Central positional nystagmus simulated by a mathematical ocular motor model of otolith-dependent modification of Listing's plane

[Vertical downbeat nystagmus in benign paroxysmal positional vertigo]

Benign paroxysmal positional vertigo (BPPV) is the most common peripheral vestibulopathy. Ppatients with BPPV contact with otorhinolaryngologists in 25% of cases. Due to the polymorphism of this pathology, an urgent task is to asess differential diagnosis of BPPV with various variants of the central positional syndrome: vestibular migraine, myofascial cervical syndrome, organic changes in the structures of the cerebellum.

OBJECTIVE

To distinguish atypical forms of BPPV with downbeating vertical nystagmus and the central positional syndrome.

Central positional nystagmus: an update

Clinically, central positional nystagmus (CPN) is often suspected when atypical forms of its peripheral counterpart, i.e., benign paroxysmal positional vertigo (BPPV), are observed, namely a linear horizontal nystagmus as in horizontal canal BPPV or a downwardly and torsionally beating nystagmus as in anterior canal BPPV. Pathophysiologically, CPN is caused by cerebellar and/or brainstem dysfunction. Recent work has provided further insights into the different clinical phenotypes and the underlying pathomechanisms. We performed a PubMed review focused on the findings on CPN using the key words "Central Positional Nystagmus", "Central Positional Vertigo", "Positional Nystagmus" OR "Positioning Nystagmus" OR "Positional Vertigo" OR "Positioning Vertigo" AND "Central" from January 2015 to August 2021. CPN may account for up to 12% of patients with positional nystagmus. Clinical data on CPN are mostly based on case reports or small retrospective case series. CPN is frequently associated with cerebellar and/or brainstem structural lesions, namely stroke, tumours or demyelination, or diffuse involvement of these structures due to degenerative or autoimmune/paraneoplastic diseases; it is also found in patients with vestibular migraine. CPN can be paroxysmal or persistent. The direction of the nystagmus is often downward in head-hanging or apogeotropic in lateral supine positions; combinations of both forms also occur. Clinically it is important to note that CPN is often associated with other central, often cerebellar ocular motor or other neurological signs; typically, it is not improved by the therapeutic liberatory manoeuvres for BPPV. These additional features are also important for the diagnosis, in particular if no structural lesions are found. Pathophysiologically, CPN is believed to reflect an abnormal integration of semicircular canal-related signals by the cerebellar nodulus, uvula and/or tonsil, ultimately providing an erroneous estimation of the head tilt and/or eye position coordinates. The natural course of CPN remains, so far, largely unknown. Symptomatic treatment of CPN consists of pharmacotherapy, e.g., with 4-aminopyridine, and causative treatment of the underlying disease if known. CPN is an important differential diagnosis to BPPV and a clinically relevant entity with heterogenous clinical presentations and various pathomechanisms and etiologies. In particular, studies on the natural course and treatment of CPN are needed.

Therapy-Resistant Atypical Downbeat Nystagmus with Vertigo Confined to Specific Head-Hanging Positions: Mapping to the Gravity Vector on a Multi-Axis Turntable

Downbeat nystagmus (DBN) observed in head-hanging positions, may be of central or peripheral origin. Central DBN in head-hanging positions is mostly due to a disorder of the vestibulo-cerebellum, whereas peripheral DBN is usually attributed to canalolithiasis of an anterior semicircular canal. Here, we describe an atypical case of a patient who, after head trauma, experienced severe and stereotypic vertigo attacks after being placed in various head-hanging positions. Vertigo lasted 10–15 s and was always associated with a robust DBN. The provocation of transient vertigo and DBN, which both showed no decrease upon repetition of maneuvers, depended on the yaw orientation relative to the trunk and the angle of backward pitch. On a motorized, multi-axis turntable, we identified the two-dimensional Helmholtz coordinates of head positions at which vertigo and DBN occurred (y-axis: horizontal, space-fixed; z-axis: vertical, and head-fixed; x-axis: torsional, head-fixed, and unchanged). This two-dimensional area of DBN-associated head positions did not change when whole-body rotations took different paths (e.g., by forwarding pitch) or were executed with different velocities. Moreover, the intensity of DBN was also independent of whole-body rotation paths and velocities. So far, therapeutic approaches with repeated liberation maneuvers and cranial vibrations were not successful. We speculate that vertigo and DBN in this patient are due to macular damage, possibly an unstable otolithic membrane that, in specific orientations relative to gravity, slips into a position causing paroxysmal stimulation or inhibition of macular hair cells.

