Vergence Parameters Research Articles

The Impact on Vergence Parameters After Smartphone Gaming.

To evaluate the impact of smartphone gaming on the vergence system of the eye. A 5-month (from March 2023 to August 2023) comparative and experimental research was conducted. Eighty-two participants with a mean age of 21.98 ± 2.26 years were present in the study. Prior to assessing accommodation and vergence system characteristics, participants underwent a comprehensive eye examination. The participants were asked to play a shooting game on a smartphone for 30 minutes at a 40 cm distance. Measurements of the vergence parameters were taken before and after the activity and afterwards were compared. Non-parametric tests were used to compare pre- and post-task measurements. The Wilcoxon signed-rank test was used to compare the variables: Negative fusional vergence (NFV), Positive fusional vergence (PFV), Near point of convergence (NPC), and Vergence Facility (VF), with the alpha error set at 5%. The mean age of the participants was 21.98 ± 2.26 years. Post-task, the vergence parameters: NPC (p < 0.001), NFV for near distance (p < 0.001), PFV for near distance (p < 0.001), and VF (p < 0.001) showed significant decrease in vergence parameters. The study shows smartphone gaming for 30 minutes affects the vergence system, leading to binocular vision anomalies in young individuals.

Diurnal Variation in Accommodation, Binocular Vergence, and Pupil Size.

Our results show significant diurnal variations in accommodative function and the magnitude of the phoria. Therefore, when comparing visual measures in clinical or laboratory settings, performing the visual examination at the same time of day (±1 hour) is encouraged. The aim of this study was to evaluate the accommodation, binocular vergence, and pupil behavior on three different times during the day. Twenty collegiate students (22.8 ± 2.1 years) participated in this study. Participants visited the laboratory on three different days at 2-hourly intervals (morning, 9:00 to 11:00 am ; afternoon, 2:00 to 4:00 pm ; evening, 7:00 to 9:00 pm ). The binocular vergence and accommodative function were measured using clinical optometric procedures, and the accommodative response and pupil function were evaluated in binocular conditions using the WAM-5500 autorefractometer. The accommodative amplitude for the right and left eyes showed statistically significant differences for the time interval ( P = .001 and P = .02, respectively), revealing higher accommodative amplitude in the morning and afternoon in comparison with the evening. Participants were more esophoric when assessed in the morning in comparison with the evening at far and near ( P = .02 and P = .01, respectively) and when assessed in the afternoon in comparison with the evening at far distance ( P = .02). The magnitude of accommodative response was higher in the morning, and it decreased throughout the day at 500 ( P < .001), 40 ( P = .05), and 20 cm ( P < .001). No statistically significant differences were obtained for any other variable. This study shows small diurnal variations in some accommodative and binocular vergence parameters, but no effects were observed for the pupil response. These outcomes are of special relevance for eye care specialists when performing repeated accommodative or binocular vergence measures. However, the diurnal variations were modest and may not influence a routine orthoptic examination.

Smartphone Use on Accommodation and Vergence Parameters: A Systematic Review

Introduction: The frequency and the trend of smartphone use increase rapidly, and 85% of Malaysians owns a smartphone and spend an average of 187 minutes per day to use the smartphone. Aims: To evaluate the potential effects of smartphone use on Accommodation and Vergence of the users. Methods: A total of 18 articles were selected in this review following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines, and the research question was formulated based on the population, intervention, control, and outcomes (PICO) method. This review was registered with PROSPERO (reference number: CRD42022293325). Data Sources: Databases namely PubMed, Web of Science, and Science-Direct were used in the article search using appropriate keywords, phrases, and Medical Subject Heading (MeSH) terms. The inclusion criteria of this review were journal articles published from January 2010 until December 2021 with full articles or abstract in English available. Results: Accommodative function has been reported to changed significantly with 20 minutes of smartphone use, with reduced amplitude, facility, relative and increased lag. In addition, vergence function has been reported to be altered significantly with receded near point of convergence. Conclusion: Smartphone use has an effect on the accommodation and vergence parameters among adults. Assessments of accommodation and vergence parameters need to be conducted in patients with the symptoms of Computer Vision Syndrome to prevent vision problems. Future reviews are required in younger cohorts with various smartphone features.

