Objective vs subjective measurements of dark vergence

Dark vergence is a resting position of vergence (tonic vergence), measured in a dark visual field to eliminate fusional, accommodative, and proximal stimuli. The vergence resting position is relevant for measures of phoria and fixation disparity. Dark vergence differs reliably among subjects: the average subject converges at a viewing distance of about 1 m, while the inter-individual range is from infinity to about 40 cm. In previous research, dark vergence was measured subjectively, i.e. observers adjusted the horizontal offset of dichoptically presented nonius targets to perceived alignment. Results of such subjective vergence tests do not necessarily agree with those of the objective measurements of eye position with eye trackers. Therefore, we made simultaneous subjective and objective measurements of dark vergence and found similar results with both methods in repeated tests in two sessions. Thus, the nonius test is sufficient for a subjective estimation of dark vergence.     

Lateral phoria at distance: contributions of accommodation.

Lateral heterophoria at distance often is cited as an index of tonic vergence, yet recent research has shown that vergence in darkness is more convergent than most measures of phoria. Two experiments were conducted to investigate the possible role of accommodation in this discrepancy. Experiment I compared measures of distance phoria of 19 young adults with measures of vergence and accommodation of the same subjects. Dark vergence was found to be correlated with but significantly more convergent than distance phoria. Hierarchical regression analysis showed that individual measures of phoria were related to subjects' negative accommodation (from the dark focus to the distant target) as well as to dark vergence. Experiment II tested 13 young adults to evaluate three simple models of the influence of accommodation on distance phoria. Distance phoria was predicted best by a model that included the subjects' negative accommodation, accommodative vergence, and dark vergence. These findings support the hypothesis that distance phoria is influenced by accommodation for the fixation target. Dark vergence is a simpler index of tonic vergence.     

Near work, visual fatigue, and variations of oculomotor tonus

One hour of near work produced adaptive changes in the resting states of accommodation and binocular vergence, which may be responsible for different aspects of visual fatigue. Two groups of college students read from either hard copy or a video display terminal (VDT). Immediately before and after reading, the subjects' distance acuity, dark vergence, dark focus, and accommodative response accuracy for a monocular stimulus were measured. After reading, subjects also rated their subjective feelings of visual fatigue. Reading produced significant changes in both accommodation and vergence, which did not differ for the hard copy and VDT modes of presentation. Dark focus and accommodative responses shifted in the myopic direction by an average of 0.6 D and at least 0.35 D, respectively; dark vergence distance shifted in the convergent direction by an average of 11.4 cm. These changes were greatest for subjects whose initial resting postures corresponded to a far distance. After reading, one third of the subjects exhibited lower visual acuity at distance. This change was significantly correlated with changes in dark focus (r = 0.35) but not with changes in dark vergence (r = -0.12). In contrast, subjective ratings of visual fatigue were not correlated with changes in the dark focus (rho = 0.13), but they were significantly correlated with changes in dark vergence (rho = 0.58). These findings indicate that ordinary near work can produce significant changes in the resting states of accommodation and vergence, whose magnitude depend on the subject's initial oculomotor resting tonus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in accommodative and vergence responses when viewing through near addition lenses.

It has been proposed that near addition lenses reduce the lag of accommodation and may slow myopia progression. In this study, we investigated the differences in accommodative response and near phoria in subjects with and without near addition lenses. Fourteen subjects (7 emmetropic and 7 myopic) participated in the study. Monocular and binocular accommodative responses to a target at 40 cm were measured with and without +2.00 diopter (D) lenses using a Canon R-1 optometer (Canon Europe N.V., Amsterdam, The Netherlands). Near dissociated phoria was measured using 3 testing methods: Maddox rod, cover test, and Von Graefe technique. The differences in accommodative response and near phoria between the 2 viewing conditions with and without the near addition lens were significant (P < 0.0001). No significant differences were revealed in accommodative response and near phoria between refractive error groups. The average accommodative responses of all subjects were 2.03 +/- 0.06 (SE) D (without +2.00 D lens) and 0.60 +/- 0.07 D (with +2.00 D lens) under monocular viewing conditions and 2.11 +/- 0.06 D (without +2.00 D lenses) and 0.77 +/- 0.07 D (with +2.00 D lenses) under binocular viewing conditions. The average near phorias of all subjects were -2.08 +/- 0.69 prism diopters (PD, without +2.00 D lenses) and -7.90 +/- 0.68 PD (with +2.00 D lenses). We discuss the effect of near addition lenses on the defocus of the retinal image, accommodative response, and near phoria. In addition, we propose that the observed difference between binocular and monocular accommodation, when viewing through near addition lenses, is caused by an increased vergence accommodation.     

