Saccadic eye movements evoked by microstimulation of the fastigial nucleus of macaque monkeys

Closed-Loop Optogenetic Perturbation of Macaque Oculomotor Cerebellum: Evidence for an Internal Saccade Model

Internal models are essential for the production of accurate movements. The accuracy of saccadic eye movements is thought to be mediated by an internal model of oculomotor mechanics encoded in the cerebellum. The cerebellum may also be part of a feedback loop that predicts the displacement of the eyes and compares it to the desired displacement in real time to ensure that saccades land on target. To investigate the role of the cerebellum in these two aspects of saccade production, we delivered saccade-triggered light pulses to channelrhodopsin-2-expressing Purkinje cells in the oculomotor vermis (OMV) of two male macaque monkeys. Light pulses delivered during the acceleration phase of ipsiversive saccades slowed the deceleration phase. The long latency of these effects and their scaling with light pulse duration are consistent with an integration of neural signals at or downstream of the stimulation site. In contrast, light pulses delivered during contraversive saccades reduced saccade velocity at short latency and were followed by a compensatory reacceleration which caused gaze to land on or near the target. We conclude that the contribution of the OMV to saccade production depends on saccade direction; the ipsilateral OMV is part of a forward model that predicts eye displacement, whereas the contralateral OMV is part of an inverse model that creates the force required to move the eyes with optimal peak velocity for the intended displacement.

Visuomotor control in mice and primates

We conduct a comparative evaluation of the visual systems from the retina to the muscles of the mouse and the macaque monkey noting the differences and similarities between these two species. The topics covered include (1) visual-field overlap, (2) visual spatial resolution, (3) V1 cortical point-image [i.e., V1 tissue dedicated to analyzing a unit receptive field], (4) object versus motion encoding, (5) oculomotor range, (6) eye, head, and body movement coordination, and (7) neocortical and cerebellar function. We also discuss blindsight in rodents and primates which provides insights on how the neocortex mediates conscious vision in these species. This review is timely because the field of visuomotor neurophysiology is expanding beyond the macaque monkey to include the mouse; there is therefore a need for a comparative analysis between these two species on how the brain generates visuomotor responses.

Bilateral control of interceptive saccades: evidence from the ipsipulsion of vertical saccades after caudal fastigial inactivation

The caudal fastigial nuclei (cFN) are the output nuclei by which the medio-posterior cerebellum influences the production of saccades toward a visual target. On the basis of the organization of their efferences to the premotor burst neurons and the bilateral control of saccades, the hypothesis was proposed that the same unbalanced activity accounts for the dysmetria of all saccades during cFN unilateral inactivation, regardless of whether the saccade is horizontal, oblique, or vertical. We further tested this hypothesis by studying, in two head-restrained macaques, the effects of unilaterally inactivating the caudal fastigial nucleus on saccades toward a target moving vertically with a constant, increasing or decreasing speed. After local muscimol injection, vertical saccades were deviated horizontally toward the injected side with a magnitude that increased with saccade size. The ipsipulsion indeed depended on the tested target speed but not its instantaneous value because it did not increase (decrease) when the target accelerated (decelerated). By subtracting the effect on contralesional horizontal saccades from the effect on ipsilesional ones, we found that the net bilateral effect on horizontal saccades was strongly correlated with the effect on vertical saccades. We explain how this correlation corroborates the bilateral hypothesis and provide arguments against the suggestion that the instantaneous saccade velocity would somehow be "encoded" by the discharge of Purkinje cells in the oculomotor vermis.NEW & NOTEWORTHY Besides causing dysmetric horizontal saccades, unilateral inactivation of caudal fastigial nucleus causes an ipsipulsion of vertical saccades. This study is the first to quantitatively describe this ipsipulsion during saccades toward a moving target. By subtracting the effects on contralesional (hypometric) and ipsilesional (hypermetric) horizontal saccades, we find that this net bilateral effect is strongly correlated with the ipsipulsion of vertical saccades, corroborating the suggestion that a common disorder affects all saccades.

Cerebellar Control of Reach Kinematics for Endpoint Precision

The cerebellum is well appreciated to impart speed, smoothness, and precision to skilled movements such as reaching. How these functions are executed by the final output stage of the cerebellum, the cerebellar nuclei, remains unknown. Here, we identify a causal relationship between cerebellar output and mouse reach kinematics and show how that relationship is leveraged endogenously to enhance reach precision. Activity in the anterior interposed nucleus (IntA) was remarkably well aligned to reach endpoint, scaling with the magnitude of limb deceleration. Closed-loop optogenetic modulation of IntA activity, triggered on reach, supported a causal role for this activity in controlling reach velocity in real time. Relating endogenous neural variability to kinematic variability, we found that IntA endpoint activity is adaptively engaged relative to variations in initial reach velocity, supporting endpoint precision. Taken together, these results provide a framework for understanding the physiology and pathophysiology of the intermediate cerebellum during precise skilled movements.

The caudal fastigial nucleus and the steering of saccades toward a moving visual target

The caudal fastigial nuclei (cFN) are the output nuclei by which the medio-posterior cerebellum influences the production of visual saccades. We investigated in two head-restrained monkeys their contribution to the generation of interceptive saccades toward a target moving centrifugally by analyzing the consequences of a unilateral inactivation (10 injection sessions). We describe here the effects on saccades made toward a centrifugal target that moved along the horizontal meridian with a constant (10, 20, or 40°/s), increasing (from 0 to 40°/s over 600 ms), or decreasing (from 40 to 0°/s over 600 ms) speed. After muscimol injection, the monkeys were unable to foveate the current location of the moving target. The horizontal amplitude of interceptive saccades was reduced during contralesional target motions and hypermetric during ipsilesional ones. For both contralesional and ipsilesional saccades, the magnitude of dysmetria increased with target speed. However, the use of accelerating and decelerating targets revealed that the dependence of dysmetria upon target velocity was not due to the current velocity but to the required amplitude of saccade. We discuss these results in the framework of two hypotheses, the so-called "dual drive" and "bilateral" hypotheses. NEW & NOTEWORTHY Unilateral inactivation of the caudal fastigial nucleus impairs the accuracy of saccades toward a moving target. Like saccades toward a static target, interceptive saccades are hypometric when directed toward the contralesional side and hypermetric when they are ipsilesional. The dysmetria depends on target velocity, but the use of accelerating or decelerating targets reveals that velocity is not the crucial parameter. We extend the bilateral fastigial control of saccades and fixation to the production of interceptive saccades.

Cerebellar control of saccade dynamics: contribution of the fastigial oculomotor region

The fastigial oculomotor region is the output by which the medioposterior cerebellum influences the generation of saccades. Recent inactivation studies reported observations suggesting an involvement in their dynamics (velocity and duration). In this work, we tested this hypothesis in the head-restrained monkey with the electrical microstimulation technique. More specifically, we studied the influence of duration, frequency, and current on the saccades elicited by fastigial stimulation and starting from a central (straight ahead) position. The results show ipsilateral or contralateral saccades whose amplitude and dynamics depend on the stimulation parameters. The duration and amplitude of their horizontal component increase with the duration of stimulation up to a maximum amplitude. Varying the stimulation frequency mostly changes their latency and the peak velocity (for contralateral saccades). Current also influences the metrics and dynamics of saccades: the horizontal amplitude and peak velocity increase with the intensity, whereas the latency decreases. The changes in peak velocity and in latency observed in contralateral saccades are not correlated. Finally, we discovered that contralateral saccades can be evoked at sites eliciting ipsilateral saccades when the stimulation frequency is reduced. However, their onset is timed not with the onset but with the offset of stimulation. These results corroborate the hypothesis that the fastigial projections toward the pontomedullary reticular formation (PMRF) participate in steering the saccade, whereas the fastigiocollicular projections contribute to the bilateral control of visual fixation. We propose that the cerebellar influence on saccade generation involves recruiting neurons and controlling the size of the active population in the PMRF.