Binocular vision parameters in handloom silk weavers

Background: Focusing the gaze for prolonged periods on a fixed distance demands high visual efficiency in handloom silk weavers and might result in various accommodative and vergence dysfunctions. The aim of the present study was to assess accommodative and vergence parameters and determine the frequency of non-strabismic binocular vision anomalies (NSBVAs) among handloom silk weavers.&#x0D; Methods: In this cross-sectional study, we recruited voluntary handloom silk weavers from the Government-aided Society of Arignar Anna Silk Co-op Society K.H.-1, Kanchipuram, aged 20–39 years. All participants underwent preliminary visual examinations and comprehensive binocular vision testing of accommodative and vergence parameters. Sensory evaluation was performed using stereopsis testing and the Worth 4 Dot test. Motor evaluation included the ocular motility assessment, heterophoria checks for distance and near, near point of convergence, negative and positive fusional vergences for distance and near, vergence facility, negative and positive relative accommodations, both monocular and binocular near points of accommodation, and accommodative facility. The monocular estimate method was used to determine the accommodative response. Quantitative data are expressed as mean (standard deviation [SD]), and qualitative data are expressed as frequency (percentage).&#x0D; Results: We recruited a total of 72 weavers, including 41 (56.9%) men and, 31 (43.1%) women with mean (SD) age, working hours, and work experience of 34.15 (4.12) years, 9 (2.5) h/day, and 17.5 (6.9) years, respectively. Of the 29 (40.3%) weavers with a refractive error, 18 (25%) had astigmatism; seven (9.7%) had myopia; and four (5.6%) had hyperopia. Of the 72 weavers, 38 (52.8%) presented with NSBVAs, including 13 (18.1%) with accommodative dysfunctions, 11 (15.3%) with vergence dysfunctions, and 14 (19.4%) with combined accommodative and vergence dysfunctions. Accommodative insufficiency was the most prevalent dysfunction among all NSBVAs. Overall, 57 (79.2%) patients reported vision-related symptoms during their weaving hours, all with NSBVAs (n = 38), 19 with normal parameters in the binocular vision test, and 15 with no symptoms.&#x0D; Conclusions: The frequency of NSBVAs was high among handloom silk weavers compared to the literature. This implies a need for comprehensive binocular vision examination for people in this occupation to rule out NSBVAs for improving their quality of life and occupational productivity. Future large-scale studies are required to determine the exact NSBVAs prevalence among workers of this near vision-related occupation.

Incidence of Accommodative Excess Reported at Binocular Vision Therapy Clinic of a Tertiary Eye Care Centre Among School Children During Pandemic Lockdown

Purpose: This study aimed to find out the aspects of incidence of accommodative excess among school-going children due to excessive near work during pandemic period in relation to various parameters like indoor-outdoor activity &amp; screen time. Methodology: A retrospective study was conducted among 176 individual children between 5-17 years of age reported with ocular or non-ocular asthenopic symptoms. A detailed test had performed to check various accommodative parameters like NPA, MEM, NRA, PRA, MAF, BAF, as well as vergence parameters like NPC, NFV and PFV. One monthly follow up had been performed all these necessary binocular vision assessments tests &amp; checked the status. Results: Out of 267 patients, 51 of them had accommodative excess (28.97%). It was found that 29% children with less screen exposure &amp; 46% of children with excessive screen exposure had developed accommodative excess which became significant (fisher’s exact 0.027). The percentage of children with less indoor activity &amp; excessive indoor activity with accommodative excess were 17% &amp; 48% respectively (p value 0.052). Among all the Accommodation parameters, Accommodative facility (100%) becomes an excellent marker followed by poor NPA (33%), NRA/PRA (27%) &amp; MEM (10%). Also children with long screen &amp; indoor exposure developed with poor NRA more easily.(18% short exposure &amp; 54% long exposure). Conclusion: Children with prolonged near work &amp; indoor activities suffered more compared to children with some outdoor activities &amp; less screen exposure. Secondary convergence insufficiency can be there along with accommodative excess. A proper vision therapy and appropriate counselling about the visual hygiene &amp; environmental modifications should always be advocated to save our future shapers of this beautiful world.

Characterizing Refractive Errors, Near Accommodative and Vergence Anomalies and Symptoms in an Optometry Clinic.

