Vestibulo-ocular reflex, optokinetic response and their interactions in the cerebellectomized cat

Nucleus incertus provides eye velocity and position signals to the vestibulo-ocular cerebellum: a new perspective of the brainstem-cerebellum-hippocampus network

The network formed by the brainstem, cerebellum, and hippocampus occupies a central position to achieve navigation. Multiple physiological functions are implicated in this complex behavior. Among these, control of the eye-head and body movements is crucial. The gaze-holding system realized by the brainstem oculomotor neural integrator (ONI) situated in the nucleus prepositus hypoglossi and fine-tuned by the contribution of different regions of the cerebellum assumes the stability of the image on the fovea. This function helps in the recognition of environmental targets and defining appropriate navigational pathways further elaborated by the entorhinal cortex and hippocampus. In this context, an enigmatic brainstem area situated in front of the ONI, the nucleus incertus (NIC), is implicated in the dynamics of brainstem-hippocampus theta oscillation and contains a group of neurons projecting to the cerebellum. These neurons are characterized by burst tonic behavior similar to the burst tonic neurons in the ONI that convey eye velocity-position signals to the cerebellar flocculus. Faced with these forgotten cerebellar projections of the NIC, the present perspective discusses the possibility that, in addition to the already described pathways linking the cerebellum and the hippocampus via the medial septum, these NIC signals related to the vestibulo-ocular reflex and gaze holding could participate in the hippocampal control of navigation.

Vestibulo-spatial navigation: pathways and sense of direction

Aims of the present article are: 1) assessing vestibular contribution to spatial navigation, 2) exploring how age, global positioning systems (GPS) use, and vestibular navigation contribute to subjective sense of direction (SOD), 3) evaluating vestibular navigation in patients with lesions of the vestibular-cerebellum (patients with downbeat nystagmus, DBN) that could inform on the signals carried by vestibulo-cerebellar-cortical pathways. We applied two navigation tasks on a rotating chair in the dark: return-to-start (RTS), where subjects drive the chair back to the origin after discrete angular displacement stimuli (path reversal), and complete-the-circle (CTC) where subjects drive the chair on, all the way round to origin (path completion). We examined 24 normal controls (20-83 yr), five patients with DBN (62-77 yr) and, as proof of principle, two patients with early dementia (84 and 76 yr). We found a relationship between SOD, assessed by Santa Barbara Sense of Direction Scale, and subject's age (positive), GPS use (negative), and CTC-vestibular-navigation-task (positive). Age-related decline in vestibular navigation was observed with the RTS task but not with the complex CTC task. Vestibular navigation was normal in patients with vestibulo-cerebellar dysfunction but abnormal, particularly CTC, in the demented patients. We conclude that vestibular navigation skills contribute to the build-up of our SOD. Unexpectedly, perceived SOD in the elderly is not inferior, possibly explained by increased GPS use by the young. Preserved vestibular navigation in cerebellar patients suggests that ascending vestibular-cerebellar projections carry velocity (not position) signals. The abnormalities in the cognitively impaired patients suggest that their vestibulo-spatial navigation is disrupted.NEW & NOTEWORTHY Our subjective sense-of-direction is influenced by how good we are at spatial navigation using vestibular cues. Global positioning systems (GPS) may inhibit sense of direction. Increased use of GPS by the young may explain why the elderly's sense of direction is not worse than the young's. Patients with vestibulo-cerebellar dysfunction (downbeat nystagmus syndrome) display normal vestibular navigation, suggesting that ascending vestibulo-cerebellar-cortical pathways carry velocity rather than position signals. Pilot data indicate that dementia disrupts vestibular navigation.

Distinct proportions of cholinergic neurons in the rat prepositus hypoglossi nucleus according to their cerebellar projection targets

