Hemiretinal Form Deprivation: Evidence for Local Control of Eye Growth and Refractive Development in Infant Monkeys

Abstract

Purpose.: To determine whether refractive development in primates is mediated by local retinal mechanisms, the authors examined the effects of hemiretinal form deprivation on ocular growth and the pattern of peripheral refractions in rhesus monkeys.

Methods.: Beginning at approximately 3 weeks of age, nine infant monkeys were reared wearing monocular diffuser lenses that eliminated form vision in the nasal field (nasal field diffuser [NFD]). Control data were obtained from the nontreated fellow eyes, 24 normal monkeys, and 19 monkeys treated with full-field diffusers. Refractive development was assessed by retinoscopy performed along the pupillary axis and at eccentricities of 15°, 30°, and 45°. Central axial dimensions and eye shape were assessed by A-scan ultrasonography and magnetic resonance imaging, respectively.

Results.: Hemiretinal form deprivation altered refractive development in a regionally selective manner, typically producing myopia in the treated hemifields. In particular, six of the NFD monkeys exhibited substantial amounts (−1.81 to −9.00 D) of relative myopia in the nasal field that were most obvious at the 15° and 30° nasal field eccentricities. The other three NFD monkeys exhibited small amounts of relative hyperopia in the treated field. The alterations in peripheral refraction were associated with local, region-specific alterations in vitreous chamber depth in the treated hemiretina.

Conclusions.: The effects of form deprivation on refractive development and eye growth in primates are mediated by mechanisms, presumably retinal, that integrate visual signals in a spatially restricted manner and exert their influence locally.

Many aspects of ocular growth and refractive development are regulated by visual feedback associated with the eye's refractive state (see Refs. 1, 2 for reviews). Knowledge of the operational properties of the vision-dependent mechanisms that influence refractive development is critical for understanding the role of vision in the genesis of common refractive errors and for developing the optimal treatment regimens for refractive error. 

In this respect, one of the most important discoveries arising from animal research is that in some species many of the effects of vision on ocular growth and refractive development appear to be mediated by mechanisms that are located entirely within the eye (see Refs. 2, 3 for reviews). The primary evidence for these retinal mechanisms has come from reduction strategy experiments that have examined the effects of a visual stimulus on refractive development when the obvious neural inputs or outputs from the eye have been eliminated. For example, in chicks and tree shrews, pharmacologic blockade and surgical section of the optic nerve do not interfere with the phenomenon of form-deprivation myopia (FDM) ^4–6 or the recovery from FDM, ⁷ and, though the set point for emmetropization is altered by optic nerve section, compensation for positive and negative lenses still occurs. ^8,9 Similarly, surgical interruption of the primary parasympathetic inputs to the eye does not prevent FDM or lens-induced changes in refractive error in chicks. ⁹ These experiments show that the visual signals that alter eye growth do not have to leave the eye and that the most obvious neural input to the eye is not essential for many aspects of vision-dependent ocular growth. 

In addition, it has been shown that these retinal mechanisms exert their influence in a spatially restricted, local manner. The most direct evidence for the local nature of these retinal mechanisms comes from experiments in which the nature of visual experience has been varied across the visual field. For example, in chicks, tree shrews, and guinea pigs that are reared with diffusers or negative lenses that only cover part of the visual field, the axial elongation and myopia are restricted to the affected part of the retina (McFadden SA. IOVS 2002;43:ARVO E-Abstract 189). ^10–14 Similarly, in chicks positive lenses that affect only the image over half the retina slow vitreous chamber elongation and produce hyperopia only in the treated portion of the retina. ¹⁰ These results demonstrate that the ocular mechanisms that regulate eye growth pool visual signals from restricted spatial regions and exert their influence locally. It is thought that the actions of these local retinal mechanisms alter the shape of the eye in response to variations in the environment to enhance the optimum focus across the retina. ^1,13,15,16  

It is not known whether local retinal mechanisms are involved in emmetropization responses in primates. In a study involving a small number of monkeys, Raviola and Wiesel ¹⁷ found that optic nerve section prevented FDM in stumptail macaques but not in rhesus monkeys. In addition, surgically eliminating the parasympathetic and sympathetic inputs to the eye did not prevent FDM in rhesus monkeys. ¹⁷ These results suggest that there are vision-dependent retinal mechanisms in rhesus monkeys that influence eye growth, but not in stumptail monkeys. This discrepancy between two closely related species is puzzling and might have come about as a result of the variability associated with the phenomenon of form-deprivation myopia in monkeys, ^18–21 possible hyperopic shifts produced by optic nerve section, ⁹ or the small number of animals studied. Regardless, it has not been clearly established that refractive development is mediated by retinal mechanisms in primates, and there have not been previous attempts to characterize the spatial summation properties of any potential retinal mechanisms in primates. Because of the significance of local retinal mechanisms to our understanding of the effects of vision on refractive development, it is critical to determine whether they exist in primates and whether they can produce predictable changes in eye shape. The great majority of what we know about local retinal mechanisms comes from studies in chicks. However, because of differences in the structure of the sclera in chicks and primates, ³ it is not reasonable to expect that similar visual manipulations will produce comparable shape changes in chicks and primates. For example, the cartilaginous portion of the chick sclera is comparatively rigid, and depriving half the retina of a chick produces a prominent bulge on the deprived side of the eye. ^5,13 Because primates have a less rigid fibrous sclera, it is possible that providing half the retina with a stimulus for growth would produce a more symmetrical, prolate axial elongation in primates. 

