Horizontal Smooth Pursuit Adaptation in Macaques After Muscimol Inactivation of the Dorsolateral Pontine Nucleus (DLPN)

Microstimulation of Interstitial Nucleus of Cajal Evokes Directionally Disconjugate Eye Movements in Monkeys With Pattern Strabismus

Purpose

Pattern strabismus is characterized by a horizontal misalignment of the eyes that varies with vertical eye position. This disorder has traditionally been described, and treated, as overaction or underaction of the oblique muscles. In recent years, evidence has accumulated that indicate that the disorder is associated with abnormal cross-talk between brainstem pathways that contribute to the horizontal and vertical components of eye movements. The present study was designed to investigate the hypothesis that the key abnormalities are at the level of, or downstream from, the interstitial nucleus of Cajal (INC).

Methods

Microstimulation was applied to the INC in two mature rhesus monkeys with "A" pattern strabismus that was experimentally induced in infancy. We asked whether the evoked movements would be vertical and conjugate, as has been previously reported in normal monkeys, or would be directionally disconjugate (i.e. with oblique or horizontal movement observed for at least one eye).

Results

Evoked movements were conjugate and vertical for a minority of sites but, for most sites, the evoked movements were directionally disconjugate. Moreover, there was typically a convergent change in horizontal strabismus when the evoked movements were upward and a divergent change when the evoked movements were downward.

Conclusions

Microstimulation of INC in monkeys with A-pattern strabismus evokes movements with the expected directional disconjugacies, implying that the key neural abnormalities are within, or downstream from, this structure. High site-to-site variability in the conjugacy/disconjugacy of evoked movements rules out the hypothesis that the abnormalities are solely peripheral.

Reduced activity of vertically acting motoneurons during convergence

Previous studies have revealed unexpected relationships between the firing rates of horizontally acting motoneurons and vergence. During a vergence task, for example, antidromically identified abducens internuclear neurons show a negative correlation between vergence angle and firing rate, which is the opposite of the modulation displayed by the medial rectus motoneurons to which they project. For a given horizontal eye position, medial rectus motoneurons discharge at a higher rate if the eyes are converged than if the same eye position is reached during a task that requires version; paradoxically, however, the horizontal rectus eye muscles show corelaxation during vergence. These complex and unexpected relationships inspired the present author to investigate whether the tonic firing rates of vertically acting motoneurons in oculomotor nucleus are correlated with vergence angle. Monkeys were trained to fixate a single, randomly selected, visual target among an array of 60 red plus-shaped LEDs, arranged at 12 different distances in three-dimensional space. The targets were arranged to permit dissociation of vertical eye position and vergence angle. Here I report, for the first time, that most vertically acting motoneurons in oculomotor nucleus show a significant negative correlation between tonic firing rate and vergence angle. This suggests the possibility that there may be a general corelaxation of extraocular muscles during vergence.NEW & NOTEWORTHY An array of 60 plus-shaped LEDs, positioned at various locations in three-dimensional space, was used to elicit conjugate and disjunctive saccades while single neurons in oculomotor nucleus were recorded from rhesus monkeys. This study demonstrates that most vertically acting motoneurons in oculomotor nucleus discharge at a lower rate when the eyes are converged.

Effects of smooth pursuit and second-order stimuli on visual motion prediction

The purpose of this study was to determine whether smooth pursuit eye movements affect visual motion prediction using a time‐to‐contact task where observers anticipate the exact instant that a partially occluded target would coincide with a stationary object. Moreover, we attempted to clarify the influence of second‐order motion on visual motion prediction during smooth pursuit. One target object moved to another stationary object (6 deg apart) at constant velocity of 3, 4, and 5 deg/s, and then the two objects disappeared 500 ms after the onset of target motion. The observers estimated the moment the moving object would overlap the stationary object and pressed a button. For the pursuit condition, both a Gaussian window and a random dots texture moved in the same direction at the same speed for the first‐order motion, whereas a Gaussian window moved over a static background composed of random dots texture for the second‐order motion. The results showed that the constant error of the time‐to‐contact shifted to a later response for the pursuit condition compared to the fixation condition, regardless of the object velocity. In addition, during smooth pursuit, the constant error for the second‐order motion shifted to an earlier response compared to the first‐order motion when the object velocity was 3 deg/s, whereas no significant difference was found at 4 and 5 deg/s. Therefore, our results suggest that visual motion prediction using a time‐to‐contact task is affected by both eye movements and motion configuration such as second‐order motion.

