Vergence deficits in patients with cerebellar lesions

Predictive posture stabilization before contact with moving objects: equivalence of smooth pursuit tracking and peripheral vision

Postural stabilization is essential to effectively interact with our environment. Humans preemptively adjust their posture to counteract impending disturbances, such as those encountered during interactions with moving objects, a phenomenon known as anticipatory postural adjustments (APAs). APAs are thought to be influenced by predictive models that incorporate object motion via retinal motion and extraretinal signals. Building on our previous work that examined APAs in relation to the perceived momentum of moving objects, here we explored the impact of object motion within different visual field sectors on the human capacity to anticipate motion and prepare APAs for contact between virtual moving objects and the limb. Participants interacted with objects moving toward them under different gaze conditions. In one condition, participants fixated on either a central point (central fixation) or left-right of the moving object (peripheral fixation), whereas in another, they followed the moving object with smooth pursuit eye movements (SPEMs). We found that APAs had the smallest magnitude in the central fixation condition and that no notable differences in APAs were apparent between the SPEM and peripheral fixation conditions. This suggests that the visual system can accurately perceive motion of objects in peripheral vision for posture stabilization. Using Bayesian model averaging, we also evaluated the contribution of different gaze variables, such as eye velocity and gain (ratio of eye and object velocity) and showed that both eye velocity and gain signals were significant predictors of APAs. Taken together, our study underscores the roles of oculomotor signals in the modulation of APAs.NEW & NOTEWORTHY We show that the human visuomotor system can detect motion in peripheral vision and make anticipatory adjustments to posture before contact with moving objects, just as effectively as when the eye movement system tracks those objects with smooth pursuit eye movements. These findings pave the way for research into how age-induced changes in spatial vision, eye movements, and motion perception could affect the control of limb movements and postural stability during motion-mediated interactions with objects.

From convergence insufficiency to functional reorganization: A longitudinal randomized controlled trial of treatment-induced connectivity plasticity

INTRODUCTION

Convergence Insufficiency (CI) is the most prevalent oculomotor dysfunction of binocular vision that negatively impacts quality of life when performing visual near tasks. Decreased resting-state functional connectivity (RSFC) is reported in the CI participants compared to binocularly normal control participants. Studies report that therapeutic interventions such as office-based vergence and accommodative therapy (OBVAT) can improve CI participants' clinical signs, visual symptoms, and task-related functional activity. However, longitudinal studies investigating the RSFC changes after such treatments in participants with CI have not been conducted. This study aimed to investigate the neural basis of OBVAT using RSFC in CI participants compared to the placebo treatment to understand how OBVAT improves visual function and symptoms.

METHODS

A total of 51 CI participants between 18 and 35 years of age were included in the study and randomly allocated to receive either 12 one-hour sessions of OBVAT or placebo treatment for 6 to 8 weeks (1 to 2 sessions per week). Resting-state functional magnetic resonance imaging and clinical assessments were evaluated at baseline and outcome for each treatment group. Region of interest (ROI) analysis was conducted in nine ROIs of the oculomotor vergence network, including the following: cerebellar vermis (CV), frontal eye fields (FEF), supplementary eye fields (SEF), parietal eye fields (PEF), and primary visual cortices (V1). Paired t-tests assessed RSFC changes in each group. A linear regression analysis was conducted for significant ROI pairs in the group-level analysis for correlations with clinical measures.

RESULTS

Paired t-test results showed increased RSFC in 10 ROI pairs after the OBVAT but not placebo treatment (p < 0.05, false discovery rate corrected). These ROI pairs included the following: Left (L)-SEF-Right (R)-V1, L-SEF-CV, R-SEF-R-PEF, R-SEF-L-V1, R-SEF-R-V1, R-SEF-CV, R-PEF-CV, L-V1-CV, R-V1-CV, and L-V1-R-V1. Significant correlations were observed between the RSFC strength of the R-SEF-R-PEF ROI pair and the following clinical visual function parameters: positive fusional vergence and near point of convergence (p < 0.05).

CONCLUSION

OBVAT, but not placebo treatment, increased the RSFC in the ROIs of the oculomotor vergence network, which was correlated with the improvements in the clinical measures of the CI participants.

Structural and spontaneous functional brain changes in visual and oculomotor areas identified by functional localization task in intermittent exotropia children

Intermittent exotropia (IXT) is characterizedby an intermittent outward deviation of the eyes. Yet, the neural substrates associated with IXT are not fully understood. This study investigated brain structure and spontaneous functional activity changes in children with IXT. All participants underwent detailed ophthalmological examinations and multimodal magnetic resonance imaging (MRI) scanning. During functional scanning, binocular visual stimuli were presented to subjects to determine brain areas involved in visual and oculomotor processing. Regions of interest(ROI) were subsequently selected based on functional activation to investigate brain structural and spontaneous functional differences between IXT children and healthy controls (HCs) using small volume correction (SVC). Reduced gray matter density (GMD) was found in the right frontal eye field (FEF) and bilateral inferior parietal lobe (IPL) in IXT children compared with HCs. Besides, reduced fractional amplitude of low-frequency fluctuations (fALFF) values were observed in the left lingual gyrus, right inferior occipital gyrus (IOG), bilateral IPL, and bilateral cerebellum in the IXT children compared to the HCs. IXT children with worse eye position control ability exhibited lower GMD and fALFF values in these areas. Finally, resting state functional connectivity (RSFC) was reduced in frontoparietal oculomotor processing areas in IXT children compared to HCs. In addition, increased cortical thickness was found in the right visual areas and bilateral IPL. These results showed that IXT-related structural and functional brain abnormalities occurred in childhood and may be related to underlying neuropathological mechanisms.