Characteristics and mechanism of apogeotropic central positional nystagmus

Here we characterize persistent apogeotropic type of central positional nystagmus, and compare it with the apogeotropic nystagmus of benign paroxysmal positional vertigo involving the lateral canal. Nystagmus was recorded in 27 patients with apogeotropic type of central positional nystagmus (22 with unilateral and five with diffuse cerebellar lesions) and 20 patients with apogeotropic nystagmus of benign paroxysmal positional vertigo. They were tested while sitting, while supine with the head straight back, and in the right and left ear-down positions. The intensity of spontaneous nystagmus was similar while sitting and supine in apogeotropic type of central positional nystagmus, but greater when supine in apogeotropic nystagmus of benign paroxysmal positional vertigo. In central positional nystagmus, when due to a focal pathology, the lesions mostly overlapped in the vestibulocerebellum (nodulus, uvula, and tonsil). We suggest a mechanism for apogeotropic type of central positional nystagmus based on the location of lesions and a model that uses the velocity-storage mechanism. During both tilt and translation, the otolith organs can relay the same gravito-inertial acceleration signal. This inherent ambiguity can be resolved by a 'tilt-estimator circuit' in which information from the semicircular canals about head rotation is combined with otolith information about linear acceleration through the velocity-storage mechanism. An example of how this mechanism works in normal subjects is the sustained horizontal nystagmus that is produced when a normal subject is rotated at a constant speed around an axis that is tilted away from the true vertical (off-vertical axis rotation). We propose that when the tilt-estimator circuit malfunctions, for example, with lesions in the vestibulocerebellum, the estimate of the direction of gravity is erroneously biased away from true vertical. If the bias is toward the nose, when the head is turned to the side while supine, there will be sustained, unwanted, horizontal positional nystagmus (apogeotropic type of central positional nystagmus) because of an inappropriate feedback signal indicating that the head is rotating when it is not.

Central positional nystagmus: Characteristics and model-based explanations

The central vestibular system operates to precisely estimate the rotational velocity and gravity orientation using the inherently ambiguous information from peripheral vestibular system. Therefore, any lesions disrupting this function can generate positional nystagmus. Central positional nystagmus (CPN) can be classified into the paroxysmal (transient) and persistent forms. The paroxysmal CPN has the features suggesting a semicircular canal origin regarding the latency, duration, and direction of nystagmus. Patients with paroxysmal CPN commonly show several different types of nystagmus classified according to the provoking positioning. The persistent form of CPN mostly appears as downbeat nystagmus while prone or supine, or apogeotropic or geotropic horizontal nystagmus when the head is turned to either side while supine. CPN may be ascribed to erroneous neural processing within the velocity-storage circuit that functions in estimating angular head velocity, gravity direction, and inertia. Paroxysmal CPN appears to be post-rotatory rebound nystagmus due to lesions involving the cerebellar nodulus and uvula. In contrast, persistent CPN may arise from erroneous gravity estimation. The overlap of lesion location responsible for both paroxysmal and persistent CPN may account for the frequent coexistence of both forms of nystagmus in a single patient.

A kinematic model for 3-D head-free gaze-shifts

Rotations of the line of sight are mainly implemented by coordinated motion of the eyes and head. Here, we propose a model for the kinematics of three-dimensional (3-D) head-unrestrained gaze-shifts. The model was designed to account for major principles in the known behavior, such as gaze accuracy, spatiotemporal coordination of saccades with vestibulo-ocular reflex (VOR), relative eye and head contributions, the non-commutativity of rotations, and Listing's and Fick constraints for the eyes and head, respectively. The internal design of the model was inspired by known and hypothesized elements of gaze control physiology. Inputs included retinocentric location of the visual target and internal representations of initial 3-D eye and head orientation, whereas outputs were 3-D displacements of eye relative to the head and head relative to shoulder. Internal transformations decomposed the 2-D gaze command into 3-D eye and head commands with the use of three coordinated circuits: (1) a saccade generator, (2) a head rotation generator, (3) a VOR predictor. Simulations illustrate that the model can implement: (1) the correct 3-D reference frame transformations to generate accurate gaze shifts (despite variability in other parameters), (2) the experimentally verified constraints on static eye and head orientations during fixation, and (3) the experimentally observed 3-D trajectories of eye and head motion during gaze-shifts. We then use this model to simulate how 2-D eye-head coordination strategies interact with 3-D constraints to influence 3-D orientations of the eye-in-space, and the implications of this for spatial vision.

