Accommodative adaptation induced by sustained disparity-vergence

Pre- and post-task measures of dark focus (DF) were used to assess the response of the accommodative system to within-task disparity-vergence in 10 late-onset myopes (LOM's) and 10 emmetropes. Disparity was induced by 3 and 6 delta base-out prisms imposed during a 3 D near-vision task undertaken for 3- and 10-min periods. Accommodation was measured after the task at 1-s intervals over a 90-s period using an objective infrared optometer to determine post-task regression of DF toward pretask values. After the 3-min task both groups show a reduction in DF adaptation with increasing disparity-vergence. After the 10-min task, disparity continues to have a significant effect on accommodative adaptation in LOM's but not in emmetropes, suggesting diminished prism adaptation in LOM's. The implications of these findings are discussed with regard to both current models of accommodation-vergence interaction and the etiology of late-onset myopia.     

Temporal aspects of accommodative adaptation

Pre- and post-task measures of dark focus (DF) were used to assess accommodative adaptation induced by brief near-vision tasks in 10 late-onset myopes (LOM's) and 10 emmetropes. The degree of adaptation was examined after tasks of 15, 30, and 45 s duration. Accommodation was measured after the task at 1 s intervals over a 90 s period using an objective infrared optometer. Emmetropic subjects showed a significant increase in DF during the first 10 s of post-task recording, which regressed to the pre-task value within a further 20 to 30 s. No significant difference in post-task accommodative regression was observed after the three task durations. Similar findings were observed for LOM's after the 15 and 30 s tasks. However, after the 45 s task, LOM's exhibited a marked negative shift in DF during the 10 to 50 s post-task period. Instillation of a beta adrenergic antagonist attenuated this negative shift, suggesting that it was mediated via beta adrenergic innervation to the ciliary muscle.     

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.     

Disparity-induced accommodation in late-onset myopia

The effect of disparity-vergence on the accommodation response (AR) was measured in 10 late-onset myopes (LOMs) and 10 emmetropic subjects during the course of a 10 minute near-vision task (working distance 33 cm). Disparity was induced by the introduction of 3 and 6 delta base-out prisms placed before the right eye, and the resulting AR for the left eye was measured at 1 min intervals using an objective infra-red optometer. LOMs showed a significant increase in AR with increasing disparity whereas emmetropes showed no significant difference in AR for all disparity conditions. The implications of these findings are discussed with reference to the aetiology of late-onset myopia.     

Accommodative error, adaptation and myopia

Accommodative adaptation, resulting from the sustained output of slow blur-driven accommodation during the course of a sustained near-vision task, has generally been assessed under open-loop conditions. This study examined whether adaptation influences closed-loop accommodation during the course of a sustained near-vision task. Accommodative adaptation was assessed in 18 fully-corrected subjects by comparing pre- and post-task values of dark accommodation recorded objectively with an infra-red optometer. Subjects performed a continuous 10 min binocular near-vision task at a viewing distance of 33 cm, with the within-task accommodative response being assessed at 1 min intervals during this period. Subjects were categorized into adaptors (N = 11) and non-adaptors (N = 7) on the basis of whether their initial 10 sec post-task adaptation exceeded +0.30D. The adapting group exhibited a significant decline in the lag of accommodation during the first 3 min of the near-task, whereas no significant change in the within-task response over time was observed in the non-adapting group. These results indicate that accommodative adaptation increases the accuracy of the within-task, closed-loop accommodative response. Furthermore, we speculate that a deficit in accommodative adaptation, being accompanied by increased retinal defocus during near fixation, may contribute to the development of nearwork-induced myopia.     

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.     

Near vision stress: vergence adaptation and accommodative fatigue

Changes in the visual system following prolonged near work were investigated. Fifteen young, normal subjects undertook a severe, two hour long, binocular near visual task at 20 cm without any breaks. Fusional stress was assessed by near 'phoria change. Refractive change was measured with an autorefractometer to investigate whether transient myopia occurred and its subsequent recovery. The near task caused vergence adaptation which was primarily due to the fusional stress of the task (accounting for 67% of its variance). The 'phoria change was to a lesser extent (40%) dependent on the accommodative "stress" of the task. Fatigue of the accommodative system resulted in increased accommodative innervation to maintain the same accurate response. Increased innervation can continue after the task on subsequent distance viewing, resulting in transient myopia (mean 0.29 DS). This transient myopia was found to be due to a transient regression of the far point towards the subject's tonic accommodation level. This can be accounted for by a shift of the tonic level as well as an increased bias towards the pre-task tonic level.     

