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Stimulation of Axial Eye Growth by Retinal Image Degradation

Lid fusion, as performed in the initial experiments [84], is an experimental manipulation with several effects: the retina no longer has access to spatial information (although it is not completely light-deprived), the mechanical pressure on the cornea is changed, and the metabolic conditions and temperature in the eye may be different. Although each of these factors could interfere with eye growth, it was found that the most important component was the deprivation of the retina of sharp vision and contrast. Accordingly, this type of myopia has been called form deprivation myopia (FDM) because form vision is no longer possible. In the meantime, it became clear that even a minor reduction of image sharpness and contrast may already stimulate axial eye growth: "deprivation myopia is a graded phenomenon" [67] and this has been shown in both chickens [3], and rhesus monkeys [67]. Therefore, the term "form deprivation myopia" may be an exaggerated description of the visual condition and could be replaced by "deprivation myopia" since this term makes no assumptions about the exact nature of the deprivation.

Deprivation myopia has been observed in almost all vertebrates that have been studied [79]. It is commonly induced by placing a frosted occluder in front of an eye for a period of several days or weeks. The speed by which deprivation myopia develops depends on the species

Fig. 1.1. If an emmetropic eye is wearing a negative lens, the focal plane is displaced behind the retina. Several animal models, including marmosets and rhesus monkeys, have shown that the eye develops compensatory axial elongation and myopia. With a positive lens, axial eye growth is inhibited, and a compensatory hyperopia develops (redrawn after [83], marmosets, left; [66], rhesus monkeys, right)

Fig. 1.1. If an emmetropic eye is wearing a negative lens, the focal plane is displaced behind the retina. Several animal models, including marmosets and rhesus monkeys, have shown that the eye develops compensatory axial elongation and myopia. With a positive lens, axial eye growth is inhibited, and a compensatory hyperopia develops (redrawn after [83], marmosets, left; [66], rhesus monkeys, right)

and the age of the animal [58]. In 1-day-old chickens, up to 20 D can be induced over 1 week of deprivation [77], but only 1D at the age of iyear [48]. Rhesus monkeys develop about 5D on average during an 8-week deprivation period at the age of 30 weeks, but only 1 D at adolescence [68]. Deprivation myopia is strikingly variable among different individuals (range 0-11D in rhesus monkeys, standard deviations about 5D [67] (a similar standard deviation is typical also in the other animal models). Although the variability cannot be explained by differences in individual treatment of the animals, it is unclear whether the variability is due to genetic factors. Epigenetic variance could also account for it (R.W. Williams, personal communication, 2003) although it is striking that both eyes respond very similarly despite the lack of visual feedback [57].

Deprivation myopia can be induced in chickens after the optic nerve has been cut [76] and in local fundal areas if only part of the visual field is deprived [78]. Local degradation of the retinal image also produces local refractive error in tree shrews [63]. There are data in both chickens [35] and tree shrews [46] showing that deprivation myopia also can be induced after the ganglion cell action potentials are blocked by intravitreal application of tetro-dotoxin, a natural sodium channel blocker. Taken together, the results show that image processing in the retina, excluding its spiking neurons, is sufficient to stimulate axial elongation.

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