Perceiving the World

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Chapter 6

• Perceiving the World

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Perception Some Key Terms

• Definition How we assemble sensations into
  meaningful patterns
• Size Constancy Perceived size of an object
  remains constant, DESPITE changes in its retinal
  image size
• Native Perception A perceptual experience based
  on innate processes
• Empirical Perception A perception strongly
  influenced by prior experience
• Shape Constancy The perceived shape of an object
  unaffected by changes in its retinal image
• Brightness Constancy Apparent brightness of an
  object stays the same under changing lighting
  conditions

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Fig. 6.1 Shape constancy. (a) When a door is open
its image actually forms a trapezoid. Shape
constancy is indicated by the fact that it is
still perceived as a rectangle. (b) With great
effort you may be able to see this design as a
collection of flat shapes. However, if you
maintain shape constancy the distorted squares
strongly suggest the surface of a sphere. (From
Spherescapes-1 by Scott Walter and Kevin McMahon,
1983.)
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Perceptual Grouping

• Figure-Ground Organization Inborn part of a
  stimulus stands out as a figure (object) against
  a plainer background (ground)
• Reversible Figure Figure and ground that can be
  reversed

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Fig. 6.2 A reversible figure-ground design. Do
you see two faces in profile, or a wineglass?
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Gestalt Principles of Organization

• Nearness Stimuli that are near each other tend
  to be grouped together
• Similarity Stimuli that are similar in size,
  shape, color, or form tend to be grouped together
• Continuation, or Continuity Perceptions tend
  toward simplicity and continuity

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Fig. 6.3 How we organize perceptions
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Gestalt Principles of Organization (cont.)

• Closure Tendency to complete a figure so that it
  has a consistent overall form
• Contiguity Nearness in time and space
  perception that one thing has caused another
• Common Region Stimuli that are found within a
  common area tend to be seen as a group

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Fig. 6.5 A challenging example of perceptual
organization. Once the camouflaged insect (known
as a giant walkingstick) becomes visible, it is
almost impossible to view the picture again
without seeing the insect.
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Depth Perception

• Definition Ability to see three-dimensional
  space and to accurately judge distances
• Visual Cliff Apparatus that looks like the edge
  of an elevated platform or cliff
• Depth Cues Features that supply information
  about distance and space
• Monocular Depth Cue Depth cue that can be sensed
  with one eye
• Binocular Depth Cue Depth cue that can be sensed
  with two eyes

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Muscular Cues for Depth Perception

• Accommodation Bending of the lens of the eye to
  focus on nearby objects
• Convergence Binocular cue when you look at
  something 50 feet or closer, your eyes must turn
  in (converge) to focus the object
• Retinal Disparity Discrepancy in the images that
  reach the right and left eyes
• Stereotopic Vision Three-dimensional sight

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Fig. 6.10 The eyes must converge, or turn in
toward the nose, to focus close objects.
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Pictoral Cues for Depth

• Features found in paintings, drawings, and
  photographs that supply information about space,
  depth, and distance
• Linear Perspective Based on apparent convergence
  of parallel lines in environment
• Overlap When one object partially blocks another
• Texture Gradients Texture changes can contribute
  to depth perception coarse texture implies
  closeness, fine texture implies distance
• Relative Motion (Motion Parallax) Nearby objects
  move a lot as your head moves distant objects
  move slightly

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Fig. 6.14 (a) Linear perspective. (b) Relative
size. (c) Light and shadow. (d) Overlap. (e)
Texture gradients. Drawings in the top row show
fairly pure examples of each of the pictorial
depth cues. In the bottom row, the pictorial
depth cues are used to assemble a more realistic
scene.
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Fig. 6.15 On a dry lake bed, relative size is
just about the only depth cue available for
judging the cameras distance from this vintage
aircraft. What do you estimate the distance to
be?
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Fig. 6.17 The apparent motion of objects viewed
during travel depends on their distance from the
observer. Apparent motion can also be influenced
by an observers point of fixation. At middle
distances, objects closer than the point of
fixation appear to move backward those beyond
the point of fixation appear to move forward.
Objects at great distances, such as the sun or
moon, always appear to move forward.
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Some Illusions

• Moon Illusion Apparent change in size that
  occurs as the moon moves from the horizon (large
  moon) to overhead (small moon)
• Apparent-Distance Hypothesis Horizon seems more
  distant than the night sky

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Fig. 6.7 An impossible figurethe three-pronged
widget.
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Fig. 6.19 The Ponzo illusion may help you
understand the moon illusion. Picture the two
white bars as resting on the railroad tracks. In
the drawing, the upper bar is the same length as
the lower bar. However, because the upper bar
appears to be farther away than the lower bar, we
perceive it as longer. The same logic applies to
the moon illusion.
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Perceptual Learning

• Change in the brain that alters how we process
  sensory information
• Perceptual Habits Ingrained patterns of
  organization and attention
• Other-Race Effect Tendency to be better at
  recognizing faces from ones own racial group
  than faces from other racial or ethnic groups
• Active Movement Self-generated action
  accelerates perceptual adaptation
• Context Information surrounding a stimulus
  affects perception
• Frames of Reference Internal standards for
  judging stimuli

