Saccade
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- For the anti-ship missile, see C-802.
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.
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[edit] Saccadic eye motion
In regards to the eye, saccades are quick, simultaneous movements of both eyes in the same direction [1]. Initiated by the frontal lobe of the brain (Brodmann area 8), saccades serve as a mechanism for fixation, refixation, rapid eye movements and the fast phase of optokinetic nystagmus [1].
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 only the central part of the retina, the fovea, has a high concentration of color sensitive photoreceptor cells called cone cells. The rest of the retina is mainly made up of monochrome photoreceptor cell called rod cells, which are especially good for motion detection. Consequently, the fovea makes up the high-resolution central part the of human 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.
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. 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.
[edit] Saccadic masking
It is a common but false belief that during the saccade, no information is passed through the optic nerve to the brain. Research has shown that some modulations of low spatial frequencies (contrast and brightness) are well-preserved or even enhanced. However, it is true that detailed imaging is suppressed during saccades. This phenomenon, known as saccadic masking or saccadic suppression, is known to occur in the time preceding a saccadic eye movement, suggesting that there are neurological functions at work rather than simple mechanical or optical functions, such as retinal blur.
A person may observe the saccadic masking effect by standing 18 inches away from a mirror and viewing his or her own left eye, then switching attention to the right eye, then back to the left and so on. 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 evidence that anything has been hidden. Of course, a second observer watching the experiment will see the subject's eyes moving back and forth in their sockets.
[edit] Comparative physiology
In birds, saccadic eye movements serve a different or at least additional 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 which 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.
[edit] See also
[edit] References
- Pettigrew JD, Wallman J. "Saccadic oscillations facilitate ocular perfusion from the avian pecten". Nature. 1990 Jan 25; 343(6256): 362-3 PMID 14756148.
