Comparison of two fixation disparity determinantions

Fixation disparity values derived from horopter measurements are compared with those determined using the disparometer on seven subjects wearing lateral prisms. Although the two measures are intrinsically different, five subjects showed similarity in the form and position of the two fixation disparity-forced vergence functions. Two subjects manifested fixation disparities that indicated the use of proximal or voluntary control of relative eye position for both base-out and base-in forms of lateral prism. Another subject showed a paradoxical response to base-out prism that could be attributed to recovery from previous application of base-in prism, adaptation to base-out prism, or both. An additional subject showed irregular alterations in vergence posture during nonius horopter measurements. The clinical implications of vergence responses other than those associated with fusion and accommodation are discussed and the effect of this form or eye movement control is related to the change in position and shape of the equidistance horopter when it is measured through lateral prism.     

Horizontal fusional amplitudes after adaptation to prism

Horizontal fusional limits and lateral phorias were measured subjectively on five normal subjects after adaptation to increments of base-in and base-out prism. Both the phoria and the fusional limits were found to change by approximately the amount of the adapting prism. The fusional amplitudes (the difference between the phoria and the fusional limits) were relatively unaffected by vergence adaptation except at the largest values of adapting prism. These results are in agreement with current models of disparity vergence eye movement control which suggest that there are independent fast and slow fusional vergence mechanisms, each with its own saturation limit.     

The Glenn A. Fry award lecture: adaptive regulation of accommodative vergence and vergence accommodation

The tonic resting level of accommodation, measured with an objective infrared optometer in an open-loop state (pinhole pupil or empty field), was increased by more than 1.75 D from baseline measures of resting focus after adapting monocularly for 1 min to a 2 D minus lens. The accommodative aftereffect disappeared in darkness, but returned when a visible stimulus reappeared in the open-loop state. Stimulation of disparity vergence with 10 delta, while accommodation was in an open-loop state, also increased the resting focus of accommodation. Similarly, the tonic resting level of vergence became more esophoric after adapting for less than 1 min to base-out prism or to a minus lens presented while the vergence loop was opened. The effects of tonic accommodation on accommodative vergence and the effects of tonic vergence on vergence accommodation were investigated with the temporal frequency responses of the AC/A and CA/C ratios to sinusoidal variations in blur (2 D) and disparity (10 delta), respectively. Accommodative vergence was unresponsive to low temporal frequency sinusoidal variations in blur (less than 0.1 Hz). Similarly, vergence accommodation was unresponsive to low temporal frequency sinusoidal variations in disparity. However, accommodative vergence and vergence accommodation were responsive to higher temporal frequency stimuli (up to 0.5 Hz). When negative feedback to the stimulated system (accommodation or vergence) was cancelled electronically, the low temporal frequency response increased for the AC/A and CA/C ratios, respectively. There was also a nonlinear increase of both AC/A and CA/C ratios as stimulus amplitude increased. It is hypothesized that the nonlinearity resulted from limitations of adaptation to small stimuli. An inverse complementary relation is suggested between the amplitude of the AC/A ratio and adaptable tonic accommodation, and between the amplitude of the CA/C ratio and adaptable tonic vergence. This model predicts that in this complementary relation, adaptable tonic elements would sustain motor responses of accommodation and vergence that were initiated by phasic elements and cross-link interactions. It also predicts that the AC/A and CA/C ratios would decrease in time as dynamic control shifted from the phasic to the adaptable tonic control mechanisms of accommodation and vergence, respectively.     

Retinal eccentricity of fusion detail affects vergence adaptation.

Adaptation of tonic vergence ("vergence adaptation" or "prism adaptation") may be produced by sustained accommodative, disparity, or proximal vergence innervation. Phoria measures were used in the present study as indices of tonic vergence adaptation to convergence and divergence stimuli in five subjects. Amblyoscope targets stimulated fusion either over the central or peripheral retina while holding accommodative and proximal stimuli constant. The magnitude and rate of tonic vergence adaptation were greater under the central fusion condition than under the peripheral fusion condition, even though the vergence stimuli were matched between central and peripheral fusion conditions. These results show that tonic vergence adaptation is influenced by the retinal eccentricity of target fusional patterns. However, additional work is required to identify the mechanism(s) underlying this eccentricity effect.     

The effect of prism adaptation on the response AC/A ratio.

