Refractive group differences in accommodation microfluctuations with changing accommodation stimulus

Accommodative response and visual fatigue following a non-congruent visual task in non-asthenopic and asthenopic individuals

PURPOSE

Asthenopia is related to near vision activities or visual tasks that dissociate accommodation from vergence. Since the results of previous studies using objective measures to diagnose asthenopia are inconsistent, this study compared optometric tests and objective metrics of accommodation in non-asthenopic and asthenopic young adults before and after a visual fatigue task.

METHODS

The accommodative response was recorded objectively for 6 min at a 3.33 D accommodative demand using an autorefractor, before and after a 5-min non-congruent visual task. Accommodation was disassociated from vergence with a ±2.00 D accommodative flipper while reading at the same distance. Optometric tests and subjective evaluations of asthenopia were performed before and after the task. Twenty-six non-presbyopic adults (23.15 ± 2.56 years) were included and identified as asthenopic (n = 14) or non-asthenopic (n = 12) based on their score on the Computer Vision Syndrome Questionnaire.

RESULTS

A mixed ANOVA found no significant difference between the groups for objective (accommodative response) or subjective metrics (feeling of fatigue, optometric tests), although all participants reported greater visual fatigue after the task. A significant effect of time (before and after the non-congruent task) was identified for the overall sample for mean accommodative lag (+0.10 D, p = 0.01), subjective visual fatigue (+1.18, p < 0.01), negative relative accommodation (-0.20 D, p = 0.02) and near negative fusional reserve (blur: +2.46Δ, p < 0.01; break: +1.89Δ, p < 0.01; recovery: +3.34Δ, p = 0.02).

CONCLUSIONS

The task-induced asthenopia, measured both objectively and subjectively, was accompanied by a change in accommodative lag, greater visual fatigue and a decrease in negative relative accommodation. Conversely, near negative fusional reserves seem to adapt to the task. No significant differences were found between the two groups with respect to accommodative metrics (objective) or subjective and optometric tests.

Accuracy and stability of accommodation and vergence responses during sustained near tasks in uncorrected hyperopes

This study investigated the accuracy and stability of accommodative and vergence functions in children with and without hyperopia while engaged in two sustained near tasks. The sustained accommodative and vergence characteristics of participants without refractive correction (n = 92, aged 5-10 years) with and without hyperopia (defined as cycloplegic retinoscopy ≥  + 1.00D and less than + 5.00D) were measured using eccentric infrared photorefraction (PowerRef 3; PlusOptix, Germany). Binocular measures of accommodation and eye position were recorded while participants engaged in 2 tasks at 25 cm for 15 min each: (1) reading small print on an Amazon Kindle and (2) watching an animated movie on liquid crystal display screen. Comprehensive visual assessment, including measurement of presenting visual acuity, amplitude of accommodation, and stereoacuity was conducted. The magnitude of accommodative and vergence responses was not related to refractive error (P > 0.05). However, there were inter-task differences in the accuracy and stability of the accommodative responses across refractive groups (P < 0.05). The relationship between accommodation and vergence was not significant in both tasks (P > 0.05). However, increased accommodative and vergence instabilities were associated with total accommodative response (P < 0.05). Despite having greater accommodative demand, uncorrected hyperopes accommodate comparably to emmetropic controls. However, uncorrected hyperopes have increased instabilities in their accommodative and vergence responses, which may adversely impact their visual experience.

Impact of text contrast polarity on the retinal activity in myopes and emmetropes using modified pattern ERG

Environmental factors favoring myopia development are still being studied and there is accumulating evidence for a significant role of nearwork. Recently, reading standard black-on-white text was found to activate the retinal OFF pathway and induce choroidal thinning, which is associated with myopia onset. Contrarily, reading white-on-black text led to thicker choroids, being protective against myopia. Respective effects on retinal processing are yet unknown. Here, we exploratively assessed the impact of contrast polarity on the retinal activity and possible interactions with eccentricity and refractive error. We recorded pattern electroretinograms in myopic and emmetropic adults while presenting a dead leaves stimulus (DLS), overlaid by masks of different size in ring or circle shape, either filled with uniform gray or text of inverted or standard contrast. In myopes, retinal responses for DLS with standard and inverted contrast were larger when the perifovea was stimulated (6-12 deg), however, including the fovea resulted in smaller amplitudes for inverted contrast than in emmetropes. The retina of emmetropes was more sensitive to inverted contrast than to standard and gray within 12 deg, but most sensitive for gray in the perifovea. This demonstrates that the refractive error influences the sensitivity to text contrast polarity, with a special role of the peripheral retina, which is in line with previous studies about blur sensitivity. Defining whether the differences derive from retinal processing or anatomical features of a myopic eye requires further investigation. Our approach might be a first step to explain how nearwork promotes the eye's elongation.

Dynamics of the accommodative response and facility with dual-focus soft contact lenses for myopia control

OBJECTIVE

To assess the impact of using dual-focus soft contact lenses for myopia control on the dynamics of the accommodative response and facility.

METHODS

24 young adult myopes were fitted with dual-focus soft contact lenses for myopia control (MiSight®) and single-vision soft contact lenses (Proclear®). The WAM-5500 open-field autorefractor was used to measure the dynamics of the accommodative response (magnitude and variability) in binocular conditions, with accommodative data being gathered from the dominant eye, at three viewing distances (500 cm, 40 cm, and 20 cm) during 90 s. Also, the binocular accommodative facility was assessed with the WAM-5500 autorefractor. All participants performed the same experimental protocol with the dual-focus (MiSight) and single-vision (Proclear) soft contact lenses, with both experimental sessions being carried in two different days and following a counterbalanced order.

RESULTS

This study showed greater lags of accommodation with the MiSight than the Proclear lenses at near distances (40 cm: 1.27 ± 0.77 vs. 0.68 ± 0.37 D, corrected p-value = 0.002, Cohen-d = 0.90; and 20 cm: 1.47 ± 0.84 vs. 1.01 ± 0.52 D, corrected p-value = 0.007, Cohen-d = 0.75), whereas a higher variability of accommodation was observed with the dual-focus than the single-vision lenses at 500 cm (0.53 ± 0.11 vs. 0.23 ± 0.10 D), 40 cm (0.82 ± 0.31 vs. 0.68 ± 0.37 D), and 20 cm (1.50 ± 0.56 vs. 1.15 ± 0.39 D) (corrected p-value < 0.001 in all cases, and Cohen-ds = 0.67-2.33). Also, a worse quantitative (27.75 ± 8.79 vs. 34.29 ± 10.08 cycles per minute, p = 0.029, Cohen-d = 0.48) and qualitative (23.68 ± 7.12 vs. 28.43 ± 7.97 score, p = 0.039, Cohen-d = 0.45) performance was observed with the MiSight when compared to the Proclear lenses.

CONCLUSIONS

The use of dual-focus soft contact lenses for myopia control alters the dynamics of accommodative response and facility in the short-term. Although this optical design has demonstrated its effectiveness for myopia control, eye care specialists should be aware of the acute effects of these lenses on accommodation performance.

Less is more: optimal recording time for measuring the steady-state accommodative response

Clinical Relevance Background: Steady-state accommodation is frequently evaluated with open-field autorefractors; however there is no consensus regarding the optimal test duration. Our results allow us to standardise the procedure followed for the assessment of the steady-state accommodative response in clinical and laboratory settings and may be of great relevance in time-constrained situations.

