Ramkumar Ramamirtham

University of Houston, Houston, TX, United States

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Publications (16)42.73 Total impact

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    ABSTRACT: Purpose. To better understand the functional significance of refractive-error measures obtained using common objective methods in laboratory animals, we compared objective and subjective measures of refractive error in adolescent rhesus monkeys. Methods. The subjects were 20 adolescent monkeys. Spherical-equivalent spectacle-plane refractive corrections were measured by retinoscopy and autorefraction while the animals were cyclopleged and anesthetized. The eye's axial dimensions were measured by A-Scan ultrasonography. Subjective measures of the eye's refractive state, with and without cycloplegia, were obtained using psychophysical methods. Specifically, we measured spatial contrast sensitivity as a function of spectacle lens power for relatively high spatial frequency gratings. The lens power that produced the highest contrast sensitivity was taken as the subjective refraction. Results. Retinoscopy and autorefraction consistently yielded higher amounts of hyperopia relative to subjective measurements obtained with or without cycloplegia. The subjective refractions were not affected by cycloplegia and on average were 1.42 ± 0.61 D and 1.24 ± 0.62 D less hyperopic than the retinoscopy and autorefraction measurements, respectively. Repeating the retinoscopy and subjective measurements through 3 mm artificial pupils produced similar differences. Conclusions. The results show that commonly used objective methods for assessing refractive errors in monkeys significantly overestimate the degree of hyperopia. It is likely that multiple factors contributed to the hyperopic bias associated with these objective measurements. However, the magnitude of the hyperopic bias was in general agreement with the “small-eye artifact” of retinoscopy.
    Optometry and Vision Science 01/2012; 89(2):168-177. · 1.90 Impact Factor
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    ABSTRACT: We analyzed the contribution of individual ocular components to vision-induced ametropias in 210 rhesus monkeys. The primary contribution to refractive-error development came from vitreous chamber depth; a minor contribution from corneal power was also detected. However, there was no systematic relationship between refractive error and anterior chamber depth or between refractive error and any crystalline lens parameter. Our results are in good agreement with previous studies in humans, suggesting that the refractive errors commonly observed in humans are created by vision-dependent mechanisms that are similar to those operating in monkeys. This concordance emphasizes the applicability of rhesus monkeys in refractive-error studies.
    Vision research 08/2010; 50(18):1867-81. · 2.29 Impact Factor
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    ABSTRACT: To determine whether visual experience can alter ocular shape and peripheral refractive error pattern, the authors investigated the effects of form deprivation on refractive development in infant rhesus monkeys. Monocular form deprivation was imposed in 10 rhesus monkeys by securing diffuser lenses in front of their treated eyes between 22 +/- 2 and 163 +/- 17 days of age. Each eye's refractive status was measured longitudinally by retinoscopy along the pupillary axis and at 15 degrees intervals along the horizontal meridian to eccentricities of 45 degrees . Control data for peripheral refraction were obtained from the nontreated fellow eyes and six untreated monkeys. Near the end of the diffuser-rearing period, the shape of the posterior globe was assessed by magnetic resonance imaging. Central axial dimensions were also determined by A-scan ultrasonography. Form deprivation produced interocular differences in central refractive errors that varied between +2.69 and -10.31 D (treated eye-fellow eye). All seven diffuser-reared monkeys that developed at least 2.00 D of relative central axial myopia also showed relative hyperopia in the periphery that increased in magnitude with eccentricity. Alterations in peripheral refraction were highly correlated with eccentricity-dependent changes in vitreous chamber depth and the shape of the posterior globe. Like humans with myopia, monkeys with form-deprivation myopia exhibit relative peripheral hyperopia and eyes that are less oblate and more prolate. Thus, in addition to producing central refractive errors, abnormal visual experience can alter the shape of the posterior globe and the pattern of peripheral refractive errors in infant primates.
