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Chapter 4
Sensation & Perception
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ISBN: 0-131-73180-7
Copyright © Allyn and Bacon 2007
Sensation and Perception
Sensation –
An early stage of perception in which neurons
in a receptor create an internal pattern of nerve
impulses that represent the conditions that
stimulated it – either inside or outside the body
Perception –
A process that makes sensory patterns
meaningful and more elaborate; it’s a person’s
interpretation of sensation. It draws upon
memory, motivation, emotion, and other
processes.
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How Does Stimulation
Become Sensation?
The brain senses the world
indirectly because the sense
organs convert stimulation
into the language of the
nervous system: neural
impulses
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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
Converting physical energy into neural
messages…stimulus reaches sense
organ, activates receptors, converts into
nerve signal
Sensory pathway –
Bundles of neurons that carry information
from the sense organs to the thalamus
then the rest of 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 (detected accurately 50% of the
time over many trials)
Difference threshold –
Smallest amount by which a stimulus can be
changed and the difference be detected half of
the time (also called just noticeable difference
– JND)
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Thresholds
Weber’s law –
The JND is always large when the
stimulus intensity is high, and small when
the stimulus intensity is low
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Signal Detection Theory
Signal detection theory –
Perceptual judgment is a combination of
sensation and decision-making processes
Stimulus event
Neural activity
Comparison with
personal standard
Action (or no action)
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Subliminal Persuasion
Studies have found that subliminal words
flashed briefly on a screen can “prime” a
person’s later responses
No controlled research has ever shown
that subliminal messages delivered to a
mass audience can influence people’s
buying habits
Copyright © Allyn and Bacon 2006
How Are the Senses Alike?
How Are They Different?
The senses all operate in
much the same way, but each
extracts different information
and sends it to its own
specialized processing region
in the brain
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The Anatomy of Visual Sensation
Retina –
Light-sensitive layer at the back of the eyeball
Photoreceptors – Light-sensitive cells in the retina
that convert light energy to neural impulses
Rods – Sensitive to dim
light but not colors
Cones – Sensitive to
colors 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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The Anatomy of Visual Sensation
Visual cortex –
Part of the brain – the occipital cortex –
where visual sensations are processed
Color –
Psychological sensation derived from the
wavelength of visible light – color, itself, is
not a property of the external world
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Transduction of Light in the Retina
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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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Copyright © Allyn and Bacon 2006
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Neural Pathways in the Human Visual
System
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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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How the Visual System Creates Color
Color blindness –
Vision disorder that prevents an
individual from discriminating certain
colors
Brightness –
Sensation caused by the intensity of
light waves
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Color Blindness
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How the Visual System Creates
Brightness
Wavelength
Intensity
(amplitude)
Color…longer
means closer to
red…
Brightness…the
greater the
amplitude…
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Two ways of sensing colors
1) Three different types of cones sense three
different parts of the visible spectrum—red,
green, and blue. This is known as the
trichromatic theory.
2) Color is processed in either-or
complementary pairs such as red/green or
yellow/blue in the bipolar cells and beyond.
This explains color blindness and negative
afterimages. It is known as the opponentprocess theory.
Copyright © Allyn and Bacon 2006
Hearing: If a Tree Falls in the
Forest...
The Physics of Sound
Frequency –
Number of cycles completed by a wave in a
given amount of time…cycles per second
are usually measured in hertz (Hz).
Low Frequency
High Frequency
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Hearing: If a Tree Falls in the
Forest...
The Physics of Sound
Amplitude –
Physical strength of a wave
High Amplitude
Low Amplitude
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So to recap…similarity between vision and
hearing…
Vision—the frequency of light waves determines color or
hue; higher frequency (short wavelength) means closer
to purple or blue and lower frequency (longer
wavelength) means closer to red. Higher amplitude
means brighter color.
Hearing—the frequency of sound waves determines
pitch; higher frequency (short wavelength) means
higher pitch tone and lower frequency (longer
wavelength) means lower pitch tone. Higher amplitude
means higher volume.
