Motion Information via the Nonfixating Eye Can Drive Optokinetic Nystagmus in Strabismus

A low-cost robotic oculomotor simulator for assessing eye tracking accuracy in health and disease

Eye tracking accuracy is affected in individuals with vision and oculomotor deficits, impeding our ability to answer important scientific and clinical questions about these disorders. It is difficult to disambiguate decreases in eye movement accuracy and changes in accuracy of the eye tracking itself. We propose the EyeRobot-a low-cost, robotic oculomotor simulator capable of emulating healthy and compromised eye movements to provide ground truth assessment of eye tracker performance, and how different aspects of oculomotor deficits might affect tracking accuracy and performance. The device can operate with eccentric optical axes or large deviations between the eyes, as well as simulate oculomotor pathologies, such as large fixational instabilities. We find that our design can provide accurate eye movements for both central and eccentric viewing conditions, which can be tracked by using a head-mounted eye tracker, Pupil Core. As proof of concept, we examine the effects of eccentric fixation on calibration accuracy and find that Pupil Core's existing eye tracking algorithm is robust to large fixation offsets. In addition, we demonstrate that the EyeRobot can simulate realistic eye movements like saccades and smooth pursuit that can be tracked using video-based eye tracking. These tests suggest that the EyeRobot, an easy to build and flexible tool, can aid with eye tracking validation and future algorithm development in healthy and compromised vision.

A competition framework for fixation-preference in strabismus

Strabismic subjects often develop the ability to fixate on a target with either eye. Previous studies have shown that fixation-preference behavior varies systematically depending on spatial location of the target. We hypothesized that, when an eccentric target is presented, oculomotor fixation-preference in strabismus may be accounted for in a competitive decision framework wherein the brain must choose between two possible retinal errors to prepare a conjugate saccade that results in one of the eyes acquiring the eccentric target. We tested this framework by recording from visuo-motor neurons in the superior colliculus (SC) of two strabismic rhesus macaque monkeys as they performed a delayed saccade task under binocular viewing conditions. In one experiment, visual targets were presented at one of two locations corresponding to the neuronal receptive field location with respect to either the viewing or the deviated eye. Robust visual sensory responses were observed when targets were presented at either location indicating the presence of competing sensory signals for eye-choice. In a second experiment, a single visual target was placed at the neuronal receptive field location where the animal switched fixation on some trials and did not on other trials. At such target locations where either eye could acquire the target, both visual and build-up activity was greater in trials when the saccade encoded by the neuron "won." These findings provide evidence for the influence of visual suppression within SC sensory activity and support the possible utilization of a competition framework, one that has been previously described for when a binocularly aligned animal chooses from among multiple targets, to drive fixation-preference behavior in strabismus.

A Dichoptic Optokinetic Nystagmus Paradigm for Interocular Suppression Quantification in Intermittent Exotropia

Purposes: To investigate the effectiveness of a dichoptic optokinetic nystagmus (dOKN) test to objectively quantify interocular suppression in intermittent exotropia (IXT) patients during the states of orthotropia and exodeviation.

Methods: The OKN motion in subjects (15 controls and 59 IXT subjects) who viewed dichoptic oppositely moving gratings with different contrast ratios was monitored and recorded by an eye tracker. Interocular suppression in control subjects was induced using neutral density (ND) filters. The OKN direction ratios were fitted to examine the changes of interocular suppression in subjects under different viewing states. Two established interocular suppression tests (phase and motion) were conducted for a comparative study.

Results: The dOKN test, which requires a minimal response from subjects, could accurately quantify the interocular suppression in both IXT and control subjects, which is in line with the established interocular suppression tests. Overall, although comparative, the strength of interocular suppression detected by the dOKN test (0.171 ± 0.088) was stronger than those of the phase (0.293 ± 0.081) and the motion tests (0.212 ± 0.068) in the control subjects with 1.5 ND filters. In IXT patients, when their eyes kept aligned, the dOKN test (0.58 ± 0.09) measured deeper visual suppression compared with the phase (0.73 ± 0.17) or the motion test (0.65 ± 0.14). Interestingly, strong interocular suppression (dOKN: 0.15 ± 0.12) was observed in IXT subjects during the periods of exodeviation, irrespective of their binocular visual function as measured by synoptophore.