Properties of smooth pursuit and visual motion reaction time to second-order motion stimuli

A large number of psychophysical and neurophysiological studies have demonstrated that smooth pursuit eye movements are tightly related to visual motion perception. This could be due to the fact that visual motion sensitive cortical areas such as meddle temporal (MT), medial superior temporal (MST) areas are involved in motion perception as well as pursuit initiation. Although the directional-discrimination and perceived target velocity tasks are used to evaluate visual motion perception, it is still uncertain whether the speed of visual motion perception, which is determined by visuomotor reaction time (RT) to a small target, is related to pursuit initiation. Therefore, we attempted to determine the relationship between pursuit latency/acceleration and the visual motion RT which was measured to the visual motion stimuli that moved leftward or rightward. The participants were instructed to fixate on a stationary target and press one of the buttons corresponding to the direction of target motion as soon as possible once the target starts to move. We applied five different visual motion stimuli including first- and second-order motion for smooth pursuit and visual motion RT tasks. It is well known that second-order motion induces lower retinal image motion, which elicits weaker responses in MT and MST compared to first-order motion stimuli. Our results showed that pursuit initiation including latency and initial eye acceleration were suppressed by second-order motion. In addition, second-order motion caused a delay in visual motion RT. The better performances in both pursuit initiation and visual motion RT were observed for first-order motion, whereas second-order (theta motion) induced remarkable deficits in both variables. Furthermore, significant Pearson's correlation and within-subjects correlation coefficients were obtained between visual motion RT and pursuit latency/acceleration. Our findings support the suggestion that there is a common neuronal pathway involved in both pursuit initiation and the speed of visual motion perception.

Functional connectivity during smooth pursuit eye movements

Smooth pursuit eye movements (SPEM) hold the image of a slowly moving stimulus on the fovea. The neural system underlying SPEM primarily includes visual, parietal, and frontal areas. In the present study, we investigated how these areas are functionally coupled and how these couplings are influenced by target motion frequency. To this end, healthy participants (n = 57) were instructed to follow a sinusoidal target stimulus moving horizontally at two different frequencies (0.2 Hz, 0.4 Hz). Eye movements and blood oxygen level-dependent (BOLD) activity were recorded simultaneously. Functional connectivity of the key areas of the SPEM network was investigated with a psychophysiological interaction (PPI) approach. How activity in five eye movement-related seed regions (lateral geniculate nucleus, V1, V5, posterior parietal cortex, frontal eye fields) relates to activity in other parts of the brain during SPEM was analyzed. The behavioral results showed clear deterioration of SPEM performance at higher target frequency. BOLD activity during SPEM versus fixation occurred in a geniculo-occipito-parieto-frontal network, replicating previous findings. PPI analysis yielded widespread, partially overlapping networks. In particular, frontal eye fields and posterior parietal cortex showed task-dependent connectivity to large parts of the entire cortex, whereas other seed regions demonstrated more regionally focused connectivity. Higher target frequency was associated with stronger activations in visual areas but had no effect on functional connectivity. In summary, the results confirm and extend previous knowledge regarding the neural mechanisms underlying SPEM and provide a valuable basis for further investigations such as in patients with SPEM impairments and known alterations in brain connectivity.NEW & NOTEWORTHY This study provides a comprehensive investigation of blood oxygen level-dependent (BOLD) functional connectivity during smooth pursuit eye movements. Results from a large sample of healthy participants suggest that key oculomotor regions interact closely with each other but also with regions not primarily associated with eye movements. Understanding functional connectivity during smooth pursuit is important, given its potential role as an endophenotype of psychoses.

