Reader’s Guide (cont.)
Vocabulary
– sensation 
– perception 
– psychophysics 
– absolute threshold 
– Weber’s law 
– signal-detection theory
Click the Speaker button
to listen to Exploring
Psychology.
1
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information. Section 1 begins on page 207 of your textbook.
• Blindfold Activity
– Walk around school
with a partner
– Hear, Smell, Taste,
Touch
– How do our other
senses
overcompensate?
2
What Is Sensation?
• The world is filled with physical changes.
• Any aspect of or change in the
environment to which an organism
responds is called a stimulus. 
• An alarm, an electric light, and an aching
muscle are all stimuli for human beings.
3
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
What Is Sensation? (cont.)
• A stimulus can be measured in many
physical ways, including its size,
duration, intensity, or wavelength. 
• A sensation occurs anytime a stimulus
activates one of your receptors. 
• The sense organs detect physical
changes in energy such as heat, light,
sound, and physical pressure.
sensation
what occurs when a stimulus
activates a receptor
4
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• Sensory
Deprivation
– This tank allows
users to escape
gravity
– Helps recover from
strenuous activity
– Too much sensory
deprivation can
lead to loss of IQ
5
What Is Sensation? (cont.)
• A sensation may be combined with other
sensations and your past experience to
yield a perception. 
• A perception is the organization of sensory
information into meaningful experiences.
perception
the organization of sensory
information into meaningful
experiences
6
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Fraser’s Spiral
Fraser’s spiral
illustrates the
difference
between
sensation and
perception. Our
perception of
this figure is
that of a spiral,
but it is actually
an illusion.
Trace the circle
carefully. Your
finger will
always come
back to its
starting point.
7
What Is Sensation? (cont.)
• What is the relationship between color
and wavelength? 
• How does changing a light’s intensity
affect your perception of its brightness? 
• The psychological study of such questions
is called psychophysics.
psychophysics
the study of the relationships
between sensory experiences
and the physical stimuli that
cause them
8
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Threshold
• How much of a stimulus is needed to notice?
• Experiments can detect the absolute threshold–
the weakest amount of a stimulus required to
produce a sensation.
absolute threshold
the weakest amount
of a stimulus that a
person can detect
half the time
9
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The Human Senses
10
Sensory Differences and Ratios
• Another type of threshold is the
difference threshold. 
• The difference threshold refers to the
minimum amount of difference a person
can detect between two stimuli. 
• A related concept is the just noticeable
difference, or JND.
difference threshold
the smallest change in a
physical stimulus that can be
detected between two stimuli
11
Sensory Differences and Ratios (cont.)
• Weber’s law states that the larger or
stronger a stimulus, the larger the
change required for a person to notice
that anything has happened to it. 
• By experimenting with variations in
sounds, temperatures, pressures, colors,
tastes, and smells, psychologists are
learning more about how each sense
responds to stimulation.
Weber’s law
the principle that the larger or
stronger a stimulus, the larger
the change required for an
observer to notice a difference
12
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Sensory Differences and Ratios (cont.)
• People who can detect minute changes
in sensation are called experts. 
• Experts who can detect small differences
work as food tasters, wine tasters, smell
experts, perfume experts, and so on.
13
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Sensory Adaptation
• What happens when
you eat a ton of
HOT CHEETOS
• Senses are most responsive to increases
and decreases, and to new events rather
than to ongoing, unchanging stimulation. 
• A good example of this sensory adaptation
is the increase in visual sensitivity that you
experience after a short time in a darkened
movie theater.
14
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Sensory Adaptation (cont.)
• Sensory adaptation allows us to
notice differences in sensations and
react to the challenges of different or
changing stimuli. 
• This principle is helpful when performing
many activities, such as the work of police,
security guards, and home inspectors. 
• These people may notice minute changes
and act appropriately.
15
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The Disappearing Circle
Sensation depends on change and contrast in the
environment. Hold your hand over one eye and stare at the
dot in the middle of the circle on the right. You should have no
trouble maintaining the image of the circle. If you do the same
with the circle on the left, however the image will fade. The
circle reappears only if you close and reopen your eye or you
shift your gaze to the X.
16
Signal-Detection Theory
• There is no sharp boundary between
stimuli that you can perceive and stimuli
you cannot perceive. 
• The signal-detection theory studies the
relations between motivation, sensitivity,
and decision making in detecting the
presence or absence
of a stimulus
(Green & Swets, 1966).
signal-detection theory
the study of people’s
tendencies to make correct
judgments in detecting the
presence of stimuli
17
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Signal-Detection Theory (cont.)
• Detection thresholds involve recognizing
some stimulus against a background of
competing stimuli. 
• In studying the difficulties faced by radar
operators, psychologists have reformulated
the concept of absolute threshold to take
into account the many factors that affect
detection of minimal stimuli. 
• As a result, signal-detection theory
abandons the idea that there is a single
true absolute threshold for a stimulus.
18
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19
The Stroop Effect
Name the color of the squares.
20
The Stroop Effect (cont.)
Name the color of the words in the figure below. 
Why was it
more difficult
to name the
color of the
words?
21
Section Assessment
Review the Vocabulary What is
the difference between sensation
and perception?
A sensation is the activation of a
sensory receptor by a stimulus. A
perception is the organization of
sensation into meaningful experiences.
22
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Section Assessment (cont.)
Think Critically Why do you think
we do not respond to all stimuli
present in our environment?
We do not respond to all stimuli
present in our environment because
our bodies would be overloaded
with stimuli. Our bodies would
soon be unable to process more
sensory information.
23
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Section Assessment (cont.)
What would happen if you asked a
four-year-old child who knew his
colors but could not read to complete
the Stroop effect activity? What if you
tried this experiment with someone
who did not speak English? What if
you used noncolor words?
24
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