Sensation and Perception Chapter 3

Sensation and Perception
Chapter 3
What is sensation ?
• Your senses are the gateway through
which your brain receives all information
about it’s environment
• A natural and automatic process
• Often taken for granted until it is
interrupted by injury or illness
• People with one nonfunctional sense are
amazingly adaptive
Overlapping Processes: Sensation
and Perception
• Sensation: the detection and sensing of
environmental stimuli (sounds, objects, odors)
Perception: occurs when we integrate,
organize, and interpret sensory information in a
meaningful way
No clear boundary line between the two
processes- psychology often regards the two
processes as a single process
• All sensation is the result of stimulation of
specialized cells, called sensory receptors, by
some form of energy.
The forms of energy most commonly in contact
with: light, sound, heat, pressure, chemical
Sensory receptors convert forms of energy into
electric impulses that are transmitted via
neurons to the brain.
The Senses
• Humans have five basic senses, each
receptive to a different form of energy:
– SIGHT: receives light energy
– HEARING: receives sound energy or sound
– TOUCH: receives mechanical energy
– SMELL: receives airborne chemical energy
– TASTE: receives chemical energy
• We are constantly bombarded by many
different forms of energy.
• Sensory receptors are highly specialized
and sensitive to specific types of energy
• For any type of stimulation to be sensed,
the stimulus energy must first be in the
form to be detected by our receptor cells
Sensory Thresholds
• Our senses are specialized in other ways- we do
not have an infinite capacity to detect all levels
of energy
To be sensed a stimulus must first be strong
enough to be detected- loud enough to be
heard, concentrated enough to be smelled,
bright enough to be seen
The point where the stimulus is strong enough
to be detected by activating a sensory receptor
cell is called threshold.
Absolute Threshold
• The minimum amount of energy in the
environment that a sensory system can
detect (half the time in trials and
Galanter’s (1962)
Absolute Thresholds
• Sense
Absolute Threshold
– Vision : A candle flame seen from 30 miles away on
a clear, dark night
– Hearing: The tick of a watch at 20 feet
– Taste: 1 teaspoon of sugar in 2 gallons
of water
– Smell ;1 drop of perfume through-out a
3-room apartment
– Touch ; A bee’s wing falling on your cheek
from a height of about ½ inch
Difference Threshold
• The smallest possible difference between
two stimuli that can be detected half the
• Also called just noticeable difference (jnd)
• The difference threshold will vary; it is not
a constant , even for a specific sensory
Weber’s Law
• A principle of sensation
• For each sense, the size of the just
noticeable difference (difference
threshold) is a constant proportion of the
size of the initial stimulus
• Whether we can detect a change in the
strength of the stimulus depends on the
intensity of the original stimulus
Sensory Adaptation
• A gradual decline in sensitivity to a
constant stimulus
Subliminal Perception
• Refers to the perception of stimuli that are
below the threshold of conscious perception or
Can subliminal messages in advertising and selfhelp tapes change people’s behavior?
Psychologists have found that the effects of
subliminal stimuli tend to be weak or short-lived,
usually lasting only seconds or minutes.
• The sense organ for vision is the eye, which
contains receptor cells that are sensitive to the
physical energy of light.
The most important sense to humans
Light is one of many different kinds of magnetic
energy that travels in the form of waves
Other forms of electromagnetic energy include
X-rays, the microwave oven, and ultraviolet light
Eye Structure: Anatomy and
• See an object- light waves reflected from
the object enter your eye and pass
through the cornea, pupil, and lense.
• A clear membrane that covers the front of
the eye, helps gather and direct incoming
• Black opening in the middle of the eye
• Surrounds the pupil, the colored structure
that we refer to when we say someone
has brown or blue eyes
• Consists of a ring of muscles that
contracts or expands to precisely control
the size of the pupil and the amount of
light entering the eye
• Behind the pupil is the lens, a transparent
structure held in place by ciliary muscles
The lens thickens and thins to bend or focus
incoming light (accommodation)
Abnormally shaped eyeballs cause improper
focus of incoming light on the retina
This results in a visual disordernearsightedness, farsightedness, or astigmatism
Nearsightedness- Myopia
• Light from a distant object is focused on
the front of the retina
Farsightedness- Hyperopia
• Light from an object or image close by is
focused behind the retina
• Presbyopia: during middle age, another
form of farsightedness caused when the
lens becomes brittle and inflexible
• An abnormally curved eyeball results in
blurry vision for lines in a particular
• Corrected with glasses that intercept and
bend the light so that the image falls
properly on the retina
The Retina
• A thin, light-sensitive membrane that lies
at the back of the eye, covering most of
its inner surface
• Contains two types of receptors for light:
rods and cones
• When exposed to light, rods and cones
undergo a chemical reaction that results in
a neural signal
• Long and thin with blunt ends
• The eye contains about 125 million rods
located in the outer edge of the retina
• Specialized function: they are very
sensitive to light
• We rely on them for our vision in dim light
and at night
• About 6 million per eye located in a small central
region of the retina (fovea)
Cones respond differently to different wave
lengths, or colors, of light
Responsible for color vision
Each cone responds to either red, green, or blue
Cones are very sensitive to small features in the
environment; visual acuity
Processing Visual Informationfrom the retina to the brain
• Rods and cones connect to specialized
neurons called bipolar cells
• The bipolar cells then funnel the collected
raw data on to the ganglion cells
• Ganglion cells are specialized neurons
• There are one million ganglion cells
How is the information transmitted
from the ganglion cells of the retina
to the brain?
