Discharge dynamics of oculomotor neural integrator neurons during conjugate and disjunctive saccades and fixation

Unilateral Pendular Nystagmus in Multiple Sclerosis: A Case Series

BACKGROUND

Acquired pendular nystagmus is most often seen in patients with demyelinating disease. Although it is often bilateral, rare cases may be monocular. There is paucity of data on the spectrum of clinical presentation, underlying mechanism, and response to treatment in patients with monocular pendular nystagmus.

METHODS

Retrospective case series of patients with monocular pendular nystagmus seen in 2 tertiary neuro-ophthalmology clinics between January 2019 and June 2022. All patients underwent a complete neuro-ophthalmological assessment and MRI.

RESULTS

We describe 5 patients (3 women) aged 31-49 with monocular pendular nystagmus. All had a diagnosis of multiple sclerosis. Three patients had horizontal and 2 had vertical pendular nystagmus. The Snellen visual acuity in the eye with pendular nystagmus varied from 20/20 to 20/200. Two patients were asymptomatic and 3 suffered visually debilitating oscillopsia. Treatment response was available for 2 patients, both of which responded well to treatment with memantine. The pendular nystagmus was observed in the eye with worse visual acuity in 4 of 5 cases (80%). Three patients had bilateral pontine lesions, and 2 had unilateral pontine lesion ipsilateral to the side of nystagmus.

CONCLUSIONS

Monocular pendular nystagmus in adults is seen most often in patients with multiple sclerosis. Asymmetry in brainstem lesions and afferent visual input may be the culprit. Treatment with memantine may result in significant improvement in symptomatic patients.

Kinematically distinct saccades are used in a context-dependent manner by larval zebrafish

Saccades are rapid eye movements that are used by all species with good vision. In this study, we set out to characterize the complete repertoire of larval zebrafish horizontal saccades to gain insight into their contributions to visually guided behavior and underlying neural control. We identified five saccade types, defined by systematic differences in kinematics and binocular coordination, which were differentially expressed across a variety of behavioral contexts. Conjugate saccades formed a large group that serves at least four functions. These include fast phases of the optokinetic nystagmus, visual scanning in stationary animals, and shifting gaze in coordination with body turns. In addition, we discovered a previously undescribed pattern of eye-body coordination in which small conjugate saccades partially oppose head rotation to maintain gaze during forward locomotion. Convergent saccades were coordinated with body movements to foveate prey targets during hunting. Detailed kinematic analysis showed that conjugate and convergent saccades differed in the millisecond coordination of the eyes and body and followed distinct velocity main sequence relationships. This challenges the prevailing view that all horizontal saccades are controlled by a common brainstem circuit and instead indicates saccade-type-specific neural control.

Dimensionality reduction reveals separate translation and rotation populations in the zebrafish hindbrain

In many brain areas, neuronal activity is associated with a variety of behavioral and environmental variables. In particular, neuronal responses in the zebrafish hindbrain relate to oculomotor and swimming variables as well as sensory information. However, the precise functional organization of the neurons has been difficult to unravel because neuronal responses are heterogeneous. Here, we used dimensionality reduction methods on neuronal population data to reveal the role of the hindbrain in visually driven oculomotor behavior and swimming. We imaged neuronal activity in zebrafish expressing GCaMP6s in the nucleus of almost all neurons while monitoring the behavioral response to gratings that rotated with different speeds. We then used reduced-rank regression, a method that condenses the sensory and motor variables into a smaller number of "features," to predict the fluorescence traces of all ROIs (regions of interest). Despite the potential complexity of the visuo-motor transformation, our analysis revealed that a large fraction of the population activity can be explained by only two features. Based on the contribution of these features to each ROI's activity, ROIs formed three clusters. One cluster was related to vergent movements and swimming, whereas the other two clusters related to leftward and rightward rotation. Voxels corresponding to these clusters were segregated anatomically, with leftward and rightward rotation clusters located selectively to the left and right hemispheres, respectively. Just as described in many cortical areas, our analysis revealed that single-neuron complexity co-exists with a simpler population-level description, thereby providing insights into the organization of visuo-motor transformations in the hindbrain.

A covered eye fails to follow an object moving in depth

To clearly view approaching objects, the eyes rotate inward (vergence), and the intraocular lenses focus (accommodation). Current ocular control models assume both eyes are driven by unitary vergence and unitary accommodation commands that causally interact. The models typically describe discrete gaze shifts to non-accommodative targets performed under laboratory conditions. We probe these unitary signals using a physical stimulus moving in depth on the midline while recording vergence and accommodation simultaneously from both eyes in normal observers. Using monocular viewing, retinal disparity is removed, leaving only monocular cues for interpreting the object's motion in depth. The viewing eye always followed the target's motion. However, the occluded eye did not follow the target, and surprisingly, rotated out of phase with it. In contrast, accommodation in both eyes was synchronized with the target under monocular viewing. The results challenge existing unitary vergence command theories, and causal accommodation-vergence linkage.

Is Primate Lens Accommodation Unilaterally or Bilaterally Controlled?

Purpose

In frontal-eyed mammals such as primates, eye movements are coordinated so that the lines of sight are directed at targets in a manner that adjusts for target distance. The lens of each eye must also be adjusted with respect to target distance to maintain precise focus. Whether the systems for controlling eye movements are monocularly or binocularly organized is currently a point of contention. We recently determined that the premotor neurons controlling the lens of one eye are bilaterally distributed in the midbrain. In this study, we examine whether this is due to premotor neurons projecting bilaterally to the preganglionic Edinger-Westphal nuclei, or by a mixture of ipsilaterally and contralaterally projecting cells supplying each nucleus.

Methods

The ciliary muscles of Macaca fasicularis monkeys were injected with recombinant forms of the N2c rabies virus, one eye with virus that produced a green fluorescent marker and the other eye with a virus that produced a red fluorescent marker.

Results

Preganglionic motoneurons in the Edinger-Westphal nucleus displayed the same marker as the ipsilateral injected muscle. Many of the premotor neurons in the supraoculomotor area and central mesencephalic reticular formation were doubly labeled. Others were labeled from either the ipsilateral or contralateral eye.

Conclusions

These results suggest that both monocular control and binocular control of lens accommodation are present. Binocular inputs yoke the accommodation in the two eyes. Monocular inputs may allow modification related to differences in each eye's target distance or differences in the capacities of the two ciliary muscles.

Abnormal Tuning in Nucleus Prepositus Hypoglossi of Monkeys With "A" Pattern Exotropia

Purpose

In many individuals with pattern strabismus, the vertical misalignment varies with horizontal eye position. It has been proposed that these cross-axis effects result from abnormal cross-talk between brainstem structures that would normally encode horizontal and vertical eye position and velocity. The nucleus prepositus hypoglossi (NPH) is an ideal structure to test this overarching hypothesis. Neurons in the NPH are believed to mathematically integrate eye velocity signals to generate a tonic signal related to horizontal eye position. We hypothesized that, in monkeys with A-pattern exotropia and vertical inconcomitance, these neurons would show an abnormally large sensitivity to vertical eye position.

Methods

Three rhesus monkeys (1 normal and 2 with A-pattern exotropia) were trained to maintain fixation on a visual target as it stepped to various locations on a tangent screen. Extracellular neural activity was recorded from neurons in the NPH. Each neuron's sensitivity to horizontal and vertical eye position was estimated using multiple linear regression and preferred directions computed for each eye.

Results

Unexpectedly, the mean preferred directions for the left eye were normal in the monkeys with A-pattern exotropia. For the right eye, there was a clear upward deviation for the right NPH and a downward deviation for the left NPH. In addition, the R² values were significantly lower for model fits for neurons recorded from the exotropic monkeys.

