Vergence Facility Research Articles

Short-term adaptation of accommodation, accommodative vergence and disparity vergence facility

Previous studies have found that subjects can increase the velocity of accommodation using visual exercises such as pencil push ups, flippers, Brock strings and the like and myriad papers have shown improvement in accommodation facility (speed) and sufficiency (amplitude) using subjective tests following vision training but few have objectively measured accommodation before and after training in either normal subjects or in patients diagnosed with accommodative infacility (abnormally slow dynamics). Accommodation is driven either directly by blur or indirectly by way of neural crosslinks from the vergence system. Until now, no study has objectively measured both accommodation and accommodative–vergence before and after vision training and the role vergence might play in modifying the speed of accommodation. In the present study, accommodation and accommodative–vergence were measured with a Purkinje Eye Tracker/optometer before and after normal subjects trained in a flipper-like task in which the stimulus stepped between 0 and 2.5diopters and back for over 200 cycles. Most subjects increased their speed of accommodation as well as their speed of accommodative vergence. Accommodative vergence led the accommodation response by approximately 77ms before training and 100ms after training and the vergence lead was most prominent in subjects with high accommodation and vergence velocities and the vergence leads tended to increase in conjunction with increases in accommodation velocity. We surmise that volitional vergence may help increase accommodation velocity by way of vergence–accommodation cross links.

Body mass index and binocular vision skills

IntroductionBody Mass Index (BMI) is of increasing interest to eye care practitioners. Associations have recently been proven between high BMI and several diseases affecting the eyes, including AMD, intracranial hypertension, optic disc cupping, and glaucoma. The symptoms of dizziness and vertigo have also been associated with high BMI. However, to these authors’ knowledge, there has been no study performed comparing BMI to binocular function. MethodsIn this analytical-descriptive study, 119 randomly selected young subjects had their BMI measured, along with refractive error, dissociated phoria, near point of convergence, vergence ranges and facility, and stereopsis. ResultsIn most situations, the subjects classified as normal and overweight, based on their BMI had better performance than those classified as underweight or obese. The worst binocular performance was found in underweight subjects. The one-way ANOVA showed only statistically significant differences between mean of near point of convergence and vergence facility, in different states of BMI. ConclusionUnlike most ocular diseases that are adversely affected by higher BMI values, most binocular vision skills are adversely affected by lower BMI values. The possible reasons for this are discussed.

Susceptibility to Visual Discomfort of 3-D Displays by Visual Performance Measures

People with some signs of binocular dysfunctioning can be susceptible to visual complaints associated with viewing stereoscopic content at large viewing distances. Two performance measurements enabled to distinguish people by their binocular status (BS) in previous research: the ratio of performance of the Wilkins rate of reading test (WRRT) between 2-D and 3-D, and the vergence facility. In an experiment, first, an extensive optometric screening was carried out to differentiate visually asymptomatic young adults with good BS (GBS) (N = 27) from those with a moderate BS (MBS) (N = 6). Second, participants had to perform the WRRT at short viewing distance followed by a questionnaire under different screen disparity settings. The results reveal that the ratio of the WRRT between 0 and -1.5 screen disparity is an appropriate indicator of participants with MBS in comparison with participants with GBS. In addition, the results show that 0.75° of screen disparity is already problematic for people with MBS. We conclude that the WRRT-ratio has potential as a BS test in consumer applications to provide individual settings for comfortable screen disparities based on viewers' BS.

A comparison of self-reported and measured autostereogram skills with clinical indicators of vergence ability in esophores

Perception of the disparity-defined form in autostereograms requires achieving and maintaining the precise vergence angle necessary to place the intended left and right images on corresponding areas of the two retinas. Most commercially available autostereograms are designed to be viewed only with one vergence posture, usually the parallel or “wall-eyed” technique1, meaning that the visual axes need to converge by one or more periods of the pattern to a point farther from the viewer than the plane of the stereogram. Converging the eyes (“cross-eyed” technique) in front of these autostereograms by one or more periods of the pattern will reverse the depth direction of the stereoscopic image, perhaps making the form more difficult to perceive. Our previous work2 compared common clinical indicators of vergence ability with subjects' self-reported and measured autostereogram skills and found significant differences between those with poor versus good self-reported and measured autostereogram skill for vergence facility, near phoria, and TNO stereoacuity for subjects who were exophoric at near. The present study was undertaken to compare clinical indicators of vergence ability to perceived and measured autostereogram skills in esophores. Our results show that esophores whose self-rated and measured autosterogram skills were “poor” demonstrated significantly poorer performance on clinical tests of vergence facility and were more likely to demonstrate differences in threshold performance for crossed vs. uncrossed disparities on the TNO stereoacuity test than were subjects with “excellent” autostereogram skills. Attempts to fuse using “wall-eyed” vs. “cross-eyed” viewing did not differ between the two groups for those subjects able to perceive the autostereogram form.