In this study, we used a rearing strategy that has provided strong evidence for the existence of local retinal mechanisms in chicks, guinea pigs, and tree shrews. Specifically, we examined the effects of hemiretinal form deprivation on ocular growth and the pattern of peripheral refraction in infant monkeys. Some of these results have been presented in abstract form. ²²  

Subjects, Materials, and Methods

Subjects

Data are presented for 56 infant rhesus monkeys (Macaca mulatta) obtained at 1 to 3 weeks of age and housed in our primate nursery that was maintained on a 12-hour light/12-hour dark lighting cycle. All the rearing and experimental procedures were reviewed and approved by the University of Houston's Institutional Animal Care and Use Committee and were in compliance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. 

The primary subjects were nine infant monkeys reared wearing goggle-mounted, nasal visual field diffusers (NFDs) over one eye and clear, zero-powered lenses in front of the fellow eye (see Ref. 23 for details of our goggle-rearing method). The NFDs consisted of a zero-powered carrier lens that was partially covered with a commercially available occlusion foil (Bangerter Occlusion Foils; Fresnel Prism and Lens Co., Mesa, AZ). The occlusion foils (manufacturer's designation, “LP” for light perception) were the strongest diffusers that we used in our previous studies of form-deprivation myopia in monkeys. ¹⁹ These diffusers, which act as high spatial frequency filters, reduced the contrast sensitivity of normal adult humans by more than 1 log unit for grating spatial frequencies of 0.125 cyc/deg, with a resultant cutoff spatial frequency near 1 cyc/deg. The vertical border of the occlusion foil was positioned approximately 1 mm to the temporal side of the center of the treated eye's entrance pupil while the eye was in primary gaze and, thus, degraded form vision throughout the nasal visual hemifield while allowing good central vision and unrestricted vision in the temporal visual hemifield. Without the occlusion foils, the carrier lenses provided monocular and binocular fields of view in the horizontal plane of 80° and 62°, respectively, and an 87° vertical field. The NFDs typically occluded slightly more than half the treated eye's monocular field. The lens-rearing period, which extended approximately from 3 weeks of age (22 ± 3 days) to 5 to 6 months of age (172 ± 15 days), encompassed the bulk of the rapid phase of emmetropization in rhesus monkeys. ²⁴  

For comparative purposes, data are also presented for 19 monkeys subjected to full-field monocular form deprivation (FFD) using the same LP diffusers that were used to construct the NFDs. The FFD monkeys wore the diffuser lenses continuously from 23 ± 3 to 150 ± 20 days of age. The data for all the FFD monkeys have been presented in previous publications. ^19,20,25  

Control data for central refractive errors and axial dimensions were obtained from 24 normal animals reared with unrestricted vision and four animals reared with clear, zero-powered lenses over both eyes (age, 145 ± 9 days). ^{5,13,19,20,23,24,26} Control data for peripheral refractions were obtained from seven of the normal animals (age, 162 ± 11 days); the data for six of these normal monkeys have been previously reported. ^25,27  

Ocular Biometry

The basic details of our biometric measurements have been described elsewhere. ^23,27 Briefly, to obtain the ocular measurements, each animal was anesthetized (15–20 mg/kg ketamine hydrochloride/0.15–0.2 mg/kg acepromazine maleate), and cycloplegia was induced using 1% tropicamide. 

Central and peripheral, spherical-equivalent refractive errors were measured by streak retinoscopy by two well-practiced investigators and averaged ²⁸ (details of our methods are described in Refs. 25 and 27). Central refraction was determined along the pupillary axis (i.e., the first Purkinje image produced by the retinoscope beam was observed in the center of the subject's entrance pupil). After measurements of central refraction, retinoscopy was performed at 15° intervals along either the vertical or the horizontal meridians out to a maximum eccentricity of 45°. Throughout this article, the eccentricities for refractive errors are specified with respect to the visual field (e.g., temporal field measurements correspond to refractive errors for the nasal retina). 

Although peripheral retinoscopy is often complicated by the phenomenon of “scissoring” or the so-called “double sliding-door effect,” using the criteria proposed by Rempt et al., ²⁹ the measures were reasonably repeatable. For example, the average limits of agreement obtained for repeated retinoscopic measures in young monkeys were 1.06, 1.03, and 1.21 D for the 15°, 30°, and 45° eccentricities, respectively. ²⁷  

Ocular axial dimensions were measured by A-scan ultrasonography implemented with either a 7-MHz (Image 2000; Mentor, Norwell, MA) or a 12-MHz (OTI Scan 1000; OTI Ophthalmic Technologies, Inc., ON, Canada) transducer. Intraocular distances were calculated from the average of 10 separate measurements using velocities of 1532, 1641, and 1532 m/s for the aqueous, lens, and vitreous, respectively. Corneal curvature was measured with a hand-held keratometer (Alcon Auto-keratometer; Alcon Systems Inc., St. Louis, MO) and/or a videotopographer (EyeSys 2000; EyeSys technologies Inc, Houston, TX). Both instruments provided repeatable and comparable measures of central corneal curvature in infant monkeys. ³⁰  

The biometric measurements were initially obtained at ages corresponding to the onset of lens wear and then at approximately 2-week intervals thereafter. Because of time constraints, peripheral refractions were obtained on either the horizontal or the vertical meridians during a given session. 

Magnetic Resonance Image Acquisition

Near the end of the diffuser-rearing period (approximately 160 days of age), magnetic resonance imaging (MRI) was performed on the NFD-reared animals using a horizontal bore scanner (7T Bruker Biospec USR 70/30; Bruker, Karlsruhe, Germany). The details of our MRI procedures have been described previously. ²⁵ Comparison data were obtained from two age-matched normal monkeys and six animals that were treated with full-field LP diffusers. The comparison data have been previously reported. ²⁵  

MRI was performed while the animals were anesthetized with 2% isoflurane gas anesthesia, which greatly reduced residual eye movements in our infant monkeys. ²⁵ During MRI, the monkeys were held in a supine position. Each animal's head was fixed on a custom-designed head holder so that the face was parallel to the floor and the eyes looked upward, with the optical axes approximately perpendicular to the bore of the magnet. The initial tripilot scan was used to localize the position of the monkey's eyes. Magnetic field homogeneity was then optimized using localized shimming with a point-resolved spectroscopy procedure. Anatomic images were acquired using a three-dimensional (3D) rapid acquisition paradigm with a relaxation enhancement sequence. T₂-weighted images were obtained with long repetition (TR, 1500 ms) and effective echo times (TE, 96 ms) to enhance the contrast between the fluids and tissues of the eye. The spatial resolution of the images was 0.195 × 0.195 × 0.5 mm in the horizontal plane. 