• The one million axons of the ganglion cells
are bundled together to form the optic
nerve, which carries messages from each
eye to the brain
• The place where the axons of the ganglion
cells join to form the optic nerve is called
the blind spot – an area that contains no
photo receptors and is insensitive to light
Optic Chiasm
• After the nerve fibers that make up the
optic nerve leave the eyes, they separate,
and some of them cross to the other side
of the head at the optic chiasm.
• The nerve fibers from the right side of
each eye travel to the right hemisphere of
the brain; those from the left side of the
eye travel to the left hemisphere
Neural pathways in the brain
• thalamus- processes information about
form, color, brightness, and depth
• midbrain- processes information about
the location of an object
• visual cortex: processes information about
form- such as angels, lines, movement
and distance of objects
Color Vision: 3 Properties of Color
• Humans can see a range of colors
• 1.Hues: different colors we see; varies with the
wavelength of light
2.Saturation: vividness or richness of the hue
3.Brightness: perceived intensity
The color of an object is determined by the
wavelength of light that the object reflects.
Color and other species
• Many animals have some color vision:
– Trichromats: can see all hues-(humans,
monkeys, apes)
– Dichromats: can only see reds or greens,
blues or yellows (most mammals)
– Monochromats: completely color blind
(reptiles, fish, insects, shellfish)
• Sound is air in motion
• When an object vibrates, it causes the air
around it to move in waves
• Waves travel 700 MPH
• Humans detect vibrations of a frequency
between 30 and 20,000 hertz (cycles per
3 Dimensions of Sound
• Pitch- determined by the frequency of
• Loudness- the result of the sound’s
intensity, or energy
• Timbre- the nature of the sound, or the
The Ear: Structure and
• The ear is divided into three areas: outer,
middle, inner
– Sounds enters through flap- pinna
– Sounds travels through the air-filled auditory
Middle Ear
• Sound enters middle ear – hits tympanic
membrane (eardrum) which vibrates with sound
• Behind the eardrum- 3 small bones- hammer,
anvil, stirrup
Stirrup vibrates onto the oval window, a
membrane which covers an opening in the
middle ear
Inner Ear
• Cochlea- transmits auditory information,
contains 3 separate fluid-filled canals
• Canals spiral inside the cochlea, which
wraps around like a snail’s shell
• Cochlea comprises the Corti- organ which
contains auditory receptor cells
Conductive Deafness
• Conductive deafness: ear is not carrying
sounds from the outer ear to the inner
• Bones of inner ear may be damaged, or
• may be an accumulation of dirt in the ear
• Person will be equally deaf to high and
low tones
Nerve Deafness
• Person has hard time hearing high -
pitched tones
• Damage to the auditory nervous system
• Hair cells in the cochlea may be damagedcan no longer transduce vibrations of the
cochlea fluid into neural impulses
• Skin has 4 main senses of touch: Cutaneous senses•
cold, warmth, pressure, pain
Free nerve fibers- responsible for temperature sensationan increase in body temperature elicits a response
Pressure-sensitive cells: cells create receptor potentials
when skin is bent or deformed
Basket nerve ending: structure that sense pressure at
the roots of hairs
Pacinian corpuscles: respond when one feels deep
pressure, such as in a massage
Taste (Gustation)
• Taste receptors: sensitive to sweet, sour, bitter,
salty tastes
Saliva- food enters mouth, mixes with liquid
Solution flows into tiny “holes” in the tongue:
taste pores
• Each pore contains slim, hair like structures
which are part of the taste bud
When stimulated, receptor potential forms
Nerves pass from tongue to the brain- tastes
have different patterns of neural activity
Smell (Olfaction)
• The sense of smell is entwined with taste
• Sense of smell can evoke distant memories and
has connection to the temporal lobe
Stimulus for smell is air-born chemicals
Olfactory receptors- lie beneath nose in two
patches of mucous membranes
Chemicals in the air dissolve in the mucous and
stimulate olfactory hair cells, cilia
Olfactory mucosa- contains free nerve endings
that sense noxious substances like ammonia
Sensory Deprivation
• What would you do if you couldn’t sense
anything at all?
• Sensory deprivation studies:
– 1. external stimuli kept to a minimum (sound
proof room with no light)
– 2. changing or distorting the subject’s
environment (may hear a constant buzzing)
– 3. a monotonous environment, with no
change in stimuli
Sensory Deprivation in Adults
• Standard symptoms: (depending on type
of deprivation)
• Problems thinking clearly and
concentrating, and score low on
intelligence tests
• Difficulty counting above 20
• Hallucinate
Sensory Deprivation in Infants
• When infants are deprived at critical
stages of development, they may never
develop normal abilities
• Orphanage babies vs. Nursing home

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