Vertical vergence in convergence

PURPOSE: To report on a 7-year-old boy with a small left-over-right deviation (-VD) which increased when the head was tilted to the left shoulder and during convergence. METHODS: The squint angles were measured by the unilateral and alternate prism cover test at distance and near fixation when the head was in ortho-position and when it was tilted. RESULTS: At distance fixation (D) there was a latent deviation of-VD 3 degrees. With near fixation (N) at 0.3 m the vertical phoria increased to-VD 18 degrees. The angle of deviation was not influenced by (N) convex lenses in front of the fixating eye despite an adequate dis-accommodation. The-VD was fairly comittant in right and left gaze. At 45 degrees head tilt to the right shoulder the-VD decreased to (D) 2 degrees and (N) 12 degrees. At 45 degrees head tilt to the left shoulder the-VD increased to (D) 18 degrees and (N) 26 degrees. A dissociated vertical deviation was excluded by the dark red glass test and by the reversed fixation test. CONCLUSION: The disturbance can be explained by a, presumably congenital, supranuclear misinnervation and has to be differentiated from other types of vertical deviation.     

Effect of suppression during tropia and phoria on phoria maintenance in intermittent exotropia

Background

Treatment for intermittent exotropia X(T) aims to keep the eye in a phoric position and to maintain the phoria. However, maintenance of phoria is difficult even after treatment, and the cause is unclear. The aim of this study was to investigate the presence of suppression during tropia and/or phoria in X(T), and to determine how the suppression affected patient’s ability to maintain phoria.

Methods

Medical records of 89 children with X(T) (mean age, 9.8?±?2.7 years) were reviewed retrospectively. According to their previous treatment for X(T), the patients were divided into four groups and compared: untreated and under observation only (28 patients), surgical treatment (32 patients), orthoptic training (eight patients) and a combined treatment of surgery and orthoptic training (21 patients). Suppression during phoria was evaluated by a physiologic diplopia test, and suppression during tropia was evaluated by a convergence test or a cover test when fusion broke. Phoria maintenance was achieved if a phoric condition was maintained even when the fusion broke at both near and far. Furthermore, the Bagolini’s red filter bar was used to quantitatively assess patient’s ability to maintain phoria at near and far distances.

Results

No subject only suppressed during phoria. Patients who suppressed under both conditions could not maintain phoria. Suppression under both conditions significantly correlated with phoria maintenance and the ability to maintain phoria (P?<?0.01, Fisher’s exact probability test). All the patients with a strong ability to maintain phoria did not suppress under either condition. As compared to the surgical treatment group, the combined treatment group had a higher percentage of patients who did not suppress under either condition and could maintain the phoria. Suppression under both conditions also significantly correlated the treatment methods (P?<?0.01, Chi-square for the independence test).

Conclusions

Suppression under both tropic and phoric conditions significantly relates to the outcome of patients’ phoria maintenance and their ability to maintain a phoric position. Suppression under both conditions is an important indication of whether X(T) shifts to constant exotropia.     

Negative vergence training in humans

Two healthy subjects (male and female, ages 22 and 25 years) spent 50 h over a period of 7 consecutive weeks training the negative vergence system. The training was performed in two 45-min daily sessions, usually immediately before and after the workday. The training was exclusively negative vergence training using devices such as variable vectograms, the aperture rule, the synoptophore, and loose or bar prisms. An extensive examination of the visual systems before, after, and periodically during the training demonstrated that the negative vergences increased at distance by 5.0 delta and at near by 9.1 delta (using hand-held prisms, bar prisms, and the synoptophore). The phorias of both subjects became more exophoric or less esophoric at both distance and near by 3.6 delta (using average of changes on the cover test, von Graefe subjective phoria technique, and the synoptophore). A haploscope equipped with a coincidence optometer showed only small increases in vergence amplitude but confirmed that the lag of accommodation became more stable after the training than it was before. Other testing suggested that the negative relative accommodation, the angle of deviation at both distance and near, positive vergences, the associated phoria, and the slope of the fixation disparity curve changed significantly over the period of training. I conclude that negative vergence training can increase the negative vergence capabilities and also affect the phoria position of the eyes via feed-back into the slow vergence system.     

