Discharges of neurons in the dorsal paraflocculus of monkeys during eye movements and visual stimulation

The impact of locomotion on the brain evolution of squirrels and close relatives

How do brain size and proportions relate to ecology and evolutionary history? Here, we use virtual endocasts from 38 extinct and extant rodent species spanning 50+ million years of evolution to assess the impact of locomotion, body mass, and phylogeny on the size of the brain, olfactory bulbs, petrosal lobules, and neocortex. We find that body mass and phylogeny are highly correlated with relative brain and brain component size, and that locomotion strongly influences brain, petrosal lobule, and neocortical sizes. Notably, species living in trees have greater relative overall brain, petrosal lobule, and neocortical sizes compared to other locomotor categories, especially fossorial taxa. Across millions of years of Eocene-Recent environmental change, arboreality played a major role in the early evolution of squirrels and closely related aplodontiids, promoting the expansion of the neocortex and petrosal lobules. Fossoriality in aplodontiids had an opposing effect by reducing the need for large brains.

Encoding of eye movements explains reward-related activity in cerebellar simple spikes

The cerebellum exhibits both motor and reward-related signals. However, it remains unclear whether reward is processed independently from the motor command or might reflect the motor consequences of the reward drive. To test how reward-related signals interact with sensorimotor processing in the cerebellum, we recorded Purkinje cell simple spike activity in the cerebellar floccular complex while monkeys were engaged in smooth pursuit eye movement tasks. The color of the target signaled the size of the reward the monkeys would receive at the end of the target motion. When the tracking task presented a single target, both pursuit and neural activity were only slightly modulated by the reward size. The reward modulations in single cells were rarely large enough to be detected. These modulations were only significant in the population analysis when we averaged across many neurons. In two-target tasks where the monkey learned to select based on the size of the reward outcome, both behavior and neural activity adapted rapidly. In both the single- and two-target tasks, the size of the reward-related modulation matched the size of the effect of reward on behavior. Thus, unlike cortical activity in eye movement structures, the reward-related signals could not be dissociated from the motor command. These results suggest that reward information is integrated with the eye movement command upstream of the Purkinje cells in the floccular complex. Thus reward-related modulations of the simple spikes are akin to modulations found in motor behavior and not to the central processing of the reward value.NEW & NOTEWORTHY Disentangling sensorimotor and reward signals is only possible if these signals do not completely overlap. We recorded activity in the floccular complex of the cerebellum while monkeys performed tasks designed to separate representations of reward from those of movement. Activity modulation by reward could be accounted for by the coding of eye movement parameters, suggesting that reward information is already integrated into motor commands upstream of the floccular complex.

Role of the Vermal Cerebellum in Visually Guided Eye Movements and Visual Motion Perception

The cerebellar cortex is a crystal-like structure consisting of an almost endless repetition of a canonical microcircuit that applies the same computational principle to different inputs. The output of this transformation is broadcasted to extracerebellar structures by way of the deep cerebellar nuclei. Visually guided eye movements are accommodated by different parts of the cerebellum. This review primarily discusses the role of the oculomotor part of the vermal cerebellum [the oculomotor vermis (OMV)] in the control of visually guided saccades and smooth-pursuit eye movements. Both types of eye movements require the mapping of retinal information onto motor vectors, a transformation that is optimized by the OMV, considering information on past performance. Unlike the role of the OMV in the guidance of eye movements, the contribution of the adjoining vermal cortex to visual motion perception is nonmotor and involves a cerebellar influence on information processing in the cerebral cortex.

Pursuit disorder and saccade dysmetria after caudal fastigial inactivation in the monkey

The caudal fastigial nuclei (cFN) are the output nuclei by which the medio-posterior cerebellum influences the production of saccadic and pursuit eye movements. We investigated the consequences of unilateral inactivation on the pursuit eye movement made immediately after an interceptive saccade toward a centrifugal target. We describe here the effects when the target moved along the horizontal meridian with a 10 or 20°/s speed. After muscimol injection, the monkeys were unable to track the present location of the moving target. During contralesional tracking, the velocity of postsaccadic pursuit was reduced. This slowing was associated with a hypometria of interceptive saccades such that gaze direction always lagged behind the moving target. No correlation was found between the sizes of saccade undershoot and the decreases in pursuit speed. During ipsilesional tracking, the effects on postsaccadic pursuit were variable across the injection sessions, whereas the interceptive saccades were consistently hypermetric. Here also, the ipsilesional pursuit disorder was not correlated with the saccade hypermetria either. The lack of correlation between the sizes of saccade dysmetria and changes of postsaccadic pursuit speed suggests that cFN activity exerts independent influences on the neural processes generating the saccadic and slow eye movements. It also suggests that the cFN is one locus where the synergy between the two motor categories develops in the context of tracking a moving visual target. We explain how the different fastigial output channels can account for these oculomotor tracking disorders. NEW & NOTEWORTHY Inactivation of the caudal fastigial nucleus impairs the ability to track a moving target. The accuracy of interceptive saccades and the velocity of postsaccadic pursuit movements are both altered, but these changes are not correlated. This absence of correlation is not compatible with an impaired common command feeding the circuits producing saccadic and pursuit eye movements. However, it suggests an involvement of caudal fastigial nuclei in their synergy to accurately track a moving target.

Synchronizing the tracking eye movements with the motion of a visual target: Basic neural processes

In primates, the appearance of an object moving in the peripheral visual field elicits an interceptive saccade that brings the target image onto the foveae. This foveation is then maintained more or less efficiently by slow pursuit eye movements and subsequent catch-up saccades. Sometimes, the tracking is such that the gaze direction looks spatiotemporally locked onto the moving object. Such a spatial synchronism is quite spectacular when one considers that the target-related signals are transmitted to the motor neurons through multiple parallel channels connecting separate neural populations with different conduction speeds and delays. Because of the delays between the changes of retinal activity and the changes of extraocular muscle tension, the maintenance of the target image onto the fovea cannot be driven by the current retinal signals as they correspond to past positions of the target. Yet, the spatiotemporal coincidence observed during pursuit suggests that the oculomotor system is driven by a command estimating continuously the current location of the target, i.e., where it is here and now. This inference is also supported by experimental perturbation studies: when the trajectory of an interceptive saccade is experimentally perturbed, a correction saccade is produced in flight or after a short delay, and brings the gaze next to the location where unperturbed saccades would have landed at about the same time, in the absence of visual feedback. In this chapter, we explain how such correction can be supported by previous visual signals without assuming "predictive" signals encoding future target locations. We also describe the basic neural processes which gradually yield the synchronization of eye movements with the target motion. When the process fails, the gaze is driven by signals related to past locations of the target, not by estimates to its upcoming locations, and a catch-up is made to reinitiate the synchronization.

