Chapter 5 Vision

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Chapter 5 Vision

• Module 5.1

• Visual Coding

• Module 5.2

• How the Brain Processes Visual Information

Module 5.5
Parallel Processing in the Visual Cortex
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Sensation and PerceptionImportant Vocabulary
Terms

• Sensation is the process of receiving,
  transducing, and coding stimulus energy in the
  world.
• Stimulus energy is physical energy, such as
  light, sound, heat
• Sense organs such as eyes, ears, skin receive
  energy

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Sensation and PerceptionImportant Vocabulary
Terms

• Reception the absorption of physical energy by
  receptors
• Transduction the conversion of physical energy
  into electrochemical energy

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Sensation and PerceptionImportant Vocabulary
Terms

• Perception the brains process of organizing and
  interpreting sensory information to give it
  meaning
• Coding a one-to-one communication between an
  aspect of a physical stimulus and an aspect of
  nervous system activity

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Neuroanatomy Handout 4 The Visual System

• Sclera (A)
• Outermost layer of eye white, fibrous,
  protective globe
• Cornea (A1) Clear disk at front of eye focuses
  light rays onto the receptor cells at back of eye
  (retina)
• Aqueous humor (B) Fluid in the very front of the
  eyeball refracts light rays onto retina

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Neuroanatomy Handout 4 The Visual System

• Iris (C) Colored part in center of eye ring
  of muscles that controls amount of light that
  gets into eye
• Pupil (D) Opening in center of eye (appears
  black) constricts or dilates with movement of
  iris

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Neuroanatomy Handout 4 The Visual System

• Lens (E) Along with cornea, focuses light onto
  receptor cells
• Vitreous humor (F) Fluid filling majority of
  eyeball refracts lightwaves
• Retina (G) lined with visual receptor cells,
  rods and cones

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Neuroanatomy Handout 4 The Visual System

• Rods (I) - most abundant in the periphery of the
  eye
• 120 million per retina
• respond to faint light
• Cones (J) - most abundant in and around the fovea
• 6 million per retina
• essential for color vision, which requires bright
  light
• Photopigments chemicals released by rods and
  cones when struck by light

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Neuroanatomy Handout 4 The Visual System

• The fovea (G2) central portion of the human
  retina which allows for acute and detailed
  vision.
• Packed tight with receptor cells

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Neuroanatomy Handout 4 The Visual System

• Rods and cones are located in the outmost layer
  of the eye.
• They communicate their messages to neurons called
  bipolar cells (K) and horizontal cells, which are
  located closer to the center of the eye.
• Bipolar cells send messages to ganglion cells (L)
  and amacrine cells that are even closer to the
  center of the eye.

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Neuroanatomy Handout 4 The Visual System

• Axons of ganglion cells (L1) join one another to
  form the optic nerve.
• The optic nerve (L2) exits through the back of
  the eye and travels to the brain.

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Neuroanatomy Handout 4 The Visual System

• Blind spot (G1) The point at which the optic
  nerve leaves the back of the eye
• it contains no receptor cells
• It does contain retinal veins and arteries (H)

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Visual Coding and the Retinal Receptors

• Perception of color is dependent upon the
  wavelength of the light.

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Visual Coding and the Retinal Receptors

• Visible wavelengths depend upon species
  receptors.
• Human range 350 nanometers (violet, short
  wavelength) to 700 nanometers (red, long
  wavelength).
• Some species can detect ultraviolet light

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Visual Coding and the Retinal Receptors

• Discrimination among colors depends upon the
  combination of responses by different neurons.
• Two major interpretations of color vision
• Trichromatic theory/Young-Helmholtz theory
• Opponent-process theory

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Visual Coding and the Retinal Receptors

• Trichromatic theory - Color perception occurs
  through the ratio of activity across the three
  types of cones
• short wavelength
• medium-wavelength
• long-wavelength

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Visual Coding and the Retinal Receptors

• More intense light increases the brightness of
  the color but does not change the ratio and thus
  does not change the perception of the color
  itself.

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Visual Coding and the Retinal Receptors

• The opponent-process theory (Ewald Hering)
  suggests that we perceive color in terms of
  paired opposites.
• white/black
• red/green
• yellow/blue
• A possible mechanism for the theory is that
  bipolar cells are excited by one set of
  wavelengths and inhibited by another.

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Negative color afterimage
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Visual Coding and the Retinal Receptors

• The opponent-process and trichromatic theories
  cant explain
• Color constancy, the ability to recognize color
  despite changes in lighting.
• Retinex theory suggests the cortex compares
  information from various parts of the retina to
  determine the brightness and color for each area.

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Visual Coding and the Retinal Receptors

• Color vision deficiency impairment in perceiving
  color differences
• X-linked trait
• Causes
• lack of a type of cone
• cone has abnormal properties
• Most common form difficulty distinguishing
  between red and green

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The Neural Basis of Visual Perception

• Ganglion cell axons form the optic nerve.

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• Optic chiasm place where the two optic nerves
  meet.
• In humans, half of the axons from each eye cross
  to the other side of the brain.
• Most axons go to the lateral geniculate nucleus,
  a smaller amount to the superior colliculus and
  fewer go to other areas.

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• Visual field the whole area of the world that
  you can see at a given time
• Receptive field the portion of the visual field
  to which any one neuron responds

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Eyes Rightby Oliver Sacks

• From The Man Who Mistook His Wife for a Hat

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The Neural Basis of Visual Perception

• Some people with damage to the primary visual
  cortex (V1) show blindsight, an ability to
  respond to visual stimuli that they report not
  seeing.

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The Neural Basis of Visual Perception

• Visual agnosia is the inability to recognize
  objects despite satisfactory vision.
• Caused by damage to the pattern pathway usually
  in the temporal cortex.
• Video link (1.5m)
• http//www.youtube.com/watch?vrwQpaHQ0hYwfeature
  related
• Prosopagnosia is the inability to recognize
  faces.
• Occurs after damage to the fusiform gyrus of the
  inferior temporal cortex.
• Video link (5.5m)
• http//www.youtube.com/watch?vXLGXAiSpN00feature
  fvw

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The Neural Basis of Visual Perception

• The visual system has neural plasticity
• Blind Learn to See with Tongue, video link
  (3m)
• https//www.youtube.com/watch?vOKd56D2mvN0