"Fixation or visual fixation is the maintaining of the visual gaze on a single location. Humans (and other animals with a fovea) typically alternate saccades and visual fixations, the notable exception being in smooth pursuit, controlled by a different neural substrate that appear to have developed for hunting prey. There are three categories of fixational eye movements: microsaccades, ocular drifts, and ocular microtremor. Fixational eye movement has been found in a number of species, including humans, other primates, cats, rabbits, turtles, salamanders, owls. Although their existence has been known since the 1950s, the role and importance of fixational eye movement is still debated."