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.
Reading involves fixating on a successive locations across the page or screen. Visual fixation is never perfectly steady: fixational eye movement occurs involuntarily. The term "fixation" can also be used to refer to the point in time and space of focus rather than to the act of fixating; a fixation in this sense is the point between any two saccades, during which the eyes are relatively stationary and virtually all visual input occurs (e.g., Martin 1974).
In the current consensus, fixational eye movement contributes to maintaining visibility, by continuously stimulating neurons in the early visual areas of the brain, which mostly respond to transient stimuli. In the absence of retinal jitter (a laboratory condition called retinal stabilization), stabilized images as a visual percept rapidly fade out and completely disappear (provided the stabilization is good enough) (Pritchard, Heron & Hebb, 1960; Coppola & Purves, 1996).
Fixation is also used in experiments in vision science or neuroscience. Human subjects are often told to fixate on an object on a monitor before any experiment takes place. This serves to direct the person's attention to the point where visual information will be presented. Experiments in neurophysiology from different laboratories have shown that fixational eye movement, particularly microsaccades, strongly modulate the activity of neurons in several visual areas of the macaque brain. This topic is currently under active investigation.
Fixational eye movement might also participate to the neural code in the early visual system, although this hypothesis is still a very recent line of research.