Saccade
Encyclopedia
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. Initiated by eye fields in the frontal and parietal lobes of the brain, saccades serve as a mechanism for fixation
, rapid eye movement and the fast phase of optokinetic nystagmus. The word appears to have been coined in the 1880s by French ophthalmologist
Émile Javal
, who used a mirror on one side of a page to observe eye movement in silent reading, and found that it involves a succession of discontinuous individual movements.
—known as the fovea
—plays a critical role in resolving objects. By moving the eye so that small parts of a scene can be sensed with greater resolution
, body resources can be used more efficiently. A human's saccades are very fast.
.
The amplitude of a saccade is the angular distance the eye travels during the movement. For amplitudes up to about 60°, the velocity of a saccade linearly depends on the amplitude (the so called "saccadic main sequence"). In saccades larger than 60°, the peak velocity starts to plateau (nonlinearly) toward the maximum velocity attainable by the eye. For instance, a 10° amplitude is associated with a velocity of 300°/sec, and 30° is associated with 500°/sec.
Saccades may rotate the eyes in any direction to relocate gaze direction (the direction of sight that corresponds to the fovea), but normally saccades do not rotate the eyes torsionally. (Torsion is clockwise or counterclockwise rotation around the line of sight when the eye is at its central primary position; defined this way, Listing's law says that when the head is motionless, torsion is kept at zero.)
Head-fixed saccades can have amplitudes of up to 90° (from one edge of the oculomotor range to the other), but in normal conditions saccades are far smaller, and any shift of gaze larger than about 20° is accompanied by a head movement. During such gaze saccades, first the eye produces a saccade to get gaze on target, whereas the head follows more slowly and the vestibulo-ocular reflex causes the eyes to roll back in the head to keep gaze on the target. During these head movements Listing's law is no longer obeyed.
Saccades, as well as microsaccades, can be distinguished from other eye movements (ocular tremor
, ocular drift, smooth pursuit) using their ballistic
nature: their top velocity is proportional to their length. This property can be used in algorithm
s for saccade detection in eye tracking
data.
, or damage to the brainstem "neural integrator" that normally holds the eyes in place. On the other hand, opsoclonus
or ocular flutter
are composed purely of fast-phase saccadic eye movements.
Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions.
Eye movement measurements are also used to investigate psychiatric disorders. For example, ADHD is characterized by an increase of antisaccade errors and an increase in delays for visually guided saccade.
.
A person may observe the saccadic masking effect by standing in front of a mirror and looking from one eye to the next (and vice versa). The subject will not experience any movement of the eyes nor any evidence that the optic nerve has momentarily ceased transmitting. Due to saccadic masking, the eye/brain system not only hides the eye movements from the individual but also hides the evidence that anything has been hidden. Of course, a second observer watching the experiment will see the subject's eyes moving back and forth. The function's main purpose is to prevent smearing of the image.
, including animals that do not have a fovea (most vertebrates do not) and animals that cannot move their eyes independently of their head (such as insects). Therefore, while saccades serve in humans and other primates to increase the effective visual resolution of a scene, there must be additional reasons for the behavior. The most frequently suggested of these reasons is to avoid blurring of the image, which would occur if the response time of a photoreceptor is longer than the time a given portion of the image is stimulating that photoreceptor as the image drifts across the eye.
In birds, saccadic eye movements serve a further function. The avian retina is highly developed. It is thicker than the mammalian retina and has a higher metabolic activity, but it lacks proper vasculature
. Therefore, the retinal cells must obtain nutrients via diffusion through the choroid
and from the vitreous humor. The pecten
is a specialised structure in the avian retina. It is a highly vascular structure that projects into the vitreous humor. Experimentally, it has been shown that during saccadic eye oscillations (which occupy up to 12% of avian viewing time), the pecten acts as an agitator, propelling perfusate towards the retina. Thus, in birds, saccadic eye movements appear to be important in retinal nutrition and respiration
.
Fixation (visual)
Fixation or visual fixation is the maintaining of the visual gaze on a single location. Humans 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...
