Intro arrow 7. Eye Movements arrow 7.2 Saccadic System
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7.2 The saccadic system

 The saccadic eye movement: The purpose of saccades is to move the eyes as quickly as possible, so that the point of interest will be centered on the fovea. They can be made not only towards visual targets, but also towards auditory or tactile stimuli, memorized objects…
Saccades are the fastest type of eye movement, reaching maximum velocities up to 500 degrees per second and are usually completed in tens of milliseconds. Despite their speed, saccade trajectories tend to be remarkably stereotyped both within and across individuals. The duration and peak velocity of saccades increase monotonically with amplitude of the movement in a consistent way which has been called the “main sequence”.
Saccade latency is defined as the delay encountered between the presentation of the stimulus and the onset of the saccade. Average latency for saccade is about 200 ms.
 Neuroanatomy of the saccadic system: The pontine and mesencephalic burst circuits provide the necessary motor signals to drive the muscles for saccades. However, eye movements are a component of the cognitive behavior of higher mammals and the decision when and where to make a saccade is usually make in the cerebral cortex when that saccade is important for visual behavior. The cortex ordinarily controls the saccadic system through the superior colliculus. The superior colliculus is a multilayered (7 layers) structure in the midbrain. It can be divided in two functional regions: the 3 superficial layers and the 4 intermediate and deep layers. The 3 superficial layers receive both direct input from the retina and a projection from striate cortex for the entire contralateral visual hemifield. Neurons of these layers answer to visual stimuli and retinotopically code the information about the contralateral visual field. In the intermediate and deep layers cell activity is primarily related to oculomotor actions, but also have multisensory responses. These cells receive visual information from prestriate, middle temporal, and parietal cortices and motor information from the frontal eye field. These layers also contain representations of the body surface and of the locations sound in space.
The substantia nigra pars reticulata sends a powerful inhibitory projection to the superior colliculus, which must be suppressed before the superior colliculus can drive a saccade.
 
 
 

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