Impairment of optokinetic (after-)nystagmus by labyrinthectomy in the rabbit

The behavior of the optokinetic system

Optokinetic responses in several species are compared, describing differences in afoveate and foveate animals, and the effects of visual testing conditions, including directions of stimulus motion. Smooth pursuit contributes to responses to full-field visual motion in foveate species; in the latter, measurement of optokinetic after-nystagmus in darkness allows investigation of the optokinetic system. The concept of optokinetic-vestibular symbiosis and velocity storage are discussed, pertinent electrophysiological studies (such as vestibular nucleus neurons that respond to both optokinetic and vestibular stimuli) are reviewed and a model is developed. The different purposes and properties of optokinetic responses (to maintain clear vision during self-rotation) and smooth pursuit (to visually track a moving target) are clarified.

Neurophysiology of the optokinetic system

This chapter provides a review of early studies into the neural substrate for optokinetic-vestibular responses. Properties and connections of retinal and brainstem neurons contributing to optokinetic responses in the afoveate rabbit are summarized. Electrophysiological and lesion studies provide support for confluence of optokinetic and vestibular signals in the vestibular nucleus to provide the brain's estimate of self-rotation. Evidence for optokinetic-vestibular symbiosis in humans comes from the observation that individuals who have lost vestibular function show no optokinetic after-nystagmus in darkness, following full-field stimulus motion. An anatomical scheme for brainstem elaboration of optokinetic responses is proposed and cerebellar contributions are reviewed.

Effect of the Stimulus Duration on the Adaptation of the Optokinetic Afternystagmus

Observing a rotating visual pattern covering a large portion of the visual field induces optokinetic nystagmus (OKN). If the lights are suddenly switched off, optokinetic afternystagmus (OKAN) occurs. OKAN is hypothesized to originate in the velocity storage mechanism (VSM), a central processing network involved in multi-sensory integration. During a sustained visual rotation, the VSM builds up a velocity signal. After the lights are turned off, the VSM discharges slowly, with OKAN as the neurophysiological correlate. It has been reported that the initial afternystagmus in the direction of the preceding stimulus (OKAN-I) can be followed by a reversed one (OKAN-II), which increases with stimulus duration up to 15 min. In 11 healthy adults, we investigated OKAN following optokinetic stimulus lasting 30 s, 3-, 5-, and 10-min. Analysis of slow-phase cumulative eye position and velocity found OKAN-II in only 5/11 participants. Those participants presented it in over 70% of their trials with longer durations, but only in 10% of their 30 s trials. While this confirms that OKAN-II manifests predominantly after sustained stimuli, it suggests that its occurrence is subject-specific. We also did not observe further increases with stimulus duration. Conversely, OKAN-II onset occurred later as stimulus duration increased (p = 0.02), while OKAN-II occurrence and peak velocity did not differ between the three longest stimuli. Previous studies on OKAN-I, used negative saturation models to account for OKAN-II. As these approaches have no foundation in the OKAN-II literature, we evaluated if a simplified version of a rigorous model of OKAN adaptation could be used in humans. Slow-phase velocity following the trials with 3-, 5-, and 10-min stimuli was fitted with a sum of two decreasing exponential functions with opposite signs (one for OKAN-I and one for OKAN-II). The model assumes separate mechanisms for OKAN-I, representing VSM discharge, and OKAN-II, described as a slower adaptation phenomenon. Although the fit was qualitatively imperfect, this is not surprising given the limited reliability of OKAN in humans. The estimated adaptation time constant seems comparable to the one describing the reversal of the vestibulo-ocular reflex during sustained rotation, suggesting a possible shared adaptive mechanism.

The Scientific Contributions of Bernard Cohen (1929-2019)

Throughout Bernard Cohen's active career at Mount Sinai that lasted over a half century, he was involved in research on vestibular control of the oculomotor, body postural, and autonomic systems in animals and humans, contributing to our understanding of such maladies as motion sickness, mal de débarquement syndrome, and orthostatic syncope. This review is an attempt to trace and connect Cohen's varied research interests and his approaches to them. His influence was vast. His scientific contributions will continue to drive research directions for many years to come.

Velocity storage mechanism drives a cerebellar clock for predictive eye velocity control

Predictive motor control is ubiquitously employed in animal kingdom to achieve rapid and precise motor action. In most vertebrates large, moving visual scenes induce an optokinetic response (OKR) control of eye movements to stabilize vision. In goldfish, the OKR was found to be predictive after a prolonged exposure to temporally periodic visual motion. A recent study showed the cerebellum necessary to acquire this predictive OKR (pOKR), but it remained unclear as to whether the cerebellum alone was sufficient. Herein we examined different fish species known to share the basic architecture of cerebellar neuronal circuitry for their ability to acquire pOKR. Carps were shown to acquire pOKR like goldfish while zebrafish and medaka did not, demonstrating the cerebellum alone not to be sufficient. Interestingly, those fish that acquired pOKR were found to exhibit long-lasting optokinetic after nystagmus (OKAN) as opposed to those that didn’t. To directly manipulate OKAN vestibular-neurectomy was performed in goldfish that severely shortened OKAN, but pOKR was acquired comparable to normal animals. These results suggest that the neuronal circuitry producing OKAN, known as the velocity storage mechanism (VSM), is required to acquire pOKR irrespective of OKAN duration. Taken together, we conclude that pOKR is acquired through recurrent cerebellum-brainstem parallel loops in which the cerebellum adjusts VSM signal flow and, in turn, receives appropriately timed eye velocity information to clock visual world motion.

Velocity storage mechanism in zebrafish larvae

The optokinetic reflex (OKR) and the angular vestibulo-ocular reflex (aVOR) complement each other to stabilize images on the retina despite self- or world motion, a joint mechanism that is critical for effective vision. It is currently hypothesized that signals from both systems integrate, in a mathematical sense, in a network of neurons operating as a velocity storage mechanism (VSM). When exposed to a rotating visual surround, subjects display the OKR, slow following eye movements frequently interrupted by fast resetting eye movements. Subsequent to light-off during optokinetic stimulation, eye movements do not stop abruptly, but decay slowly, a phenomenon referred to as the optokinetic after-response (OKAR). The OKAR is most likely generated by the VSM. In this study, we observed the OKAR in developing larval zebrafish before the horizontal aVOR emerged. Our results suggest that the VSM develops prior to and without the need for a functional aVOR. It may be critical to ocular motor control in early development as it increases the efficiency of the OKR.

