The effect of cerebellectomy on the cat's bestibulo-ocular integrator

Nucleus incertus provides eye velocity and position signals to the vestibulo-ocular cerebellum: a new perspective of the brainstem-cerebellum-hippocampus network

The network formed by the brainstem, cerebellum, and hippocampus occupies a central position to achieve navigation. Multiple physiological functions are implicated in this complex behavior. Among these, control of the eye-head and body movements is crucial. The gaze-holding system realized by the brainstem oculomotor neural integrator (ONI) situated in the nucleus prepositus hypoglossi and fine-tuned by the contribution of different regions of the cerebellum assumes the stability of the image on the fovea. This function helps in the recognition of environmental targets and defining appropriate navigational pathways further elaborated by the entorhinal cortex and hippocampus. In this context, an enigmatic brainstem area situated in front of the ONI, the nucleus incertus (NIC), is implicated in the dynamics of brainstem-hippocampus theta oscillation and contains a group of neurons projecting to the cerebellum. These neurons are characterized by burst tonic behavior similar to the burst tonic neurons in the ONI that convey eye velocity-position signals to the cerebellar flocculus. Faced with these forgotten cerebellar projections of the NIC, the present perspective discusses the possibility that, in addition to the already described pathways linking the cerebellum and the hippocampus via the medial septum, these NIC signals related to the vestibulo-ocular reflex and gaze holding could participate in the hippocampal control of navigation.

The functional operation of the vestibulo-ocular reflex

The biophysical properties of the labyrinthine semicircular canals, and the electrophysiological properties of peripheral vestibular afferent neurons over a range of stimulus frequencies, are reviewed. Resting discharge activity and adaptive properties of vestibular neurons are discussed. Central processing of vestibular signals is then examined, including push-pull organization and the velocity storage mechanism. A detailed treatment of the final common neural integrator for oculomotor signals follows with consideration of its neural substrate and how distributed networks of neurons can overcome several problems posed by conventional control-systems models, such as why neural signals, but not background discharge, are integrated. Next, the behavior of the vestibulo-ocular reflex in darkness is compared with how it satisfies visual demands during natural activities. Finally, the reflex's performance at high frequencies of head rotation is discussed.

Properties of rapid eye movements

This chapter describes the metrics and kinematics of saccades and quick phases of nystagmus, including microsaccades and large eye-head saccades. Small saccades often display dynamic overshoot, predominantly in the abducting eye. Although the function of these overshoots is unclear, the return movement is saccadic in nature. The saccade kinematics can be quantified by stereotyped relations between amplitude, duration, peak eye velocity, and peak acceleration, which vary somewhat with the initial eye position and saccade direction (centripetal vs centrifugal), possibly due to ocular plant characteristics. Saccades in a structured light environment are considerably faster than when executed in total darkness, although the origin for this facilitation is not known. The horizontal and vertical components of slant saccades are coupled, approximately matching their durations, for which possible underlying neural mechanisms are discussed. The chapter closes with a cross-species comparison of saccade characteristics.

Eye stabilization

This chapter summarizes how visual feedback could be used to stabilize the line of sight and optimize vision during attempted fixation of a stationary target. Quantitative features of oculomotor noise that causes drifts of the eye away from the target are analyzed. The sources of such noise, including the ripples in eye position due to muscle fiber twitches, and drifts of the eye away from the visual target due to vestibular imbalance, are examined. Evidence for a promptly responding stabilization system, distinct from optokinetic or pursuit eye movements is reviewed. Smooth eye movements that negate drifts of the eyes, which are discussed here, are distinct from microsaccades, which are discussed in chapter "Behavior of the saccadic system: Metrics of timing and accuracy" by Robinson.

Modeling the interaction among three cerebellar disorders of eye movements: periodic alternating, gaze-evoked and rebound nystagmus

A woman, age 44, with a positive anti-YO paraneoplastic cerebellar syndrome and normal imaging developed an ocular motor disorder including periodic alternating nystagmus (PAN), gaze-evoked nystagmus (GEN) and rebound nystagmus (RN). During fixation there was typical PAN but changes in gaze position evoked complex, time-varying oscillations of GEN and RN. To unravel the pathophysiology of this unusual pattern of nystagmus, we developed a mathematical model of normal function of the circuits mediating the vestibular-ocular reflex and gaze-holding including their adaptive mechanisms. Simulations showed that all the findings of our patient could be explained by two, small, isolated changes in cerebellar circuits: reducing the time constant of the gaze-holding integrator, producing GEN and RN, and increasing the gain of the vestibular velocity-storage positive feedback loop, producing PAN. We conclude that the gaze- and time-varying pattern of nystagmus in our patient can be accounted for by superposition of one model that produces typical PAN and another model that produces typical GEN and RN, without requiring a new oscillator in the gaze-holding system or a more complex, nonlinear interaction between the two models. This analysis suggest a strategy for uncovering gaze-evoked and rebound nystagmus in the setting of a time-varying nystagmus such as PAN. Our results are also consistent with current ideas of compartmentalization of cerebellar functions for the control of the vestibular velocity-storage mechanism (nodulus and ventral uvula) and for holding horizontal gaze steady (the flocculus and tonsil).