Cerebellar functions are modulated by cholinergic inputs, the density of which varies among cerebellar regions. Although the prepositus hypoglossi nucleus (PHN), a brainstem structure involved in controlling gaze holding, is known as one of the major sources of these cholinergic inputs, the proportions of cholinergic neurons in PHN projections to distinct cerebellar regions have not been quantitatively analyzed. In this study, we identified PHN neurons projecting to the cerebellum by applying retrograde labeling with dextran-conjugated Alexa 488 in choline acetyltransferase (ChAT)-tdTomato transgenic rats and compared the proportion of cholinergic PHN neurons in the PHN projections to four different regions of the cerebellum, namely the flocculus (FL), the uvula and nodulus (UN), lobules III-V in the vermis (VM), and the hemispheric paramedian lobule and crus 2 (PC). In the PHN, the percentage of cholinergic PHN neurons was lower in the projection to the FL than in the projection to the UN, VM or PC. Preposito-cerebellar neurons, except for preposito-FL neurons, included different proportions of cholinergic neurons at different rostrocaudal positions in the PHN. These results suggest that cholinergic PHN neurons project to not only the vestibulocerebellum but also the anterior vermis and posterior hemisphere and that the proportion of cholinergic neurons among projection neurons from the PHN differs depending on cerebellar target areas and the rostro-caudal regions of the PHN. This study provides insights regarding the involvement of cerebellar cholinergic networks in gaze holding.

Cerebellar Roles in Frequency Competitive Motor Learning of the Vestibulo-ocular Reflex

Biological motions commonly contain multiple frequency components in which each fundamental has to be adjusted by motor learning to acquire a new motor skill or maintain acquired skills. At times during this motor performance one frequency component needs to be enhanced (gain-up) while another is suppressed (gain-down). This pattern of simultaneous gain-up and -down adjustments at different frequencies is called frequency competitive motor learning. Currently we investigated cerebellar roles in this behavior utilizing the goldfish vestibulo-ocular reflex (VOR). Previously, VOR motor learning was shown in primates to be frequency selective and exhibit frequency competitive motor learning. Here we demonstrate that the goldfish VOR performs frequency competitive motor learning when high and low frequency components are trained to gain-up and gain-down, respectively. However, when the two frequency components were trained in the opposite directions only gain-up component was observed. We also found that cerebellectomy precluded any frequency competitive VOR motor learning. Complementary single unit recordings from vestibulo-cerebellar Purkinje cells revealed changes in firing modulation along with gain-down learning, but not with gain-up learning irrespective of frequency. These results demonstrate that the cerebellum is required for all frequency competitive VOR motor learning and Purkinje cell activity therein is well correlated with all gain-down behaviors independent of frequency. However, frequency competitive gain-up learning requires intact, recursive brainstem/cerebellar pathways. Collectively these findings support the idea that VOR gain-up and gain-down learning utilize separate brainstem/cerebellar circuitry that, in turn, clearly underlies the unique ability of the oculomotor system to deal with multiple frequency components.

World Statistics Drive Learning of Cerebellar Internal Models in Adaptive Feedback Control: A Case Study Using the Optokinetic Reflex

The cerebellum is widely implicated in having an important role in adaptive motor control. Many of the computational studies on cerebellar motor control to date have focused on the associated architecture and learning algorithms in an effort to further understand cerebellar function. In this paper we switch focus to the signals driving cerebellar adaptation that arise through different motor behavior. To do this, we investigate computationally the contribution of the cerebellum to the optokinetic reflex (OKR), a visual feedback control scheme for image stabilization. We develop a computational model of the adaptation of the cerebellar response to the world velocity signals that excite the OKR (where world velocity signals are used to emulate head velocity signals when studying the OKR in head-fixed experimental laboratory conditions). The results show that the filter learnt by the cerebellar model is highly dependent on the power spectrum of the colored noise world velocity excitation signal. Thus, the key finding here is that the cerebellar filter is determined by the statistics of the OKR excitation signal.

4-aminopyridine does not enhance flocculus function in tottering, a mouse model of vestibulocerebellar dysfunction and ataxia