Quantifying the ontogeny of optokinetic and vestibuloocular behaviors in zebrafish, medaka, and goldfish

We quantitatively studied the ontogeny of oculomotor behavior in larval fish as a foundation for studies linking oculomotor structure and function with genetics. Horizontal optokinetic and vestibuloocular reflexes (OKR and VOR, respectively) were measured in three different species (goldfish, zebrafish, and medaka) during the first month after hatching. For all sizes of medaka, and most zebrafish, Bode plots of OKR (0.065-3.0 Hz, +/-10 degrees/s) revealed that eye velocity closely followed stimulus velocity (gain > 0.8) at low frequency but dropped sharply above 1 Hz (gain < 0.3 at 3 Hz). Goldfish showed increased gain proportional to size across frequencies. Linearity testing with steps and sinusoids showed excellent visual performance (gain > 0.8) in medaka almost from hatching; but zebrafish and goldfish exhibited progressive improvement, with only the largest equaling medaka performance. Monocular visual stimulation in zebrafish and goldfish produced gains of 0.5 versus <0.1 for the eye viewing a moving versus stationary stimulus pattern but 0.25 versus <0.1 in medaka. Angular VOR appeared much later than OKR, initially at only high accelerations (>200 degrees /s at 0.5 Hz), first in medaka followed by larger (8.11 mm) zebrafish; but it was virtually nonexistent in goldfish. Velocity storage was not observed except for an eye velocity build-up in the largest medaka. In summary, a robust OKR was achieved shortly after hatching in all three species. In contrast, larval fish seem to be unique among vertebrates tested in their lack of significant angular VOR at stages where active movement is required for feeding and survival.

Reverse optokinetic after-nystagmus generated by gaze fixation during optokinetic stimulation

Gaze fixation during optokinetic stimulation generates an after-nystagmus with a slow component towards the reverse direction of the optokinetic stimulation. The duration and maximum slow component velocity (SCV) of this "reverse OKAN" were observed by changing the duration, velocity and direction of the optokinetic stimulation in nine normal volunteers. The duration of reverse OKAN increased with increasing stimulation time but was unaffected by changes in the stimulation velocity. The maximum SCV of reverse OKAN decreased with an increase in the stimulation velocity but was not significantly affected by changes in the optokinetic stimulation time. There was no directional difference among the horizontal, upwards and downwards reverse OKANs. The reverse OKAN was thought to be generated by a mechanism different from the velocity storage mechanism which produced optokinetic nystagmus and the first phase of OKAN. Retinal slip during the optokinetic stimulation was considered to be an input to the mechanism which generated the reverse OKAN. We hypothesize that the mechanism causing the reverse OKAN may be a generator of the second phase of OKAN, which was also intimately connected with self-motion sensation during the optokinetic stimulation.

Difference between horizontal and vertical optokinetic nystagmus in cats at upright position

The slow-phase velocity (SPV) of optokinetic nystagmus (OKN) and optokinetic after nystagmus (OKAN) in response to a velocity step of surround rotation in the horizontal direction is composed of the rapid and slow components in the cat: a rapid rise, a slow rise to a steady state, a rapid fall, and a slow decline to 0 deg/s. The rapid and slow components are attributed to the direct pathway and velocity storage neuronal mechanisms, respectively. The difference between horizontal and vertical OKN has been reported in the monkey at the upright position, but the slow and rapid components have not been distinguished. The present study compared horizontal OKN-OKAN with vertical OKN-OKAN in the cat at the upright position, distinguishing the rapid and slow components. Constant velocity rotation of a random dot pattern at a velocity of 5 to 160 deg/s was used for optokinetic stimulation.

THE RESULTS

First, the amplitude of the rapid rise was relatively small in all SPV directions and all stimulus velocities investigated, with a slight upward-SPV preference to the downward-SPV (maximum 6.4, 6.0, and 3.4 deg/s in horizontal, upward, and downward SPV directions, respectively). Second, the steady state velocity was large during horizontal OKN (maximum 69.0 deg/s), small during upward-SPV OKN (12.9 deg/s), and missing (SPV is negligibly small and irregular) during downward-SPV OKN, indicating a large directional difference of OKN. Third, the acceleration of the slow rise decreased with the stimulus velocity at higher stimulus velocities >20 deg/s during both horizontal and upward-SPV OKN, suggesting strong nonlinearity in the velocity charge system. Fourth, the decay time course of the OKAN was described by the time constant of the exponential function, and the time constant was longer during horizontal (mean, 8.3 s at a stimulus velocity of 20 deg/s) than during upward-SPV (5.4 s) OKAN, suggesting that the velocity discharge system is relatively linear compared with the velocity charge system. It is concluded that horizontal OKN-OKAN is much larger than vertical OKN-OKAN in the cat at the upright position, and this directional difference is caused mainly by the directional difference in the velocity storage mechanism, but not in the direct pathway mechanism.

Floccular complex spike response to transparent retinal slip

The flocculus of the rabbit is involved in the plasticity of compensatory eye movements. It is generally assumed that the climbing fiber input to floccular Purkinje cells encodes "retinal slip," which in turn would be a measure for the oculomotor performance error. To test this, we used transparent motion stimuli, creating a retinal slip signal that broke up this relation. We recorded the ensuing oculomotor behavior and complex spike activity of floccular Purkinje cells. Complex spike modulation in response to transparent stimulation was identical to that of a single optokinetic pattern, despite considerably different retinal slip. These results suggest that the climbing fiber code may be effectively related to the eye movement performance error, rather than to retinal slip.