The acquired axial MR images were reconstructed via in-house software (MATLAB; MathWorks, Natick, MA), which allowed the MR images to be viewed from axial, sagittal, and coronal directions. The software interpolated between the axial image slices to produce a uniform resolution of 0.195 mm in the 3D matrix, which, in terms of vitreous chamber depth, corresponds to a dioptric interval of approximately 1 D. To ensure that measurements were obtained from the appropriate image plane, the 3D volumes were rotated so that when viewed from the sagittal direction, the line connecting the equatorial poles of the lens was vertical in the superior-inferior direction. This was done to ensure that the approximate optical axis was within the axial or horizontal plane. A Canny edge detection algorithm was applied to the horizontal plane of the rotated images to determine the boundaries between ocular structures. The approximate optical axis was defined as the perpendicular in the horizontal plane through the midpoint of the line connecting the equatorial poles of the lens. The horizontal slices that contained the greatest lens thicknesses were used for all further measurements. The intersection of the presumed optical axis and the posterior lens surface were considered the approximate position of the second nodal point and was used as the reference for specifying retinal eccentricities. The primary measure of interest was the vitreous chamber depth, which was defined as the distance between the approximate position of the second nodal point and the retina. Vitreous chamber depth was determined as a function of eccentricity in 15° intervals out to eccentricities of 45° using the approximate position of the posterior nodal point as a reference. 

Statistical Analysis

Repeated-measures ANOVA (SuperANOVA; Abacus Concepts, Inc., Berkeley, CA) and multiple comparisons were used to compare the symmetry of refractive errors along the horizontal and vertical meridians in a given eye and to determine whether there were differences in refractive error as a function of eccentricity or differences in the patterns of peripheral refraction between eyes. Mixed-design, repeated-measures ANOVAs were used to determine whether there were differences between subject groups in refractive error or relative vitreous chamber depth. Probability values were adjusted using Geisser-Greenhouse corrections. Linear regression analyses of the relationship between vitreous chamber depth and refractive error were performed using statistical software (Minitab Inc., State College, PA). 

Results

Early-onset, full-field form deprivation typically produces central axial myopia in young monkeys. However, as illustrated in Figure 1A, even with consistent, severe form deprivation, the degree of myopia varies substantially between animals, with some animals developing relative hyperopia in their treated eyes. 

Figure 1.

 

Left: interocular differences in spherical-equivalent refractive corrections measured along the pupillary axis for individual control (normal monkeys reared with unrestricted vision and monkeys reared with zero-powered lenses over both eyes) and treated monkeys (right or treated eye − left or fellow eye). Mean ± 1 SD is shown to the right of the individual data. The measurements were made at the end of the period of form deprivation or around 150 days of age for the control monkeys. All the treated animals wore the same strength of diffusers (i.e., LP occlusion foils). Animals in the full-field FD group wore diffusers that degraded image quality across the entire visual field continuously throughout the treatment period. The NFD group also wore diffusers continuously; however, these diffusers only affected image quality in the nasal visual field. Right: interocular differences in refractive error plotted as a function of the interocular differences in vitreous chamber depth for the individual animals represented in the left panel. The data for the full-field FD group have been replotted from Smith et al. ^19,20 and Huang et al. ²⁵

The NFDs also interfered with central refractive development. Like full-field form deprivation, the NFDs produced a large range of central refractive errors. At the end of the treatment period, the anisometropias in the NFD-reared animals ranged from +1.81 to −9.94 D. Although five of the NFD monkeys exhibited small amounts of relative central hyperopia in the treated eyes, four of the nine treated monkeys showed substantial amounts of myopia along the pupillary axes of their treated eyes. The central myopia found in these NFD monkeys was attributed to increases in the vitreous chamber depth of the treated eyes and the relationship between the degree of anisometropia, and the interocular differences in vitreous chamber depth in NFD-reared monkeys was similar to that found in animals reared with full-field form deprivation (Fig. 1B). As a group, there were no interocular differences in corneal power (T = 0.55; P = 0.60) or corneal asphericity between the treated and fellow eyes of the NFD monkeys (T = 0.66; P = 0.53). However, the corneas in the treated eyes of the four NFD monkeys that exhibited central myopia were on average 0.56 D steeper than the corneas of their fellow eyes (T = 3.64; P = 0.04), but there were no systematic interocular differences in asphericity Q values (T = −0.88; P = 0.44) 

More important, several observations indicated that the NFDs produced localized, regionally selective changes in refractive error. First, the NFDs consistently produced asymmetries in the pattern of peripheral refractions in the horizontal meridian. Figure 2 shows longitudinal data, specifically spherical-equivalent refractive corrections plotted as a function of horizontal eccentricity, for the treated (filled circles) and fellow eyes (open circles) of five representative monkeys reared with the NFDs. At the onset of the treatment period, the absolute central refractive errors and the patterns of peripheral refraction were well matched in the two eyes of the NFD monkeys (F = 0.01; P = 0.91). Specifically, all the experimental animals exhibited moderately hyperopic central refractive errors, and there was a tendency for the eyes to be less hyperopic in the periphery, particularly in the nasal field. Moreover, there were no differences in the absolute central and peripheral refractive errors between the treated eyes of the NFD monkeys and the right eyes of the normal animals (F = 1.54; P = 0.24). 