Central paroxysmal positional nystagmus: Characteristics and possible mechanisms

OBJECTIVE

The diagnosis of central paroxysmal positional nystagmus (CPPN) is challenging, and the mechanisms require further elucidation. This study aimed to determine the characteristics and mechanisms of CPPN.

METHODS

Seventeen patients with CPPN were subjected to analyses of their clinical findings, MRI lesions, and oculographic data on spontaneous and positional nystagmus.

RESULTS

The direction of CPPN was mostly aligned with that of the head motion during the positioning, and 3 types of CPPN were identified: downbeat nystagmus on straight-head hanging, upbeat nystagmus on uprighting, and apogeotropic nystagmus during supine head roll test. The direction of CPPN was aligned with the vector sum of the rotational axes of the semicircular canals that were normally inhibited during the positioning. The intensity of evoked nystagmus was at its peak initially and then decreased exponentially over time. The time constants (TC) of the vertical CPPN ranged from 3 to 8 seconds, which corresponds to the TC of the vertical rotational vestibulo-ocular reflex. Sixteen patients (94.1%) showed more than one type of CPPN. Furthermore, persistent downbeat or apogeotropic positional nystagmus was associated in 11 patients (64.7%). Most patients with CPPN from a circumscribed brain lesion showed an involvement of the cerebellar nodulus or uvula.

CONCLUSION

CPPN may be ascribed to enhanced responses of the vestibular afferents due to lesions involving the nodulus and uvula. CPPN could be differentiated from benign paroxysmal positional nystagmus by positional nystagmus induced in multiple planes, temporal patterns of nystagmus intensity, and associated neurologic findings suggestive of central pathologies.

Three-dimensional visuo-motor control of saccades

Although the motion of the line of sight is a straightforward consequence of a particular rotation of the eye, it is much trickier to predict the rotation underlying a particular motion of the line of sight in accordance with Listing's law. Helmholtz's notion of the direction-circle together with the notion of primary and secondary reference directions in visual space provide an elegant solution to this reverse engineering problem, which the brain is faced with whenever generating a saccade. To test whether these notions indeed apply for saccades, we analyzed three-dimensional eye movements recorded in four rhesus monkeys. We found that on average saccade trajectories closely matched with the associated direction-circles. Torsional, vertical, and horizontal eye position of saccades scattered around the position predicted by the associated direction-circles with standard deviations of 0.5°, 0.3°, and 0.4°, respectively. Comparison of saccade trajectories with the likewise predicted fixed-axis rotations yielded mean coefficients of determinations (±SD) of 0.72 (±0.26) for torsion, 0.97 (±0.10) for vertical, and 0.96 (±0.11) for horizontal eye position. Reverse engineering of three-dimensional saccadic rotations based on visual information suggests that motor control of saccades, compatible with Listing's law, not only uses information on the fixation directions at saccade onset and offset but also relies on the computation of secondary reference positions that vary from saccade to saccade.

Intrinsic reference frames of superior colliculus visuomotor receptive fields during head-unrestrained gaze shifts

A sensorimotor neuron's receptive field and its frame of reference are easily conflated within the natural variability of spatial behavior. Here, we capitalized on such natural variations in 3-D eye and head positions during head-unrestrained gaze shifts to visual targets in two monkeys: to determine whether intermediate/deep layer superior colliculus (SC) receptive fields code visual targets or gaze kinematics, within four different frames of reference. Visuomotor receptive fields were either characterized during gaze shifts to visual targets from a central fixation position (32 U) or were partially characterized from each of three initial fixation points (31 U). Natural variations of initial 3-D gaze and head orientation (including torsion) provided spatial separation between four different coordinate frame models (space, head, eye, fixed-vector relative to fixation), whereas natural saccade errors provided spatial separation between target and gaze positions. Using a new statistical method based on predictive sum-of-squares, we found that in our population of 63 neurons (1) receptive field fits to target positions were significantly better than fits to actual gaze shift locations and (2) eye-centered models gave significantly better fits than the head or space frame. An intermediate frames analysis confirmed that individual neuron fits were distributed target-in-eye coordinates. Gaze position "gain" effects with the spatial tuning required for a 3-D reference frame transformation were significant in 23% (7/31) of neurons tested. We conclude that the SC primarily represents gaze targets relative to the eye but also carries early signatures of the 3-D sensorimotor transformation.