"Leaky" and "Unstable" Neural Integrator Can Coexist-Paradox Observed in Multiple Sclerosis

The mechanism for stable gaze-holding requires a neural integrator that converts pulse of neural discharge to steady firing rate. The integrator is feedback-dependent, impaired feedback manifests as either "unstable" integration when it is too much or "leaky" when it is too little. The "unstable" integrator is known to cause sinusoidal oscillations of the eyes called pendular nystagmus, whereas the "leaky" integrator causes jerky eye oscillations called gaze-evoked nystagmus. We hypothesized that integrator can be simultaneously leaky and unstable. Mechanistically, some parts of network are served by increased feedback gain (unstable network), while other part would be decreased feedback gain (leaky). Both leaky and unstable, the network converges on the ocular motor plant, leading to simultaneously present gaze-evoked jerk and sinusoidal nystagmus. We tested our hypothesis by measuring eye movements with search coil technique in 7 multiple sclerosis patients. Five of these patients had gaze-evoked nystagmus and superimposed pendular nystagmus. The gaze-evoked nystagmus depicted all the features of "leaky" integrator, that is, the drifts were always toward the null that was located at the central eye-in-orbit orientation, there were no drifts at null, and the drift velocity increased as the eyes moved farther away from the null. The pendular nystagmus had all the features of "unstable" integrator, that is, constant 4- to 6-Hz frequency, eye-in-orbit position dependence of the oscillation amplitude, and the voluntary saccade causing an oscillatory phase reset. These features were then simulated in a computational model conceptualizing our hypothesis of simultaneously leaky and unstable neural integrator.

Distinct proportions of cholinergic neurons in the rat prepositus hypoglossi nucleus according to their cerebellar projection targets

Cerebellar functions are modulated by cholinergic inputs, the density of which varies among cerebellar regions. Although the prepositus hypoglossi nucleus (PHN), a brainstem structure involved in controlling gaze holding, is known as one of the major sources of these cholinergic inputs, the proportions of cholinergic neurons in PHN projections to distinct cerebellar regions have not been quantitatively analyzed. In this study, we identified PHN neurons projecting to the cerebellum by applying retrograde labeling with dextran-conjugated Alexa 488 in choline acetyltransferase (ChAT)-tdTomato transgenic rats and compared the proportion of cholinergic PHN neurons in the PHN projections to four different regions of the cerebellum, namely the flocculus (FL), the uvula and nodulus (UN), lobules III-V in the vermis (VM), and the hemispheric paramedian lobule and crus 2 (PC). In the PHN, the percentage of cholinergic PHN neurons was lower in the projection to the FL than in the projection to the UN, VM or PC. Preposito-cerebellar neurons, except for preposito-FL neurons, included different proportions of cholinergic neurons at different rostrocaudal positions in the PHN. These results suggest that cholinergic PHN neurons project to not only the vestibulocerebellum but also the anterior vermis and posterior hemisphere and that the proportion of cholinergic neurons among projection neurons from the PHN differs depending on cerebellar target areas and the rostro-caudal regions of the PHN. This study provides insights regarding the involvement of cerebellar cholinergic networks in gaze holding.

Prevalence and Characteristics of Physiological Gaze-Evoked and Rebound Nystagmus: Implications for Testing Their Pathological Counterparts

Objective: Cerebellar diseases frequently affect the ocular motor neural velocity-to-position integrator by increasing its leakiness and thereby causing gaze-evoked nystagmus (GEN) and rebound nystagmus (RN). Minor leakiness is physiological and occasionally causes GEN in healthy humans. We aimed to evaluate the characteristics of GEN/RN in healthy subjects for better differentiation between physiological and pathological GEN/RN.

Methods: Using video-oculography, eye position was measured in 14 healthy humans at straight ahead eye position before and after, and during 30 s of ocular fixation at 4 horizontal eccentric targets between 30° and 45°. We determined the eye drift velocity and the prevalence of nystagmus before/during/after eccentric fixation.

Results: Eye drift velocities during (range: 0.62 ± 0.53°/s to 1.78 ± 0.69°/s) and after eccentric gaze (range: 0.28 ± 0.52°/s to 1.48 ± 1.02°/s) increased with the amount of gaze eccentricity (30°-45°). During continuous eccentric gaze, eye drift velocities decreased by 0.41 ± 0.18°/s at 30°, and 0.84 ± 0.38°/s at 45° gaze eccentricity. GEN was elicited in 71% of subjects at 30° gaze eccentricity. Twenty-one percent showed RN thereafter. This prevalence increased to 100% (GEN)/72% (RN) at 45° gaze eccentricity. RN found after 30° gaze eccentricity was of low velocity (0.82 ± 0.21°/s) and occurred after minor drift velocity decrease during prior eccentric gaze (0.43 ± 0.15°/s).

Conclusion: GEN and RN should be tested using horizontal gaze eccentricities of <30°, since most healthy subjects physiologically show GEN and RN at higher eccentricities. In case of an uncertain result, both the reduction of eye drift velocity during eccentric gaze and the velocity of RN can be analyzed to distinguish physiological from pathological nystagmus.

