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Chapter 4
Sensation and Reality
Psychophysics
• Study of relationship between physical stimuli
and sensations they evoke in a human
observer
• Absolute threshold: Minimum amount of
physical energy necessary to produce a
sensation
• Subliminal perception: Perception of a
stimulus below the threshold for conscious
recognition
• Difference threshold: A change in stimulus
intensity that is detectable to an observer
General Properties of Sensory Systems
• Perceptual features: Basic stimulus patterns
• Sensory coding: Converting important
features of the world into messages
understood by the brain
Just Noticeable Difference (JND)
• Any noticeable difference in a stimulus
Sensation and Perception
• Sensation: Information arriving from sense
organs (eye, ear, etc.)
• Perception: Mental process of organizing
sensations into meaningful patterns
• What can go wrong in these two processes?
Absolute thresholds define the sensory worlds of humans and animals,
sometimes with serious consequences. The endangered Florida
manatee (“sea cow”) is a peaceful, plant-eating creature that can live for
more than 60 years. For the last decade, the number of manatees killed
by boats has climbed alarmingly. The problem? Manatees have poor
sensitivity to the low-frequency sounds made by slow-moving boats.
Current laws require boats to slow down in manatee habitats, which
may actually increase the risk to these gentle giants (Gerstein, 2002).
p. 120
Using Your Senses
In partners, I will give you pieces of paper
with the names of different objects written
on them. Do not to show the words to any
other group. Have each group figure out
how much sensory information there is in
each of the written common objects. Use
all 5 of your senses in their description.
Vision: The Key Sense
• Visible spectrum: Narrow spread of the
electromagnetic spectrum to which the eyes
respond
Parts of the Eye
• Lens: Structure in the eye that focuses light
rays
• Photoreceptors: Light-sensitive cells in the
eye
More Parts of the Eye
• Retina: Light-sensitive layer of cells in the
back of the eye
– Easily damaged from excessive exposure
to light (staring at an eclipse)
• Cornea: Transparent membrane covering the
front of the eye; bends light rays inward
Accommodation
• Changes in the shape of the lens of the eye
Fig. 4-3, p. 123
Visual popout. Popout is so
basic that
babies as
young as 3
months
respond to
it.
Fig. 4-1, p. 122
Vision Problems
• Hyperopia: Difficulty focusing nearby objects
(farsightedness)
• Myopia: Difficulty focusing distant objects
(nearsightedness)
• Astigmatism: Corneal, or lens defect that
causes some areas of vision to be out of
focus; relatively common
• Presbyopia: Farsightedness caused by aging
Light Vision
• Cones: Visual receptors for colors and bright
light (daylight); 5 million in each eye
• Rods: Visual receptors for dim light; only
produce black and white; about 120 million
total
• Blind spot: Area of the retina lacking visual
receptors
Anatomy of the retina.
The retina lies behind
the vitreous humor,
which is the jelly-like
substance that fills the
eyeball. The rods and
cones are much
smaller than implied
here. The smallest are
1 micron (one millionth
of a meter) wide. The
lower-left photograph
shows rods and cones
as seen through an
electron microscope.
In the photograph the
cones are colored
green and the rods
Fig. 4-7, p. 125
Experiencing the blind spot. (a) With your right eye closed, stare
at the upper-right cross. Hold the book about 1 foot from your eye
and slowly move it back and forth. You should be able to locate a
position that causes the black spot to disappear. When it does, it
has fallen on the blind spot. With a little practice you can learn to
make people or objects you dislike disappear too! (b) Repeat the
procedure described, but stare at the lower cross. When the white
space falls on the blind spot, the black lines will appear to be
continuous. This may help you understand why you do not usually
experience a blind spot in your visual field.
