Effect of isoflurane anaesthetic time on ocular a-scan ultrasonography measures and their relationship to age and OCT measures in the Guinea pig.

A-scan ultrasonography enables precise measurement of internal ocular structures. Historically, its use has underpinned fundamental studies of eye development and aberrant eye growth in animal models of myopia; however, the procedure typically requires anaesthesia. Since anaesthesia affects intra-ocular pressure (IOP), we investigated changes in internal ocular structures with isoflurane exposure and compared measurements with those taken in awake animals using optical coherence tomography (OCT). Continuous A-scan ultrasonography was undertaken in tri-coloured guinea pigs aged 21 (n = 5), 90 (n = 5) or 160 (n = 5) days while anaesthetised (up to 36 min) with isoflurane (5% in 1.5L/min O2). Peaks were selected from ultrasound traces corresponding to the boundaries of the cornea, crystalline lens, retina, choroid and sclera. OCT scans (Zeiss Cirrus Photo 800) of the posterior eye layers were taken in 28-day-old animals (n = 19) and compared with ultrasound traces, with choroid and scleral thickness adjusted for the duration of anaesthesia based on the changes modelled in 21-day-old animals. Ultrasound traces recorded sequentially in left and right eyes in 14-day-old animals (n = 30) were compared, with each adjusted for anaesthesia duration. The thickness of the cornea was measured in enucleated eyes (n = 5) using OCT following the application of ultrasound gel (up to 20 min). Retinal thickness was the only ultrasound internal measure unaffected by anaesthesia. All other internal distances rapidly changed and were well fitted by exponential functions (either rise-to-max or decay). After 10 and 20 min of anaesthesia, the thickness of the cornea, crystalline lens and sclera increased by 17.1% and 23.3%, 0.4% and 0.6%, and 5.2% and 6.5% respectively, whilst the anterior chamber, vitreous chamber and choroid decreased by 4.4% and 6.1%, 0.7% and 1.1%, and 10.7% and 11.8% respectively. In enucleated eyes, prolonged contact of the cornea with ultrasound gel resulted in an increase in thickness of 9.3% after 10 min, accounting for approximately half of the expansion observed in live animals. At the back of the eye, ultrasound measurements of the thickness of the retina, choroid and sclera were highly correlated with those from posterior segment OCT images (R2 = 0.92, p = 1.2 × 10-13, R2 = 0.55, p = 4.0 × 10-4, R2 = 0.72, p = 5.0 × 10-6 respectively). Furthermore, ultrasound measures for all ocular components were highly correlated in left and right eyes measured sequentially, when each was adjusted for anaesthetic depth. This study shows that the depth of ocular components can change dramatically with anaesthesia. Researchers should therefore be wary of these concomitant effects and should employ adjustments to better render 'true' values.

Analytical methods for assessing retinal cell coupling using cut-loading.

Electrical coupling between retinal neurons contributes to the functional complexity of visual circuits. "Cut-loading" methods allow simultaneous assessment of cell-coupling between multiple retinal cell-types, but existing analysis methods impede direct comparison with gold standard direct dye injection techniques. In the current study, we both improved an existing method and developed two new approaches to address observed limitations. Each method of analysis was applied to cut-loaded dark-adapted Guinea pig retinae (n = 29) to assess coupling strength in the axonless horizontal cell type ('a-type', aHCs). Method 1 was an improved version of the standard protocol and described the distance of dye-diffusion (space constant). Method 2 adjusted for the geometric path of dye-transfer through cut-loaded cells and extracted the rate of dye-transfer across gap-junctions in terms of the coupling coefficient (kj). Method 3 measured the diffusion coefficient (De) perpendicular to the cut-axis. Dye transfer was measured after one of five diffusion times (1-20 mins), or with a coupling inhibitor, meclofenamic acid (MFA) (50-500µM after 20 mins diffusion). The standard protocol fits an exponential decay function to the fluorescence profile of a specified retina layer but includes non-specific background fluorescence. This was improved by measuring the fluorescence of individual cell soma and excluding from the fit non-horizontal cells located at the cut-edge (p<0.001) (Method 1). The space constant (Method 1) increased with diffusion time (p<0.01), whereas Methods 2 (p = 0.54) and 3 (p = 0.63) produced consistent results across all diffusion times. Adjusting distance by the mean cell-cell spacing within each tissue reduced the incidence of outliers across all three methods. Method 1 was less sensitive to detecting changes induced by MFA than Methods 2 (p<0.01) and 3 (p<0.01). Although the standard protocol was easily improved (Method 1), Methods 2 and 3 proved more sensitive and generalisable; allowing for detailed assessment of the tracer kinetics between different populations of gap-junction linked cell networks and direct comparison to dye-injection techniques.