, rapid eye movement and the fast phase of optokinetic nystagmus. The word appears to have been coined in the 1880s by French ophthalmologist
Ophthalmology
Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...
Émile Javal
Louis Émile Javal
Louis Émile Javal was a French ophthalmologist born in Paris. Originally trained as a civil engineer, he switched to the medical profession, receiving his degree from the University of Paris in 1868. Following graduation he travelled to Berlin and studied under Albrecht von Graefe...
, who used a mirror on one side of a page to observe eye movement in silent reading, and found that it involves a succession of discontinuous individual movements.
Function
Humans and many other animals do not look at a scene in fixed steadiness (as opposed to e.g., most birds); instead, the eyes move around, locating interesting parts of the scene and building up a mental, three-dimensional 'map' corresponding to the scene (as opposed to the graphical map of avians, that often relies upon detection of angular movement on the retina). One reason for the saccadic movement of the human eye is that the central part of the retinaRetina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
—known as the 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....
—plays a critical role in resolving objects. By moving the eye so that small parts of a scene can be sensed with greater resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
, body resources can be used more efficiently. A human's saccades are very fast.
Control
Once saccades are underway, they cannot be altered by the will. The brain does not utilize continuous feedback, where there is continuous tracking of error, but rather monitors the eye's drift away from the target, in order to return the gaze to the target; this is called a sampled data systemSampled data systems
A sampled-data system is a control system where continuous-time plant is controlled with a digital device. Under periodic sampling, the sampled-data system is time-varying but also periodic, and thus it may be modeled by a simplified discrete-time system obtained by discretizing the plant...
.
Timing and kinematics
Saccades are the fastest movements produced by the human body. The peak angular speed of the eye during a saccade reaches up to 1000°/sec in monkeys (somewhat less in humans). Saccades to an unexpected stimulus normally take about 200 milliseconds (ms) to initiate, and then last from about 20–200 ms, depending on their amplitude (20–30 ms is typical in language reading). Under certain laboratory circumstances, the latency of, or reaction time to, saccade production can be cut nearly in half (express saccades). These saccades are generated by a neuronal mechanism that bypasses time-consuming circuits and activate the eye muscles more directly. Specific pre-target oscillatory (alpha rhythms) and transient activities occurring in posterior-lateral parietal cortex and occipital cortex also characterise express saccades.The amplitude of a saccade is the angular distance the eye travels during the movement. For amplitudes up to about 60°, the velocity of a saccade linearly depends on the amplitude (the so called "saccadic main sequence"). In saccades larger than 60°, the peak velocity starts to plateau (nonlinearly) toward the maximum velocity attainable by the eye. For instance, a 10° amplitude is associated with a velocity of 300°/sec, and 30° is associated with 500°/sec.
Saccades may rotate the eyes in any direction to relocate gaze direction (the direction of sight that corresponds to the fovea), but normally saccades do not rotate the eyes torsionally. (Torsion is clockwise or counterclockwise rotation around the line of sight when the eye is at its central primary position; defined this way, Listing's law says that when the head is motionless, torsion is kept at zero.)
Head-fixed saccades can have amplitudes of up to 90° (from one edge of the oculomotor range to the other), but in normal conditions saccades are far smaller, and any shift of gaze larger than about 20° is accompanied by a head movement. During such gaze saccades, first the eye produces a saccade to get gaze on target, whereas the head follows more slowly and the vestibulo-ocular reflex causes the eyes to roll back in the head to keep gaze on the target. During these head movements Listing's law is no longer obeyed.
Saccades, as well as microsaccades, can be distinguished from other eye movements (ocular tremor
Ocular tremor
Ocular microtremor is a constant, physiological, high frequency , low amplitude eye tremor....
, ocular drift, smooth pursuit) using their ballistic
Ballistics
Ballistics is the science of mechanics that deals with the flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.A ballistic body is a body which is...
nature: their top velocity is proportional to their length. This property can be used in algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
s for saccade detection in eye tracking
Eye tracking
Eye tracking is the process of measuring either the point of gaze or the motion of an eye relative to the head. An eye tracker is a device for measuring eye positions and eye movement. Eye trackers are used in research on the visual system, in psychology, in cognitive linguistics and in product...
data.