Background:Refractive, accommodative and vergence parameters and associated anomalies cause symptoms of asthenopia. Patients consult eye care practitioners mainly due to symptoms they experience. To enhance targeted treatments from various anomalies, it is relevant to study symptoms with associating anomalies.Aim:To determine the frequencies of refractive error, accommodative and vergence anomalies, and their associations with symptoms in sample of Black South Africans.Method:This prospective, cross-sectional study comprised consecutive participants aged 10–40 years who attended the author’s optometry practice in a Black population in South Africa. Visual acuity, refraction, accommodative and vergence tests were performed. Anomalies were classified as either single measure or syndromes based on the number of failed clinical signs.Results:Participants (n = 254) had mean age 22.6 ± 7.22 years. Ninety-four were male (37%) and 160 were female (63%). The frequencies of syndrome anomalies were accommodative insufficiency 17 [(6.6%) 95% CI 3.9–10.5%)], accommodative infacility 32 [(12.6%)] 8.7–17.3%] and convergence insufficiency 22 [(8.6%, 5.1–12.3%)]. Frequencies of coexisting anomalies were refractive error and accommodative 150 (60.0%), refractive error and vergence anomalies 136 (54.4%) and vergence and accommodative disorders 155 (62.0%). Most patients were symptomatic (70.9%). Headache was the most frequent symptom (41.1%).Conclusion:Accommodative anomalies were more frequent than refractive error and vergence anomalies. The high frequency of anomalies suggests a high uptake of optometric services for asthenopia. Accommodative anomalies were the most symptomatic. The study highlights the need for diagnosing visual symptoms and coexisting anomalies. Establishment of validated study protocols for all accommodative and vergence anomalies is recommended.

Reliability of step vergence method for assessing fusional vergence in intermittent exotropia.

To evaluate the reliability of the step vergence method in measuring fusional vergence in subjects with intermittent exotropia. Thirty-two Chinese participants aged 7-20years with intermittent exotropia (excluding the convergence insufficiency type) were enrolled in this prospective study. At the eligibility screening, visual acuity, cover test and the Office Control Score were performed. For eligible participants at study visit 1, negative and positive fusional vergence at distance and near, eye dominance and the fusion maintenance test were performed. All eligible participants returned for study visit 2 on the same day (2-4h later), and the testing was repeated. The primary outcome measures were the intra-class correlation coefficient, coefficient of repeatability and smallest detectable change in the break and recovery points of negative and positive fusional vergence between the two study visits. The intra-class correlation coefficient for different vergence parameters ranged from 0.64 to 0.87. The coefficient of repeatability and the smallest detectable change for the distance positive fusional vergence break point were ±20.5 and 13.1 ∆, respectively. There was no significant difference in any vergence parameter between the first and second visits. The coefficient of repeatability and the smallest detectable change in all distance vergence parameters were high when compared to the mean value. The association between distance vergence parameters and the Office Control Score was significant only when including subjects who failed to fuse at the beginning of the test. The data demonstrate that measurement of fusional vergence with a prism bar has low repeatability in subjects with intermittent exotropia. In these individuals, convergence ability at distance is compromised, whereas other vergence parameters are not adversely affected. While the step vergence method is a valuable test in daily practice, caution is warranted when using it in clinical research.

Changes in vergence and accommodation parameters after smartphone use in healthy adults.

Purpose:To assess pre and post vergence and accommodation parameters after monitored reading on a smartphone device.Methods:This prospective comparative study was performed in a tertiary eye care center for a duration of 6 months (December 2017 – May 2018). A total of 47 healthy emmetropic subjects of age group ranging from 18-30 years were recruited for the study. Participants underwent an initial visual screening protocol, followed by accommodation and vergence parameters assessment. The subjects were given reading text of optotype N6 at 40cm working distance for 30 minutes in a smart phone device. Pre and post measurements were documented.Results:Among 47 subjects there were 17 male and 30 females with mean age group of 21.2+2.06 years. There was a statistically significant worsening of accommodative components (negative & positive relative accommodation, lag of accommodation). In vergence parameters, a statistically significant deterioration of negative (12.8 + 1.65 to 12.38 + 1.93 PD) and positive fusional vergence (15.48 + 1.53 to 16.08 +1.61 PD) was observed. The mean vergence facility also showed a statistically significant change in pre and post task measurements (13.51 + 1.64 to 10.71 + 1.91 cpm (cycles per minute)).Conclusion:The current study investigated that perusing text with a smart phone for 30 minutes shows significant effect on accommodative and vergence components for near task, with greater impact on vergence parameters. Drawn out exposure to digital screens at near, may bring about visual quality dysfunction.