Figure 2.

 

Spherical-equivalent refractive corrections that were obtained at different times during the treatment period for five representative monkeys reared with NFDs plotted as a function of visual field eccentricity along the horizontal meridian. 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. Filled symbols: treated eyes; open symbols: fellow eyes.

With time, the nontreated fellow eyes of most of the NFD monkeys exhibited changes in refractive error that were indicative of emmetropization; however, the treated eyes demonstrated clear departures from the normal emmetropization process that were most obvious in the nasal field. As illustrated by the plots in the top two rows of Figure 2, the treated eyes of three of the NFD monkeys developed relative hyperopic refractive errors in their nasal visual fields. These changes, which were first manifest as relative hyperopia in the treated eye's nasal field and as interocular refractive error differences in the nasal field, were apparent at the first measurement session after the onset of lens wear (typically 14 days). Although the relative nasal field hyperopic refractions were small in these monkeys, the interocular differences in the nasal field were consistent throughout the treatment period, and the degree of relative hyperopia in the treated eyes generally increased in magnitude with time. 

Six of the nine NFD-reared monkeys developed relative myopic errors in the nasal field. As illustrated by the representative data in the lower three rows of Figure 2, refractive development in the temporal fields of the treated eyes was largely unaffected throughout the treatment period. However, the NFDs produced obvious myopic errors in the nasal fields of most treated eyes. The resultant nasal-temporal asymmetries in refractive error in the treated eyes were noted within 30 days of the onset of lens wear, and the degree of myopia in the nasal field and, consequently, the nasal-temporal asymmetries, increased during the treatment period. Although the refractive errors along the pupillary axis were affected in most of these monkeys, the observed myopic refractive errors were typically larger at either the 15° or 30° nasal field eccentricities. 

The relative peripheral refractive errors measured along the horizontal meridian at the end of the treatment period for all the NFD monkeys are illustrated in Figure 3. The data for the treated (filled symbols) and fellow eyes (open symbols) have been normalized to the refractive error measured at 45° in the temporal field. The shaded region represents ±1 SD from the means for the right eyes of the seven normal monkeys. Typically, peripheral refractions are plotted relative to the central refraction; however, we chose to use the 45° temporal eccentricity as a reference because the central refractions were altered in many NFD animals, and the absolute differences between the fellow and treated eyes and between treated and normal monkeys were smallest at the temporal 45° eccentricity. 

Figure 3.

 

Relative ametropias plotted as a function of visual field eccentricity along the horizontal meridian for individual monkeys reared with NFDs. The data were obtained at the end of the treatment period and have been normalized to the refractive correction measured at the 45° temporal field eccentricity. Filled symbols: treated eyes; open symbols: fellow eyes. The shaded areas represent ±1 SD from the mean relative ametropias for the right eyes of seven normal monkeys. The data for six of the normal monkeys have been replotted from Hung et al. ²⁷ and Huang et al. ²⁵

The results in Figure 3 emphasize that the NFDs selectively produced relative myopia in the nasal fields of the majority of the treated eyes. For the animals represented in Figures 3D-I (lower row and last two plots on the right of the upper row), the treated-eye data were in or near the ±1 SD normal zone in the temporal field but were consistently well below the normal zone (i.e., more myopic) in the nasal field, with the greatest departures from normal occurring at the 15° and 30° nasal field eccentricities. For the other three NFD monkeys (Figs. 3A-C), the treated-eye data were, with a few exceptions, within the ±1 SD boundaries for the normal animals. However, it is interesting that for two of these animals (Fig. 3A, MKY GEN 359; Fig. 3B, MKY MIT 367), the pattern of peripheral refractions in the fellow eyes deviated more from the normal monkey data than the data for their treated eyes. Specifically, the fellow eyes were more myopic than normal at the 15° and 30° nasal eccentricities for both these animals and at the nasal 45° eccentricity for MKY MIT 367. 

A repeated-measures ANOVA confirmed that at the end of the treatment period there were significant nasal-temporal asymmetries in the peripheral refractions in the treated eyes of the NFD monkeys (F = 34.43; P = 0.003), but not in the fellow eyes (F = 0.07; P = 0.42). In addition, there were significant eccentricity-dependent differences in the absolute peripheral refractions between the treated eyes of the NFD monkeys and the right eyes of the normal monkeys (F = 4.85; P = 0.0003). 

The interocular differences in peripheral refractions for individual animals also emphasized the selective effects of the NFDs. In Figure 4, the interocular differences in refractive error obtained at the end of the treatment period are plotted as a function of eccentricity for individual treated monkeys (open symbols). The filled symbols represent the mean interocular differences for seven normal age-matched monkeys; the shaded area demarcates ±2 SD from the means. The left and right eyes of the normal monkeys were well matched at all eccentricities. However, the interocular difference plots for the treated monkeys showed obvious nasal-temporal asymmetries (F = 23.09; P = 0.0001). At the 45° temporal field eccentricity, the data for all the treated animals fell within the ±2 SD region for the normal monkeys. However, in moving from the temporal to the nasal field, the interocular differences in refractive error generally increased, with the greatest differences found at or near the 15° nasal field eccentricity. For five of the NFD monkeys, the nasal field data for the treated eyes were more myopic than for their fellow eyes, and the interocular differences fell outside the ±2 SD region for the normal monkeys at the nasal 15° and 30° eccentricities. For two other NFD animals, the treated eyes were more hyperopic than their fellow eyes at the 15° and 30° nasal field eccentricities, and these differences were also more than 2 SD away from the mean differences for normal monkeys. As noted, hyperopic anisometropia observed in the nasal fields for these monkeys reflects the fact that their fellow nontreated eyes were more myopic than the eyes of the control monkeys in the nasal visual field. 

Figure 4.