BACKGROUND

The present study aims to determine the optimal recording time to assess the magnitude and variability of the accommodative response with a binocular open-field autorefractor.

METHODS

The steady-state accommodative response of 84 healthy undergraduate students was dynamically measured using the Grand Seiko WAM-5500 autorefractor at 500 cm, 40 cm and 20 cm for 90 s while participants maintained focus on a stationary target. We examined the reliability and level of agreement between four time intervals (the first 10-s, 30-s, 60-s and 90-s) at three accommodative demands (0.2D, 2.5D and 5D).

RESULTS

The magnitude of accommodation showed an excellent level of reliability (intraclass correlation coefficient [ICC]> 0.90) for the comparisons between the 90-s, 60-s and 30-s time intervals at the three viewing distances tested. However, when including the 10-s interval, there was a moderate reduction of the reliability indices and an increase of the mean difference between two time intervals (intra-class correlation [ICC] ranging from 0.89 to 0.97; and ESs ranging from 0.16 to 0.42). For the variability of accommodation, we observed an excellent reliability for the comparisons of the 90-s and 60-s measures at the three accommodative demands (ICC: 0.95-0.96), whereas for the rest of comparisons we observed from moderate to good levels of reliability.

CONCLUSION

Our data indicate that of 30-s and 60-s are enough to accurately assess the magnitude and variability of the accommodative response with the WAM-5500 autorefractor, respectively.

Manipulating expectancies in optometry practice: Ocular accommodation and stereoacuity are sensitive to placebo and nocebo effects

INTRODUCTION

There is scientific evidence that an individual's beliefs and/or expectations play a role in the behavioural and physiological response to a given treatment. This study aimed to assess whether the dynamics of the accommodative response and stereoacuity are sensitive to experimentally induced placebo and nocebo effects.

METHODS

Nineteen healthy university students performed three experimental sessions (placebo, nocebo and control) in randomised order, with the dynamics of the accommodative response (magnitude and variability), stereoacuity and subjective measures being assessed in all sessions. For the experimental manipulation, participants ingested an inert capsule that was alleged to have positive (white capsule, placebo condition) or negative (yellow capsule, nocebo conditions) effects on the human physiology. In the control condition, participants did not ingest a capsule.

RESULTS

The data revealed that the variability of accommodation was sensitive to experimentally induced placebo and nocebo effects, showing a more stable accommodative response for the placebo compared with the nocebo condition (corrected p-value = 0.04, Cohen's d = 0.60). In addition, better stereoacuity was found with the placebo, compared with the nocebo (corrected p-value = 0.01, Cohen's d = 0.69) and control (corrected p-value = 0.03, Cohen's d = 0.59) conditions. Successful experimental manipulation was confirmed by the analysis of subjective perceptions.

CONCLUSIONS

These findings provide evidence that manipulating expectations about the efficacy of an inert treatment affect the dynamics of the accommodative response (variability of accommodation) and stereoacuity. The results have important applications in both clinical and research outcomes, where individuals´ beliefs/expectations could modulate the visual function.

Variability of Accommodative Microfluctuations in Myopic and Emmetropic Juveniles during Sustained near Work

Near work has been considered to be a potential risk factor for the onset of myopia, but with inadequate evidence. Chinese adolescents use digital devices for near work, such as study and entertainment purposes, especially during the COVID-19 pandemic. In this study, we investigated the influence of prolonged periods of near work on accommodative response, accommodative microfluctuations (AMFs), and pupil diameter between juvenile subjects of myopia and emmetropia. Sixty juveniles (30 myopes and 30 emmetropes) were recruited for the study. Participants were instructed to play a video game on a tablet PC at a distance of 33.3 cm for 40 min. Accommodative response and pupil diameter were measured with an open-field infrared refractometer in High-speed mode. Parameters of the subjects were measured once every 10 min, and analyzed by one-way repeated measure ANOVA for variation tendency. There were no significant differences between emmetropia and myopia groups with respect to age and sex (p > 0.05). The low-frequency component (LFC) of myopia gradually increased with time, reached a peak at 30 min, and then declined (p = 0.043). The high-frequency component (HFC) of myopia also reached a peak at 30 min (p = 0.036). Nevertheless, there was no significant difference in the LFC (p = 0.171) or HFC (p = 0.278) of the emmetropia group at each time point. There was no significant difference in the mean and standard deviation of the accommodative response and pupil diameter both in emmetropic and myopic juveniles. Compared with juvenile emmetropes, myopes exhibit an unstable tendency in their accommodation system for prolonged near work at a certain time point. Accommodative microfluctuations may be a sensitive, objective indicator of fatigue under sustained near work in juvenile myopes.

The effect of image resolution of display types on accommodative microfluctuations

PURPOSE

To determine whether accommodative microfluctuations (AMFs) are affected by the image resolution of the display type being observed. The effect of refractive error is also examined.

METHODS

Twenty participants, (10 myopes and 10 emmetropes) observed a target on four different displays: paper, smartphone, e-reader and visual display unit screen (VDU), whilst their accommodative responses were measured using a continuous recording infrared autorefractor. The accommodative response and AMF measures comprising low frequency components (LFC), high frequency components (HFC) and the root mean square (RMS) of the AMFs were analysed.

RESULTS

A significant increase in LFC power was observed for the paper stimulus when compared to the VDU and smartphone conditions. Myopes demonstrated a significantly higher LFC and mean accommodative response compared to emmetropes across the four displays. A significant difference in the mean AR between the displays with the lowest and highest resolution was found. A higher mean AR was found with higher resolution of the image. The HFC and RMS accommodation were not affected by display type.

CONCLUSION

The mean accommodative response and the mean LFC power appear to respond differently depending on the type of display in use. Higher resolution devices showed a reduced lag of accommodation to the accommodative demand; however, this may cause a lead of accommodation in myopes for higher resolution display types.

The random walk of accommodation fluctuations

The focusing distance of the eye fluctuates during accommodation. However, the visual role of these accommodation fluctuations is not yet fully understood. The fluctuation complexity is one of the obstacles to this long standing challenge in visual science. In this work we seek to develop a statistical approach that i) accurately describes experimental measurements and ii) directly generates randomized and realistic simulations of accommodation fluctuations for use in future experiments. To do so we use the random walk approach, which is usually appropriate to describe the dynamics of systems that combine both randomness and memory.

Fluctuations of Steady-State Accommodation Is a Marker for Screening Spasm of Near Reflex

Purpose

To determine the utility of root mean squared (RMS) deviations of steady-state accommodation as a noncycloplegic marker for spasm of near reflex (SNR) vis-à-vis regular refractive errors.

Methods

Binocular steady-state responses of accommodation, pupil, and vergence of 20 patients with accommodative spasm subtype of SNR (SNR-A; 9-23 years) and 91 with regular refractive errors (29 emmetropes, 41 myopes, 21 hyperopes; 19-38 years) was recorded in the uncorrected refractive error state for 120 seconds using a dynamic (50 frames per second), infrared photorefractor. Mean and RMS deviation of raw data was calculated for three 20-second-long epochs and their diagnostic utility was determined using standard ROC curves.