Functional changes in fusional vergence-related brain areas and correlation with clinical features in intermittent exotropia using functional magnetic resonance imaging

To explore the functional changes of the frontal eye field (FEF) and relevant brain regions and its role in the pathogenesis of intermittent exotropia (IXT) children via functional magnetic resonance imaging (fMRI). Twenty-four IXT children (mean age, 11.83 ± 1.93 years) and 28 normal control (NC) subjects (mean age, 11.11 ± 1.50 years) were recruited. During fMRI scans, the IXT children and NCs were provided with static visual stimuli (to evoke sensory fusion) and dynamic visual stimuli (to evoke motor fusion and vergence eye movements) with binocular disparity. Brain activation in the relevant brain regions and clinical characteristics were evaluated. Group differences of brain activation and brain-behavior correlations were investigated. For dynamic and static visual disparity relative to no visual disparity, reduced brain activation in the right FEF and right inferior occipital gyrus (IOG), and increased brain activation in the left middle temporal gyrus complex (MT+) were found in the IXT children compared with NCs. Significant positive correlations between the fusional vergence amplitude and the brain activation values were found in the right FEF, right IPL, and left cerebellum in the NC group. Positive correlations between brain activation values and Newcastle Control Scores (NCS) were found in the left MT+ in the IXT group. For dynamic visual disparity relative to static visual disparity, reduced brain activation in the right middle occipital gyrus, left cerebellum, and bilateral IPL was found in the IXT children compared with NCs. Significant positive correlations between brain activation values and the fusional vergence amplitude were found in the right FEF and right cerebellum in the NC group. Negative correlations between brain activation values and NCS were found in the right middle occipital gyrus, right cerebellum, left IPL, and right FEF in the IXT group. These results suggest that the reduced brain activation in the right FEF, left IPL, and cerebellum may play an important role in the pathogenesis of IXT by influencing fusional vergence function. While the increased brain activation in the left MT+ may compensate for this dysfunction in IXT children.

Central Ocular Motor Disorders: Clinical and Topographic Anatomical Diagnosis, Syndromes and Underlying Diseases

The key to the diagnosis of ocular motor disorders is a systematic clinical examination of the different types of eye movements, including eye position, spontaneous nystagmus, range of eye movements, smooth pursuit, saccades, gaze-holding function, vergence, optokinetic nystagmus, as well as testing of the function of the vestibulo-ocular reflex (VOR) and visual fixation suppression of the VOR. This is like a window which allows you to look into the brain stem and cerebellum even if imaging is normal. Relevant anatomical structures are the midbrain, pons, medulla, cerebellum and rarely the cortex. There is a simple clinical rule: vertical and torsional eye movements are generated in the midbrain, horizontal eye movements in the pons. For example, isolated dysfunction of vertical eye movements is due to a midbrain lesion affecting the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), with impaired vertical saccades only or vertical gaze-evoked nystagmus due to dysfunction of the Interstitial nucleus of Cajal (INC). Lesions of the lateral medulla oblongata (Wallenberg syndrome) lead to typical findings: ocular tilt reaction, central fixation nystagmus and dysmetric saccades. The cerebellum is relevant for almost all types of eye movements; typical pathological findings are saccadic smooth pursuit, gaze-evoked nystagmus or dysmetric saccades. The time course of the development of symptoms and signs is important for the diagnosis of underlying diseases: acute: most likely stroke; subacute: inflammatory diseases, metabolic diseases like thiamine deficiencies; chronic progressive: inherited diseases like Niemann-Pick type C with typically initially vertical and then horizontal saccade palsy or degenerative diseases like progressive supranuclear palsy. Treatment depends on the underlying disease. In this article, we deal with central ocular motor disorders. In a second article, we focus on clinically relevant types of nystagmus such as downbeat, upbeat, fixation pendular, gaze-evoked, infantile or periodic alternating nystagmus. Therefore, these types of nystagmus will not be described here in detail.

Cerebellar projections to the macaque midbrain tegmentum: Possible near response connections

Since most gaze shifts are to targets that lie at a different distance from the viewer than the current target, gaze changes commonly require a change in the angle between the eyes. As part of this response, lens curvature must also be adjusted with respect to target distance by the ciliary muscle. It has been suggested that projections by the cerebellar fastigial and posterior interposed nuclei to the supraoculomotor area (SOA), which lies immediately dorsal to the oculomotor nucleus and contains near response neurons, support this behavior. However, the SOA also contains motoneurons that supply multiply innervated muscle fibers (MIFs) and the dendrites of levator palpebrae superioris motoneurons. To better determine the targets of the fastigial nucleus in the SOA, we placed an anterograde tracer into this cerebellar nucleus in Macaca fascicularis monkeys and a retrograde tracer into their contralateral medial rectus, superior rectus, and levator palpebrae muscles. We only observed close associations between anterogradely labeled boutons and the dendrites of medial rectus MIF and levator palpebrae motoneurons. However, relatively few of these associations were present, suggesting these are not the main cerebellar targets. In contrast, labeled boutons in SOA, and in the adjacent central mesencephalic reticular formation (cMRF), densely innervated a subpopulation of neurons. Based on their location, these cells may represent premotor near response neurons that supply medial rectus and preganglionic Edinger-Westphal motoneurons. We also identified lens accommodation-related cerebellar afferent neurons via retrograde trans-synaptic transport of the N2c rabies virus from the ciliary muscle. They were found bilaterally in the fastigial and posterior interposed nuclei, in a distribution which mirrored that of neurons retrogradely labeled from the SOA and cMRF. Our results suggest these cerebellar neurons coordinate elements of the near response during symmetric vergence and disjunctive saccades by targeting cMRF and SOA premotor neurons.

