Unidirectional habituation of vestibulo-ocular responses by repeated rotational or optokinetic stimulations in the cat

Videoconference-Based Adapted Physical Exercise Training Is a Good and Safe Option for Seniors

Videoconference-based adapted physical exercise combines the benefits of supervised exercise training with staying at home, when conventional training is inaccessible. However, exercising with the use of a screen can be considered an optokinetic stimulation, and could therefore induce changes in sensory processing, affecting postural stability. The objectives of this study were to compare the effectiveness of the training delivered Face-to-Face and by Videoconferencing in improving physical capacities of older adults, and to evaluate the possible effects of the Videoconference mode on the processing of sensory information that could affect postural control. Twenty eight older adults underwent the supervised exercise program for sixteen weeks either Face-to-Face or by Videoconference. Muscular strength of knee and ankle flexors and extensors, maximum oxygen uptake, postural stability and horizontal rotational vestibulo-ocular reflex were evaluated before and after the training. Both modes of training similarly increased the VO₂ peak and strength of the motor muscles of lower limbs in all participants. The use of the Videoconference did not modify the vestibulo-ocular reflex in subjects or the importance of vision for postural control. Therefore, the Videoconference-based exercise training can be considered a safe and effective way to maintain good functional capacity in seniors.

Rebalancing the Vestibular System by Unidirectional Rotations in Patients With Chronic Vestibular Dysfunction

Introduction: Vestibular dysfunction is a common disorder that results in debilitating symptoms. Even after full compensation, the vestibulo-ocular reflex (VOR) could be further improved by using rehabilitation exercises and visual-vestibular adaptation. We hypothesized that in patients with asymmetric vestibular function, the system could be rebalanced by unidirectional rotations toward the weaker side (i.e., a pure vestibular stimulation).

Methods: Sixteen subjects (5 female and 11 male, 43.2 ± 17.0 years old) with chronic vestibular dysfunction that was non-responsive to other types of medical treatment were recruited for the study (ClinicalTrials.gov Identifier: NCT01080430). Subjects had VOR asymmetry quantified by an abnormal directional preponderance (DP) with rotation test and no previous history of central vestibular problems or fluctuating peripheral vestibular disorders. They participated either in the short-term study (one session) or the long-term study (7 visits over 5 weeks). Rehabilitation consisted of five trapezoid unidirectional rotations (peak velocity of 320°/s) toward the weaker side. Care was taken to slowly stop the rotation in order to avoid stimulation in the opposite direction during deceleration. To study the short-term effect, VOR responses were measured before and 10, 40, and 70 min after a single unidirectional rotational rehabilitation session. For long-term effects, the VOR gain was measured before and 70min after rehabilitation in each session.

Results: We observed a significant decrease in VOR asymmetry even 10 min after one rehabilitation session (short-term study). With consecutive rehabilitation sessions in the long-term study, DP further decreased to reach normal values during the first 2 sessions and only one subjects required further rehabilitation after week 4. This change in DP was due to an increase in responses during rotations toward the weaker side and a decrease in VOR responses during rotations in the other direction.

Conclusion: Our results show that unidirectional rotation can reduce the VOR imbalance and asymmetry in patients with previously compensated vestibular dysfunction and could be used as an effective supervised method for vestibular rehabilitation even in patients with longstanding vestibular dysfunction.

Neck proprioception shapes body orientation and perception of motion

This review article deals with some effects of neck muscle proprioception on human balance, gait trajectory, subjective straight-ahead (SSA), and self-motion perception. These effects are easily observed during neck muscle vibration, a strong stimulus for the spindle primary afferent fibers. We first remind the early findings on human balance, gait trajectory, SSA, induced by limb, and neck muscle vibration. Then, more recent findings on self-motion perception of vestibular origin are described. The use of a vestibular asymmetric yaw-rotation stimulus for emphasizing the proprioceptive modulation of motion perception from the neck is mentioned. In addition, an attempt has been made to conjointly discuss the effects of unilateral neck proprioception on motion perception, SSA, and walking trajectory. Neck vibration also induces persistent aftereffects on the SSA and on self-motion perception of vestibular origin. These perceptive effects depend on intensity, duration, side of the conditioning vibratory stimulation, and on muscle status. These effects can be maintained for hours when prolonged high-frequency vibration is superimposed on muscle contraction. Overall, this brief outline emphasizes the contribution of neck muscle inflow to the construction and fine-tuning of perception of body orientation and motion. Furthermore, it indicates that tonic neck-proprioceptive input may induce persistent influences on the subject's mental representation of space. These plastic changes might adapt motion sensitiveness to lasting or permanent head positional or motor changes.

