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

Divergent topographic projection of cerebral cortical areas to overlapping cerebellar lobules through distinct regions of the pontine nuclei

The massive axonal projection from the cerebrum to the cerebellum through the pontine nuclei supports the cerebrocerebellar coordination of motor and nonmotor functions. However, the cerebrum and cerebellum have distinct patterns of functional localization in their cortices. We addressed this issue by bidirectional neuronal tracing from 22 various locations of the pontine nuclei in the mouse in a comprehensive manner. Cluster analyses of the distribution patterns of labeled cortical pyramidal cells and cerebellar mossy fiber terminals classified all cases into six groups located in six different subareas of the pontine nuclei. The lateral (insular), mediorostral (cingulate and prefrontal), and caudal (visual and auditory) cortical areas of the cerebrum projected to the medial, rostral, and lateral subareas of the pontine nuclei, respectively. These pontine subareas then projected mainly to the crus I, central vermis, and paraflocculus divergently. The central (motor and somatosensory) cortical areas projected to the centrorostral, centrocaudal and caudal subareas of the pontine nuclei, which then projected mainly to the rostral and caudal lobules with a somatotopic arrangement. The results indicate a new pontine nuclei-centric view of the corticopontocerebellar projection: the generally parallel corticopontine projection to pontine nuclei subareas is relayed to the highly divergent pontocerebellar projection terminating in overlapping specific lobules of the cerebellum. Consequently, the mode of the pontine nuclei relay underlies the cerebellar functional organization.

The frequency and characteristics of saccadic dysmetria in isolated cerebellar infarction

OBJECTIVES

To investigate the frequency and pattern of horizontal saccadic dysmetria in unilateral cerebellar infarction and identify the responsible region for horizontal saccadic dysmetria.

METHODS

From the acute stroke registry of Keimyung University Dongsan Medical Center between July 2016 and October 2020, 43 patients with acute unilateral cerebellar infarction were enrolled. Eye movements were recorded during the acute period and the lesion was mapped using MRIcron software for subtraction analysis. Saccadic dysmetria was marked as hypometric when the gain is < 0.85 and hypermetric when > 1.0.

RESULTS

Among the 43 participants, 30 patients (69.8%) demonstrated saccadic dysmetria. The age was significantly higher in patients with dysmetria (66.87 ± 12.82 vs. 53.54 ± 14.09, p = 0.004). Type of dysmetria showed a significant difference according to the vascular territory of the lesion. The posterior inferior cerebellar artery (PICA) infarction group presented ipsiversive saccadic dysmetria, while the superior cerebellar artery (SCA) group showed contraversive dysmetria (p < 0.001). In the SCA group, the culmen, fastigium, and dentate were the most frequently damaged regions, while the tonsil and inferior semilunar lobule were in the PICA group.

CONCLUSION

Saccadic dysmetria was observed in a large proportion of cerebellar stroke patients, and the types of saccades were distinctive according to the vascular territory of the lesion.

Purkinje Cell Activity in the Medial and Lateral Cerebellum During Suppression of Voluntary Eye Movements in Rhesus Macaques

Volitional suppression of responses to distracting external stimuli enables us to achieve our goals. This volitional inhibition of a specific behavior is supposed to be mainly mediated by the cerebral cortex. However, recent evidence supports the involvement of the cerebellum in this process. It is currently not known whether different parts of the cerebellar cortex play differential or synergistic roles in the planning and execution of this behavior. Here, we measured Purkinje cell (PC) responses in the medial and lateral cerebellum in two rhesus macaques during pro- and anti-saccade tasks. During an antisaccade trial, non-human primates (NHPs) were instructed to make a saccadic eye movement away from a target, rather than toward it, as in prosaccade trials. Our data show that the cerebellum plays an important role not only during the execution of the saccades but also during the volitional inhibition of eye movements toward the target. Simple spike (SS) modulation during the instruction and execution periods of pro- and anti-saccades was prominent in PCs of both the medial and lateral cerebellum. However, only the SS activity in the lateral cerebellar cortex contained information about stimulus identity and showed a strong reciprocal interaction with complex spikes (CSs). Moreover, the SS activity of different PC groups modulated bidirectionally in both of regions, but the PCs that showed facilitating and suppressive activity were predominantly associated with instruction and execution, respectively. These findings show that different cerebellar regions and PC groups contribute to goal-directed behavior and volitional inhibition, but with different propensities, highlighting the rich repertoire of the cerebellar control in executive functions.