Three-dimensional analysis of eye movements during off vertical axis rotation in patients with unilateral labyrinthine loss

3D analysis of eye movements during off vertical axis rotation (OVAR) was carried out in seven subjects with unilateral labyrinthine loss (ULL). The modulation component (MOC) of these patients was not different from that of normal subjects. However, the horizontal MOC was significantly smaller when the rotation was directed towards the diseased side as opposed to the healthy side. With rotation to the diseased side, most subjects exhibited a horizontal bias component (BIC) to the opposite side as compared with the diseased side in normal subjects. The vertical and torsional BICs were also influenced to some extent by lack of input from the unilateral labyrinth. These results indicate that a unilateral otolith organ plays a major role in the production of the horizontal BIC contralateral to the direction of rotation, which is strongly related to the velocity storage mechanism in the central nervous system. In addition, the unilateral otolith organ exerts some influence on the generation of the horizontal MOC and also the BIC of vertical and torsional eye movements.

Motor dynamics encoding in cat cerebellar flocculus middle zone during optokinetic eye movements

We investigated the relationship between eye movement and simple-spike (SS) frequency of Purkinje cells in the cerebellar flocculus middle zone during the optokinetic response (OKR) in alert cats. The OKR was elicited by a sequence of a constant-speed visual pattern movement in one direction for 1 s and then in the opposite direction for 1 s. Quick-phase-free trials were selected. Sixty-six cells had direction-selective complex spike (CS) activity that was modulated during horizontal (preferring contraversive) but not vertical stimuli. The SS activity was modulated during horizontal OKR, preferring ipsiversive stimuli. Forty-one cells had well-modulated activity and were suitable for the regression model. In these cells, an inverse dynamics approach was applied, and the time course of the SS rate was reconstructed, with mean coefficient of determination 0.76, by a linear weighted superposition of the eye acceleration (mean coefficient, 0.056 spikes/s per deg/s(2)), velocity (5.10 spikes/s per deg/s), position (-2.40 spikes/s per deg), and constant (mean 34.3 spikes/s) terms, using a time delay (mean 11 ms) from the unit response to the eye response. The velocity and acceleration terms contributed to the increase in the reconstructed SS rates during ipsilateral movements, whereas the position term contributed during contralateral movements. The standard regression coefficient analyses revealed that the contribution of the velocity term (mean coefficient 0.81) was predominant over the acceleration (0.03) and position (-0.17) terms. Forward selection analysis revealed three cell types: Velocity-Position-Acceleration type (n = 27): velocity, position, and acceleration terms are significant (P < 0.05); Velocity-Position type (n = 12): velocity and position terms are significant; and Velocity-Acceleration type (n = 2): velocity and acceleration terms are significant. Using the set of coefficients obtained by regression of the response to a 5 deg/s stimulus velocity, the SS rates during higher (10, 20, and 40 deg/s) stimulus velocities were successfully reconstructed, suggesting generality of the model. The eye-position information encoded in the SS firing during the OKR was relative but not absolute in the sense that the magnitude of the position shift from the initial eye position (0 deg/s velocity) contributed to firing rate changes, but the initial eye position did not. It is concluded that 1) the SS firing frequency in the cat middle zone encodes the velocity and acceleration information for counteracting the viscosity and inertia forces respectively, during short-duration horizontal OKR and 2) the apparent position information encoded in the SS firing is not appropriate for counteracting the elastic force during the OKR.

Slow cumulative eye position to quantify optokinetic afternystagmus

In 30 normal subjects we computed the slow cumulative eye position (SCEP) of optokinetic afternystagmus (OKAN) that followed 60 seconds of full-field optokinetic stimulation at 60 degrees/s. The mean SCEP was 112.8 degrees +/- 65.0 degrees. The lower and upper fifth percentile limits for directional preponderance of the SCEP were -38.8% and 44.3%, respectively. The time constant, which we calculated by dividing the SCEP by the initial velocity, was 12.0 +/- 7.4 seconds. This value is nearly identical to the time constant obtained from semilogarithmic regression of the decay of OKAN slow-phase velocity versus time. We conclude that the SCEP is a good measure of OKAN and that it reflects the substantial amount of variability and directional asymmetry observed in the optokinetic responses of normal subjects.

Human optokinetic after-nystagmus: variability in serial test results

Test-retest variability of values for directional asymmetry in primary and secondary horizontal optokinetic after-nystagmus (OKAN I and OKAN II, respectively) was studied in 16 apparently healthy subjects. OKAN was induced by 60 s of whole-field optokinetic stimulation at speeds of 60 degrees/s and 90 degrees/s in either direction (left and right), each subject being tested on the same respective weekday once a week for 4 consecutive weeks. Values for directional asymmetry were calculated as the relative side-difference between response to drum rotation toward the right and toward the left. The subjects manifested considerable variation in values for directional asymmetry in OKAN I. This suggests prediction of a given individual's true value for directional asymmetry in OKAN I to require several measurements. On the other hand, 15/16 subjects manifested no asymmetry in OKAN II (the 16th subject was a further investigation found to have significant asymmetric caloric responses). As both OKAN I and OKAN II are known to reflect asymmetric vestibular function it is suggested that studying OKAN II may require fewer measurements of directional asymmetry, compared with studying OKAN I, when assessing the course of peripheral vestibular asymmetry.

Effects of midline medullary lesions on velocity storage and the vestibulo-ocular reflex

1. Crossing fibers were sectioned at the midline of the medulla caudal to the abducens nucleus in four cynomolgus monkeys. In two animals the lesions caused the time constant of horizontal and vertical per- and post-rotatory nystagmus to fall to 5-8 s. The slow rise in optokinetic nystagmus (OKN), as well as optokinetic after-nystagmus (OKAN) and cross-coupling of horizontal to vertical OKN and OKAN were abolished. Steady state velocities could not be maintained during off-vertical axis rotation (OVAR). Pitch and yaw nystagmus were affected similarly. We conclude that the ability to store activity related to slow phase eye velocity, i.e., "velocity storage", was lost in these monkeys for nystagmus about any axis. Velocity storage was partially affected by a small midline lesion in the same region in a third animal. There was no effect of a more superficial midline section in a fourth monkey, and it served as a control. 2. The gain (eye velocity/head velocity) of the vestibulo-ocular reflex (VOR) was unaffected by the midline lesions. Saccades were normal, as was the ability to hold the eyes in eccentric gaze positions. The gain of the fast component of OKN increased in one monkey to compensate for the loss of the slow component. 3. One animal was tested for its ability to adapt the gain of the VOR due to visual-vestibular mismatch after lesion. Average changes in gain in response to wearing magnifying (2.2x) and reducing (0.5x) lenses, were +35% and -30%, respectively. This is within the range of normal monkeys. Thus, a midline lesion that abolished velocity storage did not alter that animal's ability to adapt the gain of the VOR. 4. Lesions that reduced or abolished velocity storage interrupted crossing fibers in the rostral medulla, caudal to the abducens nuclei. Cells that contributed axons to this portion of the crossing fibers are most likely located in central portions of the medial vestibular nucleus (MVN) and/or in rostral portion of the descending vestibular nucleus (DVN). The implication is that velocity storage arises from neurons in MVN and DVN whose axons cross the midline.

