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A saccade ( , French for ''jerk'') is a quick, simultaneous movement of both
eyes Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conve ...
between two or more phases of fixation in the same direction.Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company, 1990. In contrast, in smooth pursuit movements, the eyes move smoothly instead of in jumps. The phenomenon can be associated with a shift in frequency of an emitted signal or a movement of a body part or device. Controlled cortically by the
frontal eye fields The frontal eye fields (FEF) are a region located in the frontal cortex, more specifically in Brodmann area 8 or BA8, of the primate brain. In humans, it can be more accurately said to lie in a region around the intersection of the middle fronta ...
(FEF), or subcortically by the
superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form ''tectal'' is commonly ...
, saccades serve as a mechanism for fixation, rapid eye movement, and the fast phase of
optokinetic nystagmus The optokinetic response is a combination of a slow-phase and fast-phase eye movements. It is seen when an individual tracks (pursuit movement) a moving object with their eyes, which then moves out of the field of vision, a point at which their ...
. 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.


Function

Humans and many animals do not look at a scene in fixed steadiness; 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 Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
, that often relies upon detection of angular movement on the retina). When scanning immediate surroundings or reading, human eyes make saccadic movements and stop several times, moving very quickly between each stop. The speed of movement during each saccade cannot be controlled; the eyes move as fast as they are able. One reason for the saccadic movement of the human eye is that the central part of the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
—known as the
fovea Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure. Human anatomy *Fovea centralis of the retina * Fovea buccalis or Dimple * Fovea of the femoral head * Trochlear fovea of the fr ...
—which provides the high-resolution portion of vision is very small in humans, only about 1–2 degrees of vision, but it plays a critical role in resolving objects. By moving the eye so that small parts of a scene can be sensed with greater
resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual mak ...
, body resources can be used more efficiently.


Timing and kinematics

Saccades are one of the fastest movements produced by the human eye (
blink Blinking is a bodily function; it is a semi-autonomic rapid closing of the eyelid. A single blink is determined by the forceful closing of the eyelid or inactivation of the levator palpebrae superioris and the activation of the palpebral portio ...
s may reach even higher peak velocities). The peak
angular speed Angular may refer to: Anatomy * Angular artery, the terminal part of the facial artery * Angular bone, a large bone in the lower jaw of amphibians and reptiles * Angular incisure, a small anatomical notch on the stomach * Angular gyrus, a regio ...
of the eye during a saccade reaches up to 700°/s in humans for great saccades (25° of visual angle); in some monkeys, peak speed can reach 1000°/s. 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 activates the eye muscles more directly. Specific pre-target oscillatory (
alpha rhythm Alpha waves, or the alpha rhythm, are neural oscillations in the frequency range of 8–12 Hz likely originating from the synchronous and coherent (in phase or constructive) electrical activity of thalamic pacemaker cells in humans. Historica ...
s) and transient activities occurring in posterior-lateral
parietal cortex The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus. The parietal lobe integrates sensory informa ...
and
occipital cortex The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head". The occipital lobe is the vi ...
also characterise express saccades. The amplitude of a saccade is the angular distance the eye travels during the movement. For amplitudes up to 15 or 20°, the velocity of a saccade linearly depends on the amplitude (the so-called ''saccadic main sequence'', a term borrowed from
astrophysics Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline said, Astrophysics "seeks to ascertain the nature of the h ...
; see Figure). For amplitudes larger than 20°, the peak velocity starts to plateau (nonlinearly) toward the maximum velocity attainable by the eye at around 60°. For instance, a 10° amplitude is associated with a velocity of 300°/s, and 30° is associated with 500°/s."Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 179 Encyclopædia Britannica, 1987 Therefore, for larger amplitude ranges, the main sequence can best be modeled by an inverse
power law In statistics, a power law is a Function (mathematics), functional relationship between two quantities, where a Relative change and difference, relative change in one quantity results in a proportional relative change in the other quantity, inde ...
function. The high peak velocities and the main sequence relationship can also be used to distinguish
micro- ''Micro'' (Greek letter μ ( U+03BC) or the legacy symbol µ (U+00B5)) is a unit prefix in the metric system denoting a factor of 10−6 (one millionth). Confirmed in 1960, the prefix comes from the Greek ('), meaning "small". The symbol for th ...
/saccades from other
eye movements Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms (e.g. primates, rodents, flies, birds, fish, cats, crabs, octopus) to fixate, inspect and track visual objects of inte ...
(like
ocular tremor Ocular tremor (ocular microtremor) is a constant, involuntary eye tremor of a low amplitude and high frequency. It is a type of fixational eye movement that occurs in all normal people, even when the eye appears still. The frequency of ocular mic ...
,
ocular drift Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...
, and
smooth pursuit In the scientific study of vision, smooth pursuit describes a type of eye movement in which the eyes remain fixated on a moving object. It is one of two ways that visual animals can voluntarily shift gaze, the other being saccadic eye movemen ...
). Velocity-based
algorithm In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific Computational problem, problems or to perform a computation. Algorithms are used as specificat ...
s are a common approach for saccade detection in
eye tracking Eye tracking is the process of measuring either the point of gaze (where one is looking) 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 ...
. Although, depending on the demands on timing accuracy, acceleration-based methods are more precise. 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 Listing's law, named after German mathematician Johann Benedict Listing (1808–1882), describes the three-dimensional orientation of the eye and its axes of rotation. Listing's law has been shown to hold when the head is stationary and upright and ...
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 (VOR) causes the eyes to roll back in the head to keep gaze on the target. Since the VOR can actually rotate the eyes around the line of sight, combined eye and head movements do not always obey
Listing's law Listing's law, named after German mathematician Johann Benedict Listing (1808–1882), describes the three-dimensional orientation of the eye and its axes of rotation. Listing's law has been shown to hold when the head is stationary and upright and ...
.