Bedside Assessment of Vergence in Stroke Patients.

Given the widely distributed network of midbrain, pontine, cerebellar, and cortical areas involved in the neural control of vergence, one might expect various vergence deficits in stroke patients. In this article, we investigated the localizing value of bedside vergence testing with respect to different supratentorial and infratentorial infarction locations. Three hundred five stroke patients and 50 age-matched controls were examined prospectively by means of bedside tests to assess slow and fast binocular (i.e., symmetrical) as well as slow and fast monocular (i.e., asymmetrical) convergence. Infarction locations, as identified on MRI, were correlated with vergence performance using multinomial logistic regression. Vergence deteriorated with age in both stroke patients and healthy controls. Most infarction locations did not show significant associations with vergence parameters, apart from cases with parietal lobe lesions, which exhibited insufficient asymmetrical, slow and fast vergence for both the left and the right eye. Finally, patients with severe ischemic small vessel disease showed a slight but significant decrease in their fast binocular vergence performance. There is only a limited localizing value of vergence deficits in stroke. Parietal lobe infarctions are more frequently associated with insufficient binocular and monocular vergence. Midbrain strokes were too few to draw final conclusions. However the most robust factor to emerge from our data is age. Older subjects show poor slow binocular as well as slow and fast monocular vergence. Extended white matter lesions are also correlated with deficient vergence ability suggesting a role for subcortical wide range connections in maintaining an intact vergence circuitry.

P‐2.1: Effects of vergence parameters on asthenopia of patients with convergence insufficiency

Purpose: patients with convergence insufficiency always have asthenopia during near work., but we found the signs and the symptoms did not match in part of patients. In this study, we want to investigate the effect of vergence parameters on asthenopia of patients with convergence insufficiency, and find some parameters to reflect the level of asthenopia.Method: We recruited 58 patients with convergence insufficiency from Eye Hospital in Wenzhou Medical University (34 Males, 24 Female, Age 22.67±9.33 years) randomly, excluding strabismus, eye disease, trauma, anisometropia and dysfunction of accommodation. Patients were divided into three groups according to the values of symptom questionnaires using CISS, 20 patients in severe group, 19 in moderate group, and 19 in mild group, respectively. We analyzed the near phoria (PD/PN), near blur point (PB), the relationship between recover point ( PR) and level of asthenopia. We used the Sheard's rule: PSheard=2/3*PN‐1/3PB, and the difference value between phoria and recovery point: PR‐N=PR‐PN (PD/PN was exophoria using positive value). PSheard&gt;0 means it was not match the Sheard's rule. PR‐N&lt;0 means recover point is smaller than phoria.Result: PN in sever group, moderate group and mild group was −9.50Δ±4.06Δ, −4.95Δ± 3.14Δ, −13.63Δ± 5.39Δ, respectively, the difference between each two groups are significant(F=19.46, P=&lt;0.001). PSheard in sever group, moderate group and mild group was 1.90 Δ ± 3.02 Δ, −2.07 Δ ± 3.00 Δ, 3.42 Δ ± 3.06 Δ, respectively. There are statistically significant difference between moderate group and mild group, and between moderate group and severe group (P&lt;0.05). In moderate group, PSheard &lt;0, means it matched the Sheard's rule. The other two groups could not match the Sheard's rule. PR‐N in sever, moderate, mild group was (−5.85Δ±3.24Δ; 95%CI: −7.37Δ, −4.33Δ), (0.63Δ±4.62Δ; 95%CI: −1.59Δ, 2.86Δ), (−11.63Δ±6.53Δ; 95%CI: −14.78Δ, −8.49Δ), respectively. There were statistically significant difference between each two groups(F=29.13, P&lt;0.001).Conclusions: The relationship between phoria and recover point will influence the grade of asthenopia of CI patients. Asthenopia may appear when the value of recover point is close to phoria, if PR‐N is smaller than 2.86Δ (95%CI of moderate group). And intermittent suppression may appear at near tasks to alleviate symptoms, while PR‐N is smaller than ‐8.49Δ (95%CI of mild lever group).