Floccular fossa size is not a reliable proxy of ecology and behaviour in vertebrates

The cerebellar floccular and parafloccular lobes are housed in fossae of the periotic region of the skull of different vertebrates. Experimental evidence indicates that the lobes integrate visual and vestibular information and control the vestibulo-ocular reflex, vestibulo-collic reflex, smooth pursuit and gaze holding. Multiple paleoneuroanatomy studies have deduced the behaviour of fossil vertebrates by measuring the floccular fossae (FF). These studies assumed that there are correlations between FF volume and behaviour. However, these assumptions have not been fully tested. Here, we used micro-CT scans of extant mammals (47 species) and birds (59 species) to test six possible morphological-functional associations between FF volume and ecological/behavioural traits of extant animals. Behaviour and ecology do not explain FF volume variability in four out of six variables tested. Two variables with significant results require further empirical testing. Cerebellum plasticity may explain the lack of statistical evidence for the hypotheses tested. Therefore, variation in FF volume seems to be better explained by a combination of factors such as anatomical and phylogenetic evolutionary constraints, and further empirical testing is required.

A foveal target increases catch-up saccade frequency during smooth pursuit

Images that move rapidly across the retina of the human eye blur because the retina has sluggish temporal dynamics. Voluntary smooth pursuit eye movements are modeled as matching object velocity to minimize retinal motion and prevent retinal blurring. However, "catch-up" saccades that are ubiquitous during pursuit interrupt it and disrupt clear vision. But catch-up saccades may not be a common feature of ocular pursuit, because their existence has been documented with a small moving spot, the classic pursuit stimulus, which is a weak motion stimulus that may poorly emulate larger pursuit objects. We found that spot pursuit does not generalize to that of larger objects. Observers pursued a spot or a larger virtual object with or without a superimposed spot target. Single-spot targets produced lower pursuit acceleration than larger objects. Critically, more saccadic intrusions occurred when stimuli had a central dot, even when position and velocity errors were equated, suggesting that catch-up saccades result from pursuing a single, small object or a feature on a large one. To determine what differentiates a large object from a small one, we progressively shrank the featureless virtual object and found that catch-up saccade frequency was highest when it fit in the fovea. The results suggest that pursuit of a small target or an object feature recruits a saccade mechanism that does not compensate for a weak motion signal; rather, the target compels foveation. Furthermore, catch-up saccades are likely generated by neural circuitry typically used to foveate small objects or features.

Ion Homeostasis in Rhythmogenesis: The Interplay Between Neurons and Astroglia

Proper function of all excitable cells depends on ion homeostasis. Nowhere is this more critical than in the brain where the extracellular concentration of some ions determines neurons' firing pattern and ability to encode information. Several neuronal functions depend on the ability of neurons to change their firing pattern to a rhythmic bursting pattern, whereas, in some circuits, rhythmic firing is, on the contrary, associated to pathologies like epilepsy or Parkinson's disease. In this review, we focus on the four main ions known to fluctuate during rhythmic firing: calcium, potassium, sodium, and chloride. We discuss the synergistic interactions between these elements to promote an oscillatory activity. We also review evidence supporting an important role for astrocytes in the homeostasis of each of these ions and describe mechanisms by which astrocytes may regulate neuronal firing by altering their extracellular concentrations. A particular emphasis is put on the mechanisms underlying rhythmogenesis in the circuit forming the central pattern generator (CPG) for mastication and other CPG systems. Finally, we discuss how an impairment in the ability of glial cells to maintain such homeostasis may result in pathologies like epilepsy and Parkinson's disease.

Cerebellum-dependent motor learning: lessons from adaptation of eye movements in primates

In order to ameliorate the consequences of ego motion for vision, human and nonhuman observers generate reflexive, compensatory eye movements based on visual as well as vestibular information, helping to stabilize the images of visual scenes on the retina despite ego motion. And in order to fully exploit the advantages of foveal vision, they make saccades to shift the image of an object onto the fovea and smooth pursuit eye movements to stabilize it there despite continuing object movement relative to the observer. With the exception of slow visually driven eye movements, which can be understood as manifestations of relatively straightforward feedback systems, most eye movements require a direct conversion of sensory input into appropriate motor responses in the absence of immediate sensory feedback. Hence, in order to generate appropriate oculomotor responses, the parameters linking input and output must be chosen suitably. Moreover, as the parameters may change from one manifestation of a movement to the next, for instance because of oculomotor fatigue, the choices should also be quickly modifiable. This chapter will present evidence showing that this fast parametric optimization, understood as a functionally distinct example of motor learning, is an accomplishment of specific parts of the cerebellum devoted to the control of eye movements. It will also discuss recent electrophysiological results suggesting how this specific form of motor learning may emerge from information processing in cerebellar circuits.

Visuomotor cerebellum in human and nonhuman primates

In this paper, we will review the anatomical components of the visuomotor cerebellum in human and, where possible, in non-human primates and discuss their function in relation to those of extracerebellar visuomotor regions with which they are connected. The floccular lobe, the dorsal paraflocculus, the oculomotor vermis, the uvula-nodulus, and the ansiform lobule are more or less independent components of the visuomotor cerebellum that are involved in different corticocerebellar and/or brain stem olivocerebellar loops. The floccular lobe and the oculomotor vermis share different mossy fiber inputs from the brain stem; the dorsal paraflocculus and the ansiform lobule receive corticopontine mossy fibers from postrolandic visual areas and the frontal eye fields, respectively. Of the visuomotor functions of the cerebellum, the vestibulo-ocular reflex is controlled by the floccular lobe; saccadic eye movements are controlled by the oculomotor vermis and ansiform lobule, while control of smooth pursuit involves all these cerebellar visuomotor regions. Functional imaging studies in humans further emphasize cerebellar involvement in visual reflexive eye movements and are discussed.

