Sensation and
Perception
Chapter Four
Sensation vs. Perception
 Sensation:
such as
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the process of detecting a stimulus
light waves (vision)
sound waves (hearing)
chemical molecules (smell and taste)
heat or pressure (touch)
orientation or balance (kinesthetic senses)
 Perception: the process
of integrating,
organizing and interpreting sensations

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From a sensory point of
view – mass of red,
white and blue colors
and horizontal and
vertical lines
Perception allows you
to interpret the
splotches of color and
array of lines as the
American flag
 Bottom-up processing – sensory analysis that
starts at entry levels

Used when we have no prior knowledge
 Top-down processing – construction of
perception based on experiences and
expectations
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Used when we do have prior knowledge
 Transduction
– process by which sensory
receptors convert the incoming physical
energy of stimuli (like light waves) into
neural impulses the brain can
understand
 Sensations such as “red” and “cold”
occur
only when the neural impulse reaches the
brain
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Selective attention – focusing conscious awareness
on a particular stimulus
 Attention to one thing causes inattention to
something else
 Cocktail party effect – ability to attend to only one
voice among many
Inattentional blindness – failing to see visible
objects when our attention is directed elsewhere
Change blindness – failing to notice changes in the
environment
Thresholds
 Absolute threshold –
minimum amount of a
stimulus that an
observer can reliably
detect at least 50% of
the time
Thresholds -cont
Difference threshold – minimal difference needed to
notice a stimulus change; called “just noticeable
difference”
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Noticing when the TV volume is turned down just one notch
Weber’s law – just noticeable difference will vary
depending on its relation to the original stimulus
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Size of JND is proportional to the strength of the original
stimulus
Noticing the addition of a 5-pound weight when bench
pressing 50 pounds but not when bench pressing 500
pounds
Thresholds -cont Signal detection theory – tries to explain and
predict different perceptual mistakes we make

False positive – we think we perceive a stimulus
that isn’t there
 Mistaking a stranger for someone you know on a
crowded street
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False negative – not perceiving a stimulus that is
present
 Not noticing directions on a test that tell you to
write in complete sentences
 Sensory adaptation – when a constant stimulus
is presented for a length of time, receptors fire
less frequently and the sensation often fades or
disappears
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Getting used to new running shoes or the
temperature of the water
Does not affect vision
Vision
Vision:
From the cornea to the retina
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Cornea – light waves first enter the eye here
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Pupil –opening in the middle of the iris
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Clear membrane covering the visible part of the eye
Protects the eye and helps gather and direct incoming
light waves
Changes size to let in different amounts of light
Iris – colored part of the eye
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Ring of muscle tissue that contracts or expands to control
the size of the pupil
Muscles respond to light and inner emotions – constrict in
parasympathetic calm and dilate in sympathetic arousal
Vision:
From the cornea to the retina
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Lens – transparent structure located behind the
pupil that actually focuses and bends light as it
enters the eye
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Accommodation – change in the curvature of the lens
that enables the eye to focus on objects at various
distances
Nearsightedness – results when the cornea and lens
focus on an image in front of the retina, making distant
objects appear blurry
Farsightedness – results when the cornea and lens
focus on an image behind the retina, making objects
near the eye appear blurry
Vision: The retina
 Retina – light-sensitive membrane at the back of
the eye where the transduction of light waves
into neural messages occurs
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Contains millions of sensory receptors for vision
Rods – allow you to see in poorly light
environments
 Located primarily in the retina’s periphery
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Cones- sensitive to colors and bright light
 Concentrated in the fovea – a small region in the
center of the retina
Vision: The retina
 Bipolar cells – specialized neurons that connect
the rods and cones with ganglion cells
 Ganglion cells – specialized neurons that
connect to the bipolar cells
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Bundled axons of ganglion cells form the optic
nerve
 Blind spot – point where the optic nerve leaves
the eye and where there are no rods or cones,
creating a blind spot in our vision
 Feature detectors – nerve cells in the brain that
respond to specific features of the stimulus,
such as shape, angle or movement
Theories of Color Vision
 Trichromatic or three-color theory – theorizes
that the retina has three different color
receptors – cones that detect the different
colors red (long wavelengths), blue (short
wavelengths) or green (medium wavelengths) –
which when stimulated in combination can
produce the perception of any color
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Does not explain afterimages and color blindness
