Experimental myopia in cats reared in stroboscopic illumination

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.

Recovery of dopaminergic amacrine cells after strobe light stimulation in the developing rat retina

Concerns regarding the impact of strobe light on human health and life have recently been raised. Sources of strobe light include visual display terminals, light-emitting diodes, and computer monitors. Strobe light exposure leads to visual discomfort, headaches, and poor visual performance and affects the number of dopaminergic amacrine cells (DACs) in the developing retina, as well as retinal dopamine levels in animals. DACs serve as the sole source of retinal dopamine, and dopamine release from the retina is activated by light exposure following a circadian rhythm. Using a Sprague-Dawley rat model, this study sought to determine whether changes in DACs caused by strobe light are recoverable after ceasing strobe light exposure during retinal development. From eye opening (postnatal 2 weeks), rats in the control group were reared under normal light (an unflickering 150 lux incandescent lamp with a 12 h light/dark cycle), whereas those in the experimental group (i.e., strobe-recovery group) were reared under strobe light (2 Hz for 12 h/day) exposure for 2 weeks. After postnatal week 4, normal light was provided to all animals to observe the reversibility of the effect of strobe light. Immunohistochemistry and immunoblot analysis for the rate limiting enzyme for dopamine synthesis, tyrosine hydroxylase (TH), as well as high-pressure liquid chromatography for measuring dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) were performed at postnatal weeks 4, 6, 8, and 10. The number of type I and type II TH-immunoreactive (TH-IR) cells across the entire retina was counted to evaluate whether changes in DACs induced by strobe light could recover after ceasing strobe light exposure. The number of type I TH-IR cells slightly decreased but remained at a constant level in the control group. In contrast, the number of type I TH-IR cells rapidly decreased up to postnatal week 6, but then increased after postnatal week 8 in the strobe-recovery group. Subsequently, the number of type I TH-IR cells eventually reached a number similar to that in the control group. In addition, the number of intermediate-sized TH-IR cells were increased at postnatal weeks 8 and 10 and the dopamine level was decreased at postnatal week 8 in the strobe-recovery group. However, the levels of DOPAC and TH proteins did not differ between the two groups. This suggests that changes in DACs caused by strobe light are reversible and that type II TH-IR cells may play a key role in this recovery.

Parasympathetic innervation of emmetropization

Emmetropization is affected by the temporal parameters of visual stimulation and the spectral composition of light, as well as by autonomic innervation. The goal of the current experiments is to test the hypothesis that different types of visual stimulation interact with ocular innervation in the process of emmetropization. For that, selective lesions of the autonomic nervous system were performed in chickens: involving transection of parasympathetic input to the eye from either the ciliary ganglion, innervating accommodation and pupil responses (CGX; n = 32), or pterygopalatine ganglion, innervating choroidal blood vessels and cornea (PPGX; n = 26). After 1 week of recovery, chicks were exposed to sinusoidally modulated light (3 days, 2 Hz, 680 lux) that was either achromatic (black to white [RGB], or black to yellow [RG]), or chromatic (blue to yellow [B/Y] or red to green [R/G]). Exposure to light stimulation was followed by ocular biometry (Lenstar and a Hartinger refractometer). Surgical conditions revealed a small reduction in anterior chamber depth with CGX but no other significant changes in ocular biometry/refraction under standard light conditions. With RGB achromatic stimulation, CGX eyes produced an effect on ocular components, with a further reduction in anterior chamber depth and an increase in vitreous chamber depth, while RG stimulation showed no effect. No effect was detected in PPGX under both achromatic protocols. With chromatic stimulation, CGX with R/G modulation increased eye length, while PPGX with B/Y modulation decreased eye length. We conclude that the two different types of parasympathetic innervations have antagonistic effects on eye growth and the anterior eye when challenged with the appropriate stimulus, with possible implications for the role of choroidal blood flow in emmetropization.

The Role of Dopamine in Emmetropization Modulated by Wavelength and Temporal Frequency in Guinea Pigs

Purpose

Wavelength and temporal frequency have been found to influence refractive development. This study investigated whether retinal dopamine (DA) plays a role in these processes.

