Disabling of the oculomotor neural integrator by kainic acid injections in the prepositus-vestibular complex of the cat

Neuropharmacologic aspects of the ocular motor system and the treatment of abnormal eye movements

Neuropharmacology is aiding our understanding of the control of eye movements in at least three ways. First, neurotransmitters have been identified in the pathways that coordinate gaze. Second, the technique of pharmacologic inactivation has provided a powerful method to determine the contributions of populations of neurons to specific behaviors, such as steady gaze holding. Finally, the results of basic neuropharmacologic studies have been used to identify candidate drugs for therapeutic trials of abnormal eye movements.

Mechanisms of action and targets of nitric oxide in the oculomotor system

Nitric oxide (NO) production by neurons in the prepositus hypoglossi (PH) nucleus is necessary for the normal performance of eye movements in alert animals. In this study, the mechanism(s) of action of NO in the oculomotor system has been investigated. Spontaneous and vestibularly induced eye movements were recorded in alert cats before and after microinjections in the PH nucleus of drugs affecting the NO-cGMP pathway. The cellular sources and targets of NO were also studied by immunohistochemical detection of neuronal NO synthase (NOS) and NO-sensitive guanylyl cyclase, respectively. Injections of NOS inhibitors produced alterations of eye velocity, but not of eye position, for both spontaneous and vestibularly induced eye movements, suggesting that NO produced by PH neurons is involved in the processing of velocity signals but not in the eye position generation. The effect of neuronal NO is probably exerted on a rich cGMP-producing neuropil dorsal to the nitrergic somas in the PH nucleus. On the other hand, local injections of NO donors or 8-Br-cGMP produced alterations of eye velocity during both spontaneous eye movements and vestibulo-ocular reflex (VOR), as well as changes in eye position generation exclusively during spontaneous eye movements. The target of this additional effect of exogenous NO is probably a well defined group of NO-sensitive cGMP-producing neurons located between the PH and the medial vestibular nuclei. These cells could be involved in the generation of eye position signals during spontaneous eye movements but not during the VOR.

Neural control of rotational kinematics within realistic vestibuloocular coordinate systems

Previous theoretical investigations of the three-dimensional (3-D) angular vestibuloocular reflex (VOR) have separately modeled realistic coordinate transformations in the direct velocity path or the nontrivial problems of converting angular velocity into a 3-D orientation command. We investigated the physiological and behavioral implications of combining both approaches. An ideal VOR was simulated using both a plant model with head-fixed eye muscle actions (standard plant) and one with muscular position dependencies that facilitate Listing's law (linear plant). In contrast to saccade generation, stabilization of the eye in space required a 3-D multiplicative (tensor) interaction between the various components of velocity and position in both models: in the indirect path of the standard plant version, but also in the direct path of the linear plant version. We then incorporated realistic nonorthogonal coordinate transformations (with the use of matrices) into both models. Each now malfunctioned, predicting ocular drift/retinal destabilization during and/or after the head movement, depending on the plant version. The problem was traced to the standard multiplication tensor, which was only defined for right-handed, orthonormal coordinates. We derived two solutions to this problem: 1) separating the brain stem coordinate transformation into two (sensory and motor) transformations that reordered and "undid" the nonorthogonalities of canals and muscle transformations, thus ensuring orthogonal brain stem coordinates, or 2) computing the correct tensor components for velocity-orientation multiplication in arbitrary coordinates. Both solutions provided an ideal VOR. A similar problem occurred with partial canal or muscle damage. Altering a single brain stem transformation was insufficient because the resulting coordinate changes rendered the multiplication tensor inappropriate. This was solved by either recomputing the multiplication tensor, or recomputing the appropriate internal sensory or motor matrix to normalize and reorthogonalize the brain stem. In either case, the multiplication tensor had to be correctly matched to its coordinate system. This illustrates that neural coordinate transformations affect not only serial/parallel projections in the brain, but also lateral projections associated with computations within networks/nuclei. Consequently, a simple progression from sensory to motor coordinates may not be optimal. We hypothesize that the VOR uses a dual coordinate transformation (i.e., both sensory and motor) to optimize intermediate brain stem coordinates, and then sets the appropriate internal tensor for these coordinates. We further hypothesize that each of these processes should optimally be capable of specific, experimentally identifiable adjustments for motor learning and recovery from damage.

Analysis of primate IBN spike trains using system identification techniques. I. Relationship To eye movement dynamics during head-fixed saccades

