Nonstrabismic binocular dysfunctions and cervical complaints: The possibility of a cross-dysfunction

The aim of this study is to establish a relationship between non-strabismic binocular dysfunction and neck pain. One hundred twelve participants underwent binocular vision assessment by evaluating horizontal heterophoria, horizontal and vertical fusional vergence ranges and vergence facility. The subjects were classified into two groups: binocular anomalies and normal binocular function. Neck complaints were measured with the Neck Disability Index, visual analogue scale, cervical range of motion, deep-flexor muscle activation score (AS) and performance index (PI). Our results showed that participants with low AS had significantly altered values of lateral phoria (near) (mean = -6.99 SD ± 6.96 PD) and PFV (near) blur (mean = 9.49 SD ± 5.45 PD) against those who presented normal AS (lateral phoria (near) mean = -3.64 SD ± 6.37 PD; PFV (near) blur mean = 12.84 SD ± 6.20 PD). In addition, participants with NFV (near) recovery outside the norm had a significantly lower right side-bending (mean = 35.63 SD ± 8.35 PD) than those within the standard (mean = 39.64 SD ± 9 PD). The subjects with binocular vision impairment showed a diminished response to the deep cervical musculature, with low AS and PI, as well as a tendency to suffer from cervicalgia of more than three months’ evolution and a lower range of motion.

The Normal Accommodative Convergence/Accommodation (AC/A) Ratio

BACKGROUND AND PURPOSE

To measure the Accommodative Convergence (PD)/Accommodation ratio (D) (AC/A) in a cohort of visually normal participants using common clinical methods.

PATIENTS AND METHODS

AC/A ratios of 50 visually normal subjects were measured using the distance gradient (DG), near gradient (NG), gradient using synoptophore, (SG) and heterophoria (H) methods in line with current clinical practice.

RESULTS

Median AC/A ratios for NG, DG, SG, and H were 2.0 (IQR 2.0), 1.0 (IQR 0.6), 1.0 (IQR 0.6), and 5.0 (IQR 1.7), respectively. There was a statistically significant difference in ratios calculated between all methods in the same subjects (p < 0.05). There were differences in DG vs NG, DG vs H, SG vs H, and NG vs H (p < 0.05); only DG vs SG did not differ significantly (p > 0.05). Lens power toleration was found to affect AC/A ratio in DG (p < 0.05) and latent deviation was significantly associated with (p < 0.05) AC/A ratio in NG.

CONCLUSION

Calculated AC/A ratios in this cohort were lower than historically cited normal (3-5:1) in all gradient methods. There were differences in AC/A values in the same subjects calculated with different gradient methods, indicating that these methods are not interchangeable and a universal normal range should not be applied for all methods.

The gaze stability of 4- to 10-week-old human infants

The relationship between gaze stability, retinal image quality, and visual perception is complex. Gaze instability related to pathology in adults can cause a reduction in visual acuity (e.g., Chung, LaFrance, & Bedell, 2011). Conversely, poor retinal image quality and spatial vision may be a contributing factor to gaze instability (e.g., Ukwade & Bedell, 1993). Though much is known about the immaturities in spatial vision of human infants, little is currently understood about their gaze stability. To characterize the gaze stability of young infants, adult participants and 4- to 10-week-old infants were shown a dynamic random-noise stimulus for 30-s intervals while their eye positions were recorded binocularly. After removing adultlike saccades, we used 5-s epochs of stable intersaccade gaze to estimate bivariate contour ellipse area and standard deviations of vergence. The geometric means (with standard deviations) for infants' bivariate contour ellipse area were left eye = -0.697 ± 0.534 log(°2), right eye = -0.471 ± 0.367 log(°2). For binocular vergence stability, the infant geometric means (with standard deviations) were horizontal = -1.057 ± 0.743 log(°), vertical = -1.257 ± 0.573 log(°). These values were all not significantly different from those of the adult comparison sample, suggesting that gaze instability is not a significant limiting factor in retinal image quality and spatial vision during early postnatal development.

