Human Vision

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Human Vision

• Its all in your head.
• -- Kristen Pelz 06, Biology Nerd

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The Human Eye

• Cornea
• Iris
• Lens
• Retina

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Cornea

• Filled with aqueous fluid that provides nutrition
  to the surrounding tissue
• Fixed, dome shape refracts light towards the the
  iris
• Ensures that light converges to the lens at the
  pupil

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Iris

• Colored ring of muscle fibers
• By contracting and expanding, the iris controls
  the amount of light that enters the pupil

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Lens

• Lens has natural elasticity
• Ciliary muscle stretches lens, elasticity
  retracts lens
• Presbyopia (farsightedness) is a problem during
  aging because the elasticity of the lens wears out

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Optics

• The cornea converges light on the lens
• The lens refracts and focuses light on the retina

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Retina

• Parts
• Fovea
• Blind spot
• Rods
• Cones
• 130 million photoreceptors
• Covers 72 of the sphere of the eye
• Responds chemically to photo stimulus
• Does some pre-processing for the brain

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Fovea

• The fovea is a dense, hexagonally-packed
  depression of cone-type photoreceptors at the
  back of the eye
• Only comprises 2 of the field of view, but
  contains 20 of the eyes photoreceptors
  (reference lost)
• Vital for central vision
• Saccades place the fovea on an object of interest

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Rods

• 125 million rods in the eye
• Active at low light-levels
• Black and white (only respond to intensity)
• Several rods connect to one optic nerve cell
  (ganglion cell) to elicit even more
  light-sensitivity.

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Cones
6 million cones Concentrated in the fovea in a
tight hexagonal pattern Active only at high
light-levels (when rods have saturated) Three
different types of cones respond to three
different colors of light red, green, and blue.
One cone attaches to a single ganglion cell
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How does it come together?

• Cones, concentrated in the fovea, give humans
  high-resolution color vision in the very center
  of the field of view.
• Rods, on the periphery and scattered through the
  retina, give us night vision, though not in
  color.
• When a rod or cone is stimulated by light, it
  releases a photopigment that is received by a
  ganglion cell. If there is enough pigment, the
  ganglion cell fires an action potential into the
  vision pathway along the optic nerve
• In high light-levels, the photopigment bleaches
  out, lowering the effective brightness. This is
  why it can take up to thirty minutes for ones
  eyes to adjust to indoor lighting after being
  exposed to the bright sun.
• The ganglion cells have receptive fields - tiny
  regions in the field of view that are seen by
  their attached photoreceptors (rods and cones).
  Using these receptive fields, the retina is able
  to do a great deal of preprocessing condensing
  the information from 130 million photoreceptors
  down to a signal on only 1.2 million nerve axons.
  

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The Visual Pathway

• Optic Nerve
• 1.2 million fibers
• Causes a blind-spot in the retina
• Transports information from the retina to the
  optic Chiasm

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Optic Chiasm

• The axons from the left half of the right eye
  cross over the axons from the right half of the
  left eye
• The right half of the brain handles the left half
  of the field of vision, not the left eye
• The left half of the brain handles the the right
  half of the field of vision

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Lateral Geniculate Nucleus

• In the thalamus
• Optic tract is input from optic chiasm
• Optic radiations are outputs to the brain
• The LGN combines information from other senses to
  make predictions
• Two types of cells M cells (layers 1 and 2) and
  P cells (layers 3, 4, 5 and 6)
• M cells process data quickly, with no color
  information, and ask the question where?
• P cells process data more slowly, use color
  information, and ask what?

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Visual Cortex

• Approximately 1/3 of the neo-cortex (the part of
  the brain most responsible for intelligence)
• Columns of neurons perform input-based
  learning, and specialize into different roles
• Four main regions
• Primary visual cortex (V1)
• V2
• V3
• V5 (also know as MT, or middle/medial temporal)

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Primary Visual Cortex - V1

• Spatially equivalent to the field of view
• Low-level in the computer vision hierarchy
  input of a processed image, output of simple
  geometric data (edges, connections, etc.)
• Detects fine details
• Visual Orientation
• Spatial Frequency
• Colors
• Motion
• Direction
• Speed

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V2

• Feed-forward from V1
• Processes figure/ground separation
• Orientation of illusory contours
• Mid-level in the computer vision hierarchy
  input is highly processed image data and output
  is simple scene information
• Strong feedback to V1 (for prediction?)

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V3 and V5

• V3 processes global motion
• V5 processes the movement of complex objects and
  performs high-level object recognition
• V3 and V5 both fall into high-level vision in
  the computer vision hierarchy, though V5 is
  definitely more abstract. Using V5 is thinking
  visually.

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The End
Peter Lubans March 2006 For more information,
visit www.wikipedia.org thats where I did all
of the research and obtained all of the images
for this presentation
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Sources
Optic radiations. Wikimedia, August 2005.
http//en.wikipedia.org/wiki/Optic_radiations
Last accessed March 2006. Eye. Wikimedia,
March 2006. http//en.wikipedia.org/wiki/Eye
Last accessed March 2006 Optic Nerve.
Wikimedia, Frebruary 2006. http//en.wikipedia.or
g/wiki/Optic_nerve Last accessed March
2006 Optic Chiasm. Wikimedia, February 2006.
http//en.wikipedia.org/wiki/Optic_chiasm Last
accessed March 2006 Optic Tract. Wikimedia,
January 2006. http//en.wikipedia.org/wiki/Optic_
tract Last accessed March 2006 Retina.
Wikimedia, March 2006. http//en.wikipedia.org/wi
ki/Retina Lateral Geniculate Nucleus.
Wikimedia, February 2006. http//en.wikipedia.org/
wiki/Lateral_geniculate_nucleus Last accessed
March 2006. Visual Cortex. Wikimedia, February
2006. http//en.wikipedia.org/wiki/Visual_cortex
Last accessed March 2006. Jeff Hawkins, On
Intelligence. Times Books, New York, New York,
2004.
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Images