Refractive compensation to optical defocus depends on the temporal profile of luminance modulation of the environment

Mechanisms of emmetropization and what might go wrong in myopia

Studies in animal models and humans have shown that refractive state is optimized during postnatal development by a closed-loop negative feedback system that uses retinal image defocus as an error signal, a mechanism called emmetropization. The sensor to detect defocus and its sign resides in the retina itself. The retina and/or the retinal pigment epithelium (RPE) presumably releases biochemical messengers to change choroidal thickness and modulate the growth rates of the underlying sclera. A central question arises: if emmetropization operates as a closed-loop system, why does it not stop myopia development? Recent experiments in young human subjects have shown that (1) the emmetropic retina can perfectly distinguish between real positive defocus and simulated defocus, and trigger transient axial eye shortening or elongation, respectively. (2) Strikingly, the myopic retina has reduced ability to inhibit eye growth when positive defocus is imposed. (3) The bi-directional response of the emmetropic retina is elicited with low spatial frequency information below 8 cyc/deg, which makes it unlikely that optical higher-order aberrations play a role. (4) The retinal mechanism for the detection of the sign of defocus involves a comparison of defocus blur in the blue (S-cone) and red end of the spectrum (L + M-cones) but, again, the myopic retina is not responsive, at least not in short-term experiments. This suggests that it cannot fully trigger the inhibitory arm of the emmetropization feedback loop. As a result, with an open feedback loop, myopia development becomes "open-loop".

Temporal bright light at low frequency retards lens-induced myopia in guinea pigs

Purpose

Bright light conditions are supposed to curb eye growth in animals with experimental myopia. Here we investigated the effects of temporal bright light at very low frequencies exposures on lens-induced myopia (LIM) progression.

Methods

Myopia was induced by application of -6.00 D lenses over the right eye of guinea pigs. They were randomly divided into four groups based on exposure to different lighting conditions: constant low illumination (CLI; 300 lux), constant high illumination (CHI; 8,000 lux), very low frequency light (vLFL; 300/8,000 lux, 10 min/c), and low frequency light (LFL; 300/8,000 lux, 20 s/c). Refraction and ocular dimensions were measured per week. Changes in ocular dimensions and refractions were analyzed by paired t-tests, and differences among the groups were analyzed by one-way ANOVA.

Results

Significant myopic shifts in refractive error were induced in lens-treated eyes compared with contralateral eyes in all groups after 3 weeks (all P < 0.05). Both CHI and LFL conditions exhibited a significantly less refractive shift of LIM eyes than CLI and vLFL conditions (P < 0.05). However, only LFL conditions showed significantly less overall myopic shift and axial elongation than CLI and vLFL conditions (both P < 0.05). The decrease in refractive error of both eyes correlated significantly with axial elongation in all groups (P < 0.001), except contralateral eyes in the CHI group (P = 0.231). LFL condition significantly slacked lens thickening in the contralateral eyes.

Conclusions

Temporal bright light at low temporal frequency (0.05 Hz) appears to effectively inhibit LIM progression. Further research is needed to determine the safety and the potential mechanism of temporal bright light in myopic progression.

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.

ON and OFF receptive field processing in the presence of optical scattering

The balance of ON/OFF pathway activation in the retina plays a role in emmetropization. A new myopia control lens design uses contrast reduction to down-regulate a hypothesized enhanced ON contrast sensitivity in myopes. The study thus examined ON/OFF receptive field processing in myopes and non-myopes and the impact of contrast reduction. A psychophysical approach was used to measure the combined retinal-cortical output in the form of low-level ON and OFF contrast sensitivity with and without contrast reduction in 22 participants. ON responses were lower than OFF responses (ON 1.25 ± 0.03 vs. OFF 1.39 ± 0.03 log(CS); p < 0.0001) and myopes showed generally reduced sensitivities (myopes 1.25 ± 0.05 vs. non-myopes 1.39 ± 0.05 log(CS); p = 0.05). These findings remained unaffected by contrast reduction (p > 0.05). The study suggests that perceptual differences in ON and OFF signal processing between myopes and non-myopes exist but cannot explain how contrast reduction can inhibit myopia development.

IMI-The Dynamic Choroid: New Insights, Challenges, and Potential Significance for Human Myopia

The choroid is the richly vascular layer of the eye located between the sclera and Bruch's membrane. Early studies in animals, as well as more recent studies in humans, have demonstrated that the choroid is a dynamic, multifunctional structure, with its thickness directly and indirectly subject to modulation by a variety of physiologic and visual stimuli. In this review, the anatomy and function of the choroid are summarized and links between the choroid, eye growth regulation, and myopia, as demonstrated in animal models, discussed. Methods for quantifying choroidal thickness in the human eye and associated challenges are described, the literature examining choroidal changes in response to various visual stimuli and refractive error-related differences are summarized, and the potential implications of the latter for myopia are considered. This review also allowed for the reexamination of the hypothesis that short-term changes in choroidal thickness induced by pharmacologic, optical, or environmental stimuli are predictive of future long-term changes in axial elongation, and the speculation that short-term choroidal thickening can be used as a biomarker of treatment efficacy for myopia control therapies, with the general conclusion that current evidence is not sufficient.