Automatic online control of motor adjustments in reaching and grasping

Following the princeps investigations of Marc Jeannerod on action-perception, specifically, goal-directed movement, this review article addresses visual and non-visual processes involved in guiding the hand in reaching or grasping tasks. The contributions of different sources of correction of ongoing movements are considered; these include visual feedback of the hand, as well as the often-neglected but important spatial updating and sharpening of goal localization following gaze-saccade orientation. The existence of an automatic online process guiding limb trajectory toward its goal is highlighted by a series of princeps experiments of goal-directed pointing movements. We then review psychophysical, electrophysiological, neuroimaging and clinical studies that have explored the properties of these automatic corrective mechanisms and their neural bases, and established their generality. Finally, the functional significance of automatic corrective mechanisms-referred to as motor flexibility-and their potential use in rehabilitation are discussed.

The repetition timing of high frequency afferent stimulation drives the bidirectional plasticity at central synapses in the rat medial vestibular nuclei

In this study we show that high frequency stimulation (HFS, 100Hz) of afferent fibers to the medial vestibular nucleus (MVN) can induce opposite long-term modifications of synaptic responses in the type B neurons depending upon the stimulation pattern. Long burst stimulation (LBS: 2s) and short burst stimulation (SBS: 0.55s) were applied with different burst number (BN) and inter-burst intervals (IBI). It results that both LBS and SBS can induce either N-methyl-d aspartate receptors (NMDARs)-mediated long-term potentiation (LTP) or long-term depression (LTD), depending on temporal organization of repetitive bursts. In particular, the IBI plays a relevant role in guiding the shift from LTP to LTD since by using both LBS and SBS LTP is induced by shorter IBI than LTD. By contrast, the sign of long-term effect does not depend on the mean impulse frequency evaluated within the entire stimulation period. Therefore, the patterns of repetitive vestibular activation with different ratios between periods of increased activity and periods of basal activity may lead to LTP or LTD probably causing different levels of postsynaptic Ca(2+). On the whole, this study demonstrates that glutamatergic vestibular synapse in the MVN can undergo NMDAR-dependent bidirectional plasticity and puts forward a new aspect for understanding the adaptive and compensatory plasticity of the oculomotor responses.

Vestibulo-ocular reflex and motion sickness in figure skaters

In order to determine the effect of figure skating on the functional plasticity of the vestibular system, we quantified vestibulo-ocular reflex (VOR) and motion sickness (MS) intensity in 11 female figure skaters and 11 matched control subjects. Vestibular stimulation consisted of three cycles of sinusoidal rotation (0.025 Hz, +/-60 degrees /s) and two velocity steps of 60 degrees /s (acceleration 60 degrees /s(2)). Nauseogenic stimulation consisted of a constant velocity (60 degrees /s) off vertical axis rotation (OVAR) using a 15 degrees tilt angle. Subjective sickness symptoms were rated immediately after OVAR with the Pensacola diagnostic index. During sinusoidal stimulations, the skaters' VOR, as compared with that of the controls, demonstrates a gain that is 27% lower (0.44 +/- 0.12 vs. 0.58 +/- 0.10; P < 0.01) and a phase advance (10 +/- 12 degrees vs. -0.3 +/- 6.4 degrees ; P < 0.05). During velocity steps, the VOR gain is 32% lower among the skaters (0.52 +/- 0.14 vs. 0.71 +/- 0.12; P < 0.01), but there is no difference in time constant (10.8 +/- 1.8 s vs. 10.5 +/- 2.7 s; P = 0.78). Nauseogenic stimulation evokes significantly less MS in figure skaters than in control subjects (2.8 +/- 2.8 vs. 16.2 +/- 13.7; P < 0.01). Quantitative alterations in VOR parameters observed in figure skaters probably result from vestibular habituation induced by repeated unusual stimulations when practicing figure skating.

