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.     

A longitudinal study of vergence adaptation in incipient presbyopia

BACKGROUND: To investigate vergence adaptation during the incipient phase of presbyopia, when the amplitude of accommodation approaches the level where the first reading addition is required. The study aimed to assess the ability of the vergence system to counteract changes in the component contributions to the overall vergence response with the decline in the amplitude of accommodation in presbyopia, although previous reports on the nature of changes in accommodative, tonic and proximal vergence are equivocal. METHODS: Using a 'flashed' Maddox rod technique, an assessment of vergence adaptation to 6delta base-out and 6delta base-in prism was made for 28 subjects (aged 35-45 years at the commencement of the study). The measurements were taken four times over a 2-year period. RESULTS: Using a repeated measures analysis of variance, the results show that with the decline in amplitude of accommodation, there is a statistically significant reduction in the magnitude of vergence adaptation to both base-out (p < 0.05) and base-in prism (p < 0.01). CONCLUSIONS: This study shows that with ageing, there is a decrease in the ability of the slow vergence mechanism to overcome a change in fusional vergence demand and would suggest that either the fast component of fusional vergence must cope with any change in fusional vergence demand or that the sum of the accommodative, tonic and proximal vergence responses are virtually stable with age.     

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.     

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.     

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.     

Contribution of accommodation and disparity-vergence to transient nearwork-induced myopic shifts

Many investigations have attempted to demonstrate a relationship between sustained near-vision and the aetiology of environmentally-induced myopia, but it remains unclear whether myopic development relates to the actions of either accommodation or vergence. The present study investigated the effect of varying the disparity-vergence demand during a near-vision task while maintaining the accommodative stimulus constant. We required 20 young subjects to perform a near task (viewing distance = 20 cm) for 20 min through base-in, zero power and base-out prisms. The magnitude of the prisms for each individual corresponded to one-third their near vergence range. Pre- and post-task measurements of refractive state were determined objectively using an infrared optometer, with post-task readings being recorded over the initial 50 s following task completion. The results indicated no significant difference between the transient far-point shift for the three disparity-vergence conditions. However, combining data across conditions, a significant, mean shift in the myopic direction of 0.14 D was observed during the 10-20 s period immediately following task completion which dissipated within 20-50 s post-task. These findings suggest that the transient post-task myopic shift was not related to the output of disparity-vergence.     

Distance vergence adaptation is abnormal in subjects with convergence insufficiency

It is well known that in convergence insufficiency (CI) prism adaptation is reduced in response to base-out (BO) prisms at near. There have also been some suggestions in the literature that adaptation is reduced at other distances as well. The present data show that in CI adaptation is not only reduced in response to BO at near, but also in response to base-in (BI) at near and for both BI and BO at distance. This raises the interesting question whether distance adaptation becomes reduced because of reduced near adaptation or whether these subjects have a generally reduced horizontal adaptation mechanism.     

Convergence Insufficiency Neuro-Mechanism Adult Population Study: Phoria Adaptation Results

PurposeThe purpose of this study was to compare changes in phoria adaptation between young adult binocularly normal controls (BNCs) and participants with symptomatic convergence insufficiency (CI), who were randomized to either office-based vergence accommodative therapy (OBVAT) or office-based placebo therapy (OBPT).MethodsIn the double-masked randomized clinical trial, 50 BNC and 50 CI participants were randomized to the following therapeutic interventions: OBVAT or OBPT with home reinforcement for 12 one-hour office sessions. A 6∆ base-out and 6∆ base-in phoria adaptation experiment at near (40 cm) was conducted using the flashed Maddox rod technique at baseline and at outcome. Measurements included the rate and the magnitude of phoria adaptation.ResultsAt baseline, BNC and CI participants had significantly different rates and magnitudes of base-in and base-out phoria adaptation (P < 0.001). When comparing the outcome to baseline measurements, significant main effect differences in longitudinal measurements were observed for the magnitude and the rate of phoria adaptation for both base-out and base-in experiments (P < 0.05). For the magnitude and rate of phoria adaptation, post hoc analyses using paired t-tests revealed that the CI group administered the OBVAT intervention exhibited a significant increase in the magnitude and rate of phoria adaptation compared to baseline for both base-in and base-out phoria adaptation (P < 0.01) but not for those administered OBPT.ConclusionsPhoria adaptation is significantly different at baseline between those with normal binocular vision and symptomatic CI participants. OBVAT significantly improves the rate and magnitude of both base-out and base-in phoria adaptation at near compared to OBPT. Results have clinical implications for new therapeutic interventions.     