The role of cerebellum in the child neuropsychological functioning

This chapter proposes a review of neuropsychologic and behavior findings in pediatric pathologies of the cerebellum, including cerebellar malformations, pediatric ataxias, cerebellar tumors, and other acquired cerebellar injuries during childhood. The chapter also contains reviews of the cerebellar mutism/posterior fossa syndrome, reported cognitive associations with the development of the cerebellum in typically developing children and subjects born preterm, and the role of the cerebellum in neurodevelopmental disorders such as autism spectrum disorders and developmental dyslexia. Cognitive findings in pediatric cerebellar disorders are considered in the context of known cerebellocerebral connections, internal cellular organization of the cerebellum, the idea of a universal cerebellar transform and computational internal models, and the role of the cerebellum in specific cognitive and motor functions, such as working memory, language, timing, or control of eye movements. The chapter closes with a discussion of the strengths and weaknesses of the cognitive affective syndrome as it has been described in children and some conclusions and perspectives.

Test-Retest Reliability of Functional Magnetic Resonance Imaging Activation for a Vergence Eye Movement Task

Vergence eye movements are the inward and outward rotation of the eyes responsible for binocular coordination. While studies have mapped and investigated the neural substrates of vergence, it is not well understood whether vergence eye movements evoke the blood oxygen level-dependent signal reliably in separate experimental visits. The test-retest reliability of stimulus-induced vergence eye movement tasks during a functional magnetic resonance imaging (fMRI) experiment is important for future randomized clinical trials (RCTs). In this study, we established region of interest (ROI) masks for the vergence neural circuit. Twenty-seven binocularly normal young adults participated in two functional imaging sessions measured on different days on the same 3T Siemens scanner. The fMRI experiments used a block design of sustained visual fixation and rest blocks interleaved between task blocks that stimulated eight or four vergence eye movements. The test-retest reliability of task-activation was assessed using the intraclass correlation coefficient (ICC), and that of spatial extent was assessed using the Dice coefficient. Functional activation during the vergence eye movement task of eight movements compared to rest was repeatable within the primary visual cortex (ICC = 0.8), parietal eye fields (ICC = 0.6), supplementary eye field (ICC = 0.5), frontal eye fields (ICC = 0.5), and oculomotor vermis (ICC = 0.6). The results demonstrate significant test-retest reliability in the ROIs of the vergence neural substrates for functional activation magnitude and spatial extent using the stimulus protocol of a task block stimulating eight vergence eye movements compared to sustained fixation. These ROIs can be used in future longitudinal RCTs to study patient populations with vergence dysfunctions.

Aging and eye tracking: in the quest for objective biomarkers

Recent applications of eye tracking for diagnosis, prognosis and follow-up of therapy in age-related neurological or psychological deficits have been reviewed. The review is focused on active aging, neurodegeneration and cognitive impairments. The potential impacts and current limitations of using characterizing features of eye movements and pupillary responses (oculometrics) as objective biomarkers in the context of aging are discussed. A closer look into the findings, especially with respect to cognitive impairments, suggests that eye tracking is an invaluable technique to study hidden aspects of aging that have not been revealed using any other noninvasive tool. Future research should involve a wider variety of oculometrics, in addition to saccadic metrics and pupillary responses, including nonlinear and combinatorial features as well as blink- and fixation-related metrics to develop biomarkers to trace age-related irregularities associated with cognitive and neural deficits.

A differential role for the posterior cerebellum in the adaptive control of convergence eye movements

INTRODUCTION

The vergence oculomotor system possesses two robust adaptive mechanisms; a fast "dynamic" and a slow "tonic" system that are both vital for single, clear and comfortable binocular vision. The neural substrates underlying these vergence adaptive mechanisms in humans is unclear.

METHODS

We investigated the role of the posterior cerebellum in convergence adaptation using inhibitory continuous theta-burst repetitive transcranial magnetic stimulation (cTBS) within a double-blind, sham controlled design while eye movements were recorded at 250hz via infrared oculography.

RESULTS

In a preliminary experiment we validated our stimulation protocols by reproducing results from previous work on saccadic adaptation during the classic double-step adaptive shortening paradigm. Following this, across a series of three separate experiments we observed a clear dissociation in the effect of cTBS on convergence adaptation. Dynamic adaptation was substantially reduced while tonic adaptation was unaffected. Baseline dynamic fusional vergence response were also unaffected by stimulation.

CONCLUSIONS

These results indicate a differential role for the posterior cerebellum in the adaptive control of convergence eye movements and provide initial evidence that repetitive transcranial magnetic stimulation is a viable tool to investigate the neurophysiology of vergence control. The results are discussed in the context of the current models of implicit motor adaptation of vergence and their application to clinical populations and technology design in virtual and augmented head mounted display architectures.

SIGNIFICANCE STATEMENT

The cerebellum plays a critical role in the adaptive control of motor systems. Vergence eye movements shift our gaze in depth allowing us to see in 3D and exhibit two distinct adaptive mechanisms that are engaged under a range of conditions including reading, wearing head-mounted displays and using a new spectacle prescription. It is unclear what role the cerebellum plays in these adaptive mechanisms. To answer this, we temporarily disrupted the function of the posterior cerebellum using non-invasive brain stimulation and report impairment of only one adaptive mechanism, providing evidence for neural compartmentalization. The results have implications for vergence control models and applications to comfort and experience studies in head-mounted displays and the rehabilitation of clinical populations exhibiting vergence dysfunctions.

