Accommodative defocus does not limit development of acuity in infant Macaca nemestrina monkeys

In an experiment with ten macaque monkeys (Macaca nemestrina), a combination of photorefraction and corneal reflex photography was used to measure simultaneously the plane of focus and direction of gaze while they were presented with fixation targets. The monkeys ranged in age from 2 days to 10 weeks. Some of the infants that were less than 1 month old failed to change accommodation to targets at any distance, whereas others showed limited accommodative abilities. The magnitude of the accommodative response of infants older than 4 weeks appeared to be adultlike. Infant monkey's visual acuity improves dramatically after 4 weeks. These results, which show that the improvement in spatial resolution cannot be accounted for by increased accommodative accuracy, parallel those obtained from human infants where accommodative competence is attained by about 4 months of age.