Abolition of optokinetic afternystagmus by aminoglycoside ototoxicity

We studied optokinetic afternystagmus in eight subjects with loss of or impairment of vestibular function due to ototoxic antibiotics. We found that the initial amplitude, the time constant, and the slow-phase cumulative eye position of optokinetic afternystagmus were significantly reduced in the patients. Slow-phase cumulative eye position most reliably distinguished our patients' responses from those of a normal group.

Optokinetic afternystagmus in humans: normal values of amplitude, time constant, and asymmetry

It has been suggested that the appearance of directional asymmetry and/or a reduced time constant of optokinetic afternystagmus (OKAN) might be a clinical index of vestibular imbalance. However, we do not know the limits for OKAN parameters in normal humans. Accordingly, we studied OKAN in 30 normal subjects using a "sampling" method, in which a number of values of OKAN are obtained by turning out the lights periodically during optokinetic stimulation. We found that the initial velocity of OKAN has a large intrasubject variability. Accordingly, if precision is desired so as to obtain 95% confidence that the measured mean of the initial velocity of OKAN is within 25% of the true mean in an individual subject, at least eight measurements of the initial OKAN velocity must be taken. When 12 measurements are made, all subjects had a minimum value of 5 degrees/s initial OKAN, and there was little directional asymmetry (mean of -0.47 degree/s +/- 3.13 degrees/s). The intrasubject variability of the time constant of OKAN was similar to the variability of initial OKAN velocity. However, because it is not possible to obtain repeated measures of the time constant in a short period of time, the time constant of OKAN is less likely to be useful in clinical testing.

Compensation of total loss of vestibulo-ocular reflex by enhanced optokinetic response

The gain in the optokinetic nystagmus reflex (OKR) of 27 labyrinthine-defective patients was compared with that of 27 subjects without any otoneurological abnormality, who were matched by age and sex. The patients without labyrinth function exhibited significantly higher OKR gains than the control subjects, especially so at higher age. The observed enhancement of OKR gain in the absence of vestibular function is ascribed to the effect of optokinetic training brought about by enhanced slippage of retinal images due to lack of an efficient vestibulo-ocular reflex.

Oculomotor functions of the flocculus and the vestibular nuclei after bilateral vestibular neurectomy

Ito's hypothesis of an important role of the flocculus of the vestibulocerebellum in the immediate visual control of the VOR during visual-vestibular interaction has received substantial support. Nevertheless, several parts in this hypothesis are unclear, at least in primates. In normal monkey, vestibularly driven neurones in the vestibular nuclei do not carry signals which are adequate to account for the full range of eye movement responses during optokinetic tracking (OKN) and different situations of visual-vestibular interaction (especially VOR-suppression). Thus these neurones seem not to be located at the final stage where floccular "gaze-velocity" Purkinje cells (PCs) exert their control function on the three-neurone-reflex arc. The signals of these "central" vestibular neurones (if relevant for the oculomotor output) must further be processed. After bilateral vestibular neurectomy (BVN) only a small number of vestibular nuclei neurones were found with eye velocity sensitivities during smooth pursuit tracking (SP) and OKN in the range of those of floccular PCs (also after BVN), and with the appropriate polarity of modulation. Our difficulties in finding neurones in the vestibular nuclei which, according to their neurophysiological behaviour, could be main target cells of floccular PCs, either in normal or in BVN monkeys, are discussed.

Modulatory roles of GABAergic mechanisms in post-rotatory nystagmus in the rabbit

1. To determine how the GABAergic mechanism operates in the generation of post-rotatory nystagmus, an experiment was performed with GABAergic drugs in rabbits. 2. Subconvulsive doses of picrotoxin (0.3-0.6 mg/kg, i.v.) and bicuculline (0.1 mg/kg, i.v.) decreased the number of post-rotatory nystagmus beats, whereas strychnine sulphate, at a subconvulsive dose (0.1 mg/kg i.v.), increased it. 3. Diazepam (1 mg/kg, i.v.) remarkably increased the number of post-rotatory nystagmus beats. Pretreatment with picrotoxin (0.45 mg/kg, i.v.), bicuculline (0.1 mg/kg, i.v.) or semicarbazide-HCl (180 mg/kg, i.v.) antagonized the effects of diazepam (1 mg/kg, i.v.). 4. GABAergic mechanisms may play a modulatory role in the production of nystagmus rhythm. Strychnine-sensitive neurons involved in the vestibular mechanism may behave in a different manner from picrotoxin-sensitive neurons.

