Evidence that convergence rather than accommodation controls intermittent distance exotropia

PURPOSE

This study considered whether vergence drives accommodation or accommodation drives vergence during the control of distance exotropia for near fixation. High accommodative convergence to accommodation (AC/A) ratios are often used to explain this control, but the role of convergence to drive accommodation (the CA/C relationship) is rarely considered. Atypical CA/C characteristics could equally, or better, explain common clinical findings.

METHODS

Nineteen distance exotropes, aged 4-11 years, were compared while controlling their deviation with 27 non-exotropic controls aged 5-9 years. Simultaneous vergence and accommodation responses were measured to a range of targets incorporating different combinations of blur, disparity and looming cues at four fixation distances between 2 m and 33 cm. Stimulus and response AC/A and CA/C ratios were calculated.

RESULTS

Accommodation responses for near targets (p = 0.017) and response gains (p = 0.026) were greater in the exotropes than in the controls. Despite higher clinical stimulus AC/A ratios, the distance exotropes showed lower laboratory response AC/A ratios (p = 0.02), but significantly higher CA/C ratios (p = 0.02). All the exotropes, whether the angle changed most with lenses ('controlled by accommodation') or on occlusion ('controlled by fusion'), used binocular disparity not blur as their main cue to target distance.

CONCLUSIONS

Increased vergence demand to control intermittent distance exotropia for near also drives significantly more accommodation. Minus lens therapy is more likely to act by correcting overaccommodation driven by controlling convergence, rather than by inducing blur-driven vergence. The use of convergence as a major drive to accommodation explains many clinical characteristics of distance exotropia, including apparently high near stimulus AC/A ratios.

High-resolution far-field integral-imaging camera by double snapshot

In multi-view three-dimensional imaging, to capture the elemental images of distant objects, the use of a field-like lens that projects the reference plane onto the microlens array is necessary. In this case, the spatial resolution of reconstructed images is equal to the spatial density of microlenses in the array. In this paper we report a simple method, based on the realization of double snapshots, to double the 2D pixel density of reconstructed scenes. Experiments are reported to support the proposed approach.

Ocular motor and sensory function in Parkinson's disease

PURPOSE

To evaluate the effect of dopaminergic medication and deep brain stimulation on ocular function in Parkinson's disease (PD) and to measure vision-related quality of life in subjects with PD.

DESIGN

Prospective, comparative case series.

PARTICIPANTS AND CONTROLS

Twenty-seven PD and 16 control subjects were recruited.

METHODS

Visual acuity, ocular motor function, convergence, and vision-related quality of life using the 25-item National Eye Institute Visual Function Questionnaire (VFQ-25) were measured. Visual sensory and motor measurements were obtained during the on and off states of PD dopaminergic treatment.

MAIN OUTCOME MEASURES

Convergence ability and vision-related quality of life.

RESULTS

The PD subjects had a mean age of 58.8 years; 30% were female. Their mean duration of PD was 10.9 ± 6.8 years. The control subjects had a mean age of 61.6 years; 56% were female. There was no difference in visual acuity, contrast sensitivity, or color vision of the PD subjects in their on state compared with controls. Convergence amplitudes measured with base-out prism were significantly poorer in PD subjects in their on state compared with controls (24.1 ± 8 Δ vs. 14.8 ± 10.3 Δ; P = 0.003). The mean composite VFQ-25 score was significantly worse in the PD subjects compared with the controls (87.1 ± 8.69 vs. 96.6 ± 3.05; P = 0.0001). Comparing the PD subjects in their on with their off states, there was no difference in distance exodeviation, near exodeviation, or ocular ductions. Mean convergence amplitudes and near point of convergence were better in the on state compared with the off state: 14.8 ± 10.3 Δ versus 10.7 ± 9.0 Δ (P = 0.0006) and 13.1 ± 9.1 cm versus 18.1 ± 12.2 cm (P = 0.002), respectively.

CONCLUSIONS

Convergence ability is significantly poorer in PD subjects in both the on and off states compared with controls, but improves significantly with systemic dopaminergic treatment. Ocular motor function in PD subjects fluctuates in response to treatment, which complicates ophthalmic management. Parkinson's disease subjects have a significant reduction in vision-related quality of life, especially with near activities, that is not associated with visual acuity.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Fusional convergence in childhood intermittent exotropia

PURPOSE

To evaluate fusional convergence and associations with control in children with intermittent exotropia (XT).

DESIGN

Retrospective case series.