Impact of Purkinje Cell Simple Spike Synchrony on Signal Transmission from Flocculus

Purkinje cells (PCs) in the cerebellar flocculus carry rate-coded information that ultimately drives eye movement. Floccular PCs lying nearby each other exhibit partial synchrony of their simple spikes (SS). Elsewhere in the cerebellum, PC SS synchrony has been demonstrated to influence activity of the PCs' synaptic targets, and some suggest it constitutes another vector for information transfer. We investigated in the cerebellar flocculus the extent to which the rate code and PC synchrony interact. One motivation for the study was to explain the cerebellar deficits in ataxic mice like tottering; we speculated that PC synchrony has a positive effect on rate code transmission that is lost in the mutants. Working in transgenic mice whose PCs express channelrhodopsin, we exploited a property of optogenetics to control PC synchrony: pulsed photostimulation engenders stimulus-locked spiking, whereas continuous photostimulation engenders spiking whose timing is unconstrained. We photoactivated flocculus PCs using pulsed stimuli with sinusoidally varying timing vs. continuous stimuli with sinusoidally varying intensity. Recordings of PC pairs confirmed that pulsed stimuli engendered greater PC synchrony. We quantified the efficiency of transmission of the evoked PC firing rate modulation from the amplitudes of firing rate modulation and eye movement. Rate code transmission was slightly poorer in the conditions that generated greater PC synchrony, arguing against our motivating speculation regarding the origin of ataxia in tottering. Floccular optogenetic stimulation prominently augmented a 250-300 Hz local field potential oscillation, and we demonstrate relationships between the oscillation power and the evoked PC synchrony.

Divergent projections of single pontocerebellar axons to multiple cerebellar lobules in the mouse

The basilar pontine nucleus (PN) is the key relay point for the cerebrocerebellar link. However, the projection pattern of pontocerebellar mossy fiber axons, which is essential in determining the functional organization of the cerebellar cortex, has not been fully clarified. We reconstructed the entire trajectory of 25 single pontocerebellar mossy fiber axons labeled by localized injection of biotinylated dextran amine into various locations in the PN and mapped all their terminals in an unfolded scheme of the cerebellum in 10 mice. The majority of axons (20/25 axons) entered the cerebellum through the middle cerebellar peduncle contralateral to the origin, while others entered through the ipsilateral pathway. A small number of axons (1/25 axons) had collaterals terminating in the cerebellar nuclei. Axons projected mostly to a combination of lobules, often bilaterally, and terminated in multiple zebrin (aldolase C) stripes, more frequently in zebrin-positive stripes (83.9%) than in zebrin-negative stripes, with 66.5 mossy fiber terminals on the average. Axons originating from the rostral (plus medial and lateral), central and caudal PN mainly terminated in the paraflocculus, crus I and lobule VIb-c, in the simplex lobule, crus II and paramedian lobule, and in lobules II-VIa, VIII and copula pyramidis, respectively. The results suggest that the interlobular branching pattern of pontocerebellar axons determines the group of cerebellar lobules that are involved in a related functional localization of the cerebellum. In the hemisphere, crus I may be functionally distinct from neighboring lobules (simple lobule and crus II) in the mouse cerebellum based on the pontocerebellar axonal projection pattern.