Biomechanical changes in myopic sclera correlate with underlying changes in microstructure.

Myopia alters the microstructural and biomechanical properties of the posterior sclera, which is characterized as a layered structure with potentially different inter-layer collagen fibril characteristics. Scanning acoustic microscopy (SAM) has been used to investigate how the micron-scale bulk mechanical properties of the posterior sclera are affected by myopia. Other investigators have employed second harmonic generation (SHG) imaging to characterize the collagen microstructure of tissues. In the present study, SAM and SHG imaging were used to investigate the existence of biomechanically-distinct scleral layers and identify relationships between mechanical properties and tissue microstructure in myopic guinea pig (GP) eyes. Diffusers were worn over the right eyes of six, 1-week-old GPs for one week to induce unilateral form-deprivation myopia. GPs were euthanized, enucleated, and eyes were cryosectioned. Twelve-micron-thick adjacent vertical cryosections were scanned with SAM or SHG. SAM maps of bulk modulus, mass density, and acoustic attenuation were estimated. A fiber-extraction algorithm applied to SHG images estimated collagen fiber length, width, straightness, alignment, and number density. Results revealed that the posterior sclera may exhibit biomechanically distinct layers that are affected differently in myopia. Specifically, a layered structure was observed in the mechanical-parameter maps of control eyes that was less apparent in myopic eyes. Collagen fibers in myopic eyes had smaller diameters and were more aligned. Myopia-associated biomechanical changes were most significant in the outermost and innermost scleral layers. SAM-measured mechanical parameters were correlated with collagen fiber microstructure, particularly fiber length, alignment, and number density, which may imply the biomechanical parameters estimated from SAM measurements are related to tissue microstructure. Interestingly, some changes were greatest in more-peripheral regions, suggesting interventions to strengthen the sclera may be effective away from the optic nerve and efficacy may be achieved best when intervention is applied to the outermost layer.

SWATH Based Quantitative Proteomics Reveals Significant Lipid Metabolism in Early Myopic Guinea Pig Retina.

Most of the previous myopic animal studies employed a single-candidate approach and lower resolution proteomics approaches that were difficult to detect minor changes, and generated limited systems-wide biological information. Hence, a complete picture of molecular events in the retina involving myopic development is lacking. Here, to investigate comprehensive retinal protein alternations and underlying molecular events in the early myopic stage, we performed a data-independent Sequential Window Acquisition of all Theoretical Mass Spectra (SWATH) based proteomic analysis coupled with different bioinformatics tools in pigmented guinea pigs after 4-day lens-induced myopia (LIM). Myopic eyes compared to untreated contralateral control eyes caused significant changes in refractive error and choroid thickness (p < 0.05, n = 5). Relative elongation of axial length and the vitreous chamber depth were also observed. Using pooled samples from all individuals (n = 10) to build a species-specific retinal ion library for SWATH analysis, 3202 non-redundant proteins (with 24,616 peptides) were identified at 1% global FDR. For quantitative analysis, the 10 individual retinal samples (5 pairs) were analyzed using a high resolution Triple-TOF 6600 mass spectrometry (MS) with technical replicates. In total, 37 up-regulated and 21 down-regulated proteins were found significantly changed after LIM treatment (log2 ratio (T/C) > 0.26 or < -0.26; p ≤ 0.05). Data are accepted via ProteomeXchange with identifier PXD025003. Through Ingenuity Pathways Analysis (IPA), "lipid metabolism" was found as the top function associated with the differentially expressed proteins. Based on the protein abundance and peptide sequences, expression patterns of two regulated proteins (SLC6A6 and PTGES2) identified in this pathway were further successfully validated with high confidence (p < 0.05) using a novel Multiple Reaction Monitoring (MRM) assay on a QTRAP 6500+ MS. In summary, through an integrated discovery and targeted proteomic approach, this study serves as the first report to detect and confirm novel retinal protein changes and significant biological functions in the early LIM mammalian guinea pigs. The study provides new workflow and insights for further research to myopia control.

