AP Psychology
2.3 Sensation and Perception
Sensation
• Process by which our sensory receptors
respond to light, sound, odor, textures, and
taste
• Our eyes, ears, nose, tongue, and skin
comprise an elaborate sensory system that
receives and processes information from the
environment
Sensation
• Each sense organ contains specialized cells called
receptors which detect and then convert…
– light waves - vision
– sound waves - hearing
– chemical molecules – taste, smell
– pressure – touch
… into neural impulses that are transmitted to the brain.
• adults totally deprived of sensory input for long
periods of time
– experience hallucinations
– impaired efficiency in all areas of intellectual
functioning
Perception
• Process by which the brain actively selects,
organizes and assigns meaning to incoming
neural messages sent from sensory receptors
• Example…
– Sensory point of view – the American flag is a
mass of red, white, and blue colors and horizontal
and vertical lines
– Perception – you interpret these splotches of color
and array of lines as the American flag
Basic Principles of Sensation
• Transduction
– Process by which sensory receptors convert the
incoming physical energy of stimuli (ie. Light
waves) into neural impulses the brain can
understand
– Philip Zimbardo – “process seems so immediate
and direct that it fools us into assuming the
sensation of redness is characteristic of a tomato
or the sensation of cold is a characteristic of ice
cream”
– In reality…sensations such as “red” or “cold” occur
only when neural impulses reach the brain
Transduction
Basic Principles of Sensation
• Absolute Threshold
– min. amount of a stimulus that an observer can
reliably detect at least 50% of the time
– Anything less goes unnoticed
Examples…
– Vision – candle flame 30 miles (clear, dark night)
– Hearing – watch ticking 20 feet away
– Smell – one drop of perfume in a 3-room apartment
– Taste – 1 tsp. of sugar in 2 gallons of water
– Touch – a bee wing dropped on your back from a cm
Basic Principles of Sensation
• Difference Threshold
– Minimal difference needed to notice a stimulus
change
– aka – JND / “Just Noticeable Difference”
– Example – TV volume at 50, take it down a notch
until you notice the difference
Basic Principles of Sensation
• Weber’s Law
– German Psychologist – Ersnt Weber (1795-1878)
– Observed the JND will vary dependent on its
relation to the original stimulus
– The size of the JND is proportional to the strength
of the original stimulus
– Example
• Weight lifter bench pressing 50 lbs would notice an
additional 5 pounds
• Weight lifter bench pressing 500 lbs would not notice
an additional 5 pounds
Weber’s Law
Basic Principles of Sensation
• Signal Detection Theory
– Problem with traditional thresholds
• Assumed if signal exceeded one’s AT, it would be
sensed / if below one’s AT, it would not
• Did not account for characteristics of the perceiver
– Example – notice change in volume because that is the focus
– A new theory
• No single absolute threshold
• Detection is based on a combination of stimulus
intensity, background noise, and a person’s physical
condition, biases, and level of motivation
• EX – a soldier in wartime would detect fainter stimuli
than one in peacetime
Basic Principles of Sensation
• Signal Detection Theory cont.
– When using in experiments, psychologists sort the
trials into one of four categories…
– Signal is present – the person can decide that it
present or absent (called hits or misses)
– Signal is absent – the person can still decide that
the signal is either present or absent (called false
alarms or correct rejections)
Basic Principles of Sensation
• Sensory Adaptation
– Occurs when a constant stimulus is presented for
a length of time. When this happens receptors fire
less frequently and the sensation often fades or
disappears.
– Examples…
• Jogger puts on a new pair of shoes, he or she
immediately notices the different feel. However, after
going for a jog, he or she does not notice the difference
• Swimmer dives into a pool, you notice the water is
chilly. However, after swimming for a while, you do not
notice the water temperature
The Human Visual System
• Our most important and complex sense!
• Transduces light waves into neural messages
that the brain processes into what we
consciously see.
