Effects of cerebellar disease on sequences of rapid eye movements

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.

Bilateral lesion of the cerebellar fastigial nucleus: Effects on smooth pursuit acceleration and non-reflexive visually-guided saccades

Background

“Central dizziness” due to acute bilateral midline cerebellar disease sparing the posterior vermis has specific oculomotor signs. The oculomotor region of the cerebellar fastigial nucleus (FOR) crucially controls the accuracy of horizontal visually-guided saccades and smooth pursuit eye movements. Bilateral FOR lesions elicit bilateral saccade hypermetria with preserved pursuit. It is unknown whether the initial acceleration of smooth pursuit is impaired in patients with bilateral FOR lesions.

Objective

We studied the effect of a cerebellar lesion affecting the deep cerebellar nuclei on the initial horizontal pursuit acceleration and investigated whether saccade dysmetria also affects other types of volitional saccades, i.e., memory-guided saccades and anti-saccades, which are not performed in immediate response to the visual target.

Methods

We recorded eye movements during a sinusoidal and step-ramp target motion paradigm as well as visually-guided saccades, memory-guided saccades, and anti-saccades in one patient with a circumscribed cerebellar hemorrhage and 18 healthy control subjects using a video-based eye tracker.

Results

The lesion comprised the FOR bilaterally but spared the posterior vermis. The initial pursuit acceleration was low but not significantly different from the healthy control subjects and sinusoidal pursuit was normal. Bilateral saccade hypermetria was not only seen with visually-guided saccades but also with anti-saccades and memory-guided saccades. The final eye position remained accurate.

Conclusion

We provide new insights into the contribution of the bilateral deep cerebellar nuclei on the initial acceleration of human smooth pursuit in midline cerebellar lesions. In line with experimental bilateral FOR lesion data in non-human primates, the initial pursuit acceleration in our patient was not significantly reduced, in contrast to the effects of unilateral experimental FOR lesions. Working memory and neural representation of target locations seem to remain unimpaired. Our data argue against an impaired common command feeding the circuits controlling saccadic and pursuit eye movements and support the hypothesis of independent influences on the neural processes generating both types of eye movements in the deep cerebellar nuclei.

Behavior of the saccadic system: Metrics of timing and accuracy

The behavior of saccades in response to a peripheral target is discussed. The saccade latency comprises sensory and motor processing delays of about 80ms, leaving on average more than 100ms for central processing. Many factors influence the latter. Yet, programming express saccades requires little to no central processing time. Typical saccades are hypometric by about 10%, which seems to be a deliberate strategy. A correction saccade requires only about 50ms of central processing. There is no strict dead zone for saccades, as they can be elicited by target jumps as small as 0.05deg. There seems to be no strict refractoriness in the system either, because saccade metrics can be continuously modified during the preparation interval by new target information. This suggests semi-independent processes for the "when" and "where" of saccades, which is incorporated into a neurophysiologically-inspired model. Saccades are not kept in retinotopic coordinates but are goal-directed by incorporating intervening changes in eye position. Although the updating mechanism is unclear, there is strong evidence that it involves the use of efference copy information (the outflow theory). Although the spatial percept of a target may be erroneous around saccades, the motor system seems to be more accurate. The chapter closes with a discussion on the potential function of microsaccades and slow drifts, when fixating a target.

New Cerebello-Cortical Pathway Involved in Higher-Order Oculomotor Control

The cerebellum and the basal ganglia play an important role in the control of voluntary eye movement associated with complex behavior, but little is known about how cerebellar projections project to cortical eye movement areas. Here we used retrograde transneuronal transport of rabies virus to identify neurons in the cerebellar nuclei that project via the thalamus to supplementary eye field (SEF) of the frontal cortex of macaques. After rabies injections into the SEF, many neurons in the restricted region, the ventral aspects of the dentate nucleus (DN), the caudal pole of the DN, and the posterior interpositus nucleus (PIN) were labeled disynaptically via the thalamus, whereas no neuron labeling was found in the anterior interpositus nucleus (AIN). The distribution of the labeled neurons was dorsoventrally different from that of DN and PIN neurons labeled from the motor cortex. In the basal ganglia, a large number of labeled neurons were confined to the dorsomedial portion of the internal segment of the globus pallidus (GPi) as more neurons were labeled in the inner portion of the GPi (GPii) than in the outer portion of the GPi (GPio). This is the first evidence of a projection between cerebellum/basal ganglia and the SEF that could enable the cerebellum to modulate the cognitive control of voluntary eye movement.