Top-down but not bottom-up visual scanning is affected in hereditary pure cerebellar ataxia

The aim of this study was to clarify the nature of visual processing deficits caused by cerebellar disorders. We studied the performance of two types of visual search (top-down visual scanning and bottom-up visual scanning) in 18 patients with pure cerebellar types of spinocerebellar degeneration (SCA6: 11; SCA31: 7). The gaze fixation position was recorded with an eye-tracking device while the subjects performed two visual search tasks in which they looked for a target Landolt figure among distractors. In the serial search task, the target was similar to the distractors and the subject had to search for the target by processing each item with top-down visual scanning. In the pop-out search task, the target and distractor were clearly discernible and the visual salience of the target allowed the subjects to detect it by bottom-up visual scanning. The saliency maps clearly showed that the serial search task required top-down visual attention and the pop-out search task required bottom-up visual attention. In the serial search task, the search time to detect the target was significantly longer in SCA patients than in normal subjects, whereas the search time in the pop-out search task was comparable between the two groups. These findings suggested that SCA patients cannot efficiently scan a target using a top-down attentional process, whereas scanning with a bottom-up attentional process is not affected. In the serial search task, the amplitude of saccades was significantly smaller in SCA patients than in normal subjects. The variability of saccade amplitude (saccadic dysmetria), number of re-fixations, and unstable fixation (nystagmus) were larger in SCA patients than in normal subjects, accounting for a substantial proportion of scattered fixations around the items. Saccadic dysmetria, re-fixation, and nystagmus may play important roles in the impaired top-down visual scanning in SCA, hampering precise visual processing of individual items.

N-acetylgalactosamine positive perineuronal nets in the saccade-related-part of the cerebellar fastigial nucleus do not maintain saccade gain

Perineuronal nets (PNNs) accumulate around neurons near the end of developmental critical periods. PNNs are structures of the extracellular matrix which surround synaptic contacts and contain chondroitin sulfate proteoglycans. Previous studies suggest that the chondroitin sulfate chains of PNNs inhibit synaptic plasticity and thereby help end critical periods. PNNs surround a high proportion of neurons in the cerebellar nuclei. These PNNs form during approximately the same time that movements achieve normal accuracy. It is possible that PNNs in the cerebellar nuclei inhibit plasticity to maintain the synaptic organization that produces those accurate movements.

We tested whether or not PNNs in a saccade-related part of the cerebellar nuclei maintain accurate saccade size by digesting a part of them in an adult monkey performing a task that changes saccade size (long term saccade adaptation). We use the enzyme Chondroitinase ABC to digest the glycosaminoglycan side chains of proteoglycans present in the majority of PNNs. We show that this manipulation does not result in faster, larger, or more persistent adaptation. Our result indicates that intact perineuronal nets around saccade-related neurons in the cerebellar nuclei are not important for maintaining long-term saccade gain.

Convergent synaptic inputs from the caudal fastigial nucleus and the superior colliculus onto pontine and pontomedullary reticulospinal neurons

The caudal fastigial nucleus (FN) is known to be related to the control of eye movements and projects mainly to the contralateral reticular nuclei where excitatory and inhibitory burst neurons for saccades exist [the caudal portion of the nucleus reticularis pontis caudalis (NRPc), and the rostral portion of the nucleus reticularis gigantocellularis (NRG) respectively]. However, the exact reticular neurons targeted by caudal fastigioreticular cells remain unknown. We tried to determine the target reticular neurons of the caudal FN and superior colliculus (SC) by recording intracellular potentials from neurons in the NRPc and NRG of anesthetized cats. Neurons in the rostral NRG received bilateral, monosynaptic excitation from the caudal FNs, with contralateral predominance. They also received strong monosynaptic excitation from the rostral and caudal contralateral SC, and disynaptic excitation from the rostral ipsilateral SC. These reticular neurons with caudal fastigial monosynaptic excitation were not activated antidromically from the contralateral abducens nucleus, but most of them were reticulospinal neurons (RSNs) that were activated antidromically from the cervical cord. RSNs in the caudal NRPc received very weak monosynaptic excitation from only the contralateral caudal FN, and received either monosynaptic excitation only from the contralateral caudal SC, or monosynaptic and disynaptic excitation from the contralateral caudal and ipsilateral rostral SC, respectively. These results suggest that the caudal FN helps to control also head movements via RSNs targeted by the SC, and these RSNs with SC topographic input play different functional roles in head movements.

Short-term saccadic adaptation in the macaque monkey: a binocular mechanism

Saccadic eye movements are rapid transfers of gaze between objects of interest. Their duration is too short for the visual system to be able to follow their progress in time. Adaptive mechanisms constantly recalibrate the saccadic responses by detecting how close the landings are to the selected targets. The double-step saccadic paradigm is a common method to simulate alterations in saccadic gain. While the subject is responding to a first target shift, a second shift is introduced in the middle of this movement, which masks it from visual detection. The error in landing introduced by the second shift is interpreted by the brain as an error in the programming of the initial response, with gradual gain changes aimed at compensating the apparent sensorimotor mismatch. A second shift applied dichoptically to only one eye introduces disconjugate landing errors between the two eyes. A monocular adaptive system would independently modify only the gain of the eye exposed to the second shift in order to reestablish binocular alignment. Our results support a binocular mechanism. A version-based saccadic adaptive process detects postsaccadic version errors and generates compensatory conjugate gain alterations. A vergence-based saccadic adaptive process detects postsaccadic disparity errors and generates corrective nonvisual disparity signals that are sent to the vergence system to regain binocularity. This results in striking dynamical similarities between visually driven combined saccade-vergence gaze transfers, where the disparity is given by the visual targets, and the double-step adaptive disconjugate responses, where an adaptive disparity signal is generated internally by the saccadic system.

Head-free gaze shifts provide further insights into the role of the medial cerebellum in the control of primate saccadic eye movements

This study examines how signals generated in the oculomotor cerebellum could be involved in the control of gaze shifts, which rapidly redirect the eyes from one object to another. Neurons in the caudal fastigial nucleus (cFN), the output of the oculomotor cerebellum, discharged when monkeys made horizontal head-unrestrained gaze shifts, composed of an eye saccade and a head movement. Eighty-seven percent of our neurons discharged a burst of spikes for both ipsiversive and contraversive gaze shifts. In both directions, burst end was much better timed with gaze end than was burst start with gaze start, was well correlated with eye end, and was poorly correlated with head end or the time of peak head velocity. Moreover, bursts accompanied all head-unrestrained gaze shifts whether the head moved or not. Therefore we conclude that the cFN is not part of the pathway that controls head movement. For contraversive gaze shifts, the early part of the burst was correlated with gaze acceleration. Thereafter, the burst of the neuronal population continued throughout the prolonged deceleration of large gaze shifts. For a majority of neurons, gaze duration was correlated with burst duration; for some, gaze amplitude was less well correlated with the number of spikes. Therefore we suggest that the population burst provides an acceleration boost for high acceleration (smaller) contraversive gaze shifts and helps maintain the drive required to extend the deceleration of large contraversive gaze shifts. In contrast, the ipsiversive population burst, which is less well correlated with gaze metrics but whose peak rate occurs before gaze end, seems responsible primarily for terminating the gaze shift.

