Eye movements during fixation are a special case of smooth pursuit (or visual tracking) and not a separate system.

Background: Fixational Eye Movements are commonly considered to be a separate system from the other types of eye movements, with a separate function which is holding the gaze such that a stationary visual target stays on the center of the retina. SmoothPursuit eye movements are considered to be a separate system with the function of maintaining a moving visual target on the center of the retina. Here we challenge this view by comparing the pattern of eye movements between tracking of an object that moved at different speeds and during fixation on a stationary visual target. Method: 4 observers ran a set of 5 sessions, each of a different target speed. In each session, a dot target on a PC screen was randomly selected to move at a constant speed for 2 seconds, horizontally, either to the left or to the right, or to remain stationary at the center of the screen for 2 seconds. Observers were instructed to maintain their gaze on the target. During the sessions the movements of the participants' dominant eye were recorded for off-line analysis. Results: We found that, while the kinematics of saccades during fixation varies significantly between observers, the pattern of eye movements was very similar between fixation and smooth pursuit, at low target velocities, within each of the observers. Specifically, for each observer, as target velocities decreased, saccade kinematics (such as the distribution of: rate, amplitude and direction bias) converged with the kinematics of fixation saccades (or microsaccades). Conclusions: These results suggest that eye movements during fixation, are the response of the pursuit system, tacking a target at zero velocity, and not a response of a separate system dedicated for fixating on a stationary target.