Results

RMS deviations of accommodation increased with mean refractive error in SNR-A (y = -0.23x + 0.38; r2 = 0.69; P < 0.001) and regular refractive error (y = -0.02x + 0.10; r2 = 0.14; P = 0.002) cohorts, albeit with steeper slope and higher y-intercept in the former rather than the latter cohort. RMS deviation of 0.19D reliably distinguished SNR-A from regular refractive errors with a sensitivity and specificity of 95.2% and 92.2%, respectively [mean (±1 SEM) area under ROC curve: 0.98 ± 0.01]. The sensitivity, specificity, and area under ROC curve for RMS deviations of pupil (66.7%, 80%, and 0.70 ± 0.09) and vergence (52.4%, 84.6%, and 0.68 ± 0.08) were smaller than accommodation.

Conclusions

RMS deviations of steady-state accommodation is a robust noncycloplegic marker for differentiating SNR-A from regular refractive errors. Pupil and vergence fluctuations have limited utility in this regard.

Translational Relevance

RMS deviations of accommodation may be easily obtained using commercial photorefractors, and the cut-off values reported herein may be implemented to identify SNR-A during refractive error screening.

IMI Accommodation and Binocular Vision in Myopia Development and Progression

The role of accommodation in myopia development and progression has been debated for decades. More recently, the understanding of the mechanisms involved in accommodation and the consequent alterations in ocular parameters has expanded. This International Myopia Institute white paper reviews the variations in ocular parameters that occur with accommodation and the mechanisms involved in accommodation and myopia development and progression. Convergence is synergistically linked with accommodation and the impact of this on myopia has also been critiqued. Specific topics reviewed included accommodation and myopia, role of spatial frequency, and contrast of the task of objects in the near environment, color cues to accommodation, lag of accommodation, accommodative-convergence ratio, and near phoria status. Aspects of retinal blur from the lag of accommodation, the impact of spatial frequency at near and a short working distance may all be implicated in myopia development and progression. The response of the ciliary body and its links with changes in the choroid remain to be explored. Further research is critical to understanding the factors underlying accommodative and binocular mechanisms for myopia development and its progression and to guide recommendations for targeted interventions to slow myopia progression.

The effects of colour and temporal frequency of flickering light on variability of the accommodation response in emmetropes and myopes

Background

Myopia is hypothesized to be influenced by environmental light conditions. For example, it has been shown that colour and temporal frequency of flickering light affect emmetropisation in animals. Considering the omnipresence of flickering light in our daily life, we decided to analyze the effect of colour flickers on variability of the accommodation response (VAR) in emmetropes and myopes.

Methods

We measured the dynamic accommodative responses of 19 emmetropic and 22 myopic adults using a Grand Seiko WAM-5500 open-field autorefractor. The subjects focused for more than 20 s on a black Snellen E target against three different backgrounds made up of three colour flicker combinations (red/green, red/blue and blue/green) and under five frequency conditions (0.20 Hz, 0.50 Hz, 1.00 Hz, 1.67 Hz, and 5.00 Hz).

Results

Flicker frequency and colour both had a significant effect on VAR. Lower frequencies were associated with larger variability. Colour had an effect only at low frequencies, and red/blue colour flicker resulted in the largest variability. The variability in myopes were larger than those in emmetropes.

Conclusions

These findings support the hypothesis that further studies on the colour and temporal frequency of flickering light can lead to a better understanding of the development and progression of myopia.

Dual-angle open field wavefront sensor for simultaneous measurements of the central and peripheral human eye

We have developed a novel dual-angle open field wavefront sensor. This device captures real-time foveal and peripheral Zernike aberrations, while providing natural binocular viewing conditions for the subjects. The simultaneous data recording enables accurate analysis of changes in ocular optics with accommodation overcoming any uncertainties caused by accommodative lag or lead. The instrument will be used in myopia research to study central and peripheral ocular optics during near work and to investigate the effects of optical myopia control interventions. Proof of concept measurements, performed on an artificial eye model and on 3 volunteers, showed good repeatability with foveal-peripheral data synchronization of 65 msec or better. The deviations from subjective cycloplegic refractions were not more than 0.31 D. Furthermore, we tested the dual-angle wavefront sensor in two novel measurement schemes: (1) focusing on a close target, and (2) accommodation step change.

Objective User Visual Experience Evaluation When Working with Virtual Pixel-Based 3D System and Real Voxel-Based 3D System

Volumetric display shows promising implications for healthcare related applications as an innovative technology that creates real three-dimensional (3D) image by illuminating points in three-dimensional space to generate volumetric images without image separation. We used eccentric photorefractometry to objectively study ocular performance in a practical environment by evaluating near work-induced refraction shift, accommodative microfluctuations, and pupil size for 38 young adults after viewing anaglyph, and volumetric 3D content for prolonged time. The results of our study demonstrate that participants who performed relative depth estimation task on volumetric 3D content were less likely to experience task-induced myopic refraction shift. For both 3D content types, we observed pupil constriction, that is possibly related to visual fatigue. For anaglyph 3D pupil constriction, onset was observed significantly sooner, compared to volumetric 3D. Overall, sustained work with 3D content, and small disparities or the fully eliminated possibility of accommodation-vergence conflict, not only minimizes near work-induced myopic shift, but also provide beneficial accommodation relaxation that was demonstrated in this study as hypermetropic shift for nearly half of participants.

Emmetropes and myopes differ little in their accommodation dynamics but strongly in their ciliary muscle morphology

Previous work suggested an association between near vision and myopia. We therefore investigated the accommodation process in emmetropes and myopes regarding morphologic changes of the ciliary muscle (CM) and power changes of the lens for different accommodation demands. The temporal CM of 18 emmetropic and 20 myopic students was imaged via anterior segment optical coherence tomography during far and near accommodation (2.5D, 3D, 4D). Additionally, accommodation dynamics to the stimuli pattern far-near-far (15 s each; 2.5D, 3D, 4D) were recorded with eccentric infrared photorefraction. OCT images were processed using custom-developed software facilitating the analysis of selective CM thickness (CMT) readings and CMT profiles. Anterior CMT readings were significantly smaller in myopes. Starting at 1.4 mm posterior to the scleral spur (SP), myopic CM became thicker than emmetropic. Anterior CMT changes (ΔCMT) continuously increased with accommodation demand in myopes while emmetropic ΔCMT only increased from 2.5D to 3D. Compared to emmetropes, myopes showed smaller ΔCMT but increased CM movement relative to SP. There were no significant differences between the groups for accommodation changes from far to near vision and vice versa, velocity, microfluctuations, power spectra or lag of accommodation. At 4 D, larger ΔCMT were associated with lower lens changes for disaccommodation. While CM shape, movement, and thickness showed distinct differences depending on refractive error, emmetropes and myopes did not differ in their dynamic accommodation. Further analysis is necessary to evaluate whether the CM's anatomical shape or predispositions in its intramuscular constituents are causative factors in myopigenesis.

The influence of age, refractive error, visual demand and lighting conditions on accommodative ability in Malay children and adults

PURPOSE

Near work, accommodative inaccuracy and ambient lighting conditions have all been implicated in the development of myopia. However, differences in accommodative responses with age and refractive error under different visual conditions remain unclear. This study explores differences in accommodative ability and refractive error with exposure to differing ambient illumination and visual demands in Malay schoolchildren and adults.

METHODS

Sixty young adults (21-25 years) and 60 schoolchildren (8-12 years) were recruited. Accommodative lag and accommodative fluctuations at far (6 m) and near (25 cm) were measured using the Grand Seiko WAM-5500 open-field autorefractor. The effects of mesopic room illumination on accommodation were also investigated.