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Fig. 6.9 Human infants and newborn animals refuse
to go over the edge of the visual cliff
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Animation Ames Room
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Fig. 6.24 The Ames room. From the front, the room
looks normal actually, the right-hand corner is
very short, and the left-hand corner is very
tall. In addition, the left side of the room
slants away from viewers. The diagram shows the
shape of the room and reveals why people appear
to get bigger as they cross the room toward the
nearer, shorter right corner.
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Fig. 6.27 Some interesting perceptual illusions.
Illusions are a normal part of perception.
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Fig. 6.28 Why does line (b) in the Müller-Lyer
illusion look longer than line (a)? Probably
because it looks more like a distant corner than
a nearer one. Because the vertical lines form
images of the same length, the more distant
line must be perceived as larger. As you can see
in the drawing on the right, additional depth
cues accentuate the Müller-Lyer illusion. (After
Enns Coren, 1995.)
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Illusions Is What You See What You Get?

• Illusion Length, position, motion, curvature, or
  direction is constantly misjudged
• Hallucination When people perceive objects or
  events that have no external basis in reality
• Stroboscopic Movement Illusory motion perceived
  when objects are shown in rapidly changing
  positions
• Muller-Lyer Illusion Two equal-length lines
  topped with inward or outward pointing Vs appear
  to be of different length based on experience
  with edges and corners of rooms and buildings

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Attention and Perception

• Inattentional Blindness Blindness caused by not
  attending to a stimulus
• Orientation Response Bodily changes that prepare
  an organism to receive information from a
  particular stimulus

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Perceptual Expectancies

• Bottom-Up Processing Analyzing information
  starting at the bottom (small units) and going
  upward to form a complete perception
• Top-Down Processing Pre-existing knowledge that
  is used to rapidly organize features into a
  meaningful whole
• Perceptual Set Past experiences, motives,
  contexts, or suggestions that prepare us to
  perceive in a certain way

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Fig. 6.22 The effects of prior experience on
perception. The doctored face looks far worse
when viewed right side up because it can be
related to past experience.
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Factors Affecting the Accuracy of Eyewitness
Perceptions

• Stress High levels impair accuracy
• Weapon Focus Presence of a weapon impairs
  eyewitness accuracy
• Exposure Time Less time an eyewitness has to
  observe an event, the less s/he will perceive and
  remember it
• Accuracy-Confidence Confidence is not a good
  predictor of his/her accuracy
• Cross-Racial Perceptions Eyewitnesses are better
  at identifying members of their own race than of
  other races

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More Factors Affecting the Accuracy of Eyewitness
Perceptions

• Post-Event Information Testimony reflects not
  only what was actually seen but also information
  obtained later on
• Color Perception Judgments of color made under
  monochromatic light are very unreliable
• Unconscious Transference A culprit who is
  identified may have been seen in another
  situation or context
• Alcohol Intoxication Impairs later ability to
  recall events
• Attitudes and Expectations May affect
  eyewitness perception of events

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Implications of Eyewitness Testimony

• Reality Testing Obtaining additional information
  to check your perceptions
• Habituate Tend to respond less to predictable
  and unchanging stimuli
• Dishabituation Reversal of habituation

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Extrasensory Perception (ESP) Fact or Fallacy?

• Parapsychology Study of ESP and other psi
  phenomena (events that seem to defy accepted
  scientific laws)
• Clairvoyance Purported ability to perceive
  events unaffected by distance or physical
  barriers
• Telepathy Purported ability to read minds
• Precognition Purported ability to accurately
  predict the future (Minority Report and the
  Pre-Cogs, like Agatha)
• Psychokinesis (Mind Over Matter) Purported
  ability to influence physical objects by willpower

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More ESP Issues

• Zener Cards Deck of 25 cards, each having one of
  five symbols
• Run of Luck Statistically unusual outcome that
  could occur by chance alone (e.g., getting five
  heads in a row, two jackpots within six pulls of
  a slot machine)
• Stage ESP Simulation of ESP for entertainment
  purposes
• Conclusion Existence of ESP has NOT been
  scientifically demonstrated positive results are
  usually inconclusive and easily criticized
• In sum Be skeptical! If it seems too good to be
  true, it probably is!

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Fig. 6.36 ESP cards used by J. B. Rhine, an early
experimenter in parapsychology.
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Fig. 6.37 Fake psychokinesis. (a) The performer
shows an observer several straight keys. While
doing so, he bends one of the keys by placing its
tip in the slot of another key. Normally, this is
done out of sight, behind the psychics hand.
It is clearly shown here so you can see how the
deception occurs. (b) Next, the psychic places
the two keys in the observers hand and closes
it. By skillful manipulation, the observer has
been kept from seeing the bent key. The performer
then concentrates on the keys to bend them
with psychic energy. (c) The bent key is
revealed to the observer. Miracle accomplished!
(Adapted from Randi, 1983.)
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