Contact lenses vs spectacles in myopes: is there any difference in accommodative and binocular function?

Theoretically, the accommodative and vergence demands are different between single-vision contact lenses and spectacle lenses. The aim of the present study was to determine whether these differences exist when these two correction methods are used in clinical practice. For this, different visual parameters that characterize the accommodative (accommodation amplitude, accommodative facility, and accommodative response) and binocular function (near and distance horizontal and vertical dissociated phorias, near and vertical associated phorias, near and distance negative and positive fusional vergence, vergence facility, near point of convergence, negative and positive relative accommodation, stimulus AC:A ratio and stereoacuity) were evaluated in a student population when their myopia was corrected with either spectacles or soft contact lenses (SCL). All parameters were measured on two separate occasions in 30 myopic habitual contact lens and spectacle wearers of mean age 19 ± 2.4years. Some parameters such as accommodation amplitude, accommodative response, and stimulus AC:A ratio were measured using two measurement methods which are commonly used in clinical practice. Three measurements were taken for each parameter and averaged. For the comparative statistical analysis, we used the Student's t-test (p value < 0.05). The following statistically significant differences were found with the use of SCL in comparison to spectacles: higher accommodative lags, higher negative relative accommodation, more esophoric near horizontal dissociated phoria, and lower negative fusional vergence in near vision. The results found in this study show a definite trend towards poorer accommodative and vergence function with the use of contact lenses in comparison to glasses. This downward trend, though not statistically significant in accommodative function (lower PRA values and less lens amplitude of accommodation) might suggest that temporal insufficiency in the accommodation process could be occurring while contact lenses are used, thereby possibly creating a lag in accommodation to reduce associated overconvergence. This would be manifested in more esophoric values being found in the vergence function. The higher accommodative lags found in this study with SCL could indicate that prolonged use of SCL in near tasks may provoke a continuous hyperopic retinal defocus, a risk factor for the onset and progression of myopia, as indicated in numerous studies.

Measuring visual fatigue and visual discomfort associated with 3‐D displays

Abstract— This research identifies methods for signs of visual complaints associated with stereoscopic displays. Two potential causes for contradictory results concerning these signs in previous research are hypothesized: (1) not all clinical tests are equally appropriate and (2) there is a natural variation in susceptibility amongst people with normal vision. An optometric screening differentiated participants with moderate binocular status (MBS) from those with good binocular status (GBS). In a 2 × 2 within‐subjects design (2‐D vs. 3‐D and MBS vs. GBS), a questionnaire and eight optometric tests (i.e., binocular acuity, aligning prism, fixation disparity, heterophoria, convergent and divergent fusional reserves, vergence facility, and accommodation response) were each administered before and immediately after a reading task. Results revealed that only participants with MBS in 3‐D conditions showed clinically meaningful changes in fusion range, experienced more visual discomfort, and performed worse on the reading task. Acombination of fusion range measurements and self‐report is appropriate for evaluating visual complaints of stereoscopic stills, and people with a MBS are more susceptible to visual complaints associated with stereoscopic displays. A simple measurement tool, i.e., the ratio of reading performance between 2‐D and 3‐D, to categorize people based on their binocular status is proposed.

Vergence adaptation in clinical vergence testing

Background The purposes of this investigation were to determine whether vergence adaptation occurs after vergence range testing and vergence facility testing and to determine whether vergence adaptation correlates with the results of these tests. Methods Thirty subjects participated in 3 testing sessions on different days. During each session 1 of the following was tested: base-out prism bar vergences, vergence facility (12 base-out/3 base-in binocular prism flippers for 1 minute), and 5 minutes viewing with 6 prism diopters of base-out prism. Before and after each test, the near phoria was measured using the modified Thorington method. Results There was no correlation between the amplitude of the vergence ranges and the amplitude of vergence facility. Significant vergence adaptation as indicated by an esophoric shift of approximately 3 prism diopters occurred in all testing sessions. The amplitude of vergence adaptation did not correlate with either the amplitude of the blur vergence range or vergence facility. There was a significant correlation between the amplitude of vergence adaptation and the amplitude of the break vergence range. Conclusions The lack of correlation between the blur vergence range and the vergence facility is not likely because of vergence adaptation. The lack of correlation between the break vergence range and the vergence facility may be in part caused by vergence adaptation.