The Effect of Long-Term Low-Dose Atropine on Refractive Progression in Myopic Australian School Children.

Myopia will affect half the global population by 2050 and is a leading cause of vision impairment. High-dose atropine slows myopia progression but with undesirable side-effects. Low-dose atropine is an alternative. We report the effects of 0.01% or 0.005% atropine eye drops on myopia progression in 13 Australian children aged between 2 and 18 years and observed for 2 years without and up to 5 years (mean 2.8 years) with treatment. Prior to treatment, myopia progression was either 'slow' (more positive than -0.5 D/year; mean -0.19 D/year) or 'fast' (more negative than -0.5 D/year; mean -1.01 D/year). Atropine reduced myopic progression rates (slow: -0.07 D/year, fast: -0.25 D/year, combined: before: -0.74, during: -0.18 D/year, p = 0.03). Rebound occurred in 3/4 eyes that ceased atropine. Atropine halved axial growth in the 'Slow' group relative to an age-matched model of untreated myopes (0.098 vs. 0.196 mm/year, p < 0.001) but was double that in emmetropes (0.051 mm/year, p < 0.01). Atropine did not slow axial growth in 'fast' progressors compared to the age-matched untreated myope model (0.265 vs. 0.245 mm/year, p = 0.754, Power = 0.8). Adverse effects (69% of patients) included dilated pupils (6/13) more common in children with blue eyes (5/7, p = 0.04). Low-dose atropine could not remove initial myopia offsets suggesting treatment should commence in at-risk children as young as possible.

Chick Eyes Can Recover from Lens Compensation without Visual Cues.

SIGNIFICANCE: This study shows that nonvisual mechanism(s) can guide chick eyes to recover from myopia or hyperopia bidirectionally to regain their age-matched length. Because eye growth control is phylogenetically conserved across many species, it is possible that, in general, emmetropization mechanisms are not exclusively based on a local visual feedback system. PURPOSE: Across species, growing eyes compensate for imposed defocus by modifying their growth, showing the visual controls on eye growth and emmetropization. When the spectacle lens is removed, the eyes rapidly recover back to a normal size similar to that in the untreated eyes. We asked whether this recovery process was dependent on visual feedback or whether it might be guided by intrinsic nonvisual mechanisms. METHODS: Chicks wore either a +7 (n = 16) or -7 D (n = 16) lens over one eye for 4 to 7 days; the fellow eye was left untreated. After lens removal, half were recovered in darkness and half in white light. Refractive error and ocular dimensions were measured before and after lens treatment and after recovery with a Hartinger refractometer and A-scan biometer, respectively. RESULTS: Whereas chick eyes completely recovered from prior lens treatment under normal light after 2 days, they also partially recovered from prior hyperopia (by 60%) and myopia (by 69%) after being kept in darkness for 3 days: a +7 and -7 D lens induced a difference between the eyes of +7.08 and -4.69 D, respectively. After recovery in darkness, the eyes recovered by 3.18 and 2.88 D, respectively. CONCLUSIONS: In the absence of visual cues, anisometropic eyes can modify and reverse their growth to regain a similar length to their fellow untreated eye. Because eye growth control is phylogenetically conserved across many species, it is possible that nonvisual mechanisms may contribute more generally to emmetropization and that recovery from anisometropic refractive errors may not be wholly visually controlled.

The effect of optic nerve section on form deprivation myopia in the guinea pig.