Individual objective versus subjective fixation disparity as a function of forced vergence

Inaccuracy in the vergence eye position ("fixation disparity") can occur despite a fusion stimulus. When measured with eye trackers, this inaccuracy is referred to as "objective fixation disparity". It is a matter of debate whether objective fixation disparity can be estimated with a technically simple psycho-physical procedure, i.e. the perceived offset of aligned dichoptic nonius targets, referred to as "subjective fixation disparity". To investigate the relation between these two measures, simultaneous tests were made in far vision when placing prisms in front of the eyes (for a few seconds) in order to induce forced vergence, i.e. to vary the absolute disparity (from 1 deg divergent to 3.4 deg convergent). Frequent repeated measurements in 12 observers allowed for individual analyses. Generally, fixation disparity values and the effects of prisms were much smaller in the subjective than in the objective measures. Some observers differed systematically in the characteristics of the two types of prism-induced curves. Individual regressions showed that the subjective vs. objective slope was 8% on the average (with largest individual values of 18%). This suggests that sensory fusion shifts the visual direction of the (peripheral) binocular targets by the full amount of objective fixation disparity (since single vision was achieved); however, for the (central) monocular nonius lines this shift was more or less incomplete so that the dichoptic nonius targets indicated an individual percentage of objective fixation disparity. The subjective-to-objective ratio seems to be an individual characteristic of fixation disparity in terms of the amount and in terms of the effect of prism-induced forced vergence. Therefore, on the group level the subjective measures do not allow for a precise prediction of the objective measures.

Amblyopia in High Accommodative Convergence/Accommodation Ratio Accommodative Esotropia. Influence of Bifocals on Treatment Outcome

PURPOSE

To study the influence of bifocal use on amblyopia treatment outcome in high accommodative convergence/accommodation (AC/A) ratio accommodative esotropia with deviation only at near.

DESIGN

Retrospective comparative case series.

METHODS

Setting: Tertiary referral center.

PATIENTS

Children with high AC/A ratio accommodative esotropia aged 3-8 years old, with deviation only at near with glasses, neutralized with bifocal lenses (follow-up 1 year).

INTERVENTION

Amblyopia was treated with patching. We compared bifocal and single-vision glasses users at 6 months and 1 year, with control of potential confounding variables (multiple regression).

MAIN OUTCOME MEASURES

LogMAR lines of improvement in visual acuity of the amblyopic eye, and improvement in stereoacuity.

RESULTS

Of 78 children, 61 were eligible. All patients wore single-vision glasses for 2 months (baseline visit), 46 of them changed to bifocals. Of 27 initially amblyopic children, 21 remained amblyopic at 2-month baseline (13 of them changed to bifocals). After adjustment for initial deviation, refraction, age, and amblyopia, improvement of visual acuity in the amblyopic eye was larger in the bifocal vs single-vision group at 6 months (mean 2.6 [95% confidence interval (CI): 1.9-2.9] logMAR lines vs mean 1.9 [95% CI: 0.5-2.2] logMAR lines, respectively, P = .01), but not at 1 year (mean 2.7 [95% CI: 2.2-3.1] logMAR lines vs mean 2.3 [95% CI: 1.6-3.1] logMAR lines, respectively, P = .3). Improvement of stereoacuity was not significantly different between the 2 groups.

CONCLUSIONS

Use of bifocals may provide a transient advantage, but improvement in visual acuity and stereopsis is equal with single-vision glasses over time.

Visual and binocular status in elementary school children with a reading problem

PURPOSE

This descriptive study provides a summary of the binocular anomalies seen in elementary school children identified with reading problems.

METHODS

A retrospective chart review of all children identified with reading problems and seen by the University of Waterloo, Optometry Clinic, from September 2012 to June 2013.

RESULTS

Files of 121 children (mean age 8.6 years, range 6-14 years) were reviewed. No significant refractive error was found in 81% of children. Five and 8 children were identified as strabismic at distance and near respectively. Phoria test revealed 90% and 65% of patients had normal distance and near phoria. Near point of convergencia (NPC) was <5cm in 68% of children, and 77% had stereoacuity of ≤40seconds of arc. More than 50% of the children had normal fusional vergence ranges except for near positive fusional vergencce (base out) break (46%). Tests for accommodation showed 91% of children were normal for binocular facility, and approximately 70% of children had an expected accuracy of accommodation.