The Human Visual System
• From the Cornea to the Retina…
• Cornea
– Light waves from the outside world first enter through
– Clear membrane / protects eye / helps gather and
direct incoming light waves
• Pupil
– Small opening in middle of iris
– Changes size to let in different amounts of light
The Human Visual System
• From the Cornea to the Retina…
• Iris
– Colored part of the eye
– Ring of muscle tissue that contracts or expands to
control the size of the pupil (respond to light and
emotions)
– Sympathetic (dilate) Parasympathetic (constrict)
The Human Visual System
• From the Cornea to the Retina…
• Lens
– Transparent structure behind the pupil that
focuses and bends light as it enters the eye
– Accommodation – change in the curvature of the
lens that enables the eye to focus on objects at
various distances (eye adjusts itself)
The Human Visual System
• Nearsightedness
(Myopia)
• Visual acuity problem
that results when the
cornea and lens focus
an image in the front of
the retina
• As a result, distant
objects appear blurry
• 25% of US population
The Human Visual System
• Farsightedness
(Hyperopia)
• Visual acuity problem
that results when the
cornea and lens focus
an image behind the
retina
• As a result, objects near
the eye appear blurry
• 25% of the US
population
The Human Visual System
• Retina
• Light-sensitive
membrane at the back
of the eye
• Contains millions of
sensory receptors for
vision
• Transduction of light
waves into neural
messages occurs
The Human Visual System - Retina
Rods
• Photoreceptors in the retina
• Sensitive to dim light but
not to color
• Allow you to see in poorly lit
environments
• Cats have better night vision
than humans because they
have a higher portion of
rods to cones than humans
Cones
• Photoreceptors in the retina
• Sensitive to colors and
bright light
• Concentrated in the center
of the retina (fovea)
• Images that do not fall on
the fovea tend to be
perceived as blurry or
indistinct
The Human Visual System
• Retina…
• Bipolar Cells – specialized neurons that
connect rods and cones with the ganglion cells
• Ganglion Cells – specialized neurons that
connect to the bipolar cells (bundled axons
form the optic nerve)
The Human Visual System
• Blind Spot
• Point where the optic
nerve leaves the eye
there are no rods or
cones
• Because there are no
rods or cones, we have
a tiny hole or blind spot
• Normally we are
unaware because our
eyes are always moving
The Human Visual System
• Visual Cortex
• Optic nerve carries
visual information to
• Lies in the occipital lobe
at the back of the brain
The Human Visual System
• Color Vision
– Humans can identify about 7 million different
color combinations
– Two color vision theories
• Trichromatic or three-color theory
• Opponent-process theory
The Human Visual System
• Trichromatic Theory (three-color)
– Three primary colors (red, green, blue)
– Any color can be created by combining light waves
of these three colors
– Hermann von Helmholtz (1821-1894) proposed
that the eye must have color receptors that
correspond to these primary colors
• Has been proven that the retina does have cones
especially sensitive to the three primary colors
The Human Visual System
• Opponent-process theory
– The Trichromatic theory explains how color
processing works in cones, but not what happens
in the ganglion cells and the rest of the visual
system
– The ganglion cells process color in opposing pairs
• Red-green / black-white / blue-yellow
– Visual cortex also encodes color in terms of these
three pairs
– Explains afterimages
The Human Visual System
• Afterimage
• Visual experience that
occurs after the original
source of stimulation is
no longer present
• Example
– green, black, yellow flag
– red, white, blue flag
The Human Visual System
• Color Blindness
– Genetic disorder (recessive – X) that prevents an
individual from distinguishing between certain
colors
– Have a deficiency in their cones
– Most common form is red-green deficiency
(certain pigments)
– Rare to see no color at all (only about 500
reported cases ever!!)
– 1 in 10 men / very few women
The Human Auditory System
• Hearing plays a role in language development
and social interactions. It also alerts us to
dangerous situations
• It transduces sound waves into neural
messages that the brain then processes into
what we consciously hear
• aka – audition
• Amplitude – loudness
• Frqeuency - pitch
The Human Auditory System
• Outer Ear
– Pinna
• Flap of skin and cartilage
• Catches sound waves and channels them to the auditory
canal
– Auditory Canal
• Sound waves travel down and bounce into the ear drum
– Eardrum (tympanic membrane)
• Tightly stretched membrane located at the end of the
auditory canal
• Vibrates when hit by sound waves (match the intensity
and frequency of sound waves)
Outer Ear
The Human Auditory System
• Middle Ear
– Amplifies sound waves
– Hammer, anvil, and stirrup
• Three tiny bones
• Joint action doubles the amplification of sound
– Oval Window
• Stirrup transmits the amplified vibrations to oval
window
• Small membrane separating the middle ear from inner
ear
• Relays vibrations to the cochlea
Middle Ear
The Human Auditory System
• Inner Ear
– transduces sound waves into neural messages
– Cochlea
• Spiral-shaped, fluid filled structure that contains the
basilar membrane and hair cells
• Greek word for “snail”
– Basilar membrane
