Transition dynamics between pursuit and fixation suggest different systems.

The offset of smooth pursuit eye movements is very different from the onset. The onset eye velocity is characterized by overshoot and ringing before settling to steady-state velocity. Yet, at pursuit offset, the eye velocity returns smoothly to zero. One reason for this difference may be that the pursuit system is nonlinear and behaves differently during acceleration and deceleration. After testing four subjects, we found no difference between acceleration and deceleration ringing dynamics, except for decelerations to 0 +/- 2 deg/sec, suggesting that around zero velocity, the central nervous system switches off pursuit and adopts fixation. This supports the idea that fixation and pursuit represent different neurological systems.