Properties of smooth pursuit adaptation induced by theta motion

Current study attempted to determine whether repeated smooth pursuit trials using theta motion, in which the directions of retinal image-motion and object-motion are opposed, yield pursuit adaptation. Adaptation trials consisted of 350 step-ramp trials using theta motion, and pre- and post-trials using first-order motion were conducted. As a result, initial acceleration in post-adaptation increased significantly than pre-adaptation trials. This was the case even though there was no adaptive change throughout adaptation (350 trials) using theta motion. Our results suggest that smooth pursuit could adapt to theta motion even with challenges associated with opposite retinal slip.

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.

Baseball Players' Eye Movements and Higher Coincident-Timing Task Performance

Previous studies have reported that baseball players have higher than average visual information processing abilities and outstanding motor control. The speed and position of the baseball and the batter are constantly changing, leading skilled players to acquire highly accurate visual information processing and decision-making. This study sought to clarify how movement of the eyes is associated with baseball players’ higher coincident-timing task performance. We recruited 15 right-handed baseball players and 15 age-matched track and field athletes. On a computer-based coincident-timing task, we instructed participants to stop a computer image of a moving target by pressing a button at a designated point. We presented bidirectional moving targets with various velocities, presented in a random order. The targets’ moving angular velocity varied between 100, 83, 71, 63, 56, 50, and 46 deg/s. We conducted 168 repetitions (42 reps × 4 sets) of this coincident-timing task and measured participants’ eye movements during the task using Pupil Centre Corneal Reflection. Mixed-design analysis of variance results revealed participant group effects in favor of baseball players for timing absolute error and low absolute error, as predicted from prior visual processing and decision-making research with baseball players. However, in contrast to prior research, we found significantly shorter smooth-pursuit onset latency in elite baseball players, and there were no significant group differences for saccade onset and offset latencies. This may be explained by the difference in our research paradigm with mobile targets randomly presented at various velocities from the left and right. Our data showed baseball players’ higher than normal simultaneous timing execution for making decisions and movements based on visual information, even under laboratory conditions with randomly moving mobile targets.

Abnormal Eye Position Signals in Interstitial Nucleus of Cajal in Monkeys With "A" Pattern Strabismus

Purpose

Pattern strabismus is characterized by a cross-axis pattern of horizontal and vertical misalignments. In A-pattern strabismus, for example, a divergent change in the horizontal misalignment occurs on downgaze. Work with nonhuman primate models has provided evidence that this disorder is associated with abnormal cross-talk between brainstem pathways that normally encode horizontal and vertical eye position and velocity. Neurons in the interstitial nucleus of Cajal (INC) are normally sensitive to vertical eye position; in the present study, we test the hypothesis that, in monkeys with pattern strabismus, some INC neurons will show an abnormal sensitivity to horizontal eye position.

Methods

Monkeys were rewarded for fixating a visual target that stepped to various locations on a tangent screen. Single neurons were recorded from INC in one normal monkey, and two with A-pattern strabismus. Multiple linear regression analysis was used to estimate the preferred direction for each neuron.

Results

In the normal monkey, all INC neurons had preferred directions within 20° of pure vertical (either up or down). The preferred directions were significantly more variable in the monkeys with pattern strabismus, with a minority being more sensitive to horizontal eye position than vertical eye position. In addition, the vertical eye position sensitivity was significantly less in the monkeys with strabismus.

Conclusions

In pattern strabismus, neurons in INC show neurophysiological abnormalities consistent with a failure to develop normal tuning properties. Results were consistent with the hypothesis that, in pattern strabismus, INC receives an abnormally strong signal related to horizontal eye position.

Asymmetric smooth pursuit eye movements and visual motion reaction time

Smooth pursuit eye movements often show directional asymmetry in pursuit initiation or steady-state pursuit in both humans and monkeys. It has been demonstrated that the initial part of smooth pursuit is driven by visual motion related signals in cortical areas. Parietal cortex such as middle temporal (MT) and medial superior temporal (MST) areas are known to be involved in visual motion perception as well as pursuit initiation. Therefore, the purpose of this study is to determine whether directional asymmetry in pursuit initiation is associated with visual motion perception. We used a step-ramp paradigm to induce horizontal smooth pursuit eye movements and then tested visual motion reaction time (RT). Visual motion RT was measured to the visual motion stimuli that moved leftward or rightward, which is an important parameter of our sensory motor processing based on visual motion perception. Nineteen healthy male subjects participated in the study. We found that some of our subjects showed directional asymmetries in initial pursuit acceleration between the leftward and rightward directions, which were consistent with an asymmetric bias in visual motion RT. Therefore, our results suggest that asymmetric pursuit initiation is associated with, at least in part, a bias of visual motion perception. These results could be due to a common neuronal pathway involved in both pursuit initiation and visual motion RT.