Conclusion: The dOKN test provides efficient and objective quantification of interocular suppression in IXT, and demonstrates how it fluctuates under different eye positions.

Fixation Preference for Visual and Auditory Targets in Monkeys with Strabismus

Purpose

During binocular viewing, many strabismic subjects choose the eye of fixation depending on the retinotopic location of a visual target. Here, we compare eye choice behavior when orienting to visual and non-visual (auditory) targets.

Methods

Eye movements were measured in two head-fixed exotropic strabismic monkeys in a saccadic task involving either a visual or an auditory stimulus (no visual target information or feedback) during monocular or binocular viewing. The stimulus was one of 21 visual or auditory targets arranged 10° apart in a 7 × 3 array at a distance of 57 cm in an otherwise dark room. Fixation preference was calculated by recording the incidence of using a specific eye to acquire the target at any location.

Results

Spatial patterns of fixation preference were observed in both monkeys for both visual and auditory stimuli; targets to the far right were acquired by the right eye, and targets to the far left were acquired by the left eye. For visual targets, the border for a change in fixation preference occurred in between the visual axes of the fixating and deviated eyes (variable in the two animals). In contrast, the border for fixation change remained near the cranio-center during the auditory task. During monocular viewing, fixation switching was observed only at the extremities during visual tasks; during the auditory task, fixation preference was similar to that observed during binocular viewing.

Conclusions

Fixation preference persists for invisible auditory targets. Our data suggest that visual suppression could modify underlying eye choice behavior that functions independently from vision.

Quantifying Nasotemporal Asymmetry of Interocular Suppression in Alternating Strabismus After Correction

Purpose

This study identifies and characterizes the nasotemporal hemifield difference of interocular suppression in subjects who have been successfully treated for strabismus.

Methods

Interocular suppression in the nasal and temporal hemifields were measured using two methods, namely, binocular phase combination and dichoptic motion coherence, both entailed suprathreshold stimuli. We tested 29 clinical subjects, who had strabismus (19 with exotropia and 10 with esotropia) but regained good ocular alignment (within 10 prism diopters) after surgical or refractive correction, and 10 control subjects.

Results

Both the hemifield binocular phase combination and the hemifield dichoptic motion coherence tests revealed similar nasotemporal asymmetry of interocular suppression. Subjects with previous exotropia showed deeper suppression in the nasal hemifield, and those with previous esotropia showed deeper suppression in the temporal hemifield. This finding was consistent with the hemifield suppression theory. Furthermore, there was deeper suppression but less imbalance of nasotemporal asymmetry in the hemifield dichoptic motion coherence test. Finally, clinical stereopsis and the nasotemporal asymmetry of suppression (P < 0.05 in both tests) were negatively correlated in subjects with previous exotropia and measurable stereopsis.

Conclusions

Hemifield asymmetry of interocular suppression in corrected strabismus can be measured by using static and dynamic suprathreshold stimuli. Thus, the evaluation of binocular vision in strabismus should focus on both the magnitude and the pattern of interocular suppression.

Associations between Macular Sensitivity and Fixation in Pseudophakic Children after Congenital Cataract Surgery

Purpose: To investigate the associations between macular sensitivity (MS) and fixation in pseudophakic children after congenital cataract surgery.Materials and Methods: In total 55 pseudophakic eyes and 28 healthy phakic eyes were included in this cross-sectional study. MS and fixation stability in term of 95% bivariate contour ellipse area (BCEA) were assessed with a Macular Integrity Assessment microperimeter. Central foveal thickness (CFT) and subfoveal choroidal thickness (SFCT) was measured with optical coherence tomography. MS inside and outside the 95% BCEA was compared. Influencing factors for the difference in MS between the two regions (ΔMS) were assessed.Results: The overall MS was significantly lower in pseudophakic eyes than in the controls (P < .001). In the pseudophakic group, fixation stability was stable/relatively unstable/unstable in 69.1%/16.4%/14.5% of eyes, and their MS was 27.60 ± 2.56, 25.02 ± 3.82, and 20.50 ± 7.15 dB, respectively. The unstable subgroup had significantly worse MS than the stable subgroup (P < .001). Among pseudophakic eyes, the MS inside the 95% BCEA (fixation preferred region) was significantly greater than that outside this region (P = .048), and it was more correlated with BCVA than that of the entire macula. The ΔMS became greater in those pseudophakic eyes with worse fixation stability (P < .001) and longer axial length (P = .002). Backward stepwise multiple linear regression also revealed 95%BCEA and axial length had significant influences on ΔMS (R² = 0.289, P < .001).Conclusion: MS was lower in pseudophakic eyes with poor fixation. Macular sensitivity inside and outside the fixation preferred region was different in pseudophakic children after congenital cataract surgery, and this difference increased with longer axial length and poorer fixation.

