Monkeys (
Macaca mulatta) born at the Yerkes Primate Center (Atlanta, GA) were fitted with goggles on the first day of life
(Fig. 1) . The fitting procedure was an adaptation of that originally described by Crawford and von Noorden
35 and Crawford.
36 The procedure was not stressful to the newborn macaques and did not require anesthesia or fabrication of a head mold. Padded head straps held the goggles firmly in place and prevented the infant from removing the apparatus, which was custom fabricated for each monkey from light-weight plastic. The front piece consisted of two lens holders, which unscrewed so that ultra–light-weight, 2-mm thick Fresnel plastic prisms could be inserted. Animals were observed several times per day in the primate nursery and during bottle feedings to ensure that the goggles remained clear and in proper position. The goggles did not interfere noticeably with normal play or mingling with other infant macaques. The goggle helmet was removed daily from each monkey for cleaning. During cleaning and, if necessary, adjustment of the goggle, the animal was placed briefly in a dark (light-tight) enclosure to preclude normal binocular experience. Inspection of the infant monkeys during the brief periods when the goggles were removed for cleaning disclosed that each of the animals manifested esotropia. Those reared with goggles for longer periods did not manifest noticeably larger angles of strabismus during these inspections.
The monkeys were divided into two repair groups, Early and Delayed. In each group, experimental animals wore prism goggles to induce optical strabismus of at least 11.4° (20 prism diopters) in each eye. Four experimental animals
(Table 1) wore 11.4° base-down in one eye, and 11.4° base-in in the other eye, causing a combined horizontal and vertical strabismus. A fifth experimental animal (SY), wore 11.4° base-in in each eye, causing a 22.8° (40-prism-diopter) horizontal strabismus. Normal control animals wore goggles with plano lenses
(Table 1) . The Early Repair group (two experimental and one normal control) wore the goggles for a period of 3 weeks (the equivalent of three months in humans
28 ). The Delayed Repair group (three experimental and one normal control) wore the goggles for a period of 3 months (the equivalent of 12 months in humans), or for a period of 6 months (the equivalent of 24 months in humans). At 4 to 6 months of age, the monkeys were shipped to Washington University (St. Louis, MO) where they were trained to perform visual fixation and tracking tasks without prism goggles, using a positive-feedback reward (a small bolus of fruit juice).
37 Three adult monkeys who had naturally occurring esotropia with onset at 4 weeks of age or earlier were also studied
(Table 1) . Cycloplegic refractions revealed a refractive error of less than +3.00 spherical equivalent in each of the infant and adult animals. Monocular visual acuity was measured using spatial sweep VEPs
38 39 (without correction for refractive error), documenting approximately equal vision in both eyes of the control and experimental monkeys
(Table 1) .
All experiments were performed in compliance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research and were approved by the Washington University Animal Care and Use Committee.