Accommodative vergence is driven by the phasic component of the accommodative controller

Conflicting opinions exist as to whether the phasic (reflex) component alone or both the phasic and tonic (adaptive) components of the accommodation and vergence systems drive accommodative vergence and vergence accommodation crosslinks. In this study the dissociated phoria to a 2 D target was measured before and after accommodative adaptation to discriminate the two possibilities. Results showed a significant difference in the dark-focus of accommodation pre- and post- near-vision task, indicating that accommodative adaptation had occurred. No significant change occurred in dark-vergence or in the accommodative response to the 2 D target. However, a significant decrease was found in the dissociated phoria presumably because of decreased phasic accommodation and its stimulation of accommodative vergence after the adaptation. This result is consistent with a model in which the accommodative vergence crosslink is driven by phasic accommodation only.     

Adaptation of the resting states of accommodation

Without an adequate stimulus to focus, the eyes accommodate for a "resting state" that averages about 1.6 diopter (D) (62 cm). The resting state can be made more "myopic" by adaptation to a near accommodative or vergence stimulus. In the present experiment, 21 subjects were tested for adaptation to both types of stimuli. Resting state was measured before and after adaptation in the dark (dark focus) and in the presence of an illuminated empty field. The authors found that preadaptation resting states were more myopic in the light field than in the dark test conditions; accommodative and vergence stimuli produce myopic shifts in dark and light field resting states; and a subset of the subjects show much larger aftereffects in the light field conditions. These subjects also show the largest difference between preadaptation dark- and light-field measures. Differences between dark- and light-field measures of resting state in these and other experiments may require a re-examination of the hypothesis that there is a single resting state for each subject.     

Effects of persistent viewing of 3 D TV on human visual function

目的:研究持续观看3D电视对视功能的影响,以及探索不同视功能参数之间的关系.方法:本研究录入19位受试者,每位受试者观看90分钟的2 D电视或90分钟的3 D电视,两部电视观看间隔时间为一周,顺序随机.每次观看电视前及观看后15 min内都做相关视功能检查,检查包括以下四种视功能参数:调节功能(调节反应、调节微波动、调节灵敏度、正相对调节、负相对调节、AC/A),融像性聚散功能(远距负融像性聚散功能、远距正融像性聚散功能、近距负融像性聚散功能、近距正融像性聚散功能),隐斜量(远距隐斜量、近距隐斜量),立体视.结果:无论是与观看前的基础值比较,还是与观看2D电视后的数据比较,观看3D电视后近距离调节反应增加,汇聚减少,远距隐斜量向外隐斜改变(P<0.05).结论:持续观看3D电视会对人们的视功能有一定影响,主要是打破了调节和聚散系统的平衡.厂家和公众应该引起重视,将调节功能、融像性聚散功能、隐斜量等参数作为评估3D电视安全性的指标.     