Cerebellar inputs to intraparietal cortex areas LIP and MIP: functional frameworks for adaptive control of eye movements, reaching, and arm/eye/head movement coordination

Using retrograde transneuronal transfer of rabies virus in combination with a conventional tracer (cholera toxin B), we studied simultaneously direct (thalamocortical) and polysynaptic inputs to the ventral lateral intraparietal area (LIPv) and the medial intraparietal area (MIP) in nonhuman primates. We found that these areas receive major disynaptic inputs from specific portions of the cerebellar nuclei, the ventral dentate (D), and ventrolateral interpositus posterior (IP). Area LIPv receives inputs from oculomotor domains of the caudal D and IP. Area MIP is the target of projections from the ventral D (mainly middle third), and gaze- and arm-related domains of IP involved in reaching and arm/eye/head coordination. We also showed that cerebellar cortical "output channels" to MIP predominantly stem from posterior cerebellar areas (paramedian lobe/Crus II posterior, dorsal paraflocculus) that have the required connectivity for adaptive control of visual and proprioceptive guidance of reaching, arm/eye/head coordination, and prism adaptation. These findings provide important insight about the interplay between the posterior parietal cortex and the cerebellum regarding visuospatial adaptation mechanisms and visual and proprioceptive guidance of movement. They also have potential implications for clinical approaches to optic ataxia and neglect rehabilitation.

Cerebellar contributions to the processing of saccadic errors

Saccades are fast eye movements that direct the point of regard to a target in the visual field. Repeated post-saccadic visual errors can induce modifications of the amplitude of these saccades, a process known as saccadic adaptation. Two experiments using the same paradigm were performed to study the involvement of the cerebrum and the cerebellum in the processing of saccadic errors using functional magnetic resonance imaging and in-scanner eye movement recordings. In the first active condition, saccadic adaptation was prevented using a condition in which the saccadic target was shifted to a variable position during the saccade towards it. This condition induced random saccadic errors as opposed to the second active condition in which the saccadic target was not shifted. In the baseline condition, subjects looked at a stationary dot. Both active conditions compared with baseline evoked activation in the expected saccade-related regions using a stringent statistical threshold [the frontal and parietal eye fields, primary visual area, MT/V5, and the precuneus (V6) in the cerebrum; vermis VI-VII; and lobule VI in the cerebellum, known as the oculomotor vermis). In the direct comparison between the two active conditions, significantly more cerebellar activation (vermis VIII, lobules VIII-X, left lobule VIIb) was observed with random saccadic errors (using a more relaxed statistical threshold). These results suggest a possible role for areas outside the oculomotor vermis of the cerebellum in the processing of saccadic errors. Future studies of these areas with, e.g., electrophysiological recordings, may reveal the nature of the error signals that drive the amplitude modification of saccadic eye movements.

Role of primate cerebellar hemisphere in voluntary eye movement control revealed by lesion effects

The anatomical connection between the frontal eye field and the cerebellar hemispheric lobule VII (H-VII) suggests a potential role of the hemisphere in voluntary eye movement control. To reveal the involvement of the hemisphere in smooth pursuit and saccade control, we made a unilateral lesion around H-VII and examined its effects in three Macaca fuscata that were trained to pursue visually a small target. To the step (3 degrees)-ramp (5-20 degrees/s) target motion, the monkeys usually showed an initial pursuit eye movement at a latency of 80-140 ms and a small catch-up saccade at 140-220 ms that was followed by a postsaccadic pursuit eye movement that roughly matched the ramp target velocity. After unilateral cerebellar hemispheric lesioning, the initial pursuit eye movements were impaired, and the velocities of the postsaccadic pursuit eye movements decreased. The onsets of 5 degrees visually guided saccades to the stationary target were delayed, and their amplitudes showed a tendency of increased trial-to-trial variability but never became hypo- or hypermetric. Similar tendencies were observed in the onsets and amplitudes of catch-up saccades. The adaptation of open-loop smooth pursuit velocity, tested by a step increase in target velocity for a brief period, was impaired. These lesion effects were recognized in all directions, particularly in the ipsiversive direction. A recovery was observed at 4 wk postlesion for some of these lesion effects. These results suggest that the cerebellar hemispheric region around lobule VII is involved in the control of smooth pursuit and saccadic eye movements.

An internal model of a moving visual target in the lateral cerebellum

In order to overcome the relatively long delay in processing visual feedback information when pursuing a moving visual target, it is necessary to predict the future trajectory of the target if it is to be tracked with accuracy. Predictive behaviour can be achieved through internal models, and the cerebellum has been implicated as a site for their operation. Purkinje cells in the lateral cerebellum (D zones) respond to visual inputs during visually guided tracking and it has been proposed that their neural activity reflects the operation of an internal model of target motion. Here we provide direct evidence for the existence of such a model in the cerebellum by demonstrating an internal model of a moving external target. Single unit recordings of Purkinje cells in lateral cerebellum (D2 zone) were made in cats trained to perform a predictable visually guided reaching task. For all Purkinje cells that showed tonic simple spike activity during target movement, this tonic activity was maintained during the transient disappearance of the target. Since simple spike activity could not be correlated to eye or limb movements, and the target was familiar and moved in a predictable fashion, we conclude that the Purkinje cell activity reflects the operation of an internal model based on memory of its previous motion. Such a model of the target's motion, reflected in the maintained modulation during the target's absence, could be used in a predictive capacity in the interception of a moving object.

Role of primate cerebellar lobulus petrosus of paraflocculus in smooth pursuit eye movement control revealed by chemical lesion

The primate lobulus petrosus (LP) of the cerebellar paraflocculus receives inputs from visual system-related pontine nuclei, and projects to eye movement-related cerebellar nuclei. To reveal a potential involvement of LP in oculomotor control, we lesioned LP unilaterally by local injections of ibotenic acid in three Macaca fuscata. We examined the effects of lesion on eye movements evoked by step (3 degrees )-ramp (5-15 degrees/s) moving target. To step-ramp moving target, the monkeys showed an initial slow eye movement and later a small catch-up saccade, which was followed by the post-saccadic pursuit nearly matching to the velocity of the ramp target motion. After LP lesioning, the velocity of post-saccadic pursuits in the ipsiversive and down-ward directions decreased by 20-40% in all three monkeys. These deficits lasted for at least 1 month, and some recovery was observed. In the amplitudes of catch-up saccades, no consistent changes were seen among the three monkeys after LP lesioning. These results suggest an involvement of LP in the primate smooth pursuit eye movement control.