CONCLUSION

Findings indicate that some children with an identified reading problem also present with abnormal binocular test results compared to published normal values. Further investigation should be performed to investigate the relationship between binocular vision function and reading performance.

Comparison of symmetrical prism adaptation to asymmetrical prism adaptation in those with normal binocular vision

This study sought to determine whether symmetrical compared to asymmetrical horizontal prisms (base-out or base-in) evoked different rates of phoria adaptation. Sixteen young adults with normal binocular vision participated in a symmetrical phoria adaptation experiment using a 3Δ base-out or 3Δ base-in binocular prism flipper and an asymmetrical phoria adaptation experiment using a 6Δ base-out or 6Δ base-in monocular wedge prism. The experiments were randomized and counterbalanced to reduce the influence of the prism stimulation order. Asymmetrical base-out prism adaptation was significantly faster than symmetrical prism adaptation for subjects with normal binocular vision. Asymmetrical phoria adaptation with base-in prism was not significantly different from symmetrical phoria adaptation implying that there are directional asymmetries (convergent versus divergent eye movements) in the slow fusional component of vergence. Data suggest that a potential interaction between the version system and the slow fusional vergence system may exist. Results have clinical relevance because patients with convergence or divergence insufficiency/excess may potentially show more pronounced differences between symmetrical and asymmetrical phoria adaptation compared to binocularly normal controls. These differences might also be relevant to clinical measurements such as vergence fusional range, which can be measured symmetrically (with Risley prisms in a phoroptor) or asymmetrically (with prism bar).

Human infants can generate vergence responses to retinal disparity by 5 to 10 weeks of age

Vergence is defined as a binocular eye movement during which the two eyes move in opposite directions to align to a target in depth. In adults, fine vergence control is driven primarily by interocular retinal image disparity. Although infants have not typically been shown to respond to disparity until 3 to 5 months postpartum, they have been shown to align their eyes from hours after birth. It remains unclear what drives these responses in young infants. In this experiment, 5- to 10-week-old human infants were presented with a dynamic random noise stimulus oscillating in disparity at 0.1 Hz over an amplitude of 2° for 30 s. Fourier transforms of the horizontal eye movements revealed significant disparity-driven responses at the frequency of the stimulus in over half of the tested infants. Because the stimulus updated dynamically, this experiment precluded the possibility of independent monocular fixations to a sustained target. These data demonstrate cortical binocular function in humans by five weeks, the youngest age tested here, which is as much as two months younger than previously believed.

Efficacy of vision therapy in children with learning disability and associated binocular vision anomalies

PURPOSE

To report the frequency of binocular vision (BV) anomalies in children with specific learning disorders (SLD) and to assess the efficacy of vision therapy (VT) in children with a non-strabismic binocular vision anomaly (NSBVA).

METHODS

The study was carried out at a centre for learning disability (LD). Comprehensive eye examination and binocular vision assessment was carried out for 94 children (mean (SD) age: 15 (2.2) years) diagnosed with specific learning disorder. BV assessment was done for children with best corrected visual acuity of ≥6/9 - N6, cooperative for examination and free from any ocular pathology. For children with a diagnosis of NSBVA (n=46), 24 children were randomized to VT and no intervention was provided to the other 22 children who served as experimental controls. At the end of 10 sessions of vision therapy, BV assessment was performed for both the intervention and non-intervention groups.

RESULTS

Binocular vision anomalies were found in 59 children (62.8%) among which 22% (n=13) had strabismic binocular vision anomalies (SBVA) and 78% (n=46) had a NSBVA. Accommodative infacility (AIF) was the commonest of the NSBVA and found in 67%, followed by convergence insufficiency (CI) in 25%. Post-vision therapy, the intervention group showed significant improvement in all the BV parameters (Wilcoxon signed rank test, p<0.05) except negative fusional vergence.

