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. However, this article deals with saccadic eye motion. With respect to the eye, saccades are quick, simultaneous movements of both eyes in the same direction. 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. 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 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. By moving the eye so that small parts of a scene can be sensed with greater resolution, body resources can be used more efficiently.

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. 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 that range, the peak velocity of a saccade linearly depends on the amplitude. In saccades larger than 60 degrees, the peak velocity remains constant at the maximum velocity attainable by the eye. 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 per second. These microsaccades are tiny movements, roughly 20 arcseconds in excursion and are completely imperceptible under normal circumstances. 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.

Pathophysiologic saccades
Saccadic oscillations not filling the normal function are a deviation from a healthy or normal condition.

Causes
Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions.
 * 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.
 * Opsoclonus or ocular flutter, on the other hand, are composed purely of fast-phase saccadic eye movements.

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.

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). 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.