Sensation and Perception

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Sensation and Perception
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Sensation What is it?

• The process by which a stimulus in the
  environment produces a neural impulse that the
  brain interprets as a sound, image, odor, etc.

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Transduction

• Transduction Transformation of one form of
  energy into another especially the
  transformation of stimulus information into nerve
  impulses
• Receptors Specialized neurons that are
  activated by stimulation and transduce (convert)
  it into a nerve impulse

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Transduction

• Sensory pathway Bundles of neurons that carry
  information from the sense organs to the brain

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Sensory Adaptation

• Sensory adaptation Loss of responsiveness in
  receptor cells after stimulation has remained
  unchanged for a while

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Thresholds

• Absolute threshold Amount of stimulation
  necessary for a stimulus to be detected
• Difference threshold Smallest amount by which
  a stimulus can be changed and the difference be
  detected (also called just noticeable difference
  JND)

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Thresholds

• Webers law The JND is always large when the
  stimulus intensity is high, and small when the
  stimulus intensity is low

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How Are the Senses Alike? How Are They
Different?
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Sense Stimulus Sense Organ Receptor Sensation
Vision Light waves Eye Rods and cones Color, brightness, motion
Hearing Sound waves Ear Hair cells Pitch, loudness
Skin Senses External Contact Skin Nerve endings Touch, warmth, cold
Smell Volatile Substances Nose Hair cells Odors
Taste Soluble Substances Tongue Taste buds Flavors
Pain Intense or Extreme Stimuli Pain Fibers all over body Pain receptors Pain
Kinesthetic Vestibular Body position or balance Semi-circular canals joints, tendons Hair cells specialized neurons Body position
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Sense Stimulus Sense Organ Receptor Sensation
Vision Light waves Eye Rods and cones Color, brightness, motion
Hearing Sound waves Ear Hair cells Pitch, loudness
Skin Senses External Contact Skin Nerve endings Touch, warmth, cold
Smell Volatile Substances Nose Hair cells Odors
Taste Soluble Substances Tongue Taste buds Flavors
Pain Intense or Extreme Stimuli Pain Fibers all over body Pain receptors Pain
Kinesthetic Vestibular Body position or balance Semi-circular canals joints, tendons Hair cells specialized neurons Body position
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Sense Stimulus Sense Organ Receptor Sensation
Vision Light waves Eye Rods and cones Color, brightness, motion
Hearing Sound waves Ear Hair cells Pitch, loudness
Skin Senses External Contact Skin Nerve endings Touch, warmth, cold
Smell Volatile Substances Nose Hair cells Odors
Taste Soluble Substances Tongue Taste buds Flavors
Pain Intense or Extreme Stimuli Pain Fibers all over body Pain receptors Pain
Kinesthetic Vestibular Body position or balance Semi-circular canals joints, tendons Hair cells specialized neurons Body position
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Sense Stimulus Sense Organ Receptor Sensation
Vision Light waves Eye Rods and cones Color, brightness, motion
Hearing Sound waves Ear Hair cells Pitch, loudness
Skin Senses External Contact Skin Nerve endings Touch, warmth, cold
Smell Volatile Substances Nose Hair cells Odors
Taste Soluble Substances Tongue Taste buds Flavors
Pain Intense or Extreme Stimuli Pain Fibers all over body Pain receptors Pain
Kinesthetic Vestibular Body position or balance Semi-circular canals joints, tendons Hair cells specialized neurons Body position
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Sense Stimulus Sense Organ Receptor Sensation
Vision Light waves Eye Rods and cones Color, brightness, motion
Hearing Sound waves Ear Hair cells Pitch, loudness
Skin Senses External Contact Skin Nerve endings Touch, warmth, cold
Smell Volatile Substances Nose Hair cells Odors
Taste Soluble Substances Tongue Taste buds Flavors
Pain Intense or Extreme Stimuli Pain Fibers all over body Pain receptors Pain
Kinesthetic Vestibular Body position or balance Semi-circular canals joints, tendons Hair cells specialized neurons Body position
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Sense Stimulus Sense Organ Receptor Sensation
Vision Light waves Eye Rods and cones Color, brightness, motion
Hearing Sound waves Ear Hair cells Pitch, loudness
Skin Senses External Contact Skin Nerve endings Touch, warmth, cold
Smell Volatile Substances Nose Hair cells Odors
Taste Soluble Substances Tongue Taste buds Flavors
Pain Intense or Extreme Stimuli Pain Fibers all over body Pain receptors Pain
Kinesthetic Vestibular Body position or balance Semi-circular canals joints, tendons Hair cells specialized neurons Body position
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Sense Stimulus Sense Organ Receptor Sensation
Vision Light waves Eye Rods and cones Color, brightness, motion
Hearing Sound waves Ear Hair cells Pitch, loudness
Skin Senses External Contact Skin Nerve endings Touch, warmth, cold
Smell Volatile Substances Nose Hair cells Odors
Taste Soluble Substances Tongue Taste buds Flavors
Pain Intense or Extreme Stimuli Pain Fibers all over body Pain receptors Pain
Kinesthetic Vestibular Body position or balance Semi-circular canals joints, tendons Hair cells specialized neurons Body position
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The Anatomy of Visual Sensation
Photoreceptors Light-sensitive cells in the
retina that convert light energy to neural
impulses Rods Sensitive to dimlight but not
colors Cones Sensitive tocolors but not dim
light
Fovea Area of sharpest vision in the retina
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The Anatomy of Visual Sensation

