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

Neuronal Substrates of Motor Learning in the Velocity Storage Generated During Optokinetic Stimulation in the Squirrel Monkey

Chronic motor learning in the vestibuloocular reflex (VOR) results in changes in the gain of this reflex and in other eye movements intimately associated with VOR behavior, e.g., the velocity storage generated by optokinetic stimulation (OKN velocity storage). The aim of the present study was to identify the plastic sites responsible for the change in OKN velocity storage after chronic VOR motor learning. We studied the neuronal responses of vertical eye movement flocculus target neurons (FTNs) during the optokinetic afternystagmus (OKAN) phase of the optokinetic response (OKR) before and after VOR motor learning. Our findings can be summarized as follows. 1) Chronic VOR motor learning changes the horizontal OKN velocity storage in parallel with changes in VOR gain, whereas the vertical OKN velocity storage is more complex, increasing with VOR gain increases, but not changing following VOR gain decreases. 2) FTNs contain an OKAN signal having opposite directional preferences after chronic high versus low gain learning, suggesting a change in the OKN velocity storage representation of FTNs. 3) Changes in the eye-velocity sensitivity of FTNs during OKAN are correlated with changes in the brain stem head-velocity sensitivity of the same neurons. And 4) these changes in eye-velocity sensitivity of FTNs during OKAN support the new behavior after high gain but not low gain learning. Thus we hypothesize that the changes observed in the OKN velocity storage behavior after chronic learning result from changes in brain stem pathways carrying head velocity and OKN velocity storage information, and that a parallel pathway to vertical FTNs changes its OKN velocity storage representation following low, but not high, gain VOR motor learning.

Present concepts of oculomotor organization

This chapter gives an introduction to the oculomotor system, thus providing a framework for the subsequent chapters. This chapter describes the characteristics, and outlines the structures involved, of the five basic types of eye movements, for gaze holding ("neural integrator") and eye movements in three dimensions (Listing's law, pulleys).

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.

Role of the cerebellar flocculus region in cancellation of the VOR during passive whole body rotation

A series of studies were carried out to investigate the role of the cerebellar flocculus and ventral paraflocculus in the ability to voluntarily cancel the vestibuloocular reflex (VOR). Squirrel monkeys were trained to pursue moving visual targets and to fixate a head stationary or earth stationary target during passive whole body rotation (WBR). The firing behavior of 187 horizontal eye movement-related Purkinje (Pk) cells in the flocculus region was recorded during smooth pursuit eye movements and during WBR. Half of the Pk cells encountered were eye velocity Pk cells whose firing rates were related to eye movements during smooth pursuit and WBR. Their sensitivity to eye velocity during WBR was reduced when a visual target was not present, and their response to unpredictable steps in WBR was delayed by 80-100 ms, which suggests that eye movement sensitivity depended on visual feedback. They were insensitive to WBR when the VOR was canceled. The other half of the Purkinje cells encountered were sensitive to eye velocity during pursuit and to head velocity during VOR cancellation. They resembled the gaze velocity Pk cells previously described in rhesus monkeys. The head velocity signal tended to be less than half as large as the eye velocity-related signal and was observable at a short ( approximately 40 ms) latency when the head was unpredictably accelerated during ongoing VOR cancellation. Gaze and eye velocity type Pk cells were found to be intermixed throughout the ventral paraflocculus and flocculus. Most gaze velocity Pk cells (76%) were sensitive to ipsilateral eye and head velocity, but nearly half (48%) of the eye velocity Pk cells were sensitive to contralateral eye velocity. Thus the output of flocculus region is modified in two ways during cancellation of the VOR. Signals related to both ipsilateral and contralateral eye velocity are removed, and in approximately half of the cells a relatively weak head velocity signal is added. Unilateral injections of muscimol into the flocculus region had little effect on the gain of the VOR evoked either in the presence or absence of visual targets. However, ocular pursuit velocity and the ability to suppress the VOR by fixating a head stationary target were reduced by approximately 50%. These observations suggest that the flocculus region is an essential part of the neural substrate for both visual feedback-dependent and nonvisual mechanisms for canceling the VOR during passive head movements.

