Sensation
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Eyes, ears, nose, skin, and tongue are
complex, miniaturized, living sense organs
that automatically gather information about
your environment
Transduction
◦ Process in which a sense organ changes, or
transforms, physical energy into electrical signals
that become neural impulses, which may be sent to
the brain for processing
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Adaptation
◦ The decreasing response of the sense organs as
they’re exposed to a continuous level of stimulation
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Sensation versus perception
◦ Relatively meaningless bits of information that
result when the brain processes electrical signals
that come from the sense organs
• Perceptions
– Meaningful sensory experiences that result after the
brain combines hundreds of sensations
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Stimulus: light waves
◦ Invisible (too short)
 gamma rays, x-rays, ultraviolet rays
◦ Visible (just right)
 particular segment of electromagnetic energy that we
can see because these waves are the right length to
stimulate receptors in the eye
◦ Invisible (too long)
 radar, FM, TV, shortwave, AM
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Structure and function
◦ Eyes perform two separate processes
 first: gather and focus light into precise area in the
back of eye
 second: area absorbs and transforms light waves into
electrical impulses
◦ Process called transduction
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Structure and function
◦ Vision: seven steps
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image reversed
light waves
cornea
pupil
iris
lens
retina
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Structure and function
◦ Image reversed
 in the back of the eye, objects appear upside down
 somehow the brain turns the objects right side up
◦ Light waves
 light waves are changed from broad beams to narrow,
focused ones
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Structure and function
◦ Cornea
 rounded, transparent covering over the front of your
eye
◦ Pupil
 round opening at the front of the eye that allows light
waves to pass into the eye’s interior
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Structure and function
◦ Iris
 circular muscle that surrounds the pupil and controls
the amount of light entering the eye
◦ Lens
 transparent, oval structure whose curved surface
bends and focuses light waves into an even narrower
beam
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Structure and function
◦ Retina
 located at the very back of the eyeball; a thin film that
contains cells that are extremely sensitive to light
 light-sensitive cells, called photoreceptors, begin the
process of transduction by absorbing light waves
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Retina
◦ Three layers of cells
 back layer contains two kinds of photoreceptors that
begin the process of transduction
 change light waves into electrical signals
 rod located primarily in the periphery
 cone located primarily in the center of the retina called
the fovea
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Rods
◦ Photoreceptor that contain a single chemical, called
rhodopsin
◦ Activated by small amounts of light
◦ Very light sensitive
◦ Allow us to see in dim light
◦ See only black, white, and shades of gray
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Cones
◦ Photoreceptors that contain three chemicals called
opsins
◦ Activated in bright light
◦ Allow us to see color
◦ Cones are wired individually to neighboring cells
◦ Allow us to see fine detail
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Visual pathways: eye to brain
◦ Optic nerve
◦ Primary visual cortex
◦ Visual association areas
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Visual pathways: eye to brain
◦ Optic nerve
 impulses flow through the optic nerve as it exits from
the back of the eye
 the exit point is the “blind spot”
 the optic nerves partially cross and pass through the
thalamus
 the thalamus relays impulses to the back of the
occipital lobe in the right and left hemisphere
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Visual pathways: eye to brain
◦ Primary visual cortex
 back of the occipital lobes is where primary visual
cortex transforms nerve impulses into simple visual
sensations
◦ Visual association areas
 primary visual cortex sends simple visual sensations to
neighboring association areas
 damage to the visual association area = visual agnosia:
difficulty in assembling simple visual sensations into
more complex, meaningful images
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Making colors from wavelengths
◦ Sunlight is called white light because it contains all
the light waves
◦ White light passes through a prism; separates light
waves that vary in length
◦ Visual system transforms light waves of various
lengths into millions of different colors
◦ Shorter wavelengths of violet, blue, green
◦ Longer wavelengths of yellow, orange, and red
◦ An apple is seen as red because reflection of longer
light waves that brain interprets as red
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Color vision
◦ Trichromatic theory
 three different kinds of cones in the retina
 each cone contains one of the three different lightsensitive chemicals, called opsins
 each of the three opsins is most responsive to
wavelengths that correspond to each of the three
primary colors
 blue, green, red
 all colors can be mixed from these primary colors
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Opponent-process theory
◦ Afterimage
