Measurement of visual acuity in human and monkey infants: the interface between laboratory and clinic.

The development of visual acuity has been studied in human and monkey (Macaca nemestrina) infants by a combination of preferential looking and operant techniques. Infants of both species demonstrate acuity of about 1 cycle/degree (20/600 Snellen equivalent) near birth. Acuity develops gradually in both species, reaching adult levels at 3-5 years in humans and about 1 year in monkeys. Amblyopiogenic conditions, such as strabismus, occur spontaneously in monkey infants as they do in human infants, and normal monkeys reared under deprivation conditions that mimic amblyopiogenic conditions in humans also become amblyopic. These results help to establish the infant monkey as an animal model for human visual development, and provide an important resource for sorting out the developmental causes of human amblyopia. Three major difficulties and limitations involved in the use of these techniques in clinical settings are described. These are that the testing must be done with standardized equipment by trained personnel; that it is not yet known which kinds of clinical cases can be most usefully evaluated; and that the inherent statistical limitations of the available techniques put them at the outer margin of efficiency for routine clinical use.