Theories of Color Vision
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Opponent-process theory – theorizes that ganglion
cells process color in opposing pairs of red or green,
black or white, and blue or yellow colors
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The visual cortex also encodes color in terms of these
three opponent pairs
Explains afterimages – visual experiences that occurs
after the original source of stimulation is no longer
present
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When you look at the color red for a long time, you
fatigue the sensors for red; when you switch your gaze
and look at a blank page, the opponent part of the pair
for red will fire and you will see a green afterimage
Theories of Color Vision
 Color blindness
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Typically caused by deficiency in cones
Most common is related to deficiencies in redgreen system
Hearing
Sound Waves
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Ear transforms vibrating air into nerve impulses,
which our brain decodes as sounds
The amplitude (strength) of sound waves
determines their loudness
Waves vary in frequency – number of complete
wavelengths that pass a point in a given time
Frequency determines the pitch (highness or
lowness) that we experience
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Long waves have low frequency, short waves have high
frequency
The Outer Ear
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Collects sound waves
The pinna
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The auditory canal
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Flap of skin and cartilage attached to each side of our head
Catches sound waves and channels them into the auditory canal
Sound waves travel down the auditory canal and bounce into the
ear drum
The eardrum or tympanic membrane
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Tightly stretched membrane located at the end of the auditory
canal
Eardrum vibrates when hit by sound waves; vibrations match
the intensity
The Middle Ear
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Amplifies sound waves
Hammer, anvil and stirrup (collectively called
ossicles)
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Three tiny bones in the middle ear
Joint action doubles the amplification of sound
Oval window
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Small membrane separating the middle ear from the
inner ear
Stirrup transmits amplified vibrations to the oval
window and oval window relays vibrations to the
cochlea
The Inner Ear
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Transduces sound waves into neural messages
Cochlea
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Basilar membrane
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Spiral-shaped, fluid-filled structure that contains the
basilar membrane and hair cells
Runs the length of the cochlea
Holds the hair receptors for hearing
Hair cells
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Sensory receptors embedded in the basilar membrane
Hair cells transduce the physical vibration of the sound
waves into neural impulses
Distinguishing Pitch
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Pitch – relative highness or lowness of a sound
Frequency theory – theory differences in pitch are due to the
rate of neural impulses traveling up the auditory nerve
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We sense pitch because the hair cells fire at different rates
(frequencies) in the cochlea
Explains how low-frequency tones are transmitted to the brain
Place theory – theory that differences in pitch result from
stimulation of different areas of the basilar membrane
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Higher-frequency sounds cause maximum vibrations near the
stirrup end of the basilar membrane; lower frequency sounds
cause maximum vibrations at the opposite end
Explains how high-frequency tones are transmitted to the brain
Loss of Hearing
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Conduction deafness
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Caused when the bones in the middle ear are damaged
and can’t transmit sound waves to the inner ear
Causes can include tumors, objects in ear canal, infections
or otosclerosis (genetic degeneration of the middle ear
bones)
Nerve deafness
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Caused by damage to the cochlea, hair cells or auditory
nerve
Treated with hearing aids or cochlear implants
Causes include infections, genetic defects, exposure to
loud noises, trauma, high blood pressure, diabetes and MS
Touch and the Sensory Cortex
Touch
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Touch receptors aren’t evenly distributed among the
different areas of our bodies
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More densely concentrated in the face, hands and lips
than on the legs or back
Gate-control theory of pain – the brain regulates
pain by sending signals down the spinal cord that
either open or close sensory pathways or “gates”
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Brain signals gates to open = pain is experienced or
intensified
Brain signals gates to close = pain is reduced
 Vestibular sense – provides a sense of balance
and equilibrium
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Inner ear contains receptors that are especially
important for maintaining balance
Semicircular canals are filled with fluid and lined
with hair like receptor cells that shift in response
to motion, providing the brain with important
information about the body’s posture and head
position
 Kinesthetic sense – gives us feedback about the
position and orientation of specific body parts
Chemical Senses
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Taste (or gustation)