CONCLUSION

Children with specific learning disorders have a high frequency of binocular vision disorders and vision therapy plays a significant role in improving the BV parameters. Children with SLD should be screened for BV anomalies as it could potentially be an added hindrance to the reading difficulty in this special population.

Medial Rectus Bridge Faden Operations in Accommodative and Partially Accommodative Esotropia With Convergence Excess

PURPOSE

To evaluate the results of the bilateral bridge Faden operation on the medial rectus muscles with and without recession in the treatment of accommodative and partially accommodative esotropia with convergence excess.

METHODS

A retrospective analysis was performed on the medical records of 103 patients who underwent the bridge Faden operation on both medial rectus muscles, with or without recession, for the treatment of accommodative and partially accommodative esotropia with convergence excess. Preoperative and postoperative near and distance deviations and near-distance disparities were evaluated.

RESULTS

The study population consisted of 38 (37%) girls and 65 (63%) boys. The mean age was 9.32 ± 5.83 years (range: 1 to 18 years) and the mean follow-up period was 14.49 ± 2.78 months. Fifty-one patients underwent the bridge Faden operation on both medial rectus muscles with recession (recession group) and 52 patients underwent the bridge Faden operation on both medial rectus muscles without recession (no recession group). The mean preoperative amount of esotropia at near was 43.51 ± 7.00 and 24.24 ± 3.56 prism diopters (PD) for the recession and no recession groups, respectively. The mean preoperative amount of esotropia at distance was 26.63 ± 6.86 and 9.22 ± 2.09 PD for both groups, respectively. The mean preoperative near-distance disparity was 17.14 ± 3.00 and 14.05 ± 4.14 PD for both groups, respectively. In both groups, there was a statistically significant difference in the near and distance deviations and the near-distance disparity between preoperative and postoperative values (P < .05). Postoperatively, there was no significant difference between 1 month, 6 months, and 1 year and between 6 months and 1 year (P > .05).

CONCLUSIONS

The bridge Faden operation on both medial rectus muscles either with or without recession was a successful surgical procedure in patients with accommodative and partially accommodative esotropia. During the follow-up period, the success rates did not decrease. [J Pediatr Ophthalmol Strabismus. 2017;54(6):369-374.].

2016 International Orthoptic Congress Burian Lecture: Folklore or Evidence?

The theme of the 2016 Burian Lecture is how our understanding of strabismus has been changed by the research carried out in our laboratory in Reading over the years. Accommodation and convergence are fundamental to orthoptics, but actual responses have often been very different compared to what we had expected. This paper outlines how our laboratory's understanding of common issues such as normal development of accommodation and convergence, their linkage, intermittent strabismus, anisometropia, orthoptic exercises, and risk factors for strabismus have changed. A new model of thinking about convergence and accommodation may help us to better understand and predict responses in our patients.

Changes in stimulus and response AC/A ratio with vision therapy in Convergence Insufficiency

PURPOSE

To evaluate the changes in the stimulus and response Accommodative Convergence to Accommodation (AC/A) ratio following vision therapy (VT) in Convergence Insufficiency (CI).

METHODS

Stimulus and response AC/A ratio were measured on twenty five CI participants, pre and post 10 sessions of VT. Stimulus AC/A ratio was measured using the gradient method and response AC/A ratio was calculated using modified Thorington technique with accommodative responses measured using WAM-5500 open-field autorefractor. The gradient stimulus and response AC/A cross-link ratios were compared with thirty age matched controls.

RESULTS

Mean age of the CI and control participants were 23.3±5.2 years and 22.7±4.2 years, respectively. The mean stimulus and response AC/A ratio for CI pre therapy was 2.2±0.72 and 6.3±2.0 PD/D that changed to 4.2±0.9 and 8.28±3.31 PD/D respectively post vision therapy and these changes were statistically significant (paired t-test; p<0.001). The mean stimulus and response AC/A ratio for controls was 3.1±0.81 and 8.95±2.5 PD/D respectively.

CONCLUSIONS

Stimulus and response AC/A ratio increased following VT, accompanied by clinically significant changes in vergence and accommodation parameters in subjects with convergence insufficiency. This represents the plasticity of the AC/A crosslink ratios that could be achieved with vision therapy in CI.