Conclusions

We suggest that vertical inconcomitance results from inappropriate vertical-to-horizontal cross-talk that affects the two eyes differently.

Different Activation Mechanisms of Excitatory Networks in the Rat Oculomotor Integrators for Vertical and Horizontal Gaze Holding

Gaze holding in the horizontal and vertical directions is separately controlled via the oculomotor neural integrators, the prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC), respectively. Our previous in vitro studies demonstrated that transient, high-frequency local stimulation of the PHN and the INC increased the frequency of spontaneous EPSCs that lasted for several seconds. The sustained EPSC response of PHN neurons was attributed to the activation of local excitatory networks primarily mediated via Ca²⁺-permeable AMPA (CP-AMPA) receptors and Ca²⁺-activated nonselective cation (CAN) channels. However, the contribution of CP-AMPA receptors to the activation of INC excitatory networks appeared to be small. In this study, we clarified the mechanisms of excitatory network activation in the PHN and INC using whole-cell recordings in rat brainstem slices. Although physiological and histological analyses showed that neurons that expressed CP-AMPA receptors existed not only in the PHN but also in the INC, the effect of a CP-AMPA receptor antagonist on the sustained EPSC response was significantly weaker in INC neurons than in PHN neurons. Meanwhile, the effect of an NMDA receptor antagonist on the sustained EPSC response was significantly stronger in INC neurons than in PHN neurons. Furthermore, the current and the charge transfer mediated via NMDA receptors were significantly larger in INC neurons than in PHN neurons. These results strongly suggest that these excitatory networks are activated via different synaptic mechanisms: a CP-AMPA receptor and CAN channel-dependent mechanism and an NMDA receptor-dependent mechanism in horizontal and vertical integrators, respectively.

Functional architecture underlying binocular coordination of eye position and velocity in the larval zebrafish hindbrain

BACKGROUND

The oculomotor integrator (OI) in the vertebrate hindbrain transforms eye velocity input into persistent position coding output, which plays a crucial role in retinal image stability. For a mechanistic understanding of the integrator function and eye position control, knowledge about the tuning of the OI and other oculomotor nuclei is needed. Zebrafish are increasingly used to study integrator function and sensorimotor circuits, yet the precise neuronal tuning to motor variables remains uncharacterized.

RESULTS

Here, we recorded cellular calcium signals while evoking monocular and binocular optokinetic eye movements at different slow-phase eye velocities. Our analysis reveals the anatomical distributions of motoneurons and internuclear neurons in the nucleus abducens as well as those of oculomotor neurons in caudally adjacent hindbrain volumes. Each neuron is tuned to eye position and/or velocity to variable extents and is only activated after surpassing particular eye position and velocity thresholds. While the abducens (rhombomeres 5/6) mainly codes for eye position, in rhombomeres 7/8, a velocity-to-position coding gradient exists along the rostro-caudal axis, which likely corresponds to the oculomotor structures storing velocity and position, and is in agreement with a feedforward mechanism of persistent activity generation. Position encoding neurons are recruited at eye position thresholds distributed across the behaviourally relevant dynamic range, while velocity-encoding neurons have more centred firing thresholds for velocity. In the abducens, neurons coding exclusively for one eye intermingle with neurons coding for both eyes. Many of these binocular neurons are preferentially active during conjugate eye movements and less active during monocular eye movements. This differential recruitment during monocular versus conjugate tasks represents a functional diversification in the final common motor pathway.

CONCLUSIONS

We localized and functionally characterized the repertoire of oculomotor neurons in the zebrafish hindbrain. Our findings provide evidence for a mixed but task-specific binocular code and suggest that generation of persistent activity is organized along the rostro-caudal axis in the hindbrain.

Modulation of acquired monocular pendular nystagmus in multiple sclerosis: A modeling approach

Acquired pendular nystagmus (APN) often occurs in association with the disorders affecting the visual system, such as multiple sclerosis (MS). The proposed mechanisms of APN in MS have been a delayed conduction of the visual information for ocular stabilization and unstable neural integrator for feedback controls. We determined the effects of visual inputs on the nystagmus intensity and the effects of saccades on phase shift of the nystagmus in a patient with monocular pendular nystagmus from MS. In this patient, (1) during binocular viewing in the light, the nystagmus was observed only in the eye with more severe visual loss, (2) the nystagmus disappeared in darkness, (3) monocular viewing with either eye markedly suppressed the nystagmus, (4) the nystagmus decreased when the visual inputs became less asymmetric between the eyes, and (5) saccades resulted in a phase shift of the nystagmus. From these results, we propose that the difference in the visual inputs between the eyes is responsible for monocular APN by disturbing visual integration and increasing instability of the feedback.

Neural Plasticity Following Surgical Correction of Strabismus in Monkeys

Purpose

Although widely practiced, surgical treatment of strabismus has varying levels of success and permanence. In this study we investigated adaptive responses within the brain and the extraocular muscles (EOM) that occur following surgery and therefore determine long-term success of the treatment.

Methods

Single cell responses were collected from cells in the oculomotor and abducens nuclei before and after two monkeys (M1, M2) with exotropia (divergent strabismus) underwent a strabismus correction surgery that involved weakening of the lateral rectus (LR) and strengthening of the medial rectus (MR) muscle of one eye. Eye movement and neuronal data were collected for up to 10 months after surgery during a monocular viewing smooth-pursuit task. These data were fit with a first-order equation and resulting coefficients were used to estimate the population neuronal drive (ND) to each EOM of both eyes.

Results

Surgery resulted in a ∼70% reduction in strabismus angle in both animals that reverted toward presurgical misalignment by approximately 6 months after treatment. In the first month after surgery, the ND to the treated MR reduced in one animal and ND to the LR increased in the other animal, both indicating active neural plasticity that reduced the effectiveness of the treatment. Adaptive changes in ND to the untreated eye were also identified.

Conclusions

Active neural and muscle plasticity corresponding to both the treated and the untreated eye determines longitudinal success following surgical correction of strabismus. Outcome of surgical treatment could be improved by identifying ways to enhance “positive” adaptation and limit “negative” adaptation.

Vergence and Strabismus in Neurodegenerative Disorders

Maintaining proper eye alignment is necessary to generate a cohesive visual image. This involves the coordination of complex neural networks, which can become impaired by various neurodegenerative diseases. When the vergence system is affected, this can result in strabismus and disorienting diplopia. While previous studies have detailed the effect of these disorders on other eye movements, such as saccades, relatively little is known about strabismus. Here, we focus on the prevalence, clinical characteristics, and treatment of strabismus and disorders of vergence in Parkinson’s disease, spinocerebellar ataxia, Huntington disease, and multiple system atrophy. We find that vergence abnormalities may be more common in these disorders than previously thought. In Parkinson’s disease, the evidence suggests that strabismus is related to convergence insufficiency; however, it is responsive to dopamine replacement therapy and can, therefore, fluctuate with medication “on” and “off” periods throughout the day. Diplopia is also established as a side effect of deep brain stimulation and is thought to be related to stimulation of the subthalamic nucleus and extraocular motor nucleus among other structures. In regards to the spinocerebellar ataxias, oculomotor symptoms are common in many subtypes, but diplopia is most common in SCA3 also known as Machado–Joseph disease. Ophthalmoplegia and vergence insufficiency have both been implicated in strabismus in these patients, but cannot fully explain the properties of the strabismus, suggesting the involvement of other structures as well. Strabismus has not been reported as a common finding in Huntington disease or atypical parkinsonian syndromes and more studies are needed to determine how these disorders affect binocular alignment.