 

Interocular differences in spherical-equivalent refractive corrections (right or treated eye − left or fellow eye) obtained at the end of the treatment period plotted as a function of horizontal visual field eccentricity for individual monkeys reared with NFDs (open symbols). The filled symbols and the shaded area represent, respectively, the mean interocular differences in refractive corrections and ±2 SD from the mean for seven normal monkeys. The data for six of the normal monkeys have been replotted from Hung et al. ²⁷ and Huang et al. ²⁵

A comparison of the pattern of peripheral refractions in the vertical and horizontal meridians of the treated eyes also showed that the NFDs produced regionally selective alterations in peripheral refractions. Figure 5 shows refractive error plotted as a function of eccentricity for the horizontal (left) and vertical (middle) meridians of the treated (filled symbols) and fellow (open symbols) eyes for the four NFD monkeys that developed relatively large amounts of nasal field myopia (note that the horizontal- and vertical-meridian data were obtained at similar, but not identical, ages.). In each monkey, the treated eyes were more myopic than their fellow eyes along both their vertical and horizontal meridians. However, in every case, the largest amounts of myopia in the vertical meridian were found along the pupillary axis, and the degree of myopia decreased in a systematic and relatively symmetric manner in both the superior and the inferior fields. As shown in the right, the interocular differences in refractive error were more symmetrical about the central refraction in the vertical meridian than in the horizontal meridian. Moreover, in every case, the nasal field was more myopic than either the superior or the inferior field. On the other hand, the temporal field tended to be more hyperopic than either the superior or the inferior field, particularly at the 15° and 30° eccentricities. Repeated-measures ANOVAs showed that the interocular difference in peripheral refraction in these four monkeys varied with eccentricity in both the horizontal and vertical meridians (horizontal, F = 14.80, P = 0.004; vertical, F = 8.74, P = 0.02). However, multiple comparisons showed that there were significant asymmetries between the nasal and temporal fields (F = 49.78; P = 0.001) but not between the superior and inferior fields (F = 2.18; P = 0.15). 

Figure 5.

 

Comparisons of the pattern of peripheral refractions along the horizontal (left) and vertical meridians (middle) for four representative monkeys reared with NFDs. Filled symbols: treated eyes; open symbols: fellow eyes. The age at the time of the measurement is shown in each plot. Right: interocular differences in refractive corrections (treated eye − fellow eye) plotted as a function of eccentricity for the horizontal (filled symbols) and vertical meridians (open symbols).

The MRI scans showed that the NFDs altered the shape of the treated eye's posterior globe. Figure 6 shows the horizontal MR images obtained at ages corresponding to the end of the treatment period for both eyes of a monkey reared with unrestricted vision (top), a monkey with form-deprivation myopia produced by FFD (middle), and an NFD monkey that exhibited a large amount of nasal field myopia (bottom). The white lines in each image represent the presumed optical axes and the projections used to determine vitreous chamber depth at the 15°, 30°, and 45° eccentricities. The graphs to the right show vitreous chamber depth plotted as a function of retinal eccentricity; the treated (or right) and fellow (or left) eyes are represented by the filled and open circles, respectively. For the control animal, inspection of the MRIs revealed that the two eyes had similar shapes and, as illustrated in the figure on the right, the vitreous chamber depths in the left and right eyes were similar, with both eyes showing small systematic increases in vitreous chamber depth from the nasal to the temporal hemiretinas. Full-field form deprivation produced an obvious increase in the size of the treated eye; the relative increases in vitreous chamber depth were largest in the treated eye's central retina but were observed at all eccentricities and were relatively symmetrical in the nasal and temporal hemiretinas. On the other hand, the MRIs from the NFD animal show differences in the shapes of the posterior globe between the treated and fellow eyes. Whereas the contour of the vitreoretinal interface in the fellow eye was regular and the variations in vitreous chamber depth with eccentricity were similar to those found in the control animal, the treated eye exhibited obvious departures from this normal contour that started just nasal to the central retina and that were associated with an obvious outward bulging of the temporal vitreous chamber. 

Figure 6.

 

Left: magnetic resonance images obtained in the horizontal plane at ages corresponding to the end of the diffuser-rearing period for both eyes of a normal control monkey (top), an FFD monkey with central form deprivation myopia (middle), and an NFD monkey that developed a relatively high degree of nasal field myopia (bottom). For the FFD and NFD monkeys, the treated eyes are shown on the left. Right: vitreous chamber depth plotted as a function of eccentricity for the right/treated (filled circles) and left/fellow eyes (open circles). The normal and FFD monkey data were presented previously in Huang et al. ²⁵

Figure 7 shows the horizontal MR images obtained near the end of the treatment period for five representative NFD monkeys (left). The treated eyes are shown on the left. The graphs in the right show the interocular differences in refractive error (open symbols, left ordinate) and vitreous chamber depth (filled symbols, right ordinate) plotted as a function of horizontal eccentricity. Negative numbers on the y-axis indicate that the treated eye was more myopic and had a deeper/longer vitreous chamber than its fellow eye. 

Figure 7.

 

Magnetic resonance images (left) and comparisons of the interocular differences in refractive corrections (open symbols and left ordinate scale; treated eye − fellow eye) and vitreous chamber depth (filled symbols and right ordinate scale; fellow eye − treated eye) as a function of horizontal eccentricity (right) for five representative monkeys reared with NFDs (rows). The MR images were obtained in the axial plane near the end of the treatment period. The treated eyes are shown on the left.

