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  • Topic: Saccade, Eye, Smooth pursuit
  • Pages : 6 (1659 words )
  • Download(s) : 113
  • Published : April 1, 2013
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Slide 18.
We have been able to trace this abnormality to a disorder of velocity sensitivity, which ,in turn can be localized to a small area in the brain, the middle temporal area.

There seems to be no obvious relation between smooth pursuit eye movements and what we recognise as clinical schizophrenia. But why do impaired eye tracking and schizophrenia go together?

We do not know as yet, but are searching for the answer using the time honoured methods of neurology, a medical speciality in which disparate symptoms within a patient or among family members give clues about the neurological basis of the disease. Neurologists can make sense out of the clues because of their extensive knowledge of the anatomy and physiology of the nervous system. Smooth pursuit eye movements allow the eyes to closely follow a moving object. It is one of two ways that visual animals can voluntarily shift gaze, the other being saccadic eye movements. Pursuit differs from the vestibulo-ocular reflex, which only occurs during movements of the head and serves to stabilize gaze on a stationary object. Most people are unable to initiate pursuit without a moving visual signal. The pursuit of targets moving with velocities of greater than 30°/s tend to require catch-up saccades. Smooth pursuit is asymmetric: most humans and primates tend to be better at horizontal than vertical smooth pursuit, as defined by their ability to pursue smoothly without making catch-up saccades. Most humans are also better at downward than upward pursuit.[1] Pursuit is modified by ongoing visual feedback.

Slide 19.
There are two basic methods for recording smooth pursuit eye movements, and eye movement in general. The first is with a search coil. This technique is most common in primate research, and is extremely accurate. An eye movement shifts the orientation of the coil to induce an electrical current, which is translated into horizontal and vertical eye position. The second technique is an eye tracker. This device, while somewhat more noisy, is non-invasive and is often used in human psychophysics and recently also in instructional psychology. It relies on the infrared illumination of the pupil to track eye position with a camera. -------------------------------------------------

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Slide 20.
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Neural circuitry
The neural circuitry underlying smooth pursuit is an object of debate. The first step towards the initiation of pursuit is to see a moving target. Signals from the retina ascend through the lateral geniculate nucleus and activate neurons in primary visual cortex. Primary visual cortex sends the information about the target to the middle temporal visual cortex, which responds very selectively to directions of movement. The processing of motion in this area is necessary for smooth pursuit responses. This sensory area provides the motion signal, which may or may not be smoothly pursued. A region of cortex in the frontal lobe, known as the frontal pursuit area, responds to particular vectors of pursuit, and can be electrically stimulated to induce pursuit movements. Recent evidence suggests that the superior colliculus also responds during smooth pursuit eye movement. These two areas are likely involved in providing the GO signal to initiate pursuit, as well as selecting which target to track. The GO signal from the cortex and the superior colliculus is relayed to several pontine nuclei, including the dorsolateral pontine nuclei and the nucleus reticularis tegmenti pontisThe neurons of the pons are tuned to eye velocity and are directionally selective, and can be stimulated to change the velocity of pursuit. The pontine nuclei project to the cerebellum, specifically the vermis and the paraflocculus....
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