    Investigative ophthalmology & visual science 06/2009; 50(9):4033-44. · 3.43 Impact Factor
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    ABSTRACT: To characterize peripheral refractions in infant monkeys. Cross-sectional data for horizontal refractions were obtained from 58 normal rhesus monkeys at 3 weeks of age. Longitudinal data were obtained for both the vertical and horizontal meridians from 17 monkeys. Refractive errors were measured by retinoscopy along the pupillary axis and at eccentricities of 15 degrees , 30 degrees , and 45 degrees . Axial dimensions and corneal power were measured by ultrasonography and keratometry, respectively. In infant monkeys, the degree of radial astigmatism increased symmetrically with eccentricity in all meridians. There were, however, initial nasal-temporal and superior-inferior asymmetries in the spherical equivalent refractive errors. Specifically, the refractions in the temporal and superior fields were similar to the central ametropia, but the refractions in the nasal and inferior fields were more myopic than the central ametropia, and the relative nasal field myopia increased with the degree of central hyperopia. With age, the degree of radial astigmatism decreased in all meridians, and the refractions became more symmetrical along both the horizontal and vertical meridians. Small degrees of relative myopia were evident in all fields. As in adult humans, refractive error varied as a function of eccentricity in infant monkeys and the pattern of peripheral refraction varied with the central refractive error. With age, emmetropization occurred for both central and peripheral refractive errors, resulting in similar refractions across the central 45 degrees of the visual field, which may reflect the actions of vision-dependent, growth-control mechanisms operating over a wide area of the posterior globe.
    Investigative ophthalmology & visual science 06/2008; 49(9):3747-57. · 3.43 Impact Factor
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    ABSTRACT: The purpose of this study was to investigate the relationship between refractive errors and high-order aberrations in infant rhesus monkeys. Specifically, we compared the monochromatic wave aberrations measured with a Shack-Hartman wavefront sensor between normal monkeys and monkeys with vision-induced refractive errors. Shortly after birth, both normal monkeys and treated monkeys reared with optically induced defocus or form deprivation showed a decrease in the magnitude of high-order aberrations with age. However, the decrease in aberrations was typically smaller in the treated animals. Thus, at the end of the lens-rearing period, higher than normal amounts of aberrations were observed in treated eyes, both hyperopic and myopic eyes and treated eyes that developed astigmatism, but not spherical ametropias. The total RMS wavefront error increased with the degree of spherical refractive error, but was not correlated with the degree of astigmatism. Both myopic and hyperopic treated eyes showed elevated amounts of coma and trefoil and the degree of trefoil increased with the degree of spherical ametropia. Myopic eyes also exhibited a much higher prevalence of positive spherical aberration than normal or treated hyperopic eyes. Following the onset of unrestricted vision, the amount of high-order aberrations decreased in the treated monkeys that also recovered from the experimentally induced refractive errors. Our results demonstrate that high-order aberrations are influenced by visual experience in young primates and that the increase in high-order aberrations in our treated monkeys appears to be an optical byproduct of the vision-induced alterations in ocular growth that underlie changes in refractive error. The results from our study suggest that the higher amounts of wave aberrations observed in ametropic humans are likely to be a consequence, rather than a cause, of abnormal refractive development.
    Vision Research 10/2007; 47(21):2751-66. · 2.14 Impact Factor
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    ABSTRACT: Because of the prominence of central vision in primates, it has generally been assumed that signals from the fovea dominate refractive development. To test this assumption, the authors determined whether an intact fovea was essential for either normal emmetropization or the vision-induced myopic errors produced by form deprivation. In 13 rhesus monkeys at 3 weeks of age, the fovea and most of the perifovea in one eye were ablated by laser photocoagulation. Five of these animals were subsequently allowed unrestricted vision. For the other eight monkeys with foveal ablations, a diffuser lens was secured in front of the treated eyes to produce form deprivation. Refractive development was assessed along the pupillary axis by retinoscopy, keratometry, and A-scan ultrasonography. Control data were obtained from 21 normal monkeys and three infants reared with plano lenses in front of both eyes. Foveal ablations had no apparent effect on emmetropization. Refractive errors for both eyes of the treated infants allowed unrestricted vision were within the control range throughout the observation period, and there were no systematic interocular differences in refractive error or axial length. In addition, foveal ablation did not prevent form deprivation myopia; six of the eight infants that experienced monocular form deprivation developed myopic axial anisometropias outside the control range. Visual signals from the fovea are not essential for normal refractive development or the vision-induced alterations in ocular growth produced by form deprivation. Conversely, the peripheral retina, in isolation, can regulate emmetropizing responses and produce anomalous refractive errors in response to abnormal visual experience. These results indicate that peripheral vision should be considered when assessing the effects of visual experience on refractive development.