Clinical method for measuring adaptation of tonic accommodation and vergence accommodation

Previous work indicates that tonic adapters of accommodation and vergence have indirect effects on accommodative vergence and vergence accommodation and that these crosslink responses interact with one another to produce clear and single vision. Clinical measurements of tonic accommodation, tonic vergence, vergence accommodation (CA/C ratio), and accommodative vergence (AC/A ratio) are therefore of value in determining possible binocular vision abnormalities. Currently, clinical methods are only available for measuring the tonic vergence (phoria) and the AC/A ratio. We have determined a fast and accurate method for evaluating both tonic accommodation (accommodative phoria) and vergence accommodation (CA/C ratio). In both procedures, the loop of accommodation was opened with a 0.2 cpd difference of Gaussian (DOG) target. This target was found to be incapable of stimulating any reflex accommodation, yet it is localized in space and can easily stimulate binocular fixation. Our results indicate that opening the loop of accommodation with a pinhole Maxwellian view or with a 0.2 cpd DOG produces similar response values for tonic accommodative aftereffects and vergence accommodation responses. The decay rate of tonic accommodative aftereffects as well as changes in the resting focus of accommodation caused by vergence accommodation (CA/C) can be measured clinically using dynamic retinoscopy in combination with the DOG target. Clinical values of the CA/C ratio obtained with these techniques were highly correlated (r = 0.92) with laboratory measures obtained with an objective infrared optometer and eye trac monitor.     

Fixation disparity at different viewing distances and the preferred viewing distance in a laboratory near-vision task

Previous research has shown that subjects with normal binocular vision differ reliably in the extent to which their fixation disparity changes in the exo direction when the viewing distance is shortened from 100 to 20 cm. Since fixation disparity can lead to asthenopic complaints, the present study investigates whether an exo fixation disparity induced by proximity may cause subjects to move to a longer viewing distance during a near-vision task in order to reduce exo fixation disparity. In two optometric sessions, fixation disparity and accommodation were tested at 60, 40, and 30 cm viewing distance. In a further session, subjects were required to begin a one-hour near-vision task at about 40 cm viewing distance, at which the text characters subtended a comfortable visual angle of 21 min arc. Later, the subjects were free to adopt any viewing distance. In the initial phase of the task, subjects moved back from the screen to a greater or lesser extent that was correlated with the amount of proximal exo fixation disparity: the more a subject's fixation disparity changed to exo when the viewing distance had been shortened from 60 to 30 cm the more he or she moved to longer viewing distances in the course of the near-vision task. Further, the more distant the resting position of vergence (dark vergence), the more visual complaints the subjects indicated after the task relative to before.     

Vergence adaptation in subjects with convergence excess

The main purpose of this study was to evaluate the vergence adaptive ability in subjects diagnosed with convergence excess (CE) phoria (ie, subjects with an esophoric shift from distance to near but without an intermittent tropia at near). Vergence adaptation was measured at far and near with both base-in and base-out prisms using a "flashed" Maddox rod technique in 20 control subjects and 16 subjects with CE. In addition, accommodative adaptation and the stimulus AC/A and CA/C cross-links were measured. The AC/A and CA/C ratios were found to be high and low, respectively, and accommodative adaptation was found to be reduced in CE subjects as compared with the controls (P<0.005), all as predicted by the present theory. However, vergence adaptive ability was found to be reduced in the CE subjects at both distance and near and in response to both base-in and base-out prisms (P=0.002). This finding is not in accordance with and is difficult to reconcile with the present theory of CE.     

Oculomotor function after virtual reality use differentiates symptomatic from asymptomatic individuals.

Some individuals who use virtual reality (VR) head-mounted displays (HMD) have adverse visual symptoms. PURPOSE: We measured oculomotor functions of symptomatic (n = 10) and asymptomatic (n = 10) individuals to determine if there were fundamental oculomotor performance differences. METHOD: Before and after 20 min of biocular VR-HMD use, we measured: phorias, fixation disparity, gradient accommodative convergence to accommodation ratio (AC/A), stereopsis, and nearpoint of convergence. RESULTS: We observed an exophoric shift in the nearpoint phoria of almost all subjects, whereas the farpoint phoria showed no trend. Interestingly, we observed that the phoric shift at far and near was highly correlated for the asymptomatic subjects but not for the symptomatic subjects. In addition, the (stimulus) AC/A ratio of symptomatic subjects was reduced after a period of VR-HMD use, whereas asymptomatic subjects' AC/A ratio was not reduced. CONCLUSION: The oculomotor changes among the symptomatic subjects (increased exophoria at near and reduced AC/A) appears consistent with a reduced accommodative response. In contrast, the asymptomatic subjects show changes (correlated change in phorias) which seem most consistent with adaptation in the tonic component of vergence and/or accommodation.     

