Effects of local myopic defocus on refractive development in monkeys

Abstract

Purpose: Visual signals that produce myopia are mediated by local, regionally selective mechanisms. However, little is known about spatial integration for signals that slow eye growth. The purpose of this study was to determine whether the effects of myopic defocus are integrated in a local manner in primates.

Methods: Beginning at 24 ± 2 days of age, seven rhesus monkeys were reared with monocular spectacles that produced 3 diopters (D) of relative myopic defocus in the nasal visual field of the treated eye but allowed unrestricted vision in the temporal field (NF monkeys). Seven monkeys were reared with monocular +3 D lenses that produced relative myopic defocus across the entire field of view (FF monkeys). Comparison data from previous studies were available for 11 control monkeys, 8 monkeys that experienced 3 D of hyperopic defocus in the nasal field, and 6 monkeys exposed to 3 D of hyperopic defocus across the entire field. Refractive development, corneal power, and axial dimensions were assessed at 2- to 4-week intervals using retinoscopy, keratometry, and ultrasonography, respectively. Eye shape was assessed using magnetic resonance imaging.

Results: In response to full-field myopic defocus, the FF monkeys developed compensating hyperopic anisometropia, the degree of which was relatively constant across the horizontal meridian. In contrast, the NF monkeys exhibited compensating hyperopic changes in refractive error that were greatest in the nasal visual field. The changes in the pattern of peripheral refractions in the NF monkeys reflected interocular differences in vitreous chamber shape.

Conclusions: As with form deprivation and hyperopic defocus, the effects of myopic defocus are mediated by mechanisms that integrate visual signals in a local, regionally selective manner in primates. These results are in agreement with the hypothesis that peripheral vision can influence eye shape and potentially central refractive error in a manner that is independent of central visual experience.

Figures

Figure 1

Spherical-equivalent refractive corrections obtained at different times during the treatment period for representative monkeys in the FF +3 D and NF +3 D subject groups plotted as a function of horizontal visual field eccentricity. The open and filled symbols represented the control and treated eyes, respectively. The plots on the left were obtained at the onset of the treatment period; the ages for the subsequent measures are shown in each plot. Zero eccentricity represents the pupillary axis.

Figure 2

Spherical-equivalent refractive corrections plotted as a function of horizontal visual field eccentricity for the treated (filled symbols) and fellow eyes (open symbols) of individual monkeys treated with full-field +3 D lenses (panels A–G). The data points represent the mean ± SE for the final three measurement sessions of the lens-rearing period. Panel H shows the group means (±SE) for the FF +3 D monkeys at the end of the treatment period.

Figure 3

Spherical-equivalent refractive corrections plotted as a function of horizontal visual field eccentricity for the treated (filled symbols) and fellow eyes (open symbols) of individual monkeys treated with nasal-field +3 D lenses (panels A–G). See Figure 2 legend for other details.

Figure 4

Interocular differences in refractive error (treated eye – control eye) plotted as a function of horizontal visual field eccentricity for individual monkeys that were reared with full-field (A) and nasal-field defocus (B). The shaded area represents the range of anisometropias (right eye – left eye) for age-matched normal animals.

Figure 5

Vitreous chamber depths for 3 representative animals in the full-field and nasal-field groups. For each subject group, the top and bottom rows of plots show, respectively, vitreous chamber depth and interocular differences in vitreous chamber depth (treated eye – control eye) plotted as a function of eccentricity. In the top rows of plots the filled and open symbols represent the treated and fellow control eyes, respectively. In the right panels, magnetic resonance images obtained for the horizontal plane are shown for the treated and control eyes of a monkey reared with nasal field defocus. The superimposed outlines for the treated and fellow eyes are shown in red and blue, respectively (online only). The image of the fellow eye was rotated around the presumed optic axis so that the nasal retinas are shown to the right for both eyes. The superimposed images were aligned using the lines that connected the equatorial poles of the crystalline lenses as a reference. A color version of this figure is available online at www.optvissci.com.

Figure 6

Mean (±SE) interocular differences in vitreous chamber depth obtained from MR images near the end of the treatment period plotted as a function of horizontal retinal eccentricity for the FF (left) and NF subject groups (right). The shade area represents the mean ± 2 SEs for the interocular differences in the vitreous chamber depths of age-matched normal monkeys.

Figure 7

Interocular differences in refractive error are plotted as a function of interocular differences in vitreous chamber depth (treated or right eye – fellow or left eye) for all eccentricities along the horizontal meridian for both the full-field (left) and nasal-field subject groups (right). Data are also shown for the age-matched normal controls (small circles). The solid lines represent the best fitting regression lines for the treated monkeys.

Figure 8

Average (±SE) refractions plotted as a function of horizontal eccentricity for monkeys were reared with full-field −3 D lenses (A), nasal field −3 D lenses (B), full-field +3 D lenses (C), and nasal field +3 D lenses (D). The filled and open symbols represent the treated and fellow eyes, respectively. The average (±2 SE) ametropias for age-matched normal monkeys are indicated by the shaded areas in each plot. The data for the monkeys treated with −3 D lenses were replotted from Smith et al.