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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