Functions of the nucleus of the optic tract (NOT). II. Control of ocular pursuit

Ocular pursuit in monkeys, elicited by sinusoidal and triangular (constant velocity) stimuli, was studied before and after lesions of the nucleus of the optic tract (NOT). Before NOT lesions, pursuit gains (eye velocity/target velocity) were close to unity for sinusoidal and constant-velocity stimuli at frequencies up to 1 Hz. In this range, retinal slip was less than 2 degrees. Electrode tracks made to identify the location of NOT caused deficits in ipsilateral pursuit, which later recovered. Small electrolytic lesions of NOT reduced ipsilateral pursuit gains to below 0.5 in all tested conditions. Pursuit was better, however, when the eyes moved from the contralateral side toward the center (centripetal pursuit) than from the center ipsilaterally (centrifugal pursuit), although the eyes remained in close proximity to the target with saccadic tracking. Effects of lesions on ipsilateral pursuit were not permanent, and pursuit gains had generally recovered to 60-80% of baseline after about 2 weeks. One animal had bilateral NOT lesions and lost pursuit for 4 days. Thereafter, it had a centrifugal pursuit deficit that lasted for more than 2 months. Vertical pursuit and visually guided saccades were not affected by the bilateral NOT lesions in this animal. We also compared effects of these and similar NOT lesions on optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN). Correlation of functional deficits with NOT lesions from this and previous studies showed that rostral lesions of NOT in and around the pretectal olivary nucleus, which interrupted cortical input through the brachium of the superior colliculus (BSC), affected both smooth pursuit and OKN. In two animals in which it was tested, NOT lesions that caused a deficit in pursuit also decreased the rapid and slow components of OKN slow-phase velocity and affected OKAN. It was previously shown that slightly more caudal NOT lesions were more effective in altering gain adaptation of the angular vestibulo-ocular reflex (aVOR). The present findings suggest that cortical pathways through rostral NOT play an important role in maintenance of ipsilateral ocular pursuit. Since lesions that affected ocular pursuit had similar effects on ipsilateral OKN, processing for these two functions is probably closely linked in NOT, as it is elsewhere.

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.

Induced motion: isolation and dissociation of egocentric and vection-entrained components

Induced motion (IM) is illusory motion of a stationary test target opposite to the direction of the real motion of the inducing stimulus. We define egocentric IM as an apparent motion of the test target relative to the observer, and vection-entrained IM as an apparent motion of a stationary object along with an apparent motion of the self (vection) induced by the same stimulus. These two forms of IM are often confounded, and tests for distinguishing between them have not been devised. We have devised such tests. Our test for egocentric IM relies on evidence that this form of IM is due mainly to a misregistration of eye movements when optokinetic nystagmus (OKN) is inhibited, and on evidence that OKN is evoked only by stimuli in the plane of convergence. Our test for vection-entrained IM relies on evidence that vection is evoked only by the more distant of two superimposed inducing stimuli. Thus we found egocentric IM to be induced without vection or vection-entrained IM when subjects converged on a foreground moving display with a stationary display in the background, and vection-entrained IM to be induced without egocentric IM when subjects converged on a stationary-foreground display with a moving display in the background. The two types of IM were evoked in opposite directions at the same time when subjects converged on a foreground moving display while a background display moved in the opposite direction. The two forms of IM showed no signs of interaction, and we conclude that they rely on independent motion mechanisms that operate within distinct frames of reference. A control experiment suggested that the depth adjacency effect in IM is determined by the depth adjacency of the inducing stimulus to convergence, not just to the test target.

Saccade-related Purkinje cells in the cerebellar hemispheres of the monkey

Extracellular single unit discharges of cerebellar Purkinje cells (P-cells) were recorded from the cerebellar hemispheres of two Japanese monkeys (Macaca fuscata) during spontaneous and visually guided eye movements. We found that saccade-related P-cells, whose simple-spike (SS) discharge rates were modulated in close correlation with saccadic eye movements, were localized in fairly restricted areas in the hemisphere, mostly in Crus IIa with some in the deep folia of Crus I. P-cells located in simple lobules, superficial folia of Crus I or in Crus IIp did not change their discharge rate during voluntary eye movements. Fifty-five saccade-related P-cells recorded from Crus I and II showed modulation of SS discharge rate related to both spontaneous and visually triggered saccades, with the modulation closely time-locked to the saccades. Two thirds (37/55) of saccade-related P-cells began to change their SS discharge rate 20-100 ms prior to the onset of saccades. The remaining one third (18/55) changed their activity approximately at the same time as the saccade onset. These saccade-related P-cells did not show changes in activity during smooth pursuit eye movements, and we did not find any P-cells in the cerebellar hemisphere which showed changes of activity preferentially during smooth pursuit eye movements. In about half (26/55) of the saccade-related P-cells, the pattern of modulation prior to and during saccades was biphasic: increase-decrease or decrease-increase. The other half (29/55) showed monophasic increases or decreases. For a given P-cell, the discharge pattern during saccades was similar for saccades of all directions, though there was a preferred direction in the amount of discharge rate modulation. The present findings suggest that the cerebellar hemisphere (Crus I and IIa) plays an important role in the control of voluntary saccadic eye movements, in addition to other cerebellar cortical areas (flocculus and posterior vermis) which are known to participate in the control of saccades.