RESULTS

Repeated-measures ANOVA indicated that accommodative lag at far and near differed significantly between schoolchildren and young adults [F(1.219, 35.354) = 11.857, p < 0.05]. Post hoc tests using the Bonferroni correction showed that at near, there was a greater lag in schoolchildren (0.486 ± 0.181 D) than young adults (0.259 ± 0.209 D, p < 0.05). Repeated-measures ANOVA also revealed that accommodative lag at near demands differed statistically between the non-myopic and myopic groups in young adults and schoolchildren [F(3.107, 31.431) = 12.187, p < 0.05]. Post hoc tests with Bonferroni correction showed that accommodative lag at near was significantly greater in myopic schoolchildren (0.655 ± 0.198 D) than in non-myopic schoolchildren (0.202 ± 0.141 D, p < 0.05) and myopic young adults (0.316 ± 0.172 D, p < 0.05), but no significant difference was found between myopic young adults (0.316 ± 0.172 D) and non-myopic young adults (0.242 ± 0.126 D, p > 0.05). Accommodative lag and fluctuations were greater under mesopic room conditions for all ages [all p < 0.05].

CONCLUSION

Greater accommodative lag was found in myopes than in emmetropes, in schoolchildren than in adults, and under mesopic conditions than under photopic conditions. Accommodative fluctuations were greatest in myopes and in mesopic conditions. These results suggest that differences exist in the amount of blur experienced by myopes and non-myopes at different ages and under different lighting conditions.

The Effects of Spatial Frequency on the Accommodative Responses of Myopic and Emmetropic Chinese Children

Purpose

The spatial frequency (SF) characteristics of accommodation in children are not well understood. In this study, we measured accommodative responses to grating targets to investigate the SF dependence of accommodation in children.

Methods

The effects of SF and contrast on the accommodative system were evaluated in two groups of children, including 22 with emmetropia and 20 with myopia. The contrast detection thresholds at five SFs were measured using a near-contrast sensitivity function test. The accommodative responses to grating targets with low (1.5 cycles per degree [cpd]), medium (6 cpd), and high (18 cpd) SFs were measured with a Grand Seiko WAM-5500 in dynamic mode for 30 seconds under standard and detection threshold contrast conditions. The accommodative lag and accommodative microfluctuations (AMFs) were compared between the two groups.

Results

Under standard contrast conditions, no significant difference was found in the accommodative lag across SFs (F = 2.03, P = 0.14) or between the two groups (F = 3.57, P = 0.07). The AMFs were lowest at 6 cpd in emmetropia group (F = 6.51, P = 0.003) and in total (F = 10.82, P < 0.001). Children in emmetropia group showed greater AMFs at high SFs under detection threshold contrast conditions than under standard contrast conditions (P < 0.05).

Conclusions

This study demonstrated that the instability of accommodation was SF dependent in children. The AMFs in children were smallest at the medium SF for standard contrast grating targets. Myopic children are less sensitive to the low-contrast-induced blur for high SFs than emmetropic children.

Translational Relevance

This study provides a possibility to stabilize accommodative response of children by transforming SF components of fixation targets.

Role of microfluctuations in accommodation: a novel approach to reduce non-accommodative noise

Accommodative response and its possible role in myopia development has been explored through the study of the microfluctuations (MFs) of accommodation, which are commonly divided in high (1.0 to 2.3 Hz) and low (0.1 to 0.6 Hz) frequency components. Previous research efforts have evidenced that a certain percentage of the amplitude of MFs seems not to originate in the accommodative response. We aimed to develop and test a new approach to reduce this non-accommodative noise. For this purpose, ten healthy participants were enrolled to determine the difference between the amplitude of MFs at near and distance for each range of frequencies, which was defined as the relative amplitude of MFs. The findings support the exploration of the relative rather than absolute values of the amplitude of MFs to better understand the contribution of both accommodative and non-accommodative factors to MFs.

Is 25Hz enough to accurately measure a dynamic change in the ocular accommodation?

BACKGROUND

Accommodation is often recorded at a low sampling rate using devices such as autorefractors that are designed to measure the static refractive error. It is therefore important to determine if that resolution is sufficient to accurately measure the dynamic properties of accommodation. The current study provides both theoretical and empirical evidence on the ideal sampling rate necessary to measure a dynamic response.

METHODS

Accommodative and disaccommodative step stimuli ranging from 1-3D (1D steps) were presented using a Badal optical system. Responses from 12 children (8-13 years) and 6 adults (20-35 years) were recorded using a dynamic photorefractor (DPR). Fast Fourier transformation was applied to the unsmoothed dynamic responses including position, velocity and acceleration. Also, velocity and acceleration main sequence (MS) characteristics were compared between three photorefractor conditions on 3 subjects.

RESULTS

The Nyquist sampling limit necessary to accurately estimate position, velocity and acceleration was at least 5, 10 and 70Hz, respectively. Peak velocity and acceleration were significantly underestimated at a lower rate (p<0.5). However, the slope of MS remained invariant with sampling rate (p>0.5).

CONCLUSION

Contrary to the previous findings, a dynamic accommodative response exhibited frequencies larger than 10Hz. Stimulus direction and amplitude had no influence on the frequencies present in the dynamic response. Peak velocity and acceleration can be significantly underestimated when sampled at a lower rate. Taken as a whole, low sampling rate instruments can accurately estimate static accommodation, however, caution needs to be exercised when using them for dynamic accommodation.

Is blur sensitivity altered in children with progressive myopia?

School aged children with progressive myopia show large accommodative lags to blur only cue which is suggestive of a large depth of focus (DOF). While DOF measures are lacking in this age group, their blur detection and discrimination capacities appear to be similar to their non-myopic peers. Accordingly, the current study quantified DOF and blur detection ability in progressive myopic children showing large accommodative lags compared to their non-myopic peers and adults. Blur sensitivity measures were taken from 12 children (8-13 years, 6 myopes and 6 emmetropes) and 6 adults (20-35 years). DOF was quantified using step changes in the lens induced defocus while the subjects viewed a high contrast target through a Badal lens at either 2 or 4D demand. Blur detection thresholds (BDT) were tested using a similar high contrast target in a 2-alternate forced-choice paradigm (2AFC) at both the demands. In addition to the large accommodative lags, micro fluctuations and DOF were significantly larger in myopic children compared to the other groups. However, BDTs were similar across the three groups. When limited to blur cues, the findings of a large DOF coupled with large response lags suggests that myopes are less sensitive to retinal defocus. However, in agreement to a previous study, refractive error had no influence on their BDTs suggesting that the reduced sensitivity to the defocus in a myopic eye appears to be compensated by some form of an adjustment in the higher visual processes to preserve the subjective percept even with a poor retinal image quality.

Cognitive Demand and Accommodative Microfluctuations

Previous studies have shown cognition to have an influence on accommodation. Temporal variation in the accommodative response occurs during the fixation on a stationary target. This constantly shifting response has been called accommodative micro-fluctuations (AMFs). The aim of this study is to determine the effects of increasing task cognitive demand on the ocular accommodation response. AMFs for 12 myopes and 12 emmetropes were measured under three conditions of varying cognitive demand and comprising reading of numbers (Num), simple arithmetic (SA), and complex arithmetic (CA). Fast Fourier transforms were used to analyze the different frequency band components of the AMFs. Other aspects of AMFs including root mean square accommodation values and chaos analysis was applied. A repeated measures ANOVA revealed a significant main effect of cognition in the mean power of the high frequency component (HFC) (F_2,44 = 10.03, p < 0.005). Pairwise analyses revealed that these differences exist between SA and CA tasks (p < 0.005) and the Num and CA (p < 0.005) tasks with the HFC power being the highest for the CA condition. It appears that the difficulty of a task does affect active accommodation but to a lesser extent than other factors affecting accommodation.