Nystagmus induced by stimulation of the nucleus of the optic tract in the monkey

1. The nucleus of the optic tract (NOT) was electrically stimulated in alert rhesus monkeys. In darkness stimulation evoked horizontal nystagmus with ipsilateral slow phases, followed by after-nystagmus in the same direction. The rising time course of the slow phase velocity was similar to the slow rise in optokinetic nystagmus (OKN) and to the charge time of optokinetic after-nystagmus (OKAN). The maximum velocity of the steady state nystagmus was approximately the same as that of OKAN, and the falling time course of the after-nystagmus paralleled OKAN. 2. Increases in frequency and duration of stimulation caused the rising and falling time constants of the nystagmus and after-nystagmus to become shorter. Changes in the falling time constant of the after-nystagmus were similar to changes in the time constant of OKAN produced by increases in the velocity or duration of optokinetic stimulation. 3. Stimulus-induced nystagmus was combined with OKN, OKAN and per- and post-rotatory nystagmus. The slow component of OKN as well as OKAN could be prolonged or blocked by stimulation, leaving the rapid component of OKN unaffected. Activity induced by electrical stimulation could also sum with activity arising in the semicircular canals to reduce or abolish post-rotatory nystagmus. 4. Positive stimulus sites for inducing nystagmus were located in the posterolateral pretectum. This included portions of NOT that lie in and around the brachium of the superior colliculus and adjacent regions of the dorsal terminal nucleus (DTN). 5. The data indicate that NOT stimulation had elicited the component of OKN which is responsible for the slow rise in slow phase velocity and for OKAN. The functional implication is that NOT, and possibly DTN, are major sources of visual information related to retinal slip in the animal's yaw plane for semicircular canal-related neurons in the vestibular nuclei. Analyzed in terms of a model of OKN and OKAN (Cohen et al. 1977; Waespe et al. 1983), the indirect pathway, which excites the velocity storage mechanism in the vestibular system to produce the slow component of OKN and OKAN, lies in NOT in the monkey, as it probably also does in cat, rat and rabbit. Pathways carrying activity for the rapid rise in slow phase velocity during OKN or for ocular pursuit appear to lie outside NOT.

Dynamic characteristics of optokinetically controlled eye movements following inferior olive lesions in the brown rat

1. The inferior olive was destroyed by the drug 3-acetylpyridine in brown rats. Spontaneous and optokinetic eye movements in response to constant-velocity rotation (5-80 deg/s) or sinusoidal oscillations (0.05 and 0.1 Hz with 15 deg/s peak velocity and 0.3, 0.5, 1.0 and 2 Hz with 5 deg/s peak velocity) of the visual surround were recorded 4-6 days, 40-50 days and 3-4 months after the lesion using the magnetic search coil technique. 2. Persistent oculomotor deficits were observed in rats with a lesion of more than 97% of inferior olive neurones. In cases with a less complete lesion, no or only transient deficits were observed. In these latter cases the bulk of surviving neurones was located in the caudal half of the inferior olive, which includes the dorsal cap of Kooy. 3. Eye position holding after saccadic gaze shifts in the light was strongly deficient, showing pronounced postsaccadic centripetal drift for several hundred milliseconds. Similar deficits were observed in slow-phase components following quick phases of optokinetic nystagmus. In the dark, eye position holding was also deficient. 4. Closed-loop gains of optokinetic step responses obtained from rats with inferior olive lesions could be as good as those obtained from control animals. There was, however, a trend towards smaller gain values over the range of stimulus velocities tested. The duration of optokinetic after-nystagmus was not changed. 5. The initial fast rise of slow-phase velocity of optokinetic step responses was reduced by about 30-50%, showing no recovery in the follow-up experiments up to 3-4 months after the lesion. 6. Optokinetic responses to sinusoidal oscillations of the visual surround exhibited an increasing drop in gain for frequencies between 0.1 to 0.5 Hz. In the range of 0.5-2.0 Hz gain was only about 0.2 compared to 0.7-0.8 in control animals. Phase lag of sinusoidal responses was shifted to larger values by about 25-35 deg for frequencies increasing from 0.1 to 0.5 Hz. At 1.0 Hz phase shift was reduced to about 15 deg and at 2.0 Hz no significant change in phase was observed. Both gain and phase of sinusoidal responses showed some recovery when tested 3-4 months after inferior olive lesion. 7. The results suggest that inferior olive lesions impair velocity-to-position integration, mainly as a consequence of the missing climbing fibre input to the cerebellar flocculi.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Purkinje cell activity in the flocculus of vestibular neurectomized and normal monkeys during optokinetic nystagmus (OKN) and smooth pursuit eye movements

Single unit activity was recorded in the primate flocculus after the vestibular nerves were cut (bilateral vestibular neurectomy) during optokinetic nystagmus (OKN), smooth pursuit eye movements (SP) and whole field visual stimulation with gaze fixed on a stationary target light (OKN-suppression). Following vestibular neurectomy monkeys had no vestibular responses and no optokinetic after-nystagmus (OKAN) in the horizontal plane. However, OKN slow phases still reached steady state velocities of up to 100 deg/s. After neurectomy, simple spike (SS) activity of Purkinje cells (P-cells) was modulated in relation to eye velocity, regardless of whether eye velocity was induced by a small target light moving in darkness (SP) or by a moving visual surround (OKN). In over 90% of the P-cells firing rate increased with eye velocity to the ipsilateral side and decreased with velocities to the contralateral side. Modulation in firing rate increased monotonically with increasing eye velocity. The strength of modulation was similar during SP and OKN for the same eye velocity. The change in firing rate of P-cells in response to a sudden change in optokinetic stimulus velocity contained a component related to eye velocity and a component related to eye acceleration. Only a few P-cells were also modulated with image slip velocity during OKN-suppression. The modulation of P-cells during SP and OKN was compared in normal and vestibular neurectomized monkeys. The sensitivity of floccular P-cells to eye velocity during SP was 1.14 imp X s-1/deg X s-1 in normal monkey and 1.28 imp X s-1/deg X s-1 after neurectomy. The similarity of eye velocity sensitivities demonstrates that neurectomy does not change the characteristics of floccular P-cell modulation during SP. In contrast, during OKN modulation of P-cells is quite different in normal and neurectomized monkey. In normal monkey, P-cells are modulated during steady state OKN for eye velocities above 40-60 deg/s only. This threshold velocity corresponds approximately to the maximal initial OKAN velocity (i.e. OKAN saturation velocity). After neurectomy, the threshold velocity is 0 deg/s and P-cells are modulated during steady state OKN also over ranges of eye velocities that do not cause a response in normal monkey. Sensitivities of P-cells to eye velocity during OKN for eye velocities above the threshold velocity are 1.0 imp X s-1/deg X s-1 in neurectomized monkey and 1.43 imp X s-1/deg X s-1 in normal monkey.(ABSTRACT TRUNCATED AT 400 WORDS)

