospital Queen Square London WC1N 3BG, EnglandSearch for more papers by this author G. R. BARNES, G. R. BARNES MRC Human Movement & Balance Unit National Hospital Queen Square London WC1N 3BG, EnglandSearch for more papers by this authorM. A. GREALY, M. A. GREALY MRC Human Movement & Balance Unit National Hospital Queen Square London WC1N 3BG, EnglandSearch for more papers by this author First published: May 1992 https://doi.org/10.1111/j.1749-6632.1992.tb25245.xCitations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume656, Issue1Sensing and Controlling Motion: Vestibular and Sensorimotor FunctionMay 1992Pages 687-694 RelatedInformation

Visual-Vestibular Interaction During Head-Free Pursuit of Pseudorandom Target Motion in Man1

Recordings of head and eye movement were made during pursuit of mixed-frequency, pseudorandom target motion to study the mechanism of vestibulo-ocular reflex (VOR) suppression during head-free pursuit. When high velocity stimuli were used, slow-phase gaze velocity gains decreased significantly with increases in both absolute target velocity and the velocity ratio between the frequency components. These changes occurred independently of changes in the head displacement gain, which remained relatively constant at the lower frequency and were directly attributable to impaired suppression of the VOR. Similar effects were seen when visual feedback was degraded by tachistoscopic illumination of the target. The results indicate that visual feedback, rather than an efference copy of the head velocity signal, is essential for suppression of slow-phase vestibular eye movement during head-free pursuit. When head-free and head-fixed pursuit were compared, striking similarities were seen for both slow phase gaze velocity gain and phase, indicating that gaze control during smooth pursuit is largely independent of the degree of associated head movement. This suggests that the VOR is not switched off during head-free pursuit. An estimate of the underlying VOR gain was obtained by recording the vestibular response produced by active head movements in darkness. The rather higher estimates of VOR gain obtained using an imaginary earth-fixed target paradigm were found to predict head-free gains more closely than the gains obtained during imaginary pursuit of a moving target, suggesting that such measures may be more representative of the underlying VOR gain.

Oculomotor function during space flight and susceptibility to space motion sickness

Horizontal vestibulo-ocular reflex (VOR) and saccadic eye movements (SEM) were studied in 18 subjects before and during five Space Shuttle missions to evaluate the effects of weightlessness and correlations between results and susceptibility to and actual presence of space motion sickness (SMS). Active sinusoidal head oscillation was the stimulus for VOR tests with vision (VVOR), with eyes shaded (VOR-ES), and VOR suppression (VOR-S). Eye movements were recorded by electrooculography and head position by a potentiometer. No pathological nystagmus or other abnormal eye movements were seen. No significant in-flight changes were seen in the gain, phase shift or waveform of VVOR, VOR-ES or VOR-S. Statistically significant increases in saccadic latency and decreases in saccadic velocity were seen, with no change in saccadic accuracy. Preflight differences between SMS susceptible and non-susceptible subjects were noted only in VOR-S, with less complete suppression in susceptible subjects, a finding also seen in flight. During flight, VVOR gain was significantly increased in three non-affected subjects. Saccades of SMS-affected subjects showed increased latency and velocity and decreased accuracy compared to saccades of unaffected subjects.

Vestibulo-ocular Reflex Suppression during High Velocity Head-free Pursuit in Normal Subjects

Recordings of head and smooth pursuit eye movement were made during head-free pursuit of a pseudo-random target motion stimulus. The pseudo-random stimulus was composed of 2 high velocity sinusoids, of frequency 0.4 and 1.3 Hz, with the velocity of the higher frequency being varied as a ratio of the lower frequency velocity between 0 and 2. Slow-phase gaze velocity gain for the lower frequency component decreased significantly with an increase in velocity ratio, and with an increase in target velocity above 60 degrees/s. Gaze velocity gain was frequently less than head displacement gain which remained fairly constant, indicating that the eyes had been driven in the opposite direction to head movement as a result of inability of suppress the vestibulo-ocular reflex. Similar effects were seen when visual feedback was degraded by tachistoscopic illumination of a target composed of 2 low velocity sinusoids (0.11 and 0.13 Hz). These results indicate that visual feedback, rather than head displacement, is essential for suppression of slow-phase vestibular eye movement during head-free pursuit, even at high stimulus velocities.