Types

Saccades can be categorized by intended goal in four ways: # In a ''visually guided saccade'', the eyes move toward a visual transient, or stimulus. The parameters of visually guided saccades (amplitude, latency, peak velocity, and duration) are frequently measured as a baseline when measuring other types of saccades. Visually guided saccades can be further subcategorized: #:* A ''reflexive saccade'' is triggered exogenously by the appearance of a peripheral stimulus, or by the disappearance of a fixation stimulus. #:* A ''scanning saccade'' is triggered endogenously for the purpose of exploring the visual environment. # 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 toward 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 with (or anticipate) the predictable movement of an object. As referenced to above, it is also useful to categorize saccades by latency (time between go-signal and movement onset). In this case the categorization is binary: Either a given saccade is an ''express saccade'' or it is not. The latency cut-off is approximately ~200 ms; any longer than this is outside the express saccade range.
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 (of at least several seconds), ...
s are a related type of
fixational eye movement Fixation or visual fixation is the maintaining of the gaze on a single location. An animal can exhibit visual fixation if it possess a fovea in the anatomy of their eye. The fovea is typically located at the center of the retina and is the poin ...
that are small, jerk-like, involuntary
eye movements Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms (e.g. primates, rodents, flies, birds, fish, cats, crabs, octopus) to fixate, inspect and track visual objects of inte ...
, similar to miniature versions of voluntary saccades. They typically occur during visual fixation, not only in humans, but also in animals with
foveal The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina. The fovea is responsible for sharp central vision (also called foveal vision), which is ...
vision (primates, cats, etc.). Microsaccade amplitudes vary from 2 to 120
arcminute A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...
s.


In depth

When exploring the visual environment with the gaze, humans make two to three fixations a second. Each fixation involves binocularly coordinated movements of the eyes to acquire the new target in three dimensions: horizontal and vertical, but also in-depth. In literature it has been shown how an upward or a vertical saccade is generally accompanied by a divergence of the eyes, while a downward saccade is accompanied by a convergence. The amount of this ''intra-saccadic'' ''vergence'' has a strong functional significance for the effectiveness of binocular vision. When making an upward saccade, the eyes diverged to be aligned with the most probable uncrossed disparity in that part of the visual field. On the other way around, when making a downward saccade, the eyes converged to enable alignment with crossed disparity in that part of the field. The phenomenon can be interpreted as an adaptation of rapid binocular eye movements to the statistics of the 3D environment, in order to minimize the need for corrective vergence movements at the end of saccades.


Pathophysiologic saccades

Saccadic oscillations not fitting the normal function are a deviation from a healthy or normal condition.
Nystagmus Nystagmus is a condition of involuntary (or voluntary, in some cases) eye movement. Infants can be born with it but more commonly acquire it in infancy or later in life. In many cases it may result in reduced or limited vision. Due to the invol ...
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 be due to either an imbalance in the
vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes ...
or damage to the
brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...
"neural integrator" that normally holds the eyes in place. On the other hand,
opsoclonus Opsoclonus refers to uncontrolled, irregular, and nonrhythmic eye movement. Opsoclonus consists of rapid, involuntary, multivectorial (horizontal and vertical), unpredictable, conjugate fast eye movements without inter-saccadic intervals. It is a ...
or
ocular flutter Ocular flutter is an opsoclonic disorder that results in horizontal saccades. It is caused by damage to the brainstem paramedian pontine reticular formation cells or the cerebellar neurons controlling those cells. See also * Nystagmus * Opsoclo ...
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 Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterised by excessive amounts of inattention, hyperactivity, and impulsivity that are pervasive, impairing in multiple contexts, and otherwise age-inapp ...
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 backward or forward 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 observed in humans with ocular muscle palsy. 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 or retinal error (the difference between the post-saccadic point of regard and the target position) played a role in the homeostatic regulation of saccade amplitude. Since then, much scientific research has been devoted to various experiments employing saccade adaptation.