The effect of saccades and brief fusional stimuli on phoria adaptation.

PURPOSE: The purpose of this experiment was to determine whether brief fusional stimuli and saccades similar to those seen in the alternate cover test affect phoria disadaptation. METHODS: Three cover test conditions were performed randomly. Before each test condition, subjects fused for 2 min at an angle convergent to the subjective phoria. In one test condition, subjects viewed monocularly. In another condition, subjects alternately fixated with each eye (no binocular time). In a third condition, subjects alternately fixated with each eye, and there was a 100-ms period of binocular viewing between alternations. The ocular vergence angle was monitored using scleral search coils. RESULTS: Vergence angle was plotted against time for each condition. The area under this plot was determined using a computer program. The area reflected the rate at which ocular vergence returned to the original phoria position. The mean area for the monocular condition was 300.9, the mean area for alternate fixation with no binocular time was 300.3, and the mean area for alternate fixation with binocular time was 205.2. CONCLUSIONS: Saccadic alternations do not affect phoria adaptation. However, short periods in which binocular disparate images are viewed significantly increase the rate at which phoria adaptation declines for some subjects.     

Do dissociated or associated phoria predict the comfortable prism?

Background Dissociated and associated phoria are measures of latent strabismus under artificial viewing conditions. We examined to what extent dissociated and associated phoria predict the “comfortable prism”, i.e. the prism that appears most comfortable under natural viewing conditions. Methods For associated phoria, a configuration resembling the Mallett test was employed: both eyes were presented with a fixation cross, surrounded by fusionable objects. Nonius lines served as monocular markers. For dissociated phoria, the left eye was presented with all the Mallett elements, while only a white spot was presented to the right eye. To determine the comfortable prism, all the Mallett elements, including the Nonius lines, were shown to both eyes. In each of the three tests, the observer had to adjust a pair of counterrotating prisms. To avoid any (possibly prejudiced) influence of the experimenter, the prismatic power was recorded with a potentiometer. Twenty non-strabismic subjects with a visual acuity of ≥1.0 in each eye were examined. Results The range of the intertrial mean was for dissociated phoria from +9.3 eso to −5.9 cm/m exo, for associated phoria from +11.2 eso to −3.3 cm/m exo, and for the comfortable prism from +4.8 eso to −4.1 cm/m exo (cm/m = prism dioptre). In most observers, the phoria parameters differed greatly from the comfortable prism. On average, the phoria values were shifted about 2 cm/m towards the eso direction in relation to the comfortable prism (associated phoria not less than dissociated phoria). Conclusions The deviation of both, dissociated and associated phoria, from the comfortable prism suggests that the abnormal viewing conditions under which the phoria parameters are determined induce artefacts. Accordingly, the findings cast doubt on current textbook recommendations to use dissociated or associated phoria as a basis for therapeutic prisms. Rather, patients should be allowed to determine their comfortable prism under natural viewing conditions.     

The cyclopean eye in vision: the new and old data continue to hit you right between the eyes

We argue against recent claims by Erkelens and van Ee (Vision Res., in press) and by Erkelens (Vision Res. 40 (2000) 2411) that "the concept of the cyclopean eye is em leader always irrelevant as far as vision is concerned" (p. 1157) [corrected] and that "perceived direction during monocular viewing is based on the signals of the viewing eye only" (p. 2411), respectively. In Experiment 1, we presented a pair of small lights on a visual axis and measured the absolute visual direction of the near light with reference to different parts of the face. The near light appeared in front of the bridge of the nose or very near it, contrary to what was expected from Erkelens and van Ee's claim that monocular stimuli are seen in their correct locations. In Experiment 2, we replicated Erkelens' experiments with measurements of phoria and analyses of eye movements. The results confirmed his finding that the cyclopean illusion occurred rarely in the monocular condition, but our phoria and eye movement data provided the basis for a very different interpretation. Our data show that the oculomotor signal in his particular monocular condition was considerably weaker than in his binocular condition; therefore, the rarity of the monocular cyclopean illusion is not surprising. Moreover, since both claims above are based on an over-generalization of the results of Erkelens' study, neither claim is persuasive.     