Slow-fast control of eye movements: an instance of Zeeman's model for an action

The rapid eye movements (saccades) used to transfer gaze between targets are examples of an action. The behaviour of saccades matches that of the slow-fast model of actions originally proposed by Zeeman. Here, we extend Zeeman's model by incorporating an accumulator that represents the increase in certainty of the presence of a target, together with an integrator that converts a velocity command to a position command. The saccadic behaviour of several foveate species, including human, rhesus monkey and mouse, is replicated by the augmented model. Predictions of the linear stability of the saccadic system close to equilibrium are made, and it is shown that these could be tested by applying state-space reconstruction techniques to neurophysiological recordings. Moreover, each model equation describes behaviour that can be matched to specific classes of neurons found throughout the oculomotor system, and the implication of the model is that build-up, burst and omnipause neurons are found throughout the oculomotor pathway because they constitute the simplest circuit that can produce the motor commands required to specify the trajectories of motor actions.

Vestibular and Ocular Motor Properties in Lateral Medullary Stroke Critically Depend on the Level of the Medullary Lesion

Background: Lateral medullary stroke (LMS) results in a characteristic pattern of brainstem signs including ocular motor and vestibular deficits. Thus, an impaired angular vestibulo-ocular reflex (aVOR) may be found if the vestibular nuclei are affected.

Objective: We aimed to characterize the frequency and pattern of vestibular and ocular-motor deficits in patients with LMS.

Methods: Patients with MR-confirmed acute/subacute unilateral LMS from a stroke registry were included and a bedside neuro-otological examination was performed. Video-oculography and video-based head-impulse testing (vHIT) was obtained and semicircular canal function was determined. The lesion location/extension as seen on MRI was rated and involvement of the vestibular nuclei was judged.

Results: Seventeen patients with LMS (age = 59.4 ± 14.3 years) were included. All patients had positive H.I.N.T.S. vHIT showed mild-to-moderate aVOR impairments in three patients (ipsilesional = 1; ipsilesional and contralesional = 1; contralesional = 1). Spontaneous nystagmus (n = 10/15 patients) was more often beating contralesionally than ipsilesionally (6 vs. 3) and was accompanied by upbeat nystagmus in four patients. Head-shaking nystagmus was noted in seven subjects, ipsilesionally beating in six and down-beating in one. On brain MRI, damage of the most caudal parts of the medial and/or inferior vestibular nucleus was noted in 13 patients. Only those two patients with lesions affecting the rostral medulla oblongata demonstrated an ipsilaterally impaired aVOR.

Conclusions: While subtle ocular motor signs pointed to damage of the central–vestibular pathways in all 17 patients, aVOR deficits were infrequent, restricted to those patients with rostral medullary lesions and, if present, mild to moderate only. This can be explained by lesions located too far caudally and too far ventrally to substantially affect the vestibular nuclei.

Principles of operation of a cerebellar learning circuit

We provide behavioral evidence using monkey smooth pursuit eye movements for four principles of cerebellar learning. Using a circuit-level model of the cerebellum, we link behavioral data to learning's neural implementation. The four principles are: (1) early, fast, acquisition driven by climbing fiber inputs to the cerebellar cortex, with poor retention; (2) learned responses of Purkinje cells guide transfer of learning from the cerebellar cortex to the deep cerebellar nucleus, with excellent retention; (3) functionally different neural signals are subject to learning in the cerebellar cortex versus the deep cerebellar nuclei; and (4) negative feedback from the cerebellum to the inferior olive reduces the magnitude of the teaching signal in climbing fibers and limits learning. Our circuit-level model, based on these four principles, explains behavioral data obtained by strategically manipulating the signals responsible for acquisition and recall of direction learning in smooth pursuit eye movements across multiple timescales.

Cerebellar Rebound Nystagmus Explained as Gaze-Evoked Nystagmus Relative to an Eccentric Set Point: Implications for the Clinical Examination

A brain stem/cerebellar neural integrator enables stable eccentric gaze. Cerebellar loss-of-function can cause an inability to maintain gaze eccentrically (gaze-evoked nystagmus). Moreover, after returning gaze to straight ahead, the eyes may drift toward the prior eye position (rebound nystagmus). Typically, gaze-evoked nystagmus decays during continuously held eccentric gaze. We hypothesized this adaptive behavior to be prerequisite for rebound nystagmus and thus predicted a correlation between the velocity decay of gaze-evoked nystagmus and the initial velocity of rebound nystagmus. Using video-oculography, eye position was measured in 11 patients with cerebellar degeneration at nine horizontal gaze angles (15° nasal to 25° temporal) before (baseline), during, and after attempted eccentric gaze at ± 30° for 20 s. We determined the decrease of slow-phase velocity at eccentric gaze and the slow-phase velocity of the subsequent rebound nystagmus relative to the baseline. During sustained eccentric gaze, eye drift velocity of gaze-evoked nystagmus decreased by 2.40 ± 1.47°/s. Thereafter, a uniform change of initial eye drift velocity relative to the baseline (2.40 ± 1.35°/s) occurred at all gaze eccentricities. The velocity decrease during eccentric gaze and the subsequent uniform change of eye drift were highly correlated (R² = 0.80, p < 0.001, slope = 1.09). Rebound nystagmus can be explained as gaze-evoked nystagmus relative to a set point (position with least eye drift) away from straight-ahead eye position. To improve detection at the bedside, we suggest testing rebound nystagmus not at straight-ahead eye position but at an eccentric position opposite of prior eccentric gaze (e.g., 10°), ideally using quantitative video-oculography to facilitate diagnosis of cerebellar loss-of-function.