Accommodative adaptation to monocular and binocular stimuli

Pre- and post-task measures of dark-focus (DF) were used to assess accommodative adaptation induced by a 45-s near-vision task at 33 cm. Adaptation was measured under monocular and binocular conditions for a group of 10 young emmetropic subjects (mean age 21.6 years). The accommodative response was measured objectively using an infrared optometer (Canon Autoref R-1). Post-task DF was sampled immediately after the task at 1-s intervals over a 90-s period. No significant difference in accommodative adaptation was observed between the monocular and binocular near-vision tasks. The implications of this finding are discussed with regard to the oculomotor constituents of the closed-loop accommodative response.     

Comparison of accommodative adaptation using laser and infra-red optometers

Previous investigations have reported differences in the rate of decay of accommodative adaptation depending on the form of optometer used to assess dark-focus (DF). This study has used both laser and infra-red optometers to measure pre- and post-task values of DF in 10 subjects. DF was assessed before and immediately after an 8 min near-vision task performed at the subjective near-point of accommodation. Examination of the data from the two optometers revealed significant differences in both magnitude and distribution of pre-task DF. Additionally variations in the post- to pre-task shift in DF were observed between the two methods of assessment. Using the laser optometer, six subjects exhibited positive adaptation following the near-task which was not apparent with the infra-red optometer. The data suggest that both instruments may not be sampling open-loop accommodation; the laser optometer is particularly vulnerable to extraneous influences.     

A quasistatic study of pupil and accommodation after-effects following near vision

The study examines the effect of a near task on immediate post-task measures of pupil size and accommodative state. The method of quasistatic measurement of accommodation was used and pupillary changes elicited by the accommodative stimulus were simultaneously recorded using an infrared video-pupillometer. This method requires a short (typically 2 minutes) but strong (up to 10 D in most subjects) near-vision effort, with the accommodative and pupillary responses being recorded before, during, and after the task. The results show pupil after-effects to be more pronounced than tonic accommodative after-effects. Inter-individual difference in after-effects is large. The pupil after-effect was dissociated from the tonic accommodation after-effect and lasted in some cases for more than 15 minutes. The pupil after-effect was not masked by darkness. It is proposed that monitoring the changes of pupil is valuable when assessing the after-effects of sustained near vision.     

Repeatability of post-task regression of accommodation in emmetropia and late-onset myopia

The characteristics of post-task regression of accommodation to pre-task ionic accommodation (TA) levels have been examined in a number of studies to clarify the nature of the within-task facility for accommodative adaptation. Of special interest is the recent observation that significant attenuation of post-task regression occurs in late-onset myopes (LOMs) when compared with emmetropes(EMMs). These findings have led to speculation that such attenuation may reflect a deficit in inhibitory sympathetic innervation to ciliary smooth muscle in late-onset myopia and hence a predisposition to sustained accommodative adaptation which then acts as a precursor to the induced myopia. A consequence of this study was that post-task regression may have some value in predicting those individuals who may be susceptible to post-task accommodative hysteresis. A pre-requisite for such a predictive value is that for a given individual the variation in inter-trial regression patterns is not significant. The aim of this study is principally to Investigate the inter-trial variability of post-task regression for individual subjects following a sustained near vision task, and to confirm further differences that have been reported between EMMs and LOMs with respect to the time course of post-task regressions, A modified Canon Rl infrared optometer was used to measure accommodation objectively throughout a near task and for 2 min post-task. Accommodative level was measured following 3 min fixation of a high contrast photopic Maltese cross target placed 3 D above the subject's baseline TA. Repeatability of post-task regression in 10 EMMs and 10 LOMs was assessed by taking measurements on three separate occasions, A significant difference in the post-task regression patterns between EMMs and LOMs was observed which supports previously reported work; the rate of regression to pre-task TA being slower in the LOMs. Within-subject analysis showed that for both EMMs and LOMs the differences between time-course of post-task regressions were not significant. The repeatability of post-task regression patterns thus justifies their use as indices of within-task adaptation.     

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.     