A Novel Dichoptic Optokinetic Nystagmus Paradigm to Quantify Interocular Suppression in Monocular Amblyopia

Purpose

To develop a novel dichoptic optokinetic nystagmus (OKN) paradigm and investigate its effectiveness in objectively quantifying the interocular suppression in subjects with monocular amblyopia.

Methods

Centripetal moving gratings with different contrast ratios and constant velocity were dichoptically presented to eight monocular anisometropic amblyopes and eight normal subjects. We analyzed the OKN records with an eye tracker (EyeLink; SR-Research, Ontario, Canada) to obtain the relationship between ocular-dominance of OKN and the interocular contrast ratio by fitting power curves, and examined the correlation between the effective contrast ratio for a balanced OKN and the visual acuity of the amblyopic eye in amblyopes.

Results

In normal subjects, the OKN pursuit times were roughly balanced for opposite directions when stimulated with centripetal gratings of same contrast; however, in amblyopes, the OKN pursuit times of the dominant eye exceeded that of the amblyopic eye. Increasing the contrast of one eye's grating led to an increase in its OKN dominance. The OKN directional ratio (y) could be well fitted by a power function of the interocular contrast ratio (x): y = axb. Moreover, in amblyopes, the effective contrast ratio (xb) for a balanced OKN correlated significantly positively with the visual acuity of the amblyopic eye (Spearman's correlation coefficient, 0.9698).

Conclusions

The OKN induced by dichoptic gratings moving centripetally could be used as a reliable measure to objectively quantify the interocular suppression. This paradigm, avoiding the need for subjective report from patients, offers a promising alternative index for research on the mechanisms of amblyopia and in clinical practice.

Fixational Saccades and Their Relation to Fixation Instability in Strabismic Monkeys

Purpose

To evaluate the contribution of fixational saccades toward fixation instability in strabismic monkeys.

Methods

Binocular eye movements were measured as six experimental monkeys (five strabismic monkeys and one monkey with downbeat nystagmus) and one normal monkey fixated targets of two shapes (Optotype, Disk) and two sizes (0.5°, 2°) during monocular and binocular viewing. Fixational saccades were detected using an unsupervised clustering algorithm.

Results

When compared with the normal monkey, amplitude and frequency of fixational saccades in both the viewing and nonviewing eye were greater in 3 of 5 strabismic monkeys (1-way ANOVA on ranks P < 0.001; median amplitude in the normal monkey viewing eye: 0.33°; experimental animals: median amplitude range 0.20-0.82°; median frequency in the normal monkey: 1.35/s; experimental animals: median frequency range 1.3-3.7/s). Increase in frequency of fixational saccades was largely due to quick phases of ongoing nystagmus. Fixational saccade amplitude was increased significantly (3-way ANOVA; P < 0.001) but by small magnitude depending on target shape and size (mean difference between disk and optotype targets = 0.02°; mean difference between 2° and 0.5° targets = 0.1°). Relationship between saccade amplitude and the Bivariate Contour Ellipse Area (BCEA) was nonlinear, showing saturation of saccade amplitude. Fixation instability in depth was significantly greater in strabismic monkeys (vergence BCEA: 0.63 deg2-2.15 deg2) compared with the normal animal (vergence BCEA: 0.15 deg2; P < 0.001).

Conclusions

Increased fixational instability in strabismic monkeys is only partially due to increased amplitude and more frequent fixational saccades. Target parameter effects on fixational saccades are similar to previous findings of target effects on BCEA.

