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Lecture 6

Lecture 6

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University of Toronto Scarborough
Matthias Niemeier

PSYB51 nd October 22 , 2010 Lecture 6: - we need exactly 6 muscles to control eye movement - So there are 3 dimensions to locate an object in space - Also the orientation of the object in space (3 possible axis for orientation = x-,y-,z-axis) [So axis on which an object could rotate – 3 possible axis] So an object can be described in terms of 6 dimensions: 3 for location & 3 for orientation/rotation Eye movements by itself isn’t perception but they are very significant FOR perception - So why 6 muscles?  You need 2 muscles: 1 for pulling back while another for the pulling in opposite direction (to rotate on one axis) REMEMBER: MUSCLES ONLY CONTRACT OR RELAX (2 MOVEMENTS!) So we have 3 rotation dimensions & therefore 6 muscles! So for every rotation dimension –> we have 2 muscles! - Muscles controlled by the brain indirectly - These cranial nerves are named after the nuclei out of which the axons grow e.g. Ocumotor nucleus [in the brain stem] fans out axons that form the cranial nerve # 3 (also called the ocumotor nerve) - 4 out of 6 muscles are innervated by THE OCULOMOTOR NERVE while the TROCHLEAR NERVE (CRANIAL #4) innervates the muscle of the eye on the opposite side (superior oblique) & the ABDUCENS NERVE innervates the (lateral rectus) muscle. PSYB51 nd October 22 , 2010 Superior colliculus: eye movements can be controlled through this short-cut So visual input not only going through the primary pathway we talked about in previous lectures but also directly going into the Superior colliculus and from there directly into motor control regions within the Superior colliculus and down to the muscles = this way you can make very quick eye movements Superior colliculus: A structure in the midbrain that is important in initiating and guiding eye movements. How does a smooth pursuit eye movement help us perceive moving objects? In smooth pursuit eye movements, the eyes smoothly track a moving object, allowing better extraction of visual detail from the object as well as providing an estimate of the object’s velocity. - Saccades: performed when reading for example - Fixational eye movements: what you show when you try to keep your eyes stable - Micro saccades: eye shifts a little all the time even though you try and keep the eye stable - What is the role of the vergence eye movements? Vergence eye movements are helpful when one is focussing on a near or far object. In this type of eye movement the two eyes move in opposite directions: they either diverge outward, or converge inward. [they do the movements together = both the eyes move together not independently] PSYB51 October 22 , 2010 Function of smooth eye...: B/c something that is stable on the retina will be perceived better as the photoreceptor are SLOW in the retina = it takes a certain amount of light stimulating them  So keep the eyes stable for some time to “catch” enough photons So why don’t we notice making eye movements? Why don’t we don’t really perceive these eye movements? Why is saccadic suppression important in motion perception? Saccadic suppression is important in motion perception because it eliminates the smear from retinal image motion during an eye movement. Saccadic suppression results in a brief reduction of visual sensitivity while the eye makes a saccadic eye movement. What kind of problem does the Duringcomparator solve? movement – we have reduced contrast perception and reduced motion perception  This helps us avoid/ignore retinal image smear PSYB51 October 22 , 2010 What kind of problem does the comparator solve? The comparator solves the problem of an object in motion appearing stationary because movement of the object on the retina is removed via smooth pursuit eye movements. When an eye movement is planned, one copy of the movement command goes to the eye muscles, and the other goes to an area of the visual system called the comparator. The comparator compensates for retinal image changes caused by the eye movement, inhibiting any attempt by other parts of the visual system to interpret the changes as object motion. - Efference copy = basically a motor command of the brain; actually sent to the superior colliculus and then passed on further to eye muscles. This motor command in not only sent to the eye muscles but it’s also sent back to the brain! [=so copy of the motor command] Efference copies are like assimilated sensory signals that inform the brain about what the eye muscles will most likely be doing. You send out a command for the eye muscles to contract, so the brain is informing itself that it send out this command. So that it can predict that the eye muscles within the next few seconds, will contract. - So the brain uses the efference copy (= corollary discharge signal) to basically ignore sense information that is not really interesting
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