Implications of Lateral Cerebellum in Proactive Control of Saccades

Although several lines of evidence establish the involvement of the medial and vestibular parts of the cerebellum in the adaptive control of eye movements, the role of the lateral hemisphere of the cerebellum in eye movements remains unclear. Ascending projections from the lateral cerebellum to the frontal and parietal association cortices via the thalamus are consistent with a role of these pathways in higher-order oculomotor control. In support of this, previous functional imaging studies and recent analyses in subjects with cerebellar lesions have indicated a role for the lateral cerebellum in volitional eye movements such as anti-saccades. To elucidate the underlying mechanisms, we recorded from single neurons in the dentate nucleus of the cerebellum in monkeys performing anti-saccade/pro-saccade tasks. We found that neurons in the posterior part of the dentate nucleus showed higher firing rates during the preparation of anti-saccades compared with pro-saccades. When the animals made erroneous saccades to the visual stimuli in the anti-saccade trials, the firing rate during the preparatory period decreased. Furthermore, local inactivation of the recording sites with muscimol moderately increased the proportion of error trials, while successful anti-saccades were more variable and often had shorter latency during inactivation. Thus, our results show that neuronal activity in the cerebellar dentate nucleus causally regulates anti-saccade performance. Neuronal signals from the lateral cerebellum to the frontal cortex might modulate the proactive control signals in the corticobasal ganglia circuitry that inhibit early reactive responses and possibly optimize the speed and accuracy of anti-saccades.

SIGNIFICANCE STATEMENT

Although the lateral cerebellum is interconnected with the cortical eye fields via the thalamus and the pons, its role in eye movements remains unclear. We found that neurons in the caudal part of the lateral (dentate) nucleus of the cerebellum showed the increased firing rate during the preparation of anti-saccades. Inactivation of the recording sites modestly elevated the rate of erroneous saccades to the visual stimuli in the anti-saccade trials, while successful anti-saccades during inactivation tended to have a shorter latency. Our data indicate that neuronal signals in the lateral cerebellum may proactively regulate anti-saccade generation through the pathways to the frontal cortex, and may inhibit early reactive responses and regulate the accuracy of anti-saccades.

Long-Term Predictive and Feedback Encoding of Motor Signals in the Simple Spike Discharge of Purkinje Cells

Most hypotheses of cerebellar function emphasize a role in real-time control of movements. However, the cerebellum’s use of current information to adjust future movements and its involvement in sequencing, working memory, and attention argues for predicting and maintaining information over extended time windows. The present study examines the time course of Purkinje cell discharge modulation in the monkey (Macaca mulatta) during manual, pseudo-random tracking. Analysis of the simple spike firing from 183 Purkinje cells during tracking reveals modulation up to 2 s before and after kinematics and position error. Modulation significance was assessed against trial shuffled firing, which decoupled simple spike activity from behavior and abolished long-range encoding while preserving data statistics. Position, velocity, and position errors have the most frequent and strongest long-range feedforward and feedback modulations, with less common, weaker long-term correlations for speed and radial error. Position, velocity, and position errors can be decoded from the population simple spike firing with considerable accuracy for even the longest predictive (-2000 to -1500 ms) and feedback (1500 to 2000 ms) epochs. Separate analysis of the simple spike firing in the initial hold period preceding tracking shows similar long-range feedforward encoding of the upcoming movement and in the final hold period feedback encoding of the just completed movement, respectively. Complex spike analysis reveals little long-term modulation with behavior. We conclude that Purkinje cell simple spike discharge includes short- and long-range representations of both upcoming and preceding behavior that could underlie cerebellar involvement in error correction, working memory, and sequencing.

Enhanced synaptic inhibition disrupts the efferent code of cerebellar Purkinje neurons in leaner Cav2.1 Ca 2+ channel mutant mice

