Sensation and Perception

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Sensation and Perception

• Psychology A Concise Introduction
• Richard Griggs
• Chapter 3

Prepared byJ. W. Taylor V
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Windows on the World

• We understand the world through our senses, our
  windows on the world
• Our reality, in fact, is dependent upon two basic
  processes
• Sensation Gathering information
• Perception Interpreting information

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An Example of Misperception
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The Journey

• How the Physical and Psychological Worlds Relate
• How We See and How We Hear
• How We Make Sense of What We See

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How the Physical and Psychological Worlds Relate

• The detection question
• The difference question
• The scaling question

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The Questions

• The detection question is concerned with the
  limits on our ability to detect very faint
  signals
• How intense does a light have to be for us to see
  it?
• How intense does a sound have to be for us to
  hear it?

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The Questions

• The difference question is concerned with limits
  on our detection abilities, but in this case with
  our ability to detect very small differences
  between stimuli
• What is the smallest difference in brightness
  between two lights that we can see?
• What is the smallest difference in loudness
  between two sounds that we can hear?

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The Questions

• The scaling question is concerned with how we
  perceive the magnitudes (intensities) of clearly
  detectable stimuli
• What is the relationship between the actual
  physical intensities of stimuli and our
  psychological perceptions of these intensities?

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The Detection Question

• Absolute threshold is the minimum amount of
  energy in a sensory stimulus that is detected 50
  of the time
• Subliminal stimulus is one that is detected only
  up to 49 of the time
• Any effects of subliminal persuasion are
  short-lived with no long-term consequences on our
  behavior

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Theoretical and Observed Absolute Thresholds
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Signal Detection Theory
Person's Response
Reality
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The Difference Question

• A difference threshold (also called a just
  noticeable difference, or jnd) is the minimum
  difference between two stimuli that is detected
  50 of the time
• Webers Law says that for each type of sensory
  judgment, the difference threshold is a constant
  fraction of the standard stimulus value used to
  measure it

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The Scaling Question

• Stevens Power Law states that the perceived
  magnitude of a stimulus is equal to its actual
  physical intensity raised to a constant power for
  each type of judgment
• For instance, to perceive a light as twice as
  bright, its actual intensity has to be increased
  between and 8 and 9 times
• Likewise, if an electric shock is doubled in
  intensity, we perceive it as being about 10 times
  more intense

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The Scaling Question

• Sensory adaptation is the disappearance to
  repetitive or unchanging stimuli
• This sensory adaptation has survival value, as it
  is more important to detect new stimuli (which
  may signal danger) than constant stimuli

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How We See and How We Hear

• How the eye works
• How we see color
• How the ear works
• How we distinguish pitch

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Physical Characteristics of Light and Sound Waves

• Wavelength refers to the distance in one cycle of
  a wave, from one crest to the next
• With respect to vision, human can see wavelengths
  of about 400 to 700 nanometers
• Amplitude is the amount of energy in a wave, its
  intensity, which is the height of the wave at its
  crest
• For light waves, amplitude determines its
  brightness

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A Typical Waveform and Its Characteristics
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Physical Characteristics of Light and Sound Waves

• With respect to auditory stimuli, frequency is
  the number of times a sound wave cycles in one
  second, with shorter wavelengths having higher
  frequencies
• The frequency determines the pitch of a sound
  that is how high or low the sounds is perceived
  to be
• To understand these physical characteristics,
  receptor cells must transduce them into neural
  signals that the brain can use

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How the Eye Works

• The cornea covers the eye and is the clear
  covering through which light rays pass
• The light rays are further filtered by the pupil
  through the lens before being passed to the
  retina at the back of the eye
• The lens accommodates the light waves from
  objects of different distances directly on the
  retina
• For nearsighted people, light rays from distant
  objects are focused in front of the retina,
  whereas for farsighted people, light rays from
  close objects are focused behind the retina

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How the Eye Works

• The retina is the light-sensitive layer of the
  eye and has three layers of cells
• The ganglion cells are the first layer through
  which light rays pass
• After which light rays pass through the bipolar
  cells
• And are finally processed in the receptor cells,
  which contain the visual receptor cells rods and
  cones
• The approximately 120 million rods are
  responsible for seeing in dim light and for
  peripheral vision
• The approximately 5 million cones, located in the
  center of the retina, called the fovea, are
  responsible for seeing in bright light and in
  color

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How the Eye Works

• After being processed in the retina, patterns of
  neural impulses describing the visual image are
  carried through the bipolar cells to the ganglion
  cells, which bundle together to form the optic
  nerve
• Where the optic nerve leaves the eye, there are
  no receptor cells, and thus we have a blind spot
• The optic nerve runs through the thalamus, which
  acts as a relay station to transmit sensory
  information to the correct part of the cerebral
  cortex
• Visual information is directed to the occipital
  lobe, where it is processed
• Feature detector cells recognize basic features
  of the stimulus, which are then coordinated to
  give it meaning (i.e., to perceive it)

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How the Eye Works
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How We See Color

• The Trichromatic theory contends that there are
  three types of cones, each activated by a certain
  wavelength, which corresponds approximately to
  blue, green, and red
• The Opponent-Process theory assumes that there
  are three types of cell systems
  that help us see color, and
  these systems are located
  at the post-receptor level
  of processing
• The three types of cell systems are red-green and
  blue-yellow, as well as black-white (to detect
  brightness)
• If one color in a pair is stimulated, the other
  is inhibited