Response of supraoculomotor area neurons during combined saccade-vergence movements

Combined saccade-vergence movements allow humans and other primates to align their eyes with objects of interest in three-dimensions. In the absence of saccades, vergence movements are typically slow, symmetrical movements of the two eyes in opposite directions. However, combined saccade-vergence movements produce vergence velocities that exceed values observed during vergence alone. This phenomenon is often called "vergence enhancement", or "saccade-facilitated vergence," though it is important to consider that rapid vergence changes, known as "vergence transients," are also observed during conjugate saccades. We developed a visual target array that allows monkeys to make saccades in all directions between targets spaced at distances that correspond to ~1° intervals of vergence angle relative to the monkey. We recorded the activity of vergence-sensitive neurons in the supra-oculomotor area (SOA), located dorsal and lateral to the oculomotor nucleus while monkeys made saccades with vergence amplitudes ranging from 0 to 10°. The primary focus of this study was to test the hypothesis that neurons in the SOA fire a high frequency burst of spikes during saccades that could generate the enhanced vergence. We found that individual neurons encode vergence velocity during both saccadic and non-saccadic vergence, yet firing rates were insufficient to produce the observed enhancement of vergence velocity. Our results are consistent with the hypothesis that slow vergence changes are encoded by the SOA while fast vergence movements require an additional contribution from the saccadic system. NEW & NOTEWORTHY Research into combined saccade-vergence movements has so far focused on exploring the saccadic neural circuitry, leading to diverging hypotheses regarding the role of the vergence system in this behavior. In this study, we report the first quantitative analysis of the discharge of individual neurons that encode vergence velocity in the monkey brain stem during combined saccade-vergence movements.

Comparison of three models of saccade disconjugacy in strabismus

In pattern strabismus the horizontal and vertical misalignments vary with eye position along the orthogonal axis. The disorder is typically described in terms of overaction or underaction of oblique muscles. Recent behavioral studies in humans and monkeys, however, have reported that such actions are insufficient to fully explain the patterns of directional and amplitude disconjugacy of saccades. There is mounting evidence that the oculomotor abnormalities associated with strabismus are at least partially attributable to neurophysiological abnormalities. A number of control systems models have been developed to simulate the kinematic characteristics of saccades in normal primates. In the present study we sought to determine whether these models could simulate the abnormalities of saccades in strabismus by making two assumptions: 1) in strabismus the burst generator gains differ for the two eyes and 2) abnormal crosstalk exists between the horizontal and vertical saccadic circuits in the brain stem. We tested three models, distinguished by the location of the horizontal-vertical crosstalk. All three models were able to simulate amplitude and directional saccade disconjugacy, postsaccadic drift, and a pattern strabismus for static fixation, but they made different predictions about the dynamics of saccades. By assuming that crosstalk occurs at multiple nodes, the Distributed Crosstalk Model correctly predicted the dynamics of saccades. These new models make additional predictions that can be tested with future neurophysiological experiments.NEW & NOTEWORTHY Over the past several decades, numerous control systems models have been devised to simulate the known kinematic features of saccades in normal primates. These models have proven valuable to neurophysiology, as a means of generating testable predictions. The present manuscript, as far as we are aware, is the first to present control systems models to simulate the known abnormalities of saccades in strabismus.

Comparisons of Neuronal and Excitatory Network Properties between the Rat Brainstem Nuclei that Participate in Vertical and Horizontal Gaze Holding

Gaze holding is primarily controlled by neural structures including the prepositus hypoglossi nucleus (PHN) for horizontal gaze and the interstitial nucleus of Cajal (INC) for vertical and torsional gaze. In contrast to the accumulating findings of the PHN, there is no report regarding the membrane properties of INC neurons or the local networks in the INC. In this study, to verify whether the neural structure of the INC is similar to that of the PHN, we investigated the neuronal and network properties of the INC using whole-cell recordings in rat brainstem slices. Three types of afterhyperpolarization (AHP) profiles and five firing patterns observed in PHN neurons were also observed in INC neurons. However, the overall distributions based on the AHP profile and the firing patterns of INC neurons were different from those of PHN neurons. The application of burst stimulation to a nearby site of a recorded INC neuron induced an increase in the frequency of spontaneous EPSCs. The duration of the increased EPSC frequency of INC neurons was not significantly different from that of PHN neurons. The percent of duration reduction induced by a Ca²⁺-permeable AMPA (CP-AMPA) receptor antagonist was significantly smaller in the INC than in the PHN. These findings suggest that local excitatory networks that activate sustained EPSC responses also exist in the INC, but their activation mechanisms including the contribution of CP-AMPA receptors differ between the INC and the PHN.

Neural mechanisms of oculomotor abnormalities in the infantile strabismus syndrome

Infantile strabismus is characterized by numerous visual and oculomotor abnormalities. Recently nonhuman primate models of infantile strabismus have been established, with characteristics that closely match those observed in human patients. This has made it possible to study the neural basis for visual and oculomotor symptoms in infantile strabismus. In this review, we consider the available evidence for neural abnormalities in structures related to oculomotor pathways ranging from visual cortex to oculomotor nuclei. These studies provide compelling evidence that a disturbance of binocular vision during a sensitive period early in life, whatever the cause, results in a cascade of abnormalities through numerous brain areas involved in visual functions and eye movements.

Saccadic and Postsaccadic Disconjugacy in Zebrafish Larvae Suggests Independent Eye Movement Control

Spontaneous eye movements of zebrafish larvae in the dark consist of centrifugal saccades that move the eyes from a central to an eccentric position and postsaccadic centripetal drifts. In a previous study, we showed that the fitted single-exponential time constants of the postsaccadic drifts are longer in the temporal-to-nasal (T->N) direction than in the nasal-to-temporal (N->T) direction. In the present study, we further report that saccadic peak velocities are higher and saccadic amplitudes are larger in the N->T direction than in the T->N direction. We investigated the underlying mechanism of this ocular disconjugacy in the dark with a top-down approach. A mathematic ocular motor model, including an eye plant, a set of burst neurons and a velocity-to-position neural integrator (VPNI), was built to simulate the typical larval eye movements in the dark. The modeling parameters, such as VPNI time constants, neural impulse signals generated by the burst neurons and time constants of the eye plant, were iteratively adjusted to fit the average saccadic eye movement. These simulations suggest that four pools of burst neurons and four pools of VPNIs are needed to explain the disconjugate eye movements in our results. A premotor mechanism controls the synchronous timing of binocular saccades, but the pools of burst and integrator neurons in zebrafish larvae seem to be different (and maybe separate) for both eyes and horizontal directions, which leads to the observed ocular disconjugacies during saccades and postsaccadic drifts in the dark.

The diagnostic value of saccades in movement disorder patients: a practical guide and review

Saccades are rapid eye movements designed to shift the fovea to objects of visual interest. Abnormalities of saccades offer important clues in the diagnosis of a number of movement disorders. In this review, we explore the anatomy of horizontal and vertical saccades, discuss practical aspects of their examination, and review how saccadic abnormalities in hyperkinetic and hypokinetic movement disorders aid in diagnosis, with video demonstration of classic examples. Documentation of the ease of saccade initiation, range of motion and conjugacy of saccades, speed and accuracy of saccades, dynamic saccadic trajectory, and the presence or absence of saccadic intrusions and oscillations are important components of this exam. We also provide a practical algorithm to demonstrate the value of saccades in the differential diagnosis of the movement disorders patient.