Independent roles for the dorsal paraflocculus and vermal lobule VII of the cerebellum in visuomotor coordination

Two distinct areas of cerebellar cortex, vermal lobule VII and the dorsal paraflocculus (DPFl) receive visual input. To help understand the visuomotor functions of these two regions, we compared their afferent and efferent connections using the tracers wheatgerm agglutinin horseradish peroxidase (WGA-HRP) and biotinilated dextran amine (BDA). The sources of both mossy fibre and climbing fibre input to the two areas are different. The main mossy fibre input to lobule VII is from the nucleus reticularis tegmenti pontis (NRTP), which relays visual information from the superior colliculus, while the main mossy fibre input to the DPFl is from the pontine nuclei, relaying information from cortical visual areas. The DPFl and lobule VII both also receive mossy fibre input from several common brainstem regions, but from different subsets of cells. These include visual input from the dorsolateral pons, and vestibular-oculomotor input from the medial vestibular nucleus (MVe) and the nucleus prepositus hypoglossi (Nph). The climbing fibre input to the two cerebellar regions is from different subdivisions of the inferior olivary nuclei. Climbing fibres from the caudal medial accessory olive (cMAO) project to lobule VII, while the rostral MAO (rMAO) and the principal olive (PO) project to the DPFl. The efferent projections from lobule VII and the DPF1 are to all of the recognised oculomotor and visual areas within the deep cerebellar nuclei, but to separate territories. Both regions play a role in eye movement control. The DPFl may also have a role in visually guided reaching.

The oculomotor role of the pontine nuclei and the nucleus reticularis tegmenti pontis

Cerebral cortex and the cerebellum interact closely in order to facilitate spatial orientation and the generation of motor behavior, including eye movements. This interaction is based on a massive projection system that allows the exchange of signals between the two cortices. This cerebro-cerebellar communication system includes several intercalated brain stem nuclei, whose eminent role in the organization of oculomotor behavior has only recently become apparent. This review focuses on the two major nuclei of this group taking a precerebellar position, the pontine nuclei and the nucleus reticularis tegmenti pontis, both intimately involved in the visual guidance of eye movements.

Oculomotor cerebellum

The anatomical, physiological, and behavioral evidence for the involvement of three regions of the cerebellum in oculomotor behavior is reviewed here: (1) the oculomotor vermis and paravermis of lobules V, IV, and VII; (2) the uvula and nodulus; (3) flocculus and ventral paraflocculus. No region of the cerebellum controls eye movements exclusively, but each receives sensory information relevant for the control of multiple systems. An analysis of the microcircuitry suggests how sagittal climbing fiber zones bring visual information to the oculomotor vermis; convey vestibular information to the uvula and nodulus, while optokinetic space is represented in the flocculus. The mossy fiber projections are more heterogeneous. The importance of the inferior olive in modulating Purkinje cell responses is discussed.

Recasting the Smooth Pursuit Eye Movement System

Primates use a combination of smooth pursuit and saccadic eye movements to stabilize the retinal image of selected objects within the high-acuity region near the fovea. Pursuit has traditionally been viewed as a relatively automatic behavior, driven by visual motion signals and mediated by pathways that connect visual areas in the cerebral cortex to motor regions in the cerebellum. However, recent findings indicate that this view needs to be reconsidered. Rather than being controlled primarily by areas in extrastriate cortex specialized for processing visual motion, pursuit involves an extended network of cortical areas, and, of these, the pursuit-related region in the frontal eye fields appears to exert the most direct influence. The traditional pathways through the cerebellum are important, but there are also newly identified routes involving structures previously associated with the control of saccades, including the basal ganglia, the superior colliculus, and nuclei in the brain stem reticular formation. These recent findings suggest that the pursuit system has a functional architecture very similar to that of the saccadic system. This viewpoint provides a new perspective on the processing steps that occur as descending control signals interact with circuits in the brain stem and cerebellum responsible for gating and executing voluntary eye movements. Although the traditional view describes pursuit and saccades as two distinct neural systems, it may be more accurate to consider the two movements as different outcomes from a shared cascade of sensory–motor functions.

Analysis of frequency components of cortical potentials evoked by progressive misalignment of Kanizsa squares

Cortical gamma oscillations (20-100 Hz) are thought to play an important role in encoding visual perception. If so they should emerge at about threshold. In the present investigation we examined the latter proposal. Visual responses were recorded in occipital, temporal and parietal areas (stimulus duration 512 ms). Oscillation strength and frequency were derived from FFT analysis and wavelet transforms. The specific goals of the present study are: 1: To examine the parallel between gamma oscillations and the psychometric threshold of perception of Kanizsa square (KS). The latter is gradually altered by a progressive misalignment of lower inducers (pacmen). Results show that the perception of the KS is altered by lateral displacements of the lower inducers as small as 0.1 to 0.2 degrees. In parallel, high frequency components of cortical responses gain in strength with misalignments. 2: Gamma oscillations emerge at or about the psychometric threshold. In addition, our data analysis demonstrates that gamma oscillations appear in short bursts (approx. 50 ms) in the time window between 200 and 500 ms after stimulus onset. Furthermore, controls indicated that these oscillations are of the induced-gamma type. Thus, our experiments suggest that gamma oscillations are associated with image structures and may be induced by local properties of the target.

Neuronal activity related to the visual representation of arm movements in the lateral cerebellar cortex

Testing the hypothesis that the lateral cerebellum forms a sensory representation of arm movements, we investigated cortical neuronal activity in two monkeys performing visually guided step-tracking movements with a manipulandum. A virtual target and cursor image were viewed co-planar with the manipulandum. In the normal task, manipulandum and cursor moved in the same direction; in the mirror task, the cursor was left-right reversed. In one monkey, 70- and 200-ms time delays were introduced on cursor movement. Significant task-related activity was recorded in 31 cells in one animal and 142 cells in the second: 10.2% increased activity before arm movements onset, 77.1% during arm movement, and 12.7% after the new position was reached. To test for neural representation of the visual outcome of movement, firing rate modulation was compared in normal and mirror step-tracking. Most task-related neurons (68%) showed no significant directional modulation. Of 70 directionally sensitive cells, almost one-half (n = 34, 48%) modulated firing with a consistent cursor movement direction, many fewer responding to the manipulandum direction (n = 9, 13%). For those "cursor-related" cells tested with delayed cursor movement, increased activity onset was time-locked to arm movement and not cursor movement, but activation duration was extended by an amount similar to the applied delay. Hence, activity returned to baseline about when the delayed cursor reached the target. We conclude that many cells in the lateral cerebellar cortex signaled the direction of cursor movement during active step-tracking. Such a predictive representation of the arm movement could be used in the guidance of visuo-motor actions.