The influence of age on adaptation of disparity vergence and phoria

A paucity of research exists to investigate whether the normal aging process influences the ability to adapt disparity vergence and phoria. Vergence eye movements and dissociated phoria were recorded from 49 healthy subjects (ages 20-70years) using an objective eye movement tracking system. Four-degree vergence responses were modified using a double-step protocol. Dynamics of vergence were quantified via peak velocity. The phoria adaptation experiment measured the magnitude (net change in phoria level) and rate (magnitude divided by the time constant) of phoria adaption during 5min of sustained fixation on a binocular target (40cm/8.44° from midline). The magnitude of phoria adaptation decreased as a function of age (r=-0.33; p=0.04). The ability to adapt vergence peak velocity and the rate of phoria adaptation showed no significant age-related influence (p>0.05). The data suggest that the ability to modify the disparity vergence system and the rate of phoria adaptation are not dependent on age; whereas, the magnitude of phoria adaptation decreases as part of the normal adult aging process. These results have clinical and basic science implications because one should consider age when assessing the changes in the magnitude of phoria adaptation which can be abnormal in those with oculomotor dysfunctions.

King-Devick and Pre-season Visual Function in Adolescent Athletes

PURPOSE

The King-Devick test (KD) has been studied as a remove-from-play sideline test in college-age athletes and older; however, studies in younger athletes are limited. A cross-sectional study of the KD and other vision correlates was completed on school-aged athletes during pre-season physicals for a variety of sports to determine the repeatability of the KD. The study also evaluated how convergence, alignment, or pupil function contributed to a slower King-Devick baseline reading.

METHODS

Seven hundred eighty-five athletes underwent vision screenings in a hospital or school setting by trained/certified staff as part of pre-season physicals. Six hundred nineteen had KD testing completed per the manufacturer's suggested protocol and repeated. Other baseline vision testing included visual acuity, Modified Thorington testing for alignment, convergence testing, and pupil function using the NeurOptics (NPI-200) NPi.

RESULTS

The mean fastest, error-minimized KD time for all participants was 43.9 seconds(s) (SD ± 11.6, range 24-120). Median KD time got faster (+) with age (p < 0.0001). The inter-class correlation coefficient for all scores was 0.92. The absolute mean time difference for any two tests was 3.5 s (SD ± 2.5, range 0-23). There was no association between the best KD time and reduced NPC (p = 0.63), Modified Thorington measure of alignment (p = 0.55), or NPi pupil function (p = 0.79). The Bland Altman repeated measure limits of agreement was ±6.5 seconds for those in the 10th to12th grades, and ±10.2 seconds for those in the 6th to 9th grades.

CONCLUSIONS

King-Devick score in junior high and high school athletes is variable but gets faster and more repeatable with increasing age. The KD does not correlate significantly with reduced convergence, alignment, or pupil function. Based on grouped data, a slowing of 10 seconds for younger athletes and 6 seconds for older athletes on a second administration represents a true difference in testing speed. Within-player variability should be considered when removal-from-play decisions are influenced by KD results.

[Change in the Fusion Limit by Insertion of a Middle Image Equally Dividing the Parallactic Angle of 3D Stereoscopic Images]

OBJECTIVES

The purpose of this study was to identify and clarify the requirements for 3D stereoscopic images that do not cause viewing discomfort to the human eye even when the protrusion distance is large.

METHODS

A total of 140 healthy men and women aged 14 to 79 years participated in this study. We first measured the fusion limits in these participants using two 3D stereoscopic images. We then measured the expansion of the fusion limit by inserting a middle image in a region located equally parallax from the two images.

RESULTS

The results showed that the fusion limits were significantly expanded (p<0.01) after inserting the middle image.

CONCLUSIONS

Insertion of middle images with parallax can extend the fusion limit. This method was shown to be an effective for viewing 3D stereoscopic images without causing discomfort.

