Sensation and
Perception
Psychology:
A Concise Introduction
2nd Edition
Richard Griggs
Chapter 3
Prepared by
J. W. Taylor V
Windows on the World
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We understand the world through our
senses, our “windows” on the world
Our reality, in fact, is dependent upon two
basic processes:
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Sensation: Gathering information
Perception: Interpreting information
An Example of Misperception
The Journey…

How the Physical World
Relates to the Psychological
World

How We See and How We Hear

How We Make Sense of What
We See
How the Physical World
Relates to the
Psychological World
The Detection Question
The Difference Question
The Scaling Question
The Questions
1. 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?
The Questions
2. 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?
The Questions
3. 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?
The Detection Question
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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 shortlived with no long-term consequences on our
behavior
Theoretical and Observed
Absolute Thresholds
Four Possible Outcomes in a
Signal Detection Study
Observer’s
Response
Signal
“Yes”
“No”
Present
Absent
Hit
False
Alarm
Miss
Correct
Rejection
The Difference Question
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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
Weber’s 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
The Scaling Question

Steven’s 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
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
How We See and
How We Hear
How the Eye Works
How We See Color
How the Ear Works
How We Distinguish Pitch
Physical Characteristics of
Light and Sound Waves

Wavelength refers to the
distance in one cycle of a wave,
from one crest to the next

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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
A Typical Waveform
and its Characteristics
Physical Characteristics of
Light and Sound Waves

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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
How the Eye Works

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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
How the Eye Works

The retina is the light-sensitive layer of the eye
and has three layers of cells:
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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
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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
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
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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)
How the Eye Works
How We See Color

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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

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The three types of cell systems are redgreen and blue-yellow, as well as blackwhite (to detect brightness)
If one color in a pair is stimulated, the
other is inhibited
Subtractive and
Additive Mixtures
Demonstration of
Complementary Afterimage
Demonstration of
Complementary Afterimage
How We See Color

Both theories have validity,
each at different levels of
visual information processing

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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)
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
How the Ear Works
The middle ear
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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
How the Ear Works
The inner ear
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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
How the Ear Works
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
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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
How We Distinguish Pitch
Both theories have validity

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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 (< 500 Hz) and Place 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
How We Make Sense
of What We See
Bottom-up Processing and
Top-down Processing
Perceptual Organization and
Perceptual Constancy
Depth Perception
Bottom-up Processing and
Top-down Processing
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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 brain’s use of existing
knowledge, beliefs, and expectations to interpret the
sensory stimulation
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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
Perceptual Organization
and Top-down Processing
A Context Effect on Perception
Perceptual Organization
Gestalt means “organized whole”
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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
An Example of
Figure-Ground Ambiguity
An Example of an Organizational
Perceptual Ambiguity
An Example of a
Subjective Contour
Perceptual Constancy
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Refers to the perceptual stability of
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Size
Shape
Brightness
Color
For familiar objects seen at
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Varying distances
Different angles
Different lighting conditions
Depth Perception
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Involves judging the distance
of objects from us
Binocular depth cues
require the use of both eyes
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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
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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
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
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