Long-term blue light rearing does not affect in vivo retinal function in young rhesus monkeys.

PURPOSE: Exposure to blue light is thought to be harmful to the retina. The purpose of this study was to determine the effects of long-term exposure to narrowband blue light on retinal function in rhesus monkeys. METHODS: Young rhesus monkeys were reared under short-wavelength "blue" light (n = 7; 465 nm, 183 ± 28 lx) on a 12-h light/dark cycle starting at 26 ± 2 days of age. Age-matched control monkeys were reared under broadband "white" light (n = 8; 504 ± 168 lx). Light- and dark-adapted full-field flash electroretinograms (ERGs) were recorded at 330 ± 9 days of age. Photopic stimuli were brief red flashes (0.044-5.68 cd.s/m2) on a rod-saturating blue background and the International Society for Clinical Electrophysiology of Vision (ISCEV) standard 3.0 white flash on a 30 cd/m2 white background. Monkeys were dark adapted for 20 min and scotopic stimuli were ISCEV standard white flashes of 0.01, 3.0, and 10 cd.s/m2. A-wave, b-wave, and photopic negative response (PhNR) amplitudes were measured. Light-adapted ERGs in young monkeys were compared to ERGs in adult monkeys reared in white light (n = 10; 4.91 ± 0.88 years of age). RESULTS: For red flashes on a blue background, there were no significant differences in a-wave (P = 0.46), b-wave (P = 0.75), and PhNR amplitudes (P = 0.94) between white light and blue light reared monkeys for all stimulus energies. ISCEV standard light- and dark-adapted a- and b-wave amplitudes were not significantly different between groups (P > 0.05 for all). There were no significant differences in a- and b-wave implicit times between groups for all ISCEV standard stimuli (P > 0.05 for all). PhNR amplitudes of young monkeys were significantly smaller compared to adult monkeys for all stimulus energies (P < 0.05 for all). There were no significant differences in a-wave (P = 0.19) and b-wave (P = 0.17) amplitudes between young and adult white light reared monkeys. CONCLUSIONS: Long-term exposure to narrowband blue light did not affect photopic or scotopic ERG responses in young monkeys. Findings suggest that exposure to 12 h of daily blue light for approximately 10 months does not result in altered retinal function.

Diurnal Variation and Effects of Dilation and Sedation on Intraocular Pressure in Infant Rhesus Monkeys.

PURPOSE: Intraocular pressure (IOP) is an important factor in numerous ocular conditions and research areas, including eye growth and myopia. In infant monkeys, IOP is typically measured under anesthesia. This study aimed to establish a method for awake IOP measurement in infant rhesus monkeys, determine diurnal variation, and assess the effects of dilation and sedation. METHODS: Awake IOP (iCare TonoVet) was measured every 2 h from 7:30 am to 5:30 pm to assess potential diurnal variations in infant rhesus monkeys (age 3 weeks, n = 11). The following day, and every 2 weeks to age 15 weeks, IOP was measured under three conditions: (1) awake, (2) awake and dilated (tropicamide 0.5%), and (3) sedated (ketamine and acepromazine) and dilated. Intraclass correlation coefficient (ICC) was used to determine intersession repeatability, and repeated measures. ANOVA was used to determine effects of age and condition. RESULTS: At age 3 weeks, mean (±SEM) awake IOP was 15.4 ± 0.6 and 15.2 ± 0.7 mmHg for right and left eyes, respectively (p=.59). The ICC between sessions was 0.63[-0.5 to 0.9], with a mean difference of 2.2 ± 0.3 mmHg. Diurnal IOP from 7:30 am to 5:30 pm showed no significant variation (p=.65). From 3 to 15 weeks of age, there was a significant effect of age (p=.01) and condition (p<.001). Across ages, IOP was 17.8 ± 0.7 mmHg while awake and undilated, 18.4 ± 0.2 mmHg awake and dilated, and 11.0 ± 0.3 mmHg after sedation and dilation. CONCLUSIONS: Awake IOP measurement was feasible in young rhesus monkeys. No significant diurnal variations in IOP were observed between 7:30 am and 5:30 pm at age 3 weeks. In awake monkeys, IOP was slightly higher after mydriasis and considerably lower after sedation. Findings show that IOP under ketamine/acepromazine anesthesia is significantly different than awake IOP in young rhesus monkeys.

Comparing low-coherence interferometry and A-scan ultrasonography in measuring ocular axial dimensions in young rhesus monkeys.