Differences in visual stimulation between reading and walking and implications for myopia development

Visual input plays an important role in the development of myopia (nearsightedness), a visual disorder that blurs vision at far distances. The risk of myopia progression increases with the time spent reading and decreases with outdoor activity for reasons that remain poorly understood. To investigate the stimulus parameters driving this disorder, we compared the visual input to the retina of humans performing two tasks associated with different risks of myopia progression, reading and walking. Human subjects performed the two tasks while wearing glasses with cameras and sensors that recorded visual scenes and visuomotor activity. When compared with walking, reading black text in white background reduced spatiotemporal contrast in central vision and increased it in peripheral vision, leading to a pronounced reduction in the ratio of central/peripheral strength of visual stimulation. It also made the luminance distribution heavily skewed toward negative dark contrast in central vision and positive light contrast in peripheral vision, decreasing the central/peripheral stimulation ratio of ON visual pathways. It also decreased fixation distance, blink rate, pupil size, and head-eye coordination reflexes dominated by ON pathways. Taken together with previous work, these results support the hypothesis that reading drives myopia progression by understimulating ON visual pathways.

Light Signaling and Myopia Development: A Review

INTRODUCTION

The aim of this article was to comprehensively review the relationship between light exposure and myopia with a focus on the effects of the light wavelength, illuminance, and contrast on the occurrence and progression of myopia.

METHODS

This review was performed by searching PubMed data sets including research articles and reviews utilizing the terms "light", "myopia", "refractive error", and "illuminance", and the review was concluded in November 2021. Myopia onset and progression were closely linked with emmetropization and hyperopia. To better elucidate the mechanism of myopia, some of the articles that focused on this topic were included. This article is based on previously conducted studies and does not contain any new studies with human participants or animals performed by any of the authors.

RESULTS

The pathogenesis and prevention of myopia are not completely clear. Studies have provided evidence supporting the idea that light could affect eye growth in three ways. Changing the corresponding conditions will cause changes in the growth rate and mode of the eyes, and preliminary results have shown that FR/NIR (far red/near-infrared) light is effective for myopia in juveniles.

CONCLUSION

This review discusses the results of studies on the effects of light exposure on myopia with the aims of providing clues and a theoretical basis for the use of light to control the development of myopia and offering new ideas for subsequent studies.

Retinal OFF-Pathway Overstimulation Leads to Greater Accommodation-Induced Choroidal Thinning

Purpose

To examine the interactions between accommodation and overstimulation of the retinal ON- and OFF-pathways, and their association with changes in choroidal thickness (ChT) and vascularity.

Methods

Optical coherence tomography imaging of the choroid of twenty young adults (ages 25 ± 5 years) was performed before and after a series of 30-minute-long viewing tasks, including reading a bright text on dark background (ON-pathway overstimulation) and dark text on bright background (OFF-pathway overstimulation), and a control task of viewing a movie with unbiased ON-/OFF-pathway activation. The viewing tasks were performed with relaxed, and 5 diopter (D) accommodation (induced by soft contact lenses) demands. Both reading texts were matched for the mean luminance (35 cd/m²), luminance contrast (87%), and letter size (approximately 11.8 arc minutes). The change in ChT from baseline associated with contrast polarity and accommodation was examined using linear mixed model analysis.

Results

The subfoveal ChT decreased significantly by −7 ± 1 µm with 5 D accommodation compared with relaxed accommodation (−3 ± 1 µm; P < 0.001), and by −9 ± 1 µm with OFF-pathway compared with ON-pathway overstimulation (−4 ± 1 µm; P = 0.002) and the control condition (−2 ± 1 µm; P < 0.001). Overstimulation of the OFF-pathway, but not the ON-pathway, resulted in a significantly greater choroidal thinning compared with the control condition, both at relaxed (−7 ± 1 µm; P = 0.003) and 5 D (−11 ± 1 µm; P = 0.005) accommodation levels. Similar changes were also observed for macular total, stromal, and luminal ChT.

Conclusions

Retinal OFF-pathway stimulation enhanced the choroidal thinning associated with accommodation, thereby providing a potential mechanism that involves accommodation and the retinal OFF-signaling pathway, linking near work and myopia.

Light and myopia: from epidemiological studies to neurobiological mechanisms

Myopia is far beyond its inconvenience and represents a true, highly prevalent, sight-threatening ocular condition, especially in Asia. Without adequate interventions, the current epidemic of myopia is projected to affect 50% of the world population by 2050, becoming the leading cause of irreversible blindness. Although blurred vision, the predominant symptom of myopia, can be improved by contact lenses, glasses or refractive surgery, corrected myopia, particularly high myopia, still carries the risk of secondary blinding complications such as glaucoma, myopic maculopathy and retinal detachment, prompting the need for prevention. Epidemiological studies have reported an association between outdoor time and myopia prevention in children. The protective effect of time spent outdoors could be due to the unique characteristics (intensity, spectral distribution, temporal pattern, etc.) of sunlight that are lacking in artificial lighting. Concomitantly, studies in animal models have highlighted the efficacy of light and its components in delaying or even stopping the development of myopia and endeavoured to elucidate possible mechanisms involved in this process. In this narrative review, we (1) summarize the current knowledge concerning light modulation of ocular growth and refractive error development based on studies in human and animal models, (2) summarize potential neurobiological mechanisms involved in the effects of light on ocular growth and emmetropization and (3) highlight a potential pathway for the translational development of noninvasive light-therapy strategies for myopia prevention in children.