Horizontal optokinetic ocular nystagmus in the pigmented rat

Horizontal optokinetic nystagmus was elicited in rats by rotation of a pattern of bright dots projected onto a cylinder surrounding the animal. Eye position was measured with the electromagnetic search coil technique. Optokinetic stimuli consisted either of velocity steps of pattern rotation or sinusoidal oscillations. Closed-loop gain (slow phase eye velocity/pattern velocity) of steady-stage step responses in binocular vision ranged between 0.8 and 1.0 for pattern velocities up to 20-40 degrees/s and decreased thereafter. Open-loop gain (steady-state slow phase velocity/retinal slip velocity) was dependent on retinal slip velocity and decreased linearly in double logarithmic plot from about 30 (at 0.5 degree/s) to about 9 (at 5 degrees/s). For retinal slip velocities larger than 5 degrees/s open-loop gain decayed faster and reached about 1 at 30 degrees/s. Step response profiles showed a gradual increase in slow phase eye velocity reaching steady-state after a time period roughly proportional to stimulus velocity. Initial slow phase velocity measured within 500 ms after stimulus onset reached between 2 and 4 degrees/s and was largely independent of stimulus amplitudes above 10 degrees/s. Occasionally rats showed fast rises in slow phase eye velocity at the onset of the step response profiles. Primary and secondary optokinetic afternystagmus were present. Duration of primary afternystagmus was largely independent of stimulus amplitude and lasted 8.0 +/- 4 s. Closed-loop gain of steady-state step responses in monocular vision was, for temporonasal stimuli, similar to that measured in binocular condition while for nasotemporal stimulation gain was much smaller even at low stimulus velocities. Sinusoidal modulation of slow phase velocity was linearly dependent on stimulus velocity; the linear range decreased as frequency of stimulation increased. Slow phase velocity gain was relatively constant (ca 0.8) between 0.05 and 0.3 Hz and showed only a small tendency to decrease at larger stimulus frequencies. Phase-lag increased strongly with stimulus frequency and could be fitted by assuming a response time delay of 100 ms. The results show that the rat's optokinetic system is qualitatively similar to that found in another lateral-eyed species, namely the rabbit. At a quantitative level, however, both fast and slow optokinetic response dynamics appear to be better developed in the rat than in the rabbit.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of labyrinthectomy on flash-induced nystagmus (FIN)

After bilateral labyrinthectomy the maximum slow phase velocity of flash-induced nystagmus (FIN) was diminished; the flash induced after-nystagmus was always absent. Unilateral labyrinthectomy resulted in a weaker FIN response on stimulation of both eyes, but the best response was seen after contralateral stimulation. These findings are discussed in relation to reported effects of labyrinthectomy on OKN, "central nystagmus" and vestibular nuclear discharge activity.

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.

Human optokinetic afternystagmus. Slow-phase characteristics and analysis of the decay of slow-phase velocity

Events following the extinction of lights after 1-minute exposures of naive, normal subjects to an optokinetic stimulus at 40 deg/sec have been closely examined and quantified. Mean eye displacement in each slow phase decreased from 10.12 +/- 1.61 deg during optokinetic nystagmus (OKN) to 3.36 +/- 2.32 deg during optokinetic afternystagmus (OKAN). Slow-phase duration increased from 0.26 +/- 0.03 sec during OKN to 0.45 +/- 0.195 sec during OKAN. Eye displacement per slow phase remained fairly constant during OKAN, suggesting a spatial reference for the resetting of gaze. OKAN decay is a two-component process which can be closely approximated by a sum of two exponentials, one with a short time constant of 1.15 sec and the other with a long time constant of 48.8 sec. OKAN decay commenced at a time after lights out which depended upon the presence and timing of an intervening fast phase. When a fast phase intervened, OKAN decay commenced about 230 msec after it, and about 460 msec after lights out. When lights out occurred during the fast phase, OKAN decay commenced about 340 msec later.

Flash induced nystagmus (FIN) in the rabbit: input-output relations

Flash-induced nystagmus (FIN) was elicited by monocular flicker-stimulation at 20-40 Hz. It was found more often in albino than in pigmented rabbits. Slow phase velocity (SPV) showed a gradual increase, then a steady-state level in most animals and an after-response after cessation of stimulation. incremental and decremental phase mainly showed an exponential change in SPV. Apparent time constants were in the range of 5-40 sec. Simultaneous recording of both eyes revealed greater velocity and amplitude of both phases of FIN in the stimulated eye. FIN can be specified as a type of nystagmus that is evoked by monocular essentially open-loop stimulation of the crossed ("subcortical") visual pathway. Direction-selective elements in the retina may play a specific role and their predominant orientation gives rise to nystagmus in adducting direction in the flicker-stimulated eye as if it were exposed to optokinetic stimulation in the temporonasal direction.

The horizontal optokinetic nystagmus in the cat

Horizontal optokinetic eye nystagmus (OKN) and after nystagmus (OKAN) were recorded in the alert cat (head restrained) in response to velocity steps and sinusoidal optokinetic stimuli. A strong dependency of OKN performance on stimulus pattern was found: responses were most regular and gain was high over a large range of stimulus velocities when the stimulus consisted of a high-contrast random dot pattern. Following velocity steps, OKN showed a small amplitude fast rise in slow phase velocity (SPV) which was followed by a slow build-up to steady state. The amplitude of the initial jump in SPV increased with stimulus amplitude up to 30 degrees/s and saturated afterwards. The plateau level of initial SPV ranged from 5 to 15 degrees/s. The slow build-up of SPV showed non-linearities, i.e. the time to steady state increased with stimulus amplitude and the slow rise of SPV was irregular. In most animals steady state SPV showed no signs of response saturation for step amplitudes up to 60-80 degrees/s or more. The open-loop gain (steady state SPV/retinal slip velocity) depended on retinal slip velocity and decreased from 46 at 0.5 degrees/s to 0.4 at about 60 degrees/s. OKAN I and II were consistently observed and occasionally OKAN III was noted. OKAN I durations (mean 13.8 +/- 5.1 s) and OKAN II amplitudes were independent of stimulus magnitude. Initial SPV of OKAN I was typically the same as that of OKN, i.e. no fast fall was observed. Cessation of pattern rotation in light, however, produced a fast initial decay of SPV. A least square fitting of OKAN time course was performed with various time functions. The SPV of OKAN I and II was best fitted with a damped sine wave, indicating that cat optokinetic system behaves like a second order underdamped system. Sinusoidal stimuli produced strong response non-linearities. At a given frequency gain decreased with increasing stimulus amplitudes. Gain correlated best with stimulus acceleration. In addition, strong stimuli produced characteristic response distortions. In the visual-vestibular conflict situation vectorial summation of VOR and OKN was observed only with small stimuli.