Cerebellar Purkinje cells (PCs) encode afferent information in the rate and temporal structure of their spike trains. Both spontaneous firing in these neurons and its modulation by synaptic inputs depend on Ca(2+) current carried by Ca(v)2.1 (P/Q) type channels. Previous studies have described how loss-of-function Ca(v)2.1 mutations affect intrinsic excitability and excitatory transmission in PCs. This study examines the effects of the leaner mutation on fast GABAergic transmission and its modulation of spontaneous firing in PCs. The leaner mutation enhances spontaneous synaptic inhibition of PCs, leading to transitory reductions in PC firing rate and increased spike rate variability. Enhanced inhibition is paralleled by an increase in the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) measured under voltage clamp. These differences are abolished by tetrodotoxin, implicating effects of the mutation on spike-induced GABA release. Elevated sIPSC frequency in leaner PCs is not accompanied by increased mean firing rate in molecular layer interneurons, but IPSCs evoked in PCs by direct stimulation of these neurons exhibit larger amplitude, slower decay rate, and a higher burst probability compared to wild-type PCs. Ca(2+) release from internal stores appears to be required for enhanced inhibition since differences in sIPSC frequency and amplitude in leaner and wild-type PCs are abolished by thapsigargin, an ER Ca(2+) pump inhibitor. These findings represent the first account of the functional consequences of a loss-of-function P/Q channel mutation on PC firing properties through altered GABAergic transmission. Gain in synaptic inhibition shown here would compromise the fidelity of information coding in these neurons and may contribute to impaired cerebellar function resulting from loss-of function mutations in the Ca(V)2.1 channel gene.

Transcranial magnetic stimulation and motor plasticity in human lateral cerebellum: dual effect on saccadic adaptation

The cerebellum is a key area for movement control and sensory-motor plasticity. Its medial part is considered as the exclusive cerebellar center controlling the accuracy and adaptive calibration of saccadic eye movements. However, the contribution of other zones situated in its lateral part is unknown. We addressed this question in healthy adult volunteers by using magnetic resonance imaging (MRI)-guided transcranial magnetic stimulation (TMS). The double-step target paradigm was used to adaptively lengthen or shorten saccades. TMS pulses over the right hemisphere of the cerebellum were delivered at 0, 30, or 60 ms after saccade detection in separate recording sessions. The effects on saccadic adaptation were assessed relative to a fourth session where TMS was applied to Vertex as a control site. First, TMS applied upon saccade detection before the adaptation phase reduced saccade accuracy. Second, TMS applied during the adaptation phase had a dual effect on saccadic plasticity: adaptation after-effects revealed a potentiation of the adaptive lengthening and a depression of the adaptive shortening of saccades. For the first time, we demonstrate that TMS on lateral cerebellum can influence plasticity mechanisms underlying motor performance. These findings also provide the first evidence that the human cerebellar hemispheres are involved in the control of saccade accuracy and in saccadic adaptation, with possibly different neuronal populations concerned in adaptive lengthening and shortening. Overall, these results require a reappraisal of current models of cerebellar contribution to oculomotor plasticity.

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.

Position, direction of movement, and speed tuning of cerebellar Purkinje cells during circular manual tracking in monkey

The cerebellum plays an essential role in pursuit tracking with the eye and with the hand. During smooth pursuit eye movements, both tracking position and velocity are signaled by Purkinje cells. Purkinje cell simple spike discharge is also modulated by direction and speed during linear manual tracking. This study evaluated how all three parameters, position, movement direction, and speed, are signaled in the simple spike discharge of Purkinje cells during circular manual tracking. Three rhesus monkeys intercepted and then tracked a target moving in a circle in both counterclockwise and clockwise directions across a range of constant target speeds. Two sets of analyses of the simple spike firing of 97 Purkinje cells examined the effects of position, movement direction, and speed. The first approach was the incremental improvement of regression models, initially modeling a pure position dependence, then incorporating movement direction, and finally incorporating speed dependence. The second was a model-independent approach, without any explicit assumptions about the character of the directional tuning or speed effects. Both analyses revealed the same three results: (1) Purkinje cell discharge is spatially tuned, to both the position and direction of movement, and (2) this spatial tuning is not altered by the speed, except (3) the speed scales the average firing and/or depth of modulation. The results suggest that the population of Purkinje cells forms a representation of the entire position-direction space of arm movements, and that the speed modulates the scale of that representation. This speed scaling provides insights into the cerebellar processing of movement-related timing.