METHODS

Sixty-four children (median age 7 years) with intermittent XT were identified with measures of angle of deviation, control (using a previously reported 0-to-5-point scale), and fusional convergence at a single examination. Total convergence was defined as the sum of the angle of deviation plus additional convergence in reserve. Mean values were compared with published normals. A fusion reserve ratio was calculated as "convergence reserve divided by angle of deviation." Relationships of control score with total amplitude, reserve, recovery, and fusion reserve ratio were assessed using Spearman rank correlations.

RESULTS

Convergence differed from normals in children with intermittent XT: total convergence was higher at distance (33 prism diopters [pd] vs 17 pd, P<.0001) and near (38 pd vs 18 pd, P<.0001) whereas convergence reserve was lower at distance (7 pd vs 17 pd; P<.0001). There was a strong correlation between fusion reserve ratio and control score at distance (R=-0.75, P<.0001) and near (R=-0.66, P<.0001).

CONCLUSIONS

Children with intermittent XT have subnormal convergence reserves at distance. The fusion reserve ratio correlates well with control and may be useful in grading the severity of intermittent XT.

The zone of comfort: Predicting visual discomfort with stereo displays

Recent increased usage of stereo displays has been accompanied by public concern about potential adverse effects associated with prolonged viewing of stereo imagery. There are numerous potential sources of adverse effects, but we focused on how vergence-accommodation conflicts in stereo displays affect visual discomfort and fatigue. In one experiment, we examined the effect of viewing distance on discomfort and fatigue. We found that conflicts of a given dioptric value were slightly less comfortable at far than at near distance. In a second experiment, we examined the effect of the sign of the vergence-accommodation conflict on discomfort and fatigue. We found that negative conflicts (stereo content behind the screen) are less comfortable at far distances and that positive conflicts (content in front of screen) are less comfortable at near distances. In a third experiment, we measured phoria and the zone of clear single binocular vision, which are clinical measurements commonly associated with correcting refractive error. Those measurements predicted susceptibility to discomfort in the first two experiments. We discuss the relevance of these findings for a wide variety of situations including the viewing of mobile devices, desktop displays, television, and cinema.

Short-term adaptations of the dynamic disparity vergence and phoria systems

The ability to adapt is critical to survival and varies between individuals. Adaptation of one motor system may be related to the ability to adapt another. This study sought to determine whether phoria adaptation was correlated with the ability to modify the dynamics of disparity vergence. Eye movements from ten subjects were recorded during dynamic disparity vergence modification and phoria adaptation experiments. Two different convergent stimuli were presented during the dynamic vergence modification experiment: a test stimulus (4° step) and a conditioning stimulus (4° double step). Dynamic disparity vergence responses were quantified by measuring the peak velocity (°/s). Phoria adaptation experiments measured the changes in phoria over a 5-min period of sustained fixation. The maximum velocity of phoria adaptation was determined from an exponential fit of the phoria data points. Phoria and dynamic disparity vergence peak velocity were both significantly modified (P < 0.001). The maximum velocity of phoria adaptation was significantly correlated with the changes in convergence peak velocity (r > 0.89; P < 0.001). There was a strong correlation between the ability to adaptively adjust two different oculomotor parameters: a tonic and dynamic component. Future studies should investigate additional interactions between these parameters, and the ability to adaptively change other oculomotor systems such as the saccadic or smooth pursuit system. Understanding the ability to modify phoria, dynamic disparity vergence, and other oculomotor parameters can yield insights into the plasticity of short-term adaptation mechanisms.

Surgical outcomes in convergence insufficiency-type exotropia

PURPOSE

To determine the efficacy of different types of strabismus surgeries in patients with convergence insufficiency (CI)-type exotropia, according to their response to diagnostic monocular occlusion.

DESIGN

Retrospective cohort study.

PARTICIPANTS

Sixty-five patients with CI-type exotropia with near-distance differences of ≥10 prism diopters (PD) who underwent strabismus surgery.

METHODS

Patients were divided into 3 groups according to their response to monocular occlusion: (1) true-CI group: near-distance differences ≥10 PD before and after occlusion; (2) masked-CI group: near-distance differences <10 PD before occlusion and ≥10 PD after occlusion; and (3) pseudo-CI group: near-distance differences ≥10 PD before occlusion and <10 PD after occlusion. Either bilateral lateral rectus recession based on near measurements with 1 mm augmentation (BLR) or unilateral medial rectus resection based on the near deviation with lateral rectus recession based on the distant deviation (RR) was performed.