Effect of Experimental Conditions in the Accommodation Response in Myopia

SIGNIFICANCE

The accommodative response is more affected by the type of refractive error than the method of stimulation, field of view (FOV), or stimulus depth.

PURPOSE

This study aims to analyze the effect of stimulation method, stimulus depth, and FOV on the accommodation response (AR) for emmetropes (EMM), late-onset myopes (LOM), and early-onset myopes (EOM).

METHODS

Monocular AR was measured in 26 young observers (n = 9 EMM, n = 8 LOM, n = 9 EOM) under 60 different viewing conditions that were the result of permuting the following factors: (1) stimulation method (free space or Badal lens viewing), (2) stimulus depth (flat or volumetric), (3) FOV (2.5, 4, 8, 10, and 30°), and (4) accommodative stimulus (AS: 0.17, 2.50, and 5.00 diopters [D]).

RESULTS

Mixed analysis of variance for 2.50 D of AS resulted in a significant effect of refractive group (F = 6.77, P < .01) and FOV (F = 1.26, P = .04). There was also a significant interaction between stimulus depth and FOV (F = 2.73, P = .03) and among stimulation method, FOV, and refractive group (F = 2.42, P = .02). For AS of 5.00 D, there was a significant effect of refractive group (F = 13.88, P < .01) and stimulation method (F = 5.16, P = .03). There was also a significant interaction of stimulation method, stimulus depth, and refractive group (F = 4.08, P = .03). When controlling for all interactions, LOM showed larger lags than EMM and EOM; the AR did not significantly change for fields of 8, 10, and 30°, and it did not significantly differ for different stimulation methods or stimulus depth.

CONCLUSIONS

Previously reported differences in AR when using lens-based methods compared with free space viewing may be explained by the effect of other factors such as the FOV or the depth of the stimulus. Targets with an FOV of 8 or 10° may be optimal for accurate ARs.

Vergence driven accommodation with simulated disparity in myopia and emmetropia

The formation of focused and corresponding foveal images requires a close synergy between the accommodation and vergence systems. This linkage is usually decoupled in virtual reality systems and may be dysfunctional in people who are at risk of developing myopia. We study how refractive error affects vergence-accommodation interactions in stereoscopic displays. Vergence and accommodative responses were measured in 21 young healthy adults (n=9 myopes, 22-31 years) while subjects viewed naturalistic stimuli on a 3D display. In Step 1, vergence was driven behind the monitor using a blurred, non-accommodative, uncrossed disparity target. In Step 2, vergence and accommodation were driven back to the monitor plane using naturalistic images that contained structured depth and focus information from size, blur and/or disparity. In Step 1, both refractive groups converged towards the stereoscopic target depth plane, but the vergence-driven accommodative change was smaller in emmetropes than in myopes (F_1,19=5.13, p=0.036). In Step 2, there was little effect of peripheral depth cues on accommodation or vergence in either refractive group. However, vergence responses were significantly slower (F_1,19=4.55, p=0.046) and accommodation variability was higher (F_1,19=12.9, p=0.0019) in myopes. Vergence and accommodation responses are disrupted in virtual reality displays in both refractive groups. Accommodation responses are less stable in myopes, perhaps due to a lower sensitivity to dioptric blur. Such inaccuracies of accommodation may cause long-term blur on the retina, which has been associated with a failure of emmetropization.

Effects of Near Addition Lenses and Prisms on Accommodative Microfluctuations in Chinese Children

PURPOSE

To investigate the effects of different near addition lenses and prisms on accommodative microfluctuations (AMFs) in Chinese early-onset myopic (EOM) and emmetropic (EMM) children.

METHODS

Twenty-one EMM and 27 EOM children aged between 9 and 14years participated in the study. At near, 23 children were exophoric (exo, <0 Δ), and 25 were esophoric (eso, ≥0 Δ). The AMFs and phoria through multiple addition lenses (-1.00, 0, +1.00, +2.00, and +3.00D on each eye) and prisms (base-in prism power: 3 Δ, 2 Δ, 1 Δ; and base-out prism power: 1 Δ and 2 Δ on each eye) were measured at 25cm under binocular viewing conditions with a Grand Seiko WAM-5500 auto-refractor and a modified Thorington card.

RESULTS

Higher AMFs were found in EOM than in EMM (EOM, 0.19±0.06D; EMM, 0.16±0.03D; p=0.035). Plus additions from +1.00 to +3.00D reduced the AMFs in both EMM and EOM, except +3.00D for EMM. In both refractive groups, -1.00D additions increased AMFs. Esophores showed greater AMFs compared with exophores (esophores, 0.20±0.01D; exophores, 0.16±0.01D; p=0.012). Prisms increased the AMFs in EMM and did not alter the AMFs in EOM.

CONCLUSIONS

Microfluctuations of accommodation decreased with plus addition lenses and increased with negative addition lenses. Plus addition lenses of +2.00 to +3.00D for EOM and +2.00D for EMM decreased AMFs the least at a 25-cm working distance. Esophores had higher AMFs than exophores, indicating that phoria status influences AMFs.

Sensitivity of Chaos Measures in Detecting Stress in the Focusing Control Mechanism of the Short-Sighted Eye

When fixating on a stationary object, the power of the eye’s lens fluctuates. Studies have suggested that changes in these so-called microfluctuations in accommodation may be a factor in the onset and progression of short-sightedness. Like many physiological signals, the fluctuations in the power of the lens exhibit chaotic behaviour. A breakdown or reduction in chaos in physiological systems indicates stress to the system or pathology. The purpose of this study was to determine whether the chaos in fluctuations of the power of the lens changes with refractive error, i.e. how short-sighted a subject is, and/or accommodative demand, i.e. the effective distance of the object that is being viewed. Six emmetropes (EMMs, non-short-sighted), six early-onset myopes (EOMs, onset of short-sightedness before the age of 15), and six late-onset myopes (LOMs, onset of short-sightedness after the age of 15) took part in the study. Accommodative microfluctuations were measured at 22 Hz using an SRW-5000 autorefractor at accommodative demands of 1 D (dioptres), 2 D, and 3 D. Chaos theory analysis was used to determine the embedding lag, embedding dimension, limit of predictability, and Lyapunov exponent. Topological transitivity was also tested for. For comparison, the power spectrum and standard deviation were calculated for each time record. The EMMs had a statistically significant higher Lyapunov exponent than the LOMs (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.64\pm 0.33$$\end{document}0.64±0.33 vs. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.39\pm 0.20~\hbox {D}/\hbox {s}$$\end{document}0.39±0.20D/s) and a lower embedding dimension than the LOMs (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$3.28\pm 0.46$$\end{document}3.28±0.46 vs. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$3.67\pm 0.49$$\end{document}3.67±0.49). There was insufficient evidence (non-significant p value) of a difference between EOMs and EMMs or EOMs and LOMs. The majority of time records were topologically transitive. There was insufficient evidence of accommodative demand having an effect. Power spectrum analysis and assessment of the standard deviation of the fluctuations failed to discern differences based on refractive error. Chaos differences in accommodation microfluctuations indicate that the control system for LOMs is under stress in comparison to EMMs. Chaos theory analysis is a more sensitive marker of changes in accommodation microfluctuations than traditional analysis methods.