Fig. 4-8, p. 126
More on Light Control
• Visual acuity: Sharpness of visual perception
• Fovea: Area at the center of the retina
containing only cones (50,000)
• Peripheral vision: Vision at edges of visual
field; side vision
– Many superstar athletes have excellent
peripheral vision
• Tunnel vision: Loss of peripheral vision
Color Vision
Trichromatic Theory
• Color vision theory that states we have three
cone types: red, green, blue
– Other colors produced by a combination of
these
Opponent Process Theory
• Color vision theory based on three “systems”:
red or green, blue or yellow, black or white
– Exciting one color in a pair (red) blocks the
excitation in the other member of the pair
(green)
– Afterimage: Visual sensation that remains
after stimulus is removed (seeing flashbulb
after the picture has been taken)
Example of Opponent Process
Theory
• Negative afterimages. On the following slide, stare at
the dot near the middle of the flag for at least 30
seconds. Then look immediately at a plain sheet of
white paper or a white wall. You will see the
American flag in its normal colors. Reduced
sensitivity to yellow, green, and black in the visual
system, caused by prolonged staring, results in the
appearance of complementary colors. Project the
afterimage of the flag on other colored surfaces to
get additional effects.
Fig. 4-12, p. 128
Color Blindness
• Color blindness: Inability to perceive colors;
lacks cones or has malfunctioning cones
– Total color blindness is rare
• Color weakness: Inability to distinguish some
colors
– Red-green is most common; much more
common among men than women
– Recessive, sex-linked trait on X
chromosome
Fig. 4-17, p. 130
Dark Adaptation
• Increased retinal sensitivity to light after
entering the dark; similar to going from
daylight into a dark movie theater
• Rhodopsin: Light-sensitive pigment in the
rods; involved with night vision
• Night blindness: Blindness under low-light
conditions; hazardous for driving at night
Light Adaptation
• In pairs, observe each other’s pupils. Flip a
coin to select the “subject.” I will darken the
room for a few minutes. Then turn on the
lights and estimate how many seconds it takes
your partner’s pupils to return to their original
degree of constriction.
Hearing
• Sound waves: Rhythmic movement of air
molecules
• Pitch: Higher or lower tone of a sound
• Loudness: Sound intensity
Anatomy of the ear. The
entire ear is a
mechanism for changing
waves of air pressure
into nerve impulses. The
inset in the foreground
(Cochlea “Unrolled”)
shows that as the stapes
moves the oval window,
the round window bulges
outward, allowing waves
to ripple through fluid in
the cochlea. The waves
move membranes near
the hair cells, causing
cilia or “bristles” on the
tips of the cells to bend.
The hair cells then
generate nerve impulses
carried to the brain.
Fig. 4-20, p. 133
This is how the
cochlea looks
“unrolled”. How
does this diagram
of sound waves
explain why you
can hear the bass
from a big car
stereo from far
away?
Fig. 4-22, p. 134
How Do We Detect Higher and Lower
Sounds?
• Frequency theory: As pitch rises, nerve
impulses of a corresponding frequency are
fed into the auditory nerve
• Place theory: Higher and lower tones excite
specific areas of the cochlea
Hearing Loss
• Noise Induced Hearing Loss
• Damage caused by exposing hair cells to excessively loud
sounds
– Typical at rock concerts
– By age 65, more than 40% of hair cells are gone
• Sensorineural Hearing Loss
• Caused by damage to hair cells or auditory nerve
– Hearing aids little or no help in these cases
– Cochlear implant: Electronic device that stimulates auditory
nerves directly
• Conduction Hearing Loss:
• Poor transfer of sounds from tympanic membrane to inner ear
– Compensate with amplifier (hearing aid)
Fig. 4-25, p. 135
Smell
• Olfaction: Sense of smell
• Dysosmia: Loss or impairment of sense of
smell
• Lock-and-key theory: Odors are related to
shapes of chemicals and molecules
• Pheromones: Airborne chemical signal
– Vomeronasal organ: Sense organ for
pheromones
Smell Habituation
• Ammonia, rotten eggs, vinegar and other
smelly materials can be brought to class.