Abnormal tuning of saccade-related cells in pontine reticular formation of strabismic monkeys

Strabismus is a common disorder, characterized by a chronic misalignment of the eyes and numerous visual and oculomotor abnormalities. For example, saccades are often highly disconjugate. For humans with pattern strabismus, the horizontal and vertical disconjugacies vary with eye position. In monkeys, manipulations that disturb binocular vision during the first several weeks of life result in a chronic strabismus with characteristics that closely match those in human patients. Early onset strabismus is associated with altered binocular sensitivity of neurons in visual cortex. Here we test the hypothesis that brain stem circuits specific to saccadic eye movements are abnormal. We targeted the pontine paramedian reticular formation, a structure that directly projects to the ipsilateral abducens nucleus. In normal animals, neurons in this structure are characterized by a high-frequency burst of spikes associated with ipsiversive saccades. We recorded single-unit activity from 84 neurons from four monkeys (two normal, one exotrope, and one esotrope), while they made saccades to a visual target on a tangent screen. All 24 neurons recorded from the normal animals had preferred directions within 30° of pure horizontal. For the strabismic animals, the distribution of preferred directions was normal on one side of the brain, but highly variable on the other. In fact, 12/60 neurons recorded from the strabismic animals preferred vertical saccades. Many also had unusually weak or strong bursts. These data suggest that the loss of corresponding binocular vision during infancy impairs the development of normal tuning characteristics for saccade-related neurons in brain stem.

Local population synchrony and the encoding of eye position in the primate neural integrator

Encoding horizontal eye position in the oculomotor system occurs through temporal integration of eye velocity inputs to produce tonic outputs. The nucleus prepositus is commonly believed to be the "neural integrator" that accomplishes this function through the activity of its ensemble of predominantly burst-tonic neurons. Single-unit characterizations and labeling studies of these neurons have suggested that their collective output is achieved through local feedback loops produced by direct connections between them. If this is the case, then the ensemble of burst-tonic neurons should exhibit correlated activity. To obtain electrophysiological evidence of local interactions between neurons, we simultaneously recorded pairs (n = 29) of burst-tonic neurons in the nucleus prepositus of rhesus macaque monkeys using eight-channel linear microelectrode arrays. We computed the magnitude of synchrony between their spike trains as a function of eye position during ocular fixations and as a function of distance between neurons. Importantly, we found that neurons exhibit unexpected levels of positive synchrony, which is maximal during contralateral fixations and weakest when neurons are located far apart from one another (>300 μm). Together, our results support a role for shared inputs to ipsilateral pairs of burst-tonic neurons in the encoding of eye position in the primate nucleus prepositus.

[How does the brain control eye movements? Motor and premotor neurons of the brainstem]

Knowledge of cognitive and neural architecture and processes that control eye movements has advanced enough to allow precise and quantitative analysis of hitherto unsolved phenomena. In this review, we revisit from a neuropsychological viewpoint Hering vs. Helmholtz' hypotheses on binocular coordination. Specifically, we reexamine the behavior and the neural bases of saccade-vergence movement, to move the gaze in both direction and depth under natural conditions. From the psychophysical viewpoint, neo-Heringian and neo-Helmholtzian authors have accumulated arguments favoring distinct conjugate (for saccades) and disconjugate (for vergence) systems, as well as advocating for monocularly programmed eye movements. From the neurophysiological viewpoint, which reports brain cell recordings during the execution of a given task, neo-Heringian and neo-Helmholtzian physiologists have also provided arguments in favor of both hypotheses at the level of the brainstem premotor circuitry. Bridging the two, we propose that Hering and Helmholtz were both right. The emphasis placed by the latter on adaptive processes throughout life cycle is compatible with the importance of neurobiological constraints pointed out by the former. In the meanwhile, the study of saccade-vergence eye movements recalls how much the psychophysical definition of the task determines the interpretation that is made from neurophysiological data.

Abnormal activity of neurons in abducens nucleus of strabismic monkeys

PURPOSE

Infantile strabismus is characterized by persistent misalignment of the eyes. Mounting evidence suggests that the disorder is associated with abnormalities at the neural level, but few details are known. This study investigated the signals carried by abducens neurons in monkeys with experimentally induced strabismus. We wanted to know whether the firing rates of individual neurons are exclusively related to the position and velocity of one eye and whether the overall level of activity of the abducens nucleus was in the normal range.

METHODS

We recorded 58 neurons in right and left abducens nuclei while strabismic monkeys (one esotrope and one exotrope) performed a saccade task. We analyzed the firing rates associated with static horizontal eye position and saccades by fitting the data with a dynamic equation that included position and velocity terms for each eye. Results were compared to previously published data in normal monkeys.

RESULTS

For both strabismic monkeys the overall tonic activity was 50 to 100 spikes/s lower, for every suprathreshold eye position, than what has previously been reported for normal monkeys. This was mostly the result of lower baseline activity; the slopes of rate-position curves were similar to those in previous reports in normal monkeys. The saccade velocity sensitivities were similar to those of normal monkeys, 0.35 for the esotrope and 0.40 for the exotrope. For most neurons the firing rate was more closely related to the position and velocity of the ipsilateral eye.

CONCLUSIONS

These data suggest that strabismus can be associated with reduced neural activity in the abducens nucleus.

Saccadic amplitudes during combined saccade-vergence movements result from a weighted average of the target's locations in the two retinas

Recent neurophysiological and behavioral studies have established that the saccadic amplitudes performed during combined saccade-vergence movements are unequal in the two eyes. These studies have not established, however, how the saccadic amplitude of each eye is determined. Our goal here is to fill this lacuna. We use three well-known metric attributes of saccadic movements as constraints and argue that the only quantitative model that obeys these constraints is one where each eye's saccadic amplitude is given by a weighted average of the target's locations in the two retinas. However, this theoretical result does not establish whether the weights in the weighted averaging operation are constant or whether they vary for different targets. To test the simpler of these two possibilities, namely the one of constant weights, we recorded combined saccade-vergence movements performed by human subjects. Our analysis of these movements shows that a constant-weights weighted averaging model provides an excellent description of their saccadic amplitudes. Overall, then, our conclusions are: (1) the two eyes' saccadic amplitudes are determined by weighted averages of the target's locations in the two retinas; (2) for targets within the oculomotor range of natural viewing, which was the range in our experiments, a weighted averaging model that uses constant weights accounts superbly for these saccadic amplitudes. We suggest that the weighted averaging operation that determines saccadic amplitudes is a by-product of a process whose purpose is to yoke the two eyes together. We provide a model explaining how this yoking may be achieved.

The horizontal angular vestibulo-ocular reflex: a nonlinear mechanism for context-dependent responses

Studies of the vestibulo-ocular reflex (VOR) have revealed that this type of involuntary eye movement is influenced by viewing distance. This paper presents a bilateral model for the horizontal angular VOR in the dark based on realistic physiological mechanisms. It is shown that by assigning proper nonlinear neural computations at the premotor level, the model is capable of replicating target-distance-dependent VOR responses that are in agreement with geometrical requirements. Central premotor responses in the model are also shown to be consistent with experimental observations. Moreover, the model performance after simulated unilateral canal plugging also reproduces experimental observations, an emerging property. Such local nonlinear computations could similarly generate context-dependent behaviors in other more complex motor systems.

Vergence neurons identified in the rostral superior colliculus code smooth eye movements in 3D space

The rostral superior colliculus (rSC) encodes position errors for multiple types of eye movements, including microsaccades, small saccades, smooth pursuit, and fixation. Here we address whether the rSC contributes to the development of neural signals that are suitable for controlling vergence eye movements. We use both single-unit recording and microstimulation techniques in monkey to answer this question. We found that vergence eye movements can be evoked using microstimulation in the rSC. Moreover, among the previously described neurons in rSC, we recorded a novel population of neurons that either increased (i.e., convergence neurons) or decreased (i.e., divergence neurons) their activity during vergence eye movements. In particular, these neurons dynamically encoded changes in vergence angle during vergence tracking, fixation in 3D space and the slow binocular realignment that occurs after disconjugate saccades, but were completely unresponsive during conjugate or the rapid component of disconjugate saccades (i.e., fast vergence) and conjugate smooth pursuit. Together, our microstimulation and single-neuron results suggest that the SC plays a role in the generation of signals required to precisely align the eyes toward targets in 3D space. We propose that accurate maintenance of 3D eye position, critical for the perception of stereopsis, may be mediated via the rSC.