We investigated a commercial low-coherence interferometer (LenStar LS 900 optical biometer) in measuring young rhesus monkey ocular dimensions. Ocular biometry data obtained using a LenStar and an A-scan ultrasound instrument (OPT-scan 1000) from 163 rhesus monkeys during 20-348 days of age were compared by means of coefficients of concordance and 95% limits of agreement. Linear regression was employed to examine and analyze the inter-instrument discrepancies. In young rhesus monkeys, the test-retest reliability of the LenStar was equal to or exceeded that of A-scan ultrasound (intraclass correlation = 0.86 to 0.93). The inter-instrument agreement was strong for vitreous chamber depth and axial length (coefficient of concordance = 0.95 and 0.86, respectively) and moderate for anterior chamber depth and lens thickness (coefficient of concordance = 0.74 and 0.63, respectively). The LenStar systematically underestimated ocular dimensions when compared to A-scan ultrasound (mean magnitude of difference = 0.11-0.57 mm). This difference could be minimized using linear calibration functions to equate LenStar data with ultrasound data. When this method was applied, the values between instruments were in excellent absolute agreement (mean magnitude of difference = 0.004-0.01 mm). In conclusion, the LenStar reliably measured ocular dimensions in young monkeys. When an appropriate calibration function is applied, the LenStar can be used as a substitute for A-scan ultrasonography.

Corrigendum: Long-term narrowband lighting influences activity but not intrinsically photosensitive retinal ganglion cell-driven pupil responses.

[This corrects the article DOI: 10.3389/fphys.2021.711525.].

Long-Term Narrowband Lighting Influences Activity but Not Intrinsically Photosensitive Retinal Ganglion Cell-Driven Pupil Responses.

Purpose: Light affects a variety of non-image forming processes, such as circadian rhythm entrainment and the pupillary light reflex, which are mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs). The purpose of this study was to assess the effects of long- and short-wavelength ambient lighting on activity patterns and pupil responses in rhesus monkeys. Methods: Infant rhesus monkeys were reared under either broadband "white" light (n = 14), long-wavelength "red" light (n = 20; 630 nm), or short-wavelength "blue" light (n = 21; 465 nm) on a 12-h light/dark cycle starting at 24.1 ± 2.6 days of age. Activity was measured for the first 4 months of the experimental period using a Fitbit activity tracking device and quantified as average step counts during the daytime (lights-on) and nighttime (lights-off) periods. Pupil responses to 1 s red (651 nm) and blue (456 nm) stimuli were measured after approximately 8 months. Pupil metrics included maximum constriction and the 6 s post-illumination pupil response (PIPR). Results: Activity during the lights-on period increased with age during the first 10 weeks (p < 0.001 for all) and was not significantly different for monkeys reared in white, red, or blue light (p = 0.07). Activity during the 12-h lights-off period was significantly greater for monkeys reared in blue light compared to those in white light (p = 0.02), but not compared to those in red light (p = 0.08). However, blue light reared monkeys exhibited significantly lower activity compared to both white and red light reared monkeys during the first hour of the lights-off period (p = 0.01 for both) and greater activity during the final hour of the lights-off period (p < 0.001 for both). Maximum pupil constriction and the 6 s PIPR to 1 s red and blue stimuli were not significantly different between groups (p > 0.05 for all). Conclusion: Findings suggest that long-term exposure to 12-h narrowband blue light results in greater disruption in nighttime behavioral patterns compared to narrowband red light. Normal pupil responses measured later in the rearing period suggest that ipRGCs adapt after long-term exposure to narrowband lighting.

The effects of reduced ambient lighting on lens compensation in infant rhesus monkeys.