    Investigative Ophthalmology &amp Visual Science 10/2007; 48(9):3914-22. · 3.44 Impact Factor
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    ABSTRACT: Different subspecies of rhesus monkeys (Macaca mulatta) that are derived from different geographical locations, primarily Indian and China, are commonly employed in vision research. Substantial morphological and behavioral differences have been reported between Chinese- and Indian-derived subspecies. The purpose of this study was to compare refractive development in Chinese- and Indian-derived rhesus monkeys. The subjects were 216 Indian-derived and 78 Chinese-derived normal infant rhesus monkeys. Cross-sectional data were obtained at 3 weeks of age for all subjects. In addition, longitudinal data were obtained from 10 Indian-derived (male=5, female=5) and 5 Chinese-derived monkeys (male=3, female=2) that were reared with unrestricted vision. Ocular and refractive development was assessed by retinoscopy, keratometry, video-based ophthalmophakometry, and A-scan ultrasonography. Although the course of emmetropization was very similar in these two groups of rhesus monkeys, there were consistent and significant inter-group differences in ocular dimensions and refractive error. Throughout the observation period, the Chinese-derived monkeys were on average about 0.4D less hyperopic than the Indian-derived monkeys and the Chinese-derived monkeys had longer overall axial lengths, deeper anterior and vitreous chamber depths, thicker crystalline lenses, flatter corneas and lower powered crystalline lenses. The ocular differences observed in this study presumably reflect genetic differences between subspecies but could reflect the differences in the genetic pool between isolated colonies rather than true subspecies differences. Nonetheless, the substantial ocular differences that we observed emphasize that caution must be exercised when comparing and/or pooling data from rhesus monkeys obtained from different colonies. These inter-subspecies differences might be analogous to the ethnic differences in ocular parameters that have been observed in humans.
    Vision Research 07/2007; 47(12):1668-81. · 2.14 Impact Factor
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    ABSTRACT: The purpose of this study was to characterize normal ocular development in infant monkeys and to establish both qualitative and quantitative relationships between human and monkey refractive development. The subjects were 214 normal rhesus monkeys. Cross-sectional data were obtained from 204 monkeys at about 3 weeks of age and longitudinal data were obtained from 10 representative animals beginning at about 3 weeks of age for a period of up to 5 years. Ocular development was characterized via refractive status, corneal power, crystalline lens parameters, and the eye's axial dimensions, which were determined by retinoscopy, keratometry, phakometry and A-scan ultrasonography, respectively. From birth to about 5 years of age, the growth curves for refractive error and most ocular components (excluding lens thickness and equivalent lens index) followed exponential trajectories and were highly coordinated between the two eyes. However, overall ocular growth was not a simple process of increasing the scale of each ocular component in a proportional manner. Instead the rates and relative amounts of change varied within and between ocular structures. The configuration and contribution of the major ocular components in infant and adolescent monkey eyes are qualitatively and quantitatively very comparable to those in human eyes and their development proceeds in a similar manner in both species. As a consequence, in both species the adolescent eye is not simply a scaled version of the infant eye.