Nystagmus and saccadic intrusions

We review current concepts of nystagmus and saccadic oscillations, applying a pathophysiological approach. We begin by discussing how nystagmus may arise when the mechanisms that normally hold gaze steady are impaired. We then describe the clinical and laboratory evaluation of patients with ocular oscillations. Next, we systematically review the features of nystagmus arising from peripheral and central vestibular disorders, nystagmus due to an abnormal gaze-holding mechanism (neural integrator), and nystagmus occurring when vision is compromised. We then discuss forms of nystagmus for which the pathogenesis is not well understood, including acquired pendular nystagmus and congenital forms of nystagmus. We then summarize the spectrum of saccadic disorders that disrupt steady gaze, from intrusions to flutter and opsoclonus. Finally, we review current treatment options for nystagmus and saccadic oscillations, including drugs, surgery, and optical methods. Examples of each type of nystagmus are provided in the form of figures.

A model-based theory on the origin of downbeat nystagmus

The pathomechanism of downbeat nystagmus (DBN), an ocular motor sign typical for vestibulo-cerebellar lesions, remains unclear. Previous hypotheses conjectured various deficits such as an imbalance of central vertical vestibular or smooth pursuit pathways to be causative for the generation of spontaneous upward drift. However, none of the previous theories explains the full range of ocular motor deficits associated with DBN, i.e., impaired vertical smooth pursuit (SP), gaze evoked nystagmus, and gravity dependence of the upward drift. We propose a new hypothesis, which explains the ocular motor signs of DBN by damage of the inhibitory vertical gaze-velocity sensitive Purkinje cells (PCs) in the cerebellar flocculus (FL). These PCs show spontaneous activity and a physiological asymmetry in that most of them exhibit downward on-directions. Accordingly, a loss of vertical floccular PCs will lead to disinhibition of their brainstem target neurons and, consequently, to spontaneous upward drift, i.e., DBN. Since the FL is involved in generation and control of SP and gaze holding, a single lesion, e.g., damage to vertical floccular PCs, may also explain the associated ocular motor deficits. To test our hypothesis, we developed a computational model of vertical eye movements based on known ocular motor anatomy and physiology, which illustrates how cortical, cerebellar, and brainstem regions interact to generate the range of vertical eye movements seen in healthy subjects. Model simulation of the effect of extensive loss of floccular PCs resulted in ocular motor features typically associated with cerebellar DBN: (1) spontaneous upward drift due to decreased spontaneous PC activity, (2) gaze evoked nystagmus corresponding to failure of the cerebellar loop supporting neural integrator function, (3) asymmetric vertical SP deficit due to low gain and asymmetric attenuation of PC firing, and (4) gravity-dependence of DBN caused by an interaction of otolith-ocular pathways with impaired neural integrator function.

A minute demyelinating lesion causing acute positional vertigo

Clinico-anatomical correlations in multiple sclerosis patients presenting with central positional vertigo are lacking. We report on a patient with acute onset positional vertigo mimicking benign paroxysmal positional vertigo with a single enhancing lesion in the inner part of the superior cerebellar peduncle, disclosed only after thin slice MR-imaging. This location appears to be a common cause of central positional vertigo and should be regarded as characteristic for demyelinating rather than vascular pathology. In cases presenting with positional nystagmus and vertigo without other cerebellar deficits one should look explicitly for signal abnormalities in the inner part of the superior cerebellar peduncle. High spatial resolution-MRI seems to be mandatory for lesion detection.

Positional nystagmus and vertigo due to a solitary brachium conjunctivum plaque

The authors describe two patients suffering from demyelinating central nervous system disease who developed intense vertigo and downbeat nystagmus upon tilting their heads relative to gravity. Brain MRI revealed in both cases a single, small active lesion in the right brachium conjunctivum. The disruption of otolithic signals carried in brachium conjunctivum fibres connecting the fastigial nucleus with the vestibular nuclei is thought to be causatively involved, in agreement with a recently formulated model simulating central positional nystagmus. Insufficient otolithic information results in erroneous adjustment of the Listing's plane in off-vertical head positions, thus producing nystagmic eye movements.