Vestibular, locomotor, and vestibulo-autonomic research: 50 years of collaboration with Bernard Cohen

My collaboration on the vestibulo-ocular reflex with Bernard Cohen began in 1972. Until 2017, this collaboration included studies of saccades, quick phases of nystagmus, the introduction of the concept of velocity storage, the relationship of velocity storage to motion sickness, primate and human locomotion, and studies of vasovagal syncope. These studies have elucidated the functioning of the vestibuloocular reflex, the locomotor system, the functioning of the vestibulo-sympathetic reflex, and how blood pressure and heart rate are controlled by the vestibular system. Although it is virtually impossible to review all the contributions in detail in a single paper, this article traces a thread of modeling that I brought to the collaboration, which, coupled with Bernie Cohen's expertise in vestibular and sensory-motor physiology and clinical insights, has broadened our understanding of the role of the vestibular system in a wide range of sensory-motor systems. Specifically, the paper traces how the concept of a relaxation oscillator was used to model the slow and rapid phases of ocular nystagmus. Velocity information that drives the slow compensatory eye movements was used to activate the saccadic system that resets the eyes, giving rise to the relaxation oscillator properties and simulated nystagmus as well as predicting the types of unit activity that generated saccades and nystagmic beats. The slow compensatory component of ocular nystagmus was studied in depth and gave rise to the idea that there was a velocity storage mechanism or integrator that not only is a focus for visual-vestibular interaction but also codes spatial orientation relative to gravity as referenced by the otoliths. Velocity storage also contributes to motion sickness when there are visual-vestibular as well as orientation mismatches in velocity storage. The relaxation oscillator concept was subsequently used to model the stance and swing phases of locomotion, how this impacted head and eye movements to maintain gaze in the direction of body motion, and how these were affected by Parkinson's disease. Finally, the relaxation oscillator was used to elucidate the functional form of the systolic and diastolic beats during blood pressure and how vasovagal syncope might be initiated by cerebellar-vestibular malfunction.

A neurologist and ataxia: using eye movements to learn about the cerebellum

The cerebellum, its normal functions and its diseases, and especially its relation to the control of eye movements, has been at the heart of my academic career. Here I review how this came about, with an emphasis on epiphanies, "tipping points" and the influences of mentors, colleagues and trainees. I set a path for young academicians, both clinicians and basic scientists, with some guidelines for developing a productive and rewarding career in neuroscience.

The cerebellum from the fetus to the elderly: history, advances, and future challenges

The cerebellum is now at the forefront of research in neuroscience. This is not just a coincidence, occurring about 250 years after the first description of the human cerebellum. The cerebellum contains the majority of neurons in the central nervous system and it is heavily connected with almost all cortical and subcortical areas of the supratentorial region as well as with the brainstem and the spinal cord. Cerebellar circuits are embedded in large-scale networks contributing to motor control and neurocognition. From a phenotypic standpoint, damage to cerebellar lobules interconnected with the sensorimotor cortices leads to a cerebellar motor syndrome, whereas lesions of the posterolateral cerebellum cause cognitive and neuropsychiatric impairments which may or may not be subtle. This topographic rule is valid in children and adults. Midline posterior vermal lesions cause behavioral/affective dysregulation, especially in kids. The extent of the spectrum of human cerebellar disorders is increasingly recognized from the fetus to the elderly, with recognition of consequences for the quality of life and socioeconomic costs due to lifelong morbidity of many cerebellar ataxias/pathologies. The prolonged duration of human cerebellar development makes the cerebellum especially susceptible to developmental disruption, both genetic and nongenetic. This explains the current emphasis on the clarification of the developmental course and impact of the cerebellum. The understanding of how germinal matrix zones and migration of neurons and glial cells end in a highly organized and foliated human cerebellum is essential. This is greatly accelerated by inputs from rodent developmental studies, in particular because cerebellar anatomy is conserved across species. Still, numerous questions on human fetal development remain unanswered. Although both advanced neuroimaging and genetic studies are currently leading to a better definition and understanding of the multitude of cerebellar symptoms, there is a gap, with a great need to develop therapies aiming at first, protection of the cerebellum during development, and second, restoration of cerebellar function in children and in adults. Dynamic profiles of the compensatory processes from newborns to elderly require specific studies.

Basic and translational neuro-ophthalmology of visually guided saccades: disorders of velocity

INTRODUCTION

Saccades are rapid, yoked eye movements in an effort to direct a target over fovea. The complex circuitry of saccadic eye movements has been exhaustively described. As a result clinicians can elegantly localize the pathology if it falls on the neuraxis responsible for saccades. Traditionally saccades are studied with their quantitative characteristics such as amplitude, velocity, duration, direction, latency and accuracy.

AREAS COVERED

Amongst all subtypes, the physiology of the visually guided saccades is most extensively studied. Here we will review the basic and pertinent neuro-anatomy and physiology of visually guided saccade and then discuss common or classic disorders affecting the velocity of visually guided saccades. We will then discuss the basic mechanism for saccade slowing in these disorders.

EXPERT COMMENTARY

Prompt appreciation of disorders of saccade velocity is critical to reach appropriate diagnosis. Disorders of midbrain, cerebellum, or basal ganglia can lead to prolonged transition time during gaze shift and decreased saccade velocity.