The dynamic behavior of primate (Macaca fascicularis) inhibitory burst neurons (IBNs) during head-fixed saccades was analyzed by using system identification techniques. Neurons were categorized as IBNs on the basis of their anatomic location as well as by their activity during horizontal head-fixed saccadic and smooth pursuit eye movements and vestibular nystagmus. Each IBN's latency or "dynamic lead time" (td) was determined by shifting the unit discharge in time until an optimal fit to the firing rate frequency B(t) profile was obtained by using the simple model based on eye movement dynamics,B(t) = r + b1(t); where is eye velocity. For the population of IBNs, the dynamic estimate of lead time provided a significantly lower value than a method that used the onset of the first spike. We then compared the relative abilities of different eye movement-based models to predict B(t) by using objective optimization algorithms. The most important terms for predicting B(t) were eye velocity gain (b1) and bias terms (r) mentioned above. The contributions of higher-order velocity, acceleration, and/or eye position terms to model fits were found to be negligible. The addition of a pole term [the time derivative of B(t)] in conjunction with an acceleration term significantly improved model fits to IBN spike trains, particularly when the firing rates at the beginning of each saccade [initial conditions (ICs)] were estimated as parameters. Such a model fit the data well (a fit comparable to a linear regression analysis with a R2 value of 0.5, or equivalently, a correlation coefficient of 0.74). A simplified version of this model [B(t) = rk + b1(t)], which did not contain a pole term, but in which the bias term (rk) was estimated separately for each saccade, provided nearly equivalent fits of the data. However, models in which ICs or rks were estimated separately for each saccade contained too many parameters to be considered as useful models of IBN discharges. We discovered that estimated ICs and rks were correlated with saccade amplitude for the majority of short-lead IBNs (SLIBNs; 56%) and many long-lead IBNs (LLIBNs; 42%). This observation led us to construct a more simple model that included a term that was inversely related to the amplitude of the saccade, in addition to eye velocity and constant bias terms. Such a model better described neuron discharges than more complex models based on a third-order nonlinear function of eye velocity. Given the small number of parameters required by this model (only 3) and its ability to fit the data, we suggest that it provides the most valuable description of IBN discharges. This model emphasizes that the IBN discharges are dependent on saccade amplitude and implies further that a mechanism must exist, at the motoneuron (MN) level, to offset the effect of the bias and amplitude-dependent terms. In addition, we did not find a significant difference in the variance accounted for by any of the downstream models tested for SLIBNs versus LLIBNs. Therefore we conclude that the eye movement signals encoded dynamically by SLIBNs and LLIBNs are similar in nature. Put another way, SLIBNs are not closer, dynamically, to MNs than LLIBNs.

Analysis of primate IBN spike trains using system identification techniques. II. Relationship to gaze, eye, and head movement dynamics during head-free gaze shifts

We have investigated the relationships among the firing frequency B(t) of inhibitory burst neurons (IBNs) and the metrics and dynamics of the eye, head, and gaze (eye + head) movements generated during voluntary combined eye-head gaze shifts in monkey. The same IBNs were characterized during head-fixed saccades in our first of three companion papers. In head-free gaze shifts, the number of spikes (NOS) in a burst was, for 82% of the neurons, better correlated with gaze amplitude than with the amplitude of either the eye or head components of the gaze shift. A multiple regression analysis confirmed that NOS was well correlated to the sum of head and eye amplitudes during head-free gaze shifts. Furthermore, the mean slope of the relationship between NOS and gaze amplitude was significantly less for head-free gaze shifts than for head-fixed saccades. NOS is a global parameter. To refine we used system identification techniques to evaluate a series of dynamic models in which IBN spike trains were related to gaze or eye movements. We found that gaze- and eye-based models predicted the discharges of IBNs equally well. However, the bias values required by gaze-based models were comparable to those required in our head-fixed models whereas those required by eye-based models were significantly larger. The difference in biases between gaze- and eye-based models was very strongly correlated to the mean head velocity () during gaze shifts [R = -0.93 +/- 0.15 (SD)]. This result suggested that the increased bias required by the eye-based models reflected an unmodeled input onto these cells. To pursue this argument further we investigated a series of dynamic models that included both eye velocity () and terms and this confirmed the importance of these two terms. As in our head-fixed analysis of companion paper I, the most valuable model formulation also included an eye saccade amplitude term (DeltaE) and was given by B(t) = r0 + r1DeltaE + b1 + g1 where r0, r1, b1, and g1 are constants. The amplitude of the head velocity coefficient was significantly less than that of the eye velocity coefficient. Furthermore, in our population long-lead IBNs tended to have a smaller head velocity coefficients than short-lead IBNs. We conclude that during head-free gaze shifts, the head velocity signal carried to the abducens nucleus by primate excitatory burst neurons (EBNs; if EBNs and IBNs carry similar signals) must be offset by other premotor cells.

Eye movement deficits after ibotenic acid lesions of the nucleus prepositus hypoglossi in monkeys. I. Saccades and fixation

It has been suggested that the function of the nucleus prepositus hypoglossi (nph) is the mathematical integration of velocity-coded signals to produce position-coded commands that drive abducens motoneurons and generate horizontal eye movements. In early models of the saccadic system, a single integrator provided not only the signal that maintained steady gaze after a saccade but also an efference copy of eye position, which provided a feedback signal to control the dynamics of the saccade. In this study, permanent, serial ibotenic acid lesions were made in the nph of three rhesus macaques, and their effects were studied while the alert monkeys performed a visual tracking task. Localized damage to the nph was confirmed in both Nissl and immunohistochemically stained material. The lesions clearly were correlated with long-lasting deficits in eye movement. The animals' ability to fixate in the dark was compromised quickly and uniformly so that saccades to peripheral locations were followed by postsaccadic centripetal drift. The time constant of the drift decreased to approximately one-tenth of its normal values but remained 10 times longer than that attributable to the mechanics of the eye. In contrast, saccades were affected minimally. The results are more consistent with models of the neural saccade generator that use separate feedback and position integrators than with the classical models, which use a single multipurpose element. Likewise, the data contradict models that rely on feedback from the nph. In addition, they show that the oculomotor neural integrator is not a single neural entity but is most likely distributed among a number of nuclei.