Dissociated Hysteresis of Static Ocular Counterroll in Humans

In stationary head roll positions, the eyes are cyclodivergent. We asked whether this phenomenon can be explained by a static hysteresis that differs between the eyes contra- (CE) and ipsilateral (IE) to head roll. Using a motorized turntable, healthy human subjects ( n = 8) were continuously rotated about the earth-horizontal naso-occipital axis. Starting from the upright position, a total of three full rotations at a constant velocity (2°/s) were completed (acceleration = 0.05°/s2, velocity plateau reached after 40 s). Subjects directed their gaze on a flashing laser dot straight ahead (switched on 20 ms every 2 s). Binocular three-dimensional eye movements were recorded with dual search coils that were modified (wires exiting inferiorly) to minimize torsional artifacts by the eyelids. A sinusoidal function with a first and second harmonic was fitted to torsional eye position as a function of torsional whole body position at constant turntable velocity. The amplitude and phase of the first harmonic differed significantly between the two eyes (paired t-test: P < 0.05): on average, counterroll amplitude of IE was larger [CE: 6.6 ± 1.6° (SD); IE: 8.1 ± 1.7°), whereas CE showed more position lag relative to the turntable (CE: 12.5 ± 10.7°; IE: 5.1 ± 8.7°). We conclude that cyclodivergence observed during static ocular counterroll is mainly a result of hysteresis that depends on whether eyes are contra- or ipsilateral to head roll. Static hysteresis also explains the phenomenon of residual torsion, i.e., an incomplete torsional return of the eyes when the first 360° whole body rotation was completed and subjects were back in upright position (extorsion of CE: 2.0 ± 0.10°; intorsion of IE: 1.4 ± 0.10°). A computer model that includes asymmetric backlash for each eye can explain dissociated torsional hysteresis during quasi-static binocular counterroll. We hypothesize that ocular torsional hysteresis is introduced at the level of the otolith pathways because the direction-dependent torsional position lag of the eyes is related to the head roll position and not the eye position.

Skew Deviation Revisited

Skew deviation is a vertical misalignment of the eyes caused by damage to prenuclear vestibular input to ocular motor nuclei. The resultant vertical ocular deviation is relatively comitant in nature, and is usually seen in the context of brainstem or cerebellar injury from stroke, multiple sclerosis, or trauma. Skew deviation is usually accompanied by binocular torsion, torticollis, and a tilt in the subjective visual vertical. This constellation of findings has been termed the ocular tilt reaction. In the past two decades, a clinical localizing value for skew deviation has been assigned, and a cogent vestibular mechanism for comitant and incomitant variants of skew deviation has been proposed. Our understanding of skew deviation as a manifestation of central otolithic dysfunction in different planes of three-dimensional space is evolving. The similar spectrum of vertical ocular deviations arising in patients with congenital strabismus may further expand the nosology of skew deviation to include vergence abnormalities caused by the effects of early binocular visual imbalance on the developing visual system.

Present concepts of oculomotor organization

This chapter gives an introduction to the oculomotor system, thus providing a framework for the subsequent chapters. This chapter describes the characteristics, and outlines the structures involved, of the five basic types of eye movements, for gaze holding ("neural integrator") and eye movements in three dimensions (Listing's law, pulleys).

Magnetic Resonance Imaging of Human Extraocular Muscles During Static Ocular Counter-Rolling