The potassium channel antagonist 4-aminopyridine (4-AP) improves a variety of motor abnormalities associated with disorders of the cerebellum. The most rigorous quantitative data relate to 4-AP's ability to improve eye movement deficits in humans referable to dysfunction of the cerebellar flocculus. Largely based on work in the ataxic mouse mutant tottering (which carries a mutation of the Cacna1a gene of the P/Q voltage-activated calcium channel), 4-AP is hypothesized to function by enhancing excitability or rhythmicity of floccular Purkinje cells. We tested this hypothesis by determining whether systemic or intrafloccular administration of 4-AP would ameliorate the eye movement deficits in tottering that are attributable to flocculus dysfunction, including the reductions in amplitude of the yaw-axis vestibulo-ocular reflex (VOR) and vision-enhanced vestibulo-ocular reflex (VVOR), and the optokinetic reflex (OKR) about yaw and roll axes. Because tottering's deficits increase with age, both young and elderly mutants were tested to detect any age-dependent 4-AP effects. 4-AP failed to improve VOR, VVOR, and OKR gains during sinusoidal stimuli, although it may have reduced the tendency of the mutants' responses to VOR and VVOR to decline over the course of a one-hour recording session. For constant-velocity optokinetic stimuli, 4-AP generated some enhancement of yaw OKR and upward-directed roll OKR, but the effects were also seen in normal C57BL/6 controls, and thus do not represent a specific reversal of the electrophysiological consequences of the tottering mutation. Data support a possible extra-floccular locus for the effects of 4-AP on habituation and roll OKR. Unilateral intrafloccular 4-AP injections did not affect ocular motility, except to generate mild eye elevations, consistent with reduced floccular output. Because 4-AP did not produce the effects expected if it normalized outputs of floccular Purkinje cells, there is a need for further studies to elucidate the drug's mechanism of action on cerebellar motor dysfunction.

Sparse cerebellar innervation can morph the dynamics of a model oculomotor neural integrator

The oculomotor integrator is a brainstem neural network that converts velocity signals into the position commands necessary for eye-movement control. The cerebellum can independently adjust the amplitude of eye-movement commands and the temporal characteristics of neural integration, but the percentage of integrator neurons that receive cerebellar input is very small. Adaptive dynamic systems models, configured using the genetic algorithm, show how sparse cerebellar inputs could morph the dynamics of the oculomotor integrator and independently adjust its overall response amplitude and time course. Dynamic morphing involves an interplay of opposites, in which some model Purkinje cells exert positive feedback on the network, while others exert negative feedback. Positive feedback can be increased to prolong the integrator time course at virtually any level of negative feedback. The more these two influences oppose each other, the larger become the response amplitudes of the individual units and of the overall integrator network.

Cerebellar AMPA/KA receptor antagonism by CNQX inhibits vestibuloocular reflex adaptation

Vestibuloocular reflex (VOR) performance and adaptation have been investigated during antagonism of cerebellar AMPA/quisqualate and kainate receptors (AMPA/KA) by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Injection of CNQX into the vestibulo-cerebellum of the goldfish before adaptation significantly inhibited and, at the highest dosage, completely prevented acquisition of adaptive reflex gain increases and decreases during a 3-h training period. Injection of CNQX before initiation of VOR adaptive training did not affect pre-adapted baseline performance of the reflex. Injection of CNQX, 1 to 2 h after the initiation of training did not alter the performance of adaptive gain increases that occurred before the injection. If injection of CNQX occurred at the end of adaptive training, there was an accelerated loss of the previously adapted gain changes during the retention period when the animal remained stationary in the dark. CNQX injection did not produce any permanent or long-term deficits, because goldfish could be retrained 48 h later to produce adaptive VOR gain changes similar to control animals. Thus, this work demonstrates that the AMPA/KA receptors located in the vestibulo-cerebellum of the goldfish are necessary for acquisition of short-term adaptive VOR gain increases and decreases. The deficit in adaptive capability was not the result of a deficit in performance, because CNQX did not inhibit an adaptive change that had already occurred as long as the adapting vestibular and visual stimulation continued. This adaptive performance could possibly be maintained by other glutamatergic (metabotropic) receptors located on the Purkinje cells. The retention of adapted gain increases and decreases after CNQX application was inhibited because AMPA/KA antagonism accelerated VOR gain loss after the completion of training when no vestibular or visual stimulation was present. Because the AMPA/KA receptors are located only in the molecular layer of the goldfish cerebellum, these results are, presumably, the result of AMPA/KA receptor antagonism at synapses located on the Purkinje cell dendrite tree.

