Testing visual acuity is most difficult at precisely the age that amblyopia, a preventable disorder, must be detected and treated. If intervention is delayed or inappropriate, permanent neurologic abnormalities and lifelong vision loss result. Presently, very limited methods are successfully used to test children aged 18 months to three years, which is within the "critical period" of human visual development. Our long-range goal is to develop a fully automated apparatus capable of testing key aspects of vision in this critical age group. Principles of discrimination learning and operant reinforcement have been successfully used to test vision in animals and young humans. The purpose of this application is to determine the feasibility of using a new operant approach to test visual acuity in toddlers. In this pilot project, we will develop a three choice operant procedure, and compare its performance to the two-alternative forced choice methods that are clinically available. We hypothesize that the experimental (operant) procedure will provide smaller within group variability, smaller interocular differences, and better test/retest reliability than the clinical tests of resolution (acuity cards) and recognition acuity (crowded Landolt C's). We also hypothesize that a greater percentage of the younger children will successfully complete operant testing of recognition acuity. Once we develop more reliable testing methods, future studies may evaluate whether recognition acuity provides a more sensitive measure of amblyopia in much younger children than is currently possible. Furthermore, once appropriate psychophysical techniques are developed in this pilot study, many aspects of vision (stereopsis, color vision, etc.) could be investigated in normal children or children with ocular or visual pathology.