A Rhesus Monkey With a Naturally Occurring Impairment of Disparity Vergence. II. Abnormal Near Response Cell Activity in the Supraoculomotor Area

Purpose

Convergence insufficiency is a very common disorder that can have significant adverse effects on school performance. When reading, children with this disorder often experience diplopia and headaches. We have recently obtained a rhesus monkey with a naturally occurring impairment of vergence eye movements. In the companion paper, we report behavioral testing that shows a pattern of impairments similar to what clinicians observe in human children with convergence insufficiency, including a receded near point, an exophoria that increases as target distance decreases, and difficulty maintaining an appropriate vergence angle when presented with a large field stimulus at near. For the present case report, we wondered whether these behavioral deficits would be associated with abnormal discharge patterns in brainstem neurons related to vergence eye movements.

Methods

Single unit activity was recorded from near and far response cells in the supraoculomotor area in the vergence-impaired monkey, while he performed a smooth vergence tracking task or fixated visual targets at different distances.

Results

We found an abnormally weak sensitivity to both vergence angle and vergence velocity. Nonetheless, these neurons modulated in association with contextually inappropriate slow vergence movements that occurred in the absence of saccades but not for slow divergence drifts that immediately followed converging saccades. Modulation of activity was more robust when additional depth cues were available.

Conclusions

These data suggest that disorders affecting vergence eye movements may be associated with impoverished sensory input to the near and far response cells and, perhaps, aberrant tuning in vergence-related neurons.

A Rhesus Monkey With a Naturally Occurring Impairment of Disparity Vergence. I. Behavioral Comparisons to Vergence in a Normal Animal

Purpose

Human children with disorders affecting vergence eye movements have difficulty during close work, such as reading. Patients with convergence insufficiency show a receded near point and an exophoria that is greater at near than at far. Neurologic abnormalities may underlie these symptoms, but it is difficult to test this idea directly because there is no animal model for this disorder. In the present case report, we describe behavioral testing in a rhesus monkey with a naturally occurring impairment of vergence eye movements (monkey CI).

Methods

Three monkeys were trained to perform a variety of oculomotor tasks that required saccades, vergence, and/or smooth tracking of a visual target moving in depth.

Results

Two of the monkeys (N1 and N2) were able to perform these tasks correctly. The third, monkey CI, was able to correctly perform these tasks when the required vergence angle was ≤5° but had difficulty when the task required larger convergence. This animal showed a consistent exodeviation that worsened as the target drew closer. When a variable prism was used to test disparity vergence in monkey CI, the animal showed an unstable convergence response (maximum 6°) that increased with prism correction, up to 12 prism diopters. By comparison, monkey N1 was able to achieve stable, appropriate convergence up to 26 prism diopters. Monkey CI's performance on vergence tasks improved when a large-field random checkerboard pattern was used to provide additional depth cues.

Conclusions

Monkey CI appears to have a naturally occurring disorder of vergence eye movements.

Pursuit disorder and saccade dysmetria after caudal fastigial inactivation in the monkey