The Prevalence of Strabismus,Heterophorias, and Their Associated Factors in Underserved Rural Areas of Iran

Purpose: To determine the prevalence of different types of strabismus and heterophoria and their associated factors in underserved rural areas of Iran. Methods: Two rural areas in the north and southwest of Iran were randomly sampled using multistage cluster sampling. Individuals aged over 1 year, totaling 3851, were invited to participate in the study. In addition to collecting demographic data, optometric examinations were performed for each participant. Unilateral and alternating cover tests were performed at far (6 m) and near (40 cm) to detect tropia and phoria, respectively, and the magnitude of the deviation was measured using alternating cover test and prism bar. Tropia was defined as the presence of constant unilateral or alternating horizontal strabismus (exotropia or esotropia) at either far or near fixation distances. Far and near exophoria were defined as more than 3 prism diopters (PD) and 9 PD of exophoria at far and near fixation distances, respectively. Far esophoria was defined as more than 1 PD of esophoria at far. Near esophoria was defined as any amount of esophoria at near. Results: Of 3851 selected individuals, 3314 (86.5%) participated in the study. After excluding those who met the exclusion criteria or had missing data, eventually there were 3248 subjects available for this analysis. The prevalence of tropia and phoria was 4.3% (95% CI: 3.28-5.33) and 28.37% (95% CI: 19.12 -37.61), respectively. The prevalence of exotropia and esotropia was 3.87% (95% CI: 2.77-4.98) and 0.43% (95% CI: 0.19-0.67), respectively. The prevalence of near exophoria, far exophoria, near esophoria, and far esophoria was 25.96 (95% CI: 16.71-35.2), 4.82% (95% CI: 0.23-9.41), 0.47% (95% CI: 0.2-0.75), and 0.34% (95% CI: 0.06-0.63), respectively. In this study, the prevalence of tropia had no significant association with age, sex, living place, and refractive errors (all P values >0.05), while the prevalence of phoria increased significantly with increasing age (P<0.001) and was also significantly associated with living place (P<0.001). Conclusion: The prevalence of tropia and phoria was high in our study. Moreover, the prevalence of exotropia and exophoria was higher than esotropia and esophoria. The prevalence of phoria was significantly higher in older age groups and in the participants living in the southern villages.     

The effect of prolonged monocular occlusion on latent nystagmus in the treatment of amblyopia

We recorded eye movements in 5 patients with latent nystagmus (LN) before and after 2 days of occlusion of the better eye. The slow-phase speed of the nystagmus (SPS) was in general, before occlusion, lower when the better eye fixated but, after occlusion, lower when the worse eye fixated. However, the sum of SPS during right fixation and SPS during left fixation remained constant. Oscillopsia complaints gradually disappeared during the period of occlusion. These findings indicate that the difference between the SPS during fixation with the right eye and the SPS during fixation with the left eye in LN patients is caused by a compensatory drift that decreases LN during fixation with the better eye but increases LN during fixation with the worse eye. During occlusion, this compensatory drift changes its direction and magnitude slowly over days. Hence, occlusion of the better eye in children with amblyopia and LN should be prescribed only in days per week, not in hours per day.     

Latency of saccades, vergence, and combined movements in children with early onset convergent or divergent strabismus