Electrical microstimulation of the fastigial oculomotor region in the head-unrestrained monkey

It has been shown that inactivation of the caudal fastigial nucleus (cFN) by local injection of muscimol leads to inaccurate gaze shifts in the head-unrestrained monkey and that the gaze dysmetria is primarily due to changes in the horizontal amplitude of eye saccades in the orbit. Moreover, changes in the relationship between amplitude and duration are observed for only the eye saccades and not for the head movements. These results suggest that the cFN output primarily influences a neural network involved in moving the eyes in the orbit. The present study further tested this hypothesis by examining whether head movements could be evoked by electrical microstimulation of the saccade-related region in the cFN. Long stimulation trains (200-300 ms) evoked staircase gaze shifts that were ipsi- or contralateral, depending on the stimulated site. These gaze shifts were small in amplitude and were essentially accomplished by saccadic movements of the eyes. Head movements were observed in some sites but their amplitudes were very small (mean=2.4 degrees). The occurrence of head movements and their amplitude were not enhanced by increasing stimulation frequency or intensity. In several cases, electrically evoked gaze shifts exhibited an eye-head coupling that was different from that observed in visually triggered gaze shifts. This study provides additional observations suggesting that the saccade-related region in the cFN modulates the generation of eye movements and that the deep cerebellar output region involved in influencing head movements is located elsewhere.

Bilateral representation in the deep cerebellar nuclei

The cerebellum is normally assumed to represent ipsilateral movements. We tested this by making microelectrode penetrations into the deep cerebellar nuclei (mainly nucleus interpositus) of monkeys trained to perform a reach and grasp task with either hand. Following weak single electrical stimuli, many sites produced clear bilateral facilitation of multiple forelimb muscles. The short onset latencies, which were similar for each side, suggested that at least some of the muscle responses were mediated by descending tracts originating in the brainstem, rather than via the cerebral cortex. Additionally, cerebellar neurones modulated their discharge with both ipsilateral and contralateral movements. This was so, even when we carefully excluded contralateral trials with evidence of electromyogram modulation on the ipsilateral side. We conclude that the deep cerebellar nuclei have a bilateral movement representation, and relatively direct, powerful access to limb muscles on both sides of the body. This places the cerebellum in an ideal position to coordinate bilateral movements.

Head-unrestrained gaze shifts after muscimol injection in the caudal fastigial nucleus of the monkey

The effects of unilateral cFN inactivation on horizontal and vertical gaze shifts generated from a central target toward peripheral ones were tested in two head unrestrained monkeys. After muscimol injection, the eye component was hypermetric during ipsilesional gaze shifts, hypometric during contralesional ones and deviated toward the injected side during vertical gaze shifts. The ipsilesional gaze hypermetria increased with target eccentricity until approximately 24 degrees after which it diminished and became smaller than the hypermetria of the eye component. Contrary to eye saccades, the amplitude and peak velocity of which were enhanced, the amplitude and peak velocity of head movements were reduced during ipsilesional gaze shifts. These changes in head movement were not correlated with those affecting the eye saccades. Head movements were also delayed relative to the onset of eye saccades. The alterations in head movement and the faster eye saccades likely explained the reduced head contribution to the amplitude of ipsilesional gaze shifts. The contralesional gaze hypometria increased with target eccentricity and was associated with uncorrelated reductions in eye and head peak velocities. When compared with control movements of similar amplitude, contralesional eye saccades had lower peak velocity and longer duration. This slowing likely accounted for the increase in head contribution to the amplitude of contralesional gaze shifts. These data suggest different pathways for the fastigial control of eye and head components during gaze shifts. Saccade dysmetria was not compensated by appropriate changes in head contribution, raising the issue of the feedback control of movement accuracy during combined eye-head gaze shifts.

Visual search and eye movements in patients with chronic solvent-induced toxic encephalopathy

Various aspects of visual perception have been found to be impaired in patients with occupational chronic solvent-induced toxic encephalopathy (CSE). The purpose of the study was to characterise the changes in eye movements and visual search performance in CSE patients. We measured eye movements of 13 CSE patients and 22 healthy controls during dynamic visual search task by using a fast video eye tracker. The task was to search for and identify a target letter among numerals presented in a rectangular stimulus matrix (3x3-10x10 items). Threshold search time, i.e. the duration of stimulus presentation required for identifying the target with a given probability was determined by using a psychophysical staircase method. The visual search times of the CSE patients were clearly longer, and they needed considerably more eye fixations than healthy controls to find the target. Thus, their reduced performance in this task was mainly related to the reduction in the number of items which could be processed during a single eye fixation (perceptual span). This reduction probably reflects a limited capacity of visual attention, since visual acuity, contrast sensitivity, and the oculomotor saccade velocity were found to be normal. The results suggest that motor slowness or low-level visual factors do not explain the poor performance of CSE patients in visual search tasks. The results are also discussed with respect to the effects of education, and compared to the performance in the widely used neuropsychological Trail Making Test, which uses similar stimuli and requires visual search.

Sensorimotor transformation for visually guided saccades

Visually guided movements require the brain to perform a sensorimotor transformation. The key to understanding this transformation is to understand the different roles of the superior colliculus (SC) and cerebellum (CB). The SC has a three-layered structure. Cells in the top layer have visual, but not motor, responses. However, cells in the deeper layers have both visual and motor responses. Thus, for a long time it was thought that the SC encoded both the retinal location of a sensory stimulus and the desired change in eye movement needed to acquire it. However, copious evidence has accumulated that shows that the SC encodes only the retinal location of a visual target, and not the movement needed to foveate it. Thus, the information needed to make accurate movements must come from another part of the brain, which is proposed to be the cerebellum. Here it is shown how the cerebellum could perform the sensorimotor transformation.

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.

Cortico-cerebellar coherence during a precision grip task in the monkey

We studied the synchronization of single units in macaque deep cerebellar nuclei (DCN) with local field potentials (LFPs) in primary motor cortex (M1) bilaterally during performance of a precision grip task. Analysis was restricted to periods of steady holding, during which M1 oscillations are known to be strongest. Significant coherence between DCN units and M1 LFP oscillations bilaterally was seen at approximately 10-40 Hz (contralateral M1: 25/87 units; ipsilateral: 9/87 units). Averaged coherence between DCN units and contralateral M1 LFP showed a prominent approximately 17-Hz coherence peak and an average phase of approximately -pi/2 radians, implying that the DCN units fired around the time of maximal depolarization of M1 cells. The lack of a time delay between DCN and M1 activity suggests that the cerebellum and cortex may form a pair of phase coupled oscillators. Although coherence values were low (mean peak coherence, 0.018), we used a computational model to show that this probably resulted from the nonlinearity of spike generating mechanisms within the DCN. DCN unit discharge and DCN LFPs also showed significant coherence at approximately 10-40 Hz, with similarly low magnitude (mean peak coherence, 0.012). The average coherence phase was -2.5 radians for the 6- to 14-Hz range and -1.1 radians for the 17- to 41-Hz range, suggesting different frequency-specific underlying mechanisms. Finally, 4/40 pairs of simultaneously recorded DCN units showed a significant cross-correlation peak, and 16/40 pairs showed significant unit-unit coherence. The extensive oscillatory synchronization observed between cerebellum and motor cortex may have functional importance in sensorimotor processing.