MAIN OUTCOME MEASURES

Cumulative probabilities of success, near-distance differences of exodeviation, rate of recurrence per person-year, and risk factors of recurrence.

RESULTS

There were 24 children in the true-CI group, 19 children in the masked-CI group, and 22 children in the pseudo-CI group. The cumulative probabilities of success at 2 years after BLR versus RR were 61% versus 100% in the true-CI group, 58% versus 100% in the masked-CI group, and 77% versus 71% in the pseudo-CI group. The RR procedure was significantly more successful than the BLR procedure in the true-CI and masked-CI groups.

CONCLUSIONS

Successful outcome in CI-type exotropia was closely related to the patients' response to monocular occlusion. In patients with CI-type exotropia maintained after monocular occlusion, unilateral resection-recession based on near-distance measurements is recommended.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

Intermittent exotropia: facts, opinions, and unknowns

INTRODUCTION

Intermittent exotropia (IXT) can be a controversial topic, often eliciting lively discussion. This lecture will discuss its definition, incidence, age of onset, presentation, natural variation, criteria for deterioration, goals of treatment, effectiveness of surgical treatment, types of surgical treatment, and unwanted effects of surgical treatment.

METHOD

Results from the scientific literature, opinions of respected colleagues, the opinion of the author, and the results of live polling of the audience during the John Pratt-Johnson lecture are presented.

RESULTS

IXT is defined as an exotropia that is present intermittently predominantly for distance. Its incidence is about 1% and it usually has an onset before age 5. Patients often present because of concern regarding the appearance of the eye misalignment. There is natural variation in the control of IXT, the angle of IXT, and the amount of stereopsis. Criteria that denote deterioration are increasing frequency of IXT, progressively and consistently increasing angle of IXT, loss of binocular vision, and increasing concern regarding the patient's appearance and its effect on social interaction. Goals of treatment are to retain equal or nearly equal vision, to obtain acceptable cosmesis, and to retain binocular vision. The long-term success of surgical treatment is not well proven. Persistent postoperative overcorrection is an unwanted effect of surgical treatment.

CONCLUSION

The inherent biologic variation that occurs when measuring the components of IXT makes it difficult to be dogmatic about IXT, particularly when trying to decide when deterioration is occurring.

Central crosstalk for somatic tinnitus: abnormal vergence eye movements

Background

Frequent oulomotricity problems with orthoptic testing were reported in patients with tinnitus. This study examines with objective recordings vergence eye movements in patients with somatic tinnitus patients with ability to modify their subjective tinnitus percept by various movements, such as jaw, neck, eye movements or skin pressure.

Methods

Vergence eye movements were recorded with the Eyelink II video system in 15 (23–63 years) control adults and 19 (36–62 years) subjects with somatic tinnitus.

Findings

1) Accuracy of divergence but not of convergence was lower in subjects with somatic tinnitus than in control subjects. 2) Vergence duration was longer and peak velocity was lower in subjects with somatic tinnitus than in control subjects. 3) The number of embedded saccades and the amplitude of saccades coinciding with the peak velocity of vergence were higher for tinnitus subjects. Yet, saccades did not increase peak velocity of vergence for tinnitus subjects, but they did so for controls. 4) In contrast, there was no significant difference of vergence latency between these two groups.

Interpretation

The results suggest dysfunction of vergence areas involving cortical-brainstem-cerebellar circuits. We hypothesize that central auditory dysfunction related to tinnitus percept could trigger mild cerebellar-brainstem dysfunction or that tinnitus and vergence dysfunction could both be manifestations of mild cortical-brainstem-cerebellar syndrome reflecting abnormal cross-modality interactions between vergence eye movements and auditory signals.

[Comparison of effects of periocular region dry and wet warming on visual acuity and near reflex]

PURPOSE

To investigate the influence of warming the periocular region for 10 minutes after visual display terminal (VDT) work on the near reflex, and the differences in influence between lid closure, dry and wet warming, and the amounts of water vapor.

SUBJECTS AND METHODS

Subjects in the first trial were 10 healthy volunteers (10 eyes). Following VDT, subjects wore either a warming eye steamer (water vapor: 200 mg/10 min) or an eye mask with their eyelids closed for 10 minutes. Subjects in the second trial were 5 healthy volunteers (5 eyes). Following VDT, subjects applied either a dry-warming sheet (water vapor: 0 mg/10 min) or a wet-warming sheet (water vapor: 20, 35, 160 and 200 mg/10 min) to there eyelids and the periocular region for 10 minutes. In both experiments, subjective amplitude of accommodation, pupillary reaction, convergence reaction, near vision, and breakup time were measured before and immediately after VDT work and after 2 experimental settings.