Although reduced ambient lighting (~50 lx) does not increase the degree of form-deprivation myopia (FDM) in chickens or infant monkeys, it does reduce the probability that monkeys will recover from FDM and that the normal age-dependent reduction in hyperopia will occur in monkeys reared with unrestricted vision. These findings suggest that low ambient lighting levels affect the regulatory mechanism responsible for emmetropization. To study this issue, infant rhesus monkeys (age ~ 24 days) were reared under dim light (55 ± 9 lx) with monocular -3D (dim-light lens-induced myopia, DL-LIM, n = 8) or +3D spectacle lenses (dim-light lens-induced hyperopia, DL-LIH, n = 7) until approximately 150 days of age. Refractive errors, ocular parameters and sub-foveal choroidal thickness were measured periodically and compared with normal-light-reared, lens-control monkeys (NL-LIM, n = 16; NL-LIH, n = 7). Dim light rearing significantly attenuated the degree of compensatory anisometropias in both the DL-LIM (-0.63 ± 0.77D vs. -2.11 ± 1.10D in NL-LIM) and DL-LIH treatment groups (-0.18 ± 1.93D vs. +1.71 ± 0.39D in NL-LIH). These effects came about because the treated and fellow control eyes had a lower probability of responding appropriately to the eye's effective refractive state. Vision-induced interocular differences in choroidal thickness were only observed in monkeys that exhibited compensating refractive changes, suggesting that failures in detecting the relative magnitude of optical errors underlay the abnormal refractive responses. Our findings suggest that low ambient lighting levels reduce the efficacy of the vision-dependent mechanisms that regulate refractive development.

Multiple Short Daily Periods of Normal Binocular Vision Preserve Stereopsis in Strabismus.

Purpose: Infantile strabismus impedes the development of stereopsis. In optically strabismic monkeys, 2 continuous hours of normal binocular vision per day has been shown to preserve near-normal stereopsis. In this study, we investigated whether, as in learning, multiple shorter periods of intervention would further boost performance. Methods: To simulate infantile esotropia, infant monkeys were reared with 30 prism diopters base-in starting at 4 weeks of age. Daily periods of normal binocular vision were provided by replacing prisms with plano lenses. Altogether, 14 monkeys were prism reared: 2 with continuous prism, 2 with 2 continuous hours of normal binocular vision per day, 6 with 2 noncontinuous hours, and 4 with 1 noncontinuous hour of binocular vision each day. Seven normally reared monkeys provided control data. Behavioral methods were employed to measure spatial contrast sensitivity, eye alignment, and stereopsis. Results: One monkey reared with continuous prism had poor stereopsis, and the other had no stereopsis. Ten of the 12 monkeys reared with periods of normal binocular vision had stereopsis, and those with longer and more continuous periods of binocular vision had stereopsis approaching that of normally reared monkeys. Conclusions: During early development, multiple short periods of binocular vision were effective in preserving clinically significant stereopsis in monkeys. These results suggest that by providing relatively short multiple daily intervention periods, stereopsis may be preserved in strabismic human children.

The development of and recovery from form-deprivation myopia in infant rhesus monkeys reared under reduced ambient lighting.

Although reduced ambient lighting ("dim" light) can cause myopia in emmetropizing chicks, it does not necessarily lead to myopic changes in emmetropizing rhesus monkeys. Because myopia is rarely spontaneous, a question remained whether dim light would hasten the progression of visually induced myopia. To determine the effects of dim light on the development of and recovery from form-deprivation myopia (FDM), seven 3-week-old infant rhesus monkeys were reared under dim light (mean ± SD = 55 ± 9 lx) with monocular diffuser spectacles until ~154 days of age, then maintained in dim light with unrestricted vision until ~337 days of age to allow for recovery. Refractive errors, corneal powers, ocular axial dimensions and sub-foveal choroidal thicknesses were measured longitudinally and compared to those obtained from form-deprived monkeys reared under typical laboratory lighting (504 ± 168 lx). Five of the seven subjects developed FDMs that were similar to those observed among their normal-light-reared counterparts. The average degree of form-deprivation-induced myopic anisometropia did not differ significantly between dim-light subjects (-3.88 ± 3.26D) and normal-light subjects (-4.45 ± 3.75D). However, three of the five dim-light subjects that developed obvious FDM failed to exhibit any signs of recovery and the two monkeys that were isometropic at the end of the treatment period manifest abnormal refractive errors during the recovery period. All refractive changes were associated with alterations in vitreous chamber elongation rates. It appears that dim light is not a strong myopiagenic stimulus by itself, but it can impair the optical regulation of refractive development in primates.

Topically instilled caffeine selectively alters emmetropizing responses in infant rhesus monkeys.