Retention of habituation of vestibulo-ocular reflex and sensation of rotation in humans

In humans, habituation of vestibulo-ocular reflex (VOR) by repeated caloric or rotational stimulation has been well documented. However, less attention has been directed to the effect of habituation on the sensation of self-rotation and little is known about the retention duration of vestibular habituation. To investigate these characteristics, subjects were exposed to ten sessions of angular velocity steps in yaw, with a chair rotating either alternatively in both CW and CCW directions (bidirectional protocol) or always in the same direction (unidirectional protocol), i.e., CW or CCW. The retention of habituation of VOR and sensation of rotation induced by both protocols was studied for a period up to 8 months following the end of the habituation protocols. There was a progressive decline in the VOR peak slow phase velocity and time constant throughout the sessions during both protocols. These parameters then followed an exponential recovery with a time constant of about 1 month. The duration of the sensation of rotation also habituated during repeated angular velocity steps, but it was shorter for both directions of stimulation, including after the unidirectional protocol. Sinusoidal VOR gain was not affected by vestibular habituation to velocity steps, but sinusoidal VOR phase showed an increase in phase lead at 0.02 and 0.04 Hz, which also returned to baseline values within about 1 month. We conclude that vestibular habituation is a long-lasting phenomenon. These results may be helpful for designing and scheduling the protocols for drug studies using crossover design, rehabilitation of balance disorder patients, and for the application of intermittent artificial gravity during space missions.

Are otolithic inputs interpreted better in figure skaters?

The aim of this study was to investigate whether figure skaters, as individuals who experience intense vestibular stimulation, presented modification of the otolith-ocular reflex. The reflexes of 12 figure skaters were assessed using off vertical axis rotation (OVAR). Horizontal otolith-ocular reflex during OVAR is characterized by two parameters: the eye velocity horizontal modulation, assumed to compensate for perceived lateral linear translation, and the bias, assumed to compensate for the perceived rotation. We observed that skaters presented smaller amplitude of modulation and truly compensatory bias compared with control participants. Thus, the otolithic signal during OVAR seems to be interpreted more as rotation and less as translation or inclination in figure skaters.

Efficacy of tilt-suppression in postrotatory nystagmus in cats

The phenomenon of tilting the head away from an upright position immediately after a horizontal head-rotation, thus reducing the duration of postrotatory nystagmus (PRN), has more than once been called "tilt-suppression." It represents an example of the semicircular canal-otolith interaction in the central vestibular system. In the present study we investigated how head roll-tilt influences the time constants of PRN in the horizontal and vertical planes in cats. The head/body was roll-tilted by 30 degrees toward the upright or the side down from initial roll positions immediately after termination of earth-vertical axis (EVA) rotation. Changes in head orientation either towards or away from the EVA reduced horizontal PRN. The reducing effect was small when the head was roll-tilted toward the EVA. Vertical nystagmus decreased only when the head orientation moved toward alignment with the EVA. Otolithic "tilt-suppression" may be a central neuronal mechanism that is activated to minimize the tumbling sensation of turning about a tilted axis and postural instability, but our results indicate that tilt-suppression of PRN depends on a change in head orientation with respect to the EVA.

Reversible and controlled peripheral vestibular loss by continuous infusion of ropivacaine (Narop) into the round window niche of rats

This paper describes a method for achieving a peripheral vestibular blockade in rats by instillation of local anaesthetics over the round window membrane through a permanently implanted cannula. Being rapidly reversible, the effect of the anaesthetic drug is easily controlled by a single continuous infusion, which can be repeated at any time. The method offers a unique opportunity to study the consequence of single or repeated transient vestibular loss without any use of general anaesthetics, which may be a severe confounding factor. Such studies might shed light on balance disorders related to permanent vestibular loss or episodic vestibular dysfunction. To evaluate the method, spontaneous horizontal eye movements were recorded during the first 4 h of continuous infusion. Unilateral infusion of ropivacaine gave rise to a high-frequency spontaneous nystagmus, reaching levels that have not been documented after a surgical labyrinthectomy under general anaesthesia. This vestibulo-oculomotor behaviour is consistent with a previous report using a single intratympanic instillation of lidocaine to achieve a short-lasting vestibular blockade. In the present study, it was demonstrated that the initial high-frequency nystagmus decreased during the first 100 min of infusion before stabilizing at the same level as recorded when the effect of general anaesthesia has worn off after a surgical ablation. When the transient vestibular blockade was repeated by a second infusion during the following day, the nystagmus frequency saturated on a significantly lower level than during the first blockade. Also, serial single infusions, with recovery between each functional vestibular loss, gave rise to a less severe nystagmus. It is suggested that this phenomenon is an expression of the behavioural concept of 'vestibular habituation', the neural substrate of which is rather unknown.