Visual (optokinetic) and somesthetic inputs to the cerebellum of bilaterally labyrinthectomized frogs

Unitary responses to visual (optokinetic) and somesthetic (cutaneous and propioceptive ) stimulation were recorded from the Purkinje cell layer of the cerebellum of acute (up to 30 h) and chronic (30-90 days) bilaterially labyrinthectomized frogs. Simple spikes from Purkinje cells as well as activity from cells without complex spikes were considered in this paper. The properties of the response and the distribution (restricted to the dorsal rim and auricular lobes) of the units sensitive to optokinetic stimulation of labyrinthectomized frogs, both acute and chronic, were similar to those previously reported for normal animals. The properties of the responses to somesthetic neck and limb stimulation remained similar to those of normal animals. However, there was an increase in the number of units responsive to somesthetic stimulation within the dorsal half of the corpus cerebelli (including the dorsal rim), a region experimentally deprived of vestibular afferents, neck responsive units were 33% of the total in acutely and 61% in chronically labyrinthectomized animals (compared to 5% in normal). Limb responsive units were 49% in acute and 65% in chronic animals (compared to 12% in normal). A consequence of the increase in somesthetic input was convergence of optokinetic and somesthetic inputs at the level of single units within the dorsal rim, totally absent before the lesion. The results suggest that the somesthetic spinal input might substitute for at least some features of the vestibular input to the cerebellum in bilaterally labyrinthectomized frogs.

The optokinetic reflex in the cat: modeling and computer simulation

After a brief comparative review of the dynamic characteristics of the optokinetic reflex ( OKR ) in different species and after a brief description of the main anatomical structures involved in this reflex, a mathematical model of the OKR in the cat is presented. The experimental results obtained by Godaux and Vanderkelen (1984) in the normal and in the totally cerebellectomized cat were used to validate the model and to obtain an estimation of its parameters.

Vertical optokinetic nystagmus and vestibular nystagmus in the monkey: up-down asymmetry and effects of gravity

Vertical optokinetic nystagmus (OKN) i.e., OKN in the sagittal plane, was asymmetrical in the monkey when it was induced with animals lying on their sides in a 90 degrees roll position. In typical monkeys the slow phase velocity of downward OKN (slow phases up) increased proportionally with stimulus velocity at close to unity gain to about 60 degrees/s and saturated at about 100 degrees/s. Upward OKN (slow phases down) increased with close to unity gain only to about 40 degrees/s and saturated at about 60 degrees/s. The slow phase velocity of upward OKN was usually irregular and its frequency was lower than that of downward or horizontal OKN. Upward and downward optokinetic after-nystagmus (OKAN) were also asymmetrical. Upward OKAN was weak or absent and when present it usually saturated at 10 degrees/s. Downward OKAN was stronger, increasing with a gain of about 0.7 with regard to stimulus velocity to a saturation velocity of about 50-60 degrees/s. This was usually about 10-30 degrees/s less than the saturation velocity of horizontal OKAN. The weak or absent upward OKAN indicates that stored activity related to slow phase eye velocity contributes little to the production of upward OKN. In agreement with this, there was little or no slow rise in slow phase velocity to a steady state level during upward OKN. Instead eye velocity rose to its peak velocity at the onset of stimulation. The lack of stored velocity information is probably largely responsible for the differences in regularity, gain and frequency between upward and downward OKN. Vertical vestibular nystagmus was induced by rotating monkeys in darkness with steps of velocity about a vertical axis, while they were lying on their sides in a 90 degree roll position. The velocities of the initial upward and downward slow phases were approximately equal. Gains of the vertical VOR ranged from about 0.5 to 0.98 for stimuli up to 150 degrees/s. Despite equivalent initial gains for upward and downward nystagmus, the vertical VOR was asymmetrical in that downward nystagmus had a higher frequency and generally lasted longer than upward nystagmus. Time constants of downward nystagmus (slow phases up) were about 15 s on average and were similar to those of horizontal nystagmus. Mean time constants of upward nystagmus (slow phases down) were about 8 s. This is only slightly longer than the average time constant of afferent activity in the semicircular canal nerves induced by steps of velocity.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of vestibulo-ocular reflex gain on human optokinetic responses

In 15 patients with severe bilateral vestibular impairment (mean vestibulo-ocular reflex (VOR) gain less than 0.05), constant velocity optokinetic nystagmus (OKN) gain, optokinetic after nystagmus (OKAN) initial velocity, and OKAN duration were significantly (p less than 0.0025) lower than in 20 normal subjects of similar age. In the normal subjects VOR gain was significantly correlated (p less than 0.05) with OKAN initial velocity but not with OKAN duration or OKN gain. The results of this study provide additional evidence for the existence in human subjects of an optokinetic pathway whose function, like that of the subcortical optokinetic pathway in animals, varies with VOR gain.