For each of the NFD animals there was a close correspondence between the interocular differences in vitreous chamber depth and refractive error, particularly with respect to the pattern of the nasal-temporal asymmetries. For example, the data in the top row of Figure 7 are from an NFD monkey that exhibited small amounts of relative hyperopia in the nasal field of its treated eye. Although on inspection the MR images revealed that the treated and fellow eyes were similar in shape, quantitative measurements showed that the nasal vitreous chamber (temporal field) in the treated eye was slightly longer than that in the fellow eye and that the interocular differences in vitreous chamber depth decreased systematically in the direction of the temporal retina (nasal field), paralleling the refractive-error data. As illustrated in the lower four panels of Figure 7, there were visible interocular differences in the shapes of the posterior globe in the animals with large amounts of nasal visual field myopia, and the similarities in the shapes of the interocular difference functions were particularly impressive. In each of these animals, there were few or no interocular differences in refractive error or vitreous chamber depth in the nasal retina/temporal field. There was a steep transition between the nasal and temporal portions of the functions that included the data along the pupillary axis. The interocular differences in vitreous chamber depth and refractive error were greatest in the near temporal retina/nasal visual field and decreased thereafter with increasing eccentricities. The similarities in the shapes of these functions indicate that the nasal-temporal asymmetries in refractive error resulted from changes in the shape of the posterior globe. However, differences in corneal and, possibly, lens power did contribute to the relative vertical positions of the interocular differences functions. For example, the treated-eye cornea of MKY FIN 356 (top row) was 0.59 D flatter than that of the fellow eye, whereas the treated-eye cornea of MKY KIM 364 (fourth row) was 0.88 D steeper than that of its fellow eye. In both cases, compensating for these interocular differences in corneal power would shift the interocular difference curves closer together along the vertical axis. Although the degree of astigmatism increased systematically with eccentricity in both the treated and the fellow eyes, we did not observe any nasal-temporal asymmetries in astigmatism that could have contributed to the nasal-temporal asymmetries in spherical-equivalent refraction. 

Regression analysis confirmed that the interocular differences in refractive error resulted primarily from interocular differences in vitreous chamber depth. Figure 8 shows the interocular differences in refractive error plotted against the interocular differences in vitreous chamber depth for all the peripheral measurements obtained along the horizontal meridian for all the NFD-reared monkeys. The local interocular differences in refractive error were significantly correlated with local interocular differences in vitreous chamber depth, with an r ² value of 0.74 (F = 175.9; P < 0.0001). Although the relationship between refractive error and axial length is not linear, the slope of the best-fitting function (5.03 D per mm change in vitreous chamber) agrees well with predictions based on schematic eyes. For example, the average axial length of the fellow eyes at the end of the treatment period was 16.99 mm. Assuming that the distance between the retina and the second principal plane was 91% of the eye's axial length, ²⁴ the predicted change in refractive error produced by a 1-mm change in vitreous chamber depth would be 5.6 D. 

Figure 8.

 

Interocular differences in refractive error (treated eye − fellow eye) plotted as a function of the interocular differences in vitreous chamber depth determined from the MR images (fellow eye − treated eye). Data are shown for all NFD-reared monkeys and for all eccentricities along the horizontal meridian. Solid line: best-fitting regression line.

Although there were some similarities in the alterations in the pattern of peripheral refractions and the shape of the posterior globe produced by FFDs and NFDs, there were also significant differences. Figure 9 compares the effects of FFDs and NFDs. The left column shows average (±SE) data for six monkeys that developed at least 1 D central myopic anisometropia in response full-field form deprivation. The data for the four NFD monkeys that showed the largest amounts of induced myopia are shown in the middle. These NFD animals were selected because the degree of central myopic anisometropia was similar to that in the FFD monkeys (−6.26 D vs. −6.63 D). The fellow eyes of the FFD and NFD monkeys tended to be slightly more hyperopic than of the average normal monkey (cross-hatched area in the top row). However, there were no systematic differences between the fellow eyes of the FFD and NFD monkeys in absolute refractive error (F = 0.29; P = 0.60) or the pattern of relative peripheral refractive errors (F = 0.51; P = 0.62). On the other hand, there were clear and substantial differences between the treated and fellow eyes of the FFD and NFD monkeys, and, regardless of which measure is considered (i.e., refractive errors [top and middle] or interocular differences in vitreous chamber depth [bottom]), these interocular differences varied with eccentricity, with both the FFD (F = 4.40–54.22; P = 0.006–0.08) and NFD (F = 89.18–339.74; P < 0.0009) monkeys showing larger myopic changes in the nasal field. However, there were significant eccentricity-dependent differences in the results for the NFD and FFD monkeys (ametropia, F = 6.57; P = 0.005; anisometropia, F = 6.34; P = 0.007; IOD vitreous chamber depth, F = 3.30; P = 0.05). For example, whereas the absolute refractive errors and the interocular differences in refractive error and vitreous chamber depth at the 15°, 30°, and 45° nasal field eccentricities were similar in the FFD and NFD monkeys, these measures at the 15°, 30°, and 45° temporal field eccentricities were consistently smaller in the NFD monkeys. As a consequence, as illustrated in the right of Figure 9, the nasal-temporal asymmetries in absolute refractive error (F = 13.81; P = 0.006), anisometropia (F = 12.30; P = 0.008), and interocular differences in vitreous chamber depth (F = 5.81, P = 0.04) were significantly larger in the NFD monkeys at all the eccentricities we investigated. The between-group differences in temporal field refractive errors can be attributed primarily to differences in the depths of the nasal vitreous chamber. For example, at the 15°, 30°, and 45° temporal field eccentricities, the differences in anisometropia between the FFD and NFD monkeys were 2.92, 2.71, and 1.59 D, respectively. The corresponding differences in the relative vitreous chamber depths were 0.54, 0.42, and 0.27 mm, respectively, which based on model eye calculations (5.6 D/mm), would account for 3.04, 2.36, and 1.51 D of the differences in anisometropia between the NFD and FFD monkeys. 

Figure 9.