    Vision Research 06/2007; 47(11):1424-44. · 2.14 Impact Factor
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    ABSTRACT: To characterize the temporal integration properties of the emmetropization process, the authors investigated the effects of brief daily interruptions of lens wear on the ocular compensation for negative lenses in infant rhesus monkeys. Eighteen monkeys wore -3 D lenses binocularly starting from approximately 3 weeks of age. Six of these monkeys wore the lenses continuously. For the other animals, the -3 D lenses were removed for four 15-minute periods each day. During these periods, the monkeys viewed through either zero-power lenses (n = 6) or +4.5 D lenses (n = 6). Three monkeys reared with binocular plano lenses and 16 monkeys reared normally served as controls. Refractive development was assessed by cycloplegic retinoscopy and A-scan ultrasonography. As expected, the group of animals that wore the -3 D lenses continuously exhibited clear evidence of compensating axial myopia. These predictable myopic changes were mostly eliminated by the brief, daily periods of viewing through plano lenses. Interestingly, brief periods of viewing through +4.5 D lenses produced weaker protective effects. Brief periods of unrestricted vision can prevent the axial myopia normally produced by long daily periods of imposed hyperopic defocus. Thus, the temporal integration properties of the emmetropization process normally reduce the likelihood that transient periods of hyperopic defocus will cause myopia.
    Investigative Ophthalmology &amp Visual Science 04/2007; 48(3):957-62. · 3.44 Impact Factor
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    ABSTRACT: High-order monochromatic aberrations could potentially influence vision-dependent refractive development in a variety of ways. As a first step in understanding the effects of wave aberration on refractive development, we characterized the maturational changes that take place in the high-order aberrations of infant rhesus monkey eyes. Specifically, we compared the monochromatic wave aberrations of infant and adolescent animals and measured the longitudinal changes in the high-order aberrations of infant monkeys during the early period when emmetropization takes place. Our main findings were that (1) adolescent monkey eyes have excellent optical quality, exhibiting total RMS errors that were slightly better than those for adult human eyes that have the same numerical aperture and (2) shortly after birth, infant rhesus monkeys exhibited relatively larger magnitudes of high-order aberrations predominately spherical aberration, coma, and trefoil, which decreased rapidly to assume adolescent values by about 200 days of age. The results demonstrate that rhesus monkey eyes are a good model for studying the contribution of individual ocular components to the eye's overall aberration structure, the mechanisms responsible for the improvements in optical quality that occur during early ocular development, and the effects of high-order aberrations on ocular growth and emmetropization.
    Vision Research 11/2006; 46(21):3616-33. · 2.14 Impact Factor
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    ABSTRACT: To characterize how the mechanisms that produce unilateral form-deprivation amblyopia integrate the effects of normal and abnormal vision over time, the effects of brief daily periods of unrestricted vision on the spatial vision losses produced by monocular form deprivation were investigated in infant monkeys. Beginning at 3 weeks of age, unilateral form deprivation was initiated in 18 infant monkeys by securing a diffuser spectacle lens in front of one eye and a clear plano lens in front of the fellow eye. During the treatment period (18 weeks), three infants wore the diffusers continuously. For the other experimental infants, the diffusers were removed daily and replaced with clear, zero-powered lenses for 1 (n=5), 2 (n=6), or 4 (n=4) hours. Four infants reared with binocular zero-powered lenses and four normally reared monkeys provided control data. The degree of amblyopia varied significantly with the daily duration of unrestricted vision. Continuous form deprivation caused severe amblyopia. However, 1 hour of unrestricted vision reduced the degree of amblyopia by 65%, 2 hours reduced the deficits by 90%, and 4 hours preserved near-normal spatial contrast sensitivity. The severely amblyogenic effects of form deprivation in infant primates are substantially reduced by relatively short daily periods of unrestricted vision. The manner in which the mechanisms responsible for amblyopia integrate the effects of normal and abnormal vision over time promotes normal visual development and has important implications for the management of human infants with conditions that potentially cause amblyopia.