Identification of the Purkinje cell/climbing fiber zone and its target neurons responsible for eye-movement control by the cerebellar flocculus

We identified 3 Purkinje cell/climbing fiber zones in the cat cerebellar flocculus. The zones were perpendicular to the long axes of the crooked floccular folia, forming the crooked zones. Each zone was different in axonal projection areas of its target neurons. From the neuronal networks it is theoretically expected that activity changes of a particular zone control eye movement in a particular plane: (1) the rostral and caudal zones on one side control movement in the anterior canal plane on the side of the activity changes and those on both sides control movement in all vertical planes from sagittal to transverse planes; and (2) the middle zone controls movement in the horizontal plane by reciprocal activity changes on both sides. The zone-specific climbing fiber input to a particular zone may contribute to activity changes of the zone in response to mossy fiber input spreading across several zones. Electrical stimulation of each zone evoked the same pattern of eye movement as that theoretically expected from the neuronal networks. This is the first indication that there are indeed functional differences between the Purkinje cell zones in the cerebellum. Our findings support Oscarsson's proposal that each Purkinje cell/climbing fiber zone plus its target neurons may be an operational unit for control of a given motor function.

Short latency ocular-following responses in man

The ocular-following responses elicited by brief unexpected movements of the visual scene were studied in human subjects. Response latencies varied with the type of stimulus and decreased systematically with increasing stimulus speed but, unlike those of monkeys, were not solely determined by the temporal frequency generated by sine-wave stimuli. Minimum latencies (70-75 ms) were considerably shorter than those reported for other visually driven eye movements. The magnitude of the responses to sine-wave stimuli changed markedly with stimulus speed and only slightly with spatial frequency over the ranges used. When normalized with respect to spatial frequency, all responses shared the same dependence on temporal frequency (band-pass characteristics with a peak at 16 Hz), indicating that temporal frequency, rather than speed per se, was the limiting factor over the entire range examined. This suggests that the underlying motion detectors respond to the local changes in luminance associated with the motion of the scene. Movements of the scene in the immediate wake of a saccadic eye movement were on average twice as effective as movements 600 ms later: post-saccadic enhancement. Less enhancement was seen in the wake of saccade-like shifts of the scene, which themselves elicited weak ocular following, something not seen in the wake of real saccades. We suggest that there are central mechanisms that, on the one hand, prevent the ocular-following system from tracking the visual disturbances created by saccades but, on the other, promote tracking of any subsequent disturbance and thereby help to suppress post-saccadic drift. Partitioning the visual scene into central and peripheral regions revealed that motion in the periphery can exert a weak modulatory influence on ocular-following responses resulting from motion at the center. We suggest that this may help the moving observer to stabilize his/her eyes on nearby stationary objects.

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.

Target neurons of floccular middle zone inhibition in medial vestibular nucleus

Unitary activities of 288 neurons were recorded extracellularly in the medial vestibular nucleus (MV) in anesthetized cats. In 19 neurons, located in the rostral part of the MV adjacent to the stria acustica, floccular middle zone stimulation resulted in cessation of spontaneous discharges. Systematic microstimulation in the brainstem during recording of 16 of 19 target neurons of floccular middle zone inhibition revealed that the target neurons projected to the ipsilateral abducens nucleus (ABN), and not to the contralateral ABN nor the oculomotor nucleus. The conjugate ipsilateral horizontal eye movement elicited by middle zone stimulation may be mediated by this pathway to motoneurons and internuclear neurons in the ipsilateral ABN. In additional experiments, the MV neurons responding antidromically to ipsilateral ABN stimulation and orthodromically to ipsilateral 8 nerve stimulation were recorded extracellularly. In only 7 of 36 recorded neurons, middle zone stimulation depressed the orthodromic and spontaneous activities. Many neurons were free of floccular inhibition. As to the route of floccular inhibitory control over the vestibulo-ocular reflex (VOR) during visual-vestibular stimulation, we propose that the interaction of target and VOR relay neurons takes place at the ipsilateral ABN and modulates the VOR, in addition to well known Ito's proposal that the interaction of the floccular output and the VOR takes place at secondary vestibular neurons and modulates the VOR.

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.