Methods

Guinea pigs were randomly divided into nine groups that received different lighting conditions for 4 weeks, as follows: white, green, or blue light at 0, 0.5, or 20.0 Hz. Refractions and axial lengths were measured using streak retinoscopy and A-scan ultrasound imaging. DA and its metabolites were measured by high-pressure liquid chromatography-electrochemical detection.

Results

At 0 Hz, green and blue light produced myopic and hyperopic shifts compared with that of white light. At 0.5 Hz, no significant changes were observed compared with those of green or blue light at 0 Hz, whereas white light at 0.5 Hz induced a myopic shift compared with white light at 0 or 20 Hz. At 20 Hz, green and blue light acted like white light. Among all levels of DA and its metabolites, only vitreous 3, 4-dihydroxyphenylacetic acid (DOPAC) levels and retinal DOPAC/DA ratios were dependent on wavelength, frequency, and their interaction. Specifically, retinal DOPAC/DA ratios were positively correlated with refractions in white and green light conditions. However, blue light (0, 0.5, and 20.0 Hz) produced hyperopic shifts but decreased vitreous DOPAC levels and retinal DOPAC/DA ratios.

Conclusions

The retinal DOPAC/DA ratio, indicating the metabolic efficiency of DA, is correlated with ocular growth. It may underlie myopic shifts from light exposure with a long wavelength and low temporal frequency. However, different biochemical pathways may contribute to the hyperopic shifts from short wavelength light.

Modulation of the ERK1/2-MMP-2 pathway in the sclera of guinea pigs following induction of myopia by flickering light

It has been shown that flickering light can affect the development of eyeballs. However, the exact mechanism remains unclear. The ERK1/2-MMP-2 pathway is a classic pathway involved in the modulation of the extracellular matrix (ECM) in cancer tissues. However, to the best of our knowledge, the role of this pathway in modulating the scleral ECM in myopia has not been previously examined. The present study aimed to determine the effects of the ERK1/2-MMP-2 pathway on the formation of flickering light-induced myopia (FLM). Guinea pigs were raised under illumination at a flash rate of 0.5 Hz for 6 weeks to induce FLM. Peribulbar injections of dimethylsulfoxide or PD98059 (an inhibitor of phospho-ERK1/2) were administered starting at the third week of FLM modeling. Refraction was measured prior to and following treatments. The thickness of the posterior sclera (PS) was measured under a light microscope following H&E staining. The mRNA levels of MMP-2 were detected by the reverse transcription-quantitative PCR assay. The expression levels of MMP-2 and ERK1/2 were assayed by western blot and immunohistochemical analyses. Following 6 weeks of treatment, the refraction of the FLM group became more myopic compared with that of the control group, while PD98059 treatment inhibited the changes noted in the refraction. A marked reduction in the thickness of PS was observed in the FLM group, while PD98059 inhibited the remodeling of PS. In addition, the expression levels of MMP-2 and protein levels of phospho-ERK1/2 were increased in the FLM group, while PD98059 significantly inhibited MMP-2 mRNA and protein levels. These results indicated that ERK1/2-MMP-2 may be involved in the formation of FLM in guinea pigs by regulating the remodeling of PS.

