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Lecture 5

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Lecture 5 Visual System - everything begins in the retina – part of the central nervous system => an outgrowth of the brain - retina divided down the centre, each half sends its own information => nasal portion closest to the nose => temporal portion closest to the outside of the head - tract => pathway inside the central nervous system - nerve => pathway outside the central nervous system o distinguishing bw inside and outside CNS Optic chiasm: crossover point for the two optic nerves Optic nerve: moving away from the eye; optic tract: moving closer toward the brain - temporal portion of retina come together at the optic chiasm and separate out toward the outside; continues onto the same side of the brain => vision as seen - nasal portion fibres cross at optic chiasm and move to opposite sides of the brain - in order to go through the tiny opening of the iris, objects on the right side of the visual field will go through to the left side of each retina through the iris => nasal side of right eye and temporal portion of left eye - from the left visual field, the object will hit the nasal side of the left eye, and temporal portion of right eye - the nerve fibres from the nasal portion of the right eye and temporal portion of the left eye both end up in the left cortex (for info from the right visual field) => nerve from nasal portion of right eye has to cross over - nerve fibres from nasal portion of the left eye and temporal portion of the right eye both end up in the right cortex Visual field: everything seen with the eyes - deficit in the right visual field => not problem w the right eye; problem w left cortex => blindness is in the right visual field, of both eyes 1 Levels of brain anatomy: - nervous system in the retina is the entry point => cells in the retina get excited when a spot of light hits a neuron - by area 17 of the cortex, neurons can take info from the retina and put it together => neurons in area 17 respond to particular lines of a specific orientation – will not respond to lines of other orientations - by association cortex => higher levels of processing – neurons respond to much more complex bits of info => respond to objects, faces, hands - pic on the right hand side is a map showing what a person can see (the visual field) 1. normal => inside the circle everything is white 2. cannot see anything in the right visual field or the left visual field of the right eye=> eye severed, removed, etc 3. splitting the crossover point => blindness in left visual field of the left eye and the right visual field of the right eye *if the drawing was the retina, the exact opposite would occur – since its the visual field its opposite from the retina => this problem can only be picked up from closing one eye at a time, bc vision may appear in tact => visual maps are important in diagnosis *don’t need to know #6 - complete damage to right side of occipital cortex (posterior cerebral artery stroke) => complete blindness on opposite visual field - incomplete damage – damage in a portion of the opposite visual field 7. macular sparing – spare the centre of the visual field Scotoma: small area of blindness caused by a small area of cortical damage => most of the time don’t notice, bc the brain constructs the image - natural blind spot in everybody – area where the optic
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