    Investigative Ophthalmology &amp Visual Science 07/2006; 47(6):2468-77. · 3.44 Impact Factor
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    ABSTRACT: Given the prominence of central vision in humans, it has been assumed that visual signals from the fovea dominate emmetropization. The purpose of this study was to examine the impact of peripheral vision on emmetropization. Bilateral, peripheral form deprivation was produced in 12 infant monkeys by rearing them with diffusers that had either 4- or 8-mm apertures centered on the pupils of each eye, to allow 24 degrees or 37 degrees of unrestricted central vision, respectively. At the end of the lens-rearing period, an argon laser was used to ablate the fovea in one eye of each of seven monkeys. Subsequently, all the animals were allowed unrestricted vision. Refractive error and axial dimensions were measured along the pupillary axis by retinoscopy and A-scan ultrasonography, respectively. Control data were obtained from 21 normal monkeys and 3 infants reared with binocular plano lenses. Nine of the 12 treated monkeys had refractive errors that fell outside the 10th- and 90th-percentile limits for the age-matched control subjects, and the average refractive error for the treated animals was more variable and significantly less hyperopic/more myopic (+0.03 +/- 2.39 D vs. +2.39 +/- 0.92 D). The refractive changes were symmetric in the two eyes of a given animal and axial in nature. After lens removal, all the treated monkeys recovered from the induced refractive errors. No interocular differences in the recovery process were observed in the animals with monocular foveal lesions. On the one hand, the peripheral retina can contribute to emmetropizing responses and to ametropias produced by an abnormal visual experience. On the other hand, unrestricted central vision is not sufficient to ensure normal refractive development, and the fovea is not essential for emmetropizing responses.
    Investigative Ophthalmology &amp Visual Science 12/2005; 46(11):3965-72. · 3.44 Impact Factor
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    Bai-Chuan Jiang, Ramkumar Ramamirtham
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    ABSTRACT: The purpose of this study was to determine whether the response AC/A ratio could be altered when the subject's interpupillary distance (IPD) was optically halved. We measured the changes in the AC/A ratio for 10 subjects after using the optical device for 30 min. Accommodative response was measured using a Canon R-1 optometer, and vergence response was measured with an ASL 210 Eye Movement Monitor. The average AC/A ratios were 1.20+/-0.35 (SD) (MA/D) and 0.84+/-0.39 (MA/D) before and after wearing the device, respectively. The decrease in AC/A ratio was statistically significant (p=0.01). This was mainly caused by a reduction in the slope of the accommodative vergence. The results of this study suggest that the AC/A ratio can be decreased if an IPD-narrowing device is used. A possible application of this mechanism in the study of myopia is discussed.
    Vision Research 10/2005; 45(20):2704-9. · 2.14 Impact Factor
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    ABSTRACT: Astigmatism is the most common ametropia found in humans and is often associated with large spherical ametropias. However, little is known about the etiology of astigmatism or the reason(s) for the association between spherical and astigmatic refractive errors. This study examines the frequency and characteristics of astigmatism in infant monkeys that developed axial ametropias as a result of altered early visual experience. Data were obtained from 112 rhesus monkeys that experienced a variety of lens-rearing regimens that were intended to alter the normal course of emmetropization. These visual manipulations included form deprivation (n = 13); optically imposed defocus (n = 48); and continuous ambient lighting with (n = 6) or without optically imposed defocus (n = 6). In addition, data from 19 control monkeys and 39 infants reared with an optically imposed astigmatism were used for comparison purposes. The lens-rearing period started at approximately 3 weeks of age and ended by 4 to 5 months of age. Refractive development for all monkeys was assessed periodically throughout the treatment and subsequent recovery periods by retinoscopy, keratometry, and A-scan ultrasonography. In contrast to control monkeys, the monkeys that had experimentally induced axial ametropias frequently developed significant amounts of astigmatism (mean refractive astigmatism = 0.37 +/- 0.33 D [control] vs. 1.24 +/- 0.81 D [treated]; two-sample t-test, p < 0.0001), especially when their eyes exhibited relative hyperopic shifts in refractive error. The astigmatism was corneal in origin (Pearson's r; p < 0.001 for total astigmatism and the JO and J45 components), and the axes of the astigmatism were typically oblique and bilaterally mirror symmetric. Interestingly, the astigmatism was not permanent; the majority of the monkeys exhibited substantial reductions in the amount of astigmatism at or near the end of the lens-rearing procedures. In infant monkeys, visual conditions that alter axial growth can also alter corneal shape. Similarities between the astigmatic errors in our monkeys and some astigmatic errors in humans suggest that vision-dependent changes in eye growth may contribute to astigmatism in humans.
