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, and 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 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 absence of retinal jitter (a laboratory condition called retinal stabilization), percepts tend to rapidly fade away. This has led to the view that fixational eye movement may contribute to maintaining visibility by continuously stimulating neurons in the early visual areas of the brain, which mostly respond to transient stimuli. A more recent line of research has suggested that fixational eye movements may play a deeper role in visual perception, by serving as a first computational step and participating in the encoding of information. Consistent with these hypotheses, both a restructuring of the input signal impinging onto retinal receptors and improved vision of fine spatial detail have been reported in the presence of fixational eye movements.
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.
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