Myopia

Myopia, also known as short-sightedness or near-sightedness, is a very common condition that typically starts in childhood. Severe forms of myopia (pathologic myopia) are associated with a risk of other associated ophthalmic problems. This disorder affects all populations and is reaching epidemic proportions in East Asia, although there are differences in prevalence between countries. Myopia is caused by both environmental and genetic risk factors. A range of myopia management and control strategies are available that can treat this condition, but it is clear that understanding the factors involved in delaying myopia onset and slowing its progression will be key to reducing the rapid rise in its global prevalence. To achieve this goal, improved data collection using wearable technology, in combination with collection and assessment of data on demographic, genetic and environmental risk factors and with artificial intelligence are needed. Improved public health strategies focusing on early detection or prevention combined with additional effective therapeutic interventions to limit myopia progression are also needed.

Effect of duration, and temporal modulation, of monochromatic light on emmetropization in chicks

Previous experiments disagree on the effect of monochromatic light on emmetropization. Some species respond to wavelength defocus created by longitudinal chromatic aberration and become more myopic in monochromatic red light and more hyperopic in monochromatic blue light, while other species do not. Using the chicken model, we studied the effect of the duration of light exposure, modes of lighting, and circadian interruption on emmetropization in monochromatic light. To achieve this goal, we exposed one-week-old chicks to flickering or steady monochromatic red or blue light for a short (10 days) or long (17 days) duration; other chicks were exposed to white light for 10 days. Refraction and ocular biometry were measured. Activity was measured via a motion detection algorithm and an IR camera. The results showed that in both steady and flickering light, there was a greater increase in axial length and vitreous chamber depth in chicks exposed to red or white light compared to chicks exposed to blue light. With a longer duration of exposure, axial length and vitreous chamber depth differences were no longer observed, except at an intermediate time point. Chicks exposed to red light were more active during the day compared to chicks exposed to blue light. We conclude that our results indicate that with short duration monochromatic light exposure, chicks rely on wavelength defocus to guide emmetropization. With longer exposure from hatching, our results support the notion that responses to wavelength defocus can be transient and that the difference between species may be due to differences in experimental duration and/or interference with circadian activity rhythms.

Probing the Potency of Artificial Dynamic ON or OFF Stimuli to Inhibit Myopia Development

Purpose

To determine whether equiluminant artificial dynamic ON or OFF stimuli on a computer screen can induce bidirectional changes in choroidal thickness (ChTh) in both humans and chickens, and whether such changes are associated with bidirectional changes in retinal dopamine release in chickens.

Methods

Experiment 1: Before and after ON or OFF stimulation for 1 hour, ChTh was measured with optical coherence tomography (OCT). Experiment 2: chicks (n = 14) were raised under ON or OFF stimulation for 3 hours. ChTh was determined by OCT. Experiment 3: chicks were raised for 7 days either under room light (500 lux, n = 11), dynamic ON stimulus (700 lux, n = 15), or dynamic OFF stimulus (700 lux, n = 7). In addition, negative lenses were attached to their right eyes. After experiments 2 and 3, retinal and vitreal dopamine (DA), and its metabolites, were measured by HPLC-electrochemical detection.

Results

Experiment 1: Dynamic ON stimuli caused thicker choroids (+5.3 ± 2.0 μm), whereas OFF stimuli caused choroidal thinning (-4.7 ± 0.5 μm) (right eye data only, P < 0.001). Experiment 2: After 3 hours, chickens developed thicker choroids with ON stimuli (+37.4 ± 12.4 μm) and thinner choroids with OFF stimuli (-11.3 ± 3.6 μm, difference P < 0.01). Vitreal DA, 3-methoxytyramine, and homovanillic acid levels were elevated after ON stimulation, compared with the OFF (P < 0.05). Experiment 3: After 7 days, chickens with lenses developed more myopia both with ON and OFF stimulation, compared with room light. ON stimulation increased vitreal DA compared with OFF.

Conclusions

Artificial dynamic ON or OFF stimuli had similar effects on ChTh in humans and chickens, but more work will be necessary to determine whether such stimuli can be used as novel interventions of myopia.

Temporal whole field sawtooth flicker without a spatial component elicits a myopic shift following optical defocus irrespective of waveform direction in chicks

Purpose

Myopia (short-sightedness) is the commonest visual disorder and greatest risk factor for sight threatening secondary pathologies. Myopia and hyperopia can be induced in animal models by rearing with optical lens defocus of opposite sign. The degree of refractive compensation to lens-induced defocus in chicks has been shown to be modified by directionally drifting sawtooth spatio-temporal luminance diamond plaids, with Fast-ON sawtooth spatio-temporal luminance profiles inhibiting the myopic shift in response to negative lenses, and Fast-OFF profiles inhibiting the hyperopic shift in response to positive lenses. What is unknown is whether similar sign-of-defocus dependent results produced by spatio-temporal modulation of sawtooth patterns could be achieved by rearing chicks under whole field low temporal frequency sawtooth luminance profiles at 1 or 4 Hz without a spatial component, or whether such stimuli would indiscriminately elicit a myopic shift such as that previously shown with symmetrical (or near-symmetrical) low frequency flicker across a range of species.