 

Comparisons of the average (± SE) absolute refractive errors (top: filled symbols, treated eye; open symbols, fellow eye), degree of anisometropia (middle: filled symbols, treated monkeys; open symbols, control monkeys), and interocular differences in vitreous chamber depth (bottom) plotted as a function of horizontal visual field eccentricity. Left: data are shown for six monkeys that were reared with FFDs and that developed central form-deprivation myopia. Middle: results for the four NFD monkeys that showed the largest myopic changes. Right: differences in the nasal and temporal hemifield results (temporal field − nasal field) plotted as a function of eccentricity for the FFD (open squares) and NFD (filled squares) monkeys. Top: shaded area represents ±2 SD from the mean for the normal monkeys (n = 7). The data for the FFD monkeys and six of the normal monkeys have been replotted from Huang et al. ²⁵

Discussion

The main findings of our study were that hemiretinal form deprivation altered refractive development in a regionally selective manner, typically producing myopia in the treated hemifields of infant monkeys, and that these treatment-induced changes in refractive error were associated with local, region-specific alterations in vitreous chamber depth in the treated hemiretina. In many respects, the effects of NFDs were similar to those produced by full-field form deprivation. With both manipulations there is substantial variability in the degree of myopia between animals, and the refractive-error changes are axial in nature. The primary difference between the effects of NFDs and FFDs was that the alterations produced by the NFDs were restricted primarily to the treated hemiretina. 

This pattern of results has important implications for the nature of the vision-dependent mechanisms that influence ocular growth and refractive development in primates. As discussed in previous studies involving chickens, ^5,10,13 many of the global mechanisms that have been frequently hypothesized to mediate the effects of vision on refractive development cannot easily explain these selective, local alterations in refractive development. For example, it is difficult to imagine how accommodative mechanisms or overall alterations in intraocular pressure could produce the observed nasal-field myopia. Instead, in conjunction with Raviola and Wiesel's observation that optic nerve section does not prevent form deprivation myopia in rhesus monkeys, ¹⁷ our results provide strong support for the idea that the effects of vision in primates are dominated by mechanisms that are located entirely within the eye, that integrate information in a spatially restricted manner, and that exert their influence selectively on the subjacent sclera. As hypothesized by previous investigators, the presence of these local retinal mechanisms could regulate the shape of the globe during early development to optimize refractive error across the retina. ^13,15,16 We have recently provided evidence that indicates that emmetropization occurs in the peripheral and the central retina in infant primates. ²⁷ It is reasonable to suppose that the local retinal mechanisms revealed in this study mediate this overall emmetropization process in young monkeys. However, it is important to keep in mind that our results are limited to form deprivation. Because the mechanisms that mediate the effects of form deprivation and optical defocus may not be identical, ³¹ it will be important to determine whether hemiretinal changes in refractive development can also be produced by optically imposed defocus. 

Little is known about the spatial integration properties of local retinal mechanisms. In particular, it would be valuable to know the effective summation areas of these mechanisms and whether they change with eccentricity. In many respects, the NFDs used in this study can be considered an edge stimulus and the changes in vitreous chamber depth and refractive error between the nasal, and temporal hemiretinas of our NFD-reared monkeys represent functional edge spread functions for local retinal mechanisms in the central retina. In this respect, the interocular difference plots for refractive error and vitreous chamber depth in Figure 7 and the peripheral refraction plots in Figure 2 provide a rough estimate of the upper limits of the lateral spread of the activity associated with local retinal mechanisms, at least those located near the fovea. For example, for the NFD-reared monkeys that developed relatively large amounts of nasal field myopia, the transition between affected and apparently normal retina occurs over approximately 30° of the retina, and the width of the normalized edge functions at half-height is approximately 20°. In other words, the influence of these mechanisms decreases by 50% over approximately 3 to 4 mm along the horizontal meridian of the retina (depending on age and eye size). Of course, these estimates do not take into account the potential “blurring” effects produced by the optics of the eye and, particularly, by eye movements. Consequently, it is likely that the effective zones of influence of these local retinal mechanisms are smaller than these estimates. 

In both hemifields of the FFD monkeys and in the nasal hemifield of the NFD monkeys, the degree of induced myopia declined at the larger eccentricities. The resultant relative peripheral hyperopia reflects changes in the shape of the posterior globe. Specifically, in both FFD and NFD monkeys, the treated hemiretina became less oblate/more prolate in shape. It is possible that these shape changes reflect a decrease in the ability of local mechanisms in the periphery to influence ocular shape. Regardless, the results indicate that relative peripheral hyperopia may be a consequence or side effect of central axial elongation. 

The observed hemifield changes in the refractive errors of our infant monkeys were qualitatively similar to those produced by hemiretinal form deprivation in chicks, ^5,11,13 tree shrews, ¹⁴ and guinea pigs (McFadden SA. IOVS 2002;43:E-Abstract 189) and, thus, provide another example of how the fundamental operational properties of the vision-dependent mechanisms that regulate refractive development have been conserved across species. Given the dominance of the fovea in primate vision, one might have expected that in comparison to species with more homogeneous retinas, local retinal mechanisms would be less conspicuous in primates and that refractive development would be dominated by central vision. However, as we have previously demonstrated, central vision is not essential for many aspects of vision-dependent ocular growth, and peripheral vision can dominate central refractive-error development in primates. ^26,32 The presence of local retinal mechanisms in monkeys provides a mechanistic explanation for the impact of peripheral vision on primate refractive development. Moreover, the similarity of the responses in monkeys, tree shrews, guinea pigs, and chicks to hemiretinal form deprivation indicates that the presence of a well-developed fovea does not diminish the contribution of these local retinal mechanisms to ocular growth, that the nature of these spatially localized mechanisms are fundamental from an evolutionary perspective, and that it is likely that similar mechanisms exist in humans. 