The postsaccadic unreliability of gain fields renders it unlikely that the motor system can use them to calculate target position in space

Gain fields, the eye-position modulation of visual responses, are thought to provide a mechanism by which the motor system can accurately calculate target position in space despite a constantly moving eye. Current gain-field models assume that the modulation of visual responses by eye position is accurate at all times, even around the time of a saccade. Here, we show that for at least 150 ms after a saccade, gain fields in the lateral intraparietal area (LIP) are unreliable. The majority of LIP cells with steady-state gain fields reflect the presaccadic eye position. The remainder of the cells have responses that cannot be predicted by their steady-state gain fields. Nonetheless, a monkey's oculomotor performance is accurate during this time. These results suggest that current models built upon a simple gain-field algorithm cannot be used to calculate the position of a target in space that flashes briefly after a saccade.

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.

The horizontal angular vestibulo-ocular reflex: a non-linear mechanism for context-dependent responses

A bilateral model for the horizontal angular vestibulo-ocular reflex (AVOR) is presented in this paper. It is shown that by assigning proper non-linear neural computations at the premotor level, the model is capable of replicating target-distance dependent VOR responses. Moreover, the model behavior in case of sensory plugging is also consistent with reported experimental observations.

The nucleus prepositus predominantly outputs eye movement-related information during passive and active self-motion

Maintaining a constant representation of our heading as we move through the world requires the accurate estimate of spatial orientation. As one turns (or is turned) toward a new heading, signals from the semicircular canals are relayed through the vestibular system to higher-order centers that encode head direction. To date, there is no direct electrophysiological evidence confirming the first relay point of head-motion signals from the vestibular nuclei, but previous anatomical and lesion studies have identified the nucleus prepositus as a likely candidate. Whereas burst-tonic neurons encode only eye-movement signals during head-fixed eye motion and passive vestibular stimulation, these neurons have not been studied during self-generated movements. Here, we specifically address whether burst-tonic neurons encode head motion during active behaviors. Single-unit responses were recorded from the nucleus prepositus of rhesus monkeys and compared for head-restrained and active conditions with comparable eye velocities. We found that neurons consistently encoded eye position and velocity across conditions but did not exhibit significant sensitivity to head position or velocity. Additionally, response sensitivities varied as a function of eye velocity, similar to abducens motoneurons and consistent with their role in gaze control and stabilization. Thus our results demonstrate that the primate nucleus prepositus chiefly encodes eye movement even during active head-movement behaviors, a finding inconsistent with the proposal that this nucleus makes a direct contribution to head-direction cell tuning. Given its ascending projections, however, we speculate that this eye-movement information is integrated with other inputs in establishing higher-order spatial representations.

Dynamics of eye-position signals in the dorsal visual system

BACKGROUND

Many visual areas of the primate brain contain signals related to the current position of the eyes in the orbit. These cortical eye-position signals are thought to underlie the transformation of retinal input-which changes with every eye movement-into a stable representation of visual space. For this coding scheme to work, such signals would need to be updated fast enough to keep up with the eye during normal exploratory behavior. We examined the dynamics of cortical eye-position signals in four dorsal visual areas of the macaque brain: the lateral and ventral intraparietal areas (LIP; VIP), the middle temporal area (MT), and the medial-superior temporal area (MST). We recorded extracellular activity of single neurons while the animal performed sequences of fixations and saccades in darkness.

RESULTS

The data show that eye-position signals are updated predictively, such that the representation shifts in the direction of a saccade prior to (<100 ms) the actual eye movement. Despite this early start, eye-position signals remain inaccurate until shortly after (10-150 ms) the eye movement. By using simulated behavioral experiments, we show that this brief misrepresentation of eye position provides a neural explanation for the psychophysical phenomenon of perisaccadic mislocalization, in which observers misperceive the positions of visual targets flashed around the time of saccadic eye movements.

CONCLUSIONS

Together, these results suggest that eye-position signals in the dorsal visual system are updated rapidly across eye movements and play a direct role in perceptual localization, even when they are erroneous.

Binocular coordination of eye movements--Hering's Law of equal innervation or uniocular control?

The neurophysiological basis for binocular control of eye movements in primates has been characterized by a scientific controversy that has its origin in the historical conflict of Hering and Helmholtz in the 19th century. This review focuses on two hypotheses, linked to that conflict, that seek to account for binocular coordination - Hering's Law vs. uniocular control of each eye. In an effort to manage the length of the review, the focus is on extracellular single-unit studies of premotor eye movement cells and extraocular motoneurons. In the latter half of the 20th century, these studies provided a wealth of neurophysiological data pertaining to the control of vergence and conjugate eye movements. The data were initially supportive of Hering's Law. More recent data, however, have provided support for uniocular control of each eye consistent with Helmholtz's original idea. The controversy is far from resolved. New anatomical descriptions of the disparate inputs to multiply and singly innervated extraocular muscle fibers challenge the concept of a 'final common pathway' as they suggest there may be separate groups of motoneurons involved in vergence and conjugate control of eye position. These data provide a new challenge for interpretation of uniocular premotor control networks and how they cooperate to produce coordinated eye movements.

The neural control of fast vs. slow vergence eye movements

When looking between targets located in three-dimensional space, information about relative depth is sent from the visual cortex to the motor control centers in the brainstem, which are responsible for generating appropriate motor commands to move the eyes. Surprisingly, how the neurons in the brainstem use the depth information supplied by the visual cortex to precisely aim each eye on a visual target remains highly controversial. This review will consider the results of recent studies that have focused on determining how individual neurons contribute to realigning gaze when we look between objects located at different depths. In particular, the results of new experiments provide compelling evidence that the majority of saccadic neurons dynamically encode the movement of an individual eye, and show that the time-varying discharge of the saccadic neuron population encodes the drive required to account for vergence facilitation during disconjugate saccades. Notably, these results suggest that an additional input (i.e. from a separate vergence subsystem) is not required to shape the activity of motoneurons during disconjugate saccades. Furthermore, whereas motoneurons drive both fast and slow vergence movements, saccadic neurons discharge only during fast vergence movements, emphasizing the existence of distinct premotor pathways for controlling fast vs. slow vergence. Taken together, these recent findings contradict the traditional view that the brain is circuited with independent pathways for conjugate and vergence control, and thus provide an important new insight into how the brain controls three-dimensional gaze shifts.

Responses of medial rectus motoneurons in monkeys with strabismus

PURPOSE

Monkeys reared under conditions of alternating monocular occlusion during their first few months of life show large horizontal strabismus, "A" patterns, and dissociated vertical deviation. "A" patterns manifest as an inappropriate horizontal component in the deviated eye during vertical eye movements (cross-axis movement). The objective of this study was to investigate response properties of medial rectus motoneurons (MRMNs) in relation to strabismus properties.

METHODS

Burst-tonic activity of 21 MRMNs in the oculomotor nucleus were recorded from two monkeys with exotropia as they performed horizontal and vertical smooth pursuit (0.2 Hz, ±10°) under monocular viewing conditions. Neuronal responses and horizontal component of eye movements were used to identify regression coefficients in a first-order model for each tracking condition.