The goal of this study was to examine latency of horizontal eye movements in the natural space (saccades, vergence, and combined saccade-vergence movements) in children with early onset convergent or divergent strabismus. Ten children were tested (8-11 years old): three with divergent strabismus, seven with convergent strabismus. A paradigm was used to elicit pure lateral saccades at far and near distance, pure vergence (convergence and divergence) and saccades combined with vergence movements. Horizontal eye movements from both eyes were recorded simultaneously by a photoelectric device (Oculometer, Dr. Bouis). The latency of saccades (at far and near distance), of vergence (convergence and divergence), and of combined movements greatly varies among subjects and has tendency to be longer than that observed in normal children of matched age, however, these differences reach significance in only a few cases. Children with divergent strabismus and residual gross binocular vision show abnormally longer vergence latencies than children with convergent strabismus without binocular vision. The initiation of combined movements does not show a dominant pattern, such as preceding vergence, as is found in normal children. Finally, strabismus surgery has no major effect on latencies. We conclude that there is no overall deficiency in latencies of eye movements in 3D space in children with early onset strabismus. Most likely, monocular visual input can be efficient as normal binocular vision for vergence movements. In a few subjects with divergent strabismus and fragile, intermittent binocular vision, latencies can be abnormally long, just because of the fragile binocular input and/or attention effort needs to use it. The absence of a pattern of initiation similar to normal children could be due to attention and fixation capabilities.     

Changes in cyclotorsion and vertical eye alignment during prolonged monocular occlusion

When binocular vision is prevented with monocular occlusion, the two eyes assume a position of rest related to the combination of underlying tonic innervation of the oculomotor system, cross-coupled accommodative-vergence input and vergence responses to perceptual cues for spatial location relative to the head. When the latter two are controlled, the covered eye has been shown in the majority of subjects to turn outward (exophoria) and upward (hyperphoria) after prolonged monocular occlusion. The present study investigates the change in torsional eye alignment and its relation to vertical eye alignment after eight hours of monocular occlusion. The results revealed an excyclophoria during occlusion in four out of five subjects. The patched eye also became elevated in two subjects and depressed in two others. Thus, during prolonged monocular occlusion, the relative directions of cyclophoria and vertical phoria appear to be independent. In addition, there were non-concomitant changes in vertical phoria with horizontal gaze, toward a state where the adducted eye was elevated relative to the abducted eye. Simulations with Orbit(TM) suggest that these non-concomitant changes in vertical phoria with a concomitant excyclophoria may be based upon orbital mechanics. Excyclophoria appears to be the baseline state of binocular alignment.     

Quantification of heterophoria and phoria adaptation using an automated objective system compared to clinical methods

Purpose:  To develop a system with which to quantify objectively the heterophoria to be measured throughout eye movement experiments. This study compared precision, resolution and accuracy characteristics of a limbus eye movement tracking system to the alternate cover test and the Maddox rod.
Methods:  Precision testing was performed using each technique with binocularly-normal subjects at different test sessions. Resolution was measured and compared between the three systems. The heterophoria or phoria was measured from 40 cm/16 inches objectively with the limbus eye movement system and the Maddox rod during a sustained convergence-induced phoria adaptation experiment using physical and haploscope visual targets to compare accuracy.
Results:  Precision testing quantified that the responses objectively recorded using the limbus tracking system exhibited similar standard deviations to the Maddox rod and the alternate cover test techniques. The limbus tracking method has the ability to quantify the response decay to phoria and has better resolution when compared to the clinical methods. When physical targets were used to induce phoria adaptation, the correlation was significant between the limbus eye movement tracking system and the Maddox rod after near and far adaptation, where the Maddox rod measurements were slightly more esophoric compared to those measured with the limbus tracking system.
Conclusion:  This objective limbus tracking system offers a better means by which to study the phoria and its adaptation throughout an eye movement experiment. The broader aim of this research is to establish a tool that will further the basic science of oculomotor control and binocular dysfunctions.     

The contribution of microsaccades and drifts in the maintenance of binocular steady fixation