Oral administration of the adenosine receptor (ADOR) antagonist, 7-methylxanthine (7-MX), reduces both form-deprivation and lens-induced myopia in mammalian animal models. We investigated whether topically instilled caffeine, another non-selective ADOR antagonist, retards vision-induced axial elongation in monkeys. Beginning at 24 days of age, a 1.4% caffeine solution was instilled in both eyes of 14 rhesus monkeys twice each day until the age of 135 days. Concurrent with the caffeine regimen, the monkeys were fitted with helmets that held either -3 D (-3D/pl caffeine, n = 8) or +3 D spectacle lenses (+3D/pl caffeine, n = 6) in front of their lens-treated eyes and zero-powered lenses in front of their fellow-control eyes. Refractive errors and ocular dimensions were measured at baseline and periodically throughout the lens-rearing period. Control data were obtained from 8 vehicle-treated animals also reared with monocular -3 D spectacles (-3D/pl vehicle). In addition, historical comparison data were available for otherwise untreated lens-reared controls (-3D/pl controls, n = 20; +3D/pl controls, n = 9) and 41 normal monkeys. The vehicle controls and the untreated lens-reared controls consistently developed compensating axial anisometropias (-3D/pl vehicle = -1.44 ± 1.04 D; -3D/pl controls = -1.85 ± 1.20 D; +3D/pl controls = +1.92 ± 0.56 D). The caffeine regime did not interfere with hyperopic compensation in response to +3 D of anisometropia (+1.93 ± 0.82 D), however, it reduced the likelihood that animals would compensate for -3 D of anisometropia (+0.58 ± 1.82 D). The caffeine regimen also promoted hyperopic shifts in both the lens-treated and fellow-control eyes; 26 of the 28 caffeine-treated eyes became more hyperopic than the median normal monkey (mean (±SD) relative hyperopia = +2.27 ± 1.65 D; range = +0.31 to +6.37 D). The effects of topical caffeine on refractive development, which were qualitatively similar to those produced by oral administration of 7-MX, indicate that ADOR antagonists have potential in treatment strategies for preventing and/or reducing myopia progression.

Eccentricity-dependent effects of simultaneous competing defocus on emmetropization in infant rhesus monkeys.

Dual-focus lenses that impose simultaneous competing myopic defocus over the entire visual field produce axial hyperopic shifts in refractive error. The purpose of this study was to characterize the effects of eccentricity on the ability of myopic defocus signals to influence central refractive development in infant monkeys. From 24 to 152 days of age, rhesus monkeys were reared with binocular, dual-focus lenses that had central, zero-powered zones surrounded by alternating concentric annular power zones of +3D and zero power. Between subject groups the diameter of the central, zero-powered zone was varied from 2 mm to 8 mm in 2 mm steps (+3D/pl 2 mm, n = 6; +3D/pl 4 mm, n = 6; +3D/pl 6 mm, n = 8, or + 3D/pl 8 mm, n = 6). For the treatment lens with 2, 4, 6 and 8 mm central zones, objects at eccentricities beyond 11°, 16°, 19° and 23°, respectively, were imaged exclusively through the dual-power peripheral zones. Refractive status (retinoscopy), corneal power (keratometry) and axial dimensions (ultrasonography) were measured at two-week intervals. Comparison data were obtained from monkeys reared with binocular, single-vision +3D full-field lenses (+3D FF, n = 6) and 41 normal control monkeys reared with unrestricted vision. At the end of the rearing period, with the exception of the +3D/pl 8 mm group (median = +3.64 D), the ametropias for the other lens-reared groups (medians: FF = +4.39 D, 2 mm = +5.19 D, 4 mm = +5.59 D, 6 mm = +3.50 D) were significantly more hyperopic than that for the normal monkeys (+2.50 D). These hyperopic errors were associated with shallower vitreous chambers. The key finding was that the extent and consistency of these hyperopic ametropias varied with the eccentricity of the dual-focus zones. The results confirm that myopic defocus in the near periphery can slow axial growth, but that imposed defocus beyond about 20° from the fovea does not consistently alter central refractive development.

Effects of low intensity ambient lighting on refractive development in infant rhesus monkeys (Macaca mulatta).

Studies in chickens suggest low intensity ambient lighting causes myopia. The purpose of this experiment was to examine the effects of low intensity ambient lighting (dim light) on normal refractive development in macaque monkeys. Seven infant rhesus monkeys were reared under dim light (room illumination level: ~55 lx) from 24 to ~310 days of age with otherwise unrestricted vision. Refractive error, corneal power, ocular axial dimensions, and choroidal thickness were measured in anesthetized animals at the onset of the experiment and periodically throughout the dim-light-rearing period, and were compared with those of normal-light-reared monkeys. We found that dim light did not produce myopia; instead, dim-light monkeys were hyperopic relative to normal-light monkeys (median refractive errors at ~155 days, OD: +3.13 D vs. +2.31 D; OS: +3.31D vs. +2.44 D; at ~310 days, OD: +2.75D vs. +1.78D, OS: +3.00D vs. +1.75D). In addition, dim-light rearing caused sustained thickening in the choroid, but it did not alter corneal power development, nor did it change the axial nature of the refractive errors. These results showed that, for rhesus monkeys and possibly other primates, low ambient lighting by itself is not necessarily myopiagenic, but might compromise the efficiency of emmetropization.

