Stimulus Requirements for the Decoding of Myopic and Hyperopic Defocus under Single and Competing Defocus Conditions in the Chicken

University of California, Berkeley, Berkeley, California, United States
Investigative Ophthalmology &amp Visual Science (Impact Factor: 3.66). 08/2005; 46(7):2242-52. DOI: 10.1167/iovs.04-1200
Source: PubMed

ABSTRACT The bidirectional nature of emmetropization, as observed in young chicks, implies that eyes are able to distinguish between myopic and hyperopic focusing errors. In the current study the spatial frequency and contrast dependence of this process were investigated in an experimental paradigm that allowed strict control over both parameters of the retinal image. Also investigated was the influence of accommodation.
Defocusing stimuli were presented through lens-cone devices with attached targets. These devices were monocularly applied to 5-day-old chickens for 4 days. Defocus conditions included: (1) 7 D of myopic defocus, (2) 7 D of hyperopic defocus, and (3) a combination of the two. Two high contrast target designs, a spatially rich, striped Maltese cross (target 1) and a standard Maltese cross (target 2) were used, except in some experiments where target contrast or spatial frequency content was further manipulated. To test the role of accommodation, the treated eye of some chicks underwent ciliary nerve section before attachment of the device. Refractive error (RE) was measured by retinoscopy and axial ocular dimensions measured by A-scan ultrasonography, both in chicks under anesthesia.
With imposed myopic defocus and high contrast, target 1 elicited significantly better compensation than did target 2. With imposed hyperopic defocus, both targets elicited near normal compensatory responses. Reducing image contrast to 32% for target 2 and to 16% for target 1 precluded compensation for myopic defocus, inducing myopia instead. The low-pass-filtered target also induced myopia, irrespective of the sign of imposed defocus. With competing defocus and intact accommodation, target 1 induced a transient hyperopic growth response, whereas myopia was consistently observed with target 2. When accommodation was rendered inactive, both targets induced myopia under these competitive conditions.
Compensation to myopic defocus is critically dependent on the inclusion of middle to high spatial frequencies in the stimulus and has a spatial frequency-dependent threshold contrast requirement. With competing myopic and hyperopic defocus, the former transiently dominates the latter as a determinant of ocular growth, provided that the stimulus conditions include sufficient middle to high spatial frequency information and that accommodation cues are available.

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    ABSTRACT: Purpose: Bifocal contact lenses were used to impose hyperopic and myopic defocus on the peripheral retina of marmosets. Eye growth and refractive state were compared to untreated animals and those treated with single-vision or multi-zone contact lenses from earlier studies. Methods: Thirty juvenile marmosets wore one of three experimental annular bifocal contact lens designs on their right eyes and a plano contact lens on the left eye as a control for 12 weeks from 70 days of age (10 marmosets/group). The experimental designs had plano center zones (1.5mm or 3mm) and +5D or -5D in the periphery (referred to as +5D/1.5mm, +5D/3mm and -5D/3mm). We measured the central and peripheral mean spherical refractive error (MSE), vitreous chamber depth (VC), pupil diameter (PD) and myopic progression rates prior to and during treatment. The results were compared to age-matched untreated (N=25), single-vision positive (N=19), negative (N=16), and +5/-5D multi-zone lens-reared marmosets (N=10). Results: At the end of treatment, animals in the -5D/3mm group had larger (p<0.01) and more myopic eyes (p<0.05) than animals in the +5D/1.5mm group. There was a dose-dependent relationship between the peripheral treatment zone area and the treatment-induced changes in eye growth and refractive state. Pre-treatment ocular growth rates and baseline peripheral refraction accounted for 40% of the induced refraction and axial growth rate changes. Conclusions: Eye growth and refractive state can be manipulated by altering peripheral retinal defocus. Imposing peripheral hyperopic defocus produces axial myopia, whereas peripheral myopic defocus produces axial hyperopia. The effects are smaller than using single-vision contact lenses that impose full field defocus, but support the use of bifocal or multifocal contact lenses as an effective treatment for myopia control.
    Investigative Ophthalmology &amp Visual Science 09/2014; 55(10). DOI:10.1167/iovs.14-14524 · 3.66 Impact Factor
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    ABSTRACT: Purpose. Eye growth compensates in opposite directions to single vision (SV) negative and positive lenses. We evaluated the response of the guinea pig eye to Fresnel-type lenses incorporating two different powers. Methods. 114 guinea pigs (10 groups with 9-14 in each) wore a lens over one eye and interocular differences in refractive error and ocular dimensions were measured in each of three experiments. First, the effects of three Fresnel designs (-5D/0D; +5D/0D or -5D/+5D dual powers) were compared to three SV lenses (-5D, +5D or 0D). Second, the ratio of -5D and +5D power in a Fresnel lens was varied (50:50 compared to 60:40). Third, myopia was induced by 4 days of exposure to a SV -5D lens, which was then exchanged for a Fresnel lens (+5D/-5D) or one of two SV lenses (+5D or -5D) and ocular parameters tracked for a further 3 weeks. Results. Dual power lenses induced an intermediate response between that to the two constituent powers (lenses +5D, +5D/0D, 0D, +5D/-5D, -5D/0D and -5D induced +2.1D, +0.7D, +0.1D, -0.3D, -1.6D and -5.1D in mean intraocular differences in refractive error respectively), and changing the ratio of powers induced responses equal to their weighted average. In already myopic animals, continued treatment with SV negative lenses increased their myopia (from -3.3D to -4.2D), while those switched to SV positive lenses or +5D/-5D Fresnel lenses, reduced their myopia (by 2.9D and 2.3D respectively). Conclusions. The mammalian eye integrates competing defocus to guide its refractive development and eye growth. Fresnel lenses, incorporating positive or plano power with negative power, can slow ocular growth, suggesting that such designs may control myopia progression in humans.
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