The caudal fastigial nuclei (cFN) are the output nuclei by which the medio-posterior cerebellum influences the production of saccadic and pursuit eye movements. We investigated the consequences of unilateral inactivation on the pursuit eye movement made immediately after an interceptive saccade toward a centrifugal target. We describe here the effects when the target moved along the horizontal meridian with a 10 or 20°/s speed. After muscimol injection, the monkeys were unable to track the present location of the moving target. During contralesional tracking, the velocity of postsaccadic pursuit was reduced. This slowing was associated with a hypometria of interceptive saccades such that gaze direction always lagged behind the moving target. No correlation was found between the sizes of saccade undershoot and the decreases in pursuit speed. During ipsilesional tracking, the effects on postsaccadic pursuit were variable across the injection sessions, whereas the interceptive saccades were consistently hypermetric. Here also, the ipsilesional pursuit disorder was not correlated with the saccade hypermetria either. The lack of correlation between the sizes of saccade dysmetria and changes of postsaccadic pursuit speed suggests that cFN activity exerts independent influences on the neural processes generating the saccadic and slow eye movements. It also suggests that the cFN is one locus where the synergy between the two motor categories develops in the context of tracking a moving visual target. We explain how the different fastigial output channels can account for these oculomotor tracking disorders. NEW & NOTEWORTHY Inactivation of the caudal fastigial nucleus impairs the ability to track a moving target. The accuracy of interceptive saccades and the velocity of postsaccadic pursuit movements are both altered, but these changes are not correlated. This absence of correlation is not compatible with an impaired common command feeding the circuits producing saccadic and pursuit eye movements. However, it suggests an involvement of caudal fastigial nuclei in their synergy to accurately track a moving target.

Long-term sensorimotor adaptation in the ocular following system of primates

The sudden movement of a wide-field image leads to a reflexive eye tracking response referred to as short-latency ocular following. If the image motion occurs soon after a saccade the initial speed of the ocular following is enhanced, a phenomenon known as post-saccadic enhancement. We show in macaque monkeys that repeated exposure to the same stimulus regime over a period of months leads to progressive increases in the initial speeds of ocular following. The improvement in tracking speed occurs for ocular following with and without a prior saccade. As a result of the improvement in ocular following speeds, the influence of post-saccadic enhancement wanes with increasing levels of training. The improvement in ocular following speed following repeated exposure to the same oculomotor task represents a novel form of sensori-motor learning in the context of a reflexive movement.

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.

Electrical Microstimulation of the Superior Colliculus in Strabismic Monkeys

Purpose

Visually guided saccades are disconjugate in human and nonhuman strabismic primates. The superior colliculus (SC) is a region of the brain topographically organized in visual and motor maps where the saccade goal is spatially coded. The present study was designed to investigate if a site of stimulation on the topographic motor map was evoking similar or different saccade vectors for each eye.

Methods

We used microelectrical stimulation (MS) of the SC in two strabismic (one esotrope and one exotrope) and two control macaques under binocular and monocular viewing conditions. We compared the saccade amplitudes and directions for each SC site and each condition independently of the fixating eye and then between each fixating eye. A comparison with disconjugacies of visually guided saccades was also performed.

Results

We observed different saccade vectors for the two eyes in strabismic monkeys, but conjugate saccades in normal monkeys. Evoked saccade vectors for the left eye when that eye was fixating the target were different from those of the right eye when it was fixating. The disconjugacies evoked by the MS were not identical but similar to those observed for visually guided saccades especially for the dominant eye.

Conclusions

Our results suggest that, in strabismus, the saccade generator does not interpret activation of a single location of the SC as the same desired displacement for each eye. This finding is important for advancing understanding of the development of neural circuits in strabismus.

French Abstract

Ocular motor disturbances in autism spectrum disorders: Systematic review and comprehensive meta-analysis

There has been considerable focus placed on how individuals with autism spectrum disorder (ASD) visually perceive and attend to social information, such as facial expressions or social gaze. The role of eye movements is inextricable from visual perception, however this aspect is often overlooked. We performed a series of meta-analyses based on data from 28 studies of eye movements in ASD to determine whether there is evidence for ocular motor dysfunction in ASD. Tasks assessed included visually-guided saccade tasks, gap/overlap, anti-saccade, pursuit tasks and ocular fixation. These analyses revealed evidence for ocular motor dysfunction in ASD, specifically relating to saccade dysmetria, difficulty inhibiting saccades and impaired tracking of moving targets. However there was no evidence for deficits relating to initiating eye movements, or engaging and disengaging from simple visual targets. Characterizing ocular motor abnormalities in ASD may provide insight into the functional integrity of brain networks in ASD across development, and assist our understanding of visual and social attention in ASD.

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.

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.