Visual awareness and the cerebellum: possible role of decorrelation control

The two roles in awareness most often suggested for the cerebellum are (i) keeping the details of motor skills away from forebrain computation, and (ii) signaling to the forebrain when a sensory event is not predictable from prior motor commands. However, it is unclear how current models of the cerebellum could carry out these roles. Their architecture, based on the seminal ideas of Marr and Albus, appears to need 'motor error' to learn correct motor commands. However, since motor error is the difference between the actual motor command and what the command should have been, it is a signal unavailable to the organism in principle. We propose a possible solution to this problem, termed decorrelation control, in which the cerebellum learns to decorrelate the motor command sent to the muscles from the sensory consequences of motor error. This method was tested in a linear model of oculomotor plant compensation in the vestibulo-ocular reflex. A copy of the eye-movement command was sent as mossy-fiber input to the flocculus, represented as a simple adaptive filter version of the Marr-Albus architecture. The sensory consequences of motor error were retinal slip, delivered as climbing fiber input to the flocculus. A standard anti-Hebbian learning rule was used to decorrelate the two. Simulations of the linearized problem showed the method to be effective and robust for plant compensation. Decorrelation control is thus a candidate algorithm for the basic cerebellar microcircuit, indicating how it could achieve motor learning using only signals available to the system. Such learning might then enable the cerebellum to free up visual awareness, and also, by providing a sensory signal decorrelated from motor command, supply awareness with crucial information about the external world.

Effect of cerebellar inactivation by lidocaine microdialysis on the vestibuloocular reflex in goldfish

Vestibuloocular reflex performance and adaptation were examined during vestibulocerebellar inactivation by localized lidocaine microdialysis or injection in goldfish. In the light, eye velocity perfectly compensated for head velocity (Vis-VOR) during sinusoidal yaw rotation (1/8 Hz +/- 20 degrees). In the dark, the reflex (VOR) gain was slightly reduced (gain approximately 0.8-0.9). In neither Vis-VOR nor VOR, was gain altered after 1 h of lidocaine microdialysis in the vestibulocerebellum. Before adaptation of reflex gain, the initial suppression or augmentation of Vis-VOR reflex gain produced by in-phase or out-of-phase visual-vestibular stimulation was also unaffected by cerebellar inactivation. Subsequently, 3 h of adaptive reflex training in either the in-phase or out-of-phase paradigm (acquisition phase) respectively decreased (0.30 +/- 0.09) or increased (1.60 +/- 0.08) VOR gain during artificial cerebral spinal fluid (CSF) microdialysis. However, microdialysis of lidocaine completely blocked adaptive gain changes during a 3-4 h period of continuous application. This effect was reversible because VOR gain changes were produced 1 h after lidocaine was replaced with CSF as the dialysate. After adaptive training, bilateral CSF injections (0.25 microl/side) into the vestibulocerebellum did not alter the normal retention or decay of adapted gain changes during a 3 h period in the dark (retention phase). However, injection of lidocaine into the vestibulocerebellum completely blocked retention of the adapted VOR gain returning the gain to values recorded before adaptation. In contrast to either acute or chronic surgical removal, lidocaine inactivation of the cerebellum by microdialysis did not alter either Vis-VOR and VOR behavior or interactive Vis-VOR performance over a wide range of gain extending from 0.3 to 1.4. Thus short-term VOR motor learning is a dynamic process requiring either continuous operation of brain stem cerebellar loops or, alternatively, modifiable sites within or directly influenced by the cerebellum. Our data supports the latter hypothesis, because the direct brain stem VOR pathways appear to be unaltered after cerebellar inactivation, and, hence, independent of the VOR-adapted state.

The hypothesis of the uniqueness of the oculomotor neural integrator: direct experimental evidence in the cat

