The Human Visual System

1
The Human Visual System

• Part 2 Perception

2
Visual Perception

• How one visually interprets a scene
• 4 forms of perception to be studied
• Depth
• Color
• Temporal
• Motion

3
Depth Perception

• How does one determine how far away an object is
  located?

4
Depth Perception

• Monocular Cues
• Require only 1 eye to perceive depth Cyclops.

http//www.shag-art.com/cyclops.html
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Depth Perception

• Binocular Cues
• Require 2 eyes to perceive depth.

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Monocular Cue 1

• Interposition (Overlap)
• An object that is partially covered by another
  object is farther away.

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Monocular Cue 2

• Familiar Size
• Previous knowledge of object sizes aid in judging
  distance.

Which object appears closer?
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Monocular Cue 3

• Linear Perspective
• The farther away an object is the smaller it
  appears to be.

9
Monocular Cue 4

• Atmospheric Perspective
• Objects farther off in the distance appear less
  saturated and less sharp (fuzzier) than those
  nearby.
• The more atmospheric particles between the viewer
  and a distant object the more light that is
  scattered.

10
Monocular Cue 5

• Motion Parallax
• Stationary objects that are physically closer to
  a moving viewer appear to shift faster than those
  farther away.
• Example 1
• Driving by in a car looking at objects near and
  far (animation).

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Monocular Cue 5

• Example 2
• Close one eye.
• Hold your left thumb upward at arms length.
• Hold your right thumb upward at half arms
  length.
• Position the thumbs so the right thumb blocks the
  left and move your head to one side.

• Example 3
• Close one eye.
• Hold both index fingers pointing toward each
  other.
• Circle the fingers in a bike pedaling motion.
• Stop them at eye level and move them inward to
  make them meet forming a straight line.

Observe that the background did not move, but
the thumbs appeared to move relative to each
other and the background.
Try again, but this time move your head
side-to-side to tell the distance between your
index fingers.
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Monocular Cue 6

• Shading
• Uses light falling on an object from a certain
  angle to give form and depth to an object.
• Cast shadows aid in locating an object.

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Monocular Cue 7

• Patterns
• Use contour lines to infer depth.

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Monocular Cue 8

• Accommodation
• The change of shape performed by the eye lens to
  focus on an object aids the brain in determining
  the objects distance.

Thick lens - object is near
Thin lens - object is far
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Binocular Cue 1

• Convergence
• The angle between the line of sight of each eye
  is larger as an object moves closer.
• This works for nearby objects (with accommodation)

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20
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Binocular Cue 2

• Retinal Disparity
• Each eye receives a slightly different view of a
  scene.
• The two views are used to determine the ratio of
  distances between nearby objects.
• Example
• Close one eye and position your thumbs so that
  one blocks the other with 1 cm distance between
  them.
• Switch your viewing eye.
• Open both eyes.

Threading a needle utilizes retinal disparity.
17
Color Perception

• Trichromats
• Humans have three cones that correspond to three
  ranges of the visible light in the areas of red,
  green, and blue light.

18
Human Color Vision Deficiencies

• Normal Vision
• Viewer requires 3 basic colors to match another
  color.
• 91 Males
• 99 Females

• Anomalous Trichromacy
• Also requires 3 basic colors to match another
  color, but the ratios of those three basic colors
  differ from a person with normal vision.
• 6 Males

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Human Color Vision Deficiencies

• Dichromacy
• Requires two colors to match any other color
  8-10 Caucasion males
• Four types
• Protanopes No L cones (1 males)
• Deuteranopes No I cones (1 males)
• Tritanopes No S cones (very small pop.)
• Tetartanopes Lack chromatic channel (RG or YB,
  small )

• Monochromacy
• Requires one color to match any other color
• Two types
• Cone monochromats Only one type of cone (very
  small of population)
• Rod monochromats Only the rods respond (.003
  males)

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Human Color Vision Deficiencies
21
Temporal Perception

• Negative Afterimages
• A viewer stares at an image for a period of time.
• The cones become desensitized.
• Upon looking at a plain white surface, the viewer
  perceives the previous image with complementary
  colors.

22
Temporal Perception

• Positive Afterimages - Persistence of Vision
• An intense flash of light allows a viewer to see
  a scene.
• When the light goes out the signal from the cones
  persist.
• The viewer still perceives the image.

23
Motion Perception

• Real Movement
• An object physically moves.

• Induced Movement
• The background moves behind an object causing the
  perception that the object moved.

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Motion Perception

• Stroboscopic Movement
• Quick, sequential flashes of light / images that
  imply motion.
• e.g. television, film, monitor

• Autokinetic Movement
• A still spot of light appears to move in an unlit
  background.
• e.g. star in the night sky

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Aftereffects of Movement

• Waterfall Illusion
• After staring at water falling, look at a still
  object, and it will appear to move upward.

• Spiral Aftereffect
• After staring at a rotating spiral
• a still spiral appears to move in the opposite
  direction.
• an object will appear to deform in the spiral
  motion opposite that initially observed.

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Consequences of Movement

• Perception of Causality
• One event is seen after another, thus it may be
  interpreted that the 1st event caused the 2nd.
• Personification
• Human / animal characteristics given to inanimate
  objects.

• Perception of Location
• Object 1 is seen going behind object 2, thus
  causing the viewer to extrapolate the location of
  object 1.
• Linking Successive Shots
• e.g. television, film

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Summary

• Depth Perception
• Interposition Familiar Size Linear Perspective
  Atmospheric Perspective Motion Parallax
  Shading Patterns Accommodation
• Convergence Retinal Disparity
• Color Perception
• Trichromacy

• Temporal Perception
• Desensitization negative afterimage
• Persistence of Vision positive afterimage
• Motion Perception
• Real Induced
• Stroboscopic Autokinetic
• Waterfall/Spiral Effects