Vertical and oblique saccade disconjugacy in strabismus

PURPOSE

Previous studies have shown that horizontal saccades are disconjugate in humans and monkeys with strabismus. The present study was designed to extend these results to vertical and oblique saccades. A major goal was to assess the conjugacy in terms of both amplitude and direction.

METHODS

Saccadic eye movements were recorded binocularly in three adult monkeys. One had normal eye alignment, one had exotropia resulting from a bilateral medial rectus tenotomy in the first week of life, and one had esotropia resulting from prism rearing during the first 3 months of life. We assessed the conjugacy of saccades in various directions by comparing both amplitude and direction.

RESULTS

Saccades in the strabismic monkeys were disconjugate in terms of both amplitude and direction. These effects were as large for vertical and oblique saccades as for horizontal ones. However, the pattern of disconjugacy often varied as a function of saccade direction. In some cases, saccades that appeared to be conjugate in terms of amplitude differed substantially when direction was taken into account.

CONCLUSIONS

These data indicate that the assessment of saccade disconjugacy in strabismus may yield misleading results if direction is not considered. The complex pattern of disconjugacy suggests that strabismus is associated with substantial abnormalities within the circuitry controlling saccades. Neurophysiological studies are needed to identify the specific neural substrates for these behavioral effects.

Stimulation of pontine reticular formation in monkeys with strabismus

PURPOSE

Saccade disconjugacy in strabismus could result from any of a number of factors, including abnormalities of eye muscles, the plant, motoneurons, near response cells, or atypical tuning of neurons in saccade-related areas of the brain. This study was designed to investigate the possibility that saccade disconjugacy in strabismus is associated with abnormalities in paramedian pontine reticular formation (PPRF).

METHODS

We applied microstimulation to 22 sites in PPRF and 20 sites in abducens nucleus in three rhesus macaque monkeys (one normal, one esotrope, and one exotrope).

RESULTS

When mean velocity was compared between the two eyes, a slight difference was found for 1/5 sites in the normal animal. Significant differences were found for 5/6 sites in an esotrope and 10/11 sites in an exotrope. For five sites in the strabismic monkeys, the directions of evoked movements differed by more than 40° between the two eyes. When stimulation was applied to abducens nucleus (20 sites), the ipsilateral eye moved faster for 4/6 sites in the normal animal and all nine sites in the esotrope. For the exotrope, however, the left eye always moved faster, even for three sites on the right side. For the strabismic animals, stimulation of abducens nucleus often caused a different eye to move faster than stimulation of PPRF.

CONCLUSIONS

These data suggest that PPRF is organized at least partly monocularly in strabismus and that disconjugate saccades are at least partly a consequence of unbalanced saccadic commands being sent to the two eyes.

Conjugate adaptation of smooth pursuit during monocular viewing in strabismic monkeys with exotropia

PURPOSE

Humans and monkeys are able to adapt their smooth pursuit output when challenged with consistent errors in foveal/parafoveal image motion during tracking. Visual motion information from the retina is known to be necessary for guiding smooth pursuit adaptation. The purpose of this study is to determine whether retinal motion signals delivered to one eye during smooth pursuit produce adaptation in the fellow eye. We tested smooth pursuit adaptation during monocular viewing in strabismic monkeys with exotropia.

METHODS

To induce smooth pursuit adaptation experimentally, we used a step-ramp tracking with two different velocities (adaptation paradigm), where the target begins moving at one speed (25°/s) for first 100 ms and then changes to a lower speed (5°/s) for the remainder of the trial. Typically, 100 to 200 trials were used to adapt the smooth pursuit response. Control trials employing single speed step-ramp target motion (ramp speed = 25°/s) were used before and after adaptation paradigm to estimate adaptation.

RESULTS

The magnitude of adaptation as calculated by percentage change was not significantly different (P = 0.53) for the viewing (mean, 40.3% ± 5.9%) and the nonviewing (mean, 39.7% ± 6.2%) eyes during monocular viewing conditions, even in cases with large angle (18°-20°) strabismus.

CONCLUSIONS

Our results indicate that animals with strabismus retain the ability to produce conjugate adaptation of smooth pursuit. Therefore, we suggest that a single central representation of retinal motion information in the viewing eye drives adaptation for both eyes equally.