The rectus extraocular muscle (EOM) pulleys constrain EOM paths. During visual fixation with head immobile, actively controlled pulleys are known to maintain positions causing EOM pulling directions to change by one-half the change in eye position. This pulley behavior is consistent with Listing's law (LL) of ocular torsion as observed during fixation, saccades, and pursuit. However, pulley behavior during the vestibulo-ocular reflex (VOR) has been unstudied. This experiment studied ocular counter-rolling (OCR), a static torsional VOR that violates LL but can be evoked during MRI. Tri-planar MRI was performed in 10 adult humans during central target fixation while positioned in right and left side down positions known to evoke static OCR. EOM cross-sections and paths were determined from area centroids. Paths were used to locate pulleys in three dimensions. Significant ( P < 0.025) counter-rotational repositioning of the rectus pulley arrays of both orbits was observed in the coronal plane averaging 4.1° (maximum, 8.7°) from right to left side down positions for the inferior, medial, and superior rectus pulleys. There was a trend for the lateral rectus averaging 1.4°. Torsional shift of the rectus pulley array was associated with significant contractile cross-section changes in the superior and inferior oblique muscles. Torsional rectus pulley shift during OCR, which changes pulling directions of the rectus EOMs, correlates with known insertions of the oblique EOM orbital layers on rectus pulleys. The amount of pulley reconfiguration is roughly one-half of published values of ocular torsion during static OCR, an arrangement that would cause rectus pulling directions to change by less than one-half the amount of ocular torsion.

Distributed population mechanism for the 3-D oculomotor reference frame transformation

Human saccades require a nonlinear, eye orientation-dependent reference frame transformation to transform visual codes to the motor commands for eye muscles. Primate neurophysiology suggests that this transformation is performed between the superior colliculus and brain stem burst neurons, but provides little clues as to how this is done. To understand how the brain might accomplish this, we trained a 3-layer neural net to generate accurate commands for kinematically correct 3-D saccades. The inputs to the network were a 2-D, eye-centered, topographic map of Gaussian visual receptive fields and an efference copy of eye position in 6-dimensional, push-pull "neural integrator" coordinates. The output was an eye orientation displacement command in similar coordinates appropriate to drive brain stem burst neurons. The network learned to generate accurate, kinematically correct saccades, including the eye orientation-dependent tilts in saccade motor error commands required to match saccade trajectories to their visual input. Our analysis showed that the hidden units developed complex, eye-centered visual receptive fields, widely distributed fixed-vector motor commands, and "gain field"-like eye position sensitivities. The latter evoked subtle adjustments in the relative motor contributions of each hidden unit, thereby rotating the population motor vector into the correct correspondence with the visual target input for each eye orientation: a distributed population mechanism for the visuomotor reference frame transformation. These findings were robust; there was little variation across networks with between 9 and 49 hidden units. Because essentially the same observations have been reported in the visuomotor transformations of the real oculomotor system, as well as other visuomotor systems (although interpreted elsewhere in terms of other models) we suggest that the mechanism for visuomotor reference frame transformations identified here is the same solution used in the real brain.

Eyes on Target: What Neurons Must do for the Vestibuloocular Reflex During Linear Motion

A gaze-stabilization reflex that has been conserved throughout evolution is the rotational vestibuloocular reflex (RVOR), which keeps images stable on the entire retina during head rotation. An ethological newer reflex, the translational or linear VOR (TVOR), provides fast foveal image stabilization during linear motion. Whereas the sensorimotor processing has been extensively studied in the RVOR, much less is currently known about the neural organization of the TVOR. Here we summarize the computational problems faced by the system and the potential solutions that might be used by brain stem and cerebellar neurons participating in the VORs. First and foremost, recent experimental and theoretical evidence has shown that, contrary to popular beliefs, the sensory signals driving the TVOR arise from both the otolith organs and the semicircular canals. Additional unresolved issues include a scaling by both eye position and vergence angle as well as the temporal transformation of linear acceleration signals into eye-position commands. Behavioral differences between the RVOR and TVOR, as well as distinct differences in neuroanatomical and neurophysiological properties, raise multiple functional questions and computational issues, only some of which are readily understood. In this review, we provide a summary of what is known about the functional properties and neural substrates for this oculomotor system and outline some specific hypotheses about how sensory information is centrally processed to create motor commands for the VORs.

Neural control of three-dimensional eye and head movements

Although the eyes and head can potentially rotate about any three-dimensional axis during orienting gaze shifts, behavioral recordings have shown that certain lawful strategies--such as Listing's law and Donders' law--determine which axis is used for a particular sensory input. Here, we review recent advances in understanding the neuromuscular mechanisms for these laws, the neural mechanisms that control three-dimensional head posture, and the neural mechanisms that coordinate three-dimensional eye orientation with head motion. Finally, we consider how the brain copes with the perceptual consequences of these motor acts.