Surgical Responses and Outcomes of Bilateral Medial Rectus Recession in Esotropia With Spinocerebellar Ataxia

PURPOSE

To evaluate the surgical responses and outcomes of bilateral medial rectus (BMR) recession in esotropic patients with spinocerebellar ataxia (SCA) and to compare the results with normal controls.

METHODS

The medical records of patients with SCA who underwent strabismus surgery for esotropia between 2006 and 2015 were reviewed retrospectively. Five esotropic patients with SCA (SCA group) and 10 esotropic patients without neurologic disorders (control group) who underwent BMR recession were included. Success rates, surgical responses, and the amount of preoperative and postoperative distance-near disparity were evaluated and compared between the groups.

RESULTS

The mean preoperative esodeviation was not different between the SCA and control groups (20 vs 17.3 prism diopters [PD], P = .214). However, patients with SCA showed significant undercorrection compared with controls 1 week postoperatively (4.8 vs 1.0 PD, P = .048) and at the final follow-up (6.8 vs 1.8 PD, P = .032). The surgical success rates for the SCA and control groups were 40% and 80%, respectively (P = .095). Patients with SCA demonstrated a significantly reduced surgical response compared with controls (3.15 vs 3.87 PD/mm, P = .004), and a greater amount of postoperative distance-near disparity than controls (8.0 vs 1.1 PD, P = .001).

CONCLUSIONS

A significant undercorrection was observed following BMR recession in esotropic patients with SCA. Accordingly, the authors recommend a slight overcorrection of 5 to 10 PD or adding a slanting procedure when planning strabismus surgery for this distinct group of patients. [J Pediatr Ophthalmol Strabismus. 2019;56(4):266-270.].

Functional anatomy of human extraocular muscles during fusional divergence

We employed magnetic resonance imaging to quantify human extraocular muscle contractility during centered target fusion and fusional divergence repeated with each eye viewing monocularly at 20 cm through 8Δ and at 400 cm through 4Δ base in prism. Contractility, indicated by posterior partial volume (PPV) change, was analyzed in transverse rectus and in medial and lateral superior oblique (SO) muscle compartments and by cross-sectional area change in the inferior oblique (IO). At 20 cm, 3.1 ± 0.5° (SE) diverging eye abduction in 10 subjects was associated with 4.2 ± 1.5% whole lateral rectus (LR) PPV increase ( P < 0.05) and 1.7 ± 1.1% overall medial rectus (MR) PPV decrease attributable to 3.1 ± 1.8% reduction in the superior compartment ( P < 0.025), without change in its inferior compartment or in muscles of the aligned eye. At 400 cm, 2.2 ± 0.5° diverging eye abduction in nine subjects was associated with 6.1 ± 1.3% whole LR PPV increase ( P < 10^-5) but no change in MR, with compartmentally similar relaxation in the LR and MR of the aligned eye. Unlike convergence, there were no IO or SO contractile changes for divergence to either target nor any change in rectus pulley positions. Results confirm and extend to proximal divergence the unique role of the superior MR compartment, yet no MR role for far divergence. Corelaxation of aligned eye LR and MR combined with failure of MR relaxation during divergence is consistent with the limited behavioral range of divergence. NEW & NOTEWORTHY Magnetic resonance imaging shows that the lateral rectus muscle must overcome continued contraction by its opponent the medial rectus when humans diverge their visual axes to achieve single, binocular vision. While the upper but not lower compartment of the medial rectus assists by relaxing for near targets, it does not do so when targets are far away. This behavior violates Sherrington's law of reciprocal action of antagonists and conventional assumptions about the ocular motor system.

Objective Assessment of Vergence after Treatment of Concussion-Related CI: A Pilot Study

PURPOSE

To evaluate changes in objective measures of disparity vergence after office-based vision therapy (OBVT) for concussion-related convergence insufficiency (CI) and determine the feasibility of using this objective assessment as an outcome measure in a clinical trial.

METHODS

This was a prospective, observational trial. All participants were treated with weekly OBVT with home reinforcement. Participants included two adolescents and three young adults with concussion-related, symptomatic CI. The primary outcome measure was average peak velocity for 4° symmetrical convergence steps. Other objective outcome measures of disparity vergence included time to peak velocity, latency, accuracy, settling time, and main sequence. We also evaluated saccadic eye movements using the same outcome measures. Changes in clinical measures (near point of convergence, positive fusional vergence at near, Convergence Insufficiency Symptom Survey [CISS] score) were evaluated.

RESULTS

There were statistically significant and clinically meaningful changes in all clinical measures for convergence. Four of the five subjects met clinical success criteria. For the objective measures, we found a statistically significant increase in peak velocity, response accuracy to 4° symmetrical convergence and divergence step stimuli, and the main sequence ratio for convergence step stimuli. Objective saccadic eye movements (5 and 10°) appeared normal pre-OBVT and did not show any significant change after treatment.

CONCLUSIONS

This is the first report of the use of objective measures of disparity vergence as outcome measures for concussion-related convergence insufficiency. These measures provide additional information that is not accessible with clinical tests about underlying physiological mechanisms leading to changes in clinical findings and symptoms. The study results also demonstrate that patients with concussion can tolerate the visual demands (over 200 vergence and versional eye movements) during the 25-minute testing time and suggest that these measures could be used in a large-scale randomized clinical trial of concussion-related CI as outcome measures.