Methods

Hatchling chicks (n = 166) were reared from days five to nine under one of three defocus conditions (No Lens, +10D lens, or -10D lens) and five light conditions (No Flicker, 1 Hz Fast-ON/Slow-OFF sawtooth flicker, 4 Hz Fast-ON/Slow-OFF sawtooth flicker, 1 Hz Fast-OFF/Slow-ON sawtooth flicker, or 4Hz Fast-OFF/Slow-ON sawtooth flicker). The sawtooth flicker was produced by light emitting diodes (white LEDs, 1.2 -183 Lux), and had no measurable dark phase. Biometrics (refraction and ocular axial dimensions) were measured on day nine.

Results

Both 1 Hz and 4 Hz Fast-ON and Fast-OFF sawtooth flicker induced an increase in vitreous chamber depth that was greater in the presence of negative compared to positive lens defocus. Both sawtooth profiles at both temporal frequencies inhibited the hyperopic shift in response to +10D lenses, whilst full myopic compensation (or over-compensation) in response to -10D lenses was observed.

Conclusions

Whole field low temporal frequency Fast-ON and Fast-OFF sawtooth flicker induces a generalized myopic shift, similar to that previously shown for symmetrical sine-wave and square-wave flicker. Our findings highlight that temporal modulation of retinal ON/OFF pathways per se (without a spatial component) is insufficient to produce strong sign-of-defocus dependent effect.

The role of temporal contrast and blue light in emmetropization

A previous experiment showed that blue light (as a component of white light) protected against low temporal frequency dependent eye growth. This experiment investigated the role of temporal contrast. White leghorn chicks were exposed to white (with blue) or yellow (without blue) LED lighting modulated at either low (0.2Hz) or high (10Hz) temporal frequencies. Four cone contrast conditions were used: low (16%), medium (32%), medium-high (60%) and very-high (80%). Chicks were exposed to the lighting condition for 3days (mean 680lux). Exposure to high temporal frequencies, with very high temporal contrast, reduced eye growth, regardless of spectral content. However, at low temporal frequencies, eye growth was dependent on the illuminant. At lower temporal contrast levels, growth increased regardless of temporal or spectral characteristics. To conclude, very high temporal contrast, white light, provides a "stop" signal for eye growth that overrides temporal cues for growth that manifest in yellow light.

Reading and Myopia: Contrast Polarity Matters

In myopia the eye grows too long, generating poorly focused retinal images when people try to look at a distance. Myopia is tightly linked to the educational status and is on the rise worldwide. It is still not clear which kind of visual experience stimulates eye growth in children and students when they study. We propose a new and perhaps unexpected reason. Work in animal models has shown that selective activation of ON or OFF pathways has also selective effects on eye growth. This is likely to be true also in humans. Using custom-developed software to process video frames of the visual environment in realtime we quantified relative ON and OFF stimulus strengths. We found that ON and OFF inputs were largely balanced in natural environments. However, black text on white paper heavily overstimulated retinal OFF pathways. Conversely, white text on black paper overstimulated ON pathways. Using optical coherence tomography (OCT) in young human subjects, we found that the choroid, the heavily perfused layer behind the retina in the eye, becomes about 16 µm thinner in only one hour when subjects read black text on white background but about 10 µm thicker when they read white text from black background. Studies both in animal models and in humans have shown that thinner choroids are associated with myopia development and thicker choroids with myopia inhibition. Therefore, reading white text from a black screen or tablet may be a way to inhibit myopia, while conventional black text on white background may stimulate myopia.

Circadian rhythms, refractive development, and myopia

PURPOSE

Despite extensive research, mechanisms regulating postnatal eye growth and those responsible for ametropias are poorly understood. With the marked recent increases in myopia prevalence, robust and biologically-based clinical therapies to normalize refractive development in childhood are needed. Here, we review classic and contemporary literature about how circadian biology might provide clues to develop a framework to improve the understanding of myopia etiology, and possibly lead to rational approaches to ameliorate refractive errors developing in children.

RECENT FINDINGS

Increasing evidence implicates diurnal and circadian rhythms in eye growth and refractive error development. In both humans and animals, ocular length and other anatomical and physiological features of the eye undergo diurnal oscillations. Systemically, such rhythms are primarily generated by the 'master clock' in the surpachiasmatic nucleus, which receives input from the intrinsically photosensitive retinal ganglion cells (ipRGCs) through the activation of the photopigment melanopsin. The retina also has an endogenous circadian clock. In laboratory animals developing experimental myopia, oscillations of ocular parameters are perturbed. Retinal signaling is now believed to influence refractive development; dopamine, an important neurotransmitter found in the retina, not only entrains intrinsic retinal rhythms to the light:dark cycle, but it also modulates refractive development. Circadian clocks comprise a transcription/translation feedback control mechanism utilizing so-called clock genes that have now been associated with experimental ametropias. Contemporary clinical research is also reviving ideas first proposed in the nineteenth century that light exposures might impact refraction in children. As a result, properties of ambient lighting are being investigated in refractive development. In other areas of medical science, circadian dysregulation is now thought to impact many non-ocular disorders, likely because the patterns of modern artificial lighting exert adverse physiological effects on circadian pacemakers. How, or if, such modern light exposures and circadian dysregulation contribute to refractive development is not known.