Vergence adaptation, also known as prism adaptation, is a phenomenon in which a patient's heterophoria changes after prolonged viewing through prism. The effect of prism adaptation on the accommodation-convergence relationship, quantified by the AC/A ratio, is not known. Previous studies of AC/A ratio stability and alterability have used only stimulus AC/A ratio calculations, or have measured accommodative responses to only one or two stimuli. The ideal study of AC/A ratio stability and alterability would measure accommodative responses to several accommodative stimuli, and use these along with vergence responses to calculate response AC/A ratios, rather than stimulus AC/A ratios. In addition, the gradient method should be used to avoid any effect of proximal vergence resulting from changes in target distance. This paper describes a project which investigated the effect of vergence (prism) adaptation on the gradient response AC/A ratio, using accommodative responses measured for five different accommodative stimuli. The response AC/A ratio did not significantly change following a period of adaptation to base-in prism for six of the eight subjects in this study. In addition, the response AC/A ratio did not significantly change following a period of adaptation to base-out prism for six of the eight subjects.     

Vergence adaptation in clinical vergence testing

BackgroundThe 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.MethodsThirty 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.ResultsThere 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.ConclusionsThe 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.     

Prism adaptation without binocular vision

The assumption that vergence adaptation to a prism could only take place during single binocular vision was investigated by having subjects wear a prism while fusion was dissociated. Adaptation was evaluated by its effect on lateral heterophoria. Dissociation and measurement of heterophoria were by means of an automated version of Duane's screen and parallax test. A preliminary investigation with 37 subjects fixating at 6 m while wearing 4 delta base-out and with 33 other subjects fixating at 50 cm while wearing 3 delta base-in showed that at least 25% adapted to the prism. Results of an investigation with 2 subjects fixating at 6 m (4 delta base-out and base-in) and with 3 subjects fixating at 50 cm (3 delta base-in) are presented as evidence that prism adaptation can take place. It was also found that the power of the added prism could just as well be provided by the phoria test's variable prism. In other words, there is a possibility that adaptation will occur when heterophoria is measured with a variable prism.     

The relationship between fusional vergence eye movements and fixation disparity

The fusional vergence system is under the control of a fast neural integrator which aligns the eyes and a slow neural integrator which maintains binocular alignment. These controllers are distinguished by their decay time constants and their stimuli. Previous studies indicate that the fast fusional vergence controller responds to retinal image disparity and the slow fusional vergence controller responds to the output of the fast neural integrator. Slow fusional vergence as evidenced by adaptation of the phoria is unaffected by accommodative vergence when disparity vergence is open loop. Under closed loop conditions both accommodation and disparity induced vergence influence slow fusional vergence. These results indicate that the slow vergence controller is located before the site of interaction between convergence and accommodation. Fixation disparity is described as a steady-state error of the neural integrator controlling fast fusional vergence and its amplitude is shown to be inversely related to adaptation of the phoria to prism.     

The effect of vergence adaptation on convergent accommodation

This study investigates whether, during a sustained near-vision task, the convergent accommodation (CA) derived initially from the fast (reflex) vergence response is subsequently maintained by tonic (adaptive) vergence. CA was examined during the course of a 3 min near-vision task (viewing distance 33 cm) in 10 emmetropic subjects. Vergence was stimulated by the introduction of a 6 delta base-out prism before the right eye while the open-loop accommodative response of the left eye was measured at approximately 1 s intervals using an objective infra-red optometer. In a control condition the subject viewed the target through a plano lens (0 delta). The accommodation loop was opened by the subject viewing near-type through 0.5 mm diameter pinholes. Pre- and post-task measurement of heterophoria was made using a Maddox rod and prism bar. Regression of post-task heterophoria to the pre-task value indicated that adaptation to the vergence stimulus had occurred. However, there was no significant reduction in induced CA during the task. As the onset of adaptation does not produce any fall in CA, it is concluded that both reflex and adaptive vergence act as a stimulus to CA. This finding is discussed with regard to models of accommodation-vergence interaction.     

Variation and correlation of standard clinical phoropter tests of phorias, vergence ranges, and relative accommodation in a sample of school-age children.

This paper presents a study of standard clinical phoropter tests of dissociated phorias, fusional vergence ranges, and relative accommodation ranges in 244 school-age (7.9 to 15.9 years of age) children. The tests studied included von Graefe phorias and base-in and base-out fusional vergence ranges at both distance and near, as well as negative relative accommodation (NRA) and positive relative accommodation (PRA) tests. Coefficients of correlation among the various tests are presented. Means and correlations were similar to those previously reported for adult populations. Distributions of distance lateral phorias and some distance fusional vergence parameters were leptokurtic.     

