The Visual System

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The Visual System

• Dr. Kline
• FSU

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What stimuli are required for vision?

• Light- which can be thought of as discrete
  particles (photons) or traveling waves.

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Human Visible Spectrum

• Humans can detect waves of energy traveling
  through space between 380 760 nanometers).
• Wavelengths outside this spectrum are
  undetectable to the human eye.
• Some organisms do detect wavelengths outside our
  visible spectrum.
• E.g., Rattlesnakes detect in infrared!
• bees detect ultraviolet light.

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Electromagnetic Radiation
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What are the 2 properties of light that influence
visual perception?

• 1. Wavelength is associated with our perception
  of color.
• 2. Intensity is associated with our perception of
  brightness.

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Reflectance of light

• Light is reflected off of the surface of objects
  to the eye.
• Light energy is converted into neural energy
  then processed by the brain.

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Parts of the eye

• Outer Layer
• 1. Sclera White fibrous layer
• 2. Cornea the clear protruding structure in the
  front of the eye that is curved.
• Bends light rays is responsible for 70-80 of
  eyes focusing ability.

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Cornea
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Middle Layer

• 1. Choroid vascularized layer that provides
  nutrition for retinal cells. Is located between
  the retina sclera.
• 2. Pigment epithileum a black pigment found
  between the choroid retina.
• Traps photons from light to prevent scattering
  of photons along the retina, which reduces
  distortion.

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Middle layer (contd.)

• 3. Iris smooth ring of muscle with a central
  opening (pupil). Gives us our eye color!
• 4. Pupil Pupil changes in size depending on
  intensity of light.
• -Intense light, small constricted
  pupils
• -Dim light, dilated pupils
• 5. Lens focuses light on retina (convex).
• -lens is round (nearby objects)
• -lens is flatter for distant objects

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Inner layer

• 1. Retina contains receptor cells needed for
  neural processing of light.
• A. Fovea indentation on retina.
• -fine discrimination colors detail.
• B. periphery area on either side of
  fovea of
• retina.
• -detection of light
• 2. Optic disk place where axons exit eye
  forming optic nerve.

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The Photoreceptors Rods Cones

• Cones-specialized for color vision detail
  (fovea).
• Rods-sensitive to light (periphery)
• 126 million receptors total!
• 120 million are rods 6 million are cones
• sensitivity to light enhanced by convergence
  ratio. Many rods converge on a single retinal
  ganglion cell than do cones.
• Rods-big receptive fields
• cones-small receptive fields

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How does visual information get from the eye to
the brain to be processed?

• Two vision pathways
• Geniculostriate Tectopulvinar

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Visual Pathways

• 1. Geniculostriate pathway-
• optic chiasm----LGN---Primary Visual cortex
• involved in patter perception, color
  vision
• 2. Tectopulvinar pathway-
• optic chiasm---superior colliculus---Lateral
• posterior pulvinar---PVC
• detection of light spatial orientation

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Primary Visual Cortex
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How do our eyes adapt to the dark?

• The photopic scotopic systems adjust their
  sensitivity as a function of time in the dark.
• The cones become more sensitive during the first
  5-10 minutes after being in the dark.
• Rods continue becoming more sensitive over period
  of 20-30 minutes.

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How is a dark adaptation experiment conducted?

• Step 1 The subject (S) is exposed to bright
  light of a known intensity.
• Step 2 S is then placed in total darkness
  asked to detect a spot of light (controlled
  luminance).
• Step 3 Ss detection threshold is plotted as a
  function of time spent in dark.

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We have three cone wavelengths

• 1. Short wavelength peaks at 419 nm (blues).
• 2. Medium wavelength peaks at 531 nm (greens).
• 3. Long wavelength peaks at 558 nm (reds).
• The primary colors are blue, green, red

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What are the cells in the retina?
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What is relationship between activity in the
retina the brain?

• An electrode is inserted into various parts of
  the visual system (retina, cortex) of an animal.
  
• The cells activity in response to the
  presentation of visual stimuli (lights, bars,
  complex images) to the animals retina is
  recorded.

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What is a receptive field of retinal ganglion
cells?

• The receptive field for these cells is the region
  of the retina that, when stimulated excites or
  inhibits the cells firing pattern.

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Retinal ganglion cells

• Kuffler (1953) presented spots of light to retina
  cells in the cat recorded their responses.
• The cells have a Concentric circle configuration!
• usually called center-surround cells
• On-center, off-surround cell has an excitatory
  center, inhibitory surround
• Off-center, on-surround cell has an inhibitory
  center excitatory surround

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Are receptive fields of cortical cells like those
of retinal ganglion cells?

• No!!
• Our visual cortex cells respond to more complex
  stimuli (e.g., bars of light).

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Hubel Wiesel (1950s 60s)

• Recorded cortical cells in the visual cortex of
  cats in response to visual images they presented
  to the cats retina.
• They found three types of cells with different
  receptive fields (bar detectors).

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Receptive Fields of cortical neuronsPrimary
Visual cortex

• 1. Simple Cells
• --respond to points of light or bars of
  light in a particular orientation
• 2. Complex cells
• --respond to bars of light in a particular
  orientation moving in a specific direction.
• 3. Hypercomplex Cells
• respond to bars of light in a particular
  orientation, moving in a specific direction, of
  a specific line length.

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Simple Cells
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Complex Cells
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What is the organization of the visual cortex?

• Hubel Wiesel found that the visual cortex is
  organized into columns.
• Location specific For each place on the retina
  there is a column of cells in cortex.
• Two columns next to one another in the cortex
  respond to stimulation of two adjacent points on
  the retina.

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Orientation Ocular Dominance columns in Primary
Visual Cortex
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The Visual cortex has a retinotopic map

• Visual cortex has a map of the retinas surface.
• More cortical neurons are devoted to fovea of
  retina.
• As fovea only has cones, they are widely mapped
  on cortexs surface.
• The reason cones allow us to see detail color.

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How do we see in color?

• Two Theories
• 1. Young-Helmholtz Trichromatic theory of color
  vision.
• The pattern of activity among the three
  cone-types determines the color we perceive.
• The cones all respond equally to white light
  (which contains all wavelengths).

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Evidence for Trichromatic theory

• 1. We have three types of cones sensitive to
  different wavelengths of light (short, medium,
  long wavelengths).
• 2. There are 3 types of color-blindness.
• 3. Approximately any color can be matched by
  mixing varying amounts of red, green, blue
  light.

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Additive Color mixing with lights
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Results of Additive Color Mixing
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Subtractive Additive Mixing
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Why mixing blue yellow pigments, yields the
color green.
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Color-blindness

• Results whenever we are either missing one of our
  cones or one of our cones doesnt work properly.

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2. Opponent-Process theory

• We have opposing mechanisms that allow us to
  perceive colors.
• Evidence for this theory
• 1. Incompatible colors cannot be seen Explains
  why we cant see certain colors (reddish-green,
  bluish-yellow) color afterimages.
• We have 3 opposing mechanisms red-green,
  yellow-blue, black-white. These are called
  complimentary colors put together they produce
  yellow or white.