Fos expression in the vestibular brainstem: what one marker can tell us about the network

Fos inducible transcription factor expression in rodent brains (rats and gerbils) during manipulations of vestibular input is reviewed. Stimuli included centripetal hypergravity, unilateral labyrinth lesion or semicircular canal plugging, rotational axis cross-coupling (Coriolis forces), high and low rotational vestibulo-ocular reflex gain adaptation, translabyrinth galvanic stimulation, pharmacological manipulation, and combinations thereof. Each type of stimulation elicited unique but partially redundant response patterns in the vestibulo-olivo-cerebellar (VOC) network that reflect the origin and interaction of the labyrinth inputs. On the basis of these patterns, a trained observer can predict what the animal experienced during testing; the patterns of VOC Fos expression reveal a trace of recent genomic activity. Based on principal component analysis, VOC network modules associated with lesion recovery, spatial representation and the calibration of gravity, and optokinetic influences are proposed. Probable and possible gene targets of the Fos protein are also reviewed.

Vestibulo-oculomotor behaviour in rats following a transient unilateral vestibular loss induced by lidocaine

The effects of a transient vestibular nerve blockade, achieved by intra-tympanic instillation of lidocaine, were studied in rats by recording horizontal eye movements in darkness. Evaluation of the dose-response relationship showed that a maximal effect was attained with a concentration of 4% lidocaine. Within 15 min of lidocaine instillation, a vigorous spontaneous nystagmus was observed which reached maximal frequency and velocity of the slow phase after about 20 min. Subsequently, the nystagmus failed for approximately half an hour before it reappeared. This could be avoided by providing visual feedback in between the recordings in darkness or by a contralateral instillation of 2.5% lidocaine. It is suggested that the failure reflects an overload of the vestibulo-oculomotor circuits. After recovery from the nerve blockade, when the gaze was stable, dynamic vestibular tests were performed. They revealed that a decrease of the slow phase velocity gain and the dominant time constant during, respectively, sinusoidal- and step stimulation toward the unanaesthetised side, had developed with the nerve blockade. These modulations were impaired by a nodulo-uvulectomy but not by bilateral flocculectomy, which is consistent with the concept of vestibular habituation. A GABA(B) receptor antagonist, CGP 56433A, given systemically during the nerve blockade, aggravated the vestibular asymmetry. The same effect has previously been demonstrated in both short- (days) and long-term (months) compensated rats, by antagonising the GABA(B) receptor. In summary, this study provides the first observations of vestibulo-oculomotor disturbances during the first hour after a rapid and transient unilateral vestibular loss in the rat. By using this method, it is possible to study immediate behavioural consequences and possible neural changes that might outlast the nerve blockade.

Intérêt du traitement par exercices rotatoires chez les patients atteints de syndrome de Ménière, méthode utilisée au service d’ORL des cliniques universitaires Saint-Luc

OBJECTIVE

To date, the effectiveness of balanced rehabilitation for patients with Meniere's syndrome has not been unanimously acknowledged by all physicians and physiotherapists. The purpose of this study is to assess the therapeutic efficacy of rotational exercises in the treatment of disequilibrium for patients with unilateral Meniere's syndrome.

METHODOLOGY

Rotational stimuli were used to symmetrize and reduce postrotatory nystagmic response. Three reference sources were used to assess the efficacy of this management: vestibulospinal function tests: pre- and post-treatment results at the Romberg test, the Unterberger-Fukuda stepping test, the Babinski-Weil test, and gait testing with eyes closed; rotational tests: pre- and post-treatment results; and the self-perceived impact of vertigo: assessed by the Dizziness Handicap Inventory (DHI) and a scale based on the guidelines of the Japanese Society of Equilibrium Research (JSER, 1993). The JSER scale provides quantitative vertigo evaluation; the DHI reflects the patient's perceptual evaluation of handicap.

