The Response to Visual Form Deprivation Differs with Age in Marmosets

Experimental studies with animals of different species have established that vision plays an important role in the control of eye growth and the development of refractive state (e.g., Refs. 1-5). The discovery that visual form deprivation results in axial elongation and myopia was the first evidence, and subsequent studies with defocusing lenses have strongly supported this conjecture (for recent reviews, see Refs. 6,7). Since the original observations in tree shrews,8 macaques,9 and chicks,10 many investigators have used form deprivation myopia as the paradigm to examine the mechanisms of eye growth control. In primates, form deprivation myopia has been produced by lid suture,9,11-13 translucent diffusers (also called occluders) mounted over the eye,14 or diffusing lenses, with various degrees of transmission.15 Although axial myopia is the most frequently reported result of form deprivation in primates, the susceptibility clearly differs between individuals in all studies, and responses other than myopia have occasionally been noted. Lid suture in primates produces axial hyperopia in some individuals,11,12,16 and, in other studies, myopia was induced in some animals after a brief period of form deprivation that initially resulted in hyperopia from a combination of corneal flattening and reduced axial length.13,15 Similar findings have also been reported in tree shrews,17 a species considered to be closely related to primates. Most studies in which form deprivation has been used for experimental manipulation of eye growth in primates have been performed on neonates or immature subjects. In general, more myopia is produced faster in younger animals,6 but recent studies have shown that mature subjects are also susceptible to form deprivation myopia, although it is reduced in magnitude compared with that in neonatal primates.18,19 This finding leads to the questions of how the eye-growth response characteristics change with age and whether different mechanisms are involved at different ages. In this study, we examined the effects of form deprivation by using diffusers in marmoset monkeys across a range of ages of <200 days, which spans the phase of rapid ocular growth from young juvenile through prepubescent animals. Some of the results presented herein have been published in a preliminary report (Troilo D, et al. IOVS 2002;43:ARVO E-Abstract 186). In addition to studying the effects of form deprivation as a function of age, we examined the possibility of recovery from experimentally induced myopia in marmosets. Recovery from form deprivation myopia may involve the same visual guidance mechanisms that result in emmetropization during normal development and in compensation for defocus produced by spectacle lenses, although other factors related to the size and shape of the eye may also be involved.20-22 Furthermore, recovery does not occur consistently in primates. Lid-suture-induced myopia in macaques12,23,24 and marmosets13 persists well after the end of the period of visual form deprivation. However, this appears to be related to the lid-suture manipulation. Form deprivation myopia produced with diffusers shows recovery after the deprivation is discontinued in tree shrews25 and macaques,24,26 and preliminary studies using form deprivation diffusers in marmosets reported recovery in some animals13 (Troilo D, et al. IOVS 2000;41:ARVO Abstract 691). In this article we characterize the various responses and effects of age on form deprivation myopia in a New World primate, the common marmoset. Our results suggest that age and the maturity of the eye and visual system may alter visually guided eye growth. This study also provides a guide for studying form deprivation myopia in marmosets at different ages and will be useful for additional studies in this species examining the mechanisms of eye growth control, including lens-induced refractive changes in which the experimental manipulation of vision allows some degree of feedback control.