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Subtractive and Additive Mixtures
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Demonstration of Complementary Afterimage
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How We See Color

• Both theories have validity, each at different
  levels of visual information processing
• The Trichromatic theory is correct in its account
  of how color information is processed by the
  cones
• The Opponent-Process theory is correct in its
  account of how color information is processed
  after it leaves the retina (and is processed by
  the bipolar, ganglion, and thalamic cells)

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How the Ear Works

• The outer ear
• The pinna, which is the external part of the ear,
  collects sounds and funnels them through the
  auditory canal to the tympanic membrane (the
  eardrum), which marks the boundary between the
  outer ear and the middle ear

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How the Ear Works

• The middle ear
• The malleus, incus, and stapes (also called the
  hammer, anvil and stirrup) vibrate in reaction to
  sound waves from the auditory canal
• The stapes movement creates vibrations on the
  oval window, which covers the inner ear

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How the Ear Works

• The inner ear
• The cochlea contains in the basilar membrane
  about 16,000 hair cells that are the receptor
  cells for hearing
• Fluid in the cochlea is displaced, causing the
  hair cells to move, in turn causing the sensation
  of hearing
• When these hair cells or auditory nerve fibers
  are damaged, a person suffers nerve deafness
• Conduction deafness is hearing loss due to damage
  to the mechanical system carrying sound waves to
  the cochlea

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How the Ear Works
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How We Distinguish Pitch

• Pitch is the quality of a sound perceived as high
  or low and is determined by the frequency of the
  sound wave
• Humans can perceive sound wave frequencies from
  about 20 to 20,000 Hertz
• Place theory contends that there is a specific
  place along the basilar membrane in the inner ear
  that will correspond to a particular frequency.
• Frequency theory contends that the frequency of a
  sound wave is mimicked by the firing rate of the
  hair cells across the entire basilar membrane

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How We Distinguish Pitch

• Both theories have validity
• According to the volley principle, Frequency
  theory explains our perception of sound up to
  about 5000 Hz.
• Because 5000 times per second is the upper limit
  for the firing rate using the volley principle,
  Frequency theory would not be able to explain
  how we perceive higher frequencies
• Hence, Frequency theory explains the perceptions
  of lower pitched sounds (
  theory explains how we perceive higher
  frequencies ( 5000 Hz)
• For frequencies between 500 and 5000 Hz, both
  theories are correct, and hence we have better
  pitch perception in this range

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How We Make Sense of What We See

• Bottom-up processing and top-down processing
• Perceptual organization and perceptual constancy
• Depth perception

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Bottom-up Processing and Top-down Processing

• Bottom-up processing is the processing of
  sensory information as it enters the sensory
  structures and travels to the brain
• Top-down processing is the brains use of
  existing knowledge, beliefs, and expectations to
  interpret the sensory stimulation
• Perception is subjective because of top-down
  processing
• Perceptual set occurs when we interpret an
  ambiguous stimulus in accordance with our past
  experiences
• A contextual effect occurs when we use the
  present context of sensory input to determine its
  meaning

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Perceptual Organization and Top-Down Processing
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A Context Effect on Perception
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Perceptual Organization

• Gestalt means organized whole
• Gestalt psychologists believe that the organized
  whole is greater than the sum of its individual
  pieces of sensory information
• The figure-and-ground principle states that the
  brain organizes sensory input into a figure (the
  center of attention) and a ground (the
  background)
• Closure is the tendency to complete (i.e., close)
  incomplete figures to form meaningful objects
• Subjective contours are lines or shapes that are
  perceived to be present but do not really exist

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An Example of Figure-Ground Ambiguity
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An Example of Organizational Perceptual Ambiguity
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An Example of Subjective Contour
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Grouping

• We group things together that
• Look similar Similarity
• Are closer to each other Proximity
• Are traveling in the same direction Common Fate

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Perceptual Constancy

• Refers to the perceptual stability of
• Size
• Shape
• Brightness
• Color
• For familiar objects seen at
• Varying distances
• Different angles
• Different lighting conditions

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Depth Perception

• Involves judging the distance of objects from us
• Binocular depth cues require the use of both
  eyes
• Retinal disparity refers to the fact that as the
  disparity between the two retinal images
  decreases, the distance from us increases (and
  vice versa)
• Monocular depth cues require only one eye
• Linear perspective refers to the fact that as
  parallel lines recede away from us, they appear
  to converge
• Interposition refers to the fact that if one
  object blocks our view of another, we perceive
  the blocking object as closer

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Visual Illusions

• In the Ponzo illusion, two horizontal lines are
  equal in length, but one appears longer than the
  other
• The convergence of the two lines (i.e., linear
  perspective) outside the horizontal lines
  normally indicates increasing distance

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Visual Illusions

• In the Müller-Lyer illusion, two vertical line
  segments are equal in length, but the one with
  arrow feather endings appears to be longer
• The line with arrow feather endings has the
  appearance of a corner that is receding away from
  you (the corners where two walls meet in a room),
  while the line with arrowhead endings has the
  appearance of a corner that is jutting out toward
  you (the corners where two sides of a building
  meet)
• Thus, it is our past experience with corners that
  leads the brain to believe that the line with
  arrow feather endings is farther away