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Fixation (visual)
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<p>Fixation or visual fixation is the maintaining of the <a href="/facts/Visual_perception/g7Tfaq2l">gaze</a> on a single location. An animal can exhibit visual fixation if it possess a <a href="/facts/Fovea_centralis/cCqjWzTj">fovea</a> in the anatomy of their <a href="/facts/Eye/h3lbYU55">eye</a>. The fovea is typically located at the center of the <a href="/facts/Retina/v0ISNX2S">retina</a> and is the point of clearest vision. The species in which fixational eye movement has been verified thus far include humans, primates, cats, rabbits, turtles, salamanders, and owls. Regular <a href="/facts/Eye_movement/4a9mNbP5">eye movement</a> alternates between <a href="/facts/Saccade/0YxQJpk2">saccades</a> and visual fixations, the notable exception being in <a href="/facts/Smooth_pursuit/Hf8Wq591">smooth pursuit</a>, controlled by a different <a href="/facts/Neural_substrate/RqB7mBwI">neural substrate</a> 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 <a href="/facts/Stabilized_images/5TyfcVks">retinal stabilization</a>, perceptions tend to rapidly pass away. To maintain visibility, the <a href="/facts/Nervous_system/CDxRKU2U">nervous system</a> carries out a procedure called fixational eye movement, which continuously stimulates <a href="/facts/Neurons/G7CyTVdH">neurons</a> in the early visual areas of the <a href="/facts/Brain/Xo5ABfXl">brain</a> responding to transient <a href="/facts/Stimulus_(physiology)/aBcpBfYQ">stimuli</a>. There are three categories of fixational eye movement: microsaccades, ocular drifts, and ocular microtremor. At small amplitudes the boundaries between categories become unclear, particularly between drift and tremor. </p>