The oculomotor integrator: testing of a neural network model

An important part of the vestibulo-ocular reflex is a group of cells in the caudal pons, known as the neural integrator, that converts eye-velocity commands, from the semicircular canals for example, to eye-position commands for the motoneurons of the extraocular muscles. Previously, a recurrently connected neural network model was developed by us that learns to simulate the signal processing done by the neural integrator, but it uses an unphysiological learning algorithm. We describe here a new network model that can learn the same task by using a local, Hebbian-like learning algorithm that is physiologically plausible. Through the minimization of a retinal slip error signal the model learns, given randomly selected initial synaptic weights, to both integrate simulated push-pull semicircular canal afferent signals and compensate for orbital mechanics as well. Approximately half of the model's 14 neurons are inhibitory, half excitatory. After learning, inhibitory cells tend to project contralaterally, thus forming an inhibitory commissure. The network can, of course, recover from lesions. The mature network is also able to change its gain by simulating abnormal visual-vestibular interactions. When trained with a sine wave at a single frequency, the network changed its gain at and near the training frequency but not at significantly higher or lower frequencies, in agreement with previous experimental observations. Commissural connections are essential to the functioning of this model, as was the case with our previous model. In order to determine whether a commissure plays a similar role in the real neural integrator, a series of electrical perturbations were performed on the midlines of awake, behaving juvenile rhesus monkeys and the effects on the monkeys' eye movements were examined. Eye movements were recorded using the coil system before, during, and after electrical stimulation in the midline of the pons just caudal to the abducens nuclei, which reversibly made the integrator leaky. Eye movements were also recorded from two of the monkeys before and after a midline electrolytic lesion was made at the location where stimulation produced a leaky integrator. This lesion disabled the integrator irreversibly. The eye movements that were produced by the monkeys as a result of these perturbations were then compared with eye movements produced by the model after analogous perturbations. The results are compatible with the hypothesis that integration comes about by positive feedback through lateral inhibition effected by an inhibitory commissure.

The use of system identification techniques in the analysis of oculomotor burst neuron spike train dynamics

The objective of system identification methods is to construct a mathematical model of a dynamical system in order to describe adequately the input-output relationship observed in that system. Over the past several decades, mathematical models have been employed frequently in the oculomotor field, and their use has contributed greatly to our understanding of how information flows through the implicated brain regions. However, the existing analyses of oculomotor neural discharges have not taken advantage of the power of optimization algorithms that have been developed for system identification purposes. In this article, we employ these techniques to specifically investigate the "burst generator" in the brainstem that drives saccadic eye movements. The discharge characteristics of a specific class of neurons, inhibitory burst neurons (IBNs) that project monosynaptically to ocular motoneurons, are examined. The discharges of IBNs are analyzed using different linear and nonlinear equations that express a neuron's firing frequency and history (i.e., the derivative of frequency), in terms of quantities that describe a saccade trajectory, such as eye position, velocity, and acceleration. The variance accounted for by each equation can be compared to choose the optimal model. The methods we present allow optimization across multiple saccade trajectories simultaneously. We are able to investigate objectively how well a specific equation predicts a neuron's discharge pattern as well as whether increasing the complexity of a model is justifiable. In addition, we demonstrate that these techniques can be used both to provide an objective estimate of a neuron's dynamic latency and to test whether a neuron's initial firing rate (expressed as an initial condition) is a function of a quantity describing a saccade trajectory (such as initial eye position).

How the brain keeps the eyes still

The brain can hold the eyes still because it stores a memory of eye position. The brain's memory of horizontal eye position appears to be represented by persistent neural activity in a network known as the neural integrator, which is localized in the brainstem and cerebellum. Existing experimental data are reinterpreted as evidence for an "attractor hypothesis" that the persistent patterns of activity observed in this network form an attractive line of fixed points in its state space. Line attractor dynamics can be produced in linear or nonlinear neural networks by learning mechanisms that precisely tune positive feedback.

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.

Neuronal mechanisms of two-dimensional orienting movements in the cat. I. A quantitative study of saccades and slow drifts produced in response to the electrical stimulation of the superior colliculus

To evaluate the metrics of rapid eye movements caused by the activation of distinct collicular microzones, the superior colliculus (SC) was electrically stimulated in alert behaving cats while their heads were restrained. A quantitative study of electrically induced rapid eye movements demonstrated that their amplitude and direction depended on the intensity of stimulation, the electrode location, and the initial position of the eyes, while their duration depended on the intensity of stimulation. When detailed quantitative procedures are employed, properties of saccades produced in response to the electrical stimulation of the feline SC resemble those of saccades elicited in response to the electrical stimulation of a variety of primate brain areas. Besides saccades, electrical stimulation of the feline SC gave rise to slow drifts whose amplitude and direction was also influenced by the initial position of the eyes. Because their size depended on the frequency of stimulation and their time course reflected mechanical properties of the oculomotor plant, induced slow drifts could be due to a more or less direct projection of the SC onto extraocular motoneurons. A model that includes such a variety of connections between the SC and extraocular motoneurons is presented and is shown to produce realistic combinations of fast and slow eye movements when its input is a step function of time. The present findings support the notion that an orbital mechanical factor underlies the eye position sensitivity of slow drifts and saccades evoked in response to the electrical stimulation of the SC.