1. As far as horizontal eye movements are concerned, the well-known hypothesis, not yet experimentally proved, of the common neural integrator states that the eye-position signal is generated by a common network, regardless of the type of versional movement. The aim of this study was to evaluate the validity of this hypothesis by checking whether the sensitivity to eye position of the neurones of the nucleus prepositus hypoglossi (NPH) (the main component of the system integrating the different incoming velocity signals) would be the same regardless of the type of versional movement. 2. The discharge of sixty-five NPH neurones was recorded in the alert cat during spontaneous eye movements made in the light and in response to sinusoidal rotations of the head in complete darkness. 3. For each NPH neurone, the sensitivity to eye position was determined from measurements carried out during intersaccadic fixation. The discharge rate of the studied neurone was plotted against eye position. The slope of the resulting regression line gave the sensitivity (measured during intersaccadic fixation in the light) of the neurone to eye position, which was termed K(f). 4. A new method was developed to measure the sensitivity to eye position (K(v)) of neurones during vestibular slow phases. The difficulty came from the fact that, during slow phases, eye velocity and eye position changed simultaneously and that each of those two variables could influence neuronal activity. For each neurone, the instantaneous firing rate was measured each time the eye passed through a given position during any slow phase generated during any vestibulo-ocular reflex. At a given position, the discharge rate of the neurone under study was plotted against the eye velocity. From the resulting linear regression line, two interesting values were obtained: its slope, corresponding to the sensitivity of the neurone to eye velocity, R(v), (at that given eye position) and its 'y'-intercept, F(0), the interpolated firing rate when the eye velocity was zero. This procedure was repeated for different eye positions. The values of F(0) were then plotted against the eye positions. The slope of the resulting regression line gave the sensitivity (measured during vestibular stimulation) of the neurone to eye position, which was termed K(v). 5. The errors on the individual values of K(f) and K(v) were assessed in order to allow a statistical comparison at the single unit level. 6. We found that, for each of our sixty-five neurones, the sensitivity to eye position measured during intersaccadic fixation in the light was equal to the sensitivity to eye position measured during the vestibulo-ocular reflex (VOR) elicited in complete darkness. We conclude that our results favour the hypothesis of a unique horizontal oculomotor integrator for all versional movements.

Simple-spike activity of floccular Purkinje cells responding to sinusoidal vertical rotation and optokinetic stimuli in alert cats

To understand how the cerebellar flocculus is involved in the processing of semicircular canal signals in the vertical vestibulo-ocular reflex (VOR), we analyzed the simple-spike activity of floccular Purkinje (P) cells that was modulated by sinusoidal pitch rotation, and then analyzed their activity during presentation of sinusoidal vertical optokinetic stimuli in alert, head-fixed cats. The great majority of P cells also responded to optokinetic stimuli with peak discharge near peak stimulus velocity. Eighty percent of P cells that responded to both pitch and optokinetic stimuli showed increased activity when the directions of the resultant eye movements were the same. During rapid modification of the VOR induced by visual pattern movement, modulation amplitudes of the cells tested increased together with the eye velocity increase. Maximal activation directions of these cells studied during vertical rotation in many planes were near the vertical canal planes, similar to those in our previous studies. The remaining 20% of P cells showed increased discharge for the same direction of stimulus movement. These results suggest that the activity of the majority of pitch-responding P cells contains, at least partly, a vertical eye velocity component during presentation of vestibular or optokinetic stimuli in addition to canal inputs during pitch rotation.

Further evidence for the specific involvement of the flocculus in the vertical vestibulo-ocular reflex (VOR)

We examined the simple-spike activity of floccular Purkinje (P) cells during sinusoidal pitch rotation and vertical optokinetic stimuli in alert, head-fixed cats. The great majority of pitch-responding P cells also responded to optokinetic stimuli with increased activity when the directions of the resultant eye movements were the same. During rapid modification of the VOR induced by visual pattern movement, modulation amplitudes of the cells tested increased together with the eye velocity increase. Maximal activation directions of these cells studied during vertical rotation in many planes were near the vertical canal planes. These results suggest that the activity of the majority of pitch-responding P cells contains a vertical eye velocity component during vestibular or optokinetic stimuli in addition to canal inputs during pitch rotation.

Vestibular integrators in the oculomotor system

The interstitial nucleus of Cajal (INC) and the nucleus prepositus hypoglossi (nph) are key elements in the vertical and horizontal oculomotor neural integrators, respectively. In this article, we attempt to develop possible circuits for these vestibular integrators by synthesizing recent information on the properties and connections of neurons involved in the integration process. We also examine how the cerebellar flocculus could play a role in the vertical integrator and vestibulo-ocular reflex (VOR) as well as in the modulation and plasticity of the VOR. We suggest that the circuitry for the vertical integrator involves the cerebellar flocculus in addition to the already proposed circuits distributed between the INC and the vestibular nuclei. The horizontal vestibular integrator is also distributed and seems to be characterized by functional compartmentalization. Both integrators play a wider role than simply transforming velocity-coded signals into position commands and may be pivotal in the short- and long-term modulation of the various oculomotor subsystems.