Role of MSTd extraretinal signals in smooth pursuit adaptation

The smooth pursuit (SP) system is able to adapt to challenges associated with development or system drift to maintain pursuit accuracy. Short-term adaptation of SP can be produced experimentally using a step-ramp tracking paradigm with 2 steps of velocity (double-step paradigm). Previous studies have demonstrated that the macaque cerebellum plays an essential role in SP adaptation. However, it remains uncertain whether neuronal activity in afferent structures to the cerebellum shows changes associated with SP adaptation. Therefore, we focused on the dorsal-medial part of medial superior temporal cortex (MSTd), which is part of the cortico-ponto-cerebellar pathway thought to provide extraretinal signals needed for maintaining SP. We found that 54% of the SP-related neurons showed significant changes in the first 100 ms of response correlated with adaptive changes of initial pursuit. Our results indicate that some cortical neurons in MSTd could be inside the circuit involved in SP adaptation. Furthermore, our sample of MSTd neurons started their discharge on average 103 ms after SP onset. Therefore, we suggest that extraretinal signals carried in MSTd might be due to efference copy of pursuit eye velocity signals, which reflect plastic changes in the downstream motor output pathways (e.g., the cerebellum).

Visual error signals from the pretectal nucleus of the optic tract guide motor learning for smooth pursuit

Smooth pursuit (SP) eye movements are used to maintain the image of a moving object on or near the fovea. Visual motion signals aid in driving SP and are necessary for its adaptation. The sources of visual error signals that support SP adaptation are incompletely understood but could involve neurons in cortical and brain stem areas with direction selective visual motion responses. Here we focus on the pretectal nucleus of the optic tract (NOT), which encodes retinal error information during SP. The aim of this study was to characterize the role of the NOT in SP adaptation. SP adaptation is typically produced using a double step of velocity ramp (double-step paradigm), where target speed either increases or decreases 100 ms after the beginning of a trial. In our study, we delivered a brief (200 ms) train of microelectrical stimulation (ES) in the left NOT to introduce directional error signals at the point in time where a second target speed would appear in a double-step paradigm. The target was extinguished coincidentally with the onset of the ES train. Initial eye acceleration (1st 100 ms) showed significant increases after 100 trials, which included left NOT stimulation during ongoing pursuit in an ipsiversive (leftward) direction. In contrast, initial eye acceleration showed significant decreases after repeated left NOT stimulation during contraversive (rightward) SP. Control studies performed using the same periodicity of NOT stimulation as in the preceding text but without accompanying SP did not induce changes in eye acceleration. In contrast, ES of the NOT paired with active SP produced gradual changes in eye acceleration similar to that observed in double-step paradigm. Therefore our findings support the suggestion that the NOT is an important source of visual error information for guiding motor learning during horizontal SP.

Selective defects of visual tracking in progressive supranuclear palsy (PSP): implications for mechanisms of motion vision

Smooth ocular tracking of a moving visual stimulus comprises a range of responses that encompass the ocular following response (OFR), a pre-attentive, short-latency mechanism, and smooth pursuit, which directs the retinal fovea at the moving stimulus. In order to determine how interdependent these two forms of ocular tracking are, we studied vertical OFR in progressive supranuclear palsy (PSP), a parkinsonian disorder in which vertical smooth pursuit is known to be impaired. We measured eye movements of 9 patients with PSP and 12 healthy control subjects. Subjects viewed vertically moving sine-wave gratings that had a temporal frequency of 16.7 Hz, contrast of 32%, and spatial frequencies of 0.17, 0.27 or 0.44 cycles/degree. We measured OFR amplitude as change in eye position in the 70-150 ms, open-loop interval following stimulus onset. Vertical smooth pursuit was studied as subjects attempted to track a 0.27 cycles/degree grating moving sinusoidally through several cycles at frequencies between 0.1 and 2.5 Hz. We found that OFR amplitude, and its dependence on spatial frequency, was similar in PSP patients (group mean 0.10 degree) and control subjects (0.11 degree), but the latency to onset of OFR was greater for PSP patients (group mean 99 ms) than control subjects (90 ms). When OFR amplitude was re-measured, taking into account the increased latency in PSP patients, there was still no difference from control subjects. We confirmed that smooth pursuit was consistently impaired in PSP; group mean tracking gain at 0.7 Hz was 0.29 for PSP patients and 0.63 for controls. Neither PSP patients nor control subjects showed any correlation between OFR amplitude and smooth-pursuit gain. We propose that OFR is spared because it is generated by low-level motion processing that is dependent on posterior cerebral cortex, which is less affected in PSP. Conversely, smooth pursuit depends more on projections from frontal cortex to the pontine nuclei, both of which are involved in PSP. The accessory optic pathway, which is heavily involved in PSP, seems unlikely to contribute to the OFR in humans.