Accommodative anomalies among schoolchildren in Abia State, Nigeria

Background: Ocular discomfort resulting from accommodative anomalies can impair reading efficiency, school performance and possibly a person’s quality of life. Aim: The aim of this study is to determine the prevalence of accommodative anomalies in schoolchildren in Abia State, Nigeria, and to assess possible associations with sample demographics such as age, gender and school level. Setting: The study was conducted in primary and secondary schools in Abia State, Nigeria. Methods: Case history questionnaires and vision tests were administered to 537 (mean age 13.0 ± 2.0 years) children randomly selected from nine schools in Abia State. The following vision parameters were measured: visual acuity, non-cycloplegic refraction, cover test, near point of convergence, fusional vergences, accommodative functions and ocular health evaluation. All accommodative and binocular function tests were performed following the subjective refraction with compensating lenses in place, if prescribed. Anomalies of interest such as accommodative insufficiency, accommodative excess and accommodative infacility were classified using the findings of accommodative and vergence parameters. Results: A total of 90 (16.8%) children had accommodative anomalies. Prevalence estimates include accommodative insufficiency (3.9%), accommodative excess (2.8%) and accommodative infacility (10.1%). There was no significant difference in the distribution of various accommodative anomalies between age group, gender or school level. Conclusion: The significant proportion (16.8%) of children with accommodative anomalies in the present study is an important finding, considering that paediatric vision screening programmes that only focus on visual acuity are unlikely to detect these critical visual anomalies. The result of this study is expected to direct the development of a common and broad vision screening strategy.

Vergence Endurance Test: A Pilot Study for a Concussion Biomarker.

The Vergence Endurance Test (VET), a quantitative and objective eye movement assessment, was utilized to differentiate control from concussed subjects. Nine symptomatic concussed (2 male; 30.8 ± 11 years) and 9 asymptomatic control (6 male; 25.1 ± 1.4 years) subjects participated in the VET. Symmetrical disparity vergence step targets were presented with and without visual distractors. A masked data analyst measured vergence latency, peak velocity, response amplitude, settling time, and the percentage of trials which contained blinks. A Binocular Precision Index (BPI) and a Binocular Accuracy Index (BAI) were calculated to quantify the changes that occur in the vergence parameters over the duration of the VET. Convergence and divergence peak velocity, divergence response amplitude, the percentage of trials that contained blinks during the transient portion of the response, and the BAI were significantly (p < 0.05) different between the concussed and the control subjects. For these parameters, the BAI and divergence response amplitude yielded the greatest accuracy, 78%, in their ability to discriminate between the groups. The VET objectively measures the change in vergence performance over time and shows promise as a method to diagnose a concussion. Future studies will determine whether the VET can be used to assess the extent of natural recovery and the effectiveness of therapeutic interventions.

Normative values for clinical measures used to classify accommodative and vergence anomalies in a sample of high school children in South Africa

AimTo determine normative values for stereoacuity, accommodative and vergence measures for high school populations. MethodsUsing a multi-stage random cluster sampling, 1211 children (481 males and 730 females) between 13 and 18 years of age, with a median age of 16 years, were selected. Visual acuity, stereoacuity and suppression, refractive errors, near point of convergence, heterophoria and fusional vergences, as well as, amplitude of accommodation, accommodative response, facility and relative accommodation were evaluated. ResultsMost data did not have a normal distribution. The range of normality for the vergence measures were: near point of convergence break, 5–10cm, recovery, 6–13cm, near lateral phoria, 2.5–6 prism dioptre (pd) (nasal), near vertical, orthophoria to 0.50pd, negative fusional vergence break, 12–23pd, recovery, 8–17pd, positive fusional vergence break, 16–35 and recovery 11–24pd. For accommodative measures, the range of normality for accommodative measures was: amplitude of accommodation, 12–18pd, accommodative response, plano to +0.75D, binocular accommodative facility, 5–12 cycles per minute (cpm), negative relative accommodation, 1.75–2.50DS, positive relative accommodation, −2.0 to −3.0DS and 17–69s arc for stereoacuity. ConclusionThis study provides norms for clinical measures which could be used to classify accommodative and vergence parameters for children aged 13–18 years in this population or beyond. Findings should be applied in the context of the measurement techniques and the associated limitations outlined in this report.

Objective Evaluation of Vergence Disorders and a Research-Based Novel Method for Vergence Rehabilitation.