Immunotoxin-Induced Ablation of the Intrinsically Photosensitive Retinal Ganglion Cells in Rhesus Monkeys.

Purpose: Intrinsically photosensitive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin, and are primarily involved in non-image forming functions, such as the pupillary light reflex and circadian rhythm entrainment. The goal of this study was to develop and validate a targeted ipRGC immunotoxin to ultimately examine the role of ipRGCs in macaque monkeys. Methods: An immunotoxin for the macaque melanopsin gene (OPN4), consisting of a saporin-conjugated antibody directed at the N-terminus, was prepared in solutions of 0.316, 1, 3.16, 10, and 50 µg in vehicle, and delivered intravitreally to the right eye of six rhesus monkeys, respectively. Left eyes were injected with vehicle only. The pupillary light reflex (PLR), the ipRGC-driven post illumination pupil response (PIPR), and electroretinograms (ERGs) were recorded before and after injection. For pupil measurements, 1 and 5 s pulses of light were presented to the dilated right eye while the left pupil was imaged. Stimulation included 651 nm (133 cd/m2), and 4 intensities of 456 nm (16-500 cd/m2) light. Maximum pupil constriction and the 6 s PIPR were calculated. Retinal imaging was performed with optical coherence tomography (OCT), and eyes underwent OPN4 immunohistochemistry to evaluate immunotoxin specificity and ipRGC loss. Results: Before injection, animals showed robust pupil responses to 1 and 5 s blue light. After injection, baseline pupil size increased 12 ± 17%, maximum pupil constriction decreased, and the PIPR, a marker of ipRGC activity, was eliminated in all but the lowest immunotoxin concentration. For the highest concentrations, some inflammation and structural changes were observed with OCT, while eyes injected with lower concentrations appeared normal. ERG responses showed better preserved retinal function with lower concentrations. Immunohistochemistry showed 80-100% ipRGC elimination with the higher doses being more effective; however this could be partly due to inflammation that occurred at the higher concentrations. Conclusion: Findings demonstrated that the OPN4 macaque immunotoxin was specific for ipRGCs, and induced a graded reduction in the PLR, as well as, in ipRGC-driven pupil response with concentration. Further investigation of the effects of ipRGC ablation on ocular and systemic circadian rhythms and the pupil in rhesus monkeys will provide a better understanding of the role of ipRGCs in primates.

Narrow-band, long-wavelength lighting promotes hyperopia and retards vision-induced myopia in infant rhesus monkeys.

The purpose of this investigation was to determine the effects of narrow band, long-wavelength lighting on normal refractive development and the phenomena of lens compensation and form-deprivation myopia (FDM) in infant rhesus monkeys. Starting at 24 and continuing until 151 days of age, 27 infant rhesus monkeys were reared under long-wavelength LED lighting (630 nm; illuminance = 274 ± 64 lux) with unrestricted vision (Red Light (RL) controls, n = 7) or a +3 D (+3D-RL, n = 7), -3 D (-3D-RL, n = 6) or diffuser lens (From Deprivation (FD-RL), n = 7) in front of one eye and a plano lens in front of the fellow eye. Refractive development, corneal power, and vitreous chamber depth were measured by retinoscopy, keratometry, and ultrasonography, respectively. Comparison data were obtained from normal monkeys (Normal Light (NL) controls, n = 39) and lens- (+3D-NL, n = 9; -3D-NL, n = 18) and diffuser-reared controls (FD-NL, n = 16) housed under white fluorescent lighting. At the end of the treatment period, median refractive errors for both eyes of all RL groups were significantly more hyperopic than that for NL groups (P = 0.0001 to 0.016). In contrast to fluorescent lighting, red ambient lighting greatly reduced the likelihood that infant monkeys would develop either FDM or compensating myopia in response to imposed hyperopic defocus. However, as in the +3D-NL monkeys, the treated eyes of the +3D-RL monkeys exhibited relative hyperopic shifts resulting in significant anisometropias that compensated for the monocular lens-imposed defocus (P = 0.001). The red-light-induced alterations in refractive development were associated with reduced vitreous chamber elongation and increases in choroidal thickness. The results suggest that chromatic cues play a role in vision-dependent emmetropization in primates. Narrow-band, long-wavelength lighting prevents the axial elongation typically produced by either form deprivation or hyperopic defocus, possibly by creating direction signals normally associated with myopic defocus.