Static ocular counterroll is implemented through the 3-D neural integrator

Static head roll about the naso-occipital axis is known to produce an opposite ocular counterroll with a gain of approximately 10%, but the purpose and neural mechanism of this response remain obscure. In theory counterroll could be maintained either by direct tonic vestibular inputs to motoneurons, or by a neurally integrated pulse, as observed in the saccade generator and vestibulo-ocular reflex. When simulated together with ocular drift related to torsional integrator failure, the direct tonic input model predicted that the pattern of drift would shift torsionally as in ordinary counterroll, but the integrated pulse model predicted that the equilibrium position of torsional drift would be unaffected by head roll. This was tested experimentally by measuring ocular counterroll in 2 monkeys after injection of muscimol into the mesencephalic interstitial nucleus of Cajal. Whereas 90 degrees head roll produced a mean ocular counterroll of 8.5 degrees (+/-0.7 degrees SE) in control experiments, the torsional equilibrium position observed during integrator failure failed to counterroll, showing a torsional shift of only 0.3 degrees (+/-0.6 degrees SE). This result contradicted the direct tonic input model, but was consistent with models that implement counterroll by a neurally integrated pulse.

Gravity modulates Listing's plane orientation during both pursuit and saccades

Previous studies have shown that the spatial organization of all eye orientations during visually guided saccadic eye movements (Listing's plane) varies systematically as a function of static and dynamic head orientation in space. Here we tested if a similar organization also applies to the spatial orientation of eye positions during smooth pursuit eye movements. Specifically, we characterized the three-dimensional distribution of eye positions during horizontal and vertical pursuit (0.1 Hz, +/-15 degrees and 0.5 Hz, +/-8 degrees) at different eccentricities and elevations while rhesus monkeys were sitting upright or being statically tilted in different roll and pitch positions. We found that the spatial organization of eye positions during smooth pursuit depends on static orientation in space, similarly as during visually guided saccades and fixations. In support of recent modeling studies, these results are consistent with a role of gravity on defining the parameters of Listing's law.

Three-dimensional eye position and slow phase velocity in humans with downbeat nystagmus

Downbeat nystagmus (DN), a fixation nystagmus with the fast phases directed downward, is usually caused by cerebellar lesions, but the precise etiology is not known. A disorder of the smooth-pursuit system or of central vestibular pathways has been proposed. However, both hypotheses fail to explain why DN is usually accompanied by gaze-holding nystagmus, which implies a leaky neural velocity-to-position integrator. Because three-dimensional (3-D) analysis of nystagmus slow phases provides an excellent means for testing both hypotheses, we examined 19 patients with DN during a fixation task and compared them with healthy subjects. We show that the presentation of DN patients is not uniform; they can be grouped according to their deficits: DN with vertical integrator leakage, DN with vertical and horizontal integrator leakage, and DN without integrator leakage. The 3-D analysis of the slow phases of DN patients revealed that DN is most likely neither caused by damage to central vestibular pathways carrying semicircular canal information nor by a smooth pursuit imbalance. We propose that the observed effects can be explained by partial damage of a brain stem-cerebellar loop that augments the time constant of the neural velocity to position integrators in the brain stem and neurally adjusts the orientation of Listing's plane.

Gravity dependence of ocular drift in patients with cerebellar downbeat nystagmus

Downbeat nystagmus is a frequent ocular motor sign in patients with lesions of the vestibulocerebellum. The upward drift in downbeat nystagmus is a combination of a gaze-evoked drift, due to an impaired vertical neural integrator, and a velocity bias. Using a three-dimensional turntable, we analyzed the influence of gravity on these two mechanisms. Patients with cerebellar downbeat nystagmus (n = 6) and healthy subjects (n = 12) were placed in various whole-body positions along the roll, pitch, and oblique vertical planes of the head. Ocular drift was monitored with scleral search coils. Although there was no gravity dependence of the vertical gaze-evoked drift, the vertical velocity bias consisted of two components: a gravity-dependent component that sinusoidally modulated as a function of body position along the pitch plane, and a gravity-independent component that was directed upward. The combination of the two components led to an overall drift that was minimal in supine and maximal in prone position. In healthy subjects, only the gravity-dependent component was present, but in a scaled-down manner. Our results suggest that the intact vestibulocerebellum minimizes an overacting otolith-ocular reflex elicited by pitch tilt and cancels an inherent upward ocular drift that is independent of gravity-modulated otolith signals.