PurposeWe performed video-oculography to evaluate vergence eye movement abnormalities in students diagnosed clinically with vergence disorders. We tested the efficiency of a novel rehabilitation method and evaluated its benefits with video-oculography cross-correlated with clinical tests and symptomatology.MethodsA total of 19 students (20–27 years old) underwent ophthalmologic, orthoptic examination, and a vergence test coupled with video-oculography. Eight patients were diagnosed with vergence disorders with a high symptomatology score (CISS) and performed a 5-week session of vergence rehabilitation. Vergence and rehabilitation tasks were performed with a trapezoid surface of light emitting diodes (LEDs) and adjacent buzzers (US 8851669). We used a novel Vergence double-step (Vd-s) protocol: the target stepped to a second position before the vergence movement completion. Afterward the vergence test was repeated 1 week and 1 month later.ResultsAbnormally increased intertrial variability was observed for many vergence parameters (gain, duration, and speed) for the subjects with vergence disorders. High CISS scores were correlated with variability and increased latency. After the Vd-s, variability of all parameters dropped to normal or better levels. Moreover, the convergence and divergence latency diminished significantly to levels better than normal; benefits were maintained 1 month after completion of Vd-s. CISS scores dropped to normal level, which was maintained up to 1 year.Conclusions and Translational Relevance:Intertrial variability is the major marker of vergence disorders. The Vd-s research-based method leads to normalization of vergence properties and lasting removal of symptoms. The efficiency of the method is due to the spatiotemporal parameters of repetitive trials that stimulate neural plasticity.

Characteristics of visual fatigue under the effect of 3D animation.

Visual fatigue is commonly encountered in modern life. Clinical visual fatigue characteristics caused by 2-D and 3-D animations may be different, but have not been characterized in detail. This study tried to distinguish the differential effects on visual fatigue caused by 2-D and 3-D animations. A total of 23 volunteers were subjected to accommodation and vergence assessments, followed by a 40-min video game program designed to aggravate their asthenopic symptoms. The volunteers were then assessed for accommodation and vergence parameters again and directed to watch a 5-min 3-D video program, and then assessed again for the parameters. The results support that the 3-D animations caused similar characteristics in vision fatigue parameters in some specific aspects as compared to that caused by 2-D animations. Furthermore, 3-D animations may lead to more exhaustion in both ciliary and extra-ocular muscles, and such differential effects were more evident in the high demand of near vision work. The current results indicated that an arbitrary set of indexes may be promoted in the design of 3-D display or equipments.

Reduced vergence adaptation is associated with a prolonged output of convergence accommodation in convergence insufficiency.

Convergence insufficiency (CI) is a developmental visual anomaly defined clinically by a reduced near point of convergence, a reduced capacity to view through base-out prisms (fusional convergence); coupled with asthenopic symptoms typically blur and diplopia. Experimental studies show reduced vergence parameters and tonic adaptation. Based upon current models of accommodation and vergence, we hypothesize that the reduced vergence adaptation in CI leads to excessive amounts of convergence accommodation (CA). Eleven CI participants (mean age=17.4±2.3 years) were recruited with reduced capacity to view through increasing magnitudes of base out (BO) prisms (mean fusional convergence at 40 cm=12±0.9Δ). Testing followed our previous experimental design for (n=11) binocularly normal adults. Binocular fixation of a difference of Gaussian (DoG) target (0.2 cpd) elicited CA responses during vergence adaptation to a 12Δ BO. Vergence and CA responses were obtained at 3 min intervals over a 15 min period and time course were quantified using exponential decay functions. Results were compared to previously published data on eleven binocular normals. Eight participants completed the study. CI's showed significantly reduced magnitude of vergence adaptation (CI: 2.9Δ vs. normals: 6.6Δ; p=0.01) and CA reduction (CI=0.21 D, Normals=0.55 D; p=0.03). However, the decay time constants for adaptation and CA responses were not significantly different. CA changes were not confounded by changes in tonic accommodation (Change in TA=0.01±0.2D; p=0.8). The reduced magnitude of vergence adaptation found in CI patients resulting in higher levels of CA may potentially explain their clinical findings of reduced positive fusional vergence (PFV) and the common symptom of blur.

