How do we see what we see? How do we hear what we hear?

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How do we see what we see?How do we hear what we
hear?

• A question that has intrigued philosophers since
  the beginning of recorded history

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Early Greek Philosophers

• Democritus (ca 460 B.C.) Epicurus (341-270 B.C.).
  Eidola or film theory of vision.
• Objects emit particles or copies of themselves
• For particles are continually streaming off from
  surfaces of bodies, though no diminution of the
  bodies is observed, because other particles take
  their place. And those given off for a long time
  retain the position and arrangement which their
  atoms had when they formed part of the solid
  bodies.

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Early Greek Philosophers Problems with Eidola
theory

• Why isnt matter used up?
• How does the image get in the eye?
• Why cant we see in the dark?
• Are copies given off in all directions?
• These problems led to other theories being
  proposed.

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Early Greek PhilosophersScientific progress
Problems lead to new theories

• Extramission or visual touch theories.
• Plato (ca 427-347 B.C). Eyes emit visual fire
  which coalesces with daylight to contact objects
  and returns an impression of the object.
• Problems with visual touch theories.
• Exactly what is emitted?
• How does it return impressions?
• Why cant you see in the dark?

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Early Greek PhilosophersExtramission theory
Theoretical development

• Euclids (300 B.C.) geometric theory of vision.
  Assume
• That the rectilinear rays proceeding from the eye
  diverge indefinitely
• That the figure contained by a set of visual rays
  is a cone of which the vertex is at the eye and
  the base at the surface of the object seen
• That those things are seen upon which visual rays
  fall and those things are not seen upon which
  visual rays do not fall.

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Early Greek PhilosophersExtramission theory
Theoretical development

• That things seen under a larger angle appear
  larger, those under a smaller angle appear
  smaller, and those under equal angles appear
  equal
• That things seen by higher visual rays appear
  higher, and things seen by lower rays appear
  lower
• Etc.

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Extramission theory Problems led to
modifications

• Galens answer (ca. 129-199 A.D.) as to why light
  is needed.
• When it (the air) has been illuminated by the
  sun, it is already an instrument of vision of the
  same description as the pneuma arriving from the
  brain but until it is illuminated it does not
  turn into a sympathetic instrument in accordance
  with the change effected by the outflow of pneuma
  into it.

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After the fall of the Roman Empire

• No further developments that we know of until
• The work of the Islamic scholars
• Al-Kinde (9th century) took elements of
  extramission theory and Euclids geometry and
  fused them into a coherent theory of vision.
• Alhazen (969-1039 A.D.), however, took visual
  theory to a whole new level.

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The Islamic Scholars Alhazen

• Proposed a new intromission theory.
• Took Euclid and turned it around.
• Points on a body radiate light in all directions.
• From each point of every colored body,
  illuminated by any light, issue light and color
  along every straight line that can be drawn from
  that point.

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The Islamic Scholars Alhazen

• Only those rays that enter the pupil of the eye
  produce a visual impression.
• One problem.
• Superfluity of rays problem.

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The lens is the seat of vision.
Rays from different points in space fall on same
point on the lens.
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The Islamic Scholars Alhazen
Knew about refraction.
Hypothesized only non-refracted rays penetrate
the eye
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One point in space corresponds to one point on
the lens
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Alhazen theory was not overturned until the work
of Kepler

• Kepler (1571-1630) worked out the geometric
  optics of the eye.
• Light is reflected in all directions from each
  point on a non-mirror surface.
• He traced the rays and proved that an upside-down
  and right-left reversed image should appear on
  the retina.

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Thus, some 20 centuries later we finally solved
how images were sensed by the eye

• But now we had new problems to solve.
• Images on the retina change with distance
• Images on the retina change with orientation
• Images on the retina change with angle of view

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New problem Superfluity of Images

• For each object there are potentially an infinite
  number of images that can be formed.
• How does the observer recognize individual object
  when the number of images is infinite?
• Do they have an infinite number of templates to
  match against the image?

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To answer this problem we need to consider how
signals are processed

• How do the eye and brain solve the superfluity of
  images problem?.
• Question How much information do we need to
  characterize an image?

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• Figure 4.4
• The human eye, a simplified view.

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• Figure 4.2
• The lens gets its name from Latin for lentil,
  referring to its shapean appropriate choice, as
  this cross section of the eye shows. The names of
  other parts of the eye also refer to their
  appearance.

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• Figure 4.6
• The retina lies behind the vitreous humor, which
  is the jelly-like substance that fills the
  eyeball. Note that light does not fall directly
  on the rods and cones. It must first pass through
  the outer layers of the retina, made up of
  additional nerve cells. Only about one half of
  the light falling on the front of the eye reaches
  the rods and conestestimony to the eyes amazing
  light sensitivity. The rods and cones are much
  smaller than implied here. The smallest receptors
  are 1 micron (one millionth of a meter) wide. The
  lower left photograph shows rods and cones as
  seen through an electron microscope. In the
  photograph the cones are colored green and the
  rods blue.