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.     

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.     

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.     

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.     

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.     

Voluntary effort as a stimulus to accommodation and vergence

Voluntary effort can be used to elicit changes in accommodation and vergence. We investigated the potential cross-coupling interactions of accommodative vergence/accommodation (AC/A) and vergence accommodation/vergence (CA/C) in response to voluntary effort. The responses of accommodation and vergence were measured simultaneously with a dual Purkinje image eye tracker and infrared optometer while subjects viewed a Maltese cross monocularly through a pinhole pupil and made voluntary efforts to imaginary changes in target distance. The accommodation and vergence evoked by voluntary efforts occurred in a ratio that was similar to the response AC/A ratio and different from the response CA/C ratio. One exceptional subject was found who could voluntarily accommodate with one eye occluded without the usual concomitant change in vergence. These results indicate that it is primarily the focusing response that is adjusted during voluntary efforts in the near response.     

Influence of accommodative and vergence adaptation on binocular motor disorders

Tait described four categories of binocular disorders including convergence excess, convergence insufficiency, divergence excess, and divergence insufficiency. These disorders are defined by the distance where the largest heterophoria occurs (distance or near), and the amplitude of the accommodative vergence ratio (AC/A). Insufficiency corresponds to a low AC/A ratio, whereas excess corresponds to a high AC/A. The magnitude of the AC/A ratio, which may be influenced by the adaptability of the accommodation and vergence systems, has been shown to be reciprocally related to adaptability of accommodation. Likewise, the degree of vergence accommodation has been shown to be related reciprocally to adaptability of vergence to prism. An imbalance of adaptability of accommodation and vergence systems produces abnormal cross-coupling between the two motor systems. When accommodation is more adaptable than vergence, the AC/A ratio is low and the CA/C ratio is high. Conversely, when vergence is more adaptable than accommodation, the AC/A ratio is high and the CA/C ratio is low. A method is reported for temporarily restoring moderate amplitudes of abnormal AC/A and CA/C ratios by reducing excessive adaptation with fatigue. Finally, new clinical procedures for measuring adaptation of accommodation and the CA/C ratio are presented. Taken together with current measures of vergence adaptation and AC/A ratio these procedures will permit a more complete evaluation of mutual interactions between accommodation and vergence in patients diagnosed as having excessive and insufficient vergence.     

A dynamic model of cross-coupling between accommodation and convergence: simulations of step and frequency responses.

The near triad consists of an increase in accommodation, vergence, and pupillary constriction. All three motor systems exhibit phasic and tonic responses. The tonic response adapts readily to phasic efforts of accommodation and vergence. Cross-coupling between accommodation and vergence provides a means of dynamically adjusting the tonic set points of the two motor systems to a common near or far working distance. Accommodative vergence cross-links play a dominant role in coordinating proximal changes in accommodation and convergence. The magnitude of cross-link interactions can be modified by imbalanced strength of tonic adaptation by accommodation and vergence. Reducing adaptation of tonic accommodation increases the AC/A ratio and decreases the CA/C ratio. Reducing adaptation of tonic vergence has the opposite effect. A model is able to predict these and other interactions simply by reducing the decay time constant of one of the two motor systems. For example, reducing the time constant for tonic accommodation results in an increased AC/A ratio and decreased CA/C ratio. Reducing the time constant for tonic vergence has the opposite effect. The model predicts transient step responses by accommodative vergence when the AC/A ratio is low and transient step responses of vergence accommodation when the CA/C ratio is low. It also predicts a reciprocal relationship between the AC/A and CA/C ratios. When one cross-link ratio is high the other cross-link ratio is low. Simulated frequency responses predict the low frequency roll off of low AC/A and low CA/C ratios. The step and frequency responses of cross-link ratios are shown to be the same for proximal (perceived distance) and retinal (blur and disparity) stimuli. The model suggests that physiological variations of tonic decay time constants may play an important role in determining clinically abnormal values of AC/A and CA/C ratios.     