Signal processing and distribution in cortical-brainstem pathways for smooth pursuit eye movements

Smooth pursuit (SP) eye movements are used to maintain the image of a moving object relatively stable on the fovea. Even when tracking a single target over a dark background, multiple areas including frontal eye fields (FEF) and middle temporal (MT) and medial superior temporal (MST) cortex contribute to converting visual signals into initial commands for SP. Signals in the cortical pursuit system reach the oculomotor cerebellum through brainstem centers including the dorsolateral pontine nucleus (DLPN), nucleus reticularis tegmenti pontis (NRTP), and pretectal nucleus of the optic tract (NOT). The relative information carried in these parallel pathways remains to be fully defined. We used multiple linear-regression modeling to estimate the relative sensitivities of cortical (MST, FEF), pontine (NRTP, DLPN), and NOT neurons to eye- and retinal-error parameters (position, velocity, and acceleration) during step-ramp SP of macaques (Macaca mulatta). We found that a large proportion of pursuit-related MST and DLPN neurons were most sensitive to eye-velocity or retinal error velocity. In contrast, a large proportion of FEF and rostral NRTP neurons were most sensitive to eye acceleration. Visual neurons in MST, DLPN, and NOT were most sensitive to retinal image velocity.

Behavioral Evidence of Separate Adaptation Mechanisms Controlling Saccade Amplitude Lengthening and Shortening

The accuracy of saccadic eye movements is maintained over the long term by adaptation mechanisms that decrease or increase saccade amplitude. It is still unknown whether these opposite adaptive changes rely on common mechanisms. Here, a double-step target paradigm was used to adaptively decrease (backward second target step) or increase (forward step) the amplitude of reactive saccades in one direction only. To test which sensorimotor transformation stages are subjected to these adaptive changes, we measured their transfer to antisaccades in which sensory and motor vectors are spatially dissociated. In the backward adaptation condition, all subjects showed a significant amplitude decrease for adapted prosaccades and a significant transfer of adaptation to antisaccades performed in the adapted direction, but not to oppositely directed antisaccades elicited by a target jump in the adapted direction. In the forward adaptation condition, only 14 of 19 subjects showed a significant amplitude increase for prosaccades and no significant adaptation transfer to antisaccades was detected in either the adapted or nonadapted direction. These findings suggest that, whereas the level(s) of forward adaptation cannot be resolved, the mechanisms involved in backward adaptation of reactive saccades take place at a sensorimotor level downstream from the vector inversion process of antisaccades and differ markedly from those involved in forward adaptation.

Non-human primates: model animals for developmental psychopathology

Non-human primates have been used to model psychiatric disease for several decades. The success of this paradigm has issued from comparable cognitive skills, brain morphology, and social complexity in adult monkeys and humans. Recently, interest in biological psychiatry has focused on similar brain, social, and emotional developmental processes in monkeys. In part, this is related to evidence that early postnatal experiences in human development may have profound implications for subsequent mental health. Non-human primate studies of postnatal phenomenon have generally fallen into three basic categories: experiential manipulation (largely manipulations of rearing), pharmacological manipulation (eg drug-induced psychosis), and anatomical localization (defined by strategic surgical damage). Although these efforts have been very informative each of them has certain limitations. In this review we highlight general findings from the non-human primate postnatal developmental literature and their implications for primate models in psychiatry. We argue that primates are uniquely capable of uncovering interactions between genes, environmental challenges, and development resulting in altered risk for psychopathology.