Chapter 5 Part 2

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Chapter 5 Part 2

• Sensation and Perception

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How Big IS the Moon?

• Imagine youre outside on a clear night with a
  full moon. On a table in front of you are objects
  that range in size from a BB to a beach ball.
  
• Imagine that you are picking up the one object
  that when held at arms length will JUST
  COVER the moon.

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Write down the number of the item you chose.

• BB
• Pea
• Dime
• Penny
• Nickel
• Quarter
• Golf ball

• Baseball
• Softball
• Small salad plate
• Large dinner plate
• Frisbee
• Basketball
• Beach ball

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So . . . how DOES the world out there get in?

• How do we construct our representations of the
  external world?
  
• Distal stimulus is actual object out there
  (moon)
  
• Proximal stimulus is the representation of
  objects in contact with a sense organ (visual
  image on retina)
  

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How Big is the Moon, Really?

• Most students estimate between quarter and
  softball
  
• Covers about 1/180th of the night sky
• Independent of altitude (horizon or zenith)
  time of year
  
• A pea will do it . . . really!
• Called the moon illusion

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Perceptual Constancy

• As distance between eye and object increases,
  size of retinal image decreases
  
• We compensate, usually, and perceive objects as
  unchanging despite changes in sensory stimulation
  
  
• Constancies size, shape, color, brightness

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Perceptual Constancy

• Once we have formed a stable perception of
  something, we see it as essentially the same
  regardless of differences in viewing angle,
  distance, lighting, etc.
• This tendency helps us to understand and relate
  to the world

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Size Constancy

• perception of an object as the same size
  regardless of the distance from which it is
  viewed.

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Try This

• Hold right hand at arms length
• Move toward and away from face
• Do you perceive a size change?

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Now Try This

• Hold left index finger 8 from face
• Focus on finger while moving right hand away and
  toward face
  
• Now do you perceive a size difference in your
  right hand?
  
• Whats going on?
• Size of retinal image changes in both cases
• This time, you have a reference point
• This is why artists use thumb to estimate size
  when drawing
  

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Shape Constancy

• Shape constancy
• tendency to see object as the same shape no
  matter what angle it is viewed from
  
• a door is always rectangular, right?

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Brightness Constancy

• We perceive an object as having a constant
  brightness despite differences in available
  light
  
• Has to do with percentage of light rather than
  total available light
  
• We judge brightness based on relative reflection,
  not absolute values

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Color Constancy

• Tendency to perceive familiar objects as
  retaining known color despite changes in sensory
  information
  
• Red car at night still looks red

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Depth Perception

• How far away is an object?
• Monocular cues
• Depth cues requiring only one eye
• Binocular cues
• Depth cues requiring both eyes

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Monocular Cues

• Superposition
• Linear perspective
• Aerial perspective
• Elevation
• Texture gradient
• Shadowing
• Motion parallax

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Superposition

• Monocular distance cue in which one object, by
  partially blocking another, is perceived as being
  closer.

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• Linear perspective
• Two parallel lines appear to converge at the
  horizon
  
• Aerial perspective
• More distant objects are likely to appear hazy
  and blurred

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Elevation perspective the higher on the
horizontal plane an object is, the farther away
it appears

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• Texture gradient
• Objects seen at greater distances appear to be
  smoother and less textured
  
• Shadowing
• Shadows often appear on the parts of objects that
  are more distant

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Binocular Cues

• Stereoscopic vision, derived from combining our
  two retinal images to produce a 3-D effect
  
• Retinal disparity (each eye has a slightly
  different perspective)
  
• try the floating wienie
• Convergence of the eyes as viewing distance
  decreases
  

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Sound Localization

• Just as we use monocular and binocular cues to
  sense depth and distance, we use monaural
  (one-ear) and binaural (two-ear) cues to locate
  sound.
• Monaural when one ear is needed to locate sound
• Binaural when two ears needed

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A monaural B binaural
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So How Do We Perceive Movement?
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Perception of Movement

• A complicated process involving
• Visual messages from the retina
• Messages from the muscles around the eyes as they
  shift to follow a moving object

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Motion Parallax

• Motion parallax is a depth of field cue that
  results from our motion. 
  
• As we move, objects that are closer to us move
  farther across our field of view than do objects
  that are in the distance.

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Motion Parallax

• Objects closer than fixation point appear to move
  backwards. (car wheels)
  
• Objects beyond fixation point appear to move with
  you at a decreasing speed as the object gets
  farther away.
  
• The closer an object is, the faster it appears to
  move (cockroach vs. 747)
  

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Perception of Apparent Movement

• At times our perceptual processes trick us into
  believing that an object is moving when in fact
  it is not
  
• Autokinetic illusion
• Stroboscopic motion
• Phi phenomenon

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Autokinetic Illusion

• Perception that a stationary object is actually
  moving.
  
• Occurs due to lack of a frame of reference
• Point of light in dark room

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Stroboscopic Motion

• Apparent motion resulting from flashing images in
  rapid succession (e.g., a movie or cartoon)

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Phi Phenomenon

• Apparent movement caused by flashing lights in
  sequence
  

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Visual Illusions

• Occur when sensory cues create perceptual
  experiences that do not actually exist
  
• Physical illusions such as the bent appearance of
  a stick in water
  
• Perceptual illusions occur because a stimulus
  contains misleading cues that lead to inaccurate
  perceptions
  

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Edge Enhancement Illusion
Right side of each stripe appears lighter than
left side why?
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Remember opponent process theory darker stripe
causes more inhibition therefore the appearance
of the darker region on the darker stripes
The darker stripes cause less inhibition
therefore the appearance of the lighter region on
the lighter stripes
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The Hermann Grid

• Hermann (1870) noticed this illusion while
  looking at a matrix of figures in a book.
  
• The intersections appear gray even though the
  same intensity light occurs all the way along the
  white spaces in the grid

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• The same palette is presented on
  different backgrounds
  
• The perception of differences in the
  colors of the palettes stem
  from the responses of your visual system, in part
  because stimulation of neighboring parts of the
  visual system influence one another.

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• You may also see spots in the intersections of
  the lines separating the colors in the palette.
  
• Are they gray, as in the Hermann Grid, or do they
  seem to have tinges of color associated with the
  adjacent colors in the palette? Are the spots
  equally apparent on all backgrounds?

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Personal Factors Affecting Perception

• Motivation
• Desires and needs shape perception
• Most bad behavior just a mistake, not bad
  person?
  
• Values
• Perceptions shaped by what we value
• People who share my values re family are nicer?
• Expectations
• Perceiving what we expect to perceive
• Paris in the spring

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Personal Factors Affecting Perception

• Cognitive style
• Detail Oriented vs. The Big Picture
• Experience and culture
• Aleutians have dozens of words for snow
• Americans and Muslims after 9/11
• Ex 63 vs. 10 said military action in
  Afghanistan was justified
  
• Personality
• Is the glass half full or half empty?

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Example

• Expectation perception exercise
• AKA Perceptual Set