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

Nucleus reticularis tegmenti pontis: a bridge between the basal ganglia and cerebellum for movement control

Neural processing in the basal ganglia is critical for normal movement. Diseases of the basal ganglia, such as Parkinson's disease, produce a variety of movement disorders including akinesia and bradykinesia. Many believe that the basal ganglia influence movement via thalamic projections to motor areas of the cerebral cortex and through projections to the cerebellum, which also projects to the motor cortex via the thalamus. However, lesions that interrupt these thalamic pathways to the cortex have little effect on many movements, including limb movements. Yet, limb movements are severely impaired by basal ganglia disease or damage to the cerebellum. We can explain this impairment as well as the mild effects of thalamic lesions if basal ganglia and cerebellar output reach brainstem motor regions without passing through the thalamus. In this report, we describe several brainstem pathways that connect basal ganglia output to the cerebellum via nucleus reticularis tegmenti pontis (NRTP). Additionally, we propose that widespread afferent and efferent connections of NRTP with the cerebellum could integrate processing across cerebellar regions. The basal ganglia could then alter movements via descending projections of the cerebellum. Pathways through NRTP are important for the control of normal movement and may underlie deficits associated with basal ganglia disease.

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.

A neuronal process for adaptive control of primate saccadic system

In 1980, Dr. Optican established the existence of an adaptive plasticity of saccades and its dependence on the cerebellum with Dr. Robinson. The advantage of saccades is that the neuronal mechanisms underlying their generation have been well established. This knowledge allows us to identify the neuronal elements that participate in saccade adaptation. Briefly, the superior colliculus (SC) produces a saccade command signal, which reaches motoneurons in the abducens nucleus via the brainstem burst generator. The SC saccade command also is sent to the oculomotor vermis (OMV), a saccade-related area of the cerebellar cortex, and finally converges on the same motoneurons via the caudal fastigial nucleus (cFN) and inhibitory burst neurons (IBN). During adaptation, the saccade-related burst of SC neurons does not change; however, the activity of the cerebellum and its downstream targets do. We demonstrate that the SC is the source of the error signal to the OMV, and the error signal increases the probability of complex spike occurrence and decreases simple spike activity in the OMV. This decrease, in turn, is delivered through the cFN and IBN neurons to decrease motoneuron activity and hence saccade amplitude.

Characteristics of vestibular corrective saccades in patients with slow visual saccades, vestibular disorders and controls: A descriptive analysis

OBJECTIVE

Our aim was to determine whether overt catch up saccades (OS) provoked by vestibular stimuli, as observed in the video head impulse test (vHIT), have comparable metrics as visually triggered horizontal saccades (VS), indicating a common saccadic brainstem generator.

METHODS

Three groups of patients were studied: patients with neurological disorders causing slow saccades (group 1, n = 12), patients with peripheral vestibular lesions (group 2, n = 43), and normal controls (group 3, = 24). All patients underwent vHIT and Videooculographic testing. OS velocity, acceleration, amplitude and duration and VS velocity in this group was compared between the groups.

RESULTS

There was significant reduction in the velocity of visually guided saccades in group 1, as expected from the patient selection constraints of this study. Group 1 also exhibited saccades which were longer in duration and of reduced acceleration when compared to subjects without saccadic slowing to visual targets (Group 2 and 3). There were significant positive correlations between OS acceleration and amplitude in both normal saccade groups (2 and 3) which was not observed in the slow saccade group (1).

CONCLUSIONS

The metrics of overt saccades measured by the vHIT in patients with slow saccades and normal controls are similar to visually guided saccades. This supports the hypothesis that overt saccades associated with vestibular stimuli and visually triggered saccades share common circuitry that controls metrics.

Distinguishing spinocerebellar ataxia with pure cerebellar manifestation from multiple system atrophy (MSA-C) through saccade profiles

OBJECTIVE

Patients with spinocerebellar ataxia with pure cerebellar presentation (SCD) and multiple system atrophy (MSA-C) show similar symptoms at early stages, although cerebellofugal pathology predominates in SCD, and cerebellopetal pathology in MSA-C. We studied whether saccade velocity profiles, which reflect the accelerating and braking functions of the cerebellum, can differentiate these two disorders.

METHODS

We recorded visually guided (VGS) and memory guided saccades (MGS) in 29 MSA-C patients, 12 SCD patients, and 92 age-matched normal subjects, and compared their amplitude, peak velocity and duration (accelerating and decelerating phases).

RESULTS

Hypometria predominated in VGS and MGS of MSA-C, whereas hypometria was less marked in SCD, with hypermetria frequently noted in MGS. Peak velocity was reduced, and deteriorated with advancing disease both in SCD and MSA-C groups at smaller target eccentricities. The deceleration phase was prolonged in SCD compared to MSA-C and normal groups at larger target eccentricities, which deteriorated with advancing disease.

CONCLUSION

Saccades in MSA-C were characterized by a more prominent acceleration deficit and those in SCD by a more prominent braking defect, possibly caused by the cerebellopetal and cerebellofugal pathologies, respectively.

SIGNIFICANCE

Saccade profiles provide important information regarding the accelerating and braking signals of the cerebellum in spinocerebellar ataxia.

Can the adverse effects of antiepileptic drugs be detected in saccadic eye movements?

PURPOSE

The objective of this study was to determine whether the adverse effects of antiepileptic-drugs could be assessed by the eye movements of epilepsy patients.