Myopia is induced when a growing eye wears a diffuser that deprives it of detailed spatial vision (form deprivation, FD). In chickens with optic nerve section (ONS), FD myopia still occurs, suggesting that the signals underlying myopia reside within the eye. As avian eyes differ from mammals, we asked whether local mechanisms also underlie FD myopia in a mammalian model. Young guinea pigs underwent either sham surgery followed by FD (SHAM + FD, n = 7); or ONS followed by FD (ONS + FD, n = 7); or ONS without FD (ONS, n = 9). FD was initiated 3 days after surgery with a diffuser that was worn on the surgically treated eye for 14 days. Animals with ONS + FD developed -8.9 D of relative myopia and elongated by 135 µm more than in their untreated eyes after 2 weeks of FD. These changes were significantly greater than those in SHAM + FD animals (-5.5 D and 40 µm of elongation after 14 days of FD), and reflected exaggerated elongation of the posterior vitreous chamber. The myopia reversed when FD was discontinued, despite ONS, but eyes did not recover back to normal (30 days after surgery, ONS + FD eyes still retained -3 D of relative myopia when SHAM+FD animals had returned to normal). No long-term residual myopia was present after ONS alone, ruling out a surgical artifact. Although the gross mechanism signaling myopic ocular growth and its recovery in the young mammalian eye does not require an intact optic nerve, its fine-tuning is disrupted by ONS.

Crystalline lens gradient refractive index distribution in the guinea pig.

PURPOSE: The crystalline lens undergoes morphological and functional changes with age and may also play a role in eye emmetropisation. Both the geometry and the gradient index of refraction (GRIN) distribution contribute to the lens optical properties. We studied the lens GRIN in the guinea pig, a common animal model to study myopia. METHODS: Lenses were extracted from guinea pigs (Cavia porcellus) at 18 days of age (n = 4, three monolaterally treated with negative lenses and one untreated) and 39 days of age (n = 4, all untreated). Treated eyes were myopic (-2.07 D on average) and untreated eyes hyperopic (+3.3 D), as revealed using streak retinoscopy in the live and cyclopeged animals. A custom 3D spectral domain optical coherence tomography (OCT) system (λ = 840 nm, Δλ = 50 nm) was used to image the enucleated crystalline lens at two orientations. Custom algorithms were used to estimate the lens shape and GRIN was modelled with four variables that were reconstructed using the OCT data and a minimisation algorithm. Ray tracing was used to calculate the optical power and spherical aberration assuming a homogeneous refractive index or the estimated GRIN. RESULTS: Guinea pig lenses exhibited nearly parabolic GRIN profiles. When comparing the two age groups (18- and 39 day-old) there was a significant increase in the central thickness (from 3.61 to 3.74 mm), and in the refractive index of the surface (from 1.362 to 1.366) and the nucleus (from 1.443 to 1.454). The presence of GRIN shifted the spherical aberration (-4.1 µm on average) of the lens towards negative values. CONCLUSIONS: The guinea pig lens exhibits a GRIN profile with surface and nucleus refractive indices that increase slightly during the first days of life. GRIN plays a major role in the lens optical properties and should be incorporated into computational guinea pig eye models to study emmetropisation, myopia development and ageing.

Regional changes in the elastic properties of myopic Guinea pig sclera.

Biomechanical changes in the sclera likely underlie the excessive eye elongation of axial myopia. We studied the biomechanical characteristics of myopic sclera at the microscopic level using scanning acoustic microscopy (SAM) with 7-µm in-plane resolution. Guinea pigs underwent form-deprivation (FD) in one eye from 4 to 12 days of age to induce myopia, and 12-µm-thick scleral cryosections were scanned using a custom-made SAM. Two-dimensional maps of the bulk modulus (K) and mass density (ρ) were derived from the SAM data using a frequency-domain approach. We assessed the effect on K and ρ exerted by: 1) level of induced myopia, 2) region (superior, inferior, nasal or temporal) and 3) eccentricity from the nerve using univariate and multivariate regression analyses. Induced myopia ranged between -3D and -9.3D (Mean intraocular difference of -6.2 ±â€¯1.7D, N = 11). K decreased by 0.036 GPa for every 1.0 D increase in induced myopia across vertical sections (p < 0.001). Among induced myopia right eyes, K values in the inherently more myopic superior region were 0.088 GPa less than the inferior region (p = 0.002) and K in the proximal nasal region containing the central axis were 0.10 GPa less than temporal K (p = 0.036). K also increased 0.12 GPa for every 1 mm increase in superior vertical distance (p < 0.001), an effect that was blunted after 1 week of FD. Overall, trends for ρ were less apparent than for K. ρ values increased by 20.7 mg/cm3 for every 1.00 D increase in induced myopia across horizontal sections (p < 0.001), and were greatest in the region containing the central posterior pole. ρ values in the inherently more myopic superior region were 13.1 mg/cm3 greater than that found in inferior regions among control eyes (p = 0.002), and increased by 11.2 mg/cm3 for every 1 mm increase in vertical distance (p = 0.001). This peripheral increase in ρ was blunted after 1 week of FD. Scleral material properties vary depending on the location in the sclera and the level of induced myopia. Bulk modulus was most reduced in the most myopic regions (both induced myopia and inherent regional myopia), and suggests that FD causes microscopic local decreases in sclera stiffness, while scleral mass density was most increased in the most myopic regions.