Vergence Neural Pathways: A Systematic Narrative Literature Review

Research in the neural pathway for vergence is less understood in comparison to the other four visual eye movements. The aim of this study was to review the literature on vergence neural pathways and associated disorders. A review of previous published literature though to March 2016 was conducted. Intracranial pathologies that affect entire neural functioning were found to cause convergence insufficiencies. In contrast, pathologies with a more localised intracranial lesion cause more specific vergence disorders. There is debate as to the potential presence of a "divergence centre." Detailed information on the divergence pathway is lacking and warrants further research.

Deficits in the Activation of Human Oculomotor Nuclei in Chronic Traumatic Brain Injury

Binocular eye movements form a finely tuned system that requires accurate coordination of the oculomotor dynamics of the brainstem control nuclei when tracking the fine binocular disparities required for 3D vision. They are particularly susceptible to disruption by brain injury and other neural dysfunctions. Here, we report functional magnetic resonance imaging activation of the brainstem oculomotor control nuclei by binocular saccadic and vergence eye movements, and significant reductions in their response amplitudes in mild or diffuse traumatic brain injury (dTBI). Bilateral signals were recorded from a non-TBI control group (n = 11) in the oculomotor control system of the superior colliculi, the oculomotor nuclei, the abducens nuclei, and in the supra-oculomotor area (SOA), which mediate vergence eye movements. Signals from these nuclei were significantly reduced overall in a dTBI group (n = 12) and in particular for the SOA for vergence movements, which also showed significant decreases in velocity for both the convergence and divergence directions.

Acquired Esotropia in Cerebellar Disease: A Case Series Illustrating Misdiagnosis as Isolated Lateral Rectus Paresis and Progression Over Time

We highlight an under-recognised cause of acquired esotropia with this prospective observational case series of adults with diplopia secondary to cerebellar dysfunction. We also show deterioration of cerebellar esotropia over time, which has not been previously described. Seven adults (four women) developed diplopia at a median age of 63 years (range: 31-75 years), as the initial manifestation of the underlying cerebellar disorder. Causes of cerebellar dysfunction were familial cerebellar ataxia of unknown mutation (two patients), idiopathic cerebellar ataxia (four patients), and spinocerebellar ataxia 3 (one patient). At onset, three patients had unilateral and four had bilateral lateral rectus under-action. These were initially diagnosed as lateral rectus paresis, but the diagnosis was revised, as our examination showed no slowing of abducting saccades assessed clinically and full abduction with gaze-evoked nystagmus. Esotropia was concomitant and worse for distance, although at onset one patient's esotropia was equal for near and distance. There was a trend of worsening esotropia over time, following a median interval follow-up of 4 years (range: 1-18). All patients were first observed to have cerebellar eye signs after a median interval of 5 years (range: 1-30) from presentation, i.e., impaired pursuit (7/7 patients), gaze-evoked nystagmus (7/7), hypometric saccades (3/7), downbeat nystagmus (2/7), and skew deviation (4/7). Only two patients have not developed non-ocular cerebellar eye signs, after 5 and 8 years from diplopia onset, respectively; the other five patients had gait ataxia, which could be mild. The patients were successfully treated with prisms (7/7), botulinum toxin injections (1/7), and strabismus surgery (1/7).

Esophoria or esotropia in adulthood: a sign of cerebellar dysfunction?

Convergent strabismus is a common diagnosis in early childhood, when it is mostly considered benign. If it develops later in life, strabismus can, however, be a sign of neurological disease. In these cases the underlying pathophysiological mechanisms are largely unknown. In this retrospective case-control study we analyzed the neuro-ophthalmological examination reports of 400 adult patients who presented at the German Center for Vertigo and Balance Disorders to determine an association between ocular misalignment and cerebellar dysfunction. Patients with cerebellar signs (i.e., cerebellar ataxia and/or cerebellar ocular motor signs) had a 4.49 (95 % CI [1.60; 13.78]) times higher frequency of ocular misalignment and specifically a 13.3 (95 % CI [3.80; 55.73]) times increased frequency of esophoria/esotropia (ESO) during distant gaze than patients without cerebellar dysfunction. ESO when looking into the distance was associated with saccadic smooth pursuit, dysmetria of saccades, and downbeat nystagmus (DBN) (χ (2) test, p < 0.0001 for all associations). Patients with cerebellar dysfunction also showed mildly impaired eye abduction (χ (2) test, left eye and right eye: p < 0.0001), associated with horizontal gaze-evoked nystagmus (χ (2) test, p < 0.0001). The association of ESO and DBN implicates a pathophysiological involvement of the cerebellar flocculus, while the association with dysmetric saccades suggests involvement of the oculomotor vermis. This is compatible with animal studies showing that the pathways of the flocculus/posterior interposed nucleus and vermis/nucleus fastigii are both involved in vergence movements and static binocular alignment. From a clinical point of view, a newly diagnosed esophoria/esotropia only during distant gaze may be a sign of a cerebellar disease.

The cerebellum in eye movement control: nystagmus, coordinate frames and disconjugacy

In this review we discuss several aspects of eye movement control in which the cerebellum is thought to have a key role, but have been relatively ignored. We will focus on the mechanisms underlying certain forms of cerebellar nystagmus, as well as the contributions of the cerebellum to binocular alignment in healthy and diseased states. A contemporary review of our understanding provides a basis for directions of further inquiry to address some of the uncertainties regarding the contributions of the cerebellum to ocular motor control.

What part of the cerebellum contributes to a visuospatial working memory task?

