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- Main article: Eye movements
Saccadic eye motion
The purpose of saccades can be illustrated by the human eye. Humans 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. In the human eye, one reason for saccades is that only the central part of the retina, the macula, 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. Thus, the macula 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 efficiently. If an entire scene were viewed in high resolution, the diameter of the optic nerve would need to be larger than the diameter of the eyeball itself. Subsequent processing of such a high-resolution image would require a brain many times larger than its current size.
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 a particular saccade. For amplitudes of up to about 60 degrees, the duration of a saccade linearly depends on the amplitude. In that range, the peak velocity of a saccade also linearly depends on the amplitude. In saccades larger than about 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.
- Main article: 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 the detection of modulations in low spatial frequencies, contrast and brightness are well preserved, or even enhanced during saccades. It is true, however, that modulations in high spatial frequencies and color are suppressed in association with 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 his or her gaze to the right eye, then back to the left, and so on. Movement of the eyes will not been seen, 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, it hides evidence that anything has been hidden.
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. The retinal cells must therefore 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. It has been shown experimentally 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.
1. 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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