Components of the Visual System
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Transcript of Components of the Visual System
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Components of the Visual System
• Eyes
• Visual pathways (eye to brain)
• Visual centers of the brain
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Compound & Simple Eyes
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The Diversity of Eyes
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What is light a valuable thing to sense?
• It travels essentially instantaneously through air.
• EM radiation propagates rectilinearly.
• Light is pervasive on earth.
• Light provides differential information about many terrestrial objects.
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Placement of Eyes
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Eye movements
• Head movements
• Saccades
• Tremor
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Eye Muscles
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Protection of the eye
• Encased in the optic orbit
• Surrounded by fat
• Eyelid covers eye
• Blinking cleans and moistens the eye
• Blink reflex
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Layers of the eye
• Fibrous tunic (sclera)– 1mm thick– Fibrous to contain internal pressure of eye
• Vascular tunic– Dark choroid tissue 0.2 mm thick– Dark color prevents light scatter
• Retina
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Anterior chamber
• Contains aqueous humor– Nourishes the cornea and lens– Under pressure– Glaucoma is excessively high pressure in eye
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• Iris– Two layers
• Pigmented• Vascular
• Pupil– Two sets of muscles: circular & radial– Varies in size:
• 2-8 mm in young adults (16-fold variation in light)• 5-2 mm in elderly adults
– Variations in size influence depth of field (cf. p. 44)
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• Crystalline lens– Capsule
• Modulates flow of aqueous humor• Modulates shape of lens
– Epithelial layer– Lens proper
• Grows constantly, quadrupling in size by 90 years of age.
• Subject to hardening and opacities (cataracts)• Brunescence (yellowing)
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• Vitreous chamber– Contains vitreous gel-like substance– Not renewed, so can contain floaters.
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Human & Turtle Retinas
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Macular degeneration
• Leading cause of impaired vision in industrialized nations.
• Can sometimes be arrested by laser surgery.
• One of the few health risks where African Americans have reduced rates over other racial groups.
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Diabetes
• Causes cataracts
• Growth of blood vessels in eyes.
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Objects structure light
• Objects absorb and reflect light.– Highly reflective surfaces appear light.– Poorly reflective surfaces appear dark.
• Reflectance indicates – continuities and discontinuities– texture
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Light structure is usable only if
• Light must reach the retina (~50% passes through cornea)
• The image cast on the retina must be focused and not blurred.
• The structural relations among points of light must be preserved.
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Image formation in the eye
• Optical power of cornea and crystalline lens– Variable due to change in shape of cornea
and crystalline lens
• Shape of eye
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• Presbyopia – Inability to accommodate
• Astigmatism – Irregularities in the surface of the cornea
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Photoreceptors
• Rods– ~100 million
• Cones– ~5 million
• No new cells are formed, but parts are.
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• Most fish, frogs, turtles and birds have 3-5 types of cones.
• Most mammals have only two types of cones.
• Primates have three types of cones.
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Movie made by Carlos Rozas (CanalWeb, Chile).
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• Both rods and cones contain photopigments
• Each photopigment has two parts– Opsin
• Comes in three forms
– Retinal (vitamin A derivative)• Isomerizes when it absorbs light• Isomerization slows spontaneous dark current
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Phototransduction (1 msec)
1. Retinal isomerizes when it absorbs light
2. Isomerization releases all-trans retinal which eventually decreases cGMP concentration.
3. Lower cGMP concentration lets ion channels at the synapse close.
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• Rods have one type of opsin
• Cones have three different types– 440 nanometers (aka S-cone, blue cone)
• Wavelength of light looks violet
– 530 nanometers (aka M-cone, green cone)• Wavelength of light looks green
– 560 nanometers (aka L-cone, red cone)• Wavelength of light looks yellow
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Mammalian color processing
• Blue cones are found in the retinas of most species, hence appear to be oldest in evolutionary terms.
• Two cone retinas generally have blue and green, indicating that green is next oldest in evolutionary terms.
• Primates have three cone types, indicating that the red cones are the most recent in evolutionary terms.
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