Home-Based Therapy for Symptomatic Convergence Insufficiency in Children: A Randomized Clinical Trial

PURPOSE

To compare the effectiveness of home-based (HB) computer vergence/accommodative therapy (HB-C) to HB near target push-up therapy (HB-PU) and to HB placebo treatment (HB-P) among children aged 9 to <18 years with symptomatic convergence insufficiency (CI).

METHODS

In this multicenter randomized clinical trial, participants were randomly assigned to computer therapy, near target push-ups, or placebo. All therapy was prescribed for 5 days per week at home. A successful outcome at 12 weeks was based on meeting predetermined composite criteria for the CI Symptom Survey, near point of convergence, and positive fusional vergence at near.

RESULTS

A total of 204 participants were randomly assigned to HB-C (n = 75), HB-PU (n = 85), or HB-P (n = 44). At 12 weeks, 16 of 69 (23%, 95% CI: 14-35%) in the HB-C group, 15 of 69 (22%, 95% CI: 13-33%) in the HB-PU group, and 5 of 31 (16%, 95% CI: 5-34%) in the HB-P group were classified as having a successful outcome. The difference in the percentage of participants with a successful outcome in the HB-C group compared with the HB-PU group was -4% (two-sided 97.5% CI: -19 to +11%; p = 0.56) and with the HB-P group was +5% (two-sided 97.5% CI: -12 to +22%; p = 0.52), adjusted for baseline levels of the composite outcome components.

CONCLUSIONS

The majority of participants with symptomatic CI did not have a successful outcome at 12 weeks. Some participants treated with placebo were successful. With recruitment reaching only 34% of that originally planned and differential loss to follow-up among groups, estimates of success are not precise and comparisons across groups are difficult to interpret.

Vergence findings and horizontal vergence dysfunction among first year university students in Benin City, Nigeria

PURPOSE

The study was designed to determine the prevalence of vergence dysfunctions among first year university students in Nigeria and to document the measures that define the vergence system of the visual system.

METHODS

A cross-sectional study of first year students of the University of Benin, Benin City, Nigeria, who presented for the mandatory eye examination as part of their medical examinations required for clearance was conducted. A battery of tests that defines the vergence system including near and far phoria, positive and negative fusional vergence amplitudes at far and near, near point of convergence (NPC) and AC/A ratio were measured using conventional clinical protocols.

RESULTS

The prevalence of vergence dysfunction among 212 first year university students who satisfied the inclusion criteria and gave consent to participate was 12.7%, with convergence insufficiency being the most common vergence dysfunction. Blurred vision, headache and diplopia were the most frequently reported visual symptoms.

CONCLUSION

There is a considerable prevalence of previously undiagnosed vergence dysfunctions in this population of students. The study underscored the need to carry out a thorough binocular vision assessment as part of the battery of tests administered to newly admitted students in this community to forestall any adverse effect the presence of vergence dysfunctions might have on the academic activity of university students.

Identifying visual stress during a routine eye examination

PURPOSE

To investigate whether the clinical tests used in routine eye examinations can identify adults whose reading rate increases with their preferred coloured overlay(s).

METHODS

Routine optometric tests were used to measure 73 undergraduate students' refractive error, visual acuity, stereo-acuity, amplitude of accommodation, near point of convergence, associated heterophoria at near, colour vision and ocular motility. Participants chose an overlay or combination of overlays with colour optimal for clarity, and completed the Wilkins Rate of Reading Test with and without an overlay(s) of this colour.

RESULTS

Overall, there was a significant increase in reading speed with overlay (t(72)=-5.26, p<0.0005). Twenty-six participants (36%) increased their reading rate by >5% with their chosen coloured overlay(s). Ten participants (14%) had a reading speed increase of >10%. The increase in reading speed was not significantly associated with any clinical finding.

CONCLUSION

Tests which are completed in routine eye examinations did not identify those participants who benefitted from coloured overlays in terms of reading speed.

[Surgery for convergent strabismus in childhood: why and when?]

The authors explain the reasons for and the timing of surgery for convergent strabismus, or esotropia, in children as a function of the particular type of strabismus. The goal of surgery is to correct the cross-eyed deviation by choosing the most opportune time so as to obtain the best binocular result with the minimum number of surgeries. The authors take a position in the debate over age at time of surgery for childhood esotropia, which is still controversial. Their arguments are based on recent neurophysiological and clinical data.