Effect of apparent depth cues on accommodation in a Badal optometer

BACKGROUND

The aim was to analyse the effect of peripheral depth cues on accommodation in Badal optometers.

METHODS

Monocular refractions at 0.17 and 5.00 D of accommodative stimulus were measured with the PowerRef II autorefractor (Plusoptix Inc., Atlanta, Georgia, USA). Subjects looked (randomly) at four different scenes, one real scene comprising familiar objects at different depth planes (Real) and three virtual scenes comprising different two-dimensional pictures seen through a Badal lens. The first image consisted of a photograph of the real scene taken in conditions that closely mimic a healthy standard human eye performance (out-of-focus [OoF] blur); the second image was the same photograph rendered with a depth of focus to infinity (OoF sharpness); and finally the third image consisted of a fixation target and a even white surrounding (White). In all cases the field of view was 25.0° and the fixation target was a Maltese cross subtending to two degrees.

RESULTS

Twenty-eight right eyes from healthy young subjects were measured. The achieved statistical power was 0.9. At 5.00 D of accommodative stimulus, the repeated measures analysis of variance was statistically significant (p < 0.05) and the corresponding Bonferroni post hoc tests showed the following mean accommodative response differences and standard deviation (p-value) between the real and the virtual scenes: real-white =-0.66 ± 0.92 D (p < 0.01); real-OoF sharpness = -0.43 ± 0.88 D (p = 0.07); real-OoF blur =-0.25 ± 0.93 D (p = 0.89).

CONCLUSIONS

A stimulus poor in depth cues inaccurately stimulates accommodation in Badal optometers; however, accommodation can be significantly improved in the same Badal optometer, when displaying a realistic image rich in peripheral depth cues, even though these peripheral cues (also referred to as retinal blur cues) are shown in the same plane as the fixation target. These results have important implications in stereoscopic virtual reality systems that fail to represent appropriately retinal blur.

Differences in the accommodation stimulus response curves of adult myopes and emmetropes: a summary and update

PURPOSE

To provide a summary of the classic paper "Differences in the accommodation stimulus response curves of adult myopes and emmetropes" published in Ophthalmic and Physiological Optics in 1998 and to provide an update on the topic of accommodation errors in myopia.

SUMMARY

The accommodation responses of 33 participants (10 emmetropes, 11 early onset myopes and 12 late onset myopes) aged 18-31 years were measured using the Canon Autoref R-1 free space autorefractor using three methods to vary the accommodation demand: decreasing distance (4 m to 0.25 cm), negative lenses (0 to -4 D at 4 m) and positive lenses (+4 to 0 D at 0.25 m). We observed that the greatest accommodation errors occurred for the negative lens method whereas minimal errors were observed using positive lenses. Adult progressing myopes had greater lags of accommodation than stable myopes at higher demands induced by negative lenses. Progressing myopes had shallower response gradients than the emmetropes and stable myopes; however the reduced gradient was much less than that observed in children using similar methods.

RECENT FINDINGS

This paper has been often cited as evidence that accommodation responses at near may be primarily reduced in adults with progressing myopia and not in stable myopes and/or that challenging accommodation stimuli (negative lenses with monocular viewing) are required to generate larger accommodation errors. As an analogy, animals reared with hyperopic errors develop axial elongation and myopia. Retinal defocus signals are presumably passed to the retinal pigment epithelium and choroid and then ultimately the sclera to modify eye length. A number of lens treatments that act to slow myopia progression may partially work through reducing accommodation errors.

Research of accommodative microfluctuations caused by visual fatigue based on liquid crystal and laser displays

Different levels of visual fatigue in the human eye depend on different color-formation methods and image quality. This paper uses the high-frequency component of the spectral power of accommodative microfluctuations as a major objective indicator for analyzing the effects of visual fatigue based on various displays, such as color-formation displays and 3D displays. Also, a questionnaire is used as a subjective indicator. The results are that 3D videos cause greater visual fatigue than 2D videos (p<0.001), the shutter-type 3D display causes visual fatigue more than the polarized type (p=0.012), the display of the time-sharing method causes greater visual fatigue than the spatial-formation method (p=0.008), and there is no significance between various light source modules of displays (p=0.162). In general, people with normal color discrimination have more visual fatigue than those with good color discrimination (p<0.001). Therefore, this paper uses the high-frequency component of accommodative microfluctuations to evaluate the physiological stress or strain by overexerting the visual system, and can compare the level of visual fatigue between various displays.

Retinal image quality during accommodation in adult myopic eyes

PURPOSE

Reduced retinal image contrast produced by accommodative lag is implicated with myopia development. Here, we measure accommodative error and retinal image quality from wavefront aberrations in myopes and emmetropes when they perform visually demanding and naturalistic tasks.

METHODS

Wavefront aberrations were measured in 10 emmetropic and 11 myopic adults at three distances (100, 40, and 20 cm) while performing four tasks (monocular acuity, binocular acuity, reading, and movie watching). For the acuity tasks, measurements of wavefront error were obtained near the end point of the acuity experiment. Refractive state was defined as the target vergence that optimizes image quality using a visual contrast metric (VSMTF) computed from wavefront errors.

RESULTS

Accommodation was most accurate (and image quality best) during binocular acuity whereas accommodation was least accurate (and image quality worst) while watching a movie. When viewing distance was reduced, accommodative lag increased and image quality (as quantified by VSMTF) declined for all tasks in both refractive groups. For any given viewing distance, computed image quality was consistently worse in myopes than in emmetropes, more so for the acuity than for reading/movie watching. Although myopes showed greater lags and worse image quality for the acuity experiments compared to emmetropes, acuity was not measurably worse in myopes compared to emmetropes.

CONCLUSIONS

Retinal image quality present when performing a visually demanding task (e.g., during clinical examination) is likely to be greater than for less demanding tasks (e.g., reading/movie watching). Although reductions in image quality lead to reductions in acuity, the image quality metric VSMTF is not necessarily an absolute indicator of visual performance because myopes achieved slightly better acuity than emmetropes despite showing greater lags and worse image quality. Reduced visual contrast in myopes compared to emmetropes is consistent with theories of myopia progression that point to image contrast as an inhibitory signal for ocular growth.

Corneal and crystalline lens dimensions before and after myopia onset

PURPOSE

To describe corneal and crystalline lens dimensions before, during, and after myopia onset compared with age-matched emmetropic values.

METHODS

Subjects were 732 children aged 6 to 14 years who became myopic and 596 emmetropic children participating between 1989 and 2007 in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error Study. Refractive error was measured using cycloplegic autorefraction, corneal power using a hand-held autokeratometer, crystalline lens parameters using video-based phakometry, and vitreous chamber depth (VCD) using A-scan ultrasonography. Corneal and crystalline lens parameters in children who became myopic were compared with age-, gender-, and ethnicity-matched model estimates of emmetrope values annually from 5 years before through 5 years after the onset of myopia. The comparison was made without and then with statistical adjustment of emmetrope component values to compensate for the effects of longer VCDs in children who became myopic.

RESULTS

Before myopia onset, the crystalline lens thinned, flattened, and lost power at similar rates for emmetropes and children who became myopic. The crystalline lens stopped thinning, flattening, and losing power within ±1 year of onset in children who became myopic compared with emmetropes statistically adjusted to match the longer VCDs of children who became myopic. In contrast, the cornea was only slightly steeper in children who became myopic compared with emmetropes (<0.25 D) and underwent little change across visits.

