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. Frequency is a measure of the number of occurrences of a repeating event per unit Time. However, this article deals with saccadic eye motion.
Eye saccades are quick, simultaneous movements of both eyes in the same direction. Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain [1] Initiated by the frontal lobe of the brain (Brodmann area 8), saccades serve as a mechanism for fixation, rapid eye movement and the fast phase of optokinetic nystagmus. A Brodmann area is a region of the cortex defined based on its Cytoarchitecture, or organization of cells Fixation or visual fixation is the maintaining of the visual gaze on a location Nystagmus is a type of eye movement characterized by alternating slow phase movements in one direction and Saccade -like quick phases in the other direction [1] 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. Legal residents and citizens To be French according to the first article of the Constitution is to be a citizen of France regardless of one's origin race or religion ( Ophthalmology is the branch of Medicine which deals with the diseases and surgery of the visual pathways including the Eye, Brain Louis Émile Javal ( May 5, 1839 - January 20, 1907) was a French Ophthalmologist who was born in Paris. A mirror is an object with a surface that has good Specular reflection; that is it is smooth enough to form an Image. [2]
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Function
Humans and other animals do not look at a scene in a steady way. Instead, the eyes move around, locating interesting parts of the scene and building up a mental 'map' corresponding to the scene. One reason for saccades of the human eye is that the central part of the retina, the fovea, plays a critical role in resolving objects. The vertebrate retina is a light sensitive part inside the inner layer of the Eye. The fovea, also known as the fovea centralis, is a part of the Eye, located in the center of the Macula region of the Retina. By moving the eye so that small parts of a scene can be sensed with greater resolution, body resources can be used more efficiently. Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged
Velocity and duration
The dynamics of saccadic eye motion give insight into the complexity of the mechanism that controls the motion of the eye. The saccade is the fastest movement of an external part of the human body. The peak angular speed of the eye during a saccade reaches up to 1000 degrees per second. Saccades last from about 20 to 200 milliseconds.
The duration of a saccade depends on its amplitude. Amplitude is the magnitude of change in the oscillating variable with each Oscillation, within an oscillating system The amplitude of a saccade is the angular distance that the eye needs to travel during the movement. For amplitudes up to about 60 degrees, the duration of a saccade linearly depends on the amplitude (so called "saccadic main sequence"). In saccades larger than 60 degrees, the peak velocity remains constant at the maximum velocity attainable by the eye. In Physics, velocity is defined as the rate of change of Position. Thus, the duration of these large saccades is no longer linearly dependent on the amplitude.
In addition to the kind of saccades described above, the human eye is in a constant state of vibration, oscillating back and forth at a rate of about 60 Hz. These microsaccades are tiny movements, roughly 20 arcseconds in excursion and are completely imperceptible under normal circumstances. Microsaccades are a kind of Fixational eye movement. Theyare small jerk-like involuntary Eye movements, similar tominiature versions of voluntary Saccades A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. They serve to refresh the image being cast onto the rod cells and cone cells at the back of the eye. Without microsaccades, staring fixedly at something would cause the vision to cease after a few seconds since rods and cones only respond to a change in luminance. Luminance is a photometric measure of the density of Luminous intensity in a given direction
Pathophysiologic saccades
Saccadic oscillations not filling the normal function are a deviation from a healthy or normal condition.
Causes
- Nystagmus is characterised by the combination of a smooth pursuit, which usually acts to take the eye off the point of regard, interspersed with the saccadic movement that serves to bring the eye back on target. Nystagmus is a type of eye movement characterized by alternating slow phase movements in one direction and Saccade -like quick phases in the other direction Pursuit movement is the ability of the Eyes to smoothly follow a moving object
- Opsoclonus or ocular flutter, on the other hand, are composed purely of fast-phase saccadic eye movements. Opsoclonus refers to uncontrolled Eye movements. Opsoclonus consists of rapid involuntary multivectorial (horizontal and vertical unpredictable conjugate fast eye movements Ocular flutter is an opsoclonic disorder in which the Eyes incontinuously saccadically move around the point of fixation in the field of vision.
Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions.
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 back 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 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. Saccadic masking, also known as visual saccadic suppression, is the phenomenon in Visual perception where the mind selectively blocks visual processing during 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.
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.
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 (most vertebrates do not) and animals that cannot move their eyes independently of their head (such as insects). A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class. [3] 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. Photoreceptor can refer to In anatomy/cell biology Photoreceptor cell: a photosensitive cell most commonly referring to a specialized type of neuron
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. The blood vessels are part of the Circulatory system and function to transport Blood throughout the body Therefore, the retinal cells must obtain nutrients via diffusion through the choroid and from the vitreous humor. The choroid, also known as the choroidea or choroid coat, is the vascular layer of the Eye lying between the Retina and the Sclera The pecten is a specialised structure in the avian retina. The pecten or pecten oculi is a Vascular section of the Choroid in the Eye of a Bird. 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. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called [4]
See also
- Eye movement
- Eye movement in language reading
- Eye movement in music reading
- Frontal eye fields
- Medial eye fields
- Paramedian pontine reticular formation
- Saccadic suppression of image displacement
- Whip pan
References
- ^ a b Cassin, B. The study of eye movement in language reading stretches back almost a thousand years Eye movement in music reading is the scanning of a musical score by a musician's eyes The frontal eye fields (FEF is a region located in the Premotor cortex, which is part of the frontal cortex of the primate Brain. Medial eye fields are areas in the Frontal lobe of the primate Brain that play a role in visually guided Eye movements. The paramedian pontine reticular formation, or PPRF, is part of the pontine Reticular formation, a Brain region without clearly defined borders in the center Saccadic Suppression of Image Displacement or SSID is the phenomenon in Visual perception where the brain selectively blocks visual processing during Eye movements A whip pan is a type of pan shot in which the camera moves sideways so quickly that the picture blurs into indistinct streaks and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990
- ^ Javal, É 'Essai sure la physiologie de la lecture', in Annales d'oculistique 80, pp. 61–73, 1878
- ^ Land, MF. "Motion and vision: why animals move their eyes". J Comp Physiol A. 1999 185:341–352.
- ^ Pettigrew JD, Wallman J. "Saccadic oscillations facilitate ocular perfusion from the avian pecten". Nature. 1990 Jan 25; 343(6256): 362–3 PMID 14756148.
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