• Optic nerve Bundle of neurons that carries
  visual information from the retina to the brain

Blind spot Point where the optic nerve exits
the eye and where there are no photoreceptors
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Transduction of Light in the Retina
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The Anatomy of Visual Sensation

• Visual cortex Part of the brain the occipital
  cortex where visual sensations are processed

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Neural Pathways in the Human Visual System
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How the Visual System Creates Color

• Color
• Psychological sensation derived from the
  wavelength of visible light color, itself, is
  not a property of the external world

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How the Visual System Creates Color

• Electromagnetic spectrum Entire range of
  electromagnetic energy, including radio waves,
  X-rays, microwaves, and visible light
• Visible spectrum Tiny part of the
  electromagnetic spectrum to which our eyes are
  sensitive

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Two Ways of Sensing Color

• Trichromatic theory
• Three different types of cones that sense
  different parts of the visible spectrum (i.e.,
  red, green, blue)
• Explains initial stages of color vision
• Opponent Process Theory
• From bipolar cells onward, visual system
  processes color in either-or, complementary
  fashion (i.e., red vs green or blue vs yellow)
• Sensations of one color (e.g., red) inhibits
  sensation of its complementary color (i.e.,
  green)
• Explains negative afterimages color blindness

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Afterimages

• Afterimages Sensations that linger after the
  stimulus is removed
• In the following slide, fix your eyes on the dot
  in the center of the flag

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Color Blindness
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Perception
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Perception

• Same sensory input can give rise to very
  different perceptions
• Perceptual Set
• Readiness to perceive stimuli in specific ways
• Reversible figures drawing that is compatible
  with two different interpretations

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Reversible Figures
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Reversible Figure
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Perception

• What is Perception?
• Active process in which we organize and interpret
  sensory information
• i.e., How we make sense of (or understand) what
  we see, hear, feel, taste, and smell

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Object Perception

• Distal Stimulus
• Stimuli that lie in the distance (i.e., in the
  outside world)
• Three-dimensional
• Proximal Stimulus
• Stimulus that impinges directly onto your sensory
  receptors (i.e., the retina)
• Distorted and two-dimensional

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Distal Stimulus
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Proximal Stimulus
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Images Projected on the Retina
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Object Perception

• Feature Analysis
• Analyze individual features and put them together
  to form a whole

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Bottom-Up Processing

• Start with the elements and progress to the whole
• Evidence
• Hubel and Wiesel cells in the cortex operate as
  highly specialized feature detectors

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Top-Down Processing

• Start with the whole and work towards the
  elements
• What we perceive is influenced by what we expect
  to see
• Context and prior experience are important

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Object Perception

• A number of Gestalt psychology principles can
  help explain how we organize information in order
  to perceive a coherent whole
• Figure/Ground
• Proximity
• Similarity
• Continuity
• Common Fate
• Closure
• Goodness of Form or Pragnanz

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Figure / Ground
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Proximity
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Similarity
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Continuity
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Common Fate
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Closure
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Goodness of Form or Pragnanz

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Images Projected on the Retina
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Proximal Stimulus
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Depth Perception

• Binocular Depth Cues
• Binocular disparity Each eye has a slightly
  different view of the world (i.e., the distal
  stimulus)
• The brain thus has two different BUT overlapping
  images of the world
• The difference between these two retinal (i.e.,
  proximal) images is used to compute distances to
  nearby objects
• For example
• Object at 25 feet image projected to slightly
  different locations on the right and left retina
• Closer objects project images on locations that
  are further apart on the right and left retina

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Depth Perception

• Monocular Depth Cues
• Occlusion
• Relative Size
• Familiar Size
• Linear Perspective
• Texture Gradient
• Position relative to horizon
• Motion cues for Depth
• Motion Paralax
• Optic Flow

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Proximal Stimulus
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Distal Stimulus
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Size Perception

• The size of the retinal image depends on the
  distance of the object from the observer
• Further away smaller retinal image
• Thus, to determine size, visual system must know
  how far away the object is

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Linear Perspective
A
B
Which box is bigger, A or B?
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Moon Illusion
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Final Thoughts

• Visual system is exceptionally good at analyzing
  the outside world
• i.e., perception is a faithful representation of
  the distal stimulus
• Understanding how the visual system is tricked
  (i.e., by studying common illusions) helps us to
  understand how it works