RESULTS

Patients required 11 sessions (mean value) to attain subjective improvement. Of the 23 patients, only seven required optokinetic stimulation (mean requirement: three sessions). Rotational tests and dynamic tests of the vestibulospinal function improved. The DHI and JSER results show that patients' post-rehabilitation perceptual evaluation significantly improved.

CONCLUSION

The objective and subjective measures of disequilibrium in patients with unilateral Meniere's syndrome were significantly improved.

A pattern correlation model of vestibulo-ocular reflex habituation

Through the process of habituation, the response of the vestibulo-ocular reflex (VOR) is decreased by prolonged, sinusoidal stimulation at lower frequencies (< or =0.1 Hz). Research on goldfish has uncovered frequency-specific and nonlinear behaviors associated with habituation of the goldfish VOR, which include phenomena that cannot be explained using dynamic linear and static nonlinear models. The unexplained phenomena are abrupt decreases at peak response, gain decreases far in excess of linear predictions based on phase, and violation of superposition. Their existence was attributed to a hypothetical switch that closed in the appropriate context. The pattern correlation model provides a new perspective on the process of VOR habituation. Rather than treat the stimulus as a continuous sinusoid, the pattern correlation model breaks it up into a number of discontinuous patterns. The pattern most closely correlated with the current stimulus then decreases the VOR response by the amount of that correlation times a pre-assigned weight. The pattern correlation model explains how the frequency-specific and the nonlinear behaviors may be related, and how the apparent switching phenomena may occur.

Analysis and modeling of frequency-specific habituation of the goldfish vestibulo-ocular reflex

Modification of the vestibulo-ocular reflex (VOR) by vestibular habituation is an important paradigm in the study of neural plasticity. The VOR is responsible for rotating the eyes to maintain the direction of gaze during head rotation. The response of the VOR to sinusoidal rotation is quantified by its gain (eye rotational velocity/head rotational velocity) and phase difference (eye velocity phase--inverted head velocity phase). The frequency response of the VOR in naïve animals has been previously modeled as a high-pass filter (HPF). A HPF passes signals above its corner frequency with gain 1 and phase 0 but decreases gain and increases phase lead (positive phase difference) as signal frequency decreases below its corner frequency. Modification of the VOR by habituation occurs after prolonged low-frequency rotation in the dark. Habituation causes a reduction in low-frequency VOR gain and has been simulated by increasing the corner frequency of the HPF model. This decreases gain not only at the habituating frequency but further decreases gain at all frequencies below the new corner frequency. It also causes phase lead to increase at all frequencies below the new corner frequency (up to some asymptotic value). We show that habituation of the goldfish VOR is not a broad frequency phenomena but is frequency specific. A decrease in VOR gain is produced primarily at the habituating frequency, and there is an increase in phase lead at nearby higher frequencies and a decrease in phase lead at nearby lower frequencies (phase crossover). Both the phase crossover and the frequency specific gain decrease make it impossible to simulate habituation of the VOR simply by increasing the corner frequency of the HPF model. The simplest way to simulate our data is to subtract the output of a band-pass filter (BPF) from the output of the HPF model of the naïve VOR. A BPF passes signals over a limited frequency range only. A BPF decreases gain and imparts a phase lag and lead, respectively, as frequency increases and decreases outside this range. Our model produces both the specific decrease in gain at the habituating frequency, and the phase crossover centered on the frequency of habituation. Our results suggest that VOR habituation may be similar to VOR adaptation (in which VOR modification is produced by visual-vestibular mismatch) in that both are frequency-specific phenomena.

