Fixation (visual)

Fixation or visual fixation is the maintaining of the visual gaze on a single location. An animal can exhibit visual fixation if they possess a fovea in the anatomy of their eye. The fovea is typically located at the center of the retina and is the point of clearest vision. The species in which fixational eye movement has been found thus far include humans, primates, cats, rabbits, turtles, salamanders, and owls. Regular eye movement alternates between saccades and visual fixations, the notable exception being in smooth pursuit, controlled by a different neural substrate that appears to have developed for hunting prey. The term "fixation" can either be used to refer to the point in time and space of focus or the act of fixating. Fixation, in the act of fixating, is the point between any two saccades, during which the eyes are relatively stationary and virtually all visual input occurs. In the absence of retinal jitter, a laboratory condition known as retinal stabilization, perceptions tend to rapidly fade away. To maintain visibility, the nervous system carries out a mechanism called fixational eye movement, which continuously stimulates neurons in the early visual areas of the brain responding to stimuli. There are three categories of fixational eye movements: microsaccades, ocular drifts, and ocular microtremor. Although the existence of these movements has been known since the 1950s, only recently their functions have started to become clear.

Microsaccades, also known as "flicks", are saccades, involuntarily, produced during the fixation periods. They are the largest and fastest of the fixational eye movements. Like saccades, microsaccades are usually binocular, and conjugate movements with comparable amplitudes and directions in both eyes. In the 1960s, scientists suggested the maximum amplitude for microsaccades should be 12 arcminutes to distinguish microsaccades and saccades. However, recent studies have shown that microsaccades can certainly exceed this value. Therefore, amplitude can no longer be used to distinguish microsaccades and saccades. The only way to distinguish microsaccades from saccades is by the time in which they happen: during fixation. Regular saccades are produced during the active exploration of the eye, during non-fixation tasks such as free viewing or visual search. However, microsaccades are distinguished from regular saccades because they are only produced during fixation tasks. The circularity of this definition has been the subject of ample criticism.

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