Why do the eyes alternate between slow pursuit and quick return during optokinetic nystagmus?

During optokinetic nystagmus, the eyes alternate between slow pursuit and quick return to maintain visual stability. The primary purpose of the slow phase is to pursue a moving scene, which keeps the image stationary on the retina.1

Because the eyes have a limited range of motion, they eventually reach a point where they can move no farther. At this stage, the eyes must break their current fixation and perform a quick return—a saccade—to catch up with a new part of the moving scene, such as a new stripe on a rotating drum. This to-and-fro cycle repeats indefinitely, allowing the visual system to continuously track movement despite the physical constraints of eye rotation.12