SUMMARY

The premise of this review is that circadian biology could be a productive area worthy of increased investigation, which might lead to the improved understanding of refractive development and improved therapeutic interventions.

The wavelength composition and temporal modulation of ambient lighting strongly affect refractive development in young tree shrews

Shortly after birth, the eyes of most animals (including humans) are hyperopic because the short axial length places the retina in front of the focal plane. During postnatal development, an emmetropization mechanism uses cues related to refractive error to modulate the growth of the eye, moving the retina toward the focal plane. One possible cue may be longitudinal chromatic aberration (LCA), to signal if eyes are getting too long (long [red] wavelengths in better focus than short [blue]) or too short (short wavelengths in better focus). It could be difficult for the short-wavelength sensitive (SWS, "blue") cones, which are scarce and widely spaced across the retina, to detect and signal defocus of short wavelengths. We hypothesized that the SWS cone retinal pathway could instead utilize temporal (flicker) information. We thus tested if exposure solely to long-wavelength light would cause developing eyes to slow their axial growth and remain refractively hyperopic, and if flickering short-wavelength light would cause eyes to accelerate their axial growth and become myopic. Four groups of infant northern tree shrews (Tupaia glis belangeri, dichromatic mammals closely related to primates) began 13 days of wavelength treatment starting at 11 days of visual experience (DVE). Ambient lighting was provided by an array of either long-wavelength (red, 626 ± 10 nm) or short-wavelength (blue, 464 ± 10 nm) light-emitting diodes placed atop the cage. The lights were either steady, or flickering in a pseudo-random step pattern. The approximate mean illuminance (in human lux) on the cage floor was red (steady, 527 lux; flickering, 329 lux), and blue (steady, 601 lux; flickering, 252 lux). Refractive state and ocular component dimensions were measured and compared with a group of age-matched normal animals (n = 15 for refraction (first and last days); 7 for ocular components) raised in broad spectrum white fluorescent colony lighting (100-300 lux). During the 13 day period, the refraction of the normal animals decreased from (mean ± SEM) 5.8 ± 0.7 diopters (D) to 1.5 ± 0.2 D as their vitreous chamber depth increased from 2.77 ± 0.01 mm to 2.80 ± 0.03 mm. Animals exposed to red light (both steady and flickering) remained hyperopic throughout the treatment period so that the eyes at the end of wavelength treatment were significantly hyperopic (7.0 ± 0.7 D, steady; 4.7 ± 0.8 D, flickering) compared with the normal animals (p < 0.01). The vitreous chamber of the steady red group (2.65 ± 0.03 mm) was significantly shorter than normal (p < 0.01). On average, steady blue light had little effect; the refractions paralleled the normal refractive decrease. In contrast, animals housed in flickering blue light increased the rate of refractive decrease so that the eyes became significantly myopic (-2.9 ± 1.3 D) compared with the normal eyes and had longer vitreous chambers (2.93 ± 0.04 mm). Upon return to colony lighting, refractions in all groups gradually returned toward emmetropia. These data are consistent both with the hypothesis that LCA can be an important visual cue for postnatal refractive development, and that short-wavelength temporal flicker provides an important cue for assessing and signaling defocus.

Light levels, refractive development, and myopia--a speculative review

Recent epidemiological evidence in children indicates that time spent outdoors is protective against myopia. Studies in animal models (chick, macaque, tree shrew) have found that light levels (similar to being in the shade outdoors) that are mildly elevated compared to indoor levels, slow form-deprivation myopia and (in chick and tree shrew) lens-induced myopia. Normal chicks raised in low light levels (50 lux) with a circadian light on/off cycle often develop spontaneous myopia. We propose a model in which the ambient illuminance levels produce a continuum of effects on normal refractive development and the response to myopiagenic stimuli such that low light levels favor myopia development and elevated levels are protective. Among possible mechanisms, elevation of retinal dopamine activity seems the most likely. Inputs from intrinsically-photosensitive retinal ganglion cells (ipRGCs) at elevated light levels may be involved, providing additional activation of retinal dopaminergic pathways.

Temporal integration of visual signals in lens compensation (a review)

Postnatal eye growth is controlled by visual signals. When wearing a positive lens that causes images to be focused in front of the retina (myopic defocus), the eye reduces its rate of ocular elongation and increases choroidal thickness to move the retina forward to meet the focal plane of the eye. When wearing a negative lens that causes images to be focused behind the retina (hyperopic defocus), the opposite happens. This review summarizes how the retina integrates the constantly changing visual signals in a non-linear fashion to guide eye growth in chicks: (1a) When myopic or hyperopic defocus is interrupted by a daily episode of normal vision, normal vision is more effective in reducing myopia caused by hyperopic defocus than in reducing hyperopia caused by myopic defocus; (1b) when the eye experiences alternating myopic and hyperopic defocus, the eye is more sensitive to myopic defocus than to hyperopic defocus and tends to develop hyperopia, even if the duration of hyperopic defocus is much longer than the duration of myopic defocus; (2) when the eye experiences brief, repeated episodes of defocus by wearing either positive or negative lenses, lens compensation depends on the frequency and duration of individual episodes of lens wear, not just the total daily duration of lens wear; and (3) further analysis of the time constants for the hypothesized internal emmetropization signals show that, while it takes approximately the same amount of time for the signals to rise and saturate during lens-wearing episodes, the decline of the signals between episodes depends strongly on the sign of defocus and the ocular component. Although most extensively studied in chicks, the nonlinear temporal integration of visual signals has been found in other animal models. These findings may help explain the complex etiology of myopia in school-aged children and suggest ways to slow down myopia progression.