Effect of pharmacological inactivation of nucleus reticularis tegmenti pontis on saccadic eye movements in the monkey

The superior colliculus (SC) provides signals for the generation of saccades via a direct pathway to the brain stem burst generator (BG). In addition, it sends saccade-related activity to the BG indirectly through the cerebellum via a relay in the nucleus reticularis tegmenti pontis (NRTP). Lesions of the oculomotor vermis, lobules VIc and VII, and inactivation of the caudal fastigial nucleus, the cerebellar output nucleus to which it projects, produce saccade dysmetria but have little effect on saccade peak velocity and duration. We expected similar deficits from inactivation of the NRTP. Instead, injections as small as 80 nl into the NRTP first slowed ipsiversive saccades and then gradually reduced their amplitudes. Postinjection saccades had slower peak velocities and longer durations than preinjection saccades with similar amplitudes. Contraversive saccades retained their normal kinematics. When the gains of ipsiversive saccades to 10 degrees target steps had fallen to their lowest values (0.28 +/- 0.19; mean +/- SD; n = 10 experiments), the gains of contraversive saccades to 10 degrees target steps had decreased very little (0.82 +/- 0.11). Eventually, ipsiversive saccades did not exceed 5 degrees , even to 20 degrees target steps. Moreover, these small remaining saccades apparently were made with considerable difficulty because their latencies increased substantially. When ipsiversive saccade gain was at its lowest, the gain and kinematics of vertical saccades to 10 degrees target steps exhibited inconsistent changes. We argue that our injections did not compromise the direct SC pathway. Therefore these data suggest that the cerebellar saccade pathway does not simply modulate BG activity but is required for horizontal saccades to occur at all.

Saccade Dysmetria during Functional Perturbation of the Caudal Fastigial Nucleus in the Monkey

The caudal fastigial nucleus (cFN) is the output nucleus by which the medioposterior cerebellum influences the brainstem saccade generator. In the monkey, inactivation of one cFN by local injection of muscimol impairs all saccades: ipsiversive saccades become hypermetric, contraversive saccades become hypometric, and saccades aimed at a target located in the upper or lower visual fields are biased horizontally toward the injected side. The pharmacological action of muscimol does not allow deficits that are presaccadic to be distinguished from those occurring during saccade execution. To determine the interval during which altered cFN activity affects saccade accuracy, we applied low-frequency electrical microstimulation (100 Hz for 100-300 ms) to the cFN of three monkeys while they were making saccades toward a flashed target. Similar to the effect of muscimol injection in cFN, low-frequency microstimulation biased all saccades toward the ipsilateral side. When the microstimulation was applied after target flash and before saccade onset, the ipsilateral bias was absent. However, when the stimulation was applied during the ongoing movement, the saccade trajectory was biased toward the stimulated side. The muscimol-like effect of the microstimulation suggests that the stimulation inhibits cFN activity, possibly by recruiting the inhibitory afferents from the cerebellar vermis (axons of Purkinje cells). Low-frequency microstimulation had to be applied during the saccade to bias its trajectory. These data suggest that the ipsilateral horizontal bias observed during muscimol inactivation results from an imbalance in the intrasaccadic activity between the two caudal fastigial nuclei.

Feed-forward associative learning for volitional movement control

One of the most difficult problems in motor learning is determining the source of a learning signal, sometimes called an error signal. This problem is hidden in the adaptations of simple reflexive movements by attributing its source to sensory organs. The feed-forward associative motor learning theory proposed here attributes the source to the movement system itself. When a subject performs a corrective movement after his primary movement, the proposed neural learning device learns to associate the primary motor command with the corrective motor command by using a place-coding system. In the subsequent trials, the primary movement will involve a correction due to the participation of this mechanism, thus resulting in better performance. The theory assumes three conditions, namely, that a motor center and the learning device share the same place-encoded motor information; the motor center issues a command and a learning signal simultaneously from the same unit; and a learning signal issued with a corrective command has a heterosynaptic interaction with the previous primary command. The cerebellum is a reasonable candidate for the device satisfying these conditions. The reaction time of a corrective movement, usually 100-300 ms, almost satisfies the coincidence condition for long-term depression of the granule-to-Purkinje synapses. As an application, this theory is demonstrated to account for behavioral results regarding saccadic adaptation.

Disturbed overt but normal covert shifts of attention in adult cerebellar patients

In an attempt to provide a common denominator for cognitive deficits observed in cerebellar patients, it has been suggested that they might be secondary to impaired control of attention, a 'dysmetria of attention', conceptually analogous to motor dysmetria. Albeit appealing and quite influential, the concept of attentional dysmetria as a consequence of cerebellar disease remains controversial. In an attempt to test this concept in a direct way, we compared the performance of patients with cerebellar disorders to that of normal controls on tasks requiring either overt or covert shifts of spatial attention. In the first experiment, visually guided saccades, i.e. overt shifts of spatial attention, were elicited. In the second experiment, covert shifts of attention were evoked by the need to discriminate the orientation of a Landolt C observed during controlled fixation and presented in the same locations as the saccade targets in the previous experiment. The allocation of attention was assessed by comparing acuity thresholds determined with and without spatial cueing. The patients exhibited dysmetric saccades as reflected by larger absolute position errors or a higher number of corrective saccades compared to controls. In contrast, the ability to shift attention covertly was unimpaired in the patients, as indicated by a robust improvement in visual acuity induced by spatial cueing which did not differ from the one observed in the controls and which was independent of the range of SOAs (stimulus onset asynchronies) tested. Finally, the individual amount of saccadic dysmetria did not correlate with the individual performance in the covert attentional paradigm. In summary, we conclude that the contributions of the cerebellum to attention are confined to overt manifestations based on goal-directed eye movements.

Saccade Dysmetria in Head-Unrestrained Gaze Shifts After Muscimol Inactivation of the Caudal Fastigial Nucleus in the Monkey

Lesions in the caudal fastigial nucleus (cFN) severely impair the accuracy of visually guided saccades in the head-restrained monkey. Is the saccade dysmetria a central perturbation in issuing commands for orienting gaze (eye in space) or is it a more peripheral impairment in generating oculomotor commands? This question was investigated in two head-unrestrained monkeys by analyzing the effect of inactivating one cFN on horizontal gaze shifts generated from a straight ahead fixation light-emitting diode (LED) toward a 40° eccentric target LED. After muscimol injections, when viewing the fixation LED, the starting position of the head was changed (ipsilesional and upward deviations). Ipsilesional gaze shifts were associated with a 24% increase in the eye saccade amplitude and a 58% reduction in the amplitude of the head contribution. Contralesional gaze shifts were associated with a decrease in the amplitude of both eye and head components (40 and 37% reduction, respectively). No correlation between the changes in the eye amplitude and in head contribution was observed. The amplitude of the complete head movement was decreased for ipsilesional movements (57% reduction) and unaffected for contralesional movements. For both ipsilesional and contralesional gaze shifts, the changes in eye saccade amplitude were strongly correlated with the changes in gaze amplitude and largely accounted for the gaze dysmetria. These results indicate a major role of cFN in the generation of appropriate saccadic oculomotor commands during head-unrestrained gaze shifts.