RESULTS

Comparison of position, velocity, and constant parameter coefficients, estimated from horizontal tracking data with either eye viewing, showed no significant differences (P > 0.07), indicating that neuronal activity could account for the horizontal misalignment. Comparison of the position, velocity, and constant parameter coefficients estimated from horizontal tracking and the cross-axis condition showed no significant differences (P > 0.07), suggesting that motoneuron activity could account for most of the inappropriate horizontal cross-axis movement observed in the covered eye during vertical smooth pursuit.

CONCLUSIONS

These data suggest that, in animals with sensory-induced strabismus, central innervation to extraocular muscles is responsible for setting the state of strabismus. Mechanical factors such as muscle length adaptation (for horizontal misalignment) and pulley heterotopy or static torsion (for "A" patterns) likely do not play a major role in determining properties in a sensory-induced strabismus.

Disorders of vergence eye movements

PURPOSE OF REVIEW

The aim is to re-interpret disorders of vergence in the light of recent studies that view disjunctive eye movements as but one component of three-dimensional gaze control.

RECENT FINDINGS

Most natural eye movements combine vergence with saccades, pursuit and vestibular eye movements. Electrophysiological studies in epileptic patients, as well as evidence from monkeys, indicate that frontal and parietal cortex govern vergence as a component of three-dimensional gaze. Clinicians apply Hering's law of equal innervation to interpret disjunctive movements as the superposition of conjugate and vergence commands. However, electrophysiological studies indicate that disjunctive saccades are achieved by programming each eye's movement independently. Patients with internuclear ophthalmoplegia (INO) may have preserved vergence, which can be recruited to compensate for loss of conjugacy. Vergence may also enable gaze shifts in saccadic palsy. Some forms of nystagmus suppress or change with convergence; co-contraction of the horizontal rectus muscles does not appear to be the explanation. Rather, effects of near viewing on central vestibular mechanisms or differential activation of specific types of extra-ocular muscle fiber may be responsible.

SUMMARY

Interpretation of disorders of vergence is aided by applying a scheme in which their contributions to three-dimensional gaze control is considered.

Encoding of eye position in the goldfish horizontal oculomotor neural integrator

Monocular organization of the goldfish horizontal neural integrator was studied during spontaneous scanning saccadic and fixation behaviors. Analysis of neuronal firing rates revealed a population of ipsilateral (37%), conjugate (59%), and contralateral (4%) eye position neurons. When monocular optokinetic stimuli were employed to maximize disjunctive horizontal eye movements, the sampled population changed to 57, 39, and 4%. Monocular eye tracking could be elicited at different gain and phase with the integrator time constant independently modified for each eye by either centripetal (leak) or centrifugal (instability) drifting visual stimuli. Acute midline separation between the hindbrain oculomotor integrators did not affect either monocularity or time constant tuning, corroborating that left and right eye positions are independently encoded within each integrator. Together these findings suggest that the "ipsilateral" and "conjugate/contralateral" integrator neurons primarily target abducens motoneurons and internuclear neurons, respectively. The commissural pathway is proposed to select the conjugate/contralateral eye position neurons and act as a feedforward inhibition affecting null eye position, oculomotor range, and saccade pattern.

The role of the medial longitudinal fasciculus in horizontal gaze: tests of current hypotheses for saccade-vergence interactions

Rapid shifts of the point of visual fixation between equidistant targets require equal-sized saccades of each eye. The brainstem medial longitudinal fasciculus (MLF) plays a cardinal role in ensuring that horizontal saccades between equidistant targets are tightly yoked. Lesions of the MLF--internuclear ophthalmoparesis (INO)--cause horizontal saccades to become disjunctive: adducting saccades are slow, small, or absent. However, in INO, convergence movements may remain intact. We studied horizontal gaze shifts between equidistant targets and between far and near targets aligned on the visual axis of one eye (Müller test paradigm) in five cases of INO and five control subjects. We estimated the saccadic component of each movement by measuring peak velocity and peak acceleration. We tested whether the ratio of the saccadic component of the adducting/abducting eyes stayed constant or changed for the two types of saccades. For saccades made by control subjects between equidistant targets, the group mean ratio (±SD) of adducting/abducting peak velocity was 0.96 ± 0.07 and adducting/abducting peak acceleration was 0.94 ± 0.09. Corresponding ratios for INO cases were 0.45 ± 0.10 for peak velocity and 0.27 ± 0.11 for peak acceleration, reflecting reduced saccadic pulses for adduction. For control subjects, during the Müller paradigm, the adducting/abducting ratio was 1.25 ± 0.14 for peak velocity and 1.03 ± 0.12 for peak acceleration. Corresponding ratios for INO cases were 0.82 ± 0.18 for peak velocity and 0.48 ± 0.13 for peak acceleration. When adducting/abducting ratios during Müller versus equidistant targets paradigms were compared, INO cases showed larger relative increases for both peak velocity and peak acceleration compared with control subjects. Comparison of similar-sized movements during the two test paradigms indicated that whereas INO patients could decrease peak velocity of their abducting eye during the Müller paradigm, they were unable to modulate adducting velocity in response to viewing conditions. However, the initial component of each eye's movement was similar in both cases, possibly reflecting activation of saccadic burst neurons. These findings support the hypothesis that horizontal saccades are governed by disjunctive signals, preceded by an initial, high-acceleration conjugate transient and followed by a slower vergence component.

Local neural processing and the generation of dynamic motor commands within the saccadic premotor network

The ability to accurately control movement requires the computation of a precise motor command. However, the computations that take place within premotor pathways to determine the dynamics of movements are not understood. Here we studied the local processing that generates dynamic motor commands by simultaneously recording spikes and local field potentials (LFPs) in the network that commands saccades. We first compared the information encoded by LFPs and spikes recorded from individual premotor and motoneurons (saccadic burst neurons, omnipause neurons, and motoneurons) in monkeys. LFP responses consistent with net depolarizations occurred in association with bursts of spiking activity when saccades were made in a neuron's preferred direction. In contrast, when saccades were made in a neuron's nonpreferred direction, neurons ceased spiking and the associated LFP responses were consistent with net hyperpolarizations. Surprisingly, hyperpolarizing and depolarizing LFPs encoded movement dynamics with equal robustness and accuracy. Second, we compared spiking responses at one hierarchical level of processing to LFPs at the next stage. Latencies and spike-triggered averages of LFP responses were consistent with each neuron's place within this circuit. LFPs reflected relatively local events (<500 microm) and encoded important features not available from the spiking train (i.e., hyperpolarizing response). Notably, quantification of their time-varying profiles revealed that a precise balance of depolarization and hyperpolarization underlies the production of precise saccadic eye movement commands at both motor and premotor levels. Overall, simultaneous recordings of LFPs and spiking responses provides an effective means for evaluating the local computations that take place to produce accurate motor commands.

Otolith stimulation induces c-Fos expression in vestibular and precerebellar nuclei in cats and squirrel monkeys

Vestibular information is critical for the control of balance, posture, and eye movements. Signals from the receptors, the semicircular canals and otoliths, are carried by the eighth nerve and distributed to the four nuclei of the vestibular nuclear complex, the VNC. However, anatomical and physiological data suggest that many additional brainstem nuclei are engaged in the processing of vestibular signals and generation of motor responses. To assess the role of these structures in vestibular functions, we have used the expression of the immediate early gene c-Fos as a marker for neurons activated by stimulation of the otoliths or the semicircular canals. Excitation of the otolith organs resulted in widespread c-Fos expression in the VNC, but also in other nuclei, including the external cuneate nucleus, the postpyramidal nucleus of the raphé, the nucleus prepositus hypoglossi, the subtrigeminal nucleus, the pontine nuclei, the dorsal tegmental nucleus, the locus coeruleus, and the reticular formation. Rotations that excited the semicircular canals were much less effective in inducing c-Fos expression. The large number of brainstem nuclei that showed c-Fos expression may reflect the multiple functions of the vestibular system. Some of these neurons may be involved in sensory processing of the vestibular signals, while others provide input to the vestibulo-ocular, vestibulocollic, and vestibulospinal reflexes or mediate changes in autonomic function. The data show that otolith stimulation engages brainstem structures both within and outside of the VNC, many of which project to the cerebellum.