The synkinetic interaction of convergence accommodation and accommodative convergence

An experimental paradigm is presented to test the long-standing theory that binocular vergence is supported by an addition of tonic, proximal and accommodative convergence (as measured monocularly) along with a supplemental fusional component. Results show accommodative convergence is less than, and the fusional component greater than, predicted by this traditional theory. An alternative theory is presented which includes the influence of convergence accommodation and which correctly predicts the experimental results.     

Reliability of rotary prism fusional vergence ranges.

BACKGROUND: The measurement of fusional vergence ranges is an important clinical test in the assessment of binocular vision status. Fusional vergence ranges are typically measured by recording a patient's reports of blur, break, and recovery to base-in (BI) and base-out (BO) prism. Published reliability data on fusional vergence ranges are very limited. METHODS: Eight subjects underwent four testing sessions, at which repeated measurements of fusional vergence ranges were taken. Near ranges were tested at the first session only Distance ranges were tested at all four sessions. Intra-examiner standard deviations were calculated for each fusional vergence test result (BI and BO; blur, break, and recovery) for each session. Intra-examiner standard deviations were averaged. These values were used to determine 95% limits of agreement. RESULTS: The 95% limits of agreement were between 2 delta and 2.5 delta for the distance BI break and recovery and for the near BI recovery; between 3 and 4 delta for near BI break and near BO break; between 4 and 5 delta for distance BO blur and recovery and for near BI blur; and between 5 and 5.5 delta for distance BO break and near BO blur and recovery.     

Adaptation of tonic accommodation

We compared the resting (dark) focus of accommodation before and after adapting to accommodative stimuli placed nearer or farther from an initial baseline resting focus. Short-term monocular adaptation (less than 2 min) did not result in consistent after-effects that were correlated with the adaptation stimulus. After short-term adaptation, accommodation returned to its resting level in 2-15 s. Long-term monocular adaptation (30 min) to a 6-D near stimulus resulted in a small (0.5-D) average increase in the resting focus of accommodation beyond the normal 2-15-s short-term decay. These observations illustrate a tonic adaptation of accommodation that is small and requires longer durations of adaptation than an analogous adaptation of the fusional vergence system to prism.     

The effect of incipient presbyopia on the correspondence between accommodation and vergence

PURPOSE: To investigate the accommodation-convergence relationship during the incipient phase of presbyopia. The study aimed to differentiate between the current theories of presbyopia and to explore the mechanisms by which the oculomotor system compensates for the change in the accommodation-convergence relationship contingent on a declining amplitude of accommodation. METHODS: Using a Canon R-1 open-view autorefractor and a haploscope device, measurements were made of the stimulus and response accommodative convergence/accommodation ratios and the convergence accommodation/convergence ratio of 28 subjects aged 35-45 years at the commencement of the study. Amplitude of accommodation was assessed using a push-down technique. The measurements were repeated at 4-monthly intervals over a 2-year period. RESULTS: The results showed that with the decline in the amplitude of accommodation there is an increase in the accommodative convergence response per unit of accommodative response and a decrease in the convergence accommodation response per unit of convergence. CONCLUSIONS: The results of this study fail to support the Hess-Gullstrand theory of presbyopia in that the ciliary muscle effort required to produce a unit change in accommodation increases, rather than stays constant, with age. Data show that the near vision response is limited to the maximum vergence response that can be tolerated and, despite being within the amplitude of accommodation, a stimulus may still appear blurred because the vergence component determines the proportion of available accommodation utilised during near vision.     

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.     

Adaptive disorders of accommodation and vergence in binocular dysfunction

Disturbances of binocular vision are described clinically by the Duane-White classification in terms of the magnitude of the accommodative-convergence ratio (AC/A). Convergence excess and convergence insufficiency are assumed to result from high and low AC/A ratios respectively. It is assumed that the abnormal AC/A ratio is an independent variable that underlies abnormal phorias. However, recent studies have demonstrated that the AC/A ratio is inversely related to the adaptability of tonic accommodation (lens adaptation) and directly related to adaptability of tonic vergence (prism adaptation). We have tested whether clinical categories of convergence excess and convergence insufficiency are associated with insufficient and excessive adaptation of tonic accommodation and tonic vergence. Results demonstrate greater amplitude and duration of accommodative after-effects (lens adaptation) in the convergence insufficiency than the convergence excess group. Vergence after-effects (prism adaptation) had the reverse trend for the two groups. These results indicate that adaptive disorders of accommodation and vergence may underlie binocular disorders in symptomatic patients categorized as convergence excess and convergence insufficiency.     