Deficits in saccades and fixation during muscimol inactivation of the caudal fastigial nucleus in the rhesus monkey

The caudal fastigial nucleus (cFN) is a major nucleus by which the cerebellum influences the accuracy of saccades. In head-restrained monkeys generating saccades from a fixation light-emitting diode (LED) toward a flashed target LED, we analyzed the effects of unilateral pharmacological inactivation of cFN on horizontal, vertical, and oblique saccades. When animals were viewing the fixation LED, usually after one or more correction saccades, the positions of the eyes were slightly offset in comparison with the positions maintained before the injection (average offset = 1.1 degrees). The offset was ipsilateral to the injected side and did not depend on the target location. The horizontal component of all ipsilesional saccades was hypermetric and associated with a 32-42% increase in the amplitude of the deceleration displacement without significant change in the amplitude of the acceleration displacement. The horizontal component of all contralesional saccades was hypometric and associated with a decrease in the peak velocity and in the acceleration amplitude (30-35% decrease) without significant change in the deceleration amplitude. The amplitude of vertical saccades was not systematically affected, but their trajectory was always deviated toward the injected side. They missed the target with an error that depended on saccade duration or amplitude. If any, the effects of muscimol injections on the vertical component of oblique saccades were very small. The changes in fixation and the dysmetria are both viewed as consequences of an impairment in the cFN bilateral influence on the burst neurons located in the left and right brain stem.

Control of saccadic eye movements and combined eye/head gaze shifts by the medio-posterior cerebellum

The cerebellar areas involved in the control of saccades have recently been identified in the medio-posterior cerebellum (MPC). Unit activity recordings, experimental lesions and electrical microstimulation of this region in cats and monkeys have provided a considerable amount of data and allowed the development of new computational models. In this paper, we review these data and concepts about cerebellar function, discuss their importance and limitations and suggest future directions for research. The anatomical data indicate that the MPC has more than one site of action in the visuo-oculomotor system. In contrast, most models emphasize the role of cerebellar connections with immediate pre-oculomotor circuits in the reticular formation, and only one recent model also incorporates the ascending projections of the MPC to the superior colliculus. A major challenge for future studies, in continuation with this initial attempt, is to determine whether the various cerebellar output pathways correspond to distinct contributions to the control of saccadic eye movements. Also, a series of recent studies in the cat have indicated a more general role of the MPC in the control of orienting movements in space, calling for an increasing effort to the study of the MPC in the production of head-unrestrained saccadic gaze shifts.

Distributed model of collicular and cerebellar function during saccades

How does the brain tell the eye where to go? Classical models of rapid eye movements are lumped control systems that compute analogs of physical signals such as desired eye displacement, instantaneous error, and motor drive. Components of these lumped models do not correspond well with anatomical and physiological data. We have developed a more brain-like, distributed model (called a neuromimetic model), in which the superior colliculus (SC) and cerebellum (CB) play novel roles, using information about the desired target and the movement context to generate saccades. It suggests that the SC is neither sensory nor motor; rather it encodes the desired sensory consequence of the saccade in retinotopic coordinates. It also suggests a non-computational scheme for motor control by the cerebellum, based on context learning and a novel spatial mechanism, the pilot map. The CB learns to use contextual information to initialize the pilot signal that will guide the saccade to its goal. The CB monitors feedback information to steer and stop the saccade, and thus replaces the classical notion of a displacement integrator. One consequence of this model is that no desired eye movement signal is encoded explicitly in the brain; rather it is distributed across activity in both the SC and CB. Another is that the transformation from spatially coded sensory information to temporally coded motor information is implicit in the velocity feedback loop around the CB. No explicit spatial-to-temporal transformation with a normalization step is needed.

Gaze shifts evoked by electrical stimulation of the superior colliculus in the head-unrestrained cat. II. Effect of muscimol inactivation of the caudal fastigial nucleus

The medioposterior cerebellum [vermian lobules VI and VII and caudal fastigial nucleus (cFN)] is known to play a major role in the control of saccadic gaze shifts toward a visual target. To determine the relative contribution of the cFN efferent pathways to the brainstem reticular formation and to the superior colliculus (SC), we recorded in the head-unrestrained cat the effects of cFN unilateral inactivation on gaze shifts evoked by electrical microstimulation of the deeper SC layers. Gaze shifts evoked after muscimol injection still exhibited the typical qualitative features of normal saccadic gaze shifts. Nevertheless, consistent modifications in amplitude and latency were observed. For ipsiversive movements (evoked by the SC contralateral to the inactivated cFN), these changes depended on the locus of stimulation on the motor map: for the anterior 2/3 of the SC, amplitude increased and latency tended to decrease; for the posterior 1/3 of the SC, amplitude decreased and latency increased. For the contraversive direction, amplitude moderately decreased and latency tended to increase for all but the caudal-most stimulated SC site. These modifications of SC-evoked gaze shifts during cFN inactivation differed from the ipsiversive hypermetria/contraversive hypometria pattern observed for visually triggered gaze shifts recorded during the same recording sessions. We conclude that (i) the topographical organization of gaze shift amplitude in the deeper SC layers is influenced by the cerebellum and is either severely distorted or demonstrates an amplitude reduction during inactivation of the contralateral or ipsilateral cFN, respectively; (ii) gaze shifts evoked by SC microstimulation and visually triggered gaze shifts either rely on distinct cerebellar-dependent control processes or differ by the location of the caudal-most active SC population. We present a functional scheme providing several predictions regarding the modulatory influence of the cerebellum on SC neuronal activities and on the topographical organization of fastigial-SC projections.

The role of the cerebellum in voluntary eye movements

In general the cerebellum is crucial for the control but not the initiation of movement. Voluntary eye movements are particularly useful for investigating the specific mechanisms underlying cerebellar control because they are precise and their brain-stem circuitry is already well understood. Here we describe single-unit and inactivation data showing that the posterior vermis and the caudal fastigial nucleus, to which it projects, provide a signal during horizontal saccades to make them fast, accurate, and consistent. The caudal fastigial nucleus also is necessary for the recovery of saccadic accuracy after actual or simulated neural or muscular damage causes horizontal saccades to be dysmetric. Saccade-related activity in the interpositus nucleus is related to vertical saccades. Both the caudal fastigial nucleus and the flocculus/paraflocculus are necessary for the normal smooth eye movements that pursue a small moving spot. By using eye movements, we have begun to uncover basic principles that give us insight into how the cerebellum may control movement in general.

Effects of lesions of the oculomotor vermis on eye movements in primate: saccades

We studied the effects on saccades of ablation of the dorsal cerebellar vermis (lesions centered on lobules VI and VII) in three monkeys in which the deep cerebellar nuclei were spared. One animal, with a symmetrical lesion, showed bilateral hypometric horizontal saccades. Two animals, with asymmetrical lesions, showed hypometric ipsilateral saccades, and saccades to vertically positioned targets were misdirected, usually deviating away from the side to which horizontal saccades were hypometric. Postlesion, all animals showed an increase (2- to 5-fold) in trial-to-trial variability of saccade amplitude. They also showed a change in the ratio of the amplitudes of centripetal to centrifugal saccades (orbital-position effect); usually centrifugal saccades became smaller. In the two animals with asymmetrical lesions, for saccades in the hypometric direction, latencies were markedly increased (up to approximately 500 ms). There was also an absence of express and anticipatory saccades in the hypometric direction. When overall saccade latency was increased, centrifugal saccades became relatively more delayed than centripetal saccades. The dynamic characteristics of saccades were affected to some extent in all monkeys with changes in peak velocity, eye acceleration, and especially eye deceleration. There was relatively little effect of orbital position on saccade dynamics, however, with the exception of one animal that showed an orbital position effect for eye acceleration. In a double-step adaptation paradigm, animals showed an impaired ability to adaptively adjust saccade amplitude, though increased amplitude variability postlesion may have played a role in this deficit. During a single training session, however, the latency to corrective saccades-which had been increased postlesion-gradually decreased and so enabled the animal to reach the final position of the target more quickly. Overall, both in the early postlesion period and during recovery, changes in saccade amplitude and latency tended to vary together but not with changes in saccade dynamics or adaptive capability, both of which behaved relatively independently. These findings suggest that the cerebellum can adjust saccade amplitude and saccade dynamics independently. Our results implicate the cerebellar vermis directly in every aspect of the on-line control of saccades: initiation (latency), accuracy (amplitude and direction), and dynamics (velocity and acceleration) and also in the acquisition of adaptive ocular motor behavior.