A non-visual mechanism for voluntary cancellation of the vestibulo-ocular reflex

Squirrel monkeys were trained to cancel their vestibulo-ocular reflex (VOR) by fixating a visual target that was head stationary during passive vestibular stimulation. The monkeys were seated on a vestibular turntable, and their heads were restrained. A small visual target (0.2 degrees) was projected from the vestibular turntable onto a tangent screen. The monkeys' ability to suppress their VOR by fixating a head stationary target while the turntable was moving was compared to their ability to pursue the target when it was moved in the same manner. Squirrel monkeys were better able to suppress their VOR when the turntable was moved at high velocities than they were able to pursue targets that were moving at high velocities. The gaze velocity gain during VOR cancellation began to decrease when the head velocity was above 80 degrees/s, and was greater than 0.6 when the head velocity was above 150 degrees/s. However, gaze velocity gain during smooth pursuit decreased significantly when the target velocity was greater than 60 degrees/s, and was less than 0.4 when the target velocity was 150 degrees/s or more. The latency of VOR suppression was significantly shorter than the latency of smooth pursuit while the monkey was cancelling its VOR. When an unpredictable step change in head acceleration was generated while the monkey was cancelling its VOR, the VOR evoked by the head acceleration step began to be suppressed shortly after the initiation of the step (approximately 30 ms). On the other hand, the latency of the smooth pursuit eye movement elicited when the visual target was accelerated in the same manner during VOR cancellation was approximately 100 ms. The comparison between these two results suggests that the monkeys did not use visual information related to target motion to suppress their VOR at an early latency. The monkeys' ability to suppress the VOR evoked by an unexpected change in head acceleration depended on the size of the head acceleration step. The VOR evoked by unexpected step changes in head acceleration was progressively less suppressed at an early latency as the size of the acceleration step increased, and was not suppressed at an early latency when the step change in head acceleration was greater than 500 degrees/s2. During smooth pursuit eye movements, unexpected step changes in head acceleration evoked a VOR that was suppressed at an early latency (approximately 50 ms) if the head movement was in the same direction as the ongoing smooth pursuit eye movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies of the horizontal vestibulo-ocular reflex in spaceflight.

Changes in the vestibulo-ocular reflex (VOR) during space flight have been suspected of contributing to space motion sickness. The horizontal VOR was studied in nine subjects on two space shuttle missions. Active unpaced head oscillation at 0.3 Hz was used as the stimulus to examine the gain and phase of the VOR with and without visual input, as well as the visual suppression of the reflex. No statistically significant changes were noted inflight in the gains or phase shifts of the VOR during any test condition, or between space motion sickness susceptible and nonsusceptible populations. Although VOR suppression was unaffected by spaceflight, the space motion sickness-susceptible group tended to exhibit greater error in the suppression than the nonsusceptible group. It is concluded that at this stimulus frequency, VOR gain is unaffected by spaceflight, and any minor individual changes do not seem to contribute to space motion sickness.

Suppression of OKN and VOR by afterimages and imaginary objects.

Optokinetic nystagmus (OKN) is suppressed if attention is directed to a centrally placed afterimage superimposed on a moving display. Imagining a stationary object has little or no effect. An afterimage does not provide the retinal slip and misfoveation error signals provided by a stationary object and we have shown that an effective error signal does not arise from occlusion or masking of the display by the afterimage. Although a lack of relative motion between afterimage and moving display could indicate when OKN gain is one, there is no unique relative motion signal associated with a gain of zero. Subjects could partially inhibit the vestibulo-ocular reflex (VOR) in the dark when they imagined a head-fixed object. They could suppress the response more effectively by attending to an afterimage, but the suppression was still only partial. When OKN and VOR were evoked simultaneously, pursuit movements of the eyes could not be suppressed until the vestibular inputs had subsided. We conclude that signals associated with OKN, are fully available to the mechanism that assesses the headcentric motion of objects but that signals associated with VOR are only partially available to that mechanism.