Simple spike firing in the posterior lateral cerebellar cortex of Macaque Mulatta was correlated with success-failure during a visually guided reaching task

Evidence has been accumulating which supports a role for the cerebellum in motor learning. Motor learning is though to be mediated by complex spikes acting as an error signal, which when firing in conjunction with simple spike activity modify synapses between parallel fibers and Purkinje cells. We studied the activity of neurons in the posterior lateral cerebellar cortex of macaques that were performing reaches to visual targets. We found that simple spike firing in many of these neurons was modulated by whether the monkey successfully hit the target or not. The success-failure modulation was present for reaches using either arm and could persist for several hundred milliseconds into a period when the monkey was constrained from moving its arms. This temporally extended success-failure activity could interact with complex spike firing in order to enhance learning, particularly when the motor command is temporally separated from sensory feedback.

Purkinje Cell Spike Firing in the Posterolateral Cerebellum: Correlation With Visual Stimulus, Oculomotor Response, and Error Feedback

Complex (CS)- and simple-spike (SS) discharge from single Purkinje cells (Pc) in the posterolateral cerebellum of two monkeys was recorded during a visually guided reach-touch task. A visual target appeared (TA) off-gaze at a random location on a screen. On initiation of arm reach, the target disappeared, then reappeared (TR) after a fixed delay. TR was either at the same location (baseline condition) or a shifted location at a fixed distance and direction from TA location (shift condition). Across trials, we observed one or two peaks of CS activity, depending on the reach condition. The first CS (T1 CS) peak was tuned to the location of TA on the screen, following TA by ∼150 ms. The second CS (T2 CS) peak occurred only in the shift condition, was tuned to the shift location of TR, and followed TR by ∼150 ms. The locational preferences of T1 and T2 CS peaks were the same. T1 and T2 CSs preceded saccades to TA and TR at the preferred location and occurred during reaches with either arm. T1 CSs occurred during trials in which the target appeared, and there was a saccade to target, but no subsequent arm reach followed. SS firing varied with TA/TR in the same preferred location as for the accompanying CS. We conclude that posterolateral Pc CS and SS firing changes following an off-gaze visual target appearance in a preferred location when there is a subsequent saccade to that location.

Spike firing in the lateral cerebellar cortex correlated with movement and motor parameters irrespective of the effector limb

Neuronal signals in the lateral aspect of the macaque cerebellar cortex were studied during a visually guided reaching task. During the performance of this task, the firing rate of most neurons was significantly modulated when reaching with either the ipsilateral or the contralateral arm. In some of these reach-modulated cells, we found that spike firing was correlated with the direction and speed of the reach. These correlations with motor parameters were present during reaching with either the ipsilateral or the contralateral arm. Based on these observations we suggest that spike firing in the lateral cerebellum was correlated with movement and motor parameters irrespective of the effector limb.

Neural coding of "attention for action" and "response selection" in primate anterior cingulate cortex

Noninvasive imaging techniques showed that the anterior cingulate cortex is related to higher-order cognitive and motor-related functions in humans. To elucidate the cellular mechanism of such cingulate functions, single-unit activity was recorded from three cingulate motor areas of macaque monkeys performing delayed conditional Go/No-go discrimination tasks using spatial (location) and nonspatial (color) visual cues. Unlike prefrontal neurons, only a few neurons coded the visual information on individual features (e.g., "left" or "red") in all of the rostral (CMAr), dorsal (CMAd), and ventral (CMAv) cingulate motor areas. Instead, many neurons in the CMAr exhibited the attention-like activity anticipating the second (conditioned) visual cues, with the specificity to visual category ("location" or "color"). In addition, there were a number of CMAr neurons specific to motor response (Go or No-go) in relation to the second visual cues. Some of the visual category-specific neurons in the CMAr further displayed the motor response-specific activity. On the other hand, many of the task-related CMAd and CMAv neurons seemed to be implicated directly in motor functions, such as preparation and execution of movements in Go trials. The present results suggest that the CMAr neurons may participate in cognitive and motor functions of "attention for action" and "response selection" for an appropriate action according to an intention, whereas the CMAd and CMAv neurons may be involved in "motor preparation and execution".