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Chemicals from food are absorbed by taste buds on our
tongue
Taste buds are located on papillae – bumps you can see
on your tongue
Are located all over the tongue and some parts of the
inside of the cheeks and roof of the mouth
 More densely packed taste buds = more intense taste
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Humans sense five different types of taste: sweet, salty,
sour, bitter and umami (savory or meaty taste)
Chemical Senses
 Smell (or olfaction)
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Mucous membrane at the top of each nostril
contains receptor cells that absorb airborne
chemical molecules
Receptor cells communicate neural messages to
the olfactory bulb
Impulses from the olfactory bulb don’t go to the
thalamus
 Nerve fibers connect to the brain at the amygdala
and then to the hippocampus
Rods, Cones (in Retina)
Hearing
Hair cells connected to the organ of Corti (in cochlea)
Touch
Temperature, pressure, pain nerve endings (in the
skin)
Chemical Senses
Taste
(gustation)
Sweet, sour, salty, bitter, umami taste buds (in papillae
on the tongue)
Smell
(olfaction)
Smell receptors connected to the olfactory bulb (in the
top of the nose)
Vestibular
sense
Hair like receptors in three semicircular canals (in the
inner ear)
Kinesthetic
sense
Receptors in muscles and joints
Energy Senses
Vision
Body Position Senses
Sense and Associated Receptors
Perceptual
Organization
Gestalt Principles of Organization
 Founded by Max Wertheimer in early 1900s
 Maintains that we actively process our
sensations according to consistent perceptual
rules
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Rules create whole perceptions (gestalts) that are
meaningful, symmetrical and as simple as
conditions allow
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Figure-ground
relationship –
organization of the visual
field into objects (figures)
that stand out from their
surroundings (ground)
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Your brain organizes
black markings in a book
as letters and groups
them into words and
sentences
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Letters = figure
White page = ground
Perceptual Grouping
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Law of similarity – tendency to perceive objects of a
similar size, shape or color as a unit or figure
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Law of proximity – tendency to perceive objects that
are physically close to one another as a single unit
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Organizing a crowd at a football game into home fans,
visiting fans, band members and cheerleaders
Grouping visiting fans into a single, homogenous group
Law of closure – tendency to fill in the gaps in an
incomplete image
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Scoreboard reads “HO E and VISI ORS” and your brain
fills in missing M and T to complete the words
Constancy
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Size constancy – objects closer to our eyes will
produce bigger images on our retinas, but we take
distance into account in our estimations of size
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Knowing an object doesn’t grow or shrink in size as it
moves closer or farther away
Shape constancy – objects viewed from different
angles will produce different shapes on our retinas,
but we know the shape of an object remains
constant
Brightness constancy – we perceive objects as being
a constant color even as the light reflecting off the
object changes
Shape constancy
Perceived Motion
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Stroboscopic effect – images in a series of still
pictures presented at a certain speed will appear to
be moving
Phi phenomenon – series of light bulbs turned on
and off at a particular rate will appear to be one
moving light
Autokinetic effect – if people are asked to stare at a
spot of light projected steadily onto the same place
on a wall of an otherwise dark room, they will
report seeing it move
Depth perception
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Ability to perceive
three-dimensional
space and to accurately
judge distance
Visual cliff experiment
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Supports conclusion
that perception in
humans is innate and
emerges during infancy
Monocular depth cues
 Require the use of only one eye to process
distance or depth cues
Monocular depth cues
 Linear perspective -
parallel lines appear
to converge toward a
vanishing point as
they recede into the
distance
 Carlo Crivelli’s The
Annunciation
Monocular depth cues
 Aerial perspective –
distant objects often
appear hazy and
blurred compared to
close objects
 Pieter Bruegel the
Elder’s The Harvester
Monocular depth cues
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Relative size - if two or
more objects are
assumed to be similar
in size, the object that
appears larger is
perceived as being
closer
George Seurat’s Sunday
Afternoon on the Island
of La Crande Jatte
Monocular depth cues
 Motion parallax - as you move, you use the
speed of passing objects to estimate the
distance of the objects
 On the interstate, nearby telephone poles,
fences and roadside signs seem to zip by faster
than distant hills
Binocular depth cues
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Require the use of both eyes to process distance or
depth cues
Convergence – binocular depth cue in which the
closer the object, the more the eyes converge, or
turn inward
Retinal disparity – binocular depth cue in which
separation of the eyes causes different images to fall
on each retina
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when two retinal images are very different, we
interpret the object as being close by; when they are
more nearly identical, the object is perceived as being
farther away