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

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

Differential effect of injections of kainic acid into the prepositus and the vestibular nuclei of the cat

1. In order adequately to control eye movements, oculomotoneurones have to be supplied with both an eye-velocity signal and an eye-position signal. However, all the command signals of the oculomotor system are velocity signals. Nowadays, there is general agreement about the existence of a brainstem network that would convert velocity command-signals into an eye-position signal. This circuit, because of its function, is called the oculomotor neural integrator. The most obvious symptom of its eventual failure is a gaze-holding deficit: in this case, saccades are followed by a centripetal post-saccadic drift. Although the oculomotor neural integrator is central in oculomotor theory, its precise location is still a matter for debate. 2. Previously, microinjections of kainic acid (KA) into the region of the nucleus prepositus hypoglossi (NPH) and of the medial vestibular nucleus (MVN) were found to induce a horizontal gaze-holding failure both in the cat and in the monkey. However, the relatively large volumes (1-3 microliters) and concentrations (2-4 micrograms microliters-1) used in these injections made it difficult to know if the observed deficit was due to a disturbance of the NPH or of the nearby MVN. These considerations led us to inject very small amounts of kainic acid (50 nl, 0.1 microgram microliter-1) either into the rostral part of the MVN or into different sites along the NPH of the cat. 3. The search coil technique was used to record (1) spontaneous eye movements (2) the vestibulo-ocular reflex (VOR) induced by a constant-velocity rotation (50 deg s-1 for 40 s) and the optokinetic nystagmus (OKN) elicited by rotating an optokinetic drum at 30 deg s-1 for 40 s. 4. In each injection experiment, the location of the abducens nucleus of the alert cat was mapped out by recording the antidromic field potentials evoked by the stimulation of the abducens nerve. Two micropipettes were then glued together in such a way that when the tip of the recording micropipette was in the centre of the abducens nucleus the tip of the injection micropipette was in a target area. The twin pipettes were then lowered in the brainstem until the recording micropipette reached the centre of the abducens nucleus. Kainic acid was then injected into the brainstem of the alert cat through the injection micropipette by an air pressure system. 5. Carried out according to such a protocol, KA injections into the NPH or the rostral part of the MVN consistently led to specific eye-movement changes.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of floccular Purkinje cells to sinusoidal vertical rotation and effects of muscimol infusion into the flocculus in alert cats

To understand how the flocculus is involved in the integration process in the vertical VOR, we examined the responses of floccular Purkinje cells during whole-body rotation in many vertical and horizontal planes in alert cats. The great majority of cells that responded to pitch rotation received excitation from the contralateral posterior canal, whereas the minority received excitation from the anterior canal, either on the ipsilateral or contralateral side; 35% of these vertical canal responding cells received convergent inputs, either from the ipsilateral or contralateral horizontal canal. After infusion of a GABA agonist (muscimol) into the area where many posterior canal responding Purkinje cells were recorded, the downward VOR induced by nose-up pitch was selectively impaired at low stimulus frequencies, together with an impairment of gaze holding after downward saccades. These results indicate a crucial role for posterior canal responding floccular Purkinje cells in generating downward eye position signals. Since the flocculus inhibits VOR interneurons related to the anterior and horizontal canals but not the posterior canal (Ito, 1984), our results suggest the possibility that posterior canal responding floccular Purkinje cells may form a transfloccular inhibitory pathway to anterior canal related VOR interneurons. This pathway may facilitate the positive feedback effect in the vestibular commissural inhibitory pathway between the co-planar vertical canals for the posterior canal signal, and may thereby be specifically involved in integration in the vertical VOR.

Human cyclovergence as a function of stimulus frequency and amplitude

By the use of scleral search coils a continuous record of human cyclovergence was obtained while two identical 80 degrees textured patterns, presented dichoptically, oscillated in the frontal plane in counterphase through 1, 3 and 6 degrees of cyclorotation at frequencies between 0.05 and 2 Hz. The amplitude and gain of the response decreased exponentially with increasing stimulus frequency. As stimulus amplitude increased, response amplitude also increased but gain was highest for low-amplitude cyclorotations. For an amplitude of 1 degrees and a frequency of 0.05 Hz the gain reached 0.87 for two subjects. The phase lag increased from a few degrees at a frequency of 0.05 Hz to over 100 degrees at a frequency of 2 Hz. These results suggest that cyclovergence is designed to correct for small, slow drifts in the stereoscopic alignment of the images in the two eyes. Although the disparity in the textured display was not interpreted as slant, it provided a strong stimulus for cyclovergence. The cyclovergence caused a transfer of cyclodisparity into a superimposed vertical line, which was then perceived as slanting in depth.