Adenosine receptor distribution in Rhesus monkey ocular tissue.

Adenosine receptor (ADOR) antagonists, such as 7-methylxanthine (7-MX), have been shown to slow myopia progression in humans and animal models. Adenosine receptors are found throughout the body, and regulate the release of neurotransmitters such as dopamine and glutamate. However, the role of adenosine in eye growth is unclear. Evidence suggests that 7-MX increases scleral collagen fibril diameter, hence preventing axial elongation. This study used immunohistochemistry (IHC) and reverse-transcription quantitative polymerase chain reaction (RT-qPCR) to examine the distribution of the four ADORs in the normal monkey eye to help elucidate potential mechanisms of action. Eyes were enucleated from six Rhesus monkeys. Anterior segments and eyecups were separated into components and flash-frozen for RNA extraction or fixed in 4% paraformaldehyde and processed for immunohistochemistry against ADORA1, ADORA2a, ADORA2b, and ADORA3. RNA was reverse-transcribed, and qPCR was performed using custom primers. Relative gene expression was calculated using the ΔΔCt method normalizing to liver expression, and statistical analysis was performed using Relative Expression Software Tool. ADORA1 immunostaining was highest in the iris sphincter muscle, trabecular meshwork, ciliary epithelium, and retinal nerve fiber layer. ADORA2a immunostaining was highest in the corneal epithelium, trabecular meshwork, ciliary epithelium, retinal nerve fiber layer, and scleral fibroblasts. ADORA2b immunostaining was highest in corneal basal epithelium, limbal stem cells, iris sphincter, ciliary muscle, ciliary epithelium, choroid, isolated retinal ganglion cells and scattered scleral fibroblasts. ADORA3 immunostaining was highest in the iris sphincter, ciliary muscle, ciliary epithelium, choroid, isolated retinal ganglion cells, and scleral fibroblasts. Compared to liver mRNA, ADORA1 mRNA was significantly higher in the brain, retina and choroid, and significantly lower in the iris/ciliary body. ADORA2a expression was higher in brain and retina, ADORA2b expression was higher in retina, and ADORA3 was higher in the choroid. In conclusion, immunohistochemistry and RT-qPCR indicated differential patterns of expression of the four adenosine receptors in the ocular tissues of the normal non-human primate. The presence of ADORs in scleral fibroblasts and the choroid may support mechanisms by which ADOR antagonists prevent myopia. The potential effects of ADOR inhibition on both anterior and posterior ocular structures warrant investigation.

The Adenosine Receptor Antagonist, 7-Methylxanthine, Alters Emmetropizing Responses in Infant Macaques.

Purpose: Previous studies suggest that the adenosine receptor antagonist, 7-methylxanthine (7-MX), retards myopia progression. Our aim was to determine whether 7-MX alters the compensating refractive changes produced by defocus in rhesus monkeys. Methods: Starting at age 3 weeks, monkeys were reared with -3 diopter (D; n = 10; 7-MX -3D/pl) or +3D (n = 6; 7-MX +3D/pl) spectacles over their treated eyes and zero-powered lenses over their fellow eyes. In addition, they were given 100 mg/kg of 7-MX orally twice daily throughout the lens-rearing period (age 147 ± 4 days). Comparison data were obtained from lens-reared controls (-3D/pl, n = 17; +3D/pl, n = 9) and normal monkeys (n = 37) maintained on a standard diet. Refractive status, corneal power, and axial dimensions were assessed biweekly. Results: The -3D/pl and +3D/pl lens-reared controls developed compensating myopic (-2.10 ± 1.07 D) and hyperopic anisometropias (+1.86 ± 0.54 D), respectively. While the 7-MX +3D/pl monkeys developed hyperopic anisometropias (+1.79 ± 1.11 D) that were similar to those observed in +3D/pl controls, the 7-MX -3D/pl animals did not consistently exhibit compensating myopia in their treated eyes and were on average isometropic (+0.35 ± 1.96 D). The median refractive errors for both eyes of the 7-MX -3D/pl (+5.47 D and +4.38 D) and 7-MX +3D/pl (+5.28 and +3.84 D) monkeys were significantly more hyperopic than that for normal monkeys (+2.47 D). These 7-MX-induced hyperopic ametropias were associated with shorter vitreous chambers and thicker choroids. Conclusions: In primates, 7-MX reduced the axial myopia produced by hyperopic defocus, augmented hyperopic shifts in response to myopic defocus, and induced hyperopia in control eyes. The results suggest that 7-MX has therapeutic potential in efforts to slow myopia progression.