METHODS

This study was performed prospectively in a single tertiary hospital. The inclusion criteria for this study were as follows: (1) consecutive patients with epilepsy taking antiepileptic-drugs regularly for at least 1 year, (2) the absence of structural lesions on MRI, (3) an age ≥16 years old, (4) not using medications that could influence eye movement, and (5) a normal neurological examination. The latency, peak velocity and accuracy of the saccades and the gain of the pursuits were recorded by video-based electro-oculography. We analyzed the differences in the parameters of the eye movements for 75 patients with epilepsy and 20 normal controls matched for age and sex.

RESULTS

The total latency (1017.7 ± 148.9 ms vs. 1150.7 ± 106.6 ms, p=0.0003) and accuracy [370.7% (95% CI 364.1-376.4%, range 306-408.2%), 92.7% as total accuracy normalized value vs. 383.6% (95% CI 378.8-398%, range 322.9-417.4%), 95.9% as total accuracy normalized value, p=0.0005] were significantly different between the patients with epilepsy and normal controls. For the detection of nystagmus with video-based electro-oculography, the clear cutoff values of total accuracy (≤388.7%, 97.2% as total accuracy normalized value) revealed 93.4% sensitivity and 28.6% specificity, and the clear cutoff values of total latency (≤1005.5 ms) showed 49.2% sensitivity and 78.6% specificity.

CONCLUSIONS

The total latency and accuracy of video-based electro-oculography may be screened to identify patients with a high risk of adverse effects with antiepileptic-drugs.

Eye movements in ephedrone-induced parkinsonism

Patients with ephedrone parkinsonism (EP) show a complex, rapidly progressive, irreversible, and levodopa non-responsive parkinsonian and dystonic syndrome due to manganese intoxication. Eye movements may help to differentiate parkinsonian syndromes providing insights into which brain networks are affected in the underlying disease, but they have never been systematically studied in EP. Horizontal and vertical eye movements were recorded in 28 EP and compared to 21 Parkinson's disease (PD) patients, and 27 age- and gender-matched healthy subjects using standardized oculomotor tasks with infrared videooculography. EP patients showed slow and hypometric horizontal saccades, an increased occurrence of square wave jerks, long latencies of vertical antisaccades, a high error rate in the horizontal antisaccade task, and made more errors than controls when pro- and antisaccades were mixed. Based on oculomotor performance, a direct differentiation between EP and PD was possible only by the velocity of horizontal saccades. All remaining metrics were similar between both patient groups. EP patients present extensive oculomotor disturbances probably due to manganese-induced damage to the basal ganglia, reflecting their role in oculomotor system.

A quick look at slow saccades after cardiac surgery: where is the lesion?

Saccadic palsy is a reported complication of cardiac surgery. One case that came to autopsy showed midline pontine gliosis; however, in most cases, no lesions are evident on neuroimaging. Since the saccadic palsy may range from single large slow saccades to a "staircase" of very small saccades that are normal in speed, it seems plausible that more than one mechanism is possible. Here we postulate that, in those patients who make a staircase of small saccades, loss of cerebellar Purkinje cells could cause fastigial nucleus neurons to fire prematurely, thereby decelerating saccades via inhibitory burst neurons.

Ocular motor syndromes of the brainstem and cerebellum

PURPOSE OF REVIEW

The brainstem and cerebellum contain many neuronal types that play a critical role in eye movement control. In a physiological approach, understanding how these neuronal assemblies cooperate provides strong insight into general brain functions. Furthermore, eye movements provide an interesting model for understanding neural mechanisms of sensorimotor learning, and a knowledge of the mechanisms underlying oculomotor plasticity is essential for correctly diagnosing and effectively managing patients. Finally, knowledge of the ocular motor syndromes frequently helps localize the pathological abnormality.

RECENT FINDINGS

We review the recently published works dealing with the physiological organization and pathology of slow and rapid eye movements at a brainstem and cerebellar level.

SUMMARY

The main recent findings of great interest for clinical practice or research concern the physiopathology of head shaking nystagmus, downbeat nystagmus and oculopalatal tremor; the neural substrates of three-dimensional control of eye movements and of optokinetic nystagmus; the understanding of saccade generation and of its consequences on physiological and pathological eye oscillations; and, finally, the physiological basis of saccadic adaptation.

Nicotine-induced nystagmus correlates with midpontine activation

The pathomechanism of nicotine-induced nystagmus (NIN) is unknown. The aim of this study was to delineate brain structures that are involved in NIN generation. Eight healthy volunteers inhaled nicotine in darkness during a functional magnetic resonance imaging (fMRI) experiment; eye movements were registered using video-oculography. NIN correlated with blood oxygen level-dependent (BOLD) activity levels in a midpontine site in the posterior basis pontis. NIN-induced midpontine activation may correspond to activation of the dorsomedial pontine nuclei and the nucleus reticularis tegmenti pontis, structures known to participate in the generation of multidirectional saccades and smooth pursuit eye movements.

Disorders of saccades

Saccades are rapid eye movements that assist vision by pointing the fovea of the retina, which contains the highest density of photoreceptors, at features of interest in the visual environment. A great deal is now known about the properties and neurobiology of saccades in both health and disease states. They have consequently become a valuable diagnostic and research tool. In this review, we describe the common saccadic disorders and their causes. We also highlight recent insights into the pathophysiologic mechanisms underlying these disorders and discuss how these insights have helped increase our understanding of the saccadic system as a whole.

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.