Directional asymmetry during combined saccade-vergence movements

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.

Understanding falls in progressive supranuclear palsy

INTRODUCTION

Progressive supranuclear palsy (PSP) is characterized by frequent falls which worsen with disease progression, causing substantial morbidity and mortality. Few studies have investigated which factors contribute to falls in PSP, and all have involved few participants, thus lacking necessary statistical power. The aim of this study was to identify clinical parameters most significantly associated with increasing falls in PSP, using the largest sample of patients to date.

METHODS

Comprehensive clinical data were collected from 339 not demented PSP patients meeting the NINDS-SPSP criteria, who were divided into two groups - Infrequent Fallers (IF; n = 118) with rare falls, and Frequent Fallers (FF; n = 221) who fell occasionally to multiple times a day. Of 198 clinical parameters, we hypothesized 38 to be correlated with an increasing risk of falls. These 38 parameters were analyzed via univariate regression analysis to determine the strength of their association with fall frequency. Unit odds ratios identified the magnitude with which each parameter resulted in an increasing risk of falls.

RESULTS

Twenty-five of 38 parameters analyzed were significantly associated with fall frequency based on univariate analysis. Symptom duration, clinical measures of disease severity, and several motoric and oculomotor clinical parameters were associated with FF. Examined cognitive parameters and slowing of vertical saccades were not.

CONCLUSIONS

The clinical parameters identified as associated with increased frequency of falls improve our understanding of why they occur and may help identify not demented PSP patients at risk for increasing falls.

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.

Changes in perceived egocentric direction during symmetric vergence

The Wells-Hering's laws of perceived egocentric visual direction (EVD) assume that information about eye position includes equal contributions from both eyes. An implication of this assumption is that only versional eye movements should lead to a change in perceived EVD. Previously, we showed that a differential weighting of eye-position information occurs in some individuals during asymmetric vergence. To extend this finding, we determined here whether a differential weighting of eye-position information occurs also during symmetric vergence eye movements. Open-loop pointing responses to a bright target were obtained in five subjects to estimate the contribution of each eye's position information to perceived EVD during symmetric vergence demands that ranged from 6 prism diopters base in to 18 prism diopters base out. In all five subjects, the slopes of the lines fit to the pointing responses were in the direction that was predicted from an unequal weighting of eye-position information. We conclude that symmetric vergence movements can result in a change in perceived visual direction, contrary to an assumption of the Wells-Hering's laws.

Differentiation between vergence and saccadic functional activity within the human frontal eye fields and midbrain revealed through fMRI

PURPOSE

Eye movement research has traditionally studied solely saccade and/or vergence eye movements by isolating these systems within a laboratory setting. While the neural correlates of saccadic eye movements are established, few studies have quantified the functional activity of vergence eye movements using fMRI. This study mapped the neural substrates of vergence eye movements and compared them to saccades to elucidate the spatial commonality and differentiation between these systems.

METHODOLOGY

The stimulus was presented in a block design where the 'off' stimulus was a sustained fixation and the 'on' stimulus was random vergence or saccadic eye movements. Data were collected with a 3T scanner. A general linear model (GLM) was used in conjunction with cluster size to determine significantly active regions. A paired t-test of the GLM beta weight coefficients was computed between the saccade and vergence functional activities to test the hypothesis that vergence and saccadic stimulation would have spatial differentiation in addition to shared neural substrates.

RESULTS

Segregated functional activation was observed within the frontal eye fields where a portion of the functional activity from the vergence task was located anterior to the saccadic functional activity (z>2.3; p<0.03). An area within the midbrain was significantly correlated with the experimental design for the vergence but not the saccade data set. Similar functional activation was observed within the following regions of interest: the supplementary eye field, dorsolateral prefrontal cortex, ventral lateral prefrontal cortex, lateral intraparietal area, cuneus, precuneus, anterior and posterior cingulates, and cerebellar vermis. The functional activity from these regions was not different between the vergence and saccade data sets assessed by analyzing the beta weights of the paired t-test (p>0.2).

CONCLUSION

Functional MRI can elucidate the differences between the vergence and saccade neural substrates within the frontal eye fields and midbrain.

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.

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.

The disturbance of gaze in progressive supranuclear palsy: implications for pathogenesis

Progressive supranuclear palsy (PSP) is a disease of later life that is currently regarded as a form of neurodegenerative tauopathy. Disturbance of gaze is a cardinal clinical feature of PSP that often helps clinicians to establish the diagnosis. Since the neurobiology of gaze control is now well understood, it is possible to use eye movements as investigational tools to understand aspects of the pathogenesis of PSP. In this review, we summarize each disorder of gaze control that occurs in PSP, drawing on our studies of 50 patients, and on reports from other laboratories that have measured the disturbances of eye movements. When these gaze disorders are approached by considering each functional class of eye movements and its neurobiological basis, a distinct pattern of eye movement deficits emerges that provides insight into the pathogenesis of PSP. Although some aspects of all forms of eye movements are affected in PSP, the predominant defects concern vertical saccades (slow and hypometric, both up and down), impaired vergence, and inability to modulate the linear vestibulo-ocular reflex appropriately for viewing distance. These vertical and vergence eye movements habitually work in concert to enable visuomotor skills that are important during locomotion with the hands free. Taken with the prominent early feature of falls, these findings suggest that PSP tauopathy impairs a recently evolved neural system concerned with bipedal locomotion in an erect posture and frequent gaze shifts between the distant environment and proximate hands. This approach provides a conceptual framework that can be used to address the nosological challenge posed by overlapping clinical and neuropathological features of neurodegenerative tauopathies.