Cerebellar flocculus and ventral paraflocculus Purkinje cell activity during predictive and visually driven pursuit in monkey

Purkinje cells in the flocculus and ventral paraflocculus were studied in tasks designed to distinguish predictive versus visually guided mechanisms of smooth pursuit. A sum-of-sines task allowed studies of complex predictive pursuit. A perturbation task examined visually driven pursuit during unpredictable right-angle changes in target direction. A gap task examined pursuit that was maintained when the target was turned off. Neural activity patterns were quantified using multi-linear models with sensitivities to the position, velocity, and acceleration of both motor output (eye motion) and visual input (retinal slip). During the sum-of-sines task, neural responses led eye motion by an average of 12 ms, a value larger than the 9-ms transmission delay between flocculus stimulation and eye motion. This suggests that flocculus/paraflocculus neurons drove pursuit along predictable sum-of-sines trajectories. In contrast, neural responses led eye motion by an average of only 2 ms during the perturbation task and by 6 ms during the gap task. These values suggest a follow-up role during tasks more heavily dependent on visual processing. Activity in all three tasks was explained primarily by sensitivities to eye position and velocity. Eye acceleration played a minor role during ongoing pursuit, although its influence on firing rate increased during the high accelerations following unexpected changes in target motion. Retinal slip had a relatively small influence on responses during pursuit. This was particularly true for the sum-of-sines and gap tasks where predictive control eliminated any consistent retinal-slip signals that might have been used to drive the eye. Surprisingly, the influence of retinal slip did not increase appreciably during unpredictable perturbations in target direction that generated large amounts of retinal slip. Thus although visual control signals are needed in varying amounts during the three pursuit tasks, they have been converted to motor control signals by the time they leave the flocculus/paraflocculus system. Individual neurons showed a remarkable constancy in eye-sensitivity direction across tasks that indicated direct links to oculomotor neurons. However, some neurons showed changes in sensitivity magnitude that suggested changes in control strategy for different tasks. Magnitude differences were largest for the perturbation task. We conclude that the flocculus/paraflocculus system plays a major role in driving predictive pursuit. It also processes visually driven control signals that originate in other brain regions after a slight delay.

Vertical Purkinje cells of the monkey floccular lobe: simple-spike activity during pursuit and passive whole body rotation

To understand how the simian floccular lobe is involved in vertical smooth pursuit eye movements and the vertical vestibuloocular reflex (VOR), we examined simple-spike activity of 70 Purkinje (P) cells during pursuit eye movements and passive whole body rotation. Fifty-eight cells responded during vertical and 12 during horizontal pursuit. We classified P cells as vertical gaze velocity (VG) if their modulation occurred for movements of both the eye (during vertical pursuit) and head (during pitch VOR suppression) with the modulation during one less than twice that of the other and was less during the target-fixed-in-space condition (pitch VOR X1) than during pitch VOR suppression. VG P cells constituted only a minority of vertical P cells (19%). Other vertical P cells that responded during pitch VOR suppression were classified as vertical eye and head velocity (V/) P cells (48%), regardless of the synergy of their response direction during smooth pursuit and VOR suppression. Vertical P cells that did not respond during pitch VOR suppression but did respond during rotation in vertical planes other than pitch were classified as off-pitch V/ P cells (33%). The mean eye-velocity and eye-position sensitivities of the three types of vertical P cells were similar. One-third (2/7 VG, 2/11 V/, 6/13 off-pitch V/), in addition, showed eye position sensitivity during saccade-free fixations. Maximal vestibular activation directions (MADs) were examined during VOR suppression by applying vertical whole body rotation with the monkeys oriented in different vertical planes. The MADs for VG P cells and V/ P cells with eye and vestibular sensitivity in the same direction were distributed near the pitch plane, suggesting convergence of bilateral anterior canal inputs. In contrast, MADs of off-pitch V/ P cells and V/ P cells with oppositely directed eye and vestibular sensitivity were shifted toward the roll plane, suggesting convergence of anterior and posterior canal inputs of the same side. Unlike horizontal G P cells, the modulation of many VG and V/ P cells when the target was fixed in space (pitch VOR X1) was not well predicted by the linear addition of their modulations during vertical pursuit and pitch VOR suppression. These results indicate that the populations of vertical and horizontal eye-movement P cells in the floccular lobe have markedly different discharge properties and therefore may be involved in different kinds of processing of vestibular-oculomotor interactions.

Neuronal activity in the lateral cerebellum of the cat related to visual stimuli at rest, visually guided step modification, and saccadic eye movements

1. The discharge patterns of 166 lateral cerebellar neurones were studied in cats at rest and during visually guided stepping on a horizontal circular ladder. A hundred and twelve cells were tested against one or both of two visual stimuli: a brief full-field flash of light delivered during eating or rest, and a rung which moved up as the cat approached. Forty-five cells (40%) gave a short latency response to one or both of these stimuli. These visually responsive neurones were found in hemispheral cortex (rather than paravermal) and the lateral cerebellar nucleus (rather than nucleus interpositus). 2. Thirty-seven cells (of 103 tested, 36%) responded to flash. The cortical visual response (mean onset latency 38 ms) was usually an increase in Purkinje cell discharge rate, of around 50 impulses s-1 and representing 1 or 2 additional spikes per trial (1.6 on average). The nuclear response to flash (mean onset latency 27 ms) was usually an increased discharge rate which was shorter lived and converted rapidly to a depression of discharge or return to control levels, so that there were on average only an additional 0.6 spikes per trial. A straightforward explanation of the difference between the cortical and nuclear response would be that the increased inhibitory Purkinje cell output cuts short the nuclear response. 3. A higher proportion of cells responded to rung movement, sixteen of twenty-five tested (64%). Again most responded with increased discharge, which had longer latency than the flash response (first change in dentate output ca 60 ms after start of movement) and longer duration. Peak frequency changes were twice the size of those in response to flash, at 100 impulses s-1 on average and additional spikes per trial were correspondingly 3-4 times higher. Both cortical and nuclear responses were context dependent, being larger when the rung moved when the cat was closer than further away. 4. A quarter of cells (20 of 84 tested, 24%) modulated their activity in advance of saccades, increasing their discharge rate. Four-fifths of these were non-reciprocally directionally selective. Saccade-related neurones were usually susceptible to other influences, i.e. their activity was not wholly explicable in terms of saccade parameters. 5. Substantial numbers of visually responsive neurones also discharged in relation to stepping movements while other visually responsive neurones discharged in advance of saccadic eye movements. And more than half the cells tested were active in relation both to eye movements and to stepping movements. These combinations of properties qualify even individual cerebellar neurones to participate in the co-ordination of visually guided eye and limb movements.