Disparity-driven vs blur-driven models of accommodation and convergence in binocular vision and intermittent strabismus

PURPOSE

To propose an alternative and practical model to conceptualize clinical patterns of concomitant intermittent strabismus, heterophoria, and convergence and accommodation anomalies.

METHODS

Despite identical ratios, there can be a disparity- or blur-biased "style" in three hypothetical scenarios: normal; high ratio of accommodative convergence to accommodation (AC/A) and low ratio of convergence accommodation to convergence (CA/C); low AC/A and high CA/C. We calculated disparity bias indices (DBI) to reflect these biases and provide early objective data from small illustrative clinical groups that fit these styles.

RESULTS

Normal adults (n = 56) and children (n = 24) showed disparity bias (adult DBI 0.43 [95% CI, 0.50-0.36], child DBI 0.20 [95% CI, 0.31-0.07]; P = 0.001). Accommodative esotropia (n = 3) showed less disparity-bias (DBI 0.03). In the high AC/A-low CA/C scenario, early presbyopia (n = 22) showed mean DBI of 0.17 (95% CI, 0.28-0.06), compared to DBI of -0.31 in convergence excess esotropia (n=8). In the low AC/A-high CA/C scenario near exotropia (n = 17) showed mean DBI of 0.27. DBI ranged between 1.25 and -1.67.

CONCLUSIONS

Establishing disparity or blur bias adds to AC/A and CA/C ratios to explain clinical patterns. Excessive bias or inflexibility in near-cue use increases risk of clinical problems.

Vergence responses to vertical binocular disparity during lexical identification

Humans typically make use of both eyes during reading, which necessitates precise binocular coordination in order to achieve a unified perceptual representation of written text. A number of studies have explored the magnitude and effects of naturally occurring and induced horizontal fixation disparity during reading and non-reading tasks. However, the literature concerning the processing of disparities in different dimensions, particularly in the context of reading, is considerably limited. We therefore investigated vertical vergence in response to stereoscopically presented linguistic stimuli with varying levels of vertical offset. A lexical decision task was used to explore the ability of participants to fuse binocular image disparity in the vertical direction during word identification. Additionally, a lexical frequency manipulation explored the potential interplay between visual fusion processes and linguistic processes. Results indicated that no significant motor fusional responses were made in the vertical dimension (all p-values>.11), though that did not hinder successful lexical identification. In contrast, horizontal vergence movements were consistently observed on all fixations in the absence of a horizontal disparity manipulation. These findings add to the growing understanding of binocularity and its role in written language processing, and fit neatly with previous literature regarding binocular coordination in non-reading tasks.

Vergence eye movements in patients with schizophrenia

Previous studies have shown that smooth pursuit eye movements are impaired in patients with schizophrenia. However, under normal viewing conditions, targets move not only in the frontoparallel plane but also in depth, and tracking them requires both smooth pursuit and vergence eye movements. Although previous studies in humans and non-human primates suggest that these two eye movement subsystems are relatively independent of one another, to our knowledge, there have been no prior studies of vergence tracking behavior in patients with schizophrenia. Therefore, we have investigated these eye movements in patients with schizophrenia and in healthy controls. We found that patients with schizophrenia exhibited substantially lower gains compared to healthy controls during vergence tracking at all tested speeds (e.g. 0.25 Hz vergence tracking mean gain of 0.59 vs. 0.86). Further, consistent with previous reports, patients with schizophrenia exhibited significantly lower gains than healthy controls during smooth pursuit at higher target speeds (e.g. 0.5 Hz smooth pursuit mean gain of 0.64 vs. 0.73). In addition, there was a modest (r≈0.5), but significant, correlation between smooth pursuit and vergence tracking performance in patients with schizophrenia. Our observations clearly demonstrate substantial vergence tracking deficits in patients with schizophrenia. In these patients, deficits for smooth pursuit and vergence tracking are partially correlated suggesting overlap in the central control of smooth pursuit and vergence eye movements.