A geometric method for eigenvalue problems with low-rank perturbations

We consider the problem of finding the spectrum of an operator taking the form of a low-rank (rank one or two) non-normal perturbation of a well-understood operator, motivated by a number of problems of applied interest which take this form. We use the fact that the system is a low-rank perturbation of a solved problem, together with a simple idea of classical differential geometry (the envelope of a family of curves) to completely analyse the spectrum. We use these techniques to analyse three problems of this form: a model of the oculomotor integrator due to Anastasio & Gad (2007 J. Comput. Neurosci.22, 239-254. (doi:10.1007/s10827-006-0010-x)), a continuum integrator model, and a non-local model of phase separation due to Rubinstein & Sternberg (1992 IMA J. Appl. Math.48, 249-264. (doi:10.1093/imamat/48.3.249)).

Not moving: the fundamental but neglected motor function

The function of the motor system in preventing rather than initiating movement is often overlooked. Not only are its highest levels predominantly, and tonically, inhibitory, but in general behaviour it is often intermittent, characterized by relatively short periods of activity separated by longer periods of stillness: for most of the time we are not moving, but stationary. Furthermore, these periods of immobility are not a matter of inhibition and relaxation, but require us to expend almost as much energy as when we move, and they make just as many demands on the central nervous system in controlling their performance. The mechanisms that stop movement and maintain immobility have been a greatly neglected area of the study of the brain. This paper introduces the topics to be examined in this special issue of Philosophical Transactions, discussing the various types of stopping and stillness, the problems that they impose on the motor system, the kinds of neural mechanism that underlie them and how they can go wrong.This article is part of the themed issue 'Movement suppression: brain mechanisms for stopping and stillness'.

Gaze-evoked nystagmus induced by alcohol intoxication

KEY POINTS

The cerebellum is the core structure controlling gaze stability. Chronic cerebellar diseases and acute alcohol intoxication affect cerebellar function, inducing, among others, gaze instability as gaze-evoked nystagmus. Gaze-evoked nystagmus is characterized by increased centripetal eye-drift. It is used as an important diagnostic sign for patients with cerebellar degeneration and to assess the 'driving while intoxicated' condition. We quantified the effect of alcohol on gaze-holding using an approach allowing, for the first time, the comparison of deficits induced by alcohol intoxication and cerebellar degeneration. Our results showed that alcohol intoxication induces a two-fold increase of centripetal eye-drift. We establish analysis techniques for using controlled alcohol intake as a model to support the study of cerebellar deficits. The observed similarity between the effect of alcohol and the clinical signs observed in cerebellar patients suggests a possible pathomechanism for gaze-holding deficits.

ABSTRACT

Gaze-evoked nystagmus (GEN) is an ocular-motor finding commonly observed in cerebellar disease, characterized by increased centripetal eye-drift with centrifugal correcting saccades at eccentric gaze. With cerebellar degeneration being a rare and clinically heterogeneous disease, data from patients are limited. We hypothesized that a transient inhibition of cerebellar function by defined amounts of alcohol may provide a suitable model to study gaze-holding deficits in cerebellar disease. We recorded gaze-holding at varying horizontal eye positions in 15 healthy participants before and 30 min after alcohol intake required to reach 0.6‰ blood alcohol content (BAC). Changes in ocular-motor behaviour were quantified measuring eye-drift velocity as a continuous function of gaze eccentricity over a large range (±40 deg) of horizontal gaze angles and characterized using a two-parameter tangent model. The effect of alcohol on gaze stability was assessed analysing: (1) overall effects on the gaze-holding system, (2) specific effects on each eye and (3) differences between gaze angles in the temporal and nasal hemifields. For all subjects, alcohol consumption induced gaze instability, causing a two-fold increase [2.21 (0.55), median (median absolute deviation); P = 0.002] of eye-drift velocity at all eccentricities. Results were confirmed analysing each eye and hemifield independently. The alcohol-induced transient global deficit in gaze-holding matched the pattern previously described in patients with late-onset cerebellar degeneration. Controlled intake of alcohol seems a suitable disease model to study cerebellar GEN. With alcohol resulting in global cerebellar hypofunction, we hypothesize that patients matching the gaze-holding behaviour observed here suffered from diffuse deficits in the gaze-holding system as well.

Acquired pendular nystagmus

Acquired pendular nystagmus is comprised of quasi-sinusoidal oscillations of the eyes significantly affecting gaze holding and clarity of vision. The most common causes of acquired pendular nystagmus include demyelinating disorders such as multiple sclerosis and the syndrome of ocular palatal tremor. However, several other deficits, such as pharmacological intoxication, metabolic and genetic disorders, and granulomatous disorders can lead to syndromes mimicking acquired pendular nystagmus. Study of the kinematic features of acquired pendular nystagmus has suggested a putative pathophysiology of an otherwise mysterious neurological disorder. Here we review clinical features of neurological deficits that co-occur with acquired pendular nystagmus. Subsequent discussion of the pathophysiology of individual forms of pendular nystagmus speculates on mechanisms of the underlying disease while providing insights into pharmacotherapy of nystagmus.