Neuronal activity in the flocculus of the alert monkey during sinusoidal optokinetic stimulation

1. Activity of single units was recorded in the flocculus of alert, behaving monkeys during sinusoidal optokinetic (0.02-5.0 Hz), constant velocity optokinetic, vestibular and visual-vestibular conflict stimulation. The maximal stimulus velocity for sinusoidal optokinetic stimulation at different frequencies was 40 deg/s or less (at frequencies above 1 Hz). For an amplitude series at 0.2 Hz, stimulus velocity was varied between +/- 10 to +/- 80 deg/s. In one trained monkey activity was also investigated during smooth pursuit eye movements and suppression of the vestibulo-ocular reflex by visual fixation (VOR-supp.). Only neurons which responded to 0.2 Hz (+/- 40 deg/s) optokinetic stimulation were included in the study. 2. The majority of neurons (44 out of 59) were type I Purkinje cells (PCs), which increased their simple spike activity during optokinetic cylinder rotation to the ipsilateral recording side. The responses during other, vestibular related, paradigms allowed all these neurons to be classified as so called 'gaze velocity' PCs. Three type II PCs were encountered, which responded similarly, but were only weakly modulated. 3. All type I PCs were modulated at frequencies of sinusoidal optokinetic stimulation between 0.05 and 2.5 Hz. PC's showed little or no modulation at 0.03 and 0.02 Hz. About half of the PC's still responded at 5.0 Hz. 4. Relative to eye velocity, the PC activity had a phase advance of about 30 deg between 0.1 and 2 Hz. It became larger at lower, and smaller at higher, frequencies. Eye velocity related sensitivity (imp/s/deg/s) was small at low stimulus frequencies and increased monotonically, on average from 0.16 at 0.02 Hz to 2.0 at 3.3 Hz. 5. Ten (out of 12) mossy fiber related input neurons were classified as visual neurons, since their activity could be related to the amount of retinal slip in all conditions. Neurons were clearly modulated at sinusoidal optokinetic stimulation up to 5 Hz. One input neuron, investigated during sinusoidal OKN, smooth pursuit eye movements, VOR and VOR-supp., behaved qualitatively like a 'gaze velocity' PC. The remaining input neuron encoded eye velocity at 0.2 Hz optokinetic, vestibular and visual-vestibular conflict stimulation. 6. The results show that during sinusoidal and constant velocity optokinetic stimulation 'gaze velocity' PC's do not encode eye velocity and/or eye acceleration. 7. The vestibular nuclei-flocculus complementary hypothesis (Waespe and Henn 1981) can explain PC responses to a large extent.(ABSTRACT TRUNCATED AT 400 WORDS)

Induced motion and optokinetic afternystagmus: parallel response dynamics with prolonged stimulation

Fixation of a stationary target during motion of background contours attenuates optokinetic nystagmus (OKN), while illusory induced motion (IM) of the fixated target occurs opposite the direction of contour motion. It is proposed that IM owes to a perceptually registered efferent signal for ocular pursuit which opposes an unregistered signal for OKN to achieve stable fixation. This proposal predicts parallel changes in the magnitudes of IM and optokinetic reflexes during and after optokinetic stimulation. Accordingly, leftward IM magnitude and rightward slow-phase velocity of optokinetic afternystagmus (OKAN) increased at similar rates across 90 and 160 sec of 60 deg/sec motion of background contours, and decayed at similar rates after stimulus termination. Both responses decayed more deeply following stimulation with, rather than without fixation. Neither retinal image motion nor vection can explain the IM obtained.

Baclofen and velocity storage: a model of the effects of the drug on the vestibulo-ocular reflex in the rhesus monkey

1. Baclofen had a characteristic effect on vestibular and optokinetic nystagmus in rhesus monkeys. Each aspect of nystagmus that is dependent on the velocity-storage mechanism in the vestibulo-ocular reflex (v.o.r.) was altered by the drug: (a) Baclofen reduced the dominant time constant of the v.o.r. in a dose-dependent manner up to 5 mg/kg, the highest dosage used. The alteration in v.o.r. time constant began within 15 min of injection, was maximal between 1 and 4 h, and lasted for 14-18 h. This effect mirrors changes in plasma levels of baclofen after oral doses in humans (Faigle, Keberle & Agen, 1980). (b) Slow-phase velocities of steady-state nystagmus induced by rotation about axes tilted from the vertical (off-vertical axis rotation, o.v.a.r.) were reduced after baclofen and could not be maintained at previous levels. (c) There was a dose-dependent decline in the steady-state gain of optokinetic nystagmus (o.k.n.), and at the highest dosages little o.k.n. was induced. In parallel, the peak velocity and falling time constant of optokinetic after-nystagmus (o.k.a.n.) were reduced. Since baclofen is a GABA agonist, systems utilizing GABA and acting on GABAB receptors appear to produce inhibitory control of velocity storage. 2. The step gain of the v.o.r., measured at the beginning and end of constant-velocity rotation in darkness, was unaffected by baclofen, as were saccades, quick phases of nystagmus, and the ability to hold positions of fixation or to generate linear slow phases of nystagmus. This indicates that it is possible to use baclofen to manipulate the dominant time constant of the v.o.r. and of o.k.a.n. in relative isolation from effects on other oculomotor components. 3. Baclofen caused a dose-dependent reduction in the initial jump in eye velocity at the onset of o.k.n., suggesting that the initial jump is also under inhibitory control of GABAB receptors. However, there were still occasional slow phases with velocities up to 30-40 deg/s after baclofen, and animals were capable of visually suppressing the v.o.r. This indicates that pathways responsible for causing rapid changes in slowphase velocity were capable of functioning, at least intermittently, in the presence of the drug.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of eye velocity storage during periods of suppression and reversal of eye velocity in monkeys