Pretectofugal fibers from the nucleus of the optic tract in monkeys

The nucleus of the optic tract (NOT) is the visuo-motor relay between the retina and preoculomotor structures in the pathway mediating optokinetic nystagmus (OKN). NOT lesions in monkeys produce no OKN toward the lesioned side. Then, efferent fibers from the NOT course through the brainstem and may reach the vestibular nucleus, which is proposed to be the final nucleus to the motor nucleus. In the present study, the tracer was injected through a micropipette in the NOT in four monkeys. Labeled terminals were observed ipsilaterally in the parabigeminal nucleus, superficial layers of the superior colliculus, dorsal and lateral terminal nuclei of the accessory optic system and pretectal nuclei and contralaterally in the NOT and superficial layers of the superior colliculus. Descending fibers from the NOT consisted of two major pathways: (1) fibers descended medially from the injection site through the reticularis pontis oralis to reach the lateral part of the ipsilateral nucleus reticularis tegmenti pontis; (2) fibers projecting into the dorsal cap of inferior olive, by far the greatest number of labeled fibers, descended ventrally along the lateral border of the reticularis pontis oralis and reached the medial lemniscus where they descended further and branched into the dorsolateral pontine nucleus, the lateral part of the nucleus reticularis tegmenti pontis, the peduncular pontine nucleus, the lateral pontine nucleus, the nucleus prepositus hypoglossi, the medial vestibular nucleus and finally the dorsal cap of the inferior olive. Consistent with the physiological data, the direct terminals to the medial vestibular nucleus could serve to drive the storage mechanisms and to produce OKN in the monkey.

Providing distinct vergence and version dynamics in a bilateral oculomotor network

Given reported interactions between vergence and version dynamics, ocular reflexes cannot be properly modelled as separate independent subsystems. Using a model structure compatible with known anatomy, we show that a single bilateral system can produce results consistent with observed data both at the central and ocular levels. This model provides for both vergence and conjugate integrators in a single controller, and explains the observed modulation on abducens interneurons and mesencephalic vergence cells during vergence responses. Reported interactions between version and vergence would then be a natural consequence of a shared premotor network. Major implications include: the need to record both eyes in a protocol, since cross-talk is always possible; and adaptation to monocular changes could be distributed in all motor projections to both eyes.

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.

Short-term adaptation of the phase of the vestibulo-ocular reflex (VOR) in normal human subjects

We investigated the effects of short-term vestibulo-ocular reflex (VOR) adaptation on the gain and phase of the VOR, and on eccentric gaze-holding in darkness, in five normal human subjects. For 1 h, subjects sat in a chair that rotated sinusoidally at 0.2 Hz while surrounded by a visual stimulus (optokinetic drum). The drum was rotated relative to the chair, to require a VOR with either a phase lead or lag of 45 deg (with respect to a compensatory phase of zero) with no change in gain, or a gain of 1.7 or 0.5 with no change in phase. Immediately before and after each training session, VOR gain and phase were measured in the dark with 0.2 Hz sinusoidal rotation. Gaze-holding was evaluated following 20 deg eccentric saccades in darkness. Adaptation paradigms that called only for a phase lead produced an adapted VOR with 33% of the required amount of phase change, a 20% decrease in VOR gain, and an increased centripetal drift after eccentric saccades made in darkness. Adaptation paradigms that called for a phase lag produced an adapted VOR with 29% of the required amount of phase change, no significant change in VOR gain, and a centrifugal drift after eccentric saccades. Adaptation paradigms requiring a gain of 1.7 produced a 15% increase in VOR gain with small increases in phase and in centripetal drift. Adaptation paradigms requiring a gain of 0.5 produced a 31% decrease in VOR gain with a 6 deg phase lag and a centrifugal drift. The changes in drift and phase were well correlated across all adaptation paradigms; the changes in phase and gain were not. We attribute the effects on phase and gaze-holding to changes in the time constant of the velocity-to-position ocular motor neural integrator. Phase leads and the corresponding centripetal drift are due to a leaky integrator, and phase lags and the corresponding centrifugal drift are due to an unstable integrator. These results imply that in the short-term adaptation paradigm used here, the control of drift and VOR phase are tightly coupled through the neural integrator, whereas VOR gain is controlled by another mechanism.

Eye saccade dynamics during paradoxical sleep in the cat

Cat eye movements were recorded during wakefulness and paradoxical sleep with the technique of the scleral search coil in a magnetic field. During waking, eye movements consisted of a succession of saccades and fixation phases. During paradoxical sleep, the pattern of eye movements displayed drifts of variable velocity and direction and short fixation phases, upon which saccades superimposed. These saccades displayed a repetitive, stereotyped, asymmetrical pattern. The maximum velocity/amplitude relationships, i.e. the main sequences, were determined for spontaneous and visually induced saccades of waking and for the following types of saccades during paradoxical sleep: (i) isolated saccades accompanied by ponto-geniculo-occipital (PGO) waves, (ii) isolated saccades accompanied by eye movement potentials (EMP), and (iii) saccades in bursts accompanied by PGO waves. The slope of the main sequence relationship of any type of paradoxical sleep saccade (from 21.7 degrees/s/degree for isolated saccades to 35.6 degrees/s/degree for saccades in bursts) was higher than that of any type of waking saccade (11.2 degrees/s/degree for spontaneous saccades to 14.7 degrees/s/degree for visually elicited ones). Furthermore, during paradoxical sleep, saccades in bursts were faster than isolated ones. This demonstrates that different neurophysiological mechanisms subserve the generation of waking saccades, paradoxical sleep isolated saccades and paradoxical sleep saccades in bursts, or that the oculomotor system is in a different state of excitation during these different sets of saccades. These findings throw new light on the functioning of the oculomotor system during paradoxical sleep and are discussed in terms of the functional significance of paradoxical sleep saccades and PGO waves.

Involvement of the N-methyl-D-aspartate receptors of the vestibular nucleus in the gaze-holding system of the cat

Eye movements were recorded in alert cats after injections into one of the medial vestibular nuclei (MVN) either of a N-methyl-D-aspartate (NMDA) antagonist or of a non-NMDA antagonist. A gaze-holding failure was caused by the NMDA antagonist when it was injected into the central part of the MVN but not when it was injected into the rostral part of that nucleus. By contrast, injections of the non-NMDA-receptor antagonist into the MVN did not cause any sign of gaze-holding failure. We conclude that the NMDA receptors located in the central part of the MVN are involved in the gaze-holding system.