The obvious nasal-temporal asymmetries in the shape of the posterior globe that were observed in many of our treated monkeys were similar to findings in chicks. ^5,13 However, given the anatomic differences between the scleras of chicks and monkeys, the prominent structural changes in our infant monkeys were surprising. In chicks, the local axial elongation produced by hemiretinal form deprivation caused the treated portion of the globe to bulge out. This is not a surprising finding in chicks because their scleras, composed primarily of a cartilaginous layer that actively grows during vision-induced axial elongation, are relatively rigid. ^33,34 In comparison, the scleras of monkeys consist primarily of fibrous connective tissue that is generally considered to be softer and more pliable than the chick sclera, and axial elongation is thought to come about as a consequence of remodeling of scleral tissue that leads to stretching in the tangential direction. ^35–37 In this respect, it could be argued that selective peripheral form deprivation or hemiretinal form deprivation would result in a more prolate shaped eye in monkeys and that any discrete bulges would, in essence, be smoothed out. However, the distinctive, local changes in posterior globe curvature observed in our MRI scans show that, at least in developing eyes, the monkey sclera can hold its shape. These results suggest that different types of visual experience may produce a range of distinctly differently shaped eyes in primates, similar to those that have been observed in chicks. ³⁸  

Monocular experimental manipulations of vision have been shown to produce bilateral effects in several species of laboratory animals. For example, the refractive errors for the fellow, nontreated eyes of monkeys that undergo full-field form deprivation can be more myopic or more hyperopic than normal eyes. ^19,20,39 In this respect, several observations indicate that the NFDs produced interocular effects in our monkeys. In particular, in several monkeys, the fellow, nontreated eyes developed atypical central refractive errors or abnormal patterns of peripheral refractions. For example, at the end of the treatment period, the nontreated eye's central refraction for monkey MON 368 (Fig. 2, middle row) was more hyperopic than that for any age-matched normal monkey; however, the pattern of peripheral refractive errors was not obviously altered. More interestingly, two of the three monkeys that failed to develop nasal field myopia in their treated eyes demonstrated obvious nasal-temporal asymmetries in the pattern of peripheral refractions in their fellow, nontreated eyes. In both cases (MKY GEN 359 and MKY MIT 367), the nasal-temporal asymmetries were larger in the fellow eyes than in the treated eyes, and the fellow eyes showed degrees of nasal field myopia that were outside the range of relative peripheral refractions observed in the normal monkeys. 

The nature of the mechanisms responsible for interocular effects is not well understood. It has been hypothesized that interocular effects may be mediated by humoral factors or central influences of some kind, possibly via innervations to the choroid. ^{8,19,20,39,40} It is also possible that in some cases the interocular effects are vision induced. For example, because accommodation is yoked in the two eyes of primates, rearing conditions that alter the accommodation of one eye (e.g., anisometropic treatment lenses) will potentially influence the level of accommodation in the fellow eye and potentially its refractive development, depending on which eye the animal uses to fixate objects. ²³ Similarly, conditions such as monocular form deprivation that prevent binocular vision could, by altering convergence-accommodative interactions, change the accommodative behavior of the fixating fellow eye and eventually the refractive development of that eye. ²⁰  

It seems unlikely that any of these possibilities, which would be expected to have global effects on the fellow eye, are responsible for the nasal-temporal asymmetries observed in the fellow eyes of some of our NFD monkeys. Although it is possible that a yet unknown central mechanism could produce selective regional changes in eye growth, we speculate that the fellow eye asymmetries could reflect alterations in the fixation behavior of the NFD monkeys and the effects of visual experience. We have assumed that the NFD animals would prefer to fixate primarily via the nontreated eye. However, perhaps a small number of the treated monkeys choose, for whatever reason, to fixate with their treated eyes. Logically, these animals would rotate their treated eyes in the temporal direction to maximize that eye's central field of view, which would likely result in concomitant versional eye movement of the fellow eye in the nasal direction. Chronic nasal deviation could have resulted in a selective reduction in the image quality in the temporal retina of the fellow eye. In other words, by selectively fixating with the treated eyes, a nasal-temporal asymmetry in retinal image quality might have been transferred to the fellow eye. 

The existence of independent, vision-dependent mechanisms that integrate visual information over restricted retinal regions has important implications for the role of vision in the genesis of common refractive errors and for potential optical treatment regimens for refractive error. First, because the refractive state at the fovea is dependent on ocular changes at the posterior pole and in the periphery (i.e., an expansion of the sclera in the periphery would displace the central retina in a posterior direction along the visual axis), peripheral visual signals can influence central refractive development in a manner that is independent of the nature of central vision. In this respect, there is growing evidence that the pattern of peripheral refractive errors in humans can influence central refractive development. ^38,41,42 Moreover, we have previously shown that selective peripheral form deprivation ^26,32 and relative peripheral hyperopic defocus (Smith EL III, et al. IOVS 2007;48:ARVO E-Abstract 1533) can promote central axial elongation and central myopia in infant monkeys, even in the presence of unrestricted central vision. However, because local retinal mechanisms operate in an independent manner and because vision-induced changes in the growth rate of the peripheral globe will influence central axial length, it should also be possible to influence central refractive development in a therapeutic manner by manipulating only peripheral vision. For example, to reduce myopia progression, it should be possible to eliminate peripheral visual signals that promote axial elongation or provide peripheral visual signals that normally slow ocular growth without changing or interfering with central vision. The key point is that one should be able to design a correcting lens to provide optimal central vision and, at the same time, a peripheral visual signal to reduce or accelerate central axial growth. 

Footnotes

 Supported by National Institutes of Health Grants EY-03611, EY-07551, and RR-17205 and by funds from Vision CRC and the UH Foundation.

Footnotes

 Disclosure: E.L. Smith III, P; J. Huang, None; L.-F. Hung, None; T.L. Blasdel, None; T.L. Humbird, None; K.H. Bockhorst, None

Footnotes

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