Data on differentially expressed proteins in retinal emmetropization process in guinea pig using integrated SWATH-based and targeted-based proteomics.

Myopia is generally regarded as a failure of normal emmetropization process, however, its underlying molecular mechanisms are unclear. Retinal protein profile changes using integrated SWATH and MRM-HR MS were studied in guinea pigs at 3- and 21-days of age, where the axial elongation was significantly detected. Differential proteins expressions were identified, and related to pathways which are important in postnatal development in retina, proliferation, breakdown of glycogen-energy and visual phototransduction. These results are significant as key retinal protein players and pathways that underlying emmetropization can be discovered. All raw data generated from IDA and SWATH acquisitions were accepted and published in the Peptide Atlas public repository (http://www.peptideatlas.org/) for general release (Data ID PASS00746). A more comprehensive analysis of this data can be obtained in the article "Integrated SWATH-based and targeted-based proteomics provide insights into the retinal emmetropization process in guinea pig" in Journal of Proteomics (Shan et al., 2018) [1].

Gene Expression and Pathways Underlying Form Deprivation Myopia in the Guinea Pig Sclera.

Purpose: Posterior scleral remodeling accompanies myopia. In guinea pigs developing myopia, the region around the optic nerve (peripapillary zone, PPZ) rapidly expands followed by inhibition in eye size in the periphery. We studied the differential gene expression in the sclera that accompanies these changes. Methods: Guinea pigs were form-deprived (FD) for 2 weeks to induce myopia, while the fellow eye served as a control. After 2 weeks, the PPZ and the peripheral temporal sclera were isolated in representative animals to extract the RNA. RNA sequencing was undertaken using an Illumina HiSeq 2000, with differential expression analyzed using Voom and pathways analyzed using the Ingenuity Pathway Analysis tool. RNA from additional PPZ and peripheral temporal sclera in FD and fellow eyes was used for validation of gene expression using quantitative real-time PCR (qRT-PCR). Results: In myopic sclera, 348 genes were differentially expressed between PPZ and the peripheral temporal region (corrected P < 0.05), of which 61 were differentially expressed in the PPZ between myopic and control eyes. Pathway analyses of these gene sets showed the involvement of Gαi signaling along with previously reported gamma-aminobutyric acid (GABA) and glutamate receptors among numerous novel pathways. The expression pattern of three novel genes and two myopia-related genes was validated using qRT-PCR. Conclusions: Gene expression changes are associated with the rapid elongation that occurs around the optic nerve region during the development of myopia. A prominent change in Gαi signaling, which affects cAMP synthesis and thus collagen levels, may be critical in mediating the regional changes in myopic sclera.

Integrated SWATH-based and targeted-based proteomics provide insights into the retinal emmetropization process in guinea pig.