• Runs length of cochlea / holds hair cell receptors
– Hair cells
• Sensory receptors embedded in basilar membrane
• Transduce the physical vibration of sound waves into
neural impulses
Inner Ear
The Human Auditory System
• Brain
• As hair cells bend, they
stimulate the cells of
the auditory nerve
• The auditory nerve
carries the neural
impulse to the thalamus
and then to the
temporal lobe’s
auditory cortex
Distinguishing Pitch
(highness/lowness)
• Frequency Theory
• Basilar membrane
vibrates the same
frequency as sound waves
• Explains how low
frequency sounds are
transmitted
• Does not explain faster
high frequency sounds
(neurons cannot fire
faster than 1000 times
per second)
• Place Theory
• Different frequencies
excite different hair cells
at different locations
along the basilar
membrane
• High-frequency sounds
cause max vibrations near
the stirrup
• Lower-frequency sounds
cause maximum
vibrations at the opposite
end
• Frequency Theory ^^^
• Place Theory >>>
Loss of Hearing
*most due to damage or old age
Conductive Deafness
• Caused when the tiny bones
in the middle ear are
damaged and cannot
transmit sound waves
• Hearing aides can amplify
sound and help overcome
Nerve (Sensorineural) Deafness
• Caused by damage to the
cochlea, hair cells, or
auditory nerve
• Exposure to noises such as
headphones playing at full
blast can damage cells and
cause permanent hearing
loss
• Hearing aids cannot help
nerve deafness since
damage to hair cells and
auditory nerve is almost
always irreversible
Loss of Hearing
The Chemical Sense
• Smell and taste
• Respond to chemical molecules rather than
forms of energy (light and sound waves)
• Smell and taste receptors are located near
each other and often interact
• We often have difficulty separating the two
sensations
Smell and Taste
The Chemical Senses
• Smell or Olfaction
– mucous membrane at the top of each nostril contain
receptor cells that absorb airborne chemical
molecules
– Olfactory receptors detect over 10,000 distinct smells
(flagrant flowers to leaking gas)
– Receptor cells communicate neural messages to the
olfactory bulb
– Unlike other sensations, impulses do not go through
the thalamus
– Nerve fibers connect at the brain near the amygdala
and then to the hippocampus – limbic system
(explains why smell is so closely related to vivid
memories and emotions)
Olfaction
The Chemical Senses
• Taste or Gustation
– tongue covered with bumps called papillae
– Papillae are covered with taste buds (taste buds
are also found in palate and back of throat)
– Major taste sensations
• Sweet, sour, salty, bitter, umami (proteins in meats)
– Least critical of senses, but plays role in survival by
helping us avoid harmful substances
– Aided by smell, plays a role in enjoyment
Gustation
Skin and Body Senses
• Skins Senses
– Largest sense organ
– Protects our internal organs, holds body fluids,
produces sensations of touch, warmth, and cold
and provides essential info about pain
• Body Senses
– Provide essential information about your position
and orientation in space
Skins Senses
• Touch
– Receptors not evenly distributed on body
– i.e.. more densely concentrated in hands, face,
and lips than on leg or back
– Plays role in human relationships – help
communicate feelings of support, conformity, love
• Pain
– Unpleasant sensation of physical discomfort or
suffering
– Plays a key role in survival by warning about
potential or actual injuries
Skin Senses
• Gate-control Theory of Pain
– The brain regulates pain by sending signals down
the spinal cord that either open of close sensory
pathways (gates)
– If the brain signals the gates to open, pain is
experienced or intensified
– If the brain signals the gates to close, pain is
reduced
– Psychological factors such as anxiety and fear can
intensify pain while positive emotions such as
laughter can help minimize pain
The Vestibular Sense
• Provides a sense of balance and equilibrium
• Inner ear contains receptors that are
especially important for maintaining balance
• The semicircular canals are filled with fluids
and lined with hairlike receptor cells that shift
in response to motion
– They provide the brain with information about the
body’s posture and head positions
• Vestibular and auditory systems are alike in
that both depend upon hair cells to transduce
a stimulus into a neural message
Semicircular Canals
Selection
• Selecting where to direct our attention is the
first step in perception
• Selective Attention and feature detectors help
explain why we pay attention to some stimuli
in our environment and not to other
Selective Attention
• Cognitive process of selectively concentrating
on one or more aspects of the environment
while filtering out or ignoring other
information
• Example
– Gavin is playing a new video game with his friend.
He doesn’t hear his mother call for him to come to
dinner. However, he does respond when his cell
phone rings because he is expecting a call from his
girlfriend
Selective Attention
• Perceptual Adaptation - ability of the body to adapt to
an environment by filtering out distractions.
– Example - someone who lives near a train can perceptually
adapt such that they can ignore the train whistle in order
to sleep at night. If someone came to visit, however, they
would not have perceptually adapted to the train and
would most likely not be able to sleep.