Temporal color contrast guides emmetropization in chick

As a result of longitudinal chromatic aberration (LCA), longer wavelengths are blurred when shorter wavelengths are in focus, and vice versa. As a result, LCA affects the color and temporal aspects of the retinal image with hyperopic defocus. In this experiment, we investigated how the sensitivity to temporal color contrast affects emmetropization. Ten-day-old chicks were exposed for three days to sinusoidal color modulation. The modulation was either blue/yellow flicker (BY) (n = 57) or red/green flicker (RG) (n = 60) simulating hyperopic defocus with and without a blue light component. The color contrasts tested were 0.1, 0.2, 0.3, 0.4, 0.6, and 0.8 Michelson contrast. The mean illuminance of all stimuli was 680 lux. Temporal modulation was either of a high (10 Hz) or low (0.2 Hz) temporal frequency. To test the role of short- and double-cone stimulation, an additional condition silenced these cones in RG_0.4 (D-) and was compared with RG_0.4 (D+) (n = 14). Changes in ocular components and refractive error were measured using Lenstar and a photorefractometer. With high temporal frequency BY representing an in-focus condition for shorter-wavelengths, we found that high temporal frequency BY contrast was positively correlated with vitreous expansion (R² = 0.87, p < 0.01), expanding the vitreous to compensate for hyperopic defocus. This expansion was offset by low temporal frequency RG, which represented blurred longer wavelengths. The reduction in vitreous expansion in RG_0.4, was enhanced in D+ compared to D- (p < 0.001), indicating a role for short- and/or double-cones. With high temporal frequency RG representing an in-focus condition for longer-wavelengths, we found that high temporal frequency RG contrast was also positively correlated with a linear increase in vitreous chamber depth (R² = 0.84, p < 0.01) and eye length (R² = 0.30, p ≤ 0.05), required to compensate for hyperopic defocus, but also with RG sensitive choroidal thickening (R² = 0.18: p < 0.0001). These increases in the vitreous and eye length were enhanced with D+ compared to D- (p = 0.003) showing the role of short- and double-cones in finessing the vitreous response to hyperopic defocus. Overall, the increase in vitreous chamber depth in RG was offset by reduced expansion in BY, indicating sensitivity to the shorter focal length of blue light and wavelength defocus. Predictable changes in cone contrast and temporal frequency of the retinal image that occur with LCA and defocus result in homeostatic control of emmetropization.

Interactions of cone abundancies, opsin expression, and environmental lighting with emmetropization in chickens

We had previously found that M to L cone abundancy ratios in the chicken retina are correlated with vitreous chamber depth and refractive state in chickens eyes, when they have normal visual exposure but not when they develop deprivation myopia. The finding suggests an interaction between cone abundancies and emmetropization. In the current study, we analyzed how stable this correlation was against changes in environmental variables and strain differences. We found that the correlation was preserved in two chicken strains, as long as they were raised in the laboratory facilities and not in the animal facilities of the institute. To determine the reasons for this difference, spectral and temporal lighting parameters were better adjusted in both places, whereas temperature, humidity, food, diurnal lighting cycles and illuminance were already matched. It was also verified that both strains of chickens had the same cone opsin amino acid sequences. The correlation between M to L cone abundancy and ocular biometry is highly susceptible to changes in environmental variables. Yet undetermined differences in lighting parameters were the most likely reasons. Other striking findings were that green cone opsin mRNA expression was downregulated when deprivation myopia developed. Similarly, red opsin mRNA was downregulated when chicks wore red spectacles, which made them more hyperopic. In summary, our experiments show that photoreceptor abundancies, opsin expression, and the responses to deprivation, and therefore emmetropization, are surprisingly dependent on subtle differences in lighting parameters.

Signals for defocus arise from longitudinal chromatic aberration in chick

Chicks respond to two signals from longitudinal chromatic aberration (LCA): a wavelength defocus signal and a chromatic signal. Wavelength defocus predicts reduced axial eye growth in monochromatic short-wavelength light, compared to monochromatic long-wavelength light. Wavelength defocus may also influence growth in broadband light. In contrast, a chromatic signal predicts increased growth when short-wavelength contrast > long-wavelength contrast, but only when light is broadband. We aimed to investigate the influence of blue light, temporal frequency and contrast on these signals under broadband conditions. Starting at 12 to 13 days-old, 587 chicks were exposed to the experimental illumination conditions for three days for 8h/day and spent the remainder of their day in the dark. The stimuli were flickering lights, with a temporal frequency of 0.2 or 10 Hz, low (30%) or high contrast (80%), and a variety of ratios of cone contrast simulating the effects of defocus with LCA. There were two color conditions, with blue contrast (bPlus) and without (bMinus). Stimuli in the "bPlus" condition varied the amounts of long- (L), middle- (M_) and double (D-) cone contrast, relative to short- (S-) and (UV-) cone contrast, to simulate defocus. Stimuli in the "bMinus" condition only varied the relative modulations of the L + D vs. M cones. In all cases, the average of the stimuli was white, with an illuminance of 777 lux, with cone contrast created through temporal modulation. A Lenstar LS 900 and a Hartinger refractometer were used to measure ocular components and refraction. Wavelength defocus signals with relatively high S-cone contrast resulted in reduced axial growth, and more hyperopic refractions, under low-frequency conditions (p = 0.002), in response to the myopic defocus of blue light. Chromatic signals with relatively high S-cone contrast resulted in increased axial growth and more myopic refractions, under high frequency, low contrast, conditions (p < 0.001). We conclude that the chromatic signals from LCA are dependent on the temporal frequency, phase, and relative contrast of S-cone temporal modulation, and recommend broadband spectral and temporal environments, such as the outdoor environment, to optimize the signals-for-defocus in chick.