The symptomatic patient with normal phorias at distance and near: what tests detect a binocular vision problem?

PURPOSE: Patients often manifest symptoms that appear to be related to binocular distress. Many of these patients have normal heterophoria at distance and near, making the etiology of such symptoms perplexing. We performed a visual examination of patients having normal heterophoria at distance and near in order to investigate which visual analysis tests differentiate symptomatic from asymptomatic patients. METHODS: Eighty subjects (30 symptomatic, 50 asymptomatic) with ages between 18 to 35 years of either sex and any race were pooled based on vision-symptom level determined by a 9-question standardized visual-symptoms questionnaire scored using a 100-point scale (asymptomatic: score > or = 85; symptomatic: score < or = 75). Inclusion/exclusion criteria included vision correctable to 6/6 (20/20) Snellen acuity or better in each eye, normal phorias, no latent cyclovertical heterophoria, and normal ocular health. RESULTS: Of the 30 symptomatic patients, 18 had reduced vergence facility response using 3 pd base-in/12 pd base-out loose prism at distance (n = 10; t-score = 2.41, p < 0.02, d = 76) and near (n = 15; t-score = 3.32, p < 0.01, df = 78) with a significant difference beyond the 0.02 level. No other test including measurement of accommodation, showed a significant between-group difference. However, three patients with normal vergence facility (distance and near) showed a reduced binocular accommodative facility response (+/-2.00 D at 40 cm). CONCLUSION: Given a patent with asthenopia, normal phorias, and visual acuity, a differential diagnosis may be made based primarily on using vergence facility and accommodative facility testing. From a clinical standpoint, the results expedite diagnosis of binocular vision abnormalities and direct treatment.     

Accommodative and vergence difficulties interfering with image clarity through a binocular indirect ophthalmoscope.

PURPOSE: This study sought to identify accommodative and vergence deficiencies that could explain why some students have difficulty seeing clearly when using a binocular indirect ophthalmoscope (BIO) containing its standard +2.00 D lenses. METHODS: A survey was distributed to Illinois College of Optometry 3rd- and 4th-year students. Based on the information supplied by the survey, students were divided into two groups: those who are unable ("BIO plano") and those who are able ("BIO plus") to obtain a clear image with the +2.00 D lenses in their BIO's. Forty-seven subjects participated: 22 in the BIO plano group and 25 in BIO plus group. Two of the authors, masked to subject group, measured all subjects' accommodative amplitudes and facilities, monocular estimation method (MEM) retinoscopy, negative relative accommodation and positive relative accommodation (NRA/PRA), distance and near vergence ranges, distance and near phorias, Worth 4-dot test, and near point of convergence (five times). RESULTS: The Mann-Whitney U analysis of numerical data for the two groups showed a statistically significant difference for accommodative facility in the right eye (p = 0.004). The difference between the two groups approached significance for accommodative facility with both eyes (p = 0.02), facility in the left eye (p = 0.03), distance base-out blur (p = 0.02), near base-out break (p = 0.02), and near base-out recovery (p = 0.04). For all findings in which the difference between the two groups was significant or approached significance, the BIO plus group had higher median values. When subjects were classified by difficulty with the plus side of the flippers during accommodative facility testing, there was a statistically significant difference with both eyes (Fisher exact test, p = 0.003) and with the right eye (p = 0.008) between the BIO plus and BIO plano groups. When subjects were classified by the presence or absence of an accommodative or binocular vision syndrome, categorical data analysis showed the difference between the two groups approached significance, with more BIO plano subjects having syndromes (p = 0.03). CONCLUSION: No one test absolutely defined who would have difficulty with the +2.00 D lenses in the BIO. There are several skills required; less developed plus acceptance and convergence may cause difficulties.     

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.     

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.