Differences in adaptation of tonic accommodation with refractive state

It has recently been demonstrated that intersubject variations in tonic (dark focus) levels of accommodation are related to corrected refractive state (McBrien and Millodot, 1987). The aim of the present study was to investigate the effect of sustained visual tasks on the tonic level of accommodation in different refractive groups. Eleven hyperopes, 16 emmetropes, ten early onset myopes and ten late onset myopes had their tonic accommodation measured with the objective infrared optometer Canon Autoref R-1 before and after a 15 min sustained visual counting task. The post-task tonic accommodation level was monitored for 15 min to assess the decay rate of any observed task-induced changes in tonic accommodation. Subjects repeated the experimental procedure for four task locations (6 m, pre-task tonic position, 37 cm and 20 cm). Late onset myopes showed significant positive (myopic) changes in their tonic level of accommodation at both near viewing distances, which showed no evidence of decay during the 15 min post-task monitoring period. Hyperopes, however, underwent transient "counteradaptive" decreases in their tonic level of accommodation after sustained near viewing. Emmetropes and early onset myopes showed little change in tonic levels at the two near distances. Differences between groups were also obtained at tonic and far viewing distances. Post-task changes in tonic accommodation demonstrated only a weak negative correlation with pre-task tonic accommodation levels at each task distance. It is proposed that the observed differences in adaptation of tonic accommodation among refractive groups may be related to variations in autonomic innervation of the ciliary muscle.     

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.     

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.     

Stereoacuity is affected by induced phoria but returns toward baseline during vergence adaptation.

PURPOSE: In normal binocular single vision adaptation to an induced deviation occurs over a period of several minutes. This study investigates the effect on stereoacuity during vergence adaptation. METHODS: Stereoacuity, using the Frisby near stereotest, was measured in 20 participants aged 20.2 +/- 1.9 years with normal binocular single vision. Measurements were taken immediately on inducing a phoria with 12(Delta) base-out prisms (split), and after 3, 6, and 9 minutes of adaptation. A measure of stereoacuity was also taken with the same size prisms that were used concomitantly to control for the reduced visual acuity. RESULTS: Stereoacuity was found to decrease following introduction of the 12(Delta) base-out prismatic glasses (9.4 +/- 2.5 arcsec compared with 24.4 +/- 21.4 arcsec) and then increase over the 9 minute period of adaptation (ANOVA, p = 0.0002). Using post-hoc tests with Bonferroni correction, the decrease of stereoacuity on introduction of the prism was significant (p = 0.0039), and although an increase in stereoacuity appeared to occur after 3 and 6 minutes of wearing the 12(Delta) glasses (14.9 +/- 4.3 arcsec, 12.3 +/- 4.5 arcsec), this did not return to the baseline value until 9 minutes, when the stereoacuity had increased to 12.6 +/- 10.6 arcsec (p = 0.1982). CONCLUSIONS: In these participants, inducing a deviation with base-out prisms negatively affected near stereoacuity. However, as adaptation occurred, the level of stereoacuity was seen to increase back toward the baseline measurement.     

Effect of target proximity on the open-loop accommodative response

Proximal stimuli to accommodation were assessed by comparing the open-loop accommodative responses (AR's) to stimuli placed at viewing distances of 5 m (0.2 D) and 0.33 m (3D) in 10 late-onset myopes (LOM's) and 10 emmetropes. The accommodation and vergence loops were opened by subjects viewing targets monocularly through 0.5-mm pinholes. Pre- and post-task levels of tonic accommodation (TA) were used to assess the accommodative adaptation induced by the task. For the 0.2 D condition there was no significant difference between pre-task TA, within-task AR, or post-task TA, indicating that accommodation was open-loop throughout this distal condition. When viewing the 3 D target both refractive groups exhibited significant increases in AR, emmetropes showing higher levels of proximally induced accommodation (PIA) when compared with LOM's. The data indicate that target proximity will influence AR even when both blur and vergence cues have been stabilized.     

Optimum target refraction for highly and moderately myopic patients after monofocal intraocular lens implantation