Eye position and eye velocity integrators reside in separate brainstem nuclei

Two types of central nervous system integrators are critical for oculomotor performance. The first integrates velocity commands to create position signals that hold fixation of the eye. The second stores relative velocity of the head and visual surround to stabilize gaze both during and after the occurrence of continuous self and world motion. We have used recordings from single neurons to establish that the "position" and "velocity" integrators for horizontal eye movement occupy adjacent, but nonoverlapping, locations in the goldfish medulla. Lidocaine inactivation of each integrator results in the eye movement deficits expected if horizontal eye position and velocity signals are processed separately. These observations also indicate that each brainstem compartment generates and stores these signals. Consequently, each integrator exhibits functional autonomy. Therefore, we propose that the intrinsic electrophysiological properties of the constituent neurons in each brainstem subnucleus may be sufficient for producing integrator rhythmicity.

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)

Modularity and parallel processing in the oculomotor integrator

The neural signals that hold eye position originate in a brainstem structure called the neural integrator, so-called because it is thought to compute these position signals using a process equivalent to mathematical integration. Most previous experiments have assumed that the neural integrator reacts to damage like a single mathematical integrator: the eye is expected to drift towards a unique resting point at a simple exponential rate dependent on current eye position. Physiologically, this would require a neural network with uniformly distributed internal connections. However, Cannon et al. (1983) proposed a more robust modular internal configuration, with dense local connections and sparse remote connections, computationally equivalent to a parallel array of independent sub-integrators. Damage to some sub-integrators would not affect function in the others, so that part of the position signal would remain intact, and a more complex pattern of drift would result. We evaluated this parallel integrator hypothesis by recording three-dimensional eye positions in the light and dark from five alert monkeys with partial neural integrator failure. Our previous study showed that injection of the inhibitory gamma aminobutyric acid agonist muscimol into the mesencephalic interstitial nucleus of Cajal (INC) causes almost complete failure of the integrators for vertical and torsional eye position after approximately 30 min. This study examines the more modest initial effects. Several aspects of the initial vertical drift could not be accounted for by the single integrator scheme. First, the eye did not initially drift towards a single resting position; rapid but brief drift was observed towards multiple resting positions. With time after the muscimol injection, this range of stable eye positions progressively narrowed until it eventually approximated a single point. Second, the drift had multiple time constants. Third, multiple regression analysis revealed a significant correlation between drift rate and magnitude of the previous saccade, in addition to a correlation between drift rate and position. This saccade dependence enabled animals to stabilize gaze by making a series of saccades to the same target, each with less post-saccadic drift than its predecessor. These observations were predicted and explained by a model in which each of several parallel integrators generated a fraction of the eye-position command.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct projection from the nucleus of the optic tract to the medial vestibular nucleus in the cat

The nucleus of the optic tract (NOT) serves as an important visuo-motor relay between the retina and preoculomotor structures that mediate optokinetic nystagmus. In the present study, the efferent targets of NOT were investigated using biocytin as an anterograde tracer. Following biocytin injections into NOT, labeled fibers were observed in each of the following efferent pathways: (1) those that project to the contralateral NOT via the posterior commissure; (2) those that course through the nucleus reticularis pontis oralis to terminate in the nucleus reticularis tegmenti pontis; and (3) those that descend via the medial lemniscus to the level of the medulla to terminate in the dorsolateral pontine nucleus, nucleus prepositus hypoglossi, medial vestibular nucleus and the inferior olive. Direct projections from the NOT to the medial vestibular nucleus may contribute to the residual optokinetic responses of the vestibular nucleus neurons following cerebellar or inferior olivary lesions.

Testing the common neural integrator hypothesis at the level of the individual abducens motoneurones in the alert cat

1. As far as horizontal eye movements are concerned, the well-known hypothesis of a 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 analysing the behaviour of the abducens motoneurones, the system into which the horizontal neural integrator(s) project(s). If there were a common neural integrator, the different motoneurones would receive the eye position signal through the same pathway and the sensitivity to eye position would be the same regardless of the type of versional movement. If there were multiple integrators, the sensitivity to eye position in one type of versional movement might be different from the sensitivity to eye position in another type of versional movement, at least for occasional motoneurones. 2. The discharge of thirty-one antidromically identified abducens motoneurones 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. All of the abducens motoneurones exhibited a burst of action potentials for lateral saccades. During fixation between saccades, they maintained a steady firing rate that increased as the cat fixated increasingly lateral eye positions. 4. For each abducens motoneurone, the sensitivity to eye position (Kf) was determined from measurements carried out during intersaccadic fixations. Kf was calculated from the slope of the firing rate-eye position linear regression line. 5. The discharge rate of the identified motoneurones was observed during four sinusoidal vestibular stimulations (+/- 10 deg, 0.10 Hz; +/- 20 deg, 0.10 Hz; +/- 30 deg, 0.10 Hz; +/- 40 deg, 0.10 Hz). The motoneurones exhibited a burst of activity during fast phases in the lateral direction and paused during fast phases in the opposite direction. During slow phases, motoneurones modulated their activity as a function of the vestibularly induced eye movements except for slow phases that occurred in position ranges below their recruitment threshold. In these cases their activity was cut off. 6. A new method was developed to measure the sensitivity to eye position 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.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of the vestibular commissure in the gaze holding of the cat

Recent theoretical studies have proposed that the vestibular commissure is a major component of the horizontal gaze-holding system. In order to test this hypothesis, we disrupted the vestibular commissure of the cat by a parasagittal cut running between an abducens nucleus and the adjacent medial vestibular nucleus. Successfully lesioned cats suffered of a characteristic syndrome. In the light, gaze holding was impaired when the animal was looking toward the side of the lesion, but normal when looking toward the opposite side. We conclude (1) that the vestibular commissure is a component of the gaze-holding system, (2) that the vestibular commissure is less essential for gaze holding than other structures as the nucleus prepositus hypoglossi, and (3) that the horizontal gaze-holding system consists of two halves, each being more active in ipsilateral than in contralateral gaze.