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.

Effect of vergence on human ocular following response (OFR)

The human ocular following response (OFR) is a preattentive, short-latency visual-field-holding mechanism, which is enhanced if the moving stimulus is applied in the wake of a saccade. Since most natural gaze shifts incorporate both saccadic and vergence components, we asked whether the OFR was also enhanced during vergence. Ten subjects viewed vertically moving sine-wave gratings on a video monitor at 45 cm that had a temporal frequency of 16.7 Hz, contrast of 32%, and spatial frequency of 0.17, 0.27, or 0.44 cycle/deg. In Fixation/OFR experiments, subjects fixed on a white central dot on the video monitor, which disappeared at the beginning of each trial, just as the sinusoidal grating started moving up or down. We measured the change in eye position in the 70- to 150-ms open-loop interval following stimulus onset. Group mean downward responses were larger (0.14 degrees) and made at shorter latency (85 ms) than upward responses (0.10 degrees and 96 ms). The direction of eye drifts during control trials, when gratings remained stationary, was unrelated to the prior response. During vergence/OFR experiments, subjects switched their fixation point between the white dot at 45 cm and a red spot at 15 cm, cued by the disappearance of one target and appearance of the other. When horizontal vergence velocity exceeded 15 degrees/s, motion of sinusoidal gratings commenced and elicited the vertical OFR. Subjects showed significantly (P<0.001) larger OFR when the moving stimulus was presented during convergence (group mean increase of 46%) or divergence (group mean increase of 36%) compared with following fixation. Since gaze shifts between near and far are common during natural activities, we postulate that the increase of OFR during vergence movements reflects enhancement of early cortical motion processing, which serves to stabilize the visual field as the eyes approach their new fixation point.

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.

Divergence influences triggering of both vertical and horizontal saccades

PURPOSE

In real life, divergence is frequently combined with vertical saccades. The purpose of this study was to examine the initiation of vertical and horizontal saccades, pure or combined with divergence.

METHODS

We used a gap paradigm to elicit vertical or horizontal saccades (10 degrees), pure or combined with a predictable divergence (10 degrees). Eye movements from 12 subjects were recorded with EyeLink II.

RESULTS

The major results were (i) when combined with divergence, the latency of horizontal saccades increased but not the latency of vertical saccades; (ii) for both vertical and horizontal saccades, a tight correlation between the latency of saccade and divergence was found; (iii) when the divergence was anticipated, the saccade was delayed.

CONCLUSION

We conclude that the initiation of both components of combined movements is interdependent.

Vergence nystagmus induced by motion in the ground plane: normal response characteristics

We displayed backward/forward motion on a computer monitor in the ground plane. Subjects looked at the center of a moving pattern and eye movements of both eyes were recorded with a search coil system. Involuntary nystagmus including vertical version (VV) and horizontal vergence (HV) was recorded. Dynamics of the nystagmus showed that the slow and quick phase of VV and HV were always associated with each other while the monocular horizontal eye movements composed of HV were either symmetrical or asymmetrical. Peak velocity, amplitudes and frequency of the VV and HV responses were quantified. The results suggest that involuntary HV nystagmus can be induced by simple motion from simulated optic flow in the ground plane and the HV nystagmus helps moving subjects to stabilize their gazes on the object of interest.

Tests of models for saccade-vergence interaction using novel stimulus conditions

During natural activities, two types of eye movements - saccades and vergence - are used in concert to point the fovea of each eye at features of interest. Some electrophysiological studies support the concept of independent neurobiological substrates for saccades and vergence, namely saccadic and vergence burst neurons. Discerning the interaction of these two components is complicated by the near-synchronous occurrence of saccadic and vergence components. However, by positioning the far target below the near target, it is possible to induce responses in which the peak velocity of the vertical saccadic component precedes the peak velocity of the horizontal vergence component by approximately 75 ms. When saccade-vergence responses are temporally dissociated in this way, the vergence velocity waveform changes, becoming less skewed. We excluded the possibility that such change in skewing was due to visual feedback by showing that similar behavior occurred in darkness. We then tested a saccade-related vergence burst neuron (SVBN) model proposed by Zee et al. in J Neurophysiol 68:1624-1641 (1992), in which omnipause neurons remove inhibition from both saccadic and vergence burst neurons. The technique of parameter estimation was used to calculate optimal values for responses from human subjects in which saccadic and convergence components of response were either nearly synchronized or temporally dissociated. Although the SVBN model could account for convergence waveforms when saccadic and vergence components were nearly synchronized, it could not when the components were temporally dissociated. We modified the model so that the saccadic pulse changed the parameter values of the convergence burst units if both components were synchronized. The modified model accounted for velocity waveforms of both synchronous and dissociated convergence movements. We conclude that both the saccadic pulse and omnipause neuron inhibition influence the generation of vergence movements when they are made synchronously with saccades.