Comparison of Naso-temporal Asymmetry During Monocular Smooth Pursuit, Optokinetic Nystagmus, and Ocular Following Response in Strabismic Monkeys

PURPOSE

Under monocular viewing conditions, humans and monkeys with infantile strabismus exhibit asymmetric naso-temporal (N-T) responses to motion stimuli. The goal of this study was to compare and contrast these N-T asymmetries during 3 visually mediated eye tracking tasks-optokinetic nystagmus (OKN), smooth pursuit (SP) response, and ocular following responses (OFR).

METHODS

Two adult strabismic monkeys were tested under monocular viewing conditions during OKN, SP, or OFR stimulation. OKN stimulus was unidirectional motion of a 30°x30° random dot pattern at 20°, 40°, or 80°/s for 1 minute. OFR stimulus was brief (200 ms) unidirectional motion of a 38°x28°whitenoise at 20°, 40°, or 80°/s. SP stimulus consisted of foveal step-ramp target motion at 10°, 20°, or 40°/s.

RESULTS

Mean nasalward steady state gain (0.87±0.16) was larger than temporalward gain (0.67±0.19) during monocular OKN (P<0.001). In monocular OFR, the asymmetry is manifested as a difference in OFR velocity gain (nasalward: 0.33±0.19, temporalward: 0.22±0.12; P=0.007). During monocular SP, mean nasal gain (0.97±0.2) was larger than temporal gain (0.66±0.14; P<0.001) and the mean nasalward acceleration during pursuit initiation (156±61°/s²) was larger than temporalward acceleration (118±77°/s²; P=0.04). Comparison of N-T asymmetry ratio across the 3 conditions using ANOVA showed no significant difference.

CONCLUSIONS

N-T asymmetries are identified in all 3 visual tracking paradigms in both monkeys with either eye viewing. Our data are consistent with the current hypothesis for the mechanism for N-T asymmetry that invokes an imbalance in cortical drive to brainstem circuits.

Monocular and binocular smooth pursuit in central field loss

Macular degeneration results in heterogeneous central field loss (CFL) and often has asymmetrical effects in the two eyes. As such, it is not clear to what degree the movements of the two eyes are coordinated. To address this issue, we examined smooth pursuit quantitatively in CFL participants during binocular viewing and compared it to the monocular viewing case. We also examined coordination of the two eyes during smooth pursuit and how this coordination was affected by interocular ratios of acuity and contrast, as well as CFL-specific interocular differences, such as scotoma sizes and degree of binocular overlap. We hypothesized that the coordination of eye movements would depend on the binocularity of the two eyes. To test our hypotheses, we used a modified step-ramp paradigm, and measured pursuit in both eyes while viewing was binocular, or monocular with the dominant or non-dominant eye. Data for CFL participants and age-matched controls were examined at the group, within-group, and individual levels. We found that CFL participants had a broader range of smooth pursuit gains and a significantly lower correlation between the two eyes, as compared to controls. Across both CFL and control groups, smooth pursuit gain and correlation between the eyes are best predicted by the ratio of contrast sensitivity between the eyes. For the subgroup of participants with measurable stereopsis, both smooth pursuit gain and correlation are best predicted by stereoacuity. Therefore, our results suggest that coordination between the eyes during smooth pursuit depends on binocular cooperation between the eyes.

Strabismus and the Oculomotor System: Insights from Macaque Models

Disrupting binocular vision in infancy leads to strabismus and oftentimes to a variety of associated visual sensory deficits and oculomotor abnormalities. Investigation of this disorder has been aided by the development of various animal models, each of which has advantages and disadvantages. In comparison to studies of binocular visual responses in cortical structures, investigations of neural oculomotor structures that mediate the misalignment and abnormalities of eye movements have been more recent, and these studies have shown that different brain areas are intimately involved in driving several aspects of the strabismic condition, including horizontal misalignment, dissociated deviations, A and V patterns of strabismus, disconjugate eye movements, nystagmus, and fixation switch. The responses of cells in visual and oculomotor areas that potentially drive the sensory deficits and also eye alignment and eye movement abnormalities follow a general theme of disrupted calibration, lower sensitivity, and poorer specificity compared with the normally developed visual oculomotor system.