Transient analysis of vestibular nystagmus

The significance of a nystagmus-dependent, transient component in the overall slow-phase response of the vestibulo-ocular reflex (VOR) is brought into focus. First, a simulated example is presented that shows how this transient component can bias current algorithms for the estimation of VOR parameters. Second, new algorithms are proposed that are able to estimate VOR parameters regardless of the presence of transients. Third, the new algorithms are applied to experimental data, and the results are compared with those from current algorithms. The results clearly show that the transient component can significantly alter the apparent VOR time constant, particularly when the reflex has been lesioned. The algorithms open new areas of research on the possible role of nystagmus in enhancing the compensatory function of the VOR.

A physiological study of vestibular and prepositus hypoglossi neurones projecting to the abducens nucleus in the alert cat

1. Vestibular and prepositus hypoglossi (PH) neurones projecting to the abducens (ABD) nucleus were recorded in the alert cat. Their discharge characteristics were analysed to ascertain the origin of the horizontal eye position signal present in ABD neurones. 2. Neurones were classified according to: their location with respect to the ABD nucleus; their antidromic activation from the ABD nucleus; the synaptic field potential they induced in the ABD nucleus with the spike-triggered averaging technique; and their activity during spontaneous and vestibularly induced eye movements. 3. Vestibular neurones projecting to the ABD nucleus were located in the rostral medial vestibular nucleus. They were excitatory on the contralateral and inhibitory on the ipsilateral ABD neurones. Both types of premotor vestibular neurone showed a firing rate weakly related to eye position, increasing for eye fixations in the contralateral on-direction, and decreasing with ipsilateral fixation. Position sensitivity during eye fixations was (means +/- S.D.) 1.8 +/- 0.9 spikes s-1 deg-1 for excitatory neurones and 2.2 +/- 1.3 spikes s-1 deg-1 for inhibitory neurones. Firing rate exhibited a high variability during eye fixations. Their responses during saccades in the off-direction were characterized by a pause that, although less defined, was occasionally present during saccades in the on-direction. Eye velocity sensitivity during spontaneous saccades in the on-direction was 0.17 +/- 0.15 spikes s-1 deg-1 s-1 for excitatory neurones and 0.15 +/- 0.07 spikes s-1 deg-1 s-1 for inhibitory vestibular neurones. During sinusoidal head stimulation at 0.2 Hz, vestibular neurones showed a type I discharge rate with a phase lead over eye position of 86.0 +/- 14.1 deg for excitatory and 80.2 +/- 12.5 deg for inhibitory neurones. Position sensitivity during vestibular stimulation did not differ significantly from values obtained for spontaneous eye movements. However, the velocity sensitivity of premotor vestibular neurones during head rotation was significantly higher (1.6 +/- 0.2 spikes s-1 deg-1 s-1 for excitatory and 1.5 +/- 0.3 spikes s-1 deg-1 s-1 for inhibitory neurones) than during spontaneous eye movements. 4. PH neurones projecting to the ABD nucleus were located in the rostral one-third of the nucleus. These neurones were excitatory on the ipsilateral and inhibitory on the contralateral ABD nucleus. Their firing rates were correlated mainly with eye position, increasing for abducting eye positions of the ipsilateral eye and decreasing with adduction movements.(ABSTRACT TRUNCATED AT 400 WORDS)

Firing behaviour of anticompensatory neurones in the prepositus hypoglossi nucleus of alert cat

The present results demonstrate that anticompensatory neurones could be recorded within the prepositus hypoglossi nucleus of the alert cat. These neurones, like burster-driving neurones described in paralysed cats, were characterized by a tonic increase of firing for head turning in the contraversive direction (type II) associated to bursts for each vestibular quick phase in the same direction. They were also involved in the generation of visually triggered saccades since they displayed a burst preceding contraversive saccades and a pause in the opposite direction. Thus, these neurones code eye velocity for both vestibular quick phases and visually triggered saccades in addition to head velocity. Such a firing behaviour suggests that, in addition to the classical crossed tecto-reticulo-spinal network, an ipsilateral pathway could also be involved in the generation of visually triggered rapid eye movements.

Possible excitatory and inhibitory feedback to the superior colliculus: a combined retrograde and immunocytochemical study in the prepositus hypoglossi nucleus of the guinea pig

We have investigated in the guinea pig the precise localization and the immunoreactivity of the neurones in the prepositus hypoglossi nucleus involved in a direct ascending projection onto the superior colliculus. The projecting neurones were characterized by a retrograde tracer (WGA-ApoHRP coupled to gold particles), injected in the intermediate and deep layers of the superior colliculus. After revealing gold particles, the sections were then treated using an antibody either against GABA or against glutamate, thus allowing identification of gold-filled-immunoreactive neurones. The retrogradely labelled cells were exclusively distributed on the contralateral side, and preferentially in the caudal two thirds of the prepositus hypoglossi nucleus, in its ventral and ventrolateral division. In addition, about 23% of these projecting neurones appear immunopositive when the sections are treated with a GABA antibody and around 27% are immunopositive to glutamate. Furthermore, these two classes of GABA-like or glutamate-like projecting neurones are intermingled within the prepositus hypoglossi nucleus. We conclude, in spite of a probable underestimation of these two populations, that the ascending projection is formed by an excitatory pathway that probably involves glutamate as well as an inhibitory pathway mediated by GABA. Thus we cannot consider this feedback as exclusively inhibitory as was suggested in theoretical models of the oculomotor system.

