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Fixational eye movements (also known as fixational instability, retinal jitter) are small, involuntary eye movements that occur during visual fixation. There are three categories of fixational eye movements: microsaccades, ocular drifts, and ocular microtremor. Fixational eye movements were found in a number of species, including humans, other primates, cats, rabbits, turtles, salamanders, owls, etc. Although their existence has been known since the 1950s, the role and importance of fixational eye movements is still debated.
In the current consensus, fixational eye movements contribute to maintain 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), the visual percept rapidly fades out and may even completely disappear under certain conditions (low contrast, absence of sharp edges, etc.).
Experiments in neurophysiology from different laboratories have shown that fixational eye movements, particularly microsaccades, strongly modulate the activity of neurons in several visual areas of the macaque brain. This topic is currently under active investigation.
- Roger H. S. Carpenter. Movements of the Eyes, 2nd edition (Pion, London, 1988). ISBN 0-85086-109-8
- Susana Martinez-Conde, Stephen L. Macknik & David H. Hubel. The role of fixational eye movements in visual perception. Nature Reviews Neuroscience 5(3):229-240 (2004). doi:10.1038/nrn1348 
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