Dynamic interactions between accommodation and convergence are velocity sensitive

Aftereffects of accommodation and vergence occur following approximately 1 min of adaptation to lenses and prisms respectively. This observation can be interpreted to mean that accommodation and vergence responses have phasic and tonic components. We have examined the role that these proposed subcomponents play in mutual interactions between accommodation and vergence. Both accommodative vergence (AV) and vergence accommodation (VA) were unresponsive to low temporal frequency variations (less than 0.1 Hz) in defocus and disparity respectively. However, both AV and VA were responsive to higher temporal frequency stimuli (up to 0.5 Hz). When negative feedback to the stimulated system was cancelled electronically, both AV and VA become responsive to low temporal frequency stimuli. The ratio or gain of accommodative vergence/accommodation (AC/A) and vergence accommodation/vergence (CA/C) increased nonlinearly with stimulus amplitude. Vergence aftereffects resulted from stimulation of AV and accommodative aftereffects resulted from stimulation of VA. These results are interpreted to mean there could be a complementary relationship between the amplitude of the AC/A ratio and proposed tonic adaptation of accommodation, and between the amplitude of the CA/C ratio and proposed tonic adaptation of vergence. A low saturation limit or stimulus window for tonic adaptation may account for the amplitude dependent nonlinearities of the AC/A and CA/C ratios.     

Fatigue reduces tonic accommodation

Ocular accommodation adopts a mean baseline response level of approximately 1.0 D in the absence of blur feedback (open-loop state). This baseline or tonic accommodation (TA) can be elevated following a sustained monocular accommodative response to a dioptric stimulus (lens adaptation) that exceeds the baseline open-loop level of TA. The accommodative response to the lens persists in the open-loop state (accommodative hysteresis), and eventually decays to a stable end-point. Interestingly, if the baseline TA is high, the monocularly adapted accommodative state can decay to an end-point that is below the initial pre-adapted baseline level of the TA (counter-adaptive response) (McBrien, N.A. and Millodot, M., (1988). Differences in adaptation of TA with refractive state. Invest. Ophthalmol. Vis. Sci., 29, 460-469). We have investigated the possible contribution of accommodation fatigue to the counter-adaptive change in baseline TA following sustained accommodation. Two fatigue procedures were used while viewing a target at 66 or 33 cm. In a monocular condition, accommodation was stimulated for 3 min with lens values alternating from -1.5 to +1.5 D at a rate of 0.25 Hz. In the binocular condition, convergence was stimulated with alternating prism values from 9 prism diopters (PD) base-out to 9 PD base-in. Both monocular and binocular tasks resulted in a significant reduction of TA. These results suggest that previously reported reductions of baseline TA following sustained monocular accommodation or binocular convergence could have resulted from fatigue of the accommodative system. Accommodative fatigue could be responsible for the lower values of TA observed in counter-adaptive responses to sustained accommodative or convergence effort.     

Normal values of distance heterophoria and fusional vergence ranges and effects of age

Background Although it is well known that age affects visual function, we still have much to learn about the impact of aging on binocular vision. Our aim was to establish distance heterophoria and distance fusional vergence ranges in a large non-clinical population of wide age range, to provide normal values for comparisons.Methods A cross-sectional study was performed on 271 non-clinical subjects stratified into six age groups. Distance heterophoria measurements were obtained using the von Graefe technique and distance horizontal fusional vergence ranges using Risley rotary prisms in each subject, with best spectacle correction.Results The mean heterophoria value for distance fixation was exophoric except for the 71–80 year age group, which was esophoric. No effects of age were observed on distance heterophoria and distance base-in and base-out break means. However, a significant effect of age was noted on base-in recovery and base-out recovery. Between the youngest and oldest age groups, mean base-in recovery decreased by 2.5 Δ and mean base-out recovery by 3.3 Δ.Conclusions Distance base-in and base-out recovery means diminished significantly with age such that recovery values fell outside the norm in subjects in the 61–70 year age group and older. This study establishes statistical normal values for distance heterophoria and fusional vergence in a non-clinical population. Mean values such as these are valuable tools for identifying subjects with far binocular anomalies.     

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.