The effect of alcohol on the control of the vestibulo-ocular reflex and gaze

The effects of ethyl alcohol on vestibulo-ocular reflex (VOR) and gaze functions were investigated in normal adults. Twelve normal adults were administered whisky at the rate of 1.0ml/kg (ethyl alcohol 0.43g/kg), so that the mean peak concentration of breath alcohol amounted to 0.15mg/l. Before and after alcohol administration, the subjects underwent sinusoidal rotation tests with an amplitude of 40 degrees and frequencies of 0.2-0.85Hz and eye tracking tests of unidirectional motion at 30 degrees-70 degrees/sec. For the rotation test, 7 subjects were examined at the dose of 0.65g/kg ethyl alcohol. The following results were obtained. 1. The mean value of VOR gain under the condition of performing mental arithmetic in the dark was not significantly altered by alcohol. However, the magnitude of change in VOR gain showed a highly significant correlation with the gain before drinking; whereas small values were enlarged and large values were suppressed by alcohol. 2. The accuracy of spacial gaze fixation was scarcely affected by alcohol, even at the dose of 0.65g/kg. 3. Visual fixation on a head-fixed target (fixation-induced suppression of VOR) and smooth pursuit were significantly impaired by alcohol. 4. The present study indicated that VOR is regulated even in the dark by both gaze stabilizing functions, i.e., spacial gaze fixation and smooth pursuit.

Slow conjugate eye movements: Normative data for routine diagnosis of ophthalmo-neurological disorders

With regard to the use of slow conjugate eye movements in the diagnosis of ophthalmo-neurological disorders, the suitability of various stimulus parameters for routine examinations was evaluated and normative data were obtained with Infrared-Reflection-Oculography (IROG) from 21 healthy subjects. Sinusoidal and for smooth pursuit, additionally triangular stimuli were used at various frequencies for visual as well as for vestibular stimulation with a constant amplitude of 30 degrees (± 15 deg). Sinusoidal stimulus presentation proved to be more adequate for examining smooth pursuit than triangular. For smooth pursuit and vestibulo-ocular reflex suppression (VORS) stimulus frequencies up to 0.5 Hz appeared to be most suitable for routine examinations. Normative data were also obtained for a simple VORS test consisting of manual back and forth rotation of the subjects' head by the examiner. Examinations of the vestibulo-ocular reflex (VOR) and of the visual VOR (VVOR) are recommended for particular clinical ...

Effects of visual and non-visual mechanisms on the vestibulo-ocular reflex during pseudo-random head movements in man.

1. The behaviour of the vestibulo-ocular reflex (VOR) in man was examined using pseudo-random and sinusoidal whole-body angular-motion stimuli applied about the yaw axis by a servo-controlled turn-table. 2. The VOR response was assessed in four conditions; during fixation on a head-fixed target (HFT); during attempted fixation in the dark of an imagined head-fixed (IHFT) or earth-fixed target (IEFT) and in darkness (DRK) whilst performing an auditory discrimination task. 3. When the pseudo-random stimulus was composed of four sinusoids, the three lowest frequencies (0.11, 0.24 and 0.37 Hz) were maintained constant whilst the highest frequency (F4) was varied from 0.39 to 2.08 Hz. In darkness (DRK condition) and when imagining a head-fixed target (IHFT condition) the gain of slow-phase eye velocity was not significantly affected by the frequency of the highest-frequency component, although there were significant changes in the phase for the IHFT condition. 4. During fixation of a real head-fixed target (HFT condition), both the gain and phase of eye velocity were significantly modified by the frequency (F4) of the highest-frequency component. When F4 was 0.39 Hz, all frequency components had a low gain (mean 0.05), but as F4 was increased there was a significant (P less than 0.001) increase in gain for all three low-frequency components which reached a maximum (mean 0.17) when F4 was 2.08 Hz. However, the gain for the highest frequency component always remained comparable to that obtained in response to a single discrete sinusoid of the same frequency. 5. When the stimulus was composed of only two sinusoids a similar increase in gain of the lower-frequency (0.22 Hz) component was observed in the head-fixed target condition as the frequency of the higher-frequency component was increased from 0.39 to 2.78 Hz. However, VOR gain was not significantly modified by the frequency of the higher-frequency component when subjects imagined a head-fixed or earth-fixed target in darkness. 6. The findings indicate that high levels of VOR suppression can be achieved in the head-fixed target condition with pseudo-random stimuli when all frequency components are below 0.4 Hz. But if the highest-frequency component rises above 0.8 Hz, optimum suppression is confined to the highest-frequency component, whilst suppression of the low-frequency components is significantly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)