    Optometry and Vision Science 04/2005; 82(4):248-60. · 1.90 Impact Factor
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    ABSTRACT: Although many aspects of vision-dependent eye growth are qualitatively similar in many species, the failure to observe recovery from form-deprivation myopia (FDM) in higher primates represents a significant potential departure. The purpose of this investigation was to re-examine the ability of rhesus monkeys (Macaca mulatta) to recover from FDM. Monocular form deprivation was produced either with diffuser spectacle lenses (n = 30) or by surgical eyelid closure (n = 14). The diffuser-rearing strategies were initiated at 24 +/- 3 days of age and continued for an average of 115 +/- 20 days. Surgical eyelid closure was initiated between 33 and 761 days of age and maintained for14 to 689 days. After the period of form deprivation, the animals were allowed unrestricted vision. The ability of the animals to recover from treatment-induced refractive errors was assessed periodically by retinoscopy, keratometry, and A-scan ultrasonography. Control data were obtained from 35 normal monkeys. At the onset of unrestricted vision, the deprived eyes of 18 of the diffuser-reared monkeys and 12 of the lid-sutured monkeys were at least 1.0 D less hyperopic or more myopic than their fellow eyes. The mean (diffuser = -4.06 D, lid-suture = -4.50 D) and range (diffuser = -1.0 to -10.19 D, lid-suture = -1.0 to -10.25 D) of myopic anisometropia were comparable in both treatment groups. All 18 of these diffuser-reared monkeys demonstrated recovery, with 12 animals exhibiting complete recovery. The rate of recovery, which was mediated primarily by alterations in vitreous chamber growth rate, declined with age. None of the lid-sutured monkeys exhibited clear evidence of recovery. Instead, 8 of the 12 lid-sutured monkeys exhibited progression of myopia. Like many other species, young monkeys are capable of recovering from FDM. However, the potential for recovery appears to depend on when unrestricted vision is restored, the severity of the deprivation-induced axial elongation, and possibly the method used to produce FDM.
    Investigative Ophthalmology &amp Visual Science 11/2004; 45(10):3361-72. · 3.44 Impact Factor
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    ABSTRACT: Protracted daily lighting cycles do not promote abnormal ocular enlargement in infant monkeys as they do in a variety of avian species. However, observations in humans suggest that ambient lighting at night may reduce the efficiency of the emmetropization process in primates. To test this idea, we investigated the ability of infant monkeys reared with continuous light to compensate for optically imposed changes in refractive error. Beginning at about 3 weeks of age, a hyperopic or myopic anisometropia was imposed on 12 infant rhesus monkeys by securing either a -3 D or +3 D lenses in front of one eye and a zero-powered lens in front of the fellow eye. Six of these monkeys were reared with the normal vivarium lights on continuously, whereas the other six lens-reared monkeys were maintained on a 12-h-light/12-h-dark lighting cycle. The ocular effects of the lens-rearing procedures were assessed periodically during the treatment period by cycloplegic retinoscopy, keratometry, and A-scan ultrasonography. Five of six animals in each of the lighting groups demonstrated clear evidence for compensating anisometropic growth. In both lighting groups, eyes that experienced optically imposed hyperopic defocus (-3 D lenses) exhibited faster axial growth rates and became more myopic than their fellow eyes. In contrast, eyes treated with +3 D lenses showed relatively slower axial growth rates and developed more hyperopic refractive errors. The average amount of compensating anisometropia (continuous light, 1.6 +/- 0.5 D vs. control, 2.3 +/- 0.5 D), the structural basis for the refractive errors, and the ability to recover from the induced refractive errors were also not altered by continuous light exposure. Ambient lighting at night does not appear to overtly compromise the functional integrity of the vision-dependent mechanisms that regulate emmetropization in higher primates.
    Optometry and Vision Science 06/2003; 80(5):374-82. · 1.90 Impact Factor