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.

Evaluating the influence of motor control on selective attention through a stochastic model: the paradigm of motor control dysfunction in cerebellar patient

Attention allows us to selectively process the vast amount of information with which we are confronted, prioritizing some aspects of information and ignoring others by focusing on a certain location or aspect of the visual scene. Selective attention is guided by two cognitive mechanisms: saliency of the image (bottom up) and endogenous mechanisms (top down). These two mechanisms interact to direct attention and plan eye movements; then, the movement profile is sent to the motor system, which must constantly update the command needed to produce the desired eye movement. A new approach is described here to study how the eye motor control could influence this selection mechanism in clinical behavior: two groups of patients (SCA2 and late onset cerebellar ataxia LOCA) with well-known problems of motor control were studied; patients performed a cognitively demanding task; the results were compared to a stochastic model based on Monte Carlo simulations and a group of healthy subjects. The analytical procedure evaluated some energy functions for understanding the process. The implemented model suggested that patients performed an optimal visual search, reducing intrinsic noise sources. Our findings theorize a strict correlation between the "optimal motor system" and the "optimal stimulus encoders."

Cortical processing of saccade-related efference copy signals in patients with cerebellar lesion

The updating of visual space across saccades is thought to rely on efference copies of motor commands. In humans, thalamic lesions impair performance on a saccadic double-step task, which requires the use of efference copy information, and the altering of saccade-related efference copy processing. This deficit is attributed to disruption of a pathway from the superior colliculus to the frontal eye field. However, the cerebellum is probably also involved in efference copy processing, due to its pivotal role for predictive motor control. The present study investigated the processing of efference copy information in eight patients with focal cerebellar lesions and 22 healthy controls by means of a saccadic double-step task with simultaneous event-related potential recording. Despite intact behavioural performance, a positive event-related potential component between 150 and 450 ms after first saccade onset in the updating condition, which has been interpreted in terms of the integration of efference copy signals with motor intentions for a subsequent saccade, was markedly reduced in the patients. These findings suggest that the cerebellum contributes to on-line saccade monitoring, and that cerebellar lesions alter saccade-related efference copy processing. However, given the intact behavioural performance, the reduced positivity in the patients may indicate that cerebellar damage is accounted for by either exploiting reduced saccade-related information, or making use of compensatory strategies to circumvent a deficit in using efference copy information procured by the cerebellum. The present study extends previous findings on the neural underpinnings of saccadic updating and further elucidates the mechanisms underlying cerebellar predictive motor control.

Critical role of cerebellar fastigial nucleus in programming sequences of saccades

The cerebellum plays an important role in programming accurate saccades. Cerebellar lesions affecting the ocular motor region of the fastigial nucleus (FOR) cause saccadic hypermetria; however, if a second target is presented before a saccade can be initiated (double-step paradigm), saccade hypermetria may be decreased. We tested the hypothesis that the cerebellum, especially FOR, plays a pivotal role in programming sequences of saccades. We studied patients with saccadic hypermetria because of either genetic cerebellar ataxia or surgical lesions affecting FOR and confirmed that the gain of initial saccades made to double-step stimuli was reduced compared with the gain of saccades to single target jumps. Based on measurements of the intersaccadic interval, we found that the ability to perform parallel processing of saccades was reduced or absent in all of our patients with cerebellar disease. Our results support the crucial role of the cerebellum, especially FOR, in programming sequences of saccades.

Cerebellum and ocular motor control

An intact cerebellum is a prerequisite for optimal ocular motor performance. The cerebellum fine-tunes each of the subtypes of eye movements so they work together to bring and maintain images of objects of interest on the fovea. Here we review the major aspects of the contribution of the cerebellum to ocular motor control. The approach will be based on structural–functional correlation, combining the effects of lesions and the results from physiologic studies, with the emphasis on the cerebellar regions known to be most closely related to ocular motor function: (1) the flocculus/paraflocculus for high-frequency (brief) vestibular responses, sustained pursuit eye movements, and gaze holding, (2) the nodulus/ventral uvula for low-frequency (sustained) vestibular responses, and (3) the dorsal oculomotor vermis and its target in the posterior portion of the fastigial nucleus (the fastigial oculomotor region) for saccades and pursuit initiation.