Investigating mechanisms of myopia in mice

Genetic and environmental factors have been shown to control visually-guided eye growth and influence myopia development. However, investigations into the intersection of these two factors in controlling refractive development have been limited by the lack of a genetically modifiable animal model. Technological advances have now made it possible to assess refractive state and ocular biometry in the small mouse eye and therefore to exploit the many genetic mouse mutants to investigate mechanisms of visually-guided eye growth. This review considers the benefits and challenges of studying refractive development in mice, compares the results of refractive error and ocular biometry from wild-type strains and genetic models in normal laboratory visual environments or with disrupted visual input, and discusses some of the remaining challenges in interpreting data from the mouse to validate and standardize methods between labs.

Visual backward masking performance in young adult emmetropes and myopes

PURPOSE

To investigate how temporal processing is altered in myopia and during myopic progression.

METHODS

In backward visual masking, a target's visibility is reduced by a mask presented quickly after the target. Thirty emmetropes, 40 low myopes, and 22 high myopes aged 18 to 26 years completed location and resolution masking tasks. The location task examined the ability to detect letters with low contrast and large stimulus size. The resolution task involved identifying a small letter and tested resolution and color discrimination. Target and mask stimuli were presented at nine short interstimulus intervals (12 to 259 ms) and at 1000 ms (long interstimulus interval condition).

RESULTS

In comparison with emmetropes, myopes had reduced ability in both locating and identifying briefly presented stimuli but were more affected by backward masking for a low contrast location task than for a resolution task. Performances of low and high myopes, as well as stable and progressing myopes, were similar for both masking tasks. Task performance was not correlated with myopia magnitude.

CONCLUSIONS

Myopes were more affected than emmetropes by masking stimuli for the location task. This was not affected by magnitude or progression rate of myopia, suggesting that myopes have the propensity for poor performance in locating briefly presented low contrast objects at an early stage of myopia development.

Light modulation, not choroidal vasomotor action, is a regulator of refractive compensation to signed optical blur

BACKGROUND AND PURPOSE

The nitric oxide system has two proposed sites and mechanisms of action within the ocular growth/refractive compensation platform-neuromodulatory effects on retinal physiology, and vascular/smooth muscle effects in the choroid. The relative contribution of these mechanisms are tested here with drugs that perturb the nitric oxide system and with slow flicker modulation of the ON and OFF pathways of the retina.

EXPERIMENTAL APPROACH

Intravitreal injection of saline or 900 nmol N(G) -nitro-L-arginine methyl ester or L-arginine in saline was followed by monocular defocus with ±10 D lens (or no lens), from days 5-9 under standard diurnal (SD) or daytime 1 Hz ramped flicker conditions. Biometric, electrophysiological and histological analyses were conducted.

KEY RESULTS

After 4 days of SD conditions, both drugs enhanced electroretinogram (ERG) b-wave cf. d-wave amplitudes compared with saline and reduced refractive compensation to -10 D lenses. Under flicker conditions compensation to +10 D lenses was suppressed. Choroidal thinning was observed in the drug, no lens groups under SD conditions, whereas choroidal thickening was seen in most groups under flicker conditions, irrespective of refractive outcomes.

CONCLUSIONS AND IMPLICATIONS

As choroidal thickness was not predictive of final refractive compensation across any of the variables of drug, defocus sign or light condition, it is unlikely that choroidal thickness is a primary mechanism underlying refractive compensation across the range of parameters of this study. Rather, the changes in refractive compensation observed under these particular drug and light conditions are more likely due to a neuromodulatory action on retinal ON and OFF pathways.

Potassium channel and NKCC cotransporter involvement in ocular refractive control mechanisms