Relation between fixation disparity and the asymmetry between convergent and divergent disparity step responses

The neural network model of Patel et al. [Patel, S. S., Jiang, B. C., & Ogmen, H. (2001). Vergence dynamics predict fixation disparity. Neural Computation, 13(7), 1495–1525] predicts that fixation disparity, the vergence error for a stationary fusion stimulus, is the result of asymmetrical dynamic properties of disparity vergence mechanisms: faster (slower) convergent than divergent responses give rise to an eso (exo) fixation disparity, i.e., over-convergence (under-convergence) in stationary fixation. This hypothesis was tested in the present study with an inter-individual approach: in 16 subjects we estimated the vergence step response to a 1 deg disparity stimulus with a subjective nonius procedure. Dichoptic nonius lines were flashed for 100 ms with various amounts of delay after the disparity step stimulus (0, 100, 200, 300, 400, and 1000 ms). Measured fixation disparity was significantly correlated with the prediction of Patel et al. (2001) based on the asymmetry in convergent and divergent vergence velocity ( r = .7, n = 14), which explained about 50% ( r 2) of the inter-individual variability in fixation disparity. All subjects with an exo fixation disparity (i.e., static under-convergence) had a weaker dynamic response for convergent than for divergent step stimuli. This confirms a relation between static vergence and asymmetric dynamic vergence, which both are idiosyncratic vergence parameters.

Binocular Coordination, Fixation Disparity, and Ocular Dominance

In a series of experiments we have quantified the spatial and temporal dynamics of binocular coordination. Tasks studied ranged from simple scanning and letter detection to complex visual processing in text reading. In all of these paradigms we found similar eye movement characteristics: in 70% to 90% of the observations, the saccade of the abducting eye is larger, relative differences being in the order of 5% to 15% of the amplitude. During the subsequent fixation the disparity is typically reduced by a convergence movement (about 1 deg s−1), which sometimes exceeds the initial saccade amplitude asymmetry. Interestingly, the relative vergence contributions of the eyes depend on saccade length. For progressive 2-letter reading saccades, the left (adducting) eye accounts for only 20% of the total movement as compared to about 70% for 14-letter saccades. Up to now our analysis was limited to relative rather than absolute estimates of fixation disparity. To overcome this restriction, we measured disparity using the psychophysical method of dichoptically presented nonius lines as well as direct infrared pupil-reflection registration of binocular vs monocular fixation. Both measures were independent of target eccentricity (within a range typical for reading) and produced similar subject rank orders (Spearman's \rho=0.75). When we studied vergence movements in a letter detection task using autostereograms with different levels of virtual depth, it became clear that spatiotemporal vergence parameters can be quite asymmetric for both eyes. This led to the question of whether unequal contributions to vergence may be related to ocular dominance. This hypothesis is currently being investigated with a new procedure that provides a reliable estimate of subjective visual direction (the ‘cyclopean eye’) under static viewing conditions.

A bilateral model integrating vergence and the vestibulo-ocular reflex.

The majority of previous modelling studies of vergence and the vestibulo-ocular reflex (VOR) have postulated arbitrary structures mainly on the basis of input-output behavioural relationships. Such models were developed following traditional schemes of oculomotor organization, based upon the notion of independence between different oculomotor subsystems. This impedes the simulation of complex binocular interactions and associated central activities. In contrast to preceding studies, the mathematical model for binocular control presented here was developed fully on physiological and anatomical grounds which reflect the organization and functional properties of known vergence and VOR premotor centres. Computer simulations show the model properly simulates the main observed characteristics in the discharge of several premotor and motor nuclei during slow vergence and the VOR in the dark. In particular, the model reproduces the activity profiles of abducens internuclear neurons, secondary vestibular cells, tonic prepositus hypoglossi neurons and ocular motoneurons during vergence and the VOR. It also simulates the activity of mesencephalic neurons whose discharge is modulated by vergence parameters alone. It is shown that given recent neurophysiological and behavioural findings, ocular reflexes cannot be properly modelled as separate independent subsystems whereas a single, unified modelling approach can produce results consistent with observed data. This study also shows how changes in the functional activity of shared pathways in a single two-sided structure produce vergence and conjugate integrators whose function relies on coupled loops across the brainstem: separate, dedicated operators are not necessary to replicate data. This provides evidence that challenges previous studies supporting the existence of separate vergence and conjugate integrators to transform velocity to position signals in the brainstem. A major implication of this study is that it questions the validity of testing conjugate and vergence systems independently, neglecting potential interactions.