The horizontal dark oculomotor rest position

Background

This study sought to investigate whether eye dominance and age are related to the stimulus-free oculomotor resting state described via the dark disconjugate position (near or far), the dark conjugate position (left to right), and the near dissociated phoria.

Methods

Nineteen non-presbyopes and 25 presbyopes with normal binocular vision participated in two identical sessions. The left-eye and the right-eye positions were recorded using a video-based infrared eye tracker while the subjects were in total darkness. Dark disconjugate responses and dark conjugate responses were calculated by computing the difference and the average of the left-eye and the right-eye response, respectively. The right-eye decaying to the phoria level was recorded for 15 s.

Results

A one-way ANOVA assessed statistical differences in dark conjugate and dark disconjugate positions, comparing 1) the right-eye and the left-eye sensory and/or motor dominant groups and 2) the non-presbyope and presbyope groups. The test-retests of the dark disconjugate position, the dark conjugate position and the near dissociated heterophoria were high between sessions (r?>?0.85; p?<?0.00001). For non-presbyopes the right-eye (left-eye) motor and sensory dominant subjects showed a rightward (leftward) dark conjugate position (p?<?0.01). The dark disconjugate position was receded in presbyopes compared to non-presbyopes (p?<?0.0001).

Conclusion

The data support that the left-eye, or the right-eye, motor and sensory dominance predicts the direction of the dark conjugate position. Future studies could investigate the underlying neural substrates that may, in part, contribute to the resting state of the oculomotor system in a stimulus-free environment. Knowledge of the brain-behavior governing visual-field preference has implications for understanding the natural aging process of the visual system.     

Gain changes in the accommodative convergence cross-link

The output of the accommodative vergence cross-link, the AC/A ratio, was measured before and after subjects viewed through a telestereoscope which increased their interocular separation (pd) approximately fourfold. The initial paradigm was designed to force an increase in the AC/A ratio in that subjects alternately fixated targets set at differing distances. In this paradigm, the varying vergence demands could not be met by a constant increase in phasic or tonic vergence responses. In a second paradigm, only one target was viewed at a fixed position from the eye. Now the constant vergence demand could be met by a set increase in phasic and/or tonic vergence. A significant increase in the AC/A ratio was found in the two-target study but not in the single-target study. Increases in the accommodative vergence gain did show considerable individual differences. Tonic vergence changes were interpolated from the data. This parameter increased significantly in both cases but more so when two targets were alternately fixated. This finding is consistent with current near triad models which predict that accommodative convergence input acts along with disparity vergence input to increase tonic levels of convergence. When AC/A ratios were calculated using clinical measures of the phoria at 6 and 0.4 m, increases were found in the two-target study and to a lesser degree in the single-target study. This clinical method of AC/A measure appears to have been confounded by changes in tonic vergence.     