Generation of torsional and vertical eye position signals by the interstitial nucleus of Cajal

The neural integrator, which converts eye velocity signals into position signals, is central to oculomotor theory. Similar integrators are probably necessary in any neural system that changes and maintains muscular tension. The integrator for horizontal eye position is in the pons, but the locations of the vertical and torsional integrators have not been clearly defined. Recording three-dimensional eye movements in alert monkeys during microstimulation and pharmacological inactivation of midbrain sites showed that the interstitial nucleus of Cajal generates both the torsional and vertical eye position signals. Up and down signals are linked with clockwise signals in the right brain and counterclockwise signals in the left brain. This three-dimensional coordinate system achieves orthogonality and bilateral symmetry without redundancy and optimizes energy efficiency for horizontal visual scanning.

GABA and non-GABA immunostained neurones in the nucleus prepositus and the periparabigeminal area projecting to the guinea pig superior colliculus

We have investigated the possibility that GABAergic neurones may be involved in two ascending projections to the superior colliculus, originating in the nucleus prepositus hypoglossi and in the periparabigeminal area of the mesencephalon, respectively. The projecting neurones of both structures were identified using gold-WGA-apoHRP, a retrogradely transported tracer, injected unilaterally into the superior colliculus. GABA was detected in these neurones by means of immunocytochemical staining. The results show that 25% of the projecting neurones in the prepositus hypoglossi are indeed GABA-immunoreactive. They could exert a direct inhibitory influence on the colliculus. By contrast, only a few (7%) gold-filled-GABAergic cells were detected in the periparabigeminal area, which suggests that this region cannot participate in an important inhibitory afferent system to the colliculus.

The interstitial nucleus of Cajal in the midbrain reticular formation and vertical eye movement

Bilateral lesions of the midbrain reticular formation within, and in the close vicinity of, the interstitial nucleus of Cajal (INC) result in the severe impairment of the ability to hold eccentric vertical eye position after saccades, phase advance and decreased gain of the vestibulo-ocular reflex (VOR) induced by sinusoidal vertical rotation. In addition, the INC region of alert animals contains many burst-tonic and tonic neurons whose activity is closely correlated with vertical eye movement, not only during spontaneous saccades, but also during the VOR, smooth pursuit and optokinetic eye movements. Although their activity is closely related to these conjugate vertical eye movements, it is different from the oculomotor motor neuron activity. These results indicate that the INC region is involved in, and indispensable for, some aspects of eye position generation during vertical eye movement. Further comparison of INC neuron discharge with eye movements during two special conditions indicates that the INC region alone cannot produce eye position signals. First INC neuron discharge shows no response or an 80 degrees phase advance (close to the expected value if there is no integration) in the dark compared to the light during sinusoidal vertical linear acceleration in alert cats. Second, during rapid-eye-movement (REM) sleep, the discharge of INC neurons is no longer correlated with eye position. These results imply that the INC is not the entire velocity-to-position integrator, but that it has to work with other region(s) to perform the integration. A close functional linkage has been described between vertical-eye-movement-related neurons in the INC region and vestibulo-ocular relay neurons related to the vertical semicircular canals in the vestibular nuclei. It has been suggested that both are the major constituents of the common neural integrator circuits for vertical eye movements.

Ketamine induces failure of the oculomotor neural integrator in the cat

We studied the effect of intramuscular injection of low dose of ketamine (1 mg/kg) on the spontaneous ocular movements of the cat. Ketamine is a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptors, which is used as an anesthetic agent in human surgery. We found that ketamine administration caused a failure of gaze holding: each saccade was followed by a centripetal post-saccadic drift. This defect was selective: the dynamics of the saccades was not altered (the amplitude/maximum velocity relationship was unaffected by ketamine at the dose of 1 mg/kg). We postulated that the observed effect was due to the fact that NMDA receptors were implicated in the network of the oculomotor neural integrator that converted activity related to the saccade (pulse signal) into activity responsible for gaze holding (step signal).

Serotonin-mediated inhibitory postsynaptic potential in guinea-pig prepositus hypoglossi and feedback inhibition by serotonin