 visual sensation that continues after the original
stimulus is removed
 ganglion cells in retina and cells in thalamus respond
to two pairs of colors: red-green and blue-yellow
 when excited, respond to one color of the pair
 when inhibited, respond to complementary pair
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Color blindness
◦ Inability to distinguish two or more shades in the
color spectrum
◦ Monochromatic
 total color blindness; black and white
 result of only rods and one kind of functioning cone
◦ Dichromatic
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inherited genetic defect; mostly in males
trouble distinguishing red from green
two kinds of cones
see mostly shades of green
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Stimulus
◦ Sound waves
 stimuli for hearing (audition)
 ripples of different sizes; sound waves travel through
space with varying heights and frequency
◦ Height
 distance from the bottom to the top of a sound wave;
amplitude
◦ Frequency
 number of sound waves occurring within a second
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Loudness
◦ Subjective experience of a sound’s intensity
◦ Brain calculates loudness from specific physical
energy (amplitude of sound waves)
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Pitch
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Subjective experience of a sound being high or low
Brain calculates from specific physical stimuli
Speed or frequency of sound waves
Measured in cycles (how many sound waves in a
second)
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Measuring sound waves
◦ Decibel: unit to measure loudness
◦ Threshold for hearing
 0 decibels (no sound)
 140 decibels (pain and permanent hearing loss)
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Outer, middle, and inner ear
◦ Outer ear
 consists of three structures
 external ear
 auditory canal
 tympanic membrane
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Outer, middle, and inner ear
◦ Outer ear
 external ear
 oval-shaped structure that protrudes from the side of the
head
 function
 pick up sound waves and then send them down the
auditory canal
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Outer, middle, and inner ear
◦ Outer ear
 auditory canal
 long tube that funnels sound waves down its length so
that the waves strike the tympanic membrane (ear drum)
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Outer, middle, and inner ear
◦ Outer ear
 tympanic membrane
 taut, thin structure commonly called the eardrum
 sound waves strike the tympanic membrane and cause it
to vibrate
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Outer, middle, and inner ear
◦ Middle ear
 bony cavity sealed at each end by membranes that are
connected by three tiny bones called ossicles
 hammer, anvil, and stirrup
 hammer is attached to the back of the tympanic
membrane
 anvil receives vibrations from the hammer
 stirrup makes the connection to the oval window (end
membrane)
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Outer, middle, and inner ear
◦ Inner ear
 contains two structures sealed by bone
 cochlea: involved in hearing
 vestibular system: involved in balance
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Cochlea
◦ Bony coiled exterior that resembles a snail’s shell
◦ Contains receptors for hearing
◦ Function is transduction
◦ Transforms vibrations into nerve impulses sent to
the brain for processing into auditory information
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Auditory brain areas
◦ Sensations and perceptions
◦ Two-step process occurs after the nerve impulses
reach the brain
◦ Primary auditory cortex
◦ Top edge of temporal lobe
◦ Transforms nerve impulses into basic auditory
sensations
◦ Auditory association area
◦ Combines meaningless auditory sensations into
perceptions (meaningful melodies, songs, words, or
sentences)
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Auditory cues
◦ Direction of sound
 determined by brain; calculates slight difference in
time it takes sound waves to reach the two ears
◦ Calculating pitch
 frequency theory
 applies only to low-pitched sounds
 rate ate that nerve impulses reach the brain determines
how low a sound’s pitch is
 place theory
 brain determines medium-to-higher-pitched sounds
from the place on the basilar membrane where maximum
vibration occurs
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Auditory cues
◦ Calculating loudness
 brain calculates loudness primarily from the frequency
or rate of how fast or how slow nerve impulses arrive
from the auditory nerve
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Position and balance
◦ Vestibular system is located above the cochlea in
the inner ear
◦ Includes semicircular canals
◦ Bony arches set at different angles
◦ Each semicircular canal is filled with fluid that
moves in response to movements of your head
◦ Canals have hair cells that respond to the fluid
movement
◦ Function of vestibular system
◦ Includes sensing the position of the head, keeping
the head upright, and maintaining balance
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Motion sickness (sensory mismatch between
information from the vestibular system)
◦ symptoms: feelings of discomfort, nausea, and
dizziness in a moving vehicle
◦ head bouncing, but distant objects look fairly
steady
• Meniere’s disease (malfunction of the semicircular
canals of the vestibular system)
– symptoms: dizziness, nausea, vomiting, spinning, and
piercing buzzing sounds
• Vertigo (malfunction of the semicircular canals of
the vestibular system)
– symptoms: dizziness and nausea
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Taste