Distinct neural circuits for control of movement vs. holding still

In generating a point-to-point movement, the brain does more than produce the transient commands needed to move the body part; it also produces the sustained commands that are needed to hold the body part at its destination. In the oculomotor system, these functions are mapped onto two distinct circuits: a premotor circuit that specializes in generating the transient activity that displaces the eyes and a "neural integrator" that transforms that transient input into sustained activity that holds the eyes. Different parts of the cerebellum adaptively control the motor commands during these two phases: the oculomotor vermis participates in fine tuning the transient neural signals that move the eyes, monitoring the activity of the premotor circuit via efference copy, whereas the flocculus participates in controlling the sustained neural signals that hold the eyes, monitoring the activity of the neural integrator. Here, I review the oculomotor literature and then ask whether this separation of control between moving and holding is a design principle that may be shared with other modalities of movement. To answer this question, I consider neurophysiological and psychophysical data in various species during control of head movements, arm movements, and locomotion, focusing on the brain stem, motor cortex, and hippocampus, respectively. The review of the data raises the possibility that across modalities of motor control, circuits that are responsible for producing commands that change the sensory state of a body part are distinct from those that produce commands that maintain that sensory state.

Purkinje Cells Directly Inhibit Granule Cells in Specialized Regions of the Cerebellar Cortex

Inhibition of granule cells plays a key role in gating the flow of signals into the cerebellum, and it is thought that Golgi cells are the only interneurons that inhibit granule cells. Here we show that Purkinje cells, the sole output neurons of the cerebellar cortex, also directly inhibit granule cells via their axon collaterals. Anatomical and optogenetic studies indicate that this non-canonical feedback is region specific: it is most prominent in lobules that regulate eye movement and process vestibular information. Collaterals provide fast, slow, and tonic inhibition to granule cells, and thus allow Purkinje cells to regulate granule cell excitability on multiple timescales. We propose that this feedback mechanism could regulate excitability of the input layer, contribute to sparse coding, and mediate temporal integration.

Cervical dystonia: a neural integrator disorder

Ocular motor neural integrators ensure that eyes are held steady in straight-ahead and eccentric positions of gaze. Abnormal function of the ocular motor neural integrator leads to centripetal drifts of the eyes with consequent gaze-evoked nystagmus. In 2002 a neural integrator, analogous to that in the ocular motor system, was proposed for the control of head movements. Recently, a counterpart of gaze-evoked eye nystagmus was identified for head movements; in which the head could not be held steady in eccentric positions on the trunk. These findings lead to a novel pathophysiological explanation in cervical dystonia, which proposed that the abnormalities of head movements stem from a malfunctioning head neural integrator, either intrinsically or as a result of impaired cerebellar, basal ganglia, or peripheral feedback. Here we briefly recapitulate the history of the neural integrator for eye movements, then further develop the idea of a neural integrator for head movements, and finally discuss its putative role in cervical dystonia. We hypothesize that changing the activity in an impaired head neural integrator, by modulating feedback, could treat dystonia.

Gaze holding deficits discriminate early from late onset cerebellar degeneration

The vestibulo-cerebellum calibrates the output of the inherently leaky brainstem neural velocity-to-position integrator to provide stable gaze holding. In healthy humans small-amplitude centrifugal nystagmus is present at extreme gaze-angles, with a non-linear relationship between eye-drift velocity and eye eccentricity. In cerebellar degeneration this calibration is impaired, resulting in pathological gaze-evoked nystagmus (GEN). For cerebellar dysfunction, increased eye drift may be present at any gaze angle (reflecting pure scaling of eye drift found in controls) or restricted to far-lateral gaze (reflecting changes in shape of the non-linear relationship) and resulting eyed-drift patterns could be related to specific disorders. We recorded horizontal eye positions in 21 patients with cerebellar neurodegeneration (gaze-angle = ±40°) and clinically confirmed GEN. Eye-drift velocity, linearity and symmetry of drift were determined. MR-images were assessed for cerebellar atrophy. In our patients, the relation between eye-drift velocity and gaze eccentricity was non-linear, yielding (compared to controls) significant GEN at gaze-eccentricities ≥20°. Pure scaling was most frequently observed (n = 10/18), followed by pure shape-changing (n = 4/18) and a mixed pattern (n = 4/18). Pure shape-changing patients were significantly (p = 0.001) younger at disease-onset compared to pure scaling patients. Atrophy centered around the superior/dorsal vermis, flocculus/paraflocculus and dentate nucleus and did not correlate with the specific drift behaviors observed. Eye drift in cerebellar degeneration varies in magnitude; however, it retains its non-linear properties. With different drift patterns being linked to age at disease-onset, we propose that the gaze-holding pattern (scaling vs. shape-changing) may discriminate early- from late-onset cerebellar degeneration. Whether this allows a distinction among specific cerebellar disorders remains to be determined.

Saccade detection using a particle filter

BACKGROUND

When healthy subjects track a moving target, "catch-up" saccades are triggered to compensate for the non-perfect tracking gain. The evaluation of the pursuit and/or saccade kinematics requires that saccade and pursuit components be separated from the eye movement trace. A similar situation occurs when analyzes eye movements of patients that could contain eye drifts between saccades. This task is especially difficult, because the range of saccadic amplitudes goes from microsaccades (less than 1°) to large exploratory saccades (40°).