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.

Effects of strobe light stimulation on postnatal developing rat retina

The nature and intensity of visual stimuli have changed in recent years because of television and other dynamic light sources. Although light stimuli accompanied by contrast and strength changes are thought to have an influence on visual system development, little information is available on the effects of dynamic light stimuli such as a strobe light on visual system development. Thus, this study was designed to evaluate changes caused by dynamic light stimuli during retinal development. This study used 80 Sprague-Dawley rats. From eye opening (postnatal day 14), half of the rats were maintained on a daily 12-h light/dark cycle (control group) and the remaining animals were raised under a 12-h strobe light (2 Hz)/dark cycle (strobe light-reared group). Morphological analyses and electroretinogram (ERG) were performed at postnatal weeks 3, 4, 6, 8, and 10. Among retinal neurons, tyrosine hydroxylase-immunoreactive (TH-IR, dopaminergic amacrine cells) cells showed marked plastic changes, such as variations in numbers and soma sizes. In whole-mount preparations at 6, 8, and 10 weeks, type I TH-IR cells showed a decreased number and larger somata, while type II TH-IR cells showed an increased number in strobe-reared animals. Functional assessment by scotopic ERG showed that a-wave and b-wave amplitudes increased at 6 and 8 weeks in strobe-reared animals. These results show that exposure to a strobe light during development causes changes in TH-IR cell number and morphology, leading to a disturbance in normal visual functions.

The role of luminance and chromatic cues in emmetropisation

PURPOSE

At birth most, but not all eyes, are hyperopic. Over the course of the first few years of life the refraction gradually becomes close to zero through a process called emmetropisation. This process is not thought to require accommodation, though a lag of accommodation has been implicated in myopia development, suggesting that the accuracy of accommodation is an important factor. This review will cover research on accommodation and emmetropisation that relates to the ability of the eye to use colour and luminance cues to guide the responses.

RECENT FINDINGS

There are three ways in which changes in luminance and colour contrast could provide cues: (1) The eye could maximize luminance contrast. Monochromatic light experiments have shown that the human eye can accommodate and animal eyes can emmetropise using changes in luminance contrast alone. However, by reducing the effectiveness of luminance cues in monochromatic and white light by introducing astigmatism, or by reducing light intensity, investigators have revealed that the eye also uses colour cues in emmetropisation. (2) The eye could compare relative cone contrast to derive the sign of defocus information from colour cues. Experiments involving simulations of the retinal image with defocus have shown that relative cone contrast can provide colour cues for defocus in accommodation and emmetropisation. In the myopic simulation the contrast of the red component of a sinusoidal grating was higher than that of the green and blue component and this caused relaxation of accommodation and reduced eye growth. In the hyperopic simulation the contrast of the blue component was higher than that of the green and red components and this caused increased accommodation and increased eye growth. (3) The eye could compare the change in luminance and colour contrast as the eye changes focus. An experiment has shown that changes in colour or luminance contrast can provide cues for defocus in emmetropisation. When the eye is exposed to colour flicker the eye grows almost twice as much, and becomes more myopic, compared to when the eye is exposed to luminance flicker.

SUMMARY

Neural responses of the luminance and colour mechanisms direct accommodation and emmetropisation mechanisms to different focal planes. Therefore, it is likely that the set point of refraction and accommodation is dependent on the sensitivity of the eye to changes in spatial and temporal, colour and luminance contrast.

The effect of various levels of stroboscopic illumination on the growth of guinea pig eyes

BACKGROUND

The aim was to investigate various levels of stroboscopic illumination effect on the growth of guinea pig eyes.

