The control and 3-week monkeys had stable fixation when viewing monocularly or binocularly
(Figs. 1A 1B) . The 12- and 24-week monkeys, however, showed LN (fusion maldevelopment nystagmus
8 ), more pronounced in the 24-week animals. The nystagmus was evident as nasalward slow-phase drifts, with respect to the fixating eye, interrupted by temporalward fast-phase jerks
(Figs. 1C 1D) . When the subject was fixating with the left eye (right eye occluded), both eyes drifted to the right; when fixating with the right eye (left eye occluded), both eyes drifted to the left. The wave form of the slow-phase—decreasing or linear velocity—conformed to standard LN criteria.
6 The nystagmus appeared conjugate during inspection of fixation videotape (the equivalent of clinical examination of human infants): The direction and frequency were the same in both eyes. Analysis of eye position tracings, however, revealed mild-to-moderate disconjugacies of velocity and amplitude between the two eyes. In
Figure 1C , for example, the nystagmus was more pronounced in the (esotropic) left eye of monkey GO when the animal fixated with the right eye. In
Figure 1D(monkey HA), the nystagmus was more pronounced in the esotropic right or left eye.
Figure 1Dalso illustrates (BE traces) that the nystagmus intensity tended to be less pronounced under conditions of binocular (neither eye occluded) viewing (i.e., LN was more intense than manifest LN).