Postnatal maturation of the fovea in Macaca mulatta using optical coherence tomography.

Changes in the foveal anatomy during infancy are an important component in early development of spatial vision. The present longitudinal study in rhesus monkeys was undertaken to characterize the postnatal maturation of the fovea. Starting at four weeks after birth, the retinas of the left eyes of sixteen infant monkeys were imaged using spectral domain optical coherence tomography (SD OCT). Retinal scans were repeated every 30 days during the first year of life and every 60 days thereafter. Volume scans through the fovea were registered, scaled using a three surface schematic eye, and analyzed to measure foveal pit parameters. The individual layers of the retina were manually segmented and thicknesses were measured over a transverse distance of 1250 microns from the center of the foveal pit. Based on infrared scanning laser ophthalmoscope (IR SLO) images acquired with the SD OCT system, there were significant changes in the extent of the retina scanned as the eyes matured. Using a three-surface schematic eye, the length of each scan could be computed and was validated using image registration (R2 = 0.88, slope = 1.003, p < 0.05). Over the first 18 months of life, the mean retinal thickness at the pit center had increased by 21.4% with a corresponding 20.3% decrease in pit depth. The major changes occurred within the first 120 days, but did not stabilize until a year after birth. In Macaca mulatta infants, the primary anatomical maturation of the fovea occurs within the first few months of life, as determined by longitudinal data from SD OCT measurements. The timelines for maturation of the fovea correspond well with the normal development of the lateral geniculate nucleus, cortical neurophysiology, and spatial resolution in monkeys.

Observations on the relationship between anisometropia, amblyopia and strabismus.

We investigated the potential causal relationships between anisometropia, amblyopia and strabismus, specifically to determine whether either amblyopia or strabismus interfered with emmetropization. We analyzed data from non-human primates that were relevant to the co-existence of anisometropia, amblyopia and strabismus in children. We relied on interocular comparisons of spatial vision and refractive development in animals reared with 1) monocular form deprivation; 2) anisometropia optically imposed by either contact lenses or spectacle lenses; 3) organic amblyopia produced by laser ablation of the fovea; and 4) strabismus that was either optically imposed with prisms or produced by either surgical or pharmacological manipulation of the extraocular muscles. Hyperopic anisometropia imposed early in life produced amblyopia in a dose-dependent manner. However, when potential methodological confounds were taken into account, there was no support for the hypothesis that the presence of amblyopia interferes with emmetropization or promotes hyperopia or that the degree of image degradation determines the direction of eye growth. To the contrary, there was strong evidence that amblyopic eyes were able to detect the presence of a refractive error and alter ocular growth to eliminate the ametropia. On the other hand, early onset strabismus, both optically and surgically imposed, disrupted the emmetropization process producing anisometropia. In surgical strabismus, the deviating eyes were typically more hyperopic than their fellow fixating eyes. The results show that early hyperopic anisometropia is a significant risk factor for amblyopia. Early esotropia can trigger the onset of both anisometropia and amblyopia. However, amblyopia, in isolation, does not pose a significant risk for the development of hyperopia or anisometropia.

The Effects of the Relative Strength of Simultaneous Competing Defocus Signals on Emmetropization in Infant Rhesus Monkeys.