Contribution of the rostral fastigial nucleus to the control of orienting gaze shifts in the head-unrestrained cat

The implication of the caudal part of the fastigial nucleus (cFN) in the control of saccadic shifts of the visual axis is now well established. In contrast a possible involvement of the rostral part of the fastigial nuceus (rFN) remains unknown. In the current study we investigated in the head-unrestrained cat the contribution of the rFN to the control of visually triggered saccadic gaze shifts by measuring the deficits after unilateral muscimol injection in the rFN. A typical gaze dysmetria was observed: gaze saccades directed toward the inactivated side were hypermetric, whereas those with an opposite direction were hypometric. For both movement directions, gaze dysmetria was proportional to target retinal eccentricity and could be described as a modified gain in the translation of visual signals into eye and head motor commands. Correction saccades were triggered when the target remained visible and reduced the gaze fixation error to 2.7 +/- 1.3 degrees (mean +/- SD) on average. The hypermetria of ipsiversive gaze shifts resulted predominantly from a hypermetric response of the eyes, whereas the hypometria of contraversive gaze shifts resulted from hypometric responses of both eye and head. However, even in this latter case, the eye saccade was more affected than the motion of the head. As a consequence, for both directions of gaze shift the relative contributions of the eye and head to the overall gaze displacement were altered by muscimol injection. This was revealed by a decreased contribution of the head for ipsiversive gaze shifts and an increased head contribution for contraversive movements. These modifications were associated with slight changes in the delay between eye and head movement onsets. Inactivation of the rFN also affected the initiation of eye and head movements. Indeed, the latency of ipsiversive gaze and head movements decreased to 88 and 92% of normal, respectively, whereas the latency of contraversive ones increased to 149 and 145%. The deficits induced by rFN inactivation were then compared with those obtained after muscimol injection in the cFN of the same animals. Several deficits differed according to the site of injection within the fastigial nucleus (tonic orbital eye rotation, hypermetria of ipsiversive gaze shifts and fixation offset, relationship between dysmetria and latency of contraversive gaze shifts, postural deficit). In conclusion, the present study demonstrates that the rFN is involved in the initiation and the control of combined eye-head gaze shifts. In addition our findings support a functional distinction between the rFN and cFN for the control of orienting gaze shifts. This distinction is discussed with respect to the segregated fastigiofugal projections arising from the rFN and cFN.

Orienting gaze shifts during muscimol inactivation of caudal fastigial nucleus in the cat. II. Dynamics and eye-head coupling

We have shown in the companion paper that muscimol injection in the caudal part of the fastigial nucleus (cFN) consistently leads to dysmetria of visually triggered gaze shifts that depends on movement direction. Based on the observations of a constant error and misdirected movements toward the inactivated side, we have proposed that the cFN contributes to the specification of the goal of the impending ipsiversive gaze shift. To test this hypothesis and also to better define the nature of the hypometria that affects contraversive gaze shifts, we report in this paper on various aspects of movement dynamics and of eye/head coordination patterns. Unilateral muscimol injection in cFN leads to a slight modification in the dynamics of both ipsiversive and contraversive gaze shifts (average velocity decrease = 55 degrees/s). This slowing in gaze displacements results from changes in both eye and head. In some experiments, a larger gaze velocity decrease is observed for ipsiversive gaze shifts as compared with contraversive ones, and this change is restricted to the deceleration phase. For two particular experiments testing the effect of visual feedback, we have observed a dramatic decrease in the velocity of ipsiversive gaze shifts after the animal had received visual information about its inaccurate gaze responses; but virtually no change in hypermetria was noted. These observations suggest that there is no obvious causal relationship between changes in dynamics and in accuracy of gaze shifts after muscimol injection in the cFN. Eye and head both contribute to the dysmetria of gaze. Indeed, muscimol injection leads to parallel changes in amplitude of both ocular and cephalic components. As a global result, the relative contribution of eye and head to the amplitude of ipsiversive gaze shifts remains statistically indistinguishable from that of control responses, and a small (1.6 degrees) increase in the head contribution to contraversive gaze shifts is found. The delay between eye and head movement onsets is increased by 7.3 +/- 7.4 ms for contraversive and decreased by 8.3 +/- 10.1 ms for ipsiversive gaze shifts, corresponding respectively to an increased or decreased lead time of head movement initiation. The modest changes in gaze dynamics, the absence of a link between eventual dynamics changes and dysmetria, and a similar pattern of eye-head coordination to that of control responses, altogether are compatible with the hypothesis that the hypermetria of ipsiversive gaze shifts results from an impaired specification of the metrics of the impending gaze shift. Regarding contraversive gaze shifts, the weak changes in head contribution do not seem to reflect a pathological coordination between eye and head but would rather result from the tonic deviations of gaze and head toward the inactivated side. Hence, our data suggest that the hypometria of contraversive gaze shifts also might result largely from an alteration of processes that specify the goal rather than the on-going trajectory, of saccadic gaze shifts.

Orienting gaze shifts during muscimol inactivation of caudal fastigial nucleus in the cat. I. Gaze dysmetria

The cerebellar control of orienting behavior toward visual targets was studied in the head-unrestrained cat by analyzing the deficits of saccadic gaze shifts after unilateral injection of muscimol in the caudal part of the fastigial nucleus (cFN). Gaze shifts are rendered strongly inaccurate by muscimol cFN inactivation. The characteristics of gaze dysmetria are specific to the direction of the movement with respect to the inactivated cFN. Gaze shifts directed toward the injected side are hypermetric. Irrespective of their starting position, all these ipsiversive gaze shifts overshoot the target by a constant horizontal error (or bias) to terminate at a "shifted goal" location. In particular, when gaze is directed initially at the future target's location, a response with an amplitude corresponding to the bias moves gaze away from the actual target. Additionally, when gaze is initially in between the target and this shifted goal location, the response again is directed toward the latter. This deficit of ipsiversive gaze shifts is characterized by a consistent increase in the y intercept of the relationship between horizontal gaze amplitude and horizontal retinal error. Slight increases in the slope sometimes are observed as well. Contraversive gaze shifts are markedly hypometric and, in contrast to ipsiversive responses, they do not converge onto a shifted goal but rather underestimate target eccentricity in a proportional way. This is reflected by a decrease in the slope of the relationship between horizontal gaze amplitude and horizontal retinal error, with, for some experiments, a moderate change in the y-intercept value. The same deficits are observed in a different setup, which permits the control of initial gaze position. Correction saccades rarely are observed when visual feedback is eliminated on initiation of the primary orienting response; instead, they occur frequently when the target remains visible. Like the primary contraversive saccades, they are hypometric and the ever-decreasing series of three to five correction saccades reduces the gaze fixation error but often does not completely eliminate it. We measured the position of gaze after the final correction saccade and found that fixation of a visible target is still shifted toward the inactivated cFN by 4.9 +/- 2.4 degrees. This fixation offset is correlated to, but on average 54% smaller than, the hypermetric bias of ipsiversive responses measured in the same experiments. In conclusion, the cFN contributes to the control of saccadic shifts of the visual axis toward a visual target. The hypometria of contraversive gaze shifts suggests a cFN role in adjusting a gain in the translation of retinal signals into gaze motor commands. On the basis of the convergence of ipsiversive gaze shifts onto a shifted goal, the straightness of gaze trajectory during these responses and the production of misdirected or inappropriately initiated responses toward this shifted goal, we propose that the cFN influences the processes that specify the goal of ipsiversive gaze shifts.

