Fixation (visual)
Encyclopedia
Fixation or visual fixation is the maintaining of the visual
gaze on a single location. Humans (and other animals with a fovea
) typically alternate saccade
s 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: microsaccade
s, 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 microsaccade
s, 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.
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
gaze on a single location. Humans (and other animals with a fovea
Fovea
The fovea centralis, also generally known as the fovea , is a part of the eye, located in the center of the macula region of the retina....
) typically alternate saccade
Saccade
A saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...
s 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: microsaccade
Microsaccade
Microsaccades are a kind of fixational eye movement. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during prolonged visual fixation , not only in humans, but also in animals with foveal vision . Microsaccade...
s, 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
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
, which mostly respond to transient stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
. In the absence of retinal jitter (a laboratory condition called retinal stabilization), stabilized images
Stabilized images
Stabilized Images are images on the retina that are unaffected by microsaccade or ocular microtremor . Experiments by Riggs and Ratliff ) established the remarkable finding that stabilized images result in the fading and disappearance of the visual percept...
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
Neurophysiology
Neurophysiology is a part of physiology. Neurophysiology is the study of nervous system function...
from different laboratories have shown that fixational eye movement, particularly microsaccade
Microsaccade
Microsaccades are a kind of fixational eye movement. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during prolonged visual fixation , not only in humans, but also in animals with foveal vision . Microsaccade...
s, strongly modulate the activity of neurons in several visual areas of the macaque
Macaque
The macaques constitute a genus of Old World monkeys of the subfamily Cercopithecinae. - Description :Aside from humans , the macaques are the most widespread primate genus, ranging from Japan to Afghanistan and, in the case of the barbary macaque, to North Africa...
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.