No parallel fiber volleys in the cerebellar cortex: evidence from cross-correlation analysis between Purkinje cells in a computer model and in recordings from anesthetized rats

Purkinje cells aligned on the medio-lateral axis share a large proportion of their approximately 175,000 parallel fiber inputs. This arrangement has led to the hypothesis that movement timing is coded in the cerebellum by beams of synchronously active parallel fibers. In computer simulations I show that such synchronous activation leads to a narrow spike cross-correlation between pairs of Purkinje cells. This peak was completely absent when shared parallel fiber input was active in an asynchronous mode. To determine the presence of synchronous parallel fiber beams in vivo I recorded from pairs of Purkinje cells in crus IIa of anesthetized rats. I found a complete absence of precise spike synchronization, even when both cells were strongly modulated in their spike rate by trains of air-puff stimuli to the face. These results indicate that Purkinje cell spiking is not controlled by volleys of synchronous parallel fiber inputs in the conditions examined. Instead, the data support a model by which granule cells primarily control Purkinje cell spiking via dynamic population rate changes.

Corticopontocerebellar pathway from the prearcuate region to hemispheric lobule VII of the cerebellum: an anterograde and retrograde tracing study in the monkey

The relationship between axons derived from the prearcuate region and pontine neurons projecting to contralateral cerebellar hemispheric lobule VII, lobulus petrosus (LP) of the paraflocculus or the dorsal paraflocculus (DPFl) was investigated in the monkey. The frontopontine axons were labeled with biotinylated dextran and pontocerebellar neurons with cholera toxin subunit B or fast blue. Labeled frontopontine axons and terminals were seen in the dorsal, medial, paramedian and dorsolateral parts of the pontine nuclei. The distribution of the labeled frontopontine axons overlapped that of labeled neurons projecting to hemispheric lobule VII but did not overlap that of labeled neurons projecting to LP or DPFl. The pathway from the prearcuate region to hemispheric lobule VII may provide an anatomical substrate for the involvement of hemispheric lobule VII in voluntary eye movements.

Changes in cerebellar activation pattern during two successive sequences of saccades

The changes in the cerebellar activation pattern of two successive fMRI scanning runs were determined for visually guided to-and-fro saccades in 12 healthy volunteers familiar with the study paradigm. Group and single subject-analyses revealed a constant activation of the paramedian cerebellar vermis (uvula, tonsils, tuber, folium/declive), which reflects constant ocular motor activity in both runs. A significant decrease in activation of the cerebellar hemispheres found in the second run is best explained by either a decrease in attention or the effects of motor optimization and learning. The significant, systematic changes of the cerebellar activation pattern in two successive runs were not expected, because the ocular motor task was simple, familiar, and highly automated. These findings indicate that similar effects may bias other cerebellar activation studies, in which sensorimotor tasks are repeated in a single session.

The control of rate and timing of spikes in the deep cerebellar nuclei by inhibition

Cerebellar nucleus neurons were recorded in vitro, and dynamic clamping was used to simulate inhibitory synaptic input from Purkinje cells likely to occur in vivo. Inhibitory input patterns with varying synaptic amplitudes and synchronicity were applied to determine how spike rate and spike timing can be controlled by inhibition. The excitatory input conductance was held constant to isolate the effect of dynamic inhibitory inputs on spiking. We found that the timing of individual spikes was controlled precisely by short decreases in the inhibitory conductance that were the consequence of synchronization between many inputs. The spike rate of nucleus neurons was controlled in a linear way by the rate of inhibitory inputs. The spike rate, however, also depended strongly on the amount of synchronicity present in the inhibitory inputs. An irregular spike train similar to in vivo data resulted from applied synaptic conductances when the conductance was large enough to overcome intrinsic pacemaker currents. In this situation subthreshold fluctuations in membrane potential closely followed the time course of the combined reversal potential of excitation and inhibition. This indicates that the net synaptic driving force for realistic input levels in vivo may be small and that synaptic input may operate primarily by shunting. The accurate temporal control of output spiking by inhibitory input that can be achieved in this way in the deep cerebellar nuclei may be particularly important to allow fine temporal control of movement via inhibitory output from cerebellar cortex.