Evaluation of a new criterion of binocularity

The purpose of this study was to assess a new criterion for binocular comfort analogous to the classical Sheard's criterion. Instead of equating the fusional demand with the monocular phoria as is done when Sheard's criterion is applied, the new criterion uses a calculated binocular fusional demand. The binocular demand was derived using a clinical measurement of the convergence accommodation per convergence (CA/C) ratio. Sheard's criterion was also evaluated. Other commonly used indicators of binocularity (heterophoria, vergences, accommodative amplitude, facility and response, fixation disparity, and the associated phoria) were measured. One hundred subjects (52 males, 48 females; mean age 26 years) were classified as either symptomatic or asymptomatic by an interviewing clinician. The examining clinician was intentionally masked as to the classification of the subjects. We hypothesized that the new criterion would best discriminate between the two groups of patients inasmuch as it is based on currently accepted dual-interaction models of accommodation and vergence. Our analysis confirmed that the CA/C ratio corresponded closely to those published previously (mean = 0.06 D/delta). Significant differences (p less than 0.05) were determined between the symptomatic and asymptomatic groups for gender, near phoria through a +2.00 D add, accommodative amplitude, positive vergences at near, and both the classical Sheard's and the new criterion. The new criterion was the best discriminator between the groups, identifying 72% correctly, an improvement of 6% over the classical Sheard's. However, various stepwise discriminant analysis procedures consistently failed to demonstrate that the calculated binocular fusional demand or the new criterion was superior to the near phoria or the classical Sheard's value. These results suggest potential clinical utility for new procedures based on recently described models of accommodation and vergence, but further development appears necessary.     

Fixation disparity and accommodation for stimuli closer and more distant than oculomotor tonic positions

Both the vergence and the accommodative system have individual tonic positions (also referred to as dark vergence and dark focus, respectively) where the static response may be expected to be most accurate. This was confirmed by measuring fixation disparity with nonius lines and accommodation with an autorefractometer for foveal stimuli at viewing distances of 460, 100, 60, 40, and 30 cm. Multiple regression analysis was used at each viewing distance to predict fixation disparity from dark vergence, dark focus, accommodative gain and accommodative convergence: these accommodative measures had little effect on the inter-individual variability of near fixation disparity nor on the linear slope of fixation disparity as a function viewing distance.     

Relation between distance heterophoria and tonic vergence

The distance heterophoria was measured using both the von Graefe and Maddox rod clinic procedures under standard reduced illumination conditions and in total darkness. Tonic vergence was measured in total darkness. The phoria measures were more divergent than tonic vergence, with the von Graefe being the more divergent of the two measures. The difference between the phoria and tonic vergence position is believed to be due to accommodative divergence derived from the decrease of accommodation, relative to the tonic level, required to focus the distance phoria target. Equations were derived to predict tonic vergence from the phoria value.     

Fixation disparity,accommodation, dark vergence and dark focus during inclined gaze*

We investigated several oculomotor functions at different angles of vertical inclination of the gaze direction from 15 deg upwards to 45 deg downwards. The mean accommodative resting state (measured in a dark visual field) increased when the eyes or the head were declined from 0 to 45 deg. Fixation disparity (the vergence error in minutes of arc relative to the principle visual directions) became more eso when a fusion target at a viewing distance of 40 cm was lowered: declining the gaze by 45 deg changed mean fixation disparity by 1.8 min arc with eye inclination (keeping the head upright), and by 0.9 min arc with head inclination (with eye position unchanged relative to the head). When the eyes were lowered, the individual rate of eso change in fixation disparity was correlated with the amount of the subjects’ near shifts in the resting position of vergence, measured in darkness. Significant test-retest correlations between repeated measurements showed that the effects of eye inclination on vergence varied in a reproducible way among individuals with good binocular vision.     

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

Background  

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).     

显微镜工作者近视和双眼视力障碍的患病率分析

目的：测定临床显微镜工作者屈光和双眼视力状态。

方法：这是一项以医院为基础的观察性和横断面研究。研究包括103位在特里布文教学医院工作的显微镜工作者。受试者均行全面的眼部检查,包括静态检影,动态检影和视轴评估。收集受试者显微镜下视觉状态信息。

结果：该组显微镜工作者屈光不正患病率为69.90%。68.93%受试者近视,平均近视误差为-1.58±1.89 D。研究发现61.20%受试者汇聚功能不全。调节不足与调节功能不全的发病率分别为41.30% 和40.06%。研究人群的融合性转向也有所降低。

结论：研究发现,临床显微镜工作者屈光不正尤其是近视的患病率增加。其中大多数有转斜和调节不足。大多数受试者视疲劳症状与其显微镜工作有关,这可能会影响他们的工作效率。