Although there is evidence that the cerebellum is involved in working memory (WM), it remains unclear which functions within WM the cerebellum supports and which structures are involved in WM. We tested whether the cerebellum is involved in the filtering of incoming information or in its storage. Using a statistical brain mapping approach in 29 patients with cerebellar ischemic stroke, we found that the cerebellum plays a gatekeeper role, as lesions of the tonsil, the lobus semilunaris inferior, and parts of the vermal pyramid rendered WM susceptible to irrelevant information. We conclude that the cerebellum controls incoming WM information.

Functional activity within the frontal eye fields, posterior parietal cortex, and cerebellar vermis significantly correlates to symmetrical vergence peak velocity: an ROI-based, fMRI study of vergence training

Convergence insufficiency (CI) is a prevalent binocular vision disorder with symptoms that include double/blurred vision, eyestrain, and headaches when engaged in reading or other near work. Randomized clinical trials support that Office-Based Vergence and Accommodative Therapy with home reinforcement leads to a sustained reduction in patient symptoms. However, the underlying neurophysiological basis for treatment is unknown. Functional activity and vergence eye movements were quantified from seven binocularly normal controls (BNC) and four CI patients before and after 18 h of vergence training. An fMRI conventional block design of sustained fixation vs. vergence eye movements stimulated activity in the frontal eye fields (FEF), the posterior parietal cortex (PPC), and the cerebellar vermis (CV). Comparing the CI patients' baseline measurements to the post-vergence training data sets with a paired t-test revealed the following: (1) the percent change in the BOLD signal in the FEF, PPC, and CV significantly increased (p < 0.02), (2) the peak velocity from 4° symmetrical convergence step responses increased (p < 0.01) and (3) patient symptoms assessed using the CI Symptom Survey (CISS) improved (p < 0.05). CI patient measurements after vergence training were more similar to levels observed within BNC. A regression analysis revealed the peak velocity from BNC and CI subjects before and after vergence training was significantly correlated to the percent BOLD signal change within the FEF, PPC, and CV (r = 0.6; p < 0.05). Results have clinical implications for understanding the behavioral and neurophysiological changes after vergence training in patients with CI, which may lead to the sustained reduction in visual symptoms.

Task-modulated coactivation of vergence neural substrates

While functional magnetic resonance imaging (fMRI) has identified which regions of interests (ROIs) are functionally active during a vergence movement (inward or outward eye rotation), task-modulated coactivation between ROIs is less understood. This study tested the following hypotheses: (1) significant task-modulated coactivation would be observed between the frontal eye fields (FEFs), the posterior parietal cortex (PPC), and the cerebellar vermis (CV); (2) significantly more functional activity and task-modulated coactivation would be observed in binocularly normal controls (BNCs) compared with convergence insufficiency (CI) subjects; and (3) after vergence training, the functional activity and task-modulated coactivation would increase in CIs compared with their baseline measurements. A block design of sustained fixation versus vergence eye movements stimulated activity in the FEFs, PPC, and CV. fMRI data from four CI subjects before and after vergence training were compared with seven BNCs. Functional activity was assessed using the blood oxygenation level dependent (BOLD) percent signal change. Task-modulated coactivation was assessed using an ROI-based task-modulated coactivation analysis that revealed significant correlation between the FEF, PPC, and CV ROIs. Prior to vergence training, the CIs had a reduced BOLD percent signal change compared with BNCs for the CV (p<0.05), FEFs, and PPC (p<0.01). The BOLD percent signal change increased within the CV, FEF, and PPC ROIs (p<0.001) as did the task-modulated coactivation between the FEFs and CV as well as the PPC and CV (p<0.05) when comparing the CI pre- and post-training datasets. Results from the Convergence Insufficiency Symptom Survey were correlated to the percent BOLD signal change from the FEFs and CV (p<0.05).

Muscimol inactivation of caudal fastigial nucleus and posterior interposed nucleus in monkeys with strabismus

Previously, we showed that neurons in the supraoculomotor area (SOA), known to encode vergence angle in normal monkeys, encode the horizontal eye misalignment in strabismic monkeys. The SOA receives afferent projections from the caudal fastigial nucleus (cFN) and the posterior interposed nucleus (PIN) in the cerebellum. The objectives of the present study were to investigate the potential roles of the cFN and PIN in 1) conjugate eye movements and 2) binocular eye alignment in strabismic monkeys. We used unilateral injections of the GABAA agonist muscimol to reversibly inactivate the cFN (4 injections in exotropic monkey S1 with ≈ 4° of exotropia; 5 injections in esotropic monkey S2 with ≈ 34° of esotropia) and the PIN (3 injections in monkey S1). cFN inactivation induced horizontal saccade dysmetria in all experiments (mean 39% increase in ipsilesional saccade gain and 26% decrease in contralesional gain). Also, mean contralesional smooth-pursuit gain was decreased by 31%. cFN inactivation induced a divergent change in eye alignment in both monkeys, with exotropia increasing by an average of 9.8° in monkey S1 and esotropia decreasing by an average of 11.2° in monkey S2 (P < 0.001). Unilateral PIN inactivation in monkey S1 resulted in a mean increase in the gain of upward saccades by 13% and also induced a convergent change in eye alignment, reducing exotropia by an average of 2.7° (P < 0.001). We conclude that cFN/PIN influences on conjugate eye movements in strabismic monkeys are similar to those postulated in normal monkeys and cFN/PIN play important and complementary roles in maintaining the steady-state misalignment in strabismus.