METHODS

Thirty-six two-week-old guinea pigs were randomised to one of three treatment groups (n = 12 for each). Two stroboscopic-reared groups were raised with a duty diurnal cycle of 50 per cent at a flash rate of 0.5 Hz. Illumination intensity varied between zero-to-250 lux or zero-to-500 lux during each cycle in each group, respectively. The third control group was exposed to 250 lux illumination. Refraction and biometric measurements were taken for each animal prior to and after two, four, six and eight weeks of treatment. Finally, retinal microstructure was examined.

RESULTS

There was significant correlation between refractive errors and axial elongation. After eight weeks of treatment, illumination with flickering light 0-250 lux caused a larger myopic shift with increased axial length than illumination of continuous 250 lux. Stroboscopic illumination with zero-to-500 lux caused a further myopic shift and longer axial length than stroboscopic illumination with zero-to-250 lux. In animals raised in flickering light of zero-to-250 lux or zero-to-500 lux for eight weeks, the outer segment disc membranes in photoreceptor layers were found deformed and detached.

CONCLUSION

Chronic exposure to low-frequency temporally modulated illumination-induced histological damage in the retina and induced exaggerated axial length elongation.

Effects of chronic exposure to 0.5 Hz and 5 Hz flickering illumination on the eye growth of guinea pigs

AIMS

To investigate the effect of prolonged flickering illumination exposure on the growth of the guinea pig eye.

METHODS

Thirty-six 2-week-old guinea pigs were randomized to one of the three treatment groups (n = 12 for each). Two strobe-reared groups were raised with a duty diurnal cycle of 50 % at a flash rate of 0.5 Hz and 5 Hz respectively. Illumination intensity varied between the minimum-maximum light levels of 0-600 lux during each cycle. The control group was exposed to steady 300 lux illumination. All animals underwent refraction and biometric measurements prior to and after 2, 4, 6, 8, 10 and 12 weeks of treatment. Finally, flash electroretinograms were compared, and retinal microstructures were examined.

RESULTS

There was a significant correlation between refractive errors and axial eye elongation, with myopia increasing between 1.5 and 3.4 D per mm eye elongation. After 12 weeks of treatment, the animals raised in 0.5 Hz flickering light were 5.5 ± 0.4 D more myopic than the group raised in continuous illumination, followed by the group raised at 5 Hz flicker light which was about 2.2 ± 1.3 D more myopic. In animals raised in flickering light of 5 or 0.5 Hz for 12 weeks, the implicit time of the a-wave was delayed by 4 and 8.5 ms, respectively. At this time, the outer segment disc membranes were found deformed and detached.

CONCLUSION

Chronic exposure to 0.5 and 5 Hz temporally modulated illumination induces electrophysiological and histological changes in retinal activities that alter the emmetropization of the guinea pig eye.

Myopia induced by flickering light in guinea pigs: a detailed assessment on susceptibility of different frequencies

AIM

To investigate the effectiveness and feasibility of inducing myopia in guinea pigs by flickering light (FL) stimulation with different frequencies.

METHODS

Seventy 2-week-old guinea pigs were randomly assigned to six groups: five FL groups and a control group (n=12 for each). Animals in the five FL groups were raised under 500lx illumination with a duty diurnal cycle of 50% at a flash rate of 5, 1, 0.5, 0.25 and 0.1Hz respectively. Those in the control group were reared under steady 250lx illumination. Refraction, axial length, and radius of curvature were measured before and at 2, 4, 6, 8, 10 and 12 weeks after treatment. At week 12, the eyeballs were taken out and three ocular dimensions and dry weight of sclera were measured.

RESULTS

A myopic shift and axial eye length increase developed in the five FL groups. Stimulation at 0.5Hz caused greater changes in myopic shift, axial elongation, eyeball dimension, and dry weight of sclera than stimulation at other frequencies. Compared with controls, eyes in 0.5Hz group were approximately -5.5±1.5D more myopic with increase in horizontal, vertical, axial dimensions by 0.89±0.3mm, 0.69±0.2mm, 1.12±0.2mm respectively and with increase in dry weight of sclera by 0.44mg.

CONCLUSION

Chronic exposure to periodic illumination at temporal frequency is attended by development of excessive ocular enlargement and myopic refractive error. Emmetropization could be disrupted differently by frequency alteration.