Analysis and neural network modeling of the nonlinear correlates of habituation in the vestibulo-ocular reflex

Through the process of habituation, continued exposure to low-frequency (0.01 Hz) rotation in the dark produced suppression of the low-frequency response of the vestibulo-ocular reflex (VOR) in goldfish. The response did not decay gradually, as might be expected from an error-driven learning process, but displayed several nonlinear and nonstationary features. They included asymmetrical response suppression, magnitude-dependent suppression for lower- but not higher-magnitude head rotations, and abrupt-onset suppressions suggestive of a switching mechanism. Microinjection of lidocaine into the vestibulocerebellum of habituated goldfish resulted in a temporary dishabituation. This suggests that the vestibulocerebellum mediates habituation, presumably through Purkinje cell inhibition of vestibular nuclei neurons. The habituated VOR data were simulated with a feed-forward, nonlinear neural network model of the VOR in which only Purkinje cell inhibition of vestibular nuclei neurons was varied. The model suggests that Purkinje cell inhibition may switch in to introduce nonstationarities, and cause asymmetry and magnitude-dependency in the VOR to emerge from the essential nonlinearity of vestibular nuclei neurons.

Optokinetic after-nystagmus under prolonged alteration in the direction of gravity

Optokinetic nystagmus (OKN) and after-nystagmus (OKAN) show different responses depending on the direction of gravity with respect to the head and body. In 13 normal healthy persons (10 experimental and 3 control subjects), we studied the OKAN evoked by horizontal and vertical stimuli, in upright, side-lateral, and repeated upright positions along a time course of more than 6 h. The results showed continuous modifications of OKAN, the degree increasing with time. Lateral tilt produced different effects on the intrinsic vertical and horizontal optokinetic oculomotor systems.

Unidirectional vestibular habituation in the squirrel monkey. The time course and its influence on optokinetic nystagmus

Twenty clockwise (CW) angular stimulations were given daily to squirrel monkeys over a 14-calendar day period. The gain of vestibulo-ocular reflex (VOR) to CW angular acceleration decreased after each daily session. Particularly, the maximum reduction of VOR was found after the first day's session. The reduction was retained until the time of the next day's session. But, the across-day course of the VOR reduction showed up-and-down sinusoidal patterns in all animals. With respect to the effect on optokinetic nystagmus (OKN), an increase of counter-clockwise (CCW) OK responses after repeated CW vestibular stimulations was found; however, there was no clear retention of this response modification across daily sessions.

Cross-coupled effects of repeated vestibular and optokinetic stimulations on the dynamics of the vestibulo-ocular and optokinetic reflexes in the cat

Cats were submitted to repeated step stimulations either vestibular or optokinetic. Regardless of which of the two stimuli was used, dynamic modifications were observed in both vestibulo-ocular response and optokinetic after-nystagmus (OKAN). The progressive changes in post-rotational nystagmus and OKAN were quantified by measuring the duration of their primary phase. A parallel evolution of these two parameters was found. When repeated unidirectional vestibular stimulations were used, the same asymmetry was induced in both vestibuloocular responses and OKAN. These results support the hypothesis that the vestibulo-ocular and the optokinetic reflex share a common velocity storage mechanism, although alternative hypotheses cannot be excluded.

Influence of long-term optokinetic stimulation on eye movements of the rabbit

Two kinds of optokinetic afternystagmus (OKAN) have been studied in rabbits; positive and negative OKAN. Positive OKAN is the persistence of eye movements evoked by optokinetic stimulation following the termination of the stimulus, with the slow phase of the eye movements in the same direction as the inducing stimulus. Negative OKAN is evoked by long duration optokinetic stimulation, and has a slow phase of opposite direction to the inducing stimulus. The stimulus conditions which are optimal for inducing and maintaining negative OKAN were characterized. Rabbits were placed in an optokinetic drum for periods of 12-96 h (with appropriate intervening periods for food and water). Eye movements were recorded during and after the termination of optokinetic stimulation. The optimum optokinetic stimulus velocity for the induction of negative OKAN was 5 degrees/s. The minimum duration of stimulation for the induction of negative OKAN of maximum velocity was 48 h. Once induced, the slow phase of negative OKAN attained velocities of 50-100 degrees/s. Three conditions of restraint of the rabbits were studied after negative OKAN was induced during the intervening periods when eye movements were not being recorded. These conditions were: (1) unrestrained (full freedom of movement) without visual stimulation (in a dark enclosure); (2) restrained (horizontal head and body movement prevented) without visual stimulation; and (3) restrained with visual stimulation (in the stationary optokinetic drum). Conditions 1 and 2 caused negative OKAN to dissipate within 24 h. Condition 3 caused negative OKAN to be maintained for more than 70 h. The velocity imbalance of the horizontal vestibuloocular reflex (HVOR) was measured at different times following the induction of negative OKAN. It provided a more sensitive index of the central imbalance which caused negative OKAN, than did spontaneous nystagmus. One of the consequences of optokinetic stimulation measured over a 16 h period was a decrease in the gain of the optokinetic reflex. This reduction in gain could represent a central adaptation to maintained stimulation which in the absence of continued optokinetic stimulation is expressed as a nystagmus.