Reading

Saccadic eye movement allows the mind to read quickly, but it comes with its disadvantages. It can cause the mind to skip over words because it does not see them as important to the sentence, and the mind completely leaves it from the sentence or it replaces it with the wrong word. This can be seen in "Paris in thethe Spring". This is a common psychological test, where the mind will often skip the second "the", especially when there is a line break in between the two. When speaking, the mind plans what will be said before it is said. Sometimes the mind is not able to plan in advance and the speech is rushed out. This is why there are errors like mispronunciation, stuttering, and unplanned pauses. The same thing happens when reading. The mind does not always know what will come next. This is another reason that the second "the" can be missed.


Vision


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) that would otherwise be blurred by the eye movement remain unaffected. This phenomenon, known as ''saccadic masking'' or ''saccadic suppression'', is known to begin prior to saccadic eye movements in every primate species studied, 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
chronostasis Chronostasis (from Greek χρόνος, chrónos, "time" and στάσις, stásis, "standing") is a type of temporal illusion in which the first impression following the introduction of a new event or task-demand to the brain can appear to be ext ...
. 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 or 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 an otherwise significant smearing of the image. (You can experience your eye saccade movements by using your cellphone's front-facing camera as a mirror, hold the cellphone screen a couple of inches away from your face as you saccade from one eye to the other—the cellphone's signal processing delay allows you to see the end of the saccade movement.)


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, having 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. 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 phyla, including animals that do not have a
fovea Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure. Human anatomy *Fovea centralis of the retina * Fovea buccalis or Dimple * Fovea of the femoral head * Trochlear fovea of the fr ...
(most vertebrates do) 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 cell A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...
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, has a higher metabolic activity, and has less
vasculature The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
obstruction, for greater visual acuity. Because of this, the retinal cells must obtain nutrients via diffusion through the
choroid The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye, and contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear ...
and from the
vitreous humor The vitreous body (''vitreous'' meaning "glass-like"; , ) is the clear gel that fills the space between the lens and the retina of the eyeball (the vitreous chamber) in humans and other vertebrates. It is often referred to as 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. Experiments show that, during saccadic eye oscillations (which occupy up to 12% of avian viewing time), the
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 a non- sensory, pigmented structure that projects into the vitreous humor from the point where the optic nerve enters the ...
acts as an agitator, propelling perfusate (natural lubricants) toward the retina. Thus, in birds, saccadic eye movements appear to be important in retinal nutrition and
cellular respiration Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
.


See also

*
Chronostasis Chronostasis (from Greek χρόνος, chrónos, "time" and στάσις, stásis, "standing") is a type of temporal illusion in which the first impression following the introduction of a new event or task-demand to the brain can appear to be ext ...
*
Eye movement Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms (e.g. primates, rodents, flies, birds, fish, cats, crabs, octopus) to fixate, inspect and track visual objects of interest ...
* Eye movement in language reading *
Eye movement in music reading Eye 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. The phenomenon has been studied by r ...
*
Eye tracking Eye tracking is the process of measuring either the point of gaze (where one is looking) 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 ...
*
Frame rate Frame rate (expressed in or FPS) is the frequency (rate) at which consecutive images (frames) are captured or displayed. The term applies equally to film and video cameras, computer graphics, and motion capture systems. Frame rate may also be ca ...
*
Frontal eye fields The frontal eye fields (FEF) are a region located in the frontal cortex, more specifically in Brodmann area 8 or BA8, of the primate brain. In humans, it can be more accurately said to lie in a region around the intersection of the middle fronta ...
*
List of cognitive biases Cognitive biases are systematic patterns of deviation from norm and/or rationality in judgment. They are often studied in psychology, sociology and behavioral economics. Although the reality of most of these biases is confirmed by reproducible ...
*
Supplementary eye field Supplementary eye field (SEF) is the name for the anatomical area of the dorsal medial frontal lobe of the primate cerebral cortex that is indirectly involved in the control of saccadic eye movements. Evidence for a supplementary eye field was ...
*
Medial eye fields Medial 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. Eye fields are divided into two hemispheres regulated by ...
*
Paramedian pontine reticular formation The paramedian pontine reticular formation, also known as PPRF or paraabducens nucleus, 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 ...
*
Raster scan A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image s ...
*
Saccadic masking Saccadic masking, also known as (visual) saccadic suppression, is the phenomenon in visual perception where the brain selectively blocks visual processing during eye movements in such a way that neither the motion of the eye (and subsequent motion ...
*
Saccadic suppression of image displacement Saccadic suppression of image displacement (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 sacca ...
*
Smooth pursuit In the scientific study of vision, smooth pursuit describes a type of eye movement in which the eyes remain fixated on a moving object. It is one of two ways that visual animals can voluntarily shift gaze, the other being saccadic eye movemen ...
*
Transsaccadic memory Transsaccadic memory is the neural process that allows humans to perceive their surroundings as a seamless, unified image despite rapid changes in fixation points. Transsaccadic memory is a relatively new topic of interest in the field of psychology ...
*
Whip pan A whip pan is a type of pan shot in which the camera pans 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 or a frenetic pace of action. Much like th ...


References

{{Visual system Ethology Eye Vision Articles containing video clips