Refractive states of eyes and associations between ametropia and age, breed, and axial globe length in domestic cats

OBJECTIVE

To determine the refractive states of eyes in domestic cats and to evaluate correlations between refractive error and age, breed, and axial globe measurements.

ANIMALS

98 healthy ophthalmologically normal domestic cats.

PROCEDURES

The refractive state of 196 eyes (2 eyes/cat) was determined by use of streak retinoscopy. Cats were considered ametropic when the mean refractive state was ≥ ± 0.5 diopter (D). Amplitude-mode ultrasonography was used to determine axial globe length, anterior chamber length, and vitreous chamber depth.

RESULTS

Mean ± SD refractive state of all eyes was -0.78 ± 1.37 D. Mean refractive error of cats changed significantly as a function of age. Mean refractive state of kittens (≤ 4 months old) was -2.45 ± 1.57 D, and mean refractive state of adult cats (> 1 year old) was -0.39 ± 0.85 D. Mean axial globe length, anterior chamber length, and vitreous chamber depth were 19.75 ± 1.59 mm, 4.66 ± 0.86 mm, and 7.92 ± 0.86 mm, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Correlations were detected between age and breed and between age and refractive states of feline eyes. Mean refractive error changed significantly as a function of age, and kittens had greater negative refractive error than did adult cats. Domestic shorthair cats were significantly more likely to be myopic than were domestic mediumhair or domestic longhair cats. Domestic cats should be included in the animals in which myopia can be detected at a young age, with a likelihood of progression to emmetropia as cats mature.

Chicks use changes in luminance and chromatic contrast as indicators of the sign of defocus

As the eye changes focus, the resulting changes in cone contrast are associated with changes in color and luminance. Color fluctuations should simulate the eye being hyperopic and make the eye grow in the myopic direction, while luminance fluctuations should simulate myopia and make the eye grow in the hyperopic direction. Chicks without lenses were exposed daily (9 a.m. to 5 p.m.) for three days on two consecutive weeks to 2 Hz sinusoidally modulated illumination (mean illuminance of 680 lux) to one of the following: in-phase modulated luminance flicker (LUM), counterphase-modulated red/green (R/G Color) or blue/yellow flicker (B/Y Color), combined color and luminance flicker (Color + LUM), reduced amplitude luminance flicker (Low LUM), or no flicker. After the three-day exposure to flicker, chicks were kept in a brooder under normal diurnal lighting for four days. Changes in the ocular components were measured with ultrasound and with a Hartinger Coincidence Refractometer (aus Jena, Jena, East Germany. After the first three-day exposure, luminance flicker produced more hyperopic refractions (LUM: 2.27 D) than did color flicker (R/G Color: 0.09 D; B/Y Color: -0.25 D). Changes in refraction were mainly due to changes in eye length, with color flicker producing much greater changes in eye length than luminance flicker (R/G Color: 102 μm; B/Y Color: 98 μm; LUM: 66 μm). Our results support the hypothesis that the eye can differentiate between hyperopic and myopic defocus on the basis of the effects of change in luminance or color contrast.

Effects of flickering light on refraction and changes in eye axial length of C57BL/6 mice

AIMS

To investigate the effectiveness and feasibility of inducing myopia in mice by flickering-light (FL) stimulation.

METHODS

Forty-five 28-day-old C57BL/6 (B6) mice were randomly assigned to three groups: control group, FL stimulation group and form deprivation (FD) group. Mice in the control group were raised under 250 lux illumination from 8:00 a.m. to 8:00 p.m. Mice in the FL group were raised under illumination with a duty cycle of 50% at a flash rate of 2 Hz from 8:00 a.m. to 8:00 p.m. for 6 weeks. Mice in the FD group were raised under the same conditions as the control group; the right eyes of the mice were covered with semitransparent hemispherical plastic shells serving as eye diffusers. The refractive state and axial length (AL) of the right eyes were measured by eccentric infrared photorefraction and A-scan ultrasonography, respectively, before treatment and after 2, 4, 6 or 8 weeks' treatment.

