Congenital nystagmus: control of slow tracking movements by target offset from the fovea.

Patients with congenital nystagmus (CN) are unable to respond adequately to optokinetic stimuli. This suggests that patients with CN do not use the movement of images (slip) across the retina as a control variable for their eye movements. Nevertheless, they are capable of tracking moving targets with slow eye movements. Experiments using paracentral afterimages as targets for fixation suggest that these slow tracking movements may be executed by the control of target position on the retina rather than slip across the retina: all seven patients with CN produced slow tracking movements, superimposed on nystagmic cycles. The fact that the basic parameters of the CN waveform remained unchanged under the open-loop condition constituted by afterimage tracking (only the amplitude was reduced in four out of seven patients) indicates that the timing and direction of slow and rapid components of CN do not depend on retinal feedback. During refixations between stationary targets, some patients with CN occasionally acquired the target with a slow, rather than a saccadic movement. This finding further supports the notion that patients with CN use target offset from the fovea as a very effective control variable not only for rapid, but also for slow eye movements.