PURPOSE: We investigated how the relative surface area devoted to the more positive-powered component in dual-focus lenses influences emmetropization in rhesus monkeys. METHODS: From 3 to 21 weeks of age, macaques were reared with binocular dual-focus spectacles. The treatment lenses had central 2-mm zones of zero-power and concentric annular zones that had alternating powers of either +3.0 diopters (D) and 0 D (+3 D/pL) or -3.0 D and 0 D (-3 D/pL). The relative widths of the powered and plano zones varied from 50:50 to 18:82 between treatment groups. Refractive status, corneal curvature, and axial dimensions were assessed biweekly throughout the lens-rearing period. Comparison data were obtained from monkeys reared with binocular full-field single-vision lenses (FF+3D, n = 6; FF-3D, n = 10) and from 35 normal controls. RESULTS: The median refractive errors for all of the +3 D/pL lens groups were similar to that for the FF+3D group (+4.63 D versus +4.31 D to +5.25 D; P = 0.18-0.96), but significantly more hyperopic than that for controls (+2.44 D; P = 0.0002-0.003). In the -3 D/pL monkeys, refractive development was dominated by the zero-powered portions of the treatment lenses; the -3 D/pL animals (+2.94 D to +3.13 D) were more hyperopic than the FF-3D monkeys (-0.78 D; P = 0.004-0.006), but similar to controls (+2.44 D; P = 0.14-0.22). CONCLUSIONS: The results demonstrate that even when the more positive-powered zones make up only one-fifth of a dual-focus lens' surface area, refractive development is still dominated by relative myopic defocus. Overall, the results emphasize that myopic defocus distributed across the visual field evokes strong signals to slow eye growth in primates.

Effects of Long-Wavelength Lighting on Refractive Development in Infant Rhesus Monkeys.

PURPOSE: Differences in the spectral composition of lighting between indoor and outdoor scenes may contribute to the higher prevalence of myopia in children who spend low amounts of time outdoors. Our goal was to determine whether environments dominated by long-wavelength light promote the development of myopia. METHODS: Beginning at 25 ± 2 days of age, infant monkeys were reared with long-wavelength-pass (red) filters in front of one (MRL, n = 6) or both eyes (BRL, n = 7). The filters were worn continuously until 146 ± 7 days of age. Refractive development, corneal power, and vitreous chamber depth were assessed by retinoscopy, keratometry, and ultrasonography, respectively. Control data were obtained from 6 monkeys reared with binocular neutral density (ND) filters and 33 normal monkeys reared with unrestricted vision under typical indoor lighting. RESULTS: At the end of the filter-rearing period, the median refractive error for the BRL monkeys (+4.25 diopters [D]) was significantly more hyperopic than that for the ND (+2.22 D; P = 0.003) and normal monkeys (+2.38 D; P = 0.0001). Similarly, the MRL monkeys exhibited hyperopic anisometropias that were larger than those in normal monkeys (+1.70 ± 1.55 vs. -0.013 ± 0.33 D, P < 0.0001). The relative hyperopia in the treated eyes was associated with shorter vitreous chambers. Following filter removal, the filter-reared monkeys recovered from the induced hyperopic errors. CONCLUSIONS: The observed hyperopic shifts indicate that emmetropization does not necessarily target the focal plane that maximizes luminance contrast and that reducing potential chromatic cues can interfere with emmetropization. There was no evidence that environments dominated by long wavelengths necessarily promote myopia development.

The effects of simultaneous dual focus lenses on refractive development in infant monkeys.

PURPOSE: We investigated the effects of two simultaneously imposed, competing focal planes on refractive development in monkeys. METHODS: Starting at 3 weeks of age and continuing until 150 ± 4 days of age, rhesus monkeys were reared with binocular dual-focus spectacle lenses. The treatment lenses had central 2-mm zones of zero power and concentric annular zones with alternating powers of +3.0 diopter [D] and plano (pL or 0 D) (n = 7; +3D/pL) or -3.0 D and plano (n = 7; -3D/pL). Retinoscopy, keratometry, and A-scan ultrasonography were performed every 2 weeks throughout the treatment period. For comparison purposes data were obtained from monkeys reared with full field (FF) +3.0 (n = 4) or -3.0 D (n = 5) lenses over both eyes and 33 control animals reared with unrestricted vision. RESULTS: The +3 D/pL lenses slowed eye growth resulting in hyperopic refractive errors that were similar to those produced by FF+3 D lenses (+3 D/pL = +5.25 D, FF +3 D = +4.63 D; P = 0.32), but significantly more hyperopic than those observed in control monkeys (+2.50 D, P = 0.0001). One -3 D/pL monkey developed compensating axial myopia; however, in the other -3 D/pL monkeys refractive development was dominated by the zero-powered portions of the treatment lenses. The refractive errors for the -3 D/pL monkeys were more hyperopic than those in the FF -3 D monkeys (-3 D/pL = +3.13 D, FF -3D = -1.69 D; P = 0.01), but similar to those in control animals (P = 0.15). CONCLUSIONS: In the monkeys treated with dual-focus lenses, refractive development was dominated by the more anterior (i.e., relatively myopic) image plane. The results indicate that imposing relative myopic defocus over a large proportion of the retina is an effective means for slowing ocular growth.