Myopia is generally regarded as a failure of normal emmetropization process, however, its underlying molecular mechanisms are unclear. To investigate the retinal protein profile changes during emmetropization, we studied differential protein expressions of ocular growth in young guinea pigs at 3 and 21 days old respectively, when significant axial elongation was detected (P < 0.001, n = 10). Independent pooled retinal samples of both eyes were subjected to SWATH mass spectrometry (MS) followed by bioinformatics analysis using cloud-based platforms. A comprehensive retina SWATH ion-library consisting of 3138 (22,871) unique proteins (peptides) at 1% FDR was constructed. 40 proteins were found to be significantly up-regulated and 8 proteins down-regulated during emmetropization (≥log2 of 0.43 with ≥2 peptides matched per protein; P < 0.05). Using pathway analysis, the most significant pathway identifiable was 'phototransduction' (P = 1.412e-4). Expression patterns of 7 proteins identified in this pathway were further validated and confirmed (P < 0.05) with high-resolution Multiple Reaction Monitoring (MRM-HR) MS. Combining discovery and targeted proteomics approaches, this study for the first time comprehensively profiled protein changes in the guinea pig retina during normal emmetropization-associated eye growth. The findings of this study are also relevant to the myopia development, which is the result of failed emmetropization. SIGNIFICANCE: Myopia is considered as a failure of emmetropization. However, the underlying biochemical mechanism of emmetropization, a visually guided process in which eye grows towards the optimal optical state of clear vision during early development, is not well understood. Retina is known as the key tissue to regulate this active eye growth. we studied eye growth of young guinea pigs and harvested their retinal tissues. A comprehensive SWATH ion library with identification of a total 3138 unique proteins were established, in which 48 proteins exhibited significant differential expressions between 3 and 21 days old. After MRM-HR confirmation, 'phototransduction' were found as the most active pathway during emmetropic eye growth. This study is the first in discovering key retinal protein players and pathways which are presumably orchestrated by biological mechanism(s) underlying emmetropization.

Early quantitative profiling of differential retinal protein expression in lens-induced myopia in guinea pig using fluorescence difference two-dimensional gel electrophoresis.

The current study aimed to investigate the differential protein expression in guinea pig retinas in response to lens-induced myopia (LIM) before fully compensated eye growth. Four days old guinea pigs (n=5) were subjected to ­4D LIM for 8 days. Refractive errors were measured before and at the end of the lens wear period. Ocular dimensions were also recorded using high­frequency A­scan ultrasonography. After the LIM treatment, retinas of both eyes were harvested and soluble proteins were extracted. Paired retinal protein expressions in each animal were profiled and compared using a sensitive fluorescence difference two­dimensional gel electrophoresis. The quantitative retinal proteomes of myopic and control eye were analysed using computerised DeCyder software. Those proteins that were consistently changed with at least 1.2­fold difference (P<0.05) in the same direction in all five animals were extracted, trypsin digested and identified by tandem mass spectrometry. Significant myopia was induced in guinea pigs after 8 days of lens wear. The vitreous chamber depth in lens­treated eyes was found to be significantly elongated. Typically, more than 1,000 protein spots could be detected from each retina. Thirty­two of them showed differential expression between myopic and untreated retina. Among these proteins, 21 spots were upregulated and 11 were downregulated. Eight protein spots could be successfully identified which included ß­actin, enolase 1, cytosolic malate dehydrogenase, Ras­related protein Rab­11B, protein­L­isoaspartate (D­aspartate) O­methyltransferase, PKM2 protein, X­linked eukaryotic translation initiation factor 1A and ACP1 protein. The present study serves as the first report to uncover the retinal 2D proteome expressions in mammalian guinea pig myopia model using a top­down fluorescent dyes labelling gel approach. The results showed a downregulation in glycolytic enzymes that may suggest a significant alteration of glycolysis during myopia development. Other protein candidates also suggested multiple pathways which could provide new insights for further study of the myopic eye growth.

Three-dimensional OCT based guinea pig eye model: relating morphology and optics.

Custom Spectral Optical Coherence Tomography (SOCT) provided with automatic quantification and distortion correction algorithms was used to measure the 3-D morphology in guinea pig eyes (n = 8, 30 days; n = 5, 40 days). Animals were measured awake in vivo under cyclopegia. Measurements showed low intraocular variability (<4% in corneal and anterior lens radii and <8% in the posterior lens radii, <1% interocular distances). The repeatability of the surface elevation was less than 2 µm. Surface astigmatism was the individual dominant term in all surfaces. Higher-order RMS surface elevation was largest in the posterior lens. Individual surface elevation Zernike terms correlated significantly across corneal and anterior lens surfaces. Higher-order-aberrations (except spherical aberration) were comparable with those predicted by OCT-based eye models.