NEW METHOD

In this paper we proposed a new algorithm to detect saccades based on a particle filter. The new method suppresses the baseline velocity component linked to smooth pursuit (or to eye drifts) and thus permits a constant threshold during a trial despite the smooth pursuit behavior. It also accounts for a wide range of saccade amplitudes.

RESULTS

The new method is validated with five different paradigms, microsaccades, microsaccades plus saccades with drift, linear target motion, non-linear target motion and free viewing. The sensitivity of the method to signal noise is analyzed.

COMPARISON WITH EXISTING METHODS

Traditional saccade detection algorithms using a velocity (or acceleration or jerk) threshold can be inadequate because of the baseline velocity component linked to the smooth pursuit (especially if the target motion is non-linear, i.e. not constant velocity) or to eye drifts between saccades.

CONCLUSIONS

The new method detects saccades in challenging situations involving eye movements between saccades (smooth pursuit and/or eye drifts) and unfiltered recordings.

Keeping your head on target

The mechanisms by which the human brain controls eye movements are reasonably well understood, but those for the head less so. Here, we show that the mechanisms for keeping the head aimed at a stationary target follow strategies similar to those for holding the eyes steady on stationary targets. Specifically, we applied the neural integrator hypothesis that originally was developed for holding the eyes still in eccentric gaze positions to describe how the head is held still when turned toward an eccentric target. We found that normal humans make head movements consistent with the neural integrator hypothesis, except that additional sensory feedback is needed, from proprioceptors in the neck, to keep the head on target. We also show that the complicated patterns of head movements in patients with cervical dystonia can be predicted by deficits in a neural integrator for head motor control. These results support ideas originally developed from animal studies that suggest fundamental similarities between oculomotor and cephalomotor control, as well as a conceptual framework for cervical dystonia that departs considerably from current clinical views.

Neuromimetic model of saccades for localizing deficits in an atypical eye-movement pathology

Background

When patients with ocular motor deficits come to the clinic, in numerous situations it is hard to relate their behavior to one or several deficient neural structures. We sought to demonstrate that neuromimetic models of the ocular motor brainstem could be used to test assumptions of the neural deficits linked to a patient’s behavior.

Methods

Eye movements of a patient with unexplained neurological pathology were recorded. We analyzed the patient’s behavior in terms of a neuromimetic saccadic model of the ocular motor brainstem to formulate a pathophysiological hypothesis.

Results

Our patient exhibited unusual ocular motor disorders including increased saccadic peak velocities (up to ≈1000 deg/s), dynamic saccadic overshoot, left-right asymmetrical post-saccadic drift and saccadic oscillations. We show that our model accurately reproduced the observed disorders allowing us to hypothesize that those disorders originated from a deficit in the cerebellum.

Conclusion

Our study suggests that neuromimetic models could be a good complement to traditional clinical tools. Our behavioral analyses combined with the model simulations localized four different features of abnormal eye movements to cerebellar dysfunction. Importantly, this assumption is consistent with clinical symptoms.

Gaze holding in healthy subjects

Eccentric gaze in darkness evokes minor centripetal eye drifts in healthy subjects, as cerebellar control sufficiently compensates for the inherent deficiencies of the brainstem gaze-holding network. This behavior is commonly described using a leaky integrator model, which assumes that eye velocity grows linearly with gaze eccentricity. Results from previous studies in patients and healthy subjects suggest caution when this assumption is applied to eye eccentricities larger than 20 degrees. To obtain a detailed characterization of the centripetal gaze-evoked drift, we recorded horizontal eye position in 20 healthy subjects. With their head fixed, they were asked to fixate a flashing dot (50 ms every 2 s)that was quasi-stationary displacing(0.5 deg/s) between ±40 deg horizontally in otherwise complete darkness. Drift velocity was weak at all angles tested. Linearity was assessed by dividing the range of gaze eccentricity in four bins of 20 deg each, and comparing the slopes of a linear function fitted to the horizontal velocity in each bin. The slopes of single subjects for gaze eccentricities of ±0−20 deg were, in median,0.41 times the slopes obtained for gaze eccentricities of ±20−40 deg. By smoothing the individual subjects' eye velocity as a function of gaze eccentricity, we derived a population of position-velocity curves. We show that a tangent function provides a better fit to the mean of these curves when large eccentricities are considered. This implies that the quasi-linear behavior within the typical ocular motor range is the result of a tuning procedure, which is optimized in the most commonly used range of gaze. We hypothesize that the observed non-linearity at eccentric gaze results from a saturation of the input that each neuron in the integrating network receives from the others. As a consequence, gaze-holding performance declines more rapidly at large eccentricities.