Cerebellar Purkinje cell simple spike discharge encodes movement velocity in primates during visuomotor arm tracking

Pathophysiological, lesion, and electrophysiological studies suggest that the cerebellar cortex is important for controlling the direction and speed of movement. The relationship of cerebellar Purkinje cell discharge to the control of arm movement parameters, however, remains unclear. The goal of this study was to examine how movement direction and speed and their interaction-velocity-modulate Purkinje cell simple spike discharge in an arm movement task in which direction and speed were independently controlled. The simple spike discharge of 154 Purkinje cells was recorded in two monkeys during the performance of two visuomotor tasks that required the animals to track targets that moved in one of eight directions and at one of four speeds. Single-parameter regression analyses revealed that a large proportion of cells had discharge modulation related to movement direction and speed. Most cells with significant directional tuning, however, were modulated at one speed, and most cells with speed-related discharge were modulated along one direction; this suggested that the patterns of simple spike discharge were not adequately described by single-parameter models. Therefore, a regression surface was fitted to the data, which showed that the discharge could be tuned to specific direction-speed combinations (preferred velocities). The overall variability in simple spike discharge was well described by the surface model, and the velocities corresponding to maximal and minimal discharge rates were distributed uniformly throughout the workspace. Simple spike discharge therefore appears to integrate information about both the direction and speed of arm movements, thereby encoding movement velocity.

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.

Unilateral cerebellar lesions affect initiation of ipsilateral smooth pursuit eye movements in humans

To clarify the role of the cerebellum in pursuit initiation (first 100 msec), we used infrared oculography to examine the effect of unilateral cerebellar lesions on the initial (0-20 msec) and later (80-100 msec) periods as well as the steady-state response (200-300 msec) of horizontal smooth pursuit in 10 patients with unilateral cerebellar lesions. These results were compared with those of 17 age-matched healthy subjects. Smooth pursuit was elicited with a step-ramp target movement with randomized horizontal directions and velocities of 10 degrees/sec and 30 degrees/sec. In the first 20-msec pursuit, velocity was 22% lower toward the side of the cerebellar lesion than away from it and 16% lower in the period 80 to 100 msec (normal differences, 2% and 3%). Later (200-300 msec), the ipsiversive/contraversive difference was smaller, but pursuit velocity in both directions was significantly lower in patients than in normals. No lesion affected the floccular region and/or the nodulus/uvula. Five lesions extended so far medially that they could have affected the medial deep cerebellar nucleus (fastigial nucleus). The remaining five were in the lateral hemisphere in areas previously considered uninvolved in pursuit generation. Our findings prove that the cerebellum participates in human pursuit initiation and that lesions in the lateral cerebellum possibly affect smooth pursuit.

Saccadic eye movements and regional cerebral blood flow in schizophrenic patients

This study examined saccadic eye movements, using simple stationary targets, in schizophrenic patients. The targets were eight black points or eight arabic-numbered points placed in randomized order on the circumference of a circle. Self-paced eye movements during clockwise tracking of these points, by 23 patients and 23 controls, were recorded using an infrared eye-mark recorder. Then the relationship between the saccades and clinical symptoms was investigated. Finally, the relationship between the performance of the saccades and resting regional cerebral blood flow (rCBF) was examined using single photon emission computed tomography with 99mTc-hexamethyl propyleneamine oxime (HMPAO). The results indicate that patients track with significantly fewer correct scores and more deviant scores than controls, in agreement with our previous study. There were two groups of patients: an ordinary group who obtained a full-target-hitting score at a 200-ms setting and a fast group who obtained the full score at 100 ms but not at 200 ms. Some patients displayed significantly more hypermetria than controls. Significant correlations were found between hallucination and delusion symptoms and correct score. With respect to relative rCBF, fast-group patients showed significantly decreased rCBF in the left limbic and inferior parietal areas as compared with ordinary group patients. These findings suggest that some schizophrenic patients view the stationary targets too fast and this may be related to dysfunction in the limbic-parietal association area in the left hemisphere.