Neurons in the posterior interposed nucleus of the cerebellum related to vergence and accommodation. I. Steady-state characteristics

The present study used single-unit recording and electrical microstimulation techniques in alert, trained rhesus monkeys to examine the involvement of the posterior interposed nucleus (IP) of the cerebellum in vergence and accommodative eye movements. Neurons related to vergence and ocular accommodation were encountered within a circumscribed region of the IP and their activity during changes in viewing distance was characterized. The activity of these neurons increased with decreases in vergence angle and accommodation (the far-response) but none showed changes in activity during changes in conjugate eye position and we therefore term them "far-response neurons." Far-response neurons were found within a restricted region of the IP that extended approximately 1 mm rostrocaudally and mediolaterally and 2 mm dorsal to the fourth ventricle. Microstimulation of this far-response region of the IP with low currents (<30 microA) often elicited divergence and accommodation for far. The behavior of 37 IP far-response neurons was examined during normal binocular viewing, during monocular viewing (blur cue alone), and during binocular viewing with accommodation open-loop (disparity cue alone). The activity of all cells was modulated under all three conditions. However, the change in activity of some of these neurons was significantly different under these three viewing conditions. The behavior of 70 IP far-response neurons was compared during normal binocular viewing and during viewing in which the accommodative response was significantly dissociated from the vergence response. The data from these two conditions was pooled and multiple regression analyses for each neuron generated two coefficients expressing the activity of the neuron relative to the vergence and to accommodative response respectively. On the basis of these coefficients, the overall activity of the neurons were classified as follows: 34 positively correlated with divergence, 11 positively correlated with far accommodation, 14 positively correlated with divergence and far accommodation, 9 positively correlated with divergence and accommodation, and 2 positively correlated with convergence and far accommodation. The results of this study demonstrate the involvement of a specific region of the posterior interposed nucleus of the cerebellum in vergence and accommodation. IP far-response neurons are active for vergence and accommodation irrespective of whether or not these eye movements are elicited by blur or disparity cues. The data in the present study strongly suggest that this cerebellar region is a far-response region that is involved in vergence as well as accommodative eye movements.

Changes in initiation of orienting gaze shifts after muscimol inactivation of the caudal fastigial nucleus in the cat

1. The production of a goal-directed saccadic gaze shift involves the specification of movement amplitude and direction, and the decision to trigger the movement. Behavioural and neurophysiological data suggest that these two functions involve separate processes which may interact. 2. The medio-posterior cerebellar areas are classically assigned a major contribution to the control of saccade metrics, and previous cerebellar lesion studies have revealed marked dysmetria of visually triggered gaze shifts. In contrast, these studies did not provide evidence for a cerebellar role in saccadic initiation. 3. In the present study, we investigated in the head-unrestrained cat the deficits in both the initiation and the metrics control of saccadic gaze shifts following pharmacological inactivation of the caudal part of the fastigial nucleus (cFN). 4. After cFN inactivation, latencies for contraversive gaze shifts increased to about 137 +/- 28% of normal, and latencies for ipsiversive gaze shifts decreased to about 84 +/- 8% of normal. Similar changes in head movement latency were observed, such that the temporal coupling between eye and head components remained largely unaffected. 5. Contraversive gaze shifts were more hypometric as their latency increased. In contrast, the degree of hypermetria in ipsiversive gaze shifts was unrelated to latency. 6. These results suggest a functional role of the medio-posterior cerebellum in gaze shift initiation and in storing information about the target location and/or the desired gaze shift amplitude.

Prearcuate cortex in the Cebus monkey has cortical and subcortical connections like the macaque frontal eye field and projects to fastigial-recipient oculomotor-related brainstem nuclei

The cortical and subcortical connections of the prearcuate cortex were studied in capuchin monkeys (Cebus apella, albifrons) using the anterograde and retrograde transport capabilities of the horseradish peroxidase technique. The findings demonstrate remarkable similarities to those of the macaque frontal eye field and strongly support their homology. The report then focuses on specific prearcuate projections to oculomotor-related brainstem nuclei that were shown in a companion experiment to entertain connections with the caudal oculomotor portion of the cerebellar fastigial nucleus. The principal corticocortical connections of the cebus prearcuate cortex were with dorsomedial prefrontal cortex, lateral intraparietal sulcal cortex, posterior medial parietal cortex, and superior temporal sulcal cortex, which were for the most part reciprocal and columnar in organization. The connections of the dorsal prearcuate region were heavier to the dorsomedial prefrontal and posterior medial parietal cortices, and those of the ventral region were heavier to the superior temporal sulcal cortex. The prearcuate cortex projects to several brainstem areas which also receive projections from the caudal fastigial nucleus, including the supraoculomotor periaqueductal gray matter, superior colliculus, medial nucleus reticularis tegmenti pontis, dorsomedial basilar pontine nucleus, dorsolateral basilar pontine nucleus, nucleus reticularis pontis caudalis, pontine raphe, and nucleus prepositus hypoglossi. The findings define a neuroanatomical framework within which convergence of prearcuate (putative frontal eye field) and caudal fastigial nucleus connections might occur, facilitating their potential interaction in saccadic and smooth pursuit eye movement.

Role of the cerebellum in movement control and adaptation

Three recent discoveries have substantially improved our knowledge of cerebellar function. First, the forelimb regions of the interpositus nuclei specialize in control of one particular limb movement, reach to grasp. Second, a new model indicates that vestibulo-ocular reflex adaptation requires neural changes in both the cerebellum and the brainstem. Finally, the caudal fastigial nucleus uses both short- and long-term influences to maintain saccade accuracy.

Modelling the role of the cerebellar fastigial nuclei in producing accurate saccades: the importance of burst timing

Clinical and experimental data indicate that damage to the cerebellar vermis results in permanent loss of saccadic accuracy. Models of saccade production therefore need to provide a role for the cerebellum. It has been proposed that the vermis adjusts the gain of the saccadic internal feedback loop in response to information about the amplitude of the intended saccade. A model of how the fastigial nuclei (through which vermal output is channelled) influence brainstem saccadic circuitry to achieve this effect was constructed in three stages. (1) The brainstem was represented by a version of Robinson's internal feedback model, which relates excitatory burst neuron discharge to horizontal saccade dynamics. (2) The original model was lesioned to simulate the effects of bilateral inactivation of the fastigial nuclei, namely slow hypermetric saccades. This required reducing the synaptic weight of the internal feedback pathway, and lowering the gain of the excitatory burst neurons. The resultant brainstem-only model served as a preparation for testing the effects of neuronal discharge patterns within the fastigial nuclei. (3) These discharge patterns were simulated using measurements from recent electrophysiological studies. It was found that saccadic accuracy and normal dynamics were restored in the model if the simulated burst from neurons in the contralateral fastigial nucleus were subtracted from the feedback signal (i.e. added to the command signal) early in the saccade, and the burst from neurons in the ipsilateral fastigial nucleus were added to the feedback signal later in the saccade. This pattern corresponds to the observed timing of neuronal bursts in the fastigial nuclei, and accounts qualitatively for the effects of unilateral stimulation and inactivation of both the fastigial nuclei and the cerebellar vermis. This method of producing accurate saccades also contributes to time optimal control, by increasing both saccadic acceleration and deceleration. Appropriate timing of burst onset and duration in the fastigial nuclei is essential for these roles. Evidence concerning the effects of cerebellar damage on fast movements of other parts of the body suggests that the cerebellum may use similar strategies for controlling a wide range of simple movements.