• Sensory Adaptation – decreasing responsiveness to a
stimuli due to constant stimulation
– Example – you felt your socks when you put them on, but
you stopped feeling them after a while
• Sensory Habituation – our perception of sensations is
partially due to how focused we are on them
– Example – you do not focus on your socks anymore ^^^^
Selective Attention Test #1
Selective Attention Test #2
Selective Attention
• Cocktail Party Effect
– being able to focus one's auditory attention on a
particular stimulus while filtering out a range of other
stimuli, much the same way that a partygoer can focus
on a single conversation in a noisy room
– Spotlight Effect - reading
• Cocktail Party Phenomenon
– situation in which one can attend to only part of a
noisy environment, yet highly pertinent stimuli such
as one's own name can suddenly capture attention
Change Blindness
• psychological phenomenon that occurs when
a change in a visual stimulus goes unnoticed
by the observer
• Example…an individual fails to notice a
difference between two images that are
identical except for one change
Change Blindness Intro.
Change Blindness: Door Study
Feature Detectors
• Brain has specialized neurons called feature
detectors that respond only to certain stimuli
• In occipital lobe’s visual cortex
• Respond to special features of an image
(angles, lines, curves, movements)
• Example … area behind ear perceives faces
– Prosopagnosia – damage to this area causes
inability to detect differences in faces
– Cannot recognize own face in mirror!
Gestalt Principles of Organization
• Max Wertheimer founded in early 1900s
• We process sensations according to
perceptual rules
• These rules create whole perceptions that are
meaningful, symmetrical, and as simple as
conditions will allow
Figure-Ground Perception
• Most fundamental Gestalt Principle
• Figure – main element of a scene that clearly
stands out
• Ground – less distinct background of a scene
• Example – on this page…the letters constitute
the figure, while the white page is the ground
Figure-Ground Perception
Similarity
• There is a tendency to
perceive objects of a
similar size, shape or
color as a unit or figure
• Example – crowd at
football game is a wide
assortment of different
people; you organize
them into home fans,
visitor fans, band
members, cheerleaders
Proximity
• There is a tendency to
perceive objects that
are physically close to
one another as a single
unit
• Example – home fans
and visiting fans usually
group themselves on
opposing sides of the
stadium; you would
group visiting fans into
one homogenous group
Closure
• Tendency to fill in the
gaps in an incomplete
image
• Example – scoreboards
have HOME and
VISITORS on them. If it
said HO E and VISI ORS,
you would fill in the
missing letters M and T.
Phi Phenomomenon
• Illusion movement created by presenting still
pictures in rapid succession
• Example - TV
Depth Perception
• Ability to perceive 3-D space and accurately
judge distance
• It is possible to use sound and even smell to
judge distance, we rely heavily on vision
“Visual Cliff” Experiment
• Famous experiment, Eleanor Gibson
• Placed infants old enough to crawl on a Plexiglastopped table
• Side 1 – high-contrast red-and-white check cloth
flush against the underside of the glass
(appearance of solid surface)
• Side 2 – placed the same material 4-feet below
the glass
• Created an illusion of “deep end” with a “visual
cliff”
“Visual Cliff” Experiment
• Infants almost always refused to venture
beyond the shallow side of the visual cliff
• They turned away from the “deep end” even
when their mothers held a spinning toy and
encouraged them to crawl forward
• Supports the conclusion that depth
perception in humans is innate that emerges
during infancy
Monocular (one eye) Depth Cues
• Linear Perspective
• Parallel lines appear to
converge toward a
vanishing point as they
recede into the distance
Monocular (one eye) Depth Cues
• Aerial Perspective
• Distant objects appear
hazy and blurred
compared to close
objects
Monocular (one eye) Depth Cues
• Relative Size
• Two or more objects are
assumed to be similar in
size, the object that
appears larger is
perceived as being close
Monocular (one eye) Depth Cues
• Motion Parallax
• as you move, you use
the speed of passing
objects to estimate the
distance of the objects
• Example – when you
are driving on an
interstate, nearby poles,
fences, and roadside
signs seem to zip by
faster than distant hills
Binocular (both eyes) Depth Cues
• Convergence
• Closer the object, the
more the eyes
converge, or turn
inward
Binocular (both eyes) Depth Cues
• Retinal Disparity
• Separation of the eyes cause different images to
fall on each retina
• When two retinal images are very different, we
interpret the object as being close by.
• When two retinal images are more nearly
identical, the object is perceived as being farther
away
• A person with only one eye lacks retinal disparity
and would have difficulty climbing an irregular set
of stairs