Context dependent discharge characteristics of saccade-related Purkinje cells in the cerebellar hemispheres of the monkey

In the previous paper (Mano et al, 1991), we reported the discharge patterns of saccade-related Purkinje cells during visually guided saccade task, which were recorded from posterior cerebellar hemisphere, the Crus IIa. In the present study, we analysed these P-cell's simple spike activity during the spontaneous saccade in inter-trial intervals (ITI) of visually guided saccade task, comparing with the activity during the visually guided saccade. We found that the modulation of simple spike discharges during spontaneous saccade was weaker than the modulation during the visually triggered saccade. We recorded single unit discharges of Purkinje cells from cerebellar posterior hemisphere (Crus IIa) in awake Japanese monkeys (Macaca fuscata), trained to perform simple reaction time saccade task gazing at a small light rear-projected on to a tangent screen 54 cm in front of the monkey. Horizontal and vertical eye positions were measured by a corneal search coil method. Comparison of simple spike activity associated with spontaneous saccade during ITI to the activity during visually triggered saccade clarified that the discharge patterns of simple spikes are basically the same during both types of the saccades, but the amount of the phasic modulation (increase of decrease of discharge rate) were larger for all directions (up, down, left and right) during visually guided saccade than that during spontaneous saccade in all saccade-related Purkinje cells so far examined in two monkeys. The modulation, however, cannot be assumed to have been induced by the visual stimulus per se. Because, the maximum increase of simple spike discharge rate aligned at saccade onset is larger than that aligned at target jump. And, the half width of the change was wider when aligned at target light jump than when aligned at the onset of saccades, in all the four directions, indicating the changes of the firing rate were more time-locked to the onset of saccadic eye movements than to the triggering visual stimulus. The present findings suggest that the cerebellar hemisphere plays a more important role in the control of externally triggered voluntary eye movements than in the control of self-initiated, self-paced eye movements. We discussed these findings combining with previous findings on limb movement-related P-cells (Mano et al, 1980, 1986, 1989), from view point of the general role of the cerebellar hemisphere in the control of voluntary movements.

A projection linking motor cortex with the LM-suprageniculate nuclear complex through the periaqueductal gray area which surrounds the nucleus of Darkschewitsch in the cat

Whereas a previous study by one of us (Hicks et al., 1986) suggested that periaqueductal gray (PAG) neurons projecting to the lateralis medialis-suprageniculate (LM-SG) complex might mediate transmission of affective-related nociceptive information, our present work suggests instead, a function in processes related to movement. Cells of the nucleus of Darkschewitsch (ND) are known to have reciprocal projections with the motor cortex (MX), in particular with the hand area of MX, and also to project to the rostral medial accessory olivary (MAO) nucleus (Onodera and Hicks, 1995a). That the ND might be related to saccadic oculomotor function, as well as to the control of hand movements through its connections via the olivo-cerebellar circuit, is indicated by the fact that ND receives a strong projection from the substantia nigra pars reticulata and zona incerta (SNR/ZI) and projects directly and/or indirectly to eye movement nuclei (Onodera and Hicks, 1995b). Thus, ND may function in permitting integration of eye-hand motor coordination. This study focussed on the area of PAG surrounding ND. WGA-HRP was injected into MX and many labelled terminals and large neurones were in ND, with lesser numbers being observed in the area of the PAG surrounding ND. After injections into ND and closely adjacent areas, labelled terminals were observed sparsely distributed with a restricted area of the LM-SG complex. After injections into LM-SG area, small neuronal somata were seen in the area of the PAG surrounding ND, but no labelled somata were detected in ND. Thus if the cells of this PAG area, like those of ND, have similar functions owing to their common reciprocal connections with MX, then the small neurones in PAG projecting to LM-SG may constitute an important link in the circuitry subserving visual processing and/or the regulation of orienting movements of the hand, head and eye.

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)