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)

The tectorecipient zone in the inferior olivary nucleus in the rat

Tecto-olivary and olivocerebellar projections in the rat were investigated in order to identify the tectorecipient zone in the inferior olivary nucleus and to determine whether inferior olivary neurons projecting to the cerebellar tecto-olivo-recipient zones (lobule VII, crus II, lobulus simplex, and paramedian lobule) originate in the different regions within the tectorecipient zone. An electrophysiological method and an axonal transport technique of wheat-germ agglutinin-conjugated horseradish peroxidase were used. The tectorecipient zone was identified in the caudomedial region of the medial accessory olive. Neurons projecting to lobule VII originated in the caudomedial region of the tectorecipient zone, but those to crus II, lobulus simplex, and paramedian lobule originated in its rostrolateral region. These observations suggest that there are two independent tecto-olivo-cerebellar systems: 1) superior colliculus--the medial region of the tectorecipient zone--lobule VII--the caudomedial region of the fastigial nucleus; and 2) superior colliculus--the rostralateral region of the tectorecipient region--crus II, lobulus simplex, and paramedian lobule--the dorsolateral protuberance of the fastigial nucleus.

Fastigiofugal fibers encoding horizontal and vertical components of saccades as determined by microstimulation in monkeys

To identify the routes by which oculomotor vermis signals control eye movements (saccadic signals), saccades evoked by microstimulation were studied in the region of the uncinate fasciculus (UF) and juxtarestiform body (JB) in the macaque monkey. Anatomical pathways of axons from the fastigial oculomotor region (FOR) were studied by anterograde transport of wheatgerm agglutinin conjugated horseradish peroxidase (WGA-HRP). The routes were identified by comparing maps of low threshold for evoking saccades with the anatomical map of anterogradely labeled axons arising from the FOR. Microstimulation of a region of the UF and JB demonstrated that saccadic signals are carried exclusively by decussated FOR axons which leave the cerebellum via the contralateral UF. The fibers in the JB do not carry saccadic signals. The horizontal component of saccadic signals is conveyed by fibers in the descending limb of the UF, while the vertical component is conveyed by a smaller group of fibers which separate from the UF and enter the midbrain with the contralateral superior cerebellar peduncle.

Saccadic dysmetria induced by transient functional decortication of the cerebellar vermis [corrected]

Saccadic dysmetria is seen in patients with cerebellar diseases as well as in monkeys whose vermis and fastigial nucleus are experimentally lesioned. We investigated the oculomotor signs of the vermis by blocking cerebellar impulses with bicuculline injections into the fastigial nucleus. The oculomotor abnormalities associated with the bicuculline treatment (in effect, functional as well as reversible, unilateral decortication of the vermis) were hypometric saccades toward the injection side and gaze deviation toward the opposite side. These oculomotor signs disappeared with the withdrawal of the bicuculline effect.

Divergent axon collaterals from fastigial oculomotor region to mesodiencephalic junction and paramedian pontine reticular formation in macaques

Collateralization of efferent fibers from the fastigial oculomotor region (FOR) to the paramedian pontine reticular formation (PPRF) and the mesodiencephalic junction (MDJ) was studied in macaque monkeys using a fluorescent double-labeling technique. Retrogradely-labeled neurons in the contralateral FOR were examined following injections of fast blue (FB) into the MDJ and diamidino yellow (DY) into the PPRF, or vice versa. Some FOR neurons were labeled with FB, while some other FOR neurons were labeled with DY and intermingled within the FOR. While single-labeled cells in the FOR projected either to the PPRF or to the MDJ, the presence of double-labeled cells indicated that the FOR contains neurons whose axons collateralize to project to both the MDJ and PPRF. These are regarded as the preoculomotor nuclei responsible for vertical and horizontal saccades, respectively.

Effects of fastigial stimulation upon visually-directed saccades in macaque monkeys

When the initial eye position was changed by stimulating the fastigial nucleus prior to visually-directed saccades, monkeys could not compensate for the stimulation-induced movement. The line of sight missed the target location by a distance and direction almost equal to the vector of the evoked saccade. When the stimulation was delivered 75-130 ms after the target presentation, saccades were triggered prematurely. Their initial movement reflected only the evoked saccade in some response and reflected the vector sum of the evoked and visually-directed saccades in the other. In contrast, when stimulus latencies were greater than 130 ms. saccades started toward the target location. The visuomotor processing for saccades seemed to be completed during this period, which is approximately half the latency of normal saccades. When the stimulation was applied while the eyes were already in motion, the trajectories of the saccades were strongly modified and terminated in deviated locations. These results indicate that cerebellar output impulses are projected downstream to saccade-programming circuits where visual information has already been converted into motor-command signals.

Pathways and terminations of axons arising in the fastigial oculomotor region of macaque monkeys

The majority of axons from the fastigial oculomotor region (FOR) decussated in the cerebellum at all rostrocaudal levels of the fastigial nucleus (FN) and entered the brainstem via the contralateral uncinate fasciculus (UF). Some decussated axons separated from the UF and ran medial to the contralateral superior cerebellar peduncle and ascended to the midbrain. Uncrossed FOR axons advanced rostrolaterally in the ipsilateral FN and entered the brainstem via the juxtarestiform body. The decussated fibers terminated in the brainstem nuclei that are implicated in the control of saccadic eye movements. In the midbrain, labeled terminals were found in the rostral interstitial nucleus of the medial longitudinal fasciculus, a medial part of Forel's H-field, the periaqueductal gray, the posterior commissure nucleus, and the superior colliculus of the contralateral side. In the pons and medulla, FOR fibers terminated in a caudal part of the pontine raphe, the paramedian pontine reticular formation, the nucleus reticularis tegmenti pontis, the dorsomedial pontine nucleus of the contralateral side, and the dorsomedial medullary reticular formation of both sides. In contrast, FOR projections to the vestibular complex were bilateral and were mainly to the ventral portions of the lateral and inferior vestibular nuclei. No labeled terminals were found in the following brainstem nuclei which are considered to be involved in oculomotor function: oculomotor and trochlear nuclei, interstitial nucleus of Cajal, medial and superior vestibular nuclei, periphypoglossal nuclei, and dorsolateral pontine nucleus. Labeling appeared in the red nucleus only when HRP encroached upon the posterior interposed nucleus.

Converging eye movements evoked by microstimulation of the fastigial nucleus of macaque monkeys

Stimulation of the dorsal part of the fastigial nucleus in macaques is known to evoke ipsilateral saccades, while those of the ventral part produce contralateral saccades. It was found that stimulation of the transitional zone moved the visual axis (eyes) to converge at an area (focus) in the oculomotor range, regardless of the initial eye position. These saccades were designated as 'converging saccades'. Converging saccades were directed to the focus, but the eyes did not attain the focus in one motion. Only the repetition of stimuli brought the eyes near the focus. When stimulation is applied to the transitional zone at progressively more ventral sites, the focus gradually shifted, starting from the ipsilateral hemifield to the contralateral hemifield by taking various routes.