Slow eye movements

Monkeys and humans are able to perform different types of slow eye movements. The analysis of the eye movement parameters, as well as the investigation of the neuronal activity underlying the execution of slow eye movements, offer an excellent opportunity to study higher brain functions such as motion processing, sensorimotor integration, and predictive mechanisms as well as neuronal plasticity and motor learning. As an example, since there exists a tight connection between the execution of slow eye movements and the processing of any kind of motion, these eye movements can be used as a biological, behavioural probe for the neuronal processing of motion. Global visual motion elicits optokinetic nystagmus, acting as a visual gaze stabilization system. The underlying neuronal substrate consists mainly of the cortico-pretecto-olivo-cerebellar pathway. Additionally, another gaze stabilization system depends on the vestibular input known as the vestibulo-ocular reflex. The interactions between the visual and vestibular stabilization system are essential to fulfil the plasticity of the vestibulo-ocular reflex representing a simple form of learning. Local visual motion is a necessary prerequisite for the execution of smooth pursuit eye movements which depend on the cortico-pontino-cerebellar pathway. In the wake of saccades, short-latency eye movements can be elicited by brief movements of the visual scene. Finally, eye movements directed to objects in different planes of depth consist of slow movements also. Although there is some overlap in the neuronal substrates underlying these different types of slow eye movements, there are brain areas whose activity can be associated exclusively with the execution of a special type of slow eye movement.

Projections of the lateral terminal accessory optic nucleus of the common marmoset (Callithrix jacchus)

The connections of the lateral terminal nucleus (LTN) of the accessory optic system (AOS) of the marmoset monkey were studied with anterograde 3H-amino acid light autoradiography and horseradish peroxidase retrograde labeling techniques. Results show a first and largest LTN projection to the pretectal and AOS nuclei including the ipsilateral nucleus of the optic tract, dorsal terminal nucleus, and interstitial nucleus of the superior fasciculus (posterior fibers); smaller contralateral projections are to the olivary pretectal nucleus, dorsal terminal nucleus, and LTN. A second, major bundle produces moderate-to-heavy labeling in all ipsilateral, accessory oculomotor nuclei (nucleus of posterior commissure, interstitial nucleus of Cajal, nucleus of Darkschewitsch) and nucleus of Bechterew; some of the fibers are distributed above the caudal oculomotor complex within the supraoculomotor periaqueductal gray. A third projection is ipsilateral to the pontine and mesencephalic reticular formations, nucleus reticularis tegmenti pontis and basilar pontine complex (dorsolateral nucleus only), dorsal parts of the medial terminal accessory optic nucleus, ventral tegmental area of Tsai, and rostral interstitial nucleus of the medial longitudinal fasciculus. Lastly, there are two long descending bundles: (1) one travels within the medial longitudinal fasciculus to terminate in the dorsal cap (ipsilateral >> contralateral) and medial accessory olive (ipsilateral only) of the inferior olivary complex. (2) The second soon splits, sending axons within the ipsilateral and contralateral brachium conjunctivum and is distributed to the superior and medial vestibular nuclei. The present findings are in general agreement with the documented connections of LTN with brainstem oculomotor centers in other species. In addition, there are unique connections in marmoset monkey that may have developed to serve the more complex oculomotor behavior of nonhuman primates.

Comparison of the smooth eye tracking disorder of schizophrenics with that of nonhuman primates with specific brain lesions

The smooth pursuit eye tracking deficit (ETD) often associated with schizophrenia has generated enormous interest over the last 20 years. The deficit is observed in about 80% of schizophrenics and in half of their first degree relatives. It is not affected by neuroleptic medication and is not due to inattention. A review of 52 studies (and actual records when available) on ETD in schizophrenia reveals that the deficit can consistently be described as low gain pursuit augmented with catch-up saccades and often peppered with intrusive saccades. A review of the brain areas that have been shown to be involved in pursuit provides the necessary background for the subsequent section which details the nature of the smooth tracking deficits following experimental lesions. This section reveals that the ETD following lesions of the frontal lobe is unique in that it closely resembles the ETD of schizophrenics. This finding lends further support for frontal lobe theories of schizophrenia.

Visual pontocerebellar projections in the macaque

The cerebellum plays an important role in the visual guidance of movement. In order to understand the anatomical basis of visuomotor control, we studied the projection of pontine visual cells onto the cerebellar cortex of monkeys. Wheat germ agglutinin horseradish peroxidase was injected into the dorsolateral pons two monkeys. Retrogradely labelled cells were mapped in the cerebral cortex and superior colliculus, and orthogradely labelled fibers in the cerebellar cortex. The largest number of retrogradely labelled cells in the cerebral cortex was in a group of medial extrastriate visual areas. The major cerebellar target of these dorsolateral pontine cells is the dorsal paraflocculus. There is a weaker projection to the uvula, paramedian lobe, and Crus II, and a sparse but definite projection to the ventral paraflocculus. There are virtually no projections to the flocculus. There are sparse ipsilateral pontocerebellar projections to these same regions of cerebellar cortex. In nine monkeys, we made small injections of the tracer into the cerebellar cortex and studied the location of retrogradely filled cells in the pontine nuclei and inferior olive. Injections into the dorsal paraflocculus or rostral folia of the uvula retrogradely labelled large numbers of cells in the dorsolateral region of the contralateral pontine nuclei. Labelled cells were found ipsilaterally, but in reduced numbers. Injections outside of these areas in ventral paraflocculus or paramedian lobule labelled far fewer cells in this region of the pons. We conclude that the principal source of cerebral cortical visual information arises from a medial group of extrastriate visual areas and is relayed through cells in the dorsolateral pontine nuclei. The principal target of pontine visual cells is the dorsal paraflocculus.