1. Intracellular recordings were made from neurones of the nucleus prepositus hypoglossi (PH) in slices of guinea-pig brain. Focal stimulation evoked an inhibitory postsynaptic potential (IPSP) that was typically 10-25 mV in amplitude and 1 s in duration. The IPSP reversal potential showed a Nernstian dependence on the external potassium concentration ([K+]o). 2. Spiperone blocked the IPSP with an IC50 of 40 nM, while ketanserin and (-)sulpiride had no effect. Cocaine (1 microM) prolonged the IPSP half-duration by 157%, and increased the amplitude by 28%. 3. 5-Hydroxytryptamine (5-HT, serotonin) hyperpolarized PH cells with an EC50 of 8.5 microM in control, and 135 nM in cocaine (10 microM). 8-Hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) also hyperpolarized PH cells with an EC50 of 16 nM, although the maximal effect was only 81% of the maximum 5-HT hyperpolarization. Spiperone produced a parallel, right shift of the 5-HT concentration-response curve; Schild analysis gave a Kd of 10 nM. Application of 5-HT to neurones voltage-clamped near their resting potential (about -55 mV) caused an outward current and an increase in membrane conductance. 4. The amplitude of the IPSP was reversibly decreased by non-hyperpolarizing concentrations of 5-HT and by the 5-HT1 receptor agonists 1-(m-trifluoromethylphenyl)piperazine (TFMPP) and 1-(3-chlorophenyl)piperazine (mCPP). The IC50 values for the latter two compounds were 50 nM and 1.5 microM, respectively; the maximal effect was a 90% inhibition. Neither compound affected the membrane potential nor changed the hyperpolarization induced by 5-HT. Quipizine competitively antagonized TFMPP with an estimated Kd of 165 nM. 5. When trains of stimuli were applied, an inhibition of the IPSP was observed following the first stimulus. At a frequency of 1 Hz, the inhibition was approximately 75%. This frequency-dependent 'run-down' of the IPSP was markedly attenuated by pre-treatment with TFMPP (1 microM). 6. It is concluded that the IPSP in PH cells is caused by 5-HT acting on 5-HT1A receptors to activate a potassium conductance. The release of 5-HT can be inhibited by activation of a presynaptic 5-HT1D receptor. This presynaptic receptor appears to be at least partly responsible for the run-down phenomenon, and may be involved in the physiological regulation of 5-HT synaptic transmission.

The perihypoglossal projection to the superior colliculus in the rhesus monkey

The projection of the perihypoglossal (PH) complex to the superior colliculus (SC) in the rhesus monkey was investigated using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Following physiological identification by electrical stimulation and multiunit recording, small injections of the tracer were placed within the SC of three monkeys. The largest numbers of retrogradely labeled neurons within the PH complex were found in the contralateral nucleus prepositus hypoglossi (NPH), in the laterally adjacent medial vestibular nucleus, and in the ventrally adjacent reticular formation (the nucleus reticularis supragigantocellularis). These labeled neurons are strikingly heterogeneous in size and morphology. The nuclei supragenualis and intercalatus also contain numerous labeled neurons in the 2 cases in which the injections involve the caudal SC. Large numbers of retrogradely labeled neurons as well as anterogradely transported WGA-HRP are observed also throughout the pontine and medullary reticular formation, including the midline raphe. The PH complex, particularly the NPH, is known to be involved in the coding of eye position and has been hypothesized to be a critical component of the "neural integrator." Our data demonstrate the existence of a robust projection from the PH complex to the contralateral SC in the rhesus monkey. This projection may serve as the anatomical substrate by which a corollary of eye position could reach the SC. Such a signal is a prerequisite for the computation, at the collicular level, of saccadic motor error signals observed in the SC of rhesus monkeys.

Evidence for a persistent sodium conductance in neurons from the nucleus prepositus hypoglossi

Intracellular recordings were made from 39 neurons in a slice preparation of the prepositus hypoglossi nucleus from guinea pigs. Morphological characteristics were confirmed by dying neurons with Lucifer yellow. The neurons were spontaneously active, firing in the range of 8-50 spikes/s. Spike duration was short (0.32 ms) and the spikes were followed by fast and slow afterhyperpolarizations. The current vs frequency relationship was linear during steady state firing, but showed dual firing ranges corresponding to the first, third and fifth interspike interval. The instantaneous frequency of the first few interspike intervals could reach 500 spikes/s. Depolarizing and hyperpolarizing responses to square pulses displayed initial sag and rebound responses sensitive to extracellular Cs+, pharmacologically classifying the responses as a result of a Q-like current. Substitution of Ca2+ in the medium with the inorganic calcium blockers Mn2+ or Co2+ resulted in oscillatory firing, depolarizing excursions being sensitive to tetrodotoxin (TTX). Mn2+ or Co2+ in combination with extracellular Cs+ elicited TTX-sensitive plateau potentials, blocked in Na+ -free solution. In conclusion, the prepositus neurons displayed spontaneous activity in the slice preparation and active membrane properties above as well as below the threshold of the action potential. In addition, the prepositus neurons possess a persistent sodium conductance that can be uncovered by inorganic calcium blockers. It may be involved in sustaining the spontaneous discharge.

Eye movements evoked by microstimulations in the brainstem of the alert cat

This study maps the eye movements evoked by microstimulations in the medulla of 9 alert cats. Trains of square waves (20 microA amplitude, 0.2 msec duration, 200 Hz) were delivered through glass-covered tungsten microelectrodes (0.5-1 M omega). Movements of both eyes were recorded by the magnetic field/eye coil technique. Stimulation of the prepositus hypoglossi nucleus (PH) evoked nystagmus with ipsilaterally-directed slow phases followed by after-nystagmus with contralaterally-directed slow phases. Stimulation of the medial vestibular nucleus (MVN) induced nystagmus whose slow phases were more often ipsilaterally-directed but at time contralaterally-directed. At nearly all sites where we stimulated the reticular formation underlying prepositus and vestibular nuclei (from P4.5 to P12), we recorded versional conjugate movements. They were most often ipsilaterally-directed. Some microstimulations in the region of the medial longitudinal fasciculus evoked recentering eye movements: regardless of the initial position of the gaze (to the left as well as to the right), microstimulations given at the same place induced a movement of both eyes toward their neutral position. The amplitude of this movement was proportional to the eccentricity of the pre-stimulation position of the gaze.