The Effect of Spectacle Lenses Containing Peripheral Defocus on Refractive Error and Horizontal Eye Shape in the Guinea Pig.

Purpose: It has been proposed that the peripheral retina, responding to local optical defocus, contributes to myopia and associated altered eye growth in humans. To test this hypothesis, we measured the changes in central (on-axis) and peripheral ocular dimensions in guinea pigs wearing a concentric bifocal spectacle lens design with power restricted to the periphery. Methods: Five groups of guinea pigs (n = 83) wore either a unifocal (UF) spectacle lens (-4, 0, or +4 Diopters [D]), or a peripheral defocus (PF) spectacle lens that had a plano center (diameter of 5 mm) with either -4 or +4 D in the surround (-4/0 or +4/0 D). The overall optical diameter of all lenses was 12 mm. Lenses were worn over one eye from 8 to 18 days of age for negative and plano lenses, or from 8 to 22 days of age for positive lenses. Refractive error was measured centrally and 30° off-axis in the temporal and nasal retina. The shape of the eye was analyzed from images of sectioned eyes. Results: Lenses of -4 D UF induced myopia, reflecting enhanced ocular elongation, which was centered on the optic nerve head and included the surrounding peripapillary zone (PPZ, 18° in diameter). Some ocular expansion, including within the PPZ, also was recorded with -4/0 and +4/0 D PF lenses while the +4 D UF lens inhibited rather than enhanced elongation, centrally and peripherally. Conclusions: Peripheral defocus-induced ocular expansion encompasses the PPZ, irrespective of the sign of the inducing defocus. Understanding the underlying mechanism potentially has important implications for designing multifocal lenses for controlling myopia in humans and also potentially for understanding the link between myopia and glaucoma.

Understanding and Treating Myopia: What More We Need to Know and Future Research Priorities.

Tantalizing treatment options to limit further global increases in the prevalence of myopia are emerging. However, to design more effective interventions, we still need to learn more about the underlying causes of myopia and the associated biological changes. Based on the outcomes of the 2015 International Myopia Conference, this short article summarizes what more we still need to discover and suggests possible priorities for future research.

[Study of guinea pig refractive status and ocular length during the early recovery from lens-induced myopia].

OBJECTIVE: To investigate changes in refractive status and ocular length in guinea pigs during the early time of myopic recovery for the causes of recovery. METHODS: Exp guinea pigs wore a -5.00 D lens on one eye from 4-18 days, which was then removed for 48 hours. At 18 and 20 days of age, each eye was evaluated for refractive status and ocular length of the eye. RESULTS: The right eyes treated with -5 D lenses for 12 days developed (-2.00 ± 1.50) D (P = 0.04) of myopia and had an increase in axial length of (0.033 ± 0.025) mm compared to the left eyes (P = 0.04). After 48 hours of recovery, the difference between the two eyes was reduced to (-0.72 ± 0.86) D (P = 0.13), but the ocular length still had significant difference (0.031 ± 0.022) mm (P = 0.04). During the myopia recovery early period, the refractive status and ocular length changed in the same direction in the left eyes but in the opposite way in the right eyes. CONCLUSIONS: Guinea pigs treated with -5.00 D lenses for 12 days developed explicit relative axial myopia. After removal of the lens for 48 hours, myopia significantly recovery can be due to the thickening of choroid and the reduction in ocular growth.

The relationship between image degradation and myopia in the mammalian eye.

