Effects of optically induced blur on the refractive status of young monkeys

In each of eight rhesus monkeys, one eye was defocused with a -9 D contact lens beginning before 1 month of age for periods of 2-3 months. At the end of the rearing period, interocular comparisons showed that one subject had developed a relative axial myopia (3.0 D), however, five monkeys had developed a relative axial hyperopia (2.0-3.5 D). After discontinuing the contact-lens rearing procedure, the induced refractive errors diminished over time in all subjects. These results indicate that the defocus threshold for form-deprivation myopia is relatively high and that substantial levels of optical defocus which do not exceed this threshold typically produce axial hyperopia. The recovery data suggests that monkeys have an emmetropization mechanism which is sensitive to optical defocus, but the failure of this mechanism to compensate for the refractive errors simulated during the lens-rearing procedures suggests that this mechanism has a limited operating range.