Short-term saccadic adaptation in the macaque monkey: a binocular mechanism

Saccadic eye movements are rapid transfers of gaze between objects of interest. Their duration is too short for the visual system to be able to follow their progress in time. Adaptive mechanisms constantly recalibrate the saccadic responses by detecting how close the landings are to the selected targets. The double-step saccadic paradigm is a common method to simulate alterations in saccadic gain. While the subject is responding to a first target shift, a second shift is introduced in the middle of this movement, which masks it from visual detection. The error in landing introduced by the second shift is interpreted by the brain as an error in the programming of the initial response, with gradual gain changes aimed at compensating the apparent sensorimotor mismatch. A second shift applied dichoptically to only one eye introduces disconjugate landing errors between the two eyes. A monocular adaptive system would independently modify only the gain of the eye exposed to the second shift in order to reestablish binocular alignment. Our results support a binocular mechanism. A version-based saccadic adaptive process detects postsaccadic version errors and generates compensatory conjugate gain alterations. A vergence-based saccadic adaptive process detects postsaccadic disparity errors and generates corrective nonvisual disparity signals that are sent to the vergence system to regain binocularity. This results in striking dynamical similarities between visually driven combined saccade-vergence gaze transfers, where the disparity is given by the visual targets, and the double-step adaptive disconjugate responses, where an adaptive disparity signal is generated internally by the saccadic system.

Assessing divergence in children with intermittent exotropia

PURPOSE

To report fusional divergence in children with intermittent exotropia (XT).

METHODS

We retrospectively studied 32 children with intermittent XT (aged 4 to 13 years) and 38 visually normal non-strabismic children. Fusional divergence break points were measured using a prism bar, starting from a naturally fused state. Distribution of divergence break points was evaluated. Subnormal fusional divergence was defined as below the 5th percentile in visually normal children. In children with intermittent XT, correlations were calculated between divergence break point and control score, angle of deviation, and convergence break point.

RESULTS

The distribution of fusional divergence break points in intermittent XT was normal at near but bimodal at distance. Nine percent had subnormal divergence (<10 prism diopters [pd]) at near, and 48% (<6 pd) at distance. There was a moderate correlation between divergence and convergence break points at near (r = 0.44; P=.01) but only weak inverse correlations between divergence break point and control score (r = -0.29; P = .11) and divergence break point and angle of deviation (r = 0.08; P = . 68) at near.

CONCLUSIONS

Most children with intermittent XT have normal near fusional divergence but nearly half have reduced distance fusional divergence. We found the magnitude of near divergence break point correlates with the magnitude of near convergence break point. Measuring divergence may provide useful information about fragility of fusion in patients with intermittent XT. Future studies of fusional divergence are needed to establish whether common measurement methods represent true divergence amplitudes and whether such measures have prognostic significance.

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.

Neural circuit involved in idiopathic infantile nystagmus syndrome based on FMRI

PURPOSE

To identify the neural circuitry of idiopathic infantile nystagmus syndrome (INS), characterized by an early onset alternating series of slow and rapid eye movements that can manifest in different waveforms and genetic lines. The neural circuitry of INS is currently unknown.

METHODS

A novel functional magnetic resonance imaging (fMRI) method, referred to as the null zone fMRI technique, was used to identify the neural circuitry for INS. In the null zone fMRI technique, a gaze position with minimal nystagmus within the null zone was linked to the fMRI "off" condition and a gaze position with robust nystagmus outside of the null zone was linked to the fMRI "on" condition. Eye movements were monitored with an fMRI compatible eye tracker and observed in real time to ensure subject compliance in "on" and "off" states. Subjects with INS (n = 4) included three family members (a mother and two daughters) with presumed autosomal dominant INS, as well as age- and gender-matched normal controls (n = 3).

RESULTS

Three of four subjects with INS demonstrated significant increased activation of the declive of the cerebellum, whereas no normal subjects under identical conditions showed activation of the declive of the cerebellum. Both groups showed significant activation in the occipital lobe (Brodmann areas 17, 18, 19, and cuneus).

CONCLUSION

A novel fMRI method demonstrated that the declive of the cerebellum is actively involved in INS. These are the first results to identify the cerebellum, and specifically the declive, as a possible site involved in the ocular motor dysfunction known as INS.

Cerebellum and ocular motor control

An intact cerebellum is a prerequisite for optimal ocular motor performance. The cerebellum fine-tunes each of the subtypes of eye movements so they work together to bring and maintain images of objects of interest on the fovea. Here we review the major aspects of the contribution of the cerebellum to ocular motor control. The approach will be based on structural–functional correlation, combining the effects of lesions and the results from physiologic studies, with the emphasis on the cerebellar regions known to be most closely related to ocular motor function: (1) the flocculus/paraflocculus for high-frequency (brief) vestibular responses, sustained pursuit eye movements, and gaze holding, (2) the nodulus/ventral uvula for low-frequency (sustained) vestibular responses, and (3) the dorsal oculomotor vermis and its target in the posterior portion of the fastigial nucleus (the fastigial oculomotor region) for saccades and pursuit initiation.

Cerebellar ataxia: pathophysiology and rehabilitation

This series of articles for rehabilitation in practice aims to cover a knowledge element of the rehabilitation medicine curriculum. Nevertheless they are intended to be of interest to a multidisciplinary audience. The competency addressed in this article is 'The trainee consistently demonstrates a knowledge of management approaches for specific impairments including spasticity, ataxia.'

Sustained fixation induced changes in phoria and convergence peak velocity

PURPOSE

This study sought to investigate the influence of phoria adaptation on convergence peak velocity from responses located at different initial vergence positions.

METHODS

Symmetrical 4° convergence step responses and near dissociated phoria (measured at 40 cm from the subject's midline) were recorded from six subjects with normal binocular vision using an infrared limbus tracking system with a haploscope. Two different sustained fixations (1° and 16° convergent rotation along the subject's midline) were used to study whether phoria had an influence on the peak velocity of convergence responses located at two initial vergence positions (1° or 'far' steps and 12° or 'near' steps).