Students rate the smell at the start of class
and every 10 minutes after, and should quickly
experience olfactory adaptation (habituation).
• I didn’t want to stink up the classroom, but
does anyone have any experience with this?
Gustation
• Sense of taste
– Four taste sensations: sweet, salt, sour,
bitter
– Most sensitive to bitter, less to sour, less to
salt, least sensitive to sweet
– Umami: Possible fifth taste sensation;
brothy taste
How do these work together?
• As the text points out, “flavor,” as we experience it
subjectively, is actually a combination of olfaction and
gustation. This can be demonstrated relatively easily.
• I need a volunteer who can eat something.
• Have a student volunteer taste bits of each while blindfolded.
Discriminating between different foods should be simple in
this condition. Next, test the blindfolded volunteer while he or
she pinches the nostrils closed. With more olfactory cues
reduced, correctly identifying the food bits should be more
difficult (although not impossible). Even if they correctly
identify the foods in the second test, subjects will usually
report greater difficulty. Typically, they must rely more on
texture than on “taste” when olfactory cues are reduced
Somesthetic Senses
• Sensations produced by skin, muscles, joints,
viscera, and organs of balance
• Skin senses (touch): Light touch, pressure,
pain, cold, warmth
• Kinesthetic: Detect body position and
movement
• Vestibular: Balance, acceleration, and
position in space
• Sensory organs for touch, pressure, pain,
cold, and warmth
Pain
• Visceral pain: Pain originating in internal
organs
• Referred pain: Pain felt in one part of the
body, but coming from another
• Somatic pain: Sharp, bright, fast; comes from
skin, joints, muscles, tendons
Types of Pain
• Warning system: Pain carried by large nerve
fibers; sharp, bright, fast pain that tells you
body damage may be occurring (e.g., knife
cut)
• Reminding system: Small nerve fibers;
slower, nagging, aching, widespread; gets
worse if stimulus is repeated; reminds brain
that body has been injured
Vestibular System
• Otolith organs: Sensitive to movement,
acceleration, and gravity
• Semicircular canals: Fluid-filled tubes in ears
that are sensory organs for balance
• Crista: “Float” that detects movement in
semicircular canals
• Ampulla: A wider part of the canal
Fig. 4-31, p. 141
Sensory Conflict Theory
• Motion sickness results from a mismatch
between information from vision, vestibular
system, and kinesthesis
– After spinning and stopping, fluid in
semicircular canals is still spinning, but
head is not
– Mismatch leads to sickness
• Medications, relaxation, and lying down might
help
Gate Control Theory of Pain
• Gate control theory: Pain messages from
different nerve fibers pass through the same
“neural” gate in the spinal cord.
– If gate is closed by one pain message,
other messages may not be able to pass
through
Phantom Limb
• Missing limb feels like it is present, like always,
before amputation or accident
Controlling Pain
• Fear, or high levels of anxiety, almost always
increase pain
• If you can regulate a painful stimulus, you
have control over it
• Distraction can also significantly reduce pain
• The interpretation you give a stimulus also
affects pain
Coping With Pain
• Prepared childbirth training: Promotes birth with
a minimal amount of drugs or painkillers
• Counterirritation: Using mild pain to block more
intense or long-lasting pain
CUTANEOUS TWO-POINT THRESHOLD
• 1. Be sure the subject is securely blindfolded.
• 2. Apply even, firm, but not excessive pressure with one point or two,
as directed. You should avoid causing pain to (or breaking the skin of) the
subject.
• 3. Make exact measurements when setting the two points for each
trial.
• 4. Do not repeat any trials.
• 5. Ask the subject for a response after each application of pressure.
• 6. Do not give hints or clues to help the subject. Encourage the subject
to make an immediate response.
• For each response made by the subject, put a 1 or 2 in the appropriate
box.