Neural control of eye growth and experimental myopia in primates.

Macaque monkeys become myopic when raised with fused lids to expose the retina to formless shadows during the period of postnatal eye development. The effect of the abnormal visual input is an excessive expansion of the posterior segment of the eye, a process that seems to be controlled by the nervous system. The mechanism by which the nervous system influences eye growth appears to be different in the stumptailed macaque (Macaca arctoides) and the rhesus macaque (M. mulatta). Lid-fused arctoides monkeys do not develop myopia when the ciliary muscle is paralysed or the optic nerve is cut, suggesting that the abnormal growth is caused by excessive accommodation. In contrast, paralysis of the ciliary muscle or optic nerve section does not prevent the development of myopia in the rhesus macaque, suggesting that in this species the axial growth is controlled by the retina. In both species neonatal lid fusion causes a marked increase in retinal vasoactive intestinal polypeptide (VIP). VIP is contained in a single type of amacrine cell whose dendrites spread in the middle of the inner plexiform layer. It remains to be determined whether the increase in the level of VIP is related to the abnormal axial elongation caused by lid fusion. At present we are also exploring the effects of accommodation on the growth of the eye by training juvenile arctoides monkeys to work on complex visual discrimination paradigms. Preliminary results show that performing a visual task at close range may influence the axial length and refraction in this macaque species.