Background The eye performs three types of eye movements during fixation: fast microsaccades are interrupted by slow drift movements, and tremor is superimposed on the drifts. The contribution of the microsaccades and drifts in maintaining fixation has been discussed since the late 1950s. Initially, microsaccades were thought to correct the misalignment from the optimal fixation locus induced by the drift movements, a theory still postulated in more recent work. The present study aimed to uncover to what extent each fixation movement contributes to maintain steady binocular fixation.Method Binocular fixation during a 40-s fixation task was recorded using an infrared recording technique for ten normal test persons. Start and end point of each microsaccade and drift were superimposed on a fixation map, and the distance to the preferred retinal location of fixation (PRL) was measured.Results It was found that 32.6% of the microsaccades corrected the previous drift movement towards the PRL, whereas 53.1% of the drifts corrected the endpoint of the previous microsaccade towards the PRL. The overall mean post-microsaccadic and mean post-drift distance to the PRL for the ten normal test persons were 0.46° and 0.41°, respectively; the difference was not statistically significant. Interindividually, the mean post-microsaccadic distance to the PRL ranged between 0.21° and 0.91° and the mean post-drift distance to the PRL ranged between 0.20° and 0.72°.Conclusion Neither the endpoints of the microsaccades nor the drifts bring the visual line to coincide with the centre of the PRL. Consequently, it must be the eye movements performed during the drifts (“slow control”) that keep the visual line in the centre of the foveola.     

视疲劳患者眼动参数的研究

目的:研究不同因素所致视疲劳患者眼动参数的异常,初步探讨刺激性AC/A与视疲劳之间的关系。方法:序贯选取江苏省中医院眼科门诊视疲劳患者84例,根据有无屈光因素分为A(屈光性)、B(非屈光性)两组各42例,进行6m,40cm,40cm+1D隐斜量的测量,计算各患者的计算性AC/A和梯度性AC/A,分别与Morgan正常值进行比较,并进行两两比较。结果:A组,B组的远距离隐斜均明显大于此Morgan值(P<0.05),同样A组,B组的近距离40cm处隐斜量也大于Morgan值(P<0.05),而A组,B组的梯度性AC/A值均要显著低于4/1(P<0.05),但A,B两组之间远、近距隐斜及计算性AC/A、梯度性AC/A等参数的差异无统计学意义(P>0.05)。结论:视疲劳患者较正常人群在远近距离具有更高的外隐斜,而具有较低的梯度性AC/A值,刺激性AC/A值尚不能作为不同类型视疲劳的分类指标。     

Proximal and accommodative convergence and age

The authors have estimated the phoria for distant and near fixation in two groups of subjects (mean age 27.5 ± 4.4 and 59.2 ± 8.2 years). Different accommodative stimuli were induced by adding minus lenses for distant fixation and plus lenses for near fixation. Statistical analysis of the experimental data indicates that, for distant fixation, the value of phoria per unit of accommodative stimulus is significantly lower in presbyopic than in nonpresbyopic subjects. Also, during near fixation, the accommodative convergence (AC/A ratio) is more reliable in the presbyopic subjects when the accommodative stimulus is progressively reduced. This varying behavior indicates in presbyopic subjects that proximal convergence is of greater relative importance in the determination of the fusion-free position. In nonpresbyopic subjects, accommodative convergence is the more important component.     

The spatial impact of visual distractors on saccade latency

Remote transient changes in the environment, such as the onset of visual distractors, impact on the execution of target directed saccadic eye movements. Studies that have examined the latency of the saccade response have shown conflicting results. When there was an element of target selection, saccade latency increased as the distance between distractor and target increased. In contrast, when target selection is minimized by restricting the target to appear on one axis position, latency has been found to be slowest when the distractor is shown at fixation and reduces as it moves away from this position, rather than from the target. Here we report four experiments examining saccade latency as target and distractor positions are varied. We find support for both a dependence of saccade latency on distractor distance from target and from fixation: saccade latency was longer when distractor is shown close to fixation and even longer still when shown in an opposite location (180°) to the target. We suggest that this is due to inhibitory interactions between the distractor, fixation and the target interfering with fixation disengagement and target selection.     

近视青少年配戴角膜塑形镜前后调节参数变化

目的：:观察近视青少年配戴角膜塑形镜前后调节参数及隐斜度的变化,探讨角膜塑形镜延缓近视进展的机制及近视发病病因。方法：:回顾性研究。收集2011年1月至2019年3月在中国医科大学附属第一医院眼科门诊进行角膜塑形镜配戴前后调节参数资料完整的患者51例（102眼）,比较角膜塑形镜配戴前、配戴后1、12个月或以上的正负相对...     