Dynamic characterization of agonist and antagonist oculomotoneurons during conjugate and disconjugate eye movements

In this report, we provide the first quantitative characterization of the relationship between the spike train dynamics of medial rectus oculomotoneurons (OMNs) and eye movements during conjugate and disconjugate saccades. We show that a simple, first-order model (i.e., containing eye position and velocity terms) provided an adequate model of neural discharges during both on and off-directed conjugate saccades, while a second-order model, which included a decaying slide term, significantly improved the ability to fit neuronal responses by approximately 10% (P<0.05). To understand how the same neurons drove disconjugate eye movements, we evaluated whether sensitivities estimated during conjugate saccades could be used to predict responses during disconjugate saccades. For the majority of neurons (68%), a conjugate-based model failed, and instead neurons preferentially encoded the position and velocity of the ipsilateral eye. Similar to our previous results with abducens motoneurons, we also found that position and velocity sensitivities of OMNs decreased with increasing velocity, and the simulated population drive of OMNs during disconjugate saccades was less (approximately 10%) than during conjugate saccades. Taken together, our results provide evidence that the activation of the antagonist, as well as agonist, motoneuron pools must be considered to understand the neural control of horizontal eye movements across different oculomotor behaviors. Moreover, we propose that the undersampling of smaller motoneurons (e.g., nontwitch) was likely to account for the missing drive observed during disconjugate saccades; these cells are thought to be more specialized for vergence movements and thus could provide the additional input required to command disconjugate eye movements.

Neuronal evidence for individual eye control in the primate cMRF

Previous single unit recordings and electrical stimulation have suggested that separate regions of the MRF participate in the control of vergence and conjugate eye movements. Neurons in the supraoculomotor area (SOA) have been found to encode symmetric vergence [Zhang, Y. et al. (1992). J. Neurophysiol., 67: 944-960] while neurons in the central MRF, the cMRF, located ventral to the SOA and lateral to the oculomotor nucleus are associated with conjugate eye movements [Waitzman, D.M. et al. (1996). J. Neurophysiol., 75(4): 1546-1572]. However, it remains unknown if cMRF neurons are strictly associated with conjugate movements since eye movements were recorded with a single eye coil in monkeys viewing visual stimuli at a distance of at least 50 cm. In the current study we addressed whether neurons in the cMRF might also encode vergence-related information. Interestingly, electrical stimulation elicited disconjugate saccades (contralateral eye moved more than the ipsilateral eye) from locations previously thought to elicit only conjugate saccades. Single unit recordings in this same area made in two rhesus monkeys trained to follow visual stimuli moved rapidly in depth along the axis of sight of an individual eye demonstrate that cMRF neurons do not simply encode conjugate information during disconjugate saccades; in fact our findings provide evidence that cMRF neurons are most closely associated with the movement of an individual eye. These results support the hypothesis that the midbrain shapes the activity of the pre-motor saccadic neurons by encoding integrated conjugate and vergence commands.

Dynamic Coding of Vertical Facilitated Vergence by Premotor Saccadic Burst Neurons

To redirect our gaze in three-dimensional space we frequently combine saccades and vergence. These eye movements, known as disconjugate saccades, are characterized by eyes rotating by different amounts, with markedly different dynamics, and occur whenever gaze is shifted between near and far objects. How the brain ensures the precise control of binocular positioning remains controversial. It has been proposed that the traditionally assumed “conjugate” saccadic premotor pathway does not encode conjugate commands but rather encodes monocular commands for the right or left eye during saccades. Here, we directly test this proposal by recording from the premotor neurons of the horizontal saccade generator during a dissociation task that required a vergence but no horizontal conjugate saccadic command. Specifically, saccadic burst neurons (SBNs) in the paramedian pontine reticular formation were recorded while rhesus monkeys made vertical saccades made between near and far targets. During this task, we first show that peak vergence velocities were enhanced to saccade-like speeds (e.g., >150 vs. <100°/s during saccade-free movements for comparable changes in vergence angle). We then quantified the discharge dynamics of SBNs during these movements and found that the majority of the neurons preferentially encode the velocity of the ipsilateral eye. Notably, a given neuron typically encoded the movement of the same eye during horizontal saccades that were made in depth. Taken together, our findings demonstrate that the brain stem saccadic burst generator encodes integrated conjugate and vergence commands, thus providing strong evidence for the proposal that the classic saccadic premotor pathway controls gaze in three-dimensional space.

Relationships between versional and vergent quick phases of the involuntary version-vergence nystagmus

We used ground-plane motion stimuli displayed on a computer monitor positioned below eye level to induce involuntary version-vergence nystagmus (VVN). The VVN was recorded with a search coil system. It was shown that the VVN had both vertical versional and horizontal vergence components. The VVN induced by backward motion (toward subjects) had upward versional and divergence quick phases, whereas those induced by forward motion (away from subjects) had downward and biphasic divergence-convergence quick phases. The versional and vergence components of the VVN quick phases were analyzed. A temporal dissociation of about 20 ms between version velocity peak and convergence velocity peak was revealed, which supported a modified saccade-related vergence burst neuron (SVBN) model. We suggest that the temporal dissociation may be partly because of a lower-level OKN control mechanism. Vergence peak time was dependent on version peak time. Linear relationships between vergence peak velocity and versional saccadic peak velocity were demonstrated, which was in line with the new multiplicative model. Our data support the hypothesis that the vergence system and the saccadic system can act separately but interact with each other whenever their movements occur simultaneously.

The brain stem saccadic burst generator encodes gaze in three-dimensional space

When we look between objects located at different depths the horizontal movement of each eye is different from that of the other, yet temporally synchronized. Traditionally, a vergence-specific neuronal subsystem, independent from other oculomotor subsystems, has been thought to generate all eye movements in depth. However, recent studies have challenged this view by unmasking interactions between vergence and saccadic eye movements during disconjugate saccades. Here, we combined experimental and modeling approaches to address whether the premotor command to generate disconjugate saccades originates exclusively in "vergence centers." We found that the brain stem burst generator, which is commonly assumed to drive only the conjugate component of eye movements, carries substantial vergence-related information during disconjugate saccades. Notably, facilitated vergence velocities during disconjugate saccades were synchronized with the burst onset of excitatory and inhibitory brain stem saccadic burst neurons (SBNs). Furthermore, the time-varying discharge properties of the majority of SBNs (>70%) preferentially encoded the dynamics of an individual eye during disconjugate saccades. When these experimental results were implemented into a computer-based simulation, to further evaluate the contribution of the saccadic burst generator in generating disconjugate saccades, we found that it carries all the vergence drive that is necessary to shape the activity of the abducens motoneurons to which it projects. Taken together, our results provide evidence that the premotor commands from the brain stem saccadic circuitry, to the target motoneurons, are sufficient to ensure the accurate control shifts of gaze in three dimensions.