RESULTS

The warming eye steamer significantly improved amplitude of accommodation (p<0.05), and near vision (p<0.05). Warming the periocular region with water vapor improved subjective amplitude of accommodation, and near vision, the effect of which depended on the amount of water vapor (p<0.05).

CONCLUSIONS

Warming the periocular region with water vapor improved subjective amplitude of accommodation, near vision and the effect depended on the amount of water vapor.

Saccadic lens instability increases with accommodative stimulus in presbyopes

An SRI dual Purkinje image (dPi) eye tracker was used to measure lens wobble following saccades with increasing accommodative effort as an indirect measure of ciliary muscle function in presbyopes. Ten presbyopic subjects executed 32 four-degree saccades at 1-s intervals between targets arranged in a cross on illuminated cards at each of 9 viewing distances ranging from 0.5- to 8-D accommodative demands. Post-saccadic lens wobble artifacts were extracted by subtraction of P1 (H(1)/V(1)) position signals from P4 signals (Theta(H)/Theta(V)), both of which were sampled by the eye tracker at 100 Hz. A ray tracing eye model was also employed to model the fourth Purkinje image shifts for a range of lens translations and tilts. Combining all saccades from all subjects showed a significant positive relationship between lens wobble artifact amplitude and accommodative demand. Eye model simulations indicated that artifacts of the amplitude measured could arise from either lens tilts (in the range of 2-4 degrees) or lens translations (in the range of 0.1 to 0.2 mm). Saccadic lens wobble artifacts increase with accommodative effort in presbyopes, indicating preserved ciliary muscle function and greater relaxation of zonular tension with accommodative effort. Variation across subjects may reflect differences in accommodative effort, ciliary muscle function for a given effort, and/or in intraocular anatomy.

Latitude and longitude vertical disparities

The literature on vertical disparity is complicated by the fact that several different definitions of the term "vertical disparity" are in common use, often without a clear statement about which is intended or a widespread appreciation of the properties of the different definitions. Here, we examine two definitions of retinal vertical disparity: elevation-latitude and elevation-longitude disparities. Near the fixation point, these definitions become equivalent, but in general, they have quite different dependences on object distance and binocular eye posture, which have not previously been spelt out. We present analytical approximations for each type of vertical disparity, valid for more general conditions than previous derivations in the literature: we do not restrict ourselves to objects near the fixation point or near the plane of regard, and we allow for non-zero torsion, cyclovergence, and vertical misalignments of the eyes. We use these expressions to derive estimates of the latitude and longitude vertical disparities expected at each point in the visual field, averaged over all natural viewing. Finally, we present analytical expressions showing how binocular eye position-gaze direction, convergence, torsion, cyclovergence, and vertical misalignment-can be derived from the vertical disparity field and its derivatives at the fovea.

No oculomotor plant, no final common path

The assumption that there is an oculomotor plant, a fixed relationship between motoneuron firing rate and eye position, is disproved by brainstem recording studies showing that this relationship depends on which supernuclear subsystem determines firing rate. But it remains possible that there is a final common path (FCP), a fixed relationship between firing rate and muscle force. But then, brainstem recording studies predict that lateral rectus (LR) forces (and probably medial rectus (MR) forces, as well) will be higher in converged than in unconverged gaze for a given eye position. We recently measured these forces and found that they are slightly lower in convergence, disproving the FCP hypothesis. Thus, even the relationship between motoneuron firing rate and muscle force is under supernuclear control. What peripheral oculomotor articulations could vary the relationship of firing rate to muscle force?: (1) Actively movable EOM pulleys could alter oculorotary muscle force for a given oculorotory innervation by altering muscle lengths. (2) 'Outer' motoneurons may function as gamma efferents in conjunction with palisade endings and non-twitch global EOM fibers. (3) Complex nonlinear interactions likely arise among both parallel and serially connected muscle fibers.

Vergence Dynamics and Variability of Fixation Disparity in Children and Adults

With dichoptic nonius lines presented repetitively on a computer monitor for only short moments in time (e.g., 80 ms), temporal aspects of vergence can be assessed: the variability of fixation disparity with a stationary central fusion stimulus and the velocity of convergence or divergence changes. Results and test-retest correlations of these psychophysical vergence measures are shown for children and adults. In most children older than about 7 years, the estimation of vergence velocity turned out as well as in adults in terms of amount and reliability.