CONCLUSIONS

Myopia onset is characterized by an abrupt loss of compensatory changes in the crystalline lens that continue in emmetropes throughout childhood axial elongation. The mechanism responsible for this decoupling remains speculative but might include restricted equatorial growth from internal mechanical factors.

Accommodative instability: relationship to progression of early onset myopia

BACKGROUND

In a previous study, we demonstrated that children with early onset myopia had greater instability of accommodation than a group of emmetropic children. Since that study was correlational, we were unable to determine the causal relationship between this and myopic progression. To address this, we examined the children two years later. We predicted that if accommodative instability was causing the myopic progression, instability at Visit 1 should predict the refractive error at Visit 2. Additionally, instability at Visit 1 should predict myopic progression.

METHODS

Thirteen myopic and 16 emmetropic children were included in the analysis. Dynamic measures of accommodation were made using eccentric photorefraction (PowerRefractor) while children viewed targets set at three distances (accommodative demands), namely, 0.25 metres (4.00 D demand), 0.5 metres (2.00 D demand) and 4.00 metres (0.25 D demand).

RESULTS

Both refractive error and accommodative instability at Visit 1 were highly correlated with the same measures at Visit 2. Children with myopia showed greater instability of accommodation (0.38 D) than children with emmetropia (0.26 D) at the 4.00 D target on Visit 1 and this instability of accommodation weakly predicted myopic progression.

CONCLUSIONS

The results presented in the present study suggest that instability of accommodation accompanies myopic progression, although a casual relationship cannot be established.

Accommodation steps, target spatial frequency and refractive error

PURPOSE

Errors in the accommodation response of myopes have been reported in many studies although questions remain about the exact differences in accommodation steps when compared with emmetropic individuals.

METHODS

The characteristics of the accommodation step response to large (4/1D) and small (3/2D) steps in targets with low (0.5 cpd), mid (4 cpd) and high (16 cpd) spatial frequency (SF) information was measured in myopes (MYOs) and emmetropes (EMMs).

RESULTS

In terms of step size, the larger steps showed a greater response in the 4 cpd condition than the 0.5 and 16 cpd conditions and an improved percentage correct response in the 4 cpd compared to the 16 cpd steps. In small step conditions target SF had less effect upon the magnitude of the response. In terms of refractive group differences, MYOs had a lower proportion of correct accommodation responses compared to EMMs during the small steps only, however, when correct steps were performed there were no differences in the characteristics of both large and small step responses between MYOs and EMMs.

CONCLUSIONS

These findings suggest that MYOs have some difficulty interpreting small changes in defocus to initiate or possibly fine tune a small accommodation response, however, when a correct accommodation step response is made, the MYOs accommodation plant responds in a similar manner to EMMs.

Temporal dynamics of ocular aberrations: monocular vs binocular vision

The temporal dynamics of ocular aberrations are important for the evaluation of, e.g. the accuracy of aberration estimates, the correlation to visual performance, and the requirements for real-time correction with adaptive optics. Traditionally, studies on the eye's dynamic behavior have been performed monocularly, which might have affected the results. In this study we measured aberrations and their temporal dynamics both monocularly and binocularly in the relaxed and accommodated state for six healthy subjects. Temporal frequencies up to 100 Hz were measured with a fast-acquisition Hartmann-Shack wavefront sensor having an open field-of-view configuration which allowed fixation to real targets. Wavefront aberrations were collected in temporal series of 5 s duration during binocular and monocular vision with fixation targets at 5 m and 25 cm distance. As expected, a larger temporal variability was found in the root-mean-square wavefront error when the eye accommodated, mainly for frequencies lower than 30 Hz. A statistically-significant difference in temporal behavior between monocular and binocular viewing conditions was found. However, on average it was too small to be of practical importance, although some subjects showed a notably higher variability for the monocular case during near vision. We did find differences in pupil size with mono- and binocular vision but the pupil size temporal dynamics did not behave in the same way as the aberrations' dynamics.

Accommodative fluctuations, lens tension, and ciliary body thickness in children

PURPOSE

To investigate the relationship among microfluctuations in accommodation, resting tension on the crystalline lens, ciliary body thickness, and refractive error in children.

METHODS

Subjects were 49 children, aged 8 to 15 years. Subjects wore habitual correction over their left eye and an infrared filter over the right eye during accommodative measurements. Monocular accommodation was measured continuously for two, 30-second periods using a PowerRef I at a sampling rate of 25 Hz while subjects viewed a high-contrast target at 0.25 m. The high (1.0 to 2.3 Hz) and low- (0 to 0.6 Hz) frequency components of the power spectrum from a fast Fourier transform of the accommodative response were used in analysis. Resting tension on the crystalline lens was assessed by measuring the amplitude of the oscillations of the crystalline lens after a rightward 20 degrees saccadic eye movement. Ciliary body thickness was measured 2 mm posterior to the scleral spur from images obtained with a Zeiss Visante optical coherence tomography (OCT). Cycloplegic spherical equivalent refractive error was obtained with the Grand Seiko autorefractor.

RESULTS

The mean +/- SD spherical equivalent refractive error was -1.00 D +/- 2.25 (range, -6.00 D to +3.44 D). Greater power in the log of the high-frequency component of accommodative microfluctuations was associated with thinner ciliary bodies (p = 0.03) and lower ages (p = 0.0004). More hyperopic refractive errors with greater power in the high-frequency component (p = 0.0005) and the low-frequency component (p = 0.02). No statistically significant relationship was found for the low-frequency component or root mean square of accommodative microfluctuations and refractive error.

CONCLUSIONS

High-frequency microfluctuations of accommodation appear to be suppressed with thicker ciliary bodies. These variations in accommodation need to be observed in a longitudinal study to better assess the functional significance of their relationship to ciliary body size and refractive error.

The effect of modulating ocular depth of focus upon accommodation microfluctuations in myopic and emmetropic subjects

The magnitude of accommodation microfluctuations increases in emmetropic subjects viewing low luminance targets or viewing a target through small artificial pupils. Larger microfluctuations reported in myopia may result from an abnormally large depth of focus (DoF). The effect of modulating the size of the DoF has not been investigated in myopic subjects and may help to explain the cause of the increased DoF. Accommodation microfluctuations were recorded under two experimental conditions. Firstly, 12 emmetropes (EMMs), and 24 myopes (MYOs) viewed a Maltese Cross target with luminance levels of 0.002, 0.2, 6 and 600cd/m(2) and in darkness, and second, 14 EMMs and 16 MYOs viewed a Maltese Cross target through pupil diameters of 0.5, 1, 2, 3, 4 and 5mm presented in Maxwellian view. The magnitude of the accommodation microfluctuations increased significantly with a target luminance of 0.002cd/m(2) (p<.03) and pinhole diameters of <2mm (p<.05). For all other luminance levels and pupil diameters the magnitude was constant. For both conditions, MYOs had significantly larger microfluctuations than EMMs (p<.01). Considerable inter-subject variability was observed in the degree to which the magnitude of the microfluctuations increased, for both the 0.002cd/m(2) luminance and 0.5mm pupils, however, this was not correlated with refractive error. The increase in the magnitude of the microfluctuations while viewing a low luminance target (0.002cd/m(2)) may be due to a shallower contrast gradient in the cortical image, with a consequent increase in DoF. The microfluctuations also increase when viewing through small pupils (<2mm), which increases the DoF without altering the contrast gradient. The larger microfluctuations found in the MYOs consolidates the theory that MYOs have a larger DoF than EMMs and therefore have a higher threshold for retinal image blur.