Changes in fusional vergence amplitudes after laser refractive surgery for moderate myopia

PURPOSE

To analyze the effect of bilateral laser refractive surgery on fusional vergence amplitude and near point convergence in normal orthotropic myopic patients.

SETTING

Cornea and refractive private practice, Seoul, South Korea.

DESIGN

Prospective observational study.

METHODS

Moderately myopic patients (2.0 to 6.0 diopters [D]) had bilateral laser refractive surgery with postoperative follow-ups at 1 week and 1 and 3 months. Patients with manifest strabismus, previous strabismus surgery, amblyopia, absent or impaired binocularity, or planned monovision were excluded. The preoperative and postoperative examinations included uncorrected and corrected visual acuities; a prism cover test; evaluation of the convergence, divergence, and vertical fusion amplitude using a rotary prism; and near point convergence.

RESULTS

The mean age of the 30 patients was 29 years ± 4 (SD). The mean preoperative refractive error was -4.10 ± 1.32 D in right eyes and -4.14 ± 1.37 D in left eyes. The convergence amplitude at near decreased 1 week and 1 month postoperatively, after which it progressively stabilized to near preoperative values. The convergence amplitude at far (break point) was not changed postoperatively; however, the convergence amplitude at far (recovery point) was increased at 1 month and 3 months (P=.008 and P=.003, respectively). There was a slight increase in near point convergence at 3 months (P<.001).

CONCLUSIONS

Although fusional vergence amplitudes at near and near point convergence changed after laser refractive surgery, the changes observed were small or transient. However, patients with latent phoria or strabismus should be warned before having refractive surgery.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Subjective and objective convergence of the eyes at simulated altitude of 18,000 feet preceded by short-term exposure to heat stress

Armed forces personnel including military aviators are quite often exposed concurrently to various environmental stressors like high environmental temperature and hypoxia. Literatures have suggested that exposure to one environmental stressor may modify the physiological response on subsequent exposure to same or different stressor. The present study was undertaken to investigate the impact of cross tolerance between two environmental stressors of aviation (heat and hypoxia) in ten healthy adult males in a simulated altitude chamber in a within subject experimental study. They were assessed for their convergence ability of the eyes at ground and at simulated altitude of 18,000 ft with or without pre-exposure to heat stress. Subjective convergence at simulated altitude of 18,000 ft did not show any improvement following pre-exposure to heat stress. Objective convergence was improved following pre-exposure to heat stress and was found to be 10.76 cm and 9.10 cm without and with heat stress respectively at simulated altitude of 18,000 ft. Improved objective convergence at high altitude as a result of pre-exposure to heat stress is indicative of better ocular functions. This might benefit aviators while flying at hypoxic condition.

Effect of the accommodation-vergence conflict on vergence eye movements

With the broader use of stereoscopic displays, a flurry of research activity about the accommodation-vergence conflict has emerged to highlight the implications for the human visual system. In stereoscopic displays, the introduction of binocular disparities requires the eyes to make vergence movements. In this study, we examined vergence dynamics with regard to the conflict between the stimulus-to-accommodation and the stimulus-to-vergence. In a first experiment, we evaluated the immediate effect of the conflict on vergence responses by presenting stimuli with conflicting disparity and focus on a stereoscopic display (i.e. increasing the stereoscopic demand) or by presenting stimuli with matched disparity and focus using an arrangement of displays and a beam splitter (i.e. focus and disparity specifying the same locations). We found that the dynamics of vergence responses were slower overall in the first case due to the conflict between accommodation and vergence. In a second experiment, we examined the effect of a prolonged exposure to the accommodation-vergence conflict on vergence responses, in which participants judged whether an oscillating depth pattern was in front or behind the fixation plane. An increase in peak velocity was observed, thereby suggesting that the vergence system has adapted to the stereoscopic demand. A slight increase in vergence latency was also observed, thus indicating a small decline of vergence performance. These findings offer a better understanding and document how the vergence system behaves in stereoscopic displays. We describe what stimuli in stereo-movies might produce these oculomotor effects, and discuss potential applications perspectives.