Eye-Movement Responses to Disparity Vergence Stimuli with Artificial Monocular Scotomas

The effects of artificial monocular scotomas on eye-movement responses to horizontal disparity vergence stimuli were studied in six subjects with normal binocular vision. Subjects viewed stereoscopic 1.5 degrees horizontal step disparity vergence stimuli through liquid crystal shutter glasses. The central portion of the stimulus presented to the right eye was removed to simulate monocular artificial scotomas of variable diameters (2 degrees to 10 degrees ). Eye movements were recorded with a binocular head-mounted eye tracker. Responses included pure vergence, vergence followed by saccades, and pure saccadic eye movements. The rate of responses with saccadic eye movements increased with the diameter of the artificial scotoma (p < 0.0001); there was an increase in the rate of responses starting with saccades (p < 0.0001), as well as an increase in the rate of saccades after initial vergence responses (p < 0.01). The probability of saccades after initial vergence responses was affected by the open-loop gain of the vergence response (p < 0.001). The open-loop gain decreased with increased diameters of the artificial scotomas (p < 0.0001). As the diameter of the artificial scotomas increased, the amplitude of the initial vergence eye-movement responses decreased, and the prevalence of saccadic eye movements and asymmetric vergence increased. The effects of the diameter of artificial monocular scotomas on eye-movement responses in subjects with normal binocular vision are consistent with the effects of diameter of suppression scotomas on eye-movement responses to disparity vergence stimuli in patients with infantile esotropia.

Acute superior oblique palsy in the monkey: effects of viewing conditions on ocular alignment and modelling of the ocular motor plant

We investigated the immediate and long-term changes in static eye alignment with acute superior oblique palsy (SOP) in the monkey. When the paretic eye was patched immediately after the lesion for 6-9 days, vertical alignment slowly improved. When the patch was removed and binocular viewing was allowed, alignment slowly worsened. In contrast when a monkey was not patched immediately after the lesion vertical alignment did not improve. We also show that a model of the eye plant can reproduce the observed acute deficit induced by SOP, but only by abandoning Robinson's symmetric simplification of the reciprocal innervation relationship within pairs of agonist-antagonist muscles. The model also demonstrated that physiologic variability in orbital geometry can have a large impact on SOP deficits.

Measuring fixation disparity with infrared eye-trackers

Fixation disparity, that is, misalignment of the gaze direction of both eyes, may be observed in static conditions (through standard optometric evaluation) and dynamic conditions (through eye movement recording). A computation method is presented to determine vergence angles and fixation disparity from gaze positions as commonly recorded by infrared eye-trackers when a participant looks at a personal computer (PC) screen. Eye-tracking devices provide gaze position in coordinates relative to the bidimensional screen surface. From these data, vergence angles can be calculated by trigonometric triangulations; fixation disparity is then calculated from the vergence angles. The application of the procedure to the recordings of one participant is described. To control for the effective alignment of the two eyes on the target during binocular calibration, a procedure based on the dichoptic presentation of nonius lines was used. The recordings confirm that computation and the dichoptic calibration procedures ensure reliable measures of vergence and fixation disparity. The usefulness of this approach with infrared recording of eye position is discussed.

Cyclopean geometry of binocular vision

The geometry of binocular projection is analyzed in relation to the primate visual system. An oculomotor parameterization that includes the classical vergence and version angles is defined. It is shown that the epipolar geometry of the system is constrained by binocular coordination of the eyes. A local model of the scene is adopted in which depth is measured relative to a plane containing the fixation point. These constructions lead to an explicit parameterization of the binocular disparity field involving the gaze angles as well as the scene structure. The representation of visual direction and depth is discussed with reference to the relevant psychophysical and neurophysiological literature.

The use of cues to convergence and accommodation in naïve, uninstructed participants

A remote haploscopic video refractor was used to assess vergence and accommodation responses in a group of 32 emmetropic, orthophoric, symptom free, young adults naïve to vision experiments in a minimally instructed setting. Picture targets were presented at four positions between 2 m and 33 cm. Blur, disparity and looming cues were presented in combination or separately to asses their contributions to the total near response in a within-subjects design. Response gain for both vergence and accommodation reduced markedly whenever disparity was excluded, with much smaller effects when blur and proximity were excluded. Despite the clinical homogeneity of the participant group there were also some individual differences.