Types
Saccades are measured or investigated in four ways:- In a visually guided saccade, the eyes move towards a visual onset, or stimulus. This is typically included as a baseline when measuring other types of saccades.
- In an antisaccade, the eyes move away from the visual onset. They are more delayed than visually guided saccades, and observers often make erroneous saccades in the wrong direction. A successful antisaccade requires inhibiting a reflexive saccade to the onset location, and voluntarily moving the eye in the other direction.
- In a memory guided saccade, the eyes move towards a remembered point, with no visual stimulus.
- In a sequence of predictive saccades, the eyes are kept on an object moving in a temporally and/or spatially predictive manner. In this instance, saccades often coincide (or anticipate) the regularly moving object.
Pathophysiologic saccades
Saccadic oscillations not fitting the normal function are a deviation from a healthy or normal condition. Nystagmus is characterised by the combination of 'slow phases', which usually take the eye off the point of regard, interspersed with saccade-like "quick phases" that serve to bring the eye back on target. Pathological slow phases may either be due to an imbalance in the vestibular systemVestibular system
The vestibular system, which contributes to balance in most mammals and to the sense of spatial orientation, is the sensory system that provides the leading contribution about movement and sense of balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of...
, or damage to the brainstem "neural integrator" that normally holds the eyes in place. On the other hand, opsoclonus
Opsoclonus
Opsoclonus refers to uncontrolled eye movement. Opsoclonus consists of rapid, involuntary, multivectorial , unpredictable, conjugate fast eye movements without intersaccadic intervals. It is also referred to as saccadomania or reflexive saccade...
or ocular flutter
Ocular flutter
Ocular flutter is an opsoclonic disorder in which the eyes incontinuously saccadically move around the point of fixation in the field of vision....
are composed purely of fast-phase saccadic eye movements.
Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions.
Eye movement measurements are also used to investigate psychiatric disorders. For example, ADHD is characterized by an increase of antisaccade errors and an increase in delays for visually guided saccade.
Saccade adaptation
When the brain is led to believe that the saccades it is generating are too large or too small (by an experimental manipulation in which a saccade-target steps backwards or forwards contingent on the eye movement made to acquire it), saccade amplitude gradually decreases (or increases), an adaptation (also termed gain adaptation) widely seen as a simple form of motor learning, possibly driven by an effort to correct visual error. This effect was first discovered in humans with ocular muscle weakness brought on by disease or tenectomy. In these cases, it was noticed that the patients would make hypometric (small) saccades with the affected eye, and that they were able to correct these errors over time. This led to the realization that visual error (the difference between the intended past-saccadic point of regard and the target position) played a role in the homeostatic regulation of accurate saccades. Since then, much scientific research has been devoted to various experiments employing saccade adaptation.Saccadic masking
It is a common but false belief that during the saccade, no information is passed through the optic nerve to the brain. Whereas low spatial frequencies (the 'fuzzier' parts) are attenuated, higher spatial frequencies (an image's fine details) which would otherwise be blurred out by the eye movement remain unaffected. This phenomenon, known as saccadic masking or saccadic suppression, is known to occur in the time preceding a saccadic eye movement, implying neurological reasons for the effect, rather than simply the image's motion blur. This phenomenon leads to the so-called "stopped-clock illusion", or chronostasisChronostasis
The term chronostasis refers to the illusion in which the first impression following a saccade appears to be extended in time...
.
A person may observe the saccadic masking effect by standing in front of a mirror and looking from one eye to the next (and vice versa). The subject will not experience any movement of the eyes nor any evidence that the optic nerve has momentarily ceased transmitting. Due to saccadic masking, the eye/brain system not only hides the eye movements from the individual but also hides the evidence that anything has been hidden. Of course, a second observer watching the experiment will see the subject's eyes moving back and forth. The function's main purpose is to prevent smearing of the image.