The use of cues to convergence and accommodation in naïve, uninstructed participants

A remote haploscopic video refractor was used to assess vergence and accommodation responses in a group of 32 emmetropic, orthophoric, symptom free, young adults naïve to vision experiments in a minimally instructed setting. Picture targets were presented at four positions between 2 m and 33 cm. Blur, disparity and looming cues were presented in combination or separately to asses their contributions to the total near response in a within-subjects design. Response gain for both vergence and accommodation reduced markedly whenever disparity was excluded, with much smaller effects when blur and proximity were excluded. Despite the clinical homogeneity of the participant group there were also some individual differences.

Technical note: effect of contact lenses on measurement of the accommodation microfluctuations

AIM

Dynamic measurement of accommodation in subjects with myopia usually involves recording through soft contact lenses (CLs) to correct the refractive error. Conversely, dynamic accommodation measurement in emmetropic control subjects is generally undertaken without any corrective lenses. The aim of this experiment was to determine whether CL correction affects the measurement of accommodation microfluctuations using infrared refractometry, and whether this needs to be considered in studies which attempt to compare accommodation responses between the two groups.

METHODS

Ten young emmetropic subjects viewed a high contrast Maltese cross target monocularly using the right eye at a target vergence of 0 D. The subjects viewed the target under two conditions: with CL condition and without CL condition, where the subjects viewed the target with the eye only. Accommodation responses of the right eye were recorded continuously for 2 min at a sampling rate of 52 Hz using the Shin-Nippon SRW-5000 autorefractor.

RESULTS

No significant difference (two-tailed paired t-test, t(9) = -1.499, p = 0.168) was found in mean accommodation response between the with CL (mean +/- S.D. = -0.02 +/- 0.24 D) and without CL conditions (mean +/- S.D. = +0.01 +/- 0.25 D). No significant (two-tailed paired t-test, t(9) = 0.151, p = 0.883) difference in the magnitude of the accommodation microfluctuations was found between the with CL (mean +/- S.D. = 0.162 +/- 0.04 D) and without CL condition (mean +/- S.D. = 0.169 +/- 0.04 D). Power spectrum analysis revealed no differences in the characteristics of the microfluctuations waveform between the two conditions. A control experiment carried out on a subgroup of five subjects using a negative (-3 D) CL demonstrated that there was no significant effect of the dioptric power of the CL on the magnitude of the accommodation microfluctuations (anova: F(3,15) = 0.254, p = 0.782).

CONCLUSION

Thin soft CLs do not affect the magnitude or frequency characteristics of accommodation microfluctuations when measured using the Shin-Nippon SRW-5000.

Variability of the accommodation response in early onset myopia

PURPOSE

Hyperopic retinal defocus (blur) is thought to be a cause of myopia. If the retinal image of an object is not clearly focused, the resulting blur is thought to cause the continuing lengthening of the eyeball during development causing a permanent refractive error. Both lag of accommodation, especially for near targets, and greater variability in the accommodative response, have been suggested as causes of increased hyperopic retinal blur. Previous studies of lag of accommodation show variable findings. In comparison, greater variability in the accommodative response has been demonstrated in adults with late onset myopia but has not been tested in children. This study looked at the lag and variability of accommodation in children with early onset myopia.

METHODS

Twenty-one myopic and 18 emmetropic children were tested. Dynamic measures of accommodation and pupil size were made using eccentric photorefraction (PowerRefractor) while children viewed targets set at three different accommodative demands (0.25, 2, and 4 D).

RESULTS

We found no difference in accommodative lag between groups. However, the accommodative response was more variable in the myopes than emmetropes when viewing both the near (4 D) and far (0.25 D) targets. Since pupil size and variability also varied, we analyzed the data to determine whether this could account for the inter-group differences in accommodation variability. Variation in these factors was not found to be sufficient to explain these differences. Changes in the accommodative response variability with target distance were similar to patterns reported previously in adult emmetropes and late onset myopes.

CONCLUSIONS

Children with early onset myopia demonstrate greater accommodative variability than emmetropic children, and have similar patterns of response to adult late onset myopes. This increased variability could result in an increase in retinal blur for both near and far targets. The role of accommodative variability in the etiology of myopia is discussed.

Accommodation and induced myopia in marmosets

Accommodation may indirectly influence visually guided eye growth by affecting the retinal defocus signal used to guide growth. Specifically, increased lags of accommodation associated with low stimulus-response (S-R) function slopes will impose increased hyperopic blur on the retina and may induce axial elongation and myopia. The purpose of this study was (1) to measure accommodation in awake, free viewing marmosets and (2) compare accommodation behavior in marmosets before and after inducing different amounts of myopia with binocular spectacle lenses. In untreated marmosets, the average accommodation S-R slope approached one, but showed considerable inter-individual variability (mean+/-SD: 0.964+/-0.249 for monocular viewing; 0.895+/-0.235 for binocular viewing; monocular and binocular measures not significantly different). The monocular S-R slopes were significantly reduced following a period of lens rearing that produced axial myopia (change in slope=-0.30+/-0.30, p<.01) and the reduction in slope was proportional to the amount of myopia induced (p<.01). The S-R slopes measured either under monocular or binocular conditions before induction of myopia were not well correlated with the degree of myopia induced (monocular: r=-.240, p=.453; binocular: r=-.060, p=.824). These results support the hypothesis that the reduction in S-R slope in myopes is a consequence of the myopia induced. The alternative hypothesis-that low S-R slope increases susceptibility to the development of myopia--is not supported by the weak correlation between the pre-manipulation S-R slopes and the magnitude of the myopic shift.

Characteristics of accommodative behavior during sustained reading in emmetropes and myopes

Accommodation has long been suspected to be involved in the development of myopia because near work, particularly reading, is known to be a risk factor. In this study, we measured several dynamic characteristics of accommodative behavior during extended periods of reading under close-to-natural conditions in 20 young emmetropic and stable myopic subjects. Accommodative responses, errors, and variability (including power spectrum analysis) were analyzed and related to accommodative demand and subject refractive error. All accommodative behaviors showed large inter-subject variability at all of the reading demands. Accommodative lags and variability significantly increased with closer demands for all subjects (ANOVA, p<0.05). Myopes had significantly greater variability in their accommodation responses compared to emmetropes (ANOVA, p<0.05) and had larger accommodative lags at further reading distances (unpaired t test p<0.05). Power spectrum analysis showed a significant increase in the power of accommodative microfluctuations with closer demands (ANOVA, p<0.05) and with increasing myopia at the closest reading demand (ANOVA, p<0.01). The difference in the stability of the accommodative behavior between individuals with different refractive states suggests a possible relationship between variability in accommodation and the development of myopia.

Vergence variability: a key to understanding oculomotor adaptability?

Vergence eye movements were recorded from three different populations: healthy young (ages 18-35 years), adaptive presbyopic and non-adaptive presbyopic(the presbyopic groups aged above 45 years) to determine how the variability of the eye movements made by the populations differs. The variability was determined using Shannon Entropy calculations of Wavelet transform coefficients, to yield a non-linear analysis of the vergence movement variability. The data were then fed through a k-means clustering algorithm to classify each subject, with no a priori knowledge of true subject classification. The results indicate a highly significant difference in the total entropy values between the three groups, indicating a difference in the level of information content, and thus hypothetically the oculomotor adaptability, between the three groups.Further, the frequency distribution of the entropy varied across groups.