BACKGROUND: In all species studied, myopia develops if the eye is deprived of detailed vision during development (form deprivation myopia). However, different degrees of spatial image deprivation produce different effects and have not been described in the mammalian eye. Therefore, the effect of image degradation on guinea pig emmetropisation was investigated. METHODS: Eighty-one guinea pigs wore a treatment on one eye from 6 to 13 days of age. There were four treatments: a translucent diffuser (no lines or edges were visible through the diffuser); one of five Bangerter foils (BF: 0.8, 0.6, 0.4, 0.2, light perception only), which differed in their cut-off spatial frequencies; a 'ring mount' control with no filter; or one of two neutral density filters that reduced luminance only (ND, optical density grades 0.1 and 0.6). Refractive error and ocular elongation were measured after seven days of treatment. RESULTS: The extent of induced myopia and ocular growth were related to the amount of image degradation (mean difference between the treated and untreated eyes changed in a graded manner -7.0 D to -0.2 D and from 85 µm to seven µm respectively, for spatial frequency cut-offs between zero and 24 cycles per degree). Corresponding reductions in luminance from ND filters did not increase eye growth and caused significantly less myopia than the BFs that caused a similar luminance decrement. The greatest myopia occurred when no or limited spatial information was available to the eye, but moderate myopia still occurred with spatial frequency cut-offs of six and 12 cycles per degree, well beyond the visual acuity range of guinea pigs. CONCLUSION: Excessive ocular growth and myopia are most robust when induced by spatial frequency reductions within the visual acuity range but can also be induced beyond this. Either the mechanism of ocular growth can detect supra-threshold spatial frequencies, possibly due to aliasing, or it is sensitive to small amounts of contrast degradation.

Egr-1 mRNA expression is a marker for the direction of mammalian ocular growth.

PURPOSE: The immediate early gene Egr-1 is thought to form part of the pathway that mediates abnormal ocular growth. This study investigated whether the mRNA expression levels of Egr-1 in a mammalian retina are modulated differentially, depending on the direction of ocular growth. METHODS: To induce accelerated growth and myopia, guinea pigs wore a -5 diopter (D) lens over one eye from 4 to 11 days of age. To induce inhibited growth, the lens was removed after 7 days of -5 D lens wear, and the eye allowed to recover from myopia for 3 days. Ocular parameters and Egr-1 mRNA levels were subsequently assessed, and compared to untreated fellow eyes and eyes from untreated littermates. Possible circadian changes in Egr-1 mRNA levels were also determined in 18 additional animals by taking measures every 4 hours during a 24-hour cycle. RESULTS: Ocular compensation to a -5 D lens occurred after 7 days (Δ -4.8 D, Δ +147 µm growth, N = 20). In 5 highly myopic eyes (Δ -7.4 D), Egr-1 mRNA levels in the retina were significantly downregulated relative to contralateral control (51%) and age-matched untreated (47%) eyes. Three days after the -5 D lens was removed, eyes had recovered from the myopia (Δ -0.5 D, relative change of +2.9 D, N = 4) and Egr-1 mRNA levels were significantly elevated relative to contralateral (212%) and untreated (234%) eyes, respectively. Normal Egr-1 mRNA expression was higher in the middle of the day than in the middle of the night. Immunolabeling showed strong Egr-1 reactivity in cell bodies in the inner nuclear and ganglion cell layers. CONCLUSIONS: Egr-1 mRNA levels in a mammalian retina show a bi-directional persistent response to opposing ocular growth stimuli. This suggests retinal Egr-1 might act as a signal for the direction of ocular growth in different species.

Salient distractors can induce saccade adaptation.

When saccadic eye movements consistently fail to land on their intended target, saccade accuracy is maintained by gradually adapting the movement size of successive saccades. The proposed error signal for saccade adaptation has been based on the distance between where the eye lands and the visual target (retinal error). We studied whether the error signal could alternatively be based on the distance between the predicted and actual locus of attention after the saccade. Unlike conventional adaptation experiments that surreptitiously displace the target once a saccade is initiated towards it, we instead attempted to draw attention away from the target by briefly presenting salient distractor images on one side of the target after the saccade. To test whether less salient, more predictable distractors would induce less adaptation, we separately used fixed random noise distractors. We found that both visual attention distractors were able to induce a small degree of downward saccade adaptation but significantly more to the more salient distractors. As in conventional adaptation experiments, upward adaptation was less effective and salient distractors did not significantly increase amplitudes. We conclude that the locus of attention after the saccade can act as an error signal for saccade adaptation.