An eye velocity storage mechanism has been postulated in the vestibulo-optokinetic system to account for the prolongation of vestibular nystagmus (VN) and the occurrence of optokinetic after-nystagmus (OKAN). Presentation of a subject-stationary full-field surround during VN and OKAN (= full-field fixation) rapidly reduces activity related to eye velocity of the storage mechanism. If the subject-stationary full-field surround is presented for short periods during VN or OKAN, nystagmus resumes when the animal is again in darkness, but at a lesser velocity than would be predicted from a control response. This reduction in peak eye velocity after fixation reflects a decrease in activity of the storage mechanism due to full-field fixation. This decrease in activity occurs with a shorter time constant compared to that in control trials, it has been called "dumping". We demonstrate that a subject-stationary small target light presented during VN or OKAN (= target fixation) also reduces activity of the storage mechanism with a time constant slightly greater than that for full-field fixation, but still considerably smaller than that in control trials. In 3 monkeys the time constant of discharge was reduced during the post-rotatory period from 20 s in control trials to 4.6 s by fixation of a single target light and to 2.9 s by fixation of a full-field. The time constant of discharge was reduced during OKAN from 13.2 s in control trials to 3.8 s by target fixation and to 2.6 s by full-field fixation. We report a second experimental paradigm with which the dynamics of visual-vestibular interaction involving the eye velocity storage mechanism is analysed by means of transient step responses. In this paradigm eye velocity due to activation of the storage mechanism (OKAN) is forced to reverse by a short exposure to a full-field moving in the opposite direction of the slow phases of nystagmus. Short periods of eye velocity reversal did not reduce activity of the storage mechanism more rapidly than fixation, i.e. suppression of eye velocity alone. Fixation of a full-field or of a single target light during vestibular or optokinetic stimulation reduces peak nystagmus velocity after stimulation when monkeys are in darkness. Suppression of OKN by target fixation during full-field stimulation reduces the initial eye velocity of OKAN to 15-20% compared to the OKAN velocity when OKN is allowed to occur.(ABSTRACT TRUNCATED AT 400 WORDS)

Purkinje cell activity in the primate flocculus during optokinetic stimulation, smooth pursuit eye movements and VOR-suppression

Purkinje cell (PC) activity in the flocculus of trained monkeys was recorded during: 1) Vestibular stimulation in darkness. 2) Suppression of the vestibulo-ocular reflex (VOR-supp) by fixation of a small light spot stationary with respect to the monkey. 3) Visual-vestibular conflict (i.e. the visual surround moves together with the monkey during vestibular stimulation), which leads to attenuation or suppression of vestibular nystagmus. 4) Smooth pursuit eye movements. 5) Optokinetic nystagmus (OKN). 6) Suppression of nystagmus during optokinetic stimulation (OKN-supp) by fixation of a small light spot; whereby stimulus velocity corresponds then to image slip velocity. Results were obtained from PCs, which were activated with VOR-supp during rotation to the ipsilateral side. The same PCs were also modulated during smooth pursuit and visual-vestibular conflict. No tonic modulation during constant velocity OKN occurred with slow-phase nystagmus velocities below 40-60 deg/s. Tonic responses were only seen at higher nystagmus velocities. Transient activity changes appeared at the beginning and end of optokinetic stimulation. PCs were not modulated by image slip velocity during OKN-supp. The results show that in primates the same population of floccular PCs is involved in different mechanisms of visual-vestibular interaction and that smooth pursuit and certain components of OKN slow-phase velocity share the same neural pathway. It is argued that the activity of these neurons can neither be related strictly to gaze, eye or image slip velocity; instead, their activity pattern can be best interpreted by assuming a modulation, which is complementary to that of central vestibular neurons of the vestibular nuclei, in the control of slow eye movements.