Myopia affects well over 30% of adult humans globally. However, the underlying physiological mechanism is little understood. This study tested the hypothesis that ocular growth and refractive compensation to optical defocus can be controlled by manipulation of potassium and chloride ion-driven transretinal fluid movements to the choroid. Chicks were raised with +/-10D or zero power optical defocus rendering the focal plane of the eye in front of, behind, or at the level of the retinal photoreceptors respectively. Intravitreal injections of barium chloride, a non-specific inhibitor of potassium channels in the retina and RPE or bumetanide, a selective inhibitor of the sodium-potassium-chloride cotransporter were made, targeting fluid control mechanisms. Comparison of refractive compensation to 5 mM Ba(2+) and 10(-5) M bumetanide compared with control saline injected eyes shows significant change for both positive and negative lens defocus for Ba(2+) but significant change only for negative lens defocus with bumetanide (Rx(SAL)(-10D) = -8.6 +/- .9 D; Rx(Ba2+)(-10D) = -2.9 +/- .9 D; Rx(Bum)(-10D) = -2.9 +/- .9 D; Rx(SAL)(+10D) = +8.2 +/- .9 D; Rx(Ba2+)(+10D) = +2.8 +/- 1.3 D; Rx(Bum)(+10D) = +8.0 +/- .7 D). Vitreous chamber depths showed a main effect for drug conditions with less depth change in response to defocus shown for Ba(2+) relative to Saline, while bumetanide injected eyes showed a trend to increased depth without a significant interaction with applied defocus. The results indicate that both K channels and the NKCC cotransporter play a role in refractive compensation with NKCC blockade showing far more specificity for negative, compared with positive, lens defocus. Probable sites of action relevant to refractive control include the apical retinal pigment epithelium membrane and the photoreceptor/ON bipolar synapse. The similarities between the biometric effects of NKCC inhibition and biometric reports of the blockade of the retinal ON response, suggest a possible common mechanism. The selective inhibition of refractive compensation to negative lens in chick by loop diuretics such as bumetanide suggests that these drugs may be effective in the therapeutic management of human myopia.

High susceptibility to experimental myopia in a mouse model with a retinal on pathway defect

PURPOSE

Nob mice share the same mutation in the Nyx gene that is found in humans with complete congenital stationary night blindness (CSNB1). Nob mutant mice were studied to determine whether this defect resulted in myopia, as it does in humans.

METHODS

Refractive development was measured in unmanipulated wild-type C57BL/6J (WT) and nob mice from 4 to 12 weeks of age by using an infrared photorefractor. The right eye was form deprived by means of a skull-mounted goggling apparatus at 4 weeks of age. Refractive errors were recorded every 2 weeks after goggling. The content of dopamine and the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were measured by HPLC with electrochemical detection (HPLC-ECD) in retinas of nob and WT mice under light- and dark-adapted conditions.

RESULTS

The nob mice had greater hyperopic refractive errors than did the WT mice under normal visual conditions, until 12 weeks of age when both strains had similar refractions. At 6 weeks of age, refractions became less hyperopic in the nob mice but continued to become more hyperopic in the WT mice. After 2 weeks of form deprivation (6 weeks of age), the nob mice displayed a significant myopic shift (~4 D) in refractive error relative to the opposite and control eyes, whereas WT mice required 6 weeks of goggling to elicit a similar response. As expected with loss of ON pathway transmission, light exposure did not alter DOPAC levels in the nob mice. However, dopamine and DOPAC levels were significantly lower in the nob mice compared with WT.

CONCLUSIONS

Under normal laboratory visual conditions, only minor differences in refractive development were observed between the nob and WT mice. The largest myopic shift in the nob mice resulted after form deprivation, suggesting that visual pathways dependent on nyctalopin and/or abnormally low dopaminergic activity play a role in regulating refractive development. These findings demonstrate an interaction of genetics and environment in refractive development.

Slow flash multifocal electroretinogram in myopia

PURPOSE

The purpose of this study was to investigate the characteristics of retinal function in myopes using a modified multifocal electroretinogram (mfERG) protocol, the slow flash (sf-mfERG) paradigm, which is thought to primarily reflect responses of ON- and OFF-bipolar cells and emphasize the late response components.

METHODS

Twenty-eight subjects (10 emmetropes and 18 myopes) underwent mfERG testing using VERIS 5.1.5X. The sf-mfERG stimulus array consisted of 103-scaled hexagons and flickered according to a pseudorandom binary m-sequence (2(13)-1). The stimulation sequence was slowed by inserting three dark frames such that each step in the m-sequence was four frames long (53.3ms). The amplitude and implicit time of the major sf-mfERG waveform features (N1, P1, and N2) of the first-order kernel were analysed.

RESULTS

Myopes had significantly reduced P1 and N2 amplitudes compared to the emmetropes (F(1,25)=8.818, p=0.007; F(1,25)=6.723, p=0.017). There were no significant differences in N1 amplitude or implicit time between the groups (F(1,25)=1.506, p=0.233; F(1,25)=1.291, p=0.269).

CONCLUSIONS

Late response components (P1 and N2) of the first-order sf-mfERG responses were preferentially affected in myopia, suggesting possible reduced ON- and OFF-bipolar cell activity. As bipolar cells form the first synapse of the visual system with the photoreceptors to initiate the ON- and OFF-pathways, future investigations of ON- and OFF-systems in myopia are of interest.

Delayed mfERG responses in myopia

It has been suggested that changes in the multifocal electroretinogram (mfERG) responses in myopes are primarily due to the increased axial length that accompanies myopia development. We investigated the characteristics of mfERG responses between emmetropes and myopes and determined the contribution of axial length to the mfERG data in 30 subjects (10 emmetropes and 20 myopes) using VERIS I. The amplitude and implicit time of the first positive peak (P1) of the first-order kernel were analyzed. We found that P1 implicit time in myopes was significantly longer by 1.3-3.1 ms than that of the emmetropes and this was not explained by the myopes having greater axial lengths than the emmetropes. Axial length contributed to 15% of the implicit time total variance while refractive error accounted for 27%. Delayed mfERG responses observed in myopes were not attributable to the anatomical change that accompanies myopia and may suggest underlying differences in retinal function that result from being myopic.