PURPOSE: To determine the optimum target refraction for myopic patients who want to see at both near and intermediate distances without correction after cataract surgery. SETTING: Hayashi Eye Hospital, Fukuoka, Japan. METHODS: Sixty-nine eyes of 69 patients scheduled for phacoemulsification and monofocal intraocular lens (IOL) implantation were studied. The following age groups were recruited: 50s, 60s, and 70s. With addition of a spherical lens of 1.0, 1.5, 2.0, 2.5, or 3.0 diopters (D) after best distance correction (simulation of various degrees of myopia), visual acuity at various distances was measured using the all-distance vision tester. With the assumption that visual acuity of 20/30 is necessary for near and intermediate vision, the distances at which the mean visual acuity reached 20/30 were determined. RESULTS: Six patients did not have all examinations; thus, 24 patients in the 50s group, 23 in the 60s group, and 22 in the 70s group were included in the analysis. With simulation of -1.0 or -1.5 D of myopia, sufficient near visual acuity at 0.3 m was not obtained. With -2.0 D of myopia, visual acuity better than 20/30 was obtained at 0.7 m, 0.5 m, and 0.3 m. However, with -2.5 D of myopia, visual acuity reached 20/30 at 0.5 m and 0.3 m; with -3.0 D of myopia, it reached only 20/30 at 0.3 m. There were no significant differences between the age groups in mean visual acuity from far to near distances except for intermediate visual acuity with -1.0 D and -1.5 D of myopia and for near visual acuity with -2.5 D of myopia. CONCLUSIONS: Eyes with a monofocal IOL that simulates -2.0 D of myopia achieved sufficient visual acuity for both near and intermediate distances in 3 age groups. This indicates that -2.0 D is the optimum target refraction in myopic eyes.     

不同程度近视青少年儿童双眼视功能的差异

目的通过比较不同程度近视儿童双眼协动参数的差异性及其与近视程度相关性,进一步探索近视发展的原理。方法横断面研究。研究对象包括128例7～14岁近视青少年儿童,根据等效球镜度分为低度近视组（-0.50～-3.00 D）、中度近视组（-3.25～-6.00 D）和高度近视组（>-6.00 D）。双眼协动参数测量包括远距及近距水平聚散力,远距及近距水平隐斜,梯度性调节性集合与调节的比率（AC/A比率）。采用单因素方差分析和卡方检验对3组数据进行分析。结果在融像性聚散范围中,远距散开力的破裂点、近距散开力的恢复点、远距集合力的恢复点、近距集合力的模糊点及破裂点随近视程度加深而减小（F=3.271、3.579、4.931、6.507、4.887,P<0.05）,其余参数的差异无明显统计学意义（P>0.05）;在隐斜视类型的构成比中,远距及近距隐斜类型构成比例的差异在3组中无明显统计学意义（P>0.05）,总体上近距内隐斜构成比例高于远距内隐斜（χ²=6.609,P<0.05）;梯度性AC/A随近视程度的增加而增加（F=3.625,P<0.05）,其中高度近视组明显高于低度近视组（P<0.05）。结论随着青少年儿童近视程度的增加,融像性聚散范围减少,梯度性AC/A比率增高。近视儿童表现出的近距内隐斜可能与近视发展相关。     

角膜塑形镜对近视性屈光参差儿童双眼视功能的影响

目的　探究角膜塑形镜（OK镜）对近视性屈光参差儿童双眼视功能的影响。方法　收集2018年至2019年于北京大学第三医院眼科视光中心就诊的60例近视性屈光参差儿童资料,随机分为OK组（配戴OK镜）和SP组（配戴普通框架眼镜）,每组各30例。基线时（配戴OK镜或框架眼镜前）和戴镜后6个月、12个月时,分别对受试儿童进行远距斜视度、远距融合范围、远近距立体视的检查。比较OK组和SP组儿童双眼视功能参数的差异,以及两组儿童双眼视功能参数随戴镜时间的变化情况。结果　与基线时相比,戴镜后6个月、12个月时,OK组儿童的远距融合范围、远距集合范围显著减小,近距立体视显著提高（均为P<0.05）,而远距斜视度、远距融合点、远距散开范围、远距立体视差异均无统计学意义（均为P>0.05）。与基线时相比,戴镜后6个月、12个月时,SP组儿童的近距立体视均显著提高（均为P<0.05）,而远距斜视度、远距融合点、远距融合范围、远距集合范围、远距散开范围、远距立体视差异均无统计学意义（均为P>0.05）。OK组和SP组儿童基线时和戴镜后6个月时的远距斜视度、远距融合范围、远距融合点、远距集合范围、远近距立体视差异均无统计学意义（均为P>0.05）。戴镜后12个月时,OK组儿童的远近距立体视均优于SP组（均为P<0.05）,而两组儿童其余指标差异均无统计学意义（均为P>0.05）。结论　近视性屈光参差儿童配戴OK镜后,远距融合范围减小,近距立体视提高。与框架眼镜相比,OK镜改善近视性屈光参差儿童远近距立体视的作用更好。