RESULTS

Phoria was significantly adapted after a sustained fixation task at near (16°) and far (1°) (p<0.002). A repeated measures ANOVA showed that convergence far steps were significantly faster than the near steps (p<0.03). When comparing convergence steps with the same initial vergence position, steps measured after near phoria adaptation were faster than responses after far adaptation (p<0.02). A regression analysis demonstrated that the change in phoria and the change in convergence peak velocity were significantly correlated for the far convergence steps (r = 0.97, p = 0.001). A weaker correlation was observed for the near convergence steps (r = 0.59, p = 0.20).

CONCLUSION

As a result of sustained fixation, phoria was adapted and the peak velocity of the near and far convergence steps was modified. This study has clinical considerations since prisms, which evoke phoria adaptation, can be prescribed to help alleviate visual discomfort. Future investigations should include a systematic study of how prisms may influence convergence and divergence eye movements for those prescribed with prisms within their spectacles.

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.

Vision therapy in adults with convergence insufficiency: clinical and functional magnetic resonance imaging measures

PURPOSE

This research quantified clinical measurements and functional neural changes associated with vision therapy in subjects with convergence insufficiency (CI).

METHODS

Convergence and divergence 4° step responses were compared between 13 control adult subjects with normal binocular vision and four CI adult subjects. All CI subjects participated in 18 h of vision therapy. Clinical parameters quantified throughout the therapy included: nearpoint of convergence, recovery point of convergence, positive fusional vergence at near, near dissociated phoria, and eye movements that were quantified using peak velocity. Neural correlates of the CI subjects were quantified with functional magnetic resonance imaging scans comparing random vs. predictable vergence movements using a block design before and after vision therapy. Images were quantified by measuring the spatial extent of activation and the average correlation within five regions of interests (ROI). The ROIs were the dorsolateral prefrontal cortex, a portion of the frontal lobe, part of the parietal lobe, the cerebellum, and the brain stem. All measurements were repeated 4 months to 1 year post-therapy in three of the CI subjects.

RESULTS

Convergence average peak velocities to step stimuli were significantly slower (p = 0.016) in CI subjects compared with controls; however, significant differences in average peak velocities were not observed for divergence step responses (p = 0.30). The investigation of CI subjects participating in vision therapy showed that the nearpoint of convergence, recovery point of convergence, and near dissociated phoria significantly decreased. Furthermore, the positive fusional vergence, average peak velocity from 4° convergence steps, and the amount of functional activity within the frontal areas, cerebellum, and brain stem significantly increased. Several clinical and cortical parameters were significantly correlated.

CONCLUSIONS

Convergence peak velocity was significantly slower in CI subjects compared with controls, which may result in asthenopic complaints reported by the CI subjects. Vision therapy was associated with and may have evoked clinical and cortical activity changes.

The anatomy and physiology of the ocular motor system

Accurate diagnosis of abnormal eye movements depends upon knowledge of the purpose, properties, and neural substrate of distinct functional classes of eye movement. Here, we summarize current concepts of the anatomy of eye movement control. Our approach is bottom-up, starting with the extraocular muscles and their innervation by the cranial nerves. Second, we summarize the neural circuits in the pons underlying horizontal gaze control, and the midbrain connections that coordinate vertical and torsional movements. Third, the role of the cerebellum in governing and optimizing eye movements is presented. Fourth, each area of cerebral cortex contributing to eye movements is discussed. Last, descending projections from cerebral cortex, including basal ganglionic circuits that govern different components of gaze, and the superior colliculus, are summarized. At each stage of this review, the anatomical scheme is used to predict the effects of lesions on the control of eye movements, providing clinical-anatomical correlation.

Recovery of Upper Limb Function After Cerebellar Stroke

Background and Purpose— Loss of movement coordination is the main postacute symptom after cerebellar infarction. Although the course of motor recovery has been described previously, detailed kinematic descriptions of acute stage ataxia are rare and no attempt has been made to link improvements in motor function to measures of neural recovery and lesion location. This study provides a comprehensive assessment of how lesion site and arm dysfunction are associated in the acute stage and outlines the course of upper limb motor recovery for the first 4 months after the infarction.

Methods— Sixteen adult patients with cerebellar stroke and 11 age-matched healthy controls participated. Kinematics of goal-directed and unconstrained finger-pointing movements were measured at the acute stage and in 2-week and 3-month follow-ups. MRI data were obtained for the acute and 3-month follow-up sessions. A voxel-based lesion map subtraction analysis was performed to examine the effect of ischemic lesion sites on kinematic performance.

Results— In the acute stage, nearly 70% of patients exhibited motor slowing with hand velocity and acceleration maxima below the range of the control group. MRI analysis revealed that in patients with impaired motor performance, lesions were more common in paravermal lobules IV/V and affected the deep cerebellar nuclei. Stroke affecting the superior cerebellar artery led to lower motor performance than infractions of the posterior cerebellar artery. By the 2-week-follow-up, hand kinematics had improved dramatically (gains in acceleration up to 86%). Improvements between the 2-week and the 3-month-follow-ups were less pronounced.

Conclusion— In the acute stage, arm movements were mainly characterized by abnormal slowness (bradykinesia) and not dyscoordination (ataxia). The motor signs were associated with lesions in paravermal regions of lobules IV/V and the deep cerebellar nuclei. Motor recovery was fast, with the majority of gains in upper limb function occurring in the first 2 weeks after the acute phase.