A survey of visual function in an Austrian population of school-age children with reading and writing difficulties

BackgroundTo describe and compare visual function measures of two groups of school age children (6-14 years of age) attending a specialist eyecare practice in Austria; one group referred to the practice from educational assessment centres diagnosed with reading and writing difficulties and the other, a clinical age-matched control group.MethodsRetrospective clinical data from one group of subjects with reading difficulties (n = 825) and a clinical control group of subjects (n = 328) were examined.Statistical analysis was performed to determine whether any differences existed between visual function measures from each group (refractive error, visual acuity, binocular status, accommodative function and reading speed and accuracy).ResultsStatistical analysis using one way ANOVA demonstrated no differences between the two groups in terms of refractive error and the size or direction of heterophoria at distance (p > 0.05). Using predominately one way ANOVA and chi-square analyses, those subjects in the referred group were statistically more likely to have poorer distance visual acuity, an exophoric deviation at near, a lower amplitude of accommodation, reduced accommodative facility, reduced vergence facility, a reduced near point of convergence, a lower AC/A ratio and a slower reading speed than those in the clinical control group (p < 0.05).ConclusionsThis study highlights the high proportions of visual function anomalies in a group of children with reading difficulties in an Austrian population. It confirms the importance of a full assessment of binocular visual status in order to detect and remedy these deficits in order to prevent the visual problems continuing to impact upon educational development.

A prospective study of the EYEPORT Vision Training System

Background The purpose of this study was to determine if vision training with the EYEPORT ™ system affects vision and reading performance. (Subsequent to this study, the “E.Y.E.” has been renamed “ EYEPORT ™ ”) Methods College students with normal vision (n = 31) participated in a blind, crossover study to assess within-subject and between-group effects of EYEPORT training. Half the subjects initially underwent training (10 minutes per day, 6 days a week for 3 weeks), and then crossed over to the no-train control condition (3 weeks); the other half underwent the reverse sequence. Vision and reading performance were assessed at baseline, 3 weeks, and 6 weeks. Results Multivariate analyses (Hotelling’s T 2) found small improvements in vergence and accommodative facility, reading performance, and stereopsis response time after EYEPORT training ( P < 0.025). Enhancements in reading performance and vergence facility were still present 3 weeks after cessation of training ( P < 0.001). Conclusions The EYEPORT training system shows potential to enhance visual performance and reading ability. Studies are underway to assess its efficacy in symptomatic populations.

Statistical normal values of visual parameters that characterize binocular function in children

A wide range of visual parameters used to evaluate binocular function were evaluated in a paediatric population (1056 subjects aged 6-12 years). Mean values are provided for these ages in optometric tests that directly assess the vergence system, horizontal phorias for near and far vision (measured by a modified version of the Thorington method), negative and positive vergence amplitude for near and far vision (step vergence testing), vergence facility (flippers 8 Delta BI/8 Delta BO), and near-point of convergence (penlight push-up technique and red-lens push-up technique), as well as stimulus accommodative convergence/accommodation ratio and stereoacuity (Randot test) which provide an overall evaluation of the vergence, accommodative and oculomotor systems. A statistical comparison (anova and Bonferroni post hoc test) of these values between ages was performed. The differences, although statistically significant, were not clinically meaningful, and therefore we identified two trends in the behaviour of these parameters. For all parameters, except for vergence facility, we established a single mean reference value for the age range studied. The difference between the means for vergence facility indicated the need to divide the population into two age ranges (6-8 and 8-12 years). This study establishes statistical normal values for these parameters in a paediatric population and their means are a valuable instrument for separating children with binocular anomalies from those with normal binocular vision.

Vergence facility: establishing clinical utility.

Vergence facility testing attempts to assess the ability of the fusional vergence system to respond rapidly and accurately to changing vergence demands over time [defined as the number of cycles per minute (cpm) that a stimulus can be fused through, alternating base-in (BI) and base-out (BO) prisms]. Decisions to use vergence facility as a clinical test are hampered by a lack of systematically gathered normative data. Twenty symptomatic and 20 control subjects with ages between 18 to 35 years of either sex and any race were pooled, based on vision-symptom level determined by a self-report questionnaire. Inclusion/exclusion criteria included vision correctable to 6/6 (20/20) Snellen acuity or better in each eye and normal phorias. Vergence facility response was tested over a 1-min period, using 16 combinations of BI/BO flip prisms at 4.0 and 0.4 m, based on Morgan's norms and pilot data. Horizontal vergence facility responses were not the same among those with and without symptoms, and not all magnitudes of BI/BO flip prisms produced the same response difference. A single flip prism, 3 delta BI/12 delta BO, was found to differentiate optimally between groups at distance and near. Repeatability of test results (with the 3 delta BI/12 delta BO prism) was poor at distance and good at near. In addition to providing valuable normative data, this study indicates that the vergence system nearly resets its "zero point" at any distance and sheds further light on the results of dynamic convergence and divergence stimulation on the accommodative-vergence system. From a clinical standpoint, the results improve the diagnosis of binocular vision abnormalities. The recommended near vergence facility test is easily implemented, using a commonly available flip prism (3 delta BI/12 delta BO) and having a clinical failure criterion that is easily recalled (15 cpm, sum of the BI and BO magnitudes).