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.

A double-labeling investigation of the pretectal visuo-vestibular pathways

The projections of the nucleus of the optic tract were studied in the cat by simultaneous use of two distinct retrograde tracers (Fast Blue and Diamidino Yellow) injected in the inferior olive and the prepositus hypoglossi nucleus. Following injections of diamidino yellow in one structure and fast blue in the other, a significant number of retrogradely labeled neurons projecting to either target were observed dispersed in the nucleus of the optic tract. Three populations of labeled cells were found: one which projected to the inferior olive, a second to the nucleus prepositus hypoglossi, and a third which projected by means of a bifurcating axon to both of these structures. Quantification of these results reveals that 72% of the total number of labeled neurons are labeled by the IO injection, the remaining cells being labeled by the NPH injection. Double-labeled neurons represent more than 7% of the total number of the labeled cells. Tentative inferences as to the electrophysiological properties of the nucleus of the optic tract are discussed in the context of the optokinetic system.

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)

Lesions in the cat prepositus complex: effects on the vestibulo-ocular reflex and saccades

The effects of bilateral electrolytic lesions within and around the prepositus hypoglossi (p.h.) nucleus on horizontal saccades in the dark and on the horizontal sinusoidal vestibulo-ocular reflex (v.o.r.) in the dark were studied. After p.h. lesion, including its rostral part between P 7 and P 8, the v.o.r. showed a phase lead as much as about 90 deg at 0.10 Hz. A significant gain reduction paralleled that phase lead at lower frequencies. A large post-saccadic drift was also observed, the time constant of which ranged from 0.3 to 0.6 s. After p.h. lesion extending from P 8 to P 11 (but sparing the rostral part of the p.h.), no significant gain or phase lead change was observed. Post-saccadic drift was either missing or weak. A bilateral medial vestibular nucleus (m.v.n.) lesion from P 7 to P 11 produced a marked gain decrease, paralleled by a marked phase advance. A post-saccadic drift was observed (tau = 0.6 s). A surgical mid-line lesion from P 7 to P 11 (depth: about 2 mm) was followed by no remarkable change in the gain and in the phase of the v.o.r. No post-saccadic drift was observed after such lesion. It was concluded that (i) both the horizontal v.o.r. integration processing, and the horizontal saccadic integration processing were destroyed when an electrolytic lesion was made 'in the region of' the rostral part of the p.h. nucleus, and that (ii) the posterior four-fifths of the p.h. was the location of neither the horizontal v.o.r. integrator nor the horizontal saccadic integrator.

Vestibular nuclei activity and eye movements in the alert monkey during sinusoidal optokinetic stimulation

In the alert monkey (Macaca fascicularis) vestibular nuclei neurons and eye movements were recorded during sinusoidal optokinetic stimulation in the horizontal plane at frequencies between 0.02-3.3 Hz. Maximal stimulus velocity was generally kept constant at 40 deg/s, except for frequencies above 1 Hz. Eye movements showed a nystagmus-like pattern up to 0.2 Hz with a gain (change in eye position/change in cylinder position) greater than 0.8; at frequencies above 1 Hz the gain dropped to 0.35 at 3.3 Hz. A decrease in gain was accompanied by an increasing phase lag. Recordings in the vestibular nuclei were obtained from 'vestibular only' and 'vestibular plus saccade' neurons. Neurons with a strong eye position signal ('vestibular plus position') were excluded. The vast majority (87%) of neurons were not modulated at 0.2 Hz or higher frequencies of sinusoidal optokinetic stimulation, and were classified as 'low-frequency' type neurons. Compared to the response at constant stimulus velocity, sensitivity (imp X s-1/deg X s-1) dropped to 72% at 0.03 Hz and 16% at 0.1 Hz. A few neurons (13%) responding at 0.2 Hz (sensitivity on average 65% of the constant velocity response) were classified as 'high-frequency' type neurons. They did not respond above 1.0 Hz and showed no modulation with individual eye movements. The results suggest that the activity in the groups of vestibular nuclei neurons tested here is insufficient to account for the eye movements in response to sinusoidal optokinetic stimulation at frequencies above 0.1 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

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.