Effect of alertness on the vestibulo-ocular reflex and on the slow rise in optokinetic nystagmus in rabbits

The effect of alertness on the time constant of the vestibulo-ocular reflex (VOR) and the time constant of the slow rise of the optokinetic nystagmus (OKN) was studied in nine pigmented rabbits. When the rabbits were alerted by vibration, the time constant of the VOR was prolonged, and that of the slow rise in OKN was shortened, whereas the gain of VOR and OKN remained largely unaffected. These findings agree with the suggestion that the state of alertness affects the vestibular system by way of the so-called velocity storage mechanism.

The influence of interstimulus interval on the development of vestibular habituation to repeated velocity steps

In order to clarify the problem of which stimulus parameters affect vestibular habituation, a group of cats was submitted to repeated velocity steps involving changes in either the step amplitude or the interval between two consecutive steps. In the first two experiments, the protocol was the same as in a previous study which used steps of 160 degrees/s separated by 60 s, except that the steps were of 80 degrees/s and 16 degrees/s. In the remaining experiments the step amplitude was kept constant (160 degrees/s) and the interstimulus interval was changed: each step was delivered either immediately after the reversal of the nystagmus elicited by the preceding step (only a few beats in the reversed direction were allowed to occur) or immediately before (no beats in the reversed direction). Vestibular habituation was found to occur in both experiments of the first series. Nevertheless, the marked initial suppression of the response, that was reported as one aspect of vestibular habituation to steps of 160 degrees/s, was clearly present when steps of 80 degrees/s were used, but was not as clear when the step amplitude was reduced to 16 degrees/s. The experiments of the second series showed that a typical vestibular habituation still occurs when steps are delivered just after the reversal of nystagmus. On the contrary, no habituation was observed when steps preceded the reversal of nystagmus. It was concluded that the presence of an anticompensatory phase is interpreted by the central nervous system as a sign that the response evoked in a reflex way is functionally meaningless or even detrimental. An habituation process is then started to suppress the response.

Human optokinetic afternystagmus. Effects of repeated stimulation

Normal human subjects were exposed to repeated optokinetic afternystagmus (OKAN) testing in either one direction or alternating directions of stripe movement. Sessions were conducted at intervals of either one week or several weeks. Repeated exposure to OKAN stimulation in one direction produced significant response decrements in cumulative displacement, short time constant, long time constant, and the coefficient of the long time constant component (C). The data suggest that the decrease in C and cumulative displacement occurred most noticeably between trials 3 and 4 of the first session. Retesting after 1 week, and up to 8 weeks later revealed no recovery. Repeated exposure to alternating leftward and rightward stimuli resulted in response decrement in both cumulative displacement and C. Responses to leftward stimuli were indistinguishable from responses to rightward stimuli.

Differential effects of diazepam upon vestibulo- and visual-oculomotor responses in the rabbit

The effects of intravenously administered diazepam (0.6 mg/kg) on vestibulo-ocular reflex (VOR), optokinetic nystagmus (OKN), optovestibular reflex (OVR) and their after-nystagmus were examined in rabbits. These reflexes were evoked by velocity step of 20 degrees/sec of chair or drum rotation. Slow phase eye velocity (SPEV) of OVR shows algebraic summation of those of VOR and OKN. Although SPEVs of VOR, OKN and OVR significantly decreased at 10 and 30 min after diazepam injection (p less than 0.05, t-test), OKN shows most distinctive reduction. SPEV reduction of OVR after diazepam administration was also equal to the algebraic summation of VOR and OKN reductions.