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How Sound Waves Become Auditory
Sensations
Tympanic membrane –
The eardrum
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How Sound Waves Become Auditory
Sensations
Cochlea –
Where sound
waves are
transduced
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How Sound Waves Become Auditory
Sensations
Cochlea
Basilar membrane –
Thin strip of tissue
sensitive to
vibrations
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How Sound Waves Become Auditory
Sensations
Auditory nerve –
Neural pathway
connecting the ear
and the brain
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How Sound Waves Become Auditory
Sensations
Auditory cortex –
Portion of the temporal lobe that
processes sounds
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The Psychology of Pitch,
Loudness, and Timbre
Timbre –
Quality of a sound wave that derives from
the wave’s complexity (zigzags in the
sound waves’ form). It helps you identify
sounds, for example even though the
same song can be sung by different
singers and they are singing the same
exact notes, you can tell the difference in
voice due to timbre.
Copyright © Allyn and Bacon 2006
Deafness
Conduction deafness –
An inability to hear resulting from damage to structures
of the middle or inner ear. This is not genetic.
Nerve deafness –
An inability to hear, linked to a deficit in the body’s
ability to transmit impulses from the cochlea to the
brain, usually involving the auditory nerve or higher
auditory processing centers. This tends to be the case
in individuals born deaf.
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Position and Movement
Vestibular sense –
Sense of body orientation with respect to
gravity
Kinesthetic sense –
Sense of body position and movement of
body parts relative to each other
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Smell
Olfaction –
Sense of smell
Olfactory bulbs –
Brain sites of olfactory processing
Pheromones –
Chemical signals released by organisms
to communicate with other members of
the species
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Smell
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Taste
Gustation – The sense of taste
Taste buds –
Receptors for taste
(primarily on the
upper side of the
tongue)
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The Skin Senses
Touch
Warmth
Cold
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Pain
Placebos –
Substances that appear to be drugs due
to effect but are not
Placebo effect –
A response to a placebo caused by
subjects’ belief that they are taking real
drugs or medications
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What is the Relationship
Between Perception
and Sensation?
Perception brings meaning
to sensation, so perception
produces an interpretation of
the external world, not a
perfect representation of it
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What is the Relationship
Between Perception
and Sensation?
Percept –
Meaningful product of a perception
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The Machinery of
Perceptual Processing
Feature detectors –
Cells in the cortex that specialize in
extracting certain features of a stimulus
Binding problem –
A major unsolved mystery in cognitive
psychology, concerning the physical
processes used by the brain to combine
many aspects of sensation to a single
percept
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Bottom-Up and
Top-Down Processing
Bottom-up processing –
Analysis that emphasizes characteristics
of the stimulus, rather than internal
concepts
Top-down processing –
Emphasizes perceiver's expectations,
memories, and other cognitive factors
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Perceptual Constancies
Perceptual constancy –
Ability to recognize the same object
under different conditions, such as
changes in illumination, distance, or
location
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Perceptual Ambiguity and Distortion
Illusions –
Demonstrably incorrect experience of a
stimulus pattern, shared by others in the
same perceptual environment
Ambiguous figures –
Images that are capable of more than
one interpretation
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Perceptual Illusions—The Necker Cube
Do you see
or
?
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Perceptual Illusions—The Hermann Grid
Do you see small gray squares between
the black squares?
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Theoretical Explanations for
Perception
Learning-based inference –
View that perception is primarily shaped
by learning, rather than innate factors
Perceptual set –
Readiness to detect a particular stimulus
in a given context
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Cultural Influences on Perception
A
B
Which box is bigger, A or B?
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The Gestalt Approach
Gestalt psychology –
View that much of perception is shaped by
innate factors built into the brain; it focuses on
perceptual wholes and patterns, not just parts
Figure –
Part of a pattern that commands attention
Ground –
Part of a pattern that does not command
attention; the background
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The Gestalt Approach
Subjective contours –
Boundaries that are perceived but do not
appear in the stimulus pattern
Closure –
Tendency to fill in gaps in figures and
see incomplete figures as complete
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The Gestalt Laws of Perceptual
Grouping—Max Wertheimer
Similarity
Proximity
Continuity
Common fate
Prägnanz
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Law of Prägnanz
A
BIRD
IN THE
THE HAND
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Binocular Cues for Depth Perception
These depend on the use of two eyes
1) Binocular convergence: this has to do
with the feeling in your eye muscles as
you focus on objects of different
distances
2) Retinal disparity: this has to do with the
slightly different perspective in each eye
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Monocular Cues for Depth Perception
This has to do with being able to judge distance
using just one eye.
1) Relative size
2) Light and shadow
3) Interposition
4) Relative motion
5) Atmospheric perspective
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End of Chapter 4
Copyright © Allyn and Bacon 2007