Effects of prolonged optokinetic stimulation on oculomotor and locomotor balance functions

Squirrel monkeys were exposed to optokinetic stimulus (90 degrees/sec constant speed, unidirectional) for 60 min. Eye movements during and after the stimulus exposure were recorded. Comparison of the data between very early and late stages of exposure showed the oculomotor gain increase and the nystagmic frequency decline. Slow phase eye velocity of bilaterally labyrinthectomized squirrel monkeys in the late exposure stage could reach almost to the level of normal animals. Post-stimulus analysis in normal monkeys showed that amphetamine enhanced the optokinetic after-nystagmus duration, the maximum slow phase eye velocity, and the time constant. In contrast, the effect of amphetamine on reversed optokinetic after-nystagmus was not at the significant level in all parameters studied. The manifestation of directional reciprocity of optokinetic after-nystagmus was inconsistent. In bilaterally labyrinthectomized animals, ipsilateral optokinetic after-nystagmus did not appear after the stimulus cessation. Instead, immediate reversed optokinetic after-nystagmus appeared. When the normal animal was kept in the light after the stimulus cessation, slow phase eye velocity of reversed optokinetic after-nystagmus declined relatively rapidly. Reversed optokinetic after-nystagmus and vestibular evoked nystagmus were summated or deducted, in velocity domain, depending upon the direction. Optokinetically induced system imbalance did not depict when the monkey's spinal locomotor function was measured by the platform runway test with the availability of vision.

Flash-induced nystagmus (FIN) and the vestibular system in the rabbit

Flash-induced nystagmus (FIN) was found more often in albino than in pigmented rabbits. Incremental and decremental phase mainly showed exponential change in SPV, but linear change was seen as well with apparent time constant of 5-40 sec. FIN was unaltered by rendering the stimulated eye akinetic. Lesioning the nucleus of the optic tract (NOT) on one side abolished FIN and OKN responses from the other side. Bilateral labyrinthectomy led to a diminished response and after-responses were lost. Algebraic addition was shown if FIN was combined with vestibular reflex eye movements. The gain of these reflexes was enhanced. FIN may be specified as a type of nystagmus that is evoked by monocular essentially open-loop stimulation of the crossed ('subcortical') optic pathway. Direction-selective elements in the retina may play a specific role in that their predominant orientation gives rise to abduction nystagmus during on/off stimulation by flashes.

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.

Visual motor rehabilitation in children with cerebral palsy

Cerebral palsied (CP) children were given intensive visuo-oculomotor training in order to improve their visuo-oculomotor control, using children's films as a visual stimulus. A comparative study was conducted on a group of normal children of the same age. Results showed that training does improve visuo-oculomotor system control as illustrated by (1) a marked increase in smooth pursuit precision and maximum velocity, (2) an improvement of saccadic movement precision and stability, and (3) a shortening of the saccadic reaction time. The highest performance was observed under conditions in which the child pointed at and followed the visuo-acoustic target with his arm extended.

The efferent connections of the nucleus of the optic tract and the superior colliculus in the rabbit

3H-leucine injections were made in tectal and pretectal areas in the rabbit. After injections in the nucleus of the optic tract (NOT) labeled fibers were distributed bilaterally to the superior colliculus, the dorsal part of the medial geniculate nucleus (MGd), and the pulvinar nucleus, and ipsilaterally to the external layer of the ventral lateral geniculate nucleus (LGv), the dorsal geniculate nucleus (LGd) pars beta, the reticular thalamic nucleus, and the lateral and medial terminal nucleus (LTN, MTN). Many labeled fibers were distributed to the lateral and some to the medial parts of the pontine nuclei. more caudally, coarse labeled fiber bundles descended ipsilaterally, distributing fibers to the prepositus hypoglossi and abducens nucleus and to the caudally adjoining medial reticular formation. Many labeled fibers were also present in the inferior olive, especially ipsilaterally in the dorsal cap and the ventrally adjoining pars beta, and a few in the contralateral dorsal cap area. Contralaterally, some descending fibers terminated in the dorsal part of the facial nucleus, in which motoneurons are located innervating the orbicularis oculi muscle. The superficial layers of superior colliculus distributed fibers bilaterally to the internal layer in the ventral lateral geniculate nucleus (LGv), the LGd alpha (lateral part), the MGd, the pulvinar, and more caudally to the ipsilateral parabigeminal and lateral pontine nuclei. The deep collicular layers distributed fibers ipsilaterally to MG (internal division), pulvinar, and the internal layer of LGv. Furthermore, ascending connections were found to the suprageniculate nucleus, the zona incerta, the mediodorsal nucleus, and some intralaminar and midline nuclei. Descending fibers terminated in the mesencephalic lateral tegmentum, pontine nuclei, and ventrally in the pontine and high medullary reticular formation. Contralaterally fibers were distributed to the nucleus reticularis tegmenti pontis (NRTP), the medial reticular formation, and the inferior olive just lateral to the nucleus beta. In one case fibers were also distributed to the lateral part of the contralateral facial nucleus in which motoneurons are located innervating the upper lip muscles.

Modification of the balance and gain of the vestibulo-ocular reflex in the cat

The characteristics of the vestibulo-ocular reflex (VOR) of a normal cat can be modified in response to visual demands. Two aspects of the VOR are modifiable independently by a normal cat: the gain and the balance. An imbalance results in a spontaneous nystagmus and an asymmetric VOR. Neither the gain nor the balance of a dark-reared cat's VOR is susceptible to visual modification. A cat whose crossed visual pathways are severed at the level of the optic chiasm is able to modify the gain of the VOR but not its balance. Both dark-reared and split-chiasm cats have only very short-lasting optokinetic after-nystagmus.

Electrophysiological correlates of the reversed postoptokinetic nystagmus in the rabbit: activity of vestibular and floccular neurons

Unit activity changes accompanying the optokinetic nystagmus (OKN) and reversed postoptokinetic nystagmus (RPN) in the rabbit were examined in 180 vestibular and floccular neurons. After initial charging of the RPN generator by 60-min optokinetic stimulation, a sequence of 1-min optokinetic stimulation (OKN) followed by 1-min darkness (RPN) and 1-min illumination of the stationary optokinetic drum (L), was cycled while corresponding unit activity changes were recorded during 3--5 cycles and evaluated with a computer. About 50% of vestibular neurons (type A) increased their activity during OKN and/or decreased it during RPN with respect to the L period, whereas 24% (type B) reached in a reciprocal manner. The remaining neurons were either unaffected or responded in an atypical way. Most floccular neurons (75%) were activated during ipsilateral optokinetic stimulation, but were not significantly affected by RPN. It is suggested that the neural trace of RPN develops in the vestibular complex and vestibulocerebellum as a part of the process compensating for the effect of continued optokinetic stimulation.