M L Crawford

Spectacle lenses alter eye growth and the refractive status of young monkeys

The influence of visual experience on ocular development in higher primates is not well understood. To investigate the possible role of defocus in regulating ocular growth, spectacle lenses were used to optically simulate refractive anomalies in young monkeys (for example, myopia or nearsightedness). Both positive and negative lenses produced compensating ocular growth that reduced the lens-induced refractive errors and, at least for low lens powers, minimized any refractive-error differences between the two eyes. These results indicate that the developing primate visual system can detect the presence of refractive anomalies and alter each eyes growth to eliminate these refractive errors. Moreover, these results support the hypothesis that spectacle lenses can alter eye development in young children.

Behavioral studies of the sensitive periods of development of visual functions in monkeys

The age-dependent effects of monocular form deprivation on psychophysically determined visual functions were investigated in rhesus monkeys. Monocular form deprivation was initiated at various ages from 1 to 25 months and maintained for 18 months. The effects of form deprivation varied across the visual functions evaluated. Absolute scotopic sensitivity was depressed by form deprivation initiated only at 1 or 2 months of age. Photopic, increment-threshold spectral sensitivity functions showed alterations in sensitivity levels for monkeys treated at 5 months of age or earlier, with no effect thereafter. Monocular form deprivation at 1 to 5 months of age resulted in profound deficits in spatial modulation sensitivity. The effects on spatial vision decreased systematically as the age of onset was delayed from age 6 to 18 months. Finally, binocular summation measures revealed an absence of binocular vision even for the monkey form-deprived at 25 months of age, i.e. the experimental treatment series failed to define the upper limit of the age-range for the effects of monocular form deprivation on binocular summation mechanisms. Comparisons of the age-dependent effects of monocular form deprivation across the various functions demonstrated that the sensitive period, i.e. the period of life during which a visual function may be altered by monocular form deprivation, was different for each of the psychophysical measures of visual function.

Glutathione content is altered in Müller cells of monkey eyes with experimental glaucoma

Extracellular levels of glutamate are thought to be increased in glaucoma and thus contribute to retinal damage. An increase in glutamate concentration or duration in the extracellular retinal space is expected to impact glutathione content in Müller cells since glutamate is the rate-limiting constituent in glutathione synthesis. We have investigated whether glutathione content is changed in retinal Müller cells of monkeys with experimental glaucoma using immunocytochemistry and image analysis. Müller cells in glaucomatous retinas showed significantly greater immunoreactivity (27-57%) for glutathione than those in fellow control retinas, increasing with the duration of elevated intraocular pressure (IOP). This pattern of labeling was prominent in all four monkeys studied. The presence of more glutathione in Müller cells of glaucomatous retinas is consistent with an increase in extracellular glutamate and an increase in transport and metabolism of glutamate.

The Influence of the Period of Deprivation on Experimental Refractive Errors

The eye’s optical and axial components normally grow in a regulated manner so that the eye maintains an approximately emmetropic refractive condition throughout development. But, if the potential for a clear retinal image is prevented during development by an ocular abnormality (or by experimental manipulation in laboratory animals), the coordinated growth of the eye is disrupted resulting in an anomalous refractive error (Rabin et al., 1981; von Noorden and Lewis, 1987; also see Criswell and Goss, 1983; and Yinon, 1984, for recent reviews). Since these resulting refractive errors can aggravate existing amblyopiogenic factors, optimal treatment and management procedures for amblyopia require an understanding of the vision-dependent mechanisms that influence the emmetropization process.