◦ Chemical sense because the stimuli are various
chemicals
◦ Tongue
◦ Surface of the tongue
◦ Taste buds
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Tongue
◦ Five basic tastes
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sweet
salty
sour
bitter
umami: meaty-cheesy taste
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Surface of the tongue
◦ Chemicals, which are the stimuli for taste, break
down into molecules
◦ Molecules mix with saliva and run into narrow
trenches on the surface of the tongue
◦ Molecules then stimulate the taste buds
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Taste buds
Shaped like miniature onions
Receptors for taste
Chemicals dissolved in saliva activate taste buds
Produce nerve impulses that reach areas of the
brain’s parietal lobe
◦ Brain transforms impulses into sensations of taste
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Flavor
◦ Combination of taste and smell
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Smell, or olfaction
◦ Steps for olfaction
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stimulus
olfactory cells
sensation and memories
functions of olfaction
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Smell, or olfaction
◦ Stimulus
 we smell volatile substances
 volatile substances are released molecules in the air at
room temperature
 examples: skunk spray, perfumes, warm brownies; not
glass or steel
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Smell, or olfaction
◦ Olfactory cells
 receptors for smell located in a one-inch-square patch
of tissue in the uppermost part of the nasal passages
 olfactory cells are covered in mucus that dissolves
volatile molecules and stimulates the cells
 the cells trigger nerve impulses that travel to the brain,
which interprets the impulses as different smells
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Smell, or olfaction
◦ Sensations and memories
 nerve impulses travel to the olfactory bulb
 impulses are relayed to the primary olfactory cortex
 cortex transforms nerve impulses into olfactory
sensations
 we can identify as many as 10,000 different odors
 we stop smelling our deodorants or perfumes because
of decreased responding (adaptation)
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Smell, or olfaction
◦ Functions of olfaction
 one function: to intensify the taste of food
 second function: to warn of potentially dangerous
foods
 third function: to elicit strong memories; emotional
feelings
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Touch
◦ Includes pressure, temperature, and pain
◦ Beneath the outer layer of skin are a half-dozen
miniature sensors that are receptors for the sense
of touch
◦ Change mechanical pressure or temperature
variations into nerve impulses that are sent to the
brain for processing
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Receptors in the skin
◦ Skin
◦ Hair receptors
◦ Free nerve endings
◦ Pacinian corpuscle
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Skin
◦ Outermost layer
◦ Thin film of dead cells containing no receptors
◦ Just below are first receptors, which look like
groups of thread-like extensions
◦ Middle and fatty layer
◦ Variety of receptors with different shapes and
functions
◦ Some are hair receptors
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Hair receptors
◦ Free nerve endings wrapped around the base of
each hair follicle
◦ Hair follicles fire with a burst of activity when first
bent
◦ If hair remains bent for a period of time, the
receptors will cease firing
◦ Sensory adaptation
◦ Example: wearing a watch
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Free nerve endings
◦ Near bottom of the outer layer of skin
◦ Have nothing protecting or surrounding them
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Pacinian corpuscle
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In fatty layer of skin
Largest touch sensor
Highly sensitive to touch
Responds to vibration and adapts very quickly
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Brain areas
◦ Somatosensory cortex
◦ Located in the parietal lobe
◦ Transforms nerve impulses into sensations of
touch, temperature, and pain
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What causes pain?
◦ Pain: unpleasant sensory and emotional experience
that may result from tissue damage, one’s thoughts
or beliefs, or environmental stressors
◦ Pain results from many different stimuli
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How does the mind stop pain?
◦ Gate control theory of pain
◦ Nonpainful nerve impulses compete with pain
impulses in trying to reach the brain
◦ Creates a bottleneck or neutral gate
◦ Shifting attention or rubbing an injured area
decreases the passage of painful impulses
◦ Result: pain is dulled
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Endorphins
◦ Chemicals produced by the brain and secreted in
response to injury or severe physical or
psychological stress
◦ Pain-reducing properties of endorphins are similar
to those of morphine
◦ Brain produces endorphins in situations that evoke
great fear, anxiety, stress, or bodily injury as well as
intense aerobic activity
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Dread
◦ Connected to pain centers in brain
◦ Not the act itself that people fear
◦ Time waiting before event causes dread
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Acupuncture
◦ Trained practitioners insert thin needles into
various points on the body’s surface and then
manually twirl or electrically stimulate the needles
◦ After 10 to 20 minutes of stimulation, patients
often report a reduction in various kinds of pain