Signalling properties of identified deep cerebellar nuclear neurons related to eye and head movements in the alert cat

1. The spike activity of deep cerebellar nuclear neurons was recorded in the alert cat during spontaneous and during vestibularly and visually induced eye movements. 2. Neurons were classified according to their location in the nuclei, their antidromic activation from projection sites, their sensitivity to eye position and velocity during spontaneous eye movements, and their responses to vestibular and optokinetic stimuli. 3. Type I EPV (eye position and velocity) neurons were located mainly in the posterior part of the fastigial nucleus and activated antidromically almost exclusively from the medial longitudinal fasciculus close to the oculomotor complex. These neurons, reported here for the first time, increased their firing rate during saccades and eye fixations towards the contralateral hemifield. Their position sensitivity to eye fixations in the horizontal plane was 5.3 +/- 2.6 spikes s-1 deg-1 (mean +/- S.D.). Eye velocity sensitivity during horizontal saccades was 0.71 +/- 0.52 spikes s-1 deg-1 s-1. Variability of their firing rate during a given eye fixation was higher than that shown by abducens motoneurons. 4. Type I EPV neurons increased their firing rate during ipsilateral head rotations at 0.5 Hz with a mean phase lead over eye position of 95.3 +/- 9.5 deg. They were also activated by contralateral optokinetic stimulation at 30 deg s-1. Their sensitivity to eye position and velocity in the horizontal plane during vestibular and optokinetic stimuli yielded values similar to those obtained for spontaneous eye movements. 5. Type II neurons were located in both fastigial and dentate nuclei and were activated antidromically from the restiform body, the medial longitudinal fasciculus close to the oculomotor complex, the red nucleus and the pontine nuclei. Type II neurons were not related to spontaneous eye movements. These neurons increased their firing rate in response to contralateral head rotation and during ipsilateral optokinetic stimulation, and decreased it with the oppositely directed movements. 6. Saccade-related neurons were located mostly in the fastigial and dentate nuclei. Fastigial neurons were activated antidromically from the medial longitudinal fasciculus, while dentate neurons were activated from the red nucleus. These neurons fired a burst of spikes whose duration was significantly related to saccade duration. Dentate neurons responded during the omni-directional saccades, while some fastigial neurons fired more actively during contralateral saccades. 7. These three types of neuron represent the output channel for oculomotor signals of the posterior vermis and paravermis. It is proposed that type I EPV neurons correspond to a group of premotor neurons directly involved in oculomotor control.(ABSTRACT TRUNCATED AT 400 WORDS)

Generation of smooth-pursuit eye movements: neuronal mechanisms and pathways

This article reviews the current state of knowledge of the primate smooth-pursuit system. The emphasis is on the neuronal mechanisms and pathways that control pursuit eye movements in the monkey. The review covers the neuronal structures believed to be involved in pursuit generation from striate cortex to the final premotoneuron structures in the brainstem. Information gathered from physiological and anatomical work is stressed.

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

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

Neuronal activity in the lateral cerebellum of trained monkeys, related to visual stimuli or to eye movements

1. The responses of neurones in the lateral cerebellar cortex to visual stimuli and to eye movements were recorded in rhesus monkeys trained to perform visually guided arm and eye movements in a tracking task. 2. Twenty-two of 134 units recorded (16%) modulated their discharge in response to a bright Xenon flash. They were mainly located in the dorsal paraflocculus. Among those identified as Purkinje cells both simple spike and climbing fibre responses to the flash were seen. (72% of the units were related to arm movements; these were centred in the paramedian lobule, and have been described fully in Marple-Horvat & Stein (1987).) 3. The visual responsiveness of one of the units varied according to the phase of the monkey's task. Around the time that the target stepped, which was the monkey's cue to move, its sensitivity to other stimuli disappeared. 4. Only two neurones responded to the movements of the tracking target. These responses were conditional upon the monkey using visual signals to guide his movements; they did not respond to the target step if he moved before the target did. 5. Fourteen units (10%) located in crus I and II and lobulus simplex correlated strongly with the velocity of horizontal eye movements. Only one of these also responded to visual stimuli. 6. Thus most neurones were found to carry only visual, or eye movement, or limb movement information rather than combinations of these signals; they were located in different but overlapping regions of lateral cerebellar cortex. Visually responsive neurones are probably involved in planning the visual goal of movements, while eye and arm movement neurones probably help to create co-ordinative structures for executing voluntary eye and arm movements.

Cerebellotectal pathways in the macaque: implications for collicular generation of saccades

The cerebellum is thought to modulate saccadic activity in the primate in order to maintain targeting accuracy, and the cerebellotectal pathway has been posited to play a role in this modulation. However, anatomical descriptions of this pathway in primates are sketchy and conflicting. To determine whether the organization of the cerebellotectal projection in primates is similar to that found in other species, neuroanatomical tracer transport techniques were utilized in two species of macaque monkey to label cerebellotectal somata and fiber terminations. Two pathways were found. One, the fastigiotectal pathway, is derived from cells in the caudal fastigial nucleus and projects bilaterally to the rostral end of the intermediate gray layer. The other pathway is derived from cells in the posterior interposed nucleus and the adjacent posterior wing of the dentate nucleus, and it terminates contralaterally throughout the ventral half of the intermediate gray and the deep gray layers. Both of these pathways terminate within the layers of the superior colliculus containing premotor, saccade-related neurons, but the differences in the distribution of their terminals and cells of origin suggest that these two pathways have different functions. Furthermore, the pattern of connections of these two pathways indicates that they do not function as a traditional feedback circuit. We suggest that the cerebellotectal pathways may instead modulate collicular activity in a more complex manner. For example, it may provide signals necessary for corrective saccades or for maintaining spatial registry between the different sensory representations supplied to the superior colliculus and its presaccadic output, which is organized into a motor map.

Vestibular and visual signals in the ventral paraflocculus of the cerebellum in rabbits

Extracellular microelectrode recordings from single cerebellar neurons were made in the ventral paraflocculus of anesthetized, paralyzed pigmented rabbits during vestibular and visual stimulation. The discharge of 6 out of 207 neurons was modulated during vestibular or visual stimulation. The activity of 5 neurons was modulated during vertical vestibular stimulation. The discharge of only one neuron was modulated exclusively during vertical optokinetic stimulation. The information from both the vestibular and visual systems which is received by the ventral paraflocculus appears to differ from that which is received by the flocculus in rabbits.