Vergence facility and target type.

Target suppression-cues are considered important for valid binocular accommodative facility response. However, there are no comparable recommendations concerning the effect of target type on vergence facility response. Ten subjects ages 16 to 19 years of either sex and any race were pooled, based on the lack of any asthenopic symptoms determined by a verbal interview with the investigator. Inclusion/exclusion criteria included vision correctable to 6/6 (20/20) Snellen acuity or better in each eye, and near-normal phorias. Vergence facility response was tested over a 1-min period using 8 delta base-in (BI) and 20 delta base-out (BO) loose prisms at near (0.4 M) for 3 different vertically oriented targets: 6/9 (20/30) Snellen letters, back-illuminated anaglyphic shapes, and modified Wirt circles. For the group, the mean facility response was similar among the target types [Snellen letters: 9.5 cycles per minute (cpm) +/- 5.6; anaglyphic shapes: 9.0 cpm +/- 6.3; and Wirt circles: 9.4 cpm +/- 4.5]. Group response-differences were not significantly different by one-way ANOVA polynomial regression testing at the 0.05 level (F-value = 0.03, p = 0.97, df = 2). Whereas a binocular accommodative facility target must have additional vectographic or anaglyphic suppression-cues, vergence facility testing may incorporate a simple and available vertical row of 6/9 (20/30) Snellen letters, which provide inherent fusional suppression-cues, for a valid binocular response.

Stereopsis, Accommodative and Vergence Facility: Do They Relate to Dyslexia?

The relation between visual functions and reading performance has been the subject of conflicting opinion over the years. The purpose of this study is 2-fold: first, to examine factors other than visual acuity, i.e., stereopsis, accommodative facility and vergence facility, which might show a greater impact on reading achievement; and second, to utilize well defined dyslexic and normal controls to ensure that the reading deficits were not caused by overt neurological, psychological, educational, or environmental factors. Thirteen normal readers and 13 dyslexics were carefully matched by rigorous quantitative criteria. The performance of both groups on tests of visual acuity and stereopsis was similar. The dyslexic readers tended to show better accommodative facility. Dyslexics performed significantly worse than the matched normal readers on a test of vergence facility. These results, in agreement with those reported by other studies, indicate that less efficient dynamic vergence facility may contribute to reading impairment, unlike other static functions such as visual acuity and stereopsis. It may be that the vergence problems of the dyslexics are related to sequential oculo-motor abnormalities. The dyslexics' vergence problems may also be partly responsible for their large number of small eye movements.

The Measurement of Visual Efficiency Standards for Pilots in the United States Air Force

Military aviation requires a high degree of visual skill and efficiency. The current visual standards for flying status in the United States Air Force employ static tests of visual function. Visual factors utilized in flight are continually changing to meet the demands of the situation. This study evaluates the performance of two carefully matched samples of people on a test of dynamic visual change, i.e., vergence facility. We compared the ability of a sample of military pilots to make rapid fusional movements in a base-in and base-out direction with a group of nonpilots matched for other normal visual characteristics. The number of times each subject could reestablish fusion while viewing an anaglyph target through alternating 16 delta base-out and 4 delta base-in prism lenses was recorded over a 1-minute time interval. We found the pilot's ability to initiate and sustain fusional vergence movements to be similar to that of their nonrated military counterparts. Our work does not indicate that this test of visual efficiency would discriminate between the pilots and nonpilots in our sample if it were included in a test battery to identify individuals with exceptional visual proficiency.

Vergence Facility

We examined 310 schoolchildren to determine what normal values might be expected for successive convergence and divergence ocular movements. Vergence facility may affect the efficiency of visual information processing. The population consisted of children in grades 1 through 8. The subjects wore anaglyph glasses and viewed a nonvariable square-X-circle anaglyph target alternately through a 16 delta base-out and 4 delta base-in prism flipper. The subjects were directed to keep the target "clear and single." The total number of vergence cycles was recorded for 1 min. Students in higher grade levels were able to complete more vergence changes than students in beginning grades. The average number of changes accomplished per minute at each age level is presented for this population.

Portsea Modified Clinical Technique: Results from an Expanded Optometric Screening Protocol for Children

The results from screening 5,597 Australian school children using the Portsea Modified Clinical Technique (PMCT) are presented. The new protocol provides information about visual acuity, refractive status, strabismus, heterophoria, ocular motility, convergence, accommodative facility, fusional vergence facility, stereopsis, colour vision and ocular health. Population statistics have been determined for refraction, heterophoria, convergence insufficiency, accommodative infacility and vergence infacility. The screening results are compared to those from previous studies where possible.