Abstract
Purpose: :
Newborn monkeys reared viewing non-corresponding binocular images develop the infantile strabismus syndrome in a few weeks. Here we measure development of the ocular following response (OFR) after different durations of non-correspondence.
Methods: :
Eight infant macaques were fitted with prism goggles at age 3 wks to induce image non-correspondence. The goggles were removed after 3 wks (n=2), 6 wks (n=2), 9 wks (n=2) or 12 wks (n=2). At age 1 year the monkeys were trained to perform visual tasks and eye movements were recorded using eye coils. Horizontal OFR was evoked by ramp motion (5-75 deg/sec) of a large-field (45 deg x 45 deg) display of random dots (each subtending 1-3 deg arc) 50 msec after a centering-saccade. Exposure to horizontal dot motion was limited to 200 msec followed by a blanking of the display for 500 msec. Ramp motion direction and velocity were randomized.
Results: :
The shortest-duration monkeys (3 wks) regained normal eye alignment and exhibited symmetric pursuit/OFR. In contrast, the intermediate and long-duration monkeys (6, 9 and 12 wks) exhibited persistent heterotropias and pursuit/OFR asymmetries. The severity of the OFR asymmetry increased with increasing duration. For each group, a nasal bias index (NBI) was calculated as (Vn - Vt) / (Vn + Vt) x 100 [Vn = nasally-directed and Vt = temporally-directed mean eye velocity]. The 3 wk monkeys had a NBI of -4.0. The 6 wk, 9 and 12 wk monkeys had NBIs of 13, 34 and 44, respectively.
Conclusions: :
Longer durations of binocular decorrelation in infancy cause greater maldevelopment of the OFR. These results reinforce the importance of restoring normal eye alignment in infancy within a short period of time.
Keywords: strabismus • development • eye movements: saccades and pursuits