Spatial updating
When a visual stimulus is seen before a saccade, subjects are still able to make another saccade back to that image, even if it is no longer visible. This shows that the brain is somehow able to take into account the intervening eye movement. It is thought that the brain does this by temporarily recording a copy of the command for the eye movement, and comparing this to the remembered image of the target. This is called spatial updating. Neurophysiologists who have recorded from cortical areas for saccades during spatial updating have found that memory related signals get remapped during each saccade.Trans-saccadic perception
It is also thought that perceptual memory is updated during saccades so that information gathered across fixations can be compared and synthesized. However, the entire visual image is not updated during each saccade, only 3-4 features or objects if they are attended to. Some scientists believe that this is the same as visual working memory, but as in spatial updating the eye movement has to be accounted for. The process of retaining information across a saccade is called trans-saccadic memory and the process of integrating information from more than one fixation is called trans-saccadic integration.Comparative physiology
Saccades are a widespread phenomenon across animals with image-forming visual systems. They have been observed in animals across three phylaPhylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
, including animals that do not have a fovea (most vertebrates do not) and animals that cannot move their eyes independently of their head (such as insects). Therefore, while saccades serve in humans and other primates to increase the effective visual resolution of a scene, there must be additional reasons for the behavior. The most frequently suggested of these reasons is to avoid blurring of the image, which would occur if the response time of a photoreceptor is longer than the time a given portion of the image is stimulating that photoreceptor as the image drifts across the eye.
In birds, saccadic eye movements serve a further function. The avian retina is highly developed. It is thicker than the mammalian retina and has a higher metabolic activity, but it lacks proper vasculature
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
. Therefore, the retinal cells must obtain nutrients via diffusion through the choroid
Choroid
The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissue, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye , while in the outlying areas it narrows to 0.1 mm...
and from the vitreous humor. The pecten
Pecten oculi
The pecten or pecten oculi is a comb-like structure of blood vessels belonging to the choroid in the eye of a bird. It is non-sensory and is a pigmented structure that projects into the vitreous body from the point where the optic nerve enters the eyeball. The pecten is believed to both nourish the...
is a specialised structure in the avian retina. It is a highly vascular structure that projects into the vitreous humor. Experimentally, it has been shown that during saccadic eye oscillations (which occupy up to 12% of avian viewing time), the pecten acts as an agitator, propelling perfusate towards the retina. Thus, in birds, saccadic eye movements appear to be important in retinal nutrition and respiration
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
.
See also
- Eye movement
- Eye movement in language readingEye movement in language readingEye movement in reading involves visual processing of words. This was first described by the French ophthalmologist Louis Émile Javal in the late 19th century. He reported that eyes do not move continuously along a line of text, but make short rapid movements intermingled with short stops...
- Eye movement in music readingEye movement in music readingEye movement in music reading is the scanning of a musical score by a musician's eyes. This usually occurs as the music is read during performance, although musicians sometimes scan music silently to study it, and sometimes perform from memory without score. The phenomenon has been studied by...
- Frontal eye fieldsFrontal eye fieldsThe frontal eye fields is a region located in the premotor cortex, which is part of the frontal cortex of the primate brain.-Function:...
- Medial eye fieldsMedial eye fieldsMedial eye fields are areas in the frontal lobe of the primate brain that play a role in visually guided eye movement. Most Neuroscientists refer to this area as the supplementary eye fields.-See also:*Saccade*Pursuit movement*Supplementary eye fields...
- Paramedian pontine reticular formationParamedian pontine reticular formationThe paramedian pontine reticular formation, or PPRF, is part of the pontine reticular formation, a brain region without clearly defined borders in the center of the pons. It is involved in the coordination of eye movements, particularly horizontal gaze and saccades.- Input, Output, and Function...
- Saccadic suppression of image displacementSaccadic suppression of image displacementSaccadic Suppression of Image Displacement or SSID, is the phenomenon in visual perception where the brain selectively blocks visual processing during eye movements in such a way that large changes in object location in the visual scene during a saccade or blink are not detected.The phenomenon...
- Whip panWhip panA whip pan is a type of pan shot in which the camera moves sideways so quickly that the picture blurs into indistinct streaks. It is commonly used as a transition between shots, and can indicate the passage of time and/or a frenetic pace of action....