Comparison of Saccade-Associated Neuronal Activity in the Primate Central Mesencephalic and Paramedian Pontine Reticular Formations

The oculomotor system must convert signals representing the target of an intended eye movement into appropriate input to drive the individual extraocular muscles. Neural models propose that this transformation may involve either a decomposition of the intended eye displacement signal into horizontal and vertical components or an implicit process whereby component signals do not predominate until the level of the motor neurons. Thus decomposition models predict that premotor neurons should primarily encode component signals while implicit models predict encoding of off-cardinal optimal directions by premotor neurons. The central mesencephalic reticular formation (cMRF) and paramedian pontine reticular formation (PPRF) are two brain stem regions that likely participate in the development of motor activity since both structures are anatomically connected to nuclei that encode movement goal (superior colliculus) and generate horizontal eye movements (abducens nucleus). We compared cMRF and PPRF neurons and found they had similar relationships to saccade dynamics, latencies, and movement fields. Typically, the direction preference of these premotor neurons was horizontal, suggesting they were related to saccade components. To confirm this supposition, we studied the neurons during a series of oblique saccades that caused “component stretching” and thus allowed the vectorial (overall) saccade velocity to be dissociated from horizontal component velocity. The majority of cMRF and PPRF neurons encoded component velocity across all saccades, supporting decomposition models that suggest horizontal and vertical signals are generated before the level of the motoneurons. However, we also found novel vectorial eye velocity encoding neurons that may have important implications for saccade control.

A reevaluation of the inverse dynamic model for eye movements

To construct an appropriate motor command from signals that provide a representation of desired action, the nervous system must take into account the dynamic characteristics of the motor plant to be controlled. In the oculomotor system, signals specifying desired eye velocity are thought to be transformed into motor commands by an inverse dynamic model of the eye plant that is shared for all types of eye movements and implemented by a weighted combination of eye velocity and position signals. Neurons in the prepositus hypoglossi and adjacent medial vestibular nuclei (PH-BT neurons) were traditionally thought to encode the "eye position" component of this inverse model. However, not only are PH-BT responses inconsistent with this theoretical role, but compensatory eye movement responses to translation do not show evidence for processing by a common inverse dynamic model. Prompted by these discrepancies between theoretical notions and experimental observations, we reevaluated these concepts using multiple-frequency rotational and translational head movements. Compatible with the notion of a common inverse model, we show that PH-BT responses are unique among all premotor cell types in bearing a consistent relationship to the motor output during eye movements driven by different sensory stimuli. However, because their responses are dynamically identical to those of motoneurons, PH-BT neurons do not simply represent an internal component of the inverse model, but rather its output. They encode and distribute an estimate of the motor command, a signal critical for accurate motor execution and learning.

Correlation of cross-axis eye movements and motoneuron activity in non-human primates with "A" pattern strabismus

PURPOSE

The authors showed earlier that animals reared with certain types of visual sensory deprivation during their first few months of life develop large horizontal strabismus, A/V patterns, and dissociated vertical deviation (DVD). Cross-axis eye movements were observed in the nonfixating eye that reflected pattern strabismus and DVD. The purpose of this study was to investigate whether neuronal activity within the oculomotor nucleus could be driving the abnormal cross-axis eye movements observed in the nonfixating eye.

METHODS

Burst-tonic activity was recorded from oculomotor nucleus neurons in three animals with A-pattern exotropia as they performed horizontal or vertical smooth pursuit during monocular viewing. Two animals were reared by alternate monocular occlusion for 4 months, and one animal was reared by binocular deprivation for 3 weeks.

RESULTS

In this study, efforts were focused on neurons modulated for vertical eye movements. Vertical burst-tonic motoneurons were strongly correlated with vertical eye movements regardless of whether the movement was purposeful, as in vertical smooth pursuit, or whether it was inappropriate, as in a vertical component observed in the nonfixating eye during horizontal smooth pursuit. Quantitative analysis of position and velocity sensitivities of the cells measured during the different tracking conditions suggested that motoneuron activity was sufficient to account for most of the inappropriate vertical cross-axis component.

CONCLUSIONS

Results suggest that, in animals with sensory-induced strabismus, innervation to extraocular muscles from motor nuclei produce the inappropriate cross-axis eye movements, resulting in change in ocular misalignment with gaze position associated with pattern strabismus and DVD.

Multiplicative computation in the vestibulo-ocular reflex (VOR)

Multiplicative computation is a basic operation that is crucial for neural information processing, but examples of multiplication by neural pathways that perform well-defined sensorimotor transformations are scarce. Here in behaving monkeys, we identified a multiplication of vestibular and eye position signals in the vestibulo-ocular reflex (VOR). Monkeys were trained to maintain fixation on visual targets at different horizontal locations and received brief unilateral acoustic clicks (1 ms, rarefaction, 85 approximately 110 db NHL) that were delivered into one of their external ear canals. We found that both the click-evoked horizontal eye movement responses and the click-evoked neuronal responses of the abducens neurons exhibited linear dependencies on horizontal conjugate eye position, indicating that the interaction of vestibular and horizontal conjugate eye position was multiplicative. Latency analysis further indicated that the site of the multiplication was within the direct VOR pathways. Based on these results, we propose a novel neural mechanism that implements the VOR gain modulation by fixation distance and gaze eccentricity. In this mechanism, the vestibular signal from a single labyrinth interacts multiplicatively with the position signals of each eye (Principle of Multiplication). These effects, however, interact additively with the other labyrinth (Principle of Addition). Our analysis suggests that the new mechanism can implement the VOR gain modulation by fixation distance and gaze eccentricity within the direct VOR pathways.

Binocular coordination of saccades in children with vertigo: dependency on the vergence state

The present study examines the quality of binocular coordination of saccades at far and near distance in 15 children with symptoms of vertigo headache and equilibrium disorders; these children show normal vestibular function but abnormal convergence eye movements (e.g., long time preparation, slow execution and poor accuracy, see ). The results show normal binocular saccade coordination at far distance, but large abnormal disconjugacy for saccades at near distance. During combined saccade-vergence movements (studied in six of these children), convergence remains abnormally slow. This supports the interpretation according to which poor binocular yoking of the saccades is linked to the reduced ability to produce fast convergence during the saccade; a learning mechanism based on rapid vergence would help to reduce the abducting-adducting asymmetry of the saccades. An alternative interpretation would be reduced learning ability for monocular adjustment of the saccade signals.

Oscillatory and Intrinsic Membrane Properties of Guinea Pig Nucleus Prepositus Hypoglossi Neurons In Vitro

Numerous models of the oculomotor neuronal integrator located in the prepositus hypoglossi nucleus (PHN) involve both highly tuned recurrent networks and intrinsic neuronal properties; however, there is little experimental evidence for the relative role of these two mechanisms. The experiments reported here show that all PHN neurons (PHNn) show marked phasic behavior, which is highly oscillatory in ∼25% of the population. The behavior of this subset of PHNn, referred to as type D PHNn, is clearly different from that of the medial vestibular nucleus neurons, which transmit the bulk of head velocity-related sensory vestibular inputs without integrating them. We have investigated the firing and biophysical properties of PHNn and developed data-based realistic neuronal models to quantitatively illustrate that their active conductances can produce the oscillatory behavior. Although some individual type D PHNn are able to show some features of mathematical integration, the lack of robustness of this behavior strongly suggests that additional network interactions, likely involving all types of PHNn, are essential for the neuronal integrator. Furthermore, the relationship between the impulse activity and membrane potential of type D PHNn is highly nonlinear and frequency-dependent, even for relatively small-amplitude responses. These results suggest that some of the synaptic input to type D PHNn is likely to evoke oscillatory responses that will be nonlinearly amplified as the spike discharge rate increases. It would appear that the PHNn have specific intrinsic properties that, in conjunction with network interconnections, enhance the persistent neural activity needed for their function.