Low frequency temporal modulation of light promotes a myopic shift in refractive compensation to all spectacle lenses

Emmetropization, the process by which ocular growth of young animals adapts to ensure focussed retinal images, can be disrupted by high frequency flicker, causing a hypermetropic shift. Emmetropization can also be disrupted differentially, in a sign dependent manner, by pharmacological alteration of the balance of activation of the ON and OFF retinal sub-systems in normal light or by rearing in an environment with a moving spatiotemporally varied diamond pattern (yielding local sawtooth illumination on the retina). Thus the aim of this experiment was to determine whether low frequency temporal modulation alone was sufficient to cause defocus sign-dependent interference with compensation. Chicks were reared for 6 or 7 days with monocular +/-10 D, 0 D, or No Lenses in a 12h light/dark cycle. Luminance of the environment was temporally modulated during the light cycle with a non-square wave profile pulse of 250 msec duration, with the illumination fluctuating between 1.5 and 180 lux at 1 Hz, 2 Hz, 4 Hz or with no flicker (0 Hz-180 lux). Final refractive state and ocular dimensions, measured using retinoscopy and A-scan ultrasonography, demonstrated that in the absence of temporal luminance modulation (0 Hz), chicks compensated to induced defocus in the expected sign-dependent manner. However, under 1, 2 and 4 Hz flickering light conditions, there was an overall myopic offset of approximately 6D across lens groups with refractive compensation to positive lenses more strongly inhibited. This myopic offset was reflected by increases in the depth of both vitreous and anterior chambers. However, luminance modulation had no effect on refraction or ocular parameters in the No Lens conditions. This is a hitherto unreported strong interaction between lens wear and low frequency temporally modulated light, with the refractive compensation mechanism being overridden by a generalized myopic shift.

Stimulating human accommodation without changes in focus

PURPOSE

Inspired by the finding in chickens that preferential stimulation of the ON retinal system suppresses myopia induced by negative spectacle lens wear and that stimulation of the OFF system suppresses the hyperopia induced by positive lens wear, we sought to determine whether stimulation of the ON-OFF retinal systems could drive directional accommodation responses in humans. If emmetropisation and accommodation use similar image processing algorithms, more accommodation would be expected with OFF stimulation.

METHODS

Accommodation responses were measured while viewing a computer-generated pattern designed to stimulate the ON-OFF systems. The stimulus comprised a rectangular field (12 x 9.5 cm) on a black background filled with 196 discs (diameters: 0.4-1.0 cm). These were presented on an LCD monitor in a dark room at a viewing distance of 55 cm (1.8 D). Thirteen subjects aged 21-37 years took part. The individual discs had saw-tooth shaped temporal luminance profiles with the same time period but with random phases with respect to each other, so that the mean brightness of the stimulus was constant. To eliminate accommodation responses based on other cues (i.e. proximity) a 0.5 mm artificial pupil was used to open the accommodation loop. Refraction in the vertical pupil meridian was continuously recorded with an infrared photorefractor (the PowerRefractor). To verify that computer-based stimuli presented within our experimental design were effective in driving accommodation, previously studied stimuli were also tested: changes in size (looming) and incremental low pass filtering.

RESULTS

Preferential stimulation of the ON or OFF subsystems produced a convincing depth illusion in all subjects (which was psychophysically confirmed in four subjects). Although the stimulus appeared to move in depth it did not produce accommodation responses that were consistent with that, i.e. the accommodation system did not appear to fluctuate in rhythm with the temporal oscillations of the stimulus. As the target appeared to loom it induced a greater accommodation response then when it appeared to recede. The looming target produced changes in the accommodation response in nine of 13 subjects that were consistent with its perceived change in proximity (although the target did not actually move in depth). Incremental low pass filtering produced non-directional drifts of accommodation in all subjects. Combinations of the stimuli (i.e. looming and low pass filtering, ON/OFF and looming) were not more effective stimuli to accommodation. After removal of the artificial pupil (closed loop conditions), accommodation was no longer induced with any of these stimuli.

CONCLUSIONS

Although the preferential ON or OFF stimulation produced a pronounced illusion of motion in depth despite constant average brightness, proximal accommodation was induced in only one subject. Therefore, the ON/OFF stimulation appeared to have only minor input into proximal accommodation. Potential inputs into reflex accommodation need to be defined in further studies.

Inhibition of retinal ON/OFF systems differentially affects refractive compensation to defocus

Hatchling chicks reared with defocusing spectacle lenses compensate for the applied defocus, both refractively and through changes in eye growth, in about 1 week. In this experiment, we show that pharmacological inhibition of the retinal ON or OFF responses to light stimulation with isomers of alpha-aminoadipic acid results in a sign-dependent pattern of interference with the refractive compensation mechanism. An intravitreal injection of 2.5 microM L-alpha-aminoadipic acid inhibited the ERG ON response and inhibited refractive compensation to negative lens defocus, but not to positive or zero power lens defocus. D-alpha-aminoadipic acid in the same dose reduced the retinal OFF response and inhibited refractive compensation to positive lens defocus, but not to negative or zero power lenses. Thus the pharmacological manipulation of induced refractive change suggests that the retinal ON and OFF subsystems play independent roles in the emmetropization process.