Cues and Control Strategies in Visually Guided Tracking

Detailed quantitative models are required to investigate the neurological basis of motor behavior. Previous studies of visually guided manual tracking have either identified a variety of control signals (cues) for planning tracking movements or analyzed how a single cue is used (i.e., one-tracking strategy). A systematic, quantitative analysis of the effects and interactions of cues in terms of human manual-tracking performance is presented here together with measurements of concomitant eye movements. These measurements help define the routes by which information reaches the CNS, and the analysis elucidates how the control signals are processed and combined. The results quantify not only the large improvement in performance observed when the target waveform being tracked is predictable but also the extent to which this improvement depends on the availability of current information about target movements and positional error. Target information is shown to provide short-term prediction independent of the error signals used in on-line negative feedback control.

The subarcuate fossa and cerebellum of extant primates: comparative study of a skull-brain interface

The subarcuate fossa of the petrosal bone houses the petrosal lobule of the cerebellar paraflocculus. Although the subarcuate fossa can be extensive, little is known about its relative size and distribution in primates. Studies indicate parafloccular involvement with cerebellar areas coordinating vestibular, visual, auditory, and locomotor systems. Hypotheses have proposed a role for the paraflocculus in vestibular-oculomotor integration, caudal muscle control, autonomic function, and visual-manual predation. This study examines the morphology and relative extent of the subarcuate fossa/petrosal lobule in a range of living primates. Methods include study of postmortem specimens representing nine mammalian orders, and qualification of the volume of the subarcuate fossa and endocranial cavity in 155 dry primate crania of 36 genera. Results show that, in mammals, the size and morphology of the petrosal lobule is directly related to that of the subarcuate fossa. Craniometric analysis shows that the ratio of subarcuate fossa volume to endocranial volume is largest in lemuriforms. The largest ratio is in Microcebus and Hapalemur. Lorisids show a significant reduction in the size of the subarcuate fossa to almost 50% below the lemuriform mean. Tarsius is near the lemuriform mean. Among platyrrhines, the ratio is high, but significantly reduced compared to lemuiforms. The highest platyrrhine ratio is seen in Ateles, the lowest in Saimiri and Alouatta. Atelids are significantly elevated compared to cebids. In cercopithecids, the fossa is significantly reduced compared to platyrrhines. The trend toward reduction of the cercopithecid fossa is most pronounced in Theropithecus and least evident in Presbytis. In hominoids, the fossa is present only in Hylobates. In great apes and humans, other than Gorilla, the petromastoid canal occupies a similar location to the subarcuate fossa of other primates, but is not homologous to it. Neither the subarcuate fossa nor the petromastoid canal are present in Gorilla. A graded reduction of the subarcuate fossa/petrosal lobule is evident among primates which evolved later in time. The relative size of this cerebellar lobule within primates may reflect size-related factors and/or degree of neocortical evolution as these relate to usage of a specific sensory-mediated locomotor behavior. The subarcuate fossa may serve as an indicator to the differentiation of the petrosal lobule of the paraflocculus in fossil forms.

Representations of the body surface by climbing fiber responses in the dorsal paraflocculus of the cat

Mechanical stimulation elicited climbing fiber responses in 22% of the 377 Purkinje cells isolated in the dorsal paraflocculus of cats anesthetized with sodium pentobarbital. The responsive units were not evenly distributed throughout the dorsal paraflocculus. In the posterior division, 37% (65/177) of the cells had climbing fiber responses elicited by tactile stimulation, whereas in the lateral and accessory divisions the units were mainly unresponsive to tactile stimulation. Eleven of the 174 cells were responsive to tactile stimulation in the lateral division and only one climbing fiber response was driven in the 26 cells isolated in the accessory division. The climbing fiber representation was mainly of the ipsilateral forelimb and hindlimb with only a few responses representing the face. About half of the hindlimb representation was encountered in a patch located in the most medial part of the posterior division. No consistent location was identified between animals for the rest of the representations, which were intermingled among unresponsive units. The receptive fields of the majority of the forepaw and hindpaw representation included areas of the lateral toes; only a few responses represented the medial parts of the paw.

Cerebellar involvement in the coordination control of the oculo-manual tracking system: effects of cerebellar dentate nucleus lesion

When the hand of the observer is used as a visual target, oculomotor performance evaluated in terms of tracking accuracy, delay and maximal ocular velocity is higher than when the subject tracks a visual target presented on a screen. The coordination control exerted by the motor system of the arm on the oculomotor system has two sources: the transfer of kinaesthetic information originating in the arm which increases the mutual coupling between the arm and the eyes and information from the arm movement efferent copy which synchronizes the motor activities of both subsystems (Gauthier et al. 1988; Gauthier and Mussa-Ivaldi 1988). We investigated the involvement of the cerebellum in coordination control during a visuo-oculo-manual tracking task. Experiments were conducted on baboons trained to track visual targets with the eyes and/or the hand. The role of the cerebellum was determined by comparing tracking performance defined in terms of delay, accuracy (position or velocity tracking errors) and maximal velocity, before and after lesioning the cerebellar dentate nucleus. Results showed that in the intact animal, ocular tracking was more saccadic when the monkey followed an external target than when it moved the target with its hand. After lesioning, eye-alone tracking of a visual target as well as eye-and-hand-tracking with the hand contralateral to the lesion was little if at all affected. Conversely, ocular tracking of the hand ipsilateral to the lesion side became more saccadic and the correlation between eye and hand movement decreased considerably while the delay between target and eyes increased. In normal animals, the delay between the eyes and the hand was close to zero, and maximal smooth pursuit velocity was around 100 degrees per second with close to unity gain; in eye-alone tracking the delay and maximal smooth pursuit velocity were 200 ms and 50 deg per second, respectively. After lesioning, delay and maximum velocity were respectively around 210 ms and 40 deg per second, that is close to the values measured in eye-alone tracking. Thus, after dentate lesioning, the oculomotor system was unable to use information from the motor system of the arm to enhance its performance. We conclude that the cerebellum is involved in the "coordination control" between the oculomotor and manual motor systems in visuo-oculo-manual tracking tasks.