Ocular translation and cyclotorsion due to changes in fixation distance

When an observer, looking straight ahead with one eye occluded, aligns a nearby target with a more distant one, and then alternately fixates upon the near and far targets, the iris and pupil of the viewing eye shift back and forth laterally by as much as 200 μm, and produce fixation-dependent misalignment of the targets. These changes in steady-state eye position—usually temporal during near fixation, nasal during far fixation—apparently involve translation of the optic axis, and are accompanied by rotation of the iris around the optic axis (up to 2° cyclotorsion). Unbalanced relaxation of both the superior and inferior oblique muscles during near fixation would be adequate to account for the observed eye movements.     

Errors of binocular fixation are common in normal subjects during natural conditions.

PURPOSE: To investigate the accuracy of fixation after symmetrical vergence eye movements along the midline during natural full-field viewing conditions using a video method of eye position measurement. METHODS: The accuracy of binocular fixation after symmetrical vergence eye movements during natural conditions was measured on 29 young adults using a precise head-mounted video eye movement measuring system. All subjects had normal binocular vision and good visual acuity. Measurements were taken for both near and far fixation after vergence changes of 5 degrees, 10 degrees, and 15 degrees using three rates of change, approximately 0.25, 0.5, and 1 Hz. RESULTS: The amplitude of the vergence movement tended to be hypometric, resulting in underconvergence for near fixations, and overconvergence for distance fixation. For far fixations, most errors (82%) were from -120 to +120 min arc, and for near, most errors (85%) were from -30 to +120 min arc. For far fixations, there was a significant effect of the size of vergence change (F1,28 = 61.8; p < 0.001), the rate of change (F1,28 = 7.08; p = 0.013), and the interaction between these two factors (F1,28 = 7.17; p = 0.012) on resulting errors, with the eyes showing greater overconvergence on the target for the larger and faster fixation changes. For near fixations, there was a significant effect (F1,28 = 15.9; p < 0.001) for the angle of change with the faster vergence changes producing relatively more convergence, thus reducing the mean vergence error. No subject reported diplopia during any conditions despite our measures showing vergence errors of up to 5 degrees. CONCLUSIONS: Vergence errors of up to +/-2 degrees, without diplopia, were common in subjects with normal binocular single vision. Errors of 5 degrees were rare but present. In all, the largest number of errors occurred as a failure of divergence for far fixations, consistent with previous studies that have suggested differences in the neural control of pathways for convergence and divergence, or possibly caused by differences in the anatomical properties of the medial and lateral rectus muscles and their associated fascia. The absence of diplopia during the period of fixation could only be partly associated with the visual suppression associated with vergence eye movements that has been reported by others because it was still present after the vergence movement was completed. The natural viewing conditions in this study that included a full visual field and multiple disparities may have contributed to this effect.     

The latency of saccades,vergence, and combined eye movements in children and in adults

PURPOSE: To examine the latency of eye movements in three-dimensional space (saccade, vergence, and combined saccade-vergence) in children and adults. METHODS: Fifteen normal children (4.5-12 years of age) and 15 normal adults (22-44 years of age) were tested. A standard paradigm was used to elicit pure lateral saccades at far and close viewing distance, pure vergence (convergence and divergence), and saccade combined with vergence movements. Horizontal eye movements from both eyes were recorded simultaneously by the oculometer, a photoelectric device. RESULTS: The mean latency in saccades, vergence, and combined eye movements was longer in children than in adults; the variability of such latency values was also larger in children. There was a progressive decrease with age in mean latency. All latencies approached or reached adult levels at approximately 10 to 12 years of age. Latency of saccades at close viewing distance was shorter than that at far in both adults and children. Convergence latency was longer than divergence latency in adults and most of the children. Latency of components of combined movements was longer than that of corresponding pure movements. Children initiated combined movements by triggering the vergence component first, whereas adults did not show a dominant pattern. The percentage of synchronous start of the two components was significantly higher in adults. CONCLUSIONS: Saccade- and vergence-triggering mechanisms are distinct and mature progressively and in parallel with age. The capacity for synchronization of the two components of combined eye movements develops more slowly and remains below adult level, even at the age of 12 years.