RESULTS

After 6 weeks' exposure to FL, the refraction became more myopic compared with the control group as indicated by longer AL compared with the control group (p < 0.05); the FD eyes were more myopic than the FL eyes (p < 0.05). However, some mice lost their eye diffusers, and lens opacities were found.

CONCLUSION

Myopia can be induced by FL in B6 mice. The myopic shift induced by FL is less than that induced by FD, but FL causes fewer side effects, and is safery and easier to manipulate.

The choroidal blood flow response after flicker stimulation in chicks

Form-deprivation myopia (FDM) can be prevented by exposing the animal to stroboscopic illumination (10 Hz). Flicker illumination is known to stimulate the release of dopamine (DA) from the retina. We hypothesize that DA was released and diffused into the choroid. To prove this hypothesis, we decided to undertake an investigation in chicks and measure choroidal blood flow (ChBF) during stimulation of the ocular fundus with diffuse flicker. White Leghorn chicks (2 weeks old) were used for this study. Different flash stimulations (5 Hz approximately 50 Hz) were given for 3 minutes, then ChBF was recorded with the PeriFlux flowmeter simultaneously and continuously for 5 minutes. Some birds are recorded up to 120 minutes to find out any late-onset effect. The ChBF was increased after flicker stimulation. The difference was statistically significant in 10 Hz, 20 Hz, and 30 Hz. The ChBF can maintain 20% higher for 60 minutes. Therefore, flicker illumination preventing FDM may be induced by the hyperactivity of ganglion cells, then stimulates the release of DA from the retina and suppresses the development of myopia.

Constant light affects retinal dopamine levels and blocks deprivation myopia but not lens-induced refractive errors in chickens

Chickens were raised with either translucent occluders or lenses, both under normal light cycles (12-h light/12-h dark) and in constant light (CL). Under normal light cycles, eyes with occluders became very myopic, and eyes with lenses became either relatively hyperopic (positive lenses) or myopic (negative lenses). After the treatment, retinal dopamine (DA), DOPAC, and serotonin levels were measured by high-pressure liquid chromatography (HPLC-EC). A significant drop in daytime retinal DOPAC (-20%) was observed after 1 week of deprivation, and in both DOPAC (-40%) and DA (-30%) after 2 weeks of deprivation. No changes in retinal serotonin levels were found. Retinal DA or DOPAC content remained unchanged after 2 or 4 days of lens wearing even though the lenses had already exerted their maximal effect on axial eye growth. When the chickens were raised in CL, development of deprivation myopia was reduced (8 days CL) or entirely blocked (13 days CL). Lens-induced changes in eye growth were not different after either 6 or 11 days in CL, compared to animals raised in a normal light cycle. Thirteen days of CL resulted in a dramatic reduction of DA and DOPAC levels, but serotonin levels were also lowered. The results suggest that lens-induced changes in refraction may not be dependent on dopaminergic pathways whereas deprivation myopia requires normal diurnal DA rhythms to develop.

Retinal influences on sclera underlie visual deprivation myopia

Visual deprivation of chicks rapidly induces an increased growth of the vitreous chamber which results in axial myopia, even after a day or so. This experimental myopia provides an opportunity to trace the causal path from altered visual experience to altered eye growth. With respect to the visual factors involved, the gross activity of the retinal neurons seems an important variable. The eyes of animals raised wearing translucent occluders do not become myopic or elongated if stroboscopic illumination is present. Conversely, the eyes do become myopic and elongated if deprived of all form vision, if reared in a featureless environment or if deprived in only part of the visual field. The deprivation must, however, be continuous if myopia is to result; even as little as two hours of normal vision per day almost completely eliminates the effect of 12 hours of deprivation. The control of eye growth by vision seems to take place in local regions of the eye. Deprivation of various parts of the visual field produces myopia and elongation in only the deprived part, even in animals in which the optic nerve has been cut. It seems that at the level of the sclera ocular elongation (and thus myopia) results from local growth, because we find increases in synthesis of DNA and protein as well as more new cells. The protein synthesis increase is also local, in the sense that the posterior sclera grows much more than other parts, explaining why deprivation causes ocular elongation and myopia, rather than simply larger eyes. These results suggest that some factor in part of the retina can influence the growth of the subjacent sclera.