Neurological basis for eye movements of the blind

When normal subjects fix their eyes upon a stationary target, their gaze is not perfectly still, due to small movements that prevent visual fading. Visual loss is known to cause greater instability of gaze, but reported comparisons with normal subjects using reliable measurement techniques are few. We measured binocular gaze using the magnetic search coil technique during attempted fixation (monocular or binocular viewing) of 4 individuals with childhood-onset of monocular visual loss, 2 individuals with late-onset monocular visual loss due to age-related macular degeneration, 2 individuals with bilateral visual loss, and 20 healthy control subjects. We also measured saccades to visual or somatosensory cues. We tested the hypothesis that gaze instability following visual impairment is caused by loss of inputs that normally optimize the performance of the neural network (integrator), which ensures both monocular and conjugate gaze stability. During binocular viewing, patients with early-onset monocular loss of vision showed greater instability of vertical gaze in the eye with visual loss and, to a lesser extent, in the normal eye, compared with control subjects. These vertical eye drifts were much more disjunctive than upward saccades. In individuals with late monocular visual loss, gaze stability was more similar to control subjects. Bilateral visual loss caused eye drifts that were larger than following monocular visual loss or in control subjects. Accurate saccades could be made to somatosensory cues by an individual with acquired blindness, but voluntary saccades were absent in an individual with congenital blindness. We conclude that the neural gaze-stabilizing network, which contains neurons with both binocular and monocular discharge preferences, is under adaptive visual control. Whereas monocular visual loss causes disjunctive gaze instability, binocular blindness causes both disjunctive and conjugate gaze instability (drifts and nystagmus). Inputs that bypass this neural network, such as projections to motoneurons for upward saccades, remain conjugate.

What Sherrington missed: the ubiquity of the neural integrator

Despite its numerous illustrations unequivocally demonstrating the phenomenon, in Sherrington's Integrative Action of the Nervous System, he considered "integration" only in its spatial and coordinative aspects, and failed to notice time integration as an equally pervasive feature of all motor systems. First demonstrated in the oculomotor system by Robinson and others, in the vestibulo-ocular reflex, and then as a necessary component of the oculomotor "final common path" (another Sherringtonian concept), integration is manifested at two further levels: in generating optokinetic responses and in the mechanism of saccadic decision. But integration is not a purely oculomotor phenomenon: behind it lie two fundamental motor principles. First, that the brain operates in terms of change, implying differentiation in sensory systems and integration in motor ones. Second, that the molecular physiology of muscle contraction means that remaining still requires not only continual expenditure of energy but also continual computational effort--a firm and precise integrator.

Synaptic mechanism for the sustained activation of oculomotor integrator circuits in the rat prepositus hypoglossi nucleus: contribution of Ca2+-permeable AMPA receptors

Sustained neural activity is involved in several brain functions. Although recurrent/feedback excitatory networks are proposed as a neural mechanism for this sustained activity, the synaptic mechanisms have not been fully clarified. To address this issue, we investigated the excitatory synaptic responses of neurons in the prepositus hypoglossi nucleus (PHN), a brainstem structure involved as an oculomotor neural integrator, using whole-cell voltage-clamp recordings in rat slice preparations. Under a blockade of inhibitory synaptic transmissions, the application of "burst stimulation" (100 Hz, 20 pulses) to a brainstem area projecting to the PHN induced an increase in the frequency of EPSCs in PHN neurons that lasted for several seconds. Sustained EPSC responses were observed even when the burst stimulation was applied in the vicinity of a recorded neuron within the PHN that was isolated from the slices. Pharmacologically, the sustained EPSC responses were reduced by 1-naphthyl acetyl spermine (50 μm), a blocker of Ca(2+)-permeable AMPA (CP-AMPA) receptors. Analysis of the current-voltage (I-V) relationship of the current responses to iontophoretic application of kainate revealed that more than one-half of PHN neurons exhibited an inwardly rectifying I-V relationship. Furthermore, PHN neurons exhibiting inwardly rectifying current responses showed higher Ca(2+) permeability. The sustained EPSC responses were also reduced by flufenamic acid (200 μm), a blocker of Ca(2+)-activated nonselective cation (CAN) channels. These results indicate that the sustained EPSC responses are attributable to the sustained activation of local excitatory networks in the PHN, which arises from the activation of CP-AMPA receptors and CAN channels in PHN neurons.

Using input minimization to train a cerebellar model to simulate regulation of smooth pursuit

Cerebellar learning appears to be driven by motor error, but whether or not error signals are provided by climbing fibers (CFs) remains a matter of controversy. Here we show that a model of the cerebellum can be trained to simulate the regulation of smooth pursuit eye movements by minimizing its inputs from parallel fibers (PFs), which carry various signals including error and efference copy. The CF spikes act as "learn now" signals. The model can be trained to simulate the regulation of smooth pursuit of visual objects following circular or complex trajectories and provides insight into how Purkinje cells might encode pursuit parameters. In minimizing both error and efference copy, the model demonstrates how cerebellar learning through PF input minimization (InMin) can make movements more accurate and more efficient. An experimental test is derived that would distinguish InMin from other models of cerebellar learning which assume that CFs carry error signals.

A nonlinear model of the neural integrator improves detection of deficits in the human VOR

A nonlinear model has been proposed to describe the set-point-dependent characteristics of the neural integrator (NI) in the oculomotor system. It was shown to yield improved prediction of slow-phase eye position in the vestibulo-ocular reflex (VOR) of normal subjects, when compared to the classical linear model of the NI. In this paper, we compare the parameters of this nonlinear NI model fitted to VOR data from: 1) compensated subjects diagnosed with vestibular deficiencies such as vestibular neuronitis and Meniere's disease and 2) normal (symptom-free) subjects. The identified models exhibit more severe nonlinearity in VOR patients than the normal controls. Several of the identified parameters in patients unmask asymmetries and more context dependence in the NI and in the VOR gain that are consistent with the lesioned side and could serve to support detection of lesions even after compensation.