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Sensation & Perception
Psychology 2012 – Spring 2004
Sensation & Perception
Sensation – detection of basic sensory
experiences

Sounds, touch, smells
Perception – interpretation of
sensations in meaningful ways

Interpreting sounds as music, touch as the
cat rubbing against you, smells as dinner
cooking
Sensation & Perception
Sensory receptors – specialized cells unique
to each sense organ

They respond to a particular form of sensory
stimulation

Transduction – the process of converting different
forms of energy into electrical impulses that are
transmitted via neurons to the brain

Done by the sensory receptors
Sensory Thresholds
Absolute threshold – the smallest stimulus that can
be detected half the time
Difference threshold – the smallest difference
between 2 stimuli that can be detected half the time

AKA the just-noticeable difference (jnd)

Weber’s law – the ability to detect a difference between 2
stimuli depends on the strength of the original stimulus
Sensory adaptation – sensory receptor cells become
less responsive to a constant stimulus
Vision
Wavelength – the
distance from one
wave peak to
another
Parts of the Eye
Cornea – clear membrane covering the
visible part of the eye; helps gather and direct
incoming light
Pupil – opening in the middle of the iris that
changes size to let in different amounts of
light
Iris – colored part of the eye; controls the size
of the pupil

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Dim light = iris widens; more light is let in
Bright light = iris narrows; less light let in
Parts of the Eye
Lens – structure located behind the pupil that focuses
light entering the eye

Accommodation – process by which the lens changes shape
to focus incoming light on the retina

Problems with the shape of the eyeball = either
nearsightedness (long, thin eyeball) or farsightedness (short,
fat eyeball)
Retina – membrane located in the back of the eye
containing the sensory receptors for vision

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Rods – receptors of the eye that are highly sensitive to light,
but not color; responsible for peripheral & night vision
Cones – pointed sensory receptors of the eye that detect
color vision and fine details

Most cones are concentrated on the fovea (in the center of the
retina)
Visual Processing in the
Retina
Bipolar cells – specialized neurons in the
retina that connect with the rods & cones and
the ganglion cells
Ganglion cells – specialized neurons in the
retina that connect to the bipolar cells; axons
of these cells make up the optic nerve

Each ganglion cell combines, analyzes, and
encodes information from the rods & cones before
sending info to the brain
From the Eye to the
Brain
Optic nerve – exits from the back of the eye and
carries information to the visual cortex of the brain

Made up of a million axons of ganglion cells
Optic chiasm – where the optic nerve fibers from
each eye meet and partly cross over to the opposite
side of the brain
Thalamus – where most of the optic nerve axons go;
processes information about form, color, brightness,
and depth
Visual cortex – information from the cortex goes here,
where it is decoded and interpreted
Color
The perceptual experience of different
wavelengths of light

Hue – color; different wavelengths


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Brightness – intensity of color


We see colors from 400 nanometers (violet) to 700 nanometers
(red)
White object – reflecting all colors; black object – absorbs all
colors; red object – reflects red light
Amplitude (height) of the light wave
Saturation – the purity of color (light wave)

Red = more saturation than pink
Theories of Color
Perception
Trichromatic Theory – the sensation of color is a result of the
cones being especially sensitive to either red (long
wavelengths), green (medium), or blue (short) light

Color blindness – inability to sense a certain color of light due to a
deficiency in cone sensitivity
Opponent-Process Theory – the sensation of color is due to
opposing pairs of color receptors (red/green, blue/yellow, and
black/white); when one color in the pair is stimulated, the other
is inhibited

Afterimages – seeing the other color (e.g., green) in a color pair
once the original color (e.g., red) is no longer present
Integrated Explanation of Color Vision – cones of the retina
adhere to the trichromatic theory, while the ganglion cells
adhere to the opponent process theory
Hearing/Audition
Pitch – highness or lowness of sound
(frequency – wavelength)

Hertz – units used to measure frequency
Loudness – determined by intensity
(amplitude - height) of sound wave

Decibels – units used to measure loudness
Timbre – the complexity of sound
(combination of frequencies)
Hearing
Outer Ear – part of the ear collecting sound waves
 Pinna, ear canal, & eardrum
 Eardrum – membrane at the end of the outer ear
that vibrates when hit by sound waves
Middle Ear – amplifies sound waves via the hammer,
the anvil, and the stirrup
Inner ear – part of the ear that changes sound into
neural impulses
 Cochlea – structure that contains sensory
receptors for sound

Basilar membrane – membrane in the cochlea containing
hair cells (receptors for sound)
Distinguishing Pitch
Frequency theory – the view that the basilar
membrane vibrates at the same frequency as
the sound wave involved

Works for low frequency sounds
Place theory – the view that different
frequencies cause larger vibrations at
different locations along the basilar
membrane

Works for higher frequency sounds
Smell
Olfactory receptors –
where odor molecules are
sensed

Bundles of axons make up
olfactory nerves
Olfactory bulb – where
sensation of smell is
registered; in the olfactory
cortex at the front of the
brain
Smell Sensitivity
Sense of smell varies among animals
Dogs have 200 million olfactory rods,
spread out in a much bigger nose
Humans differ greatly in ability to detect
smells
The most sensitive people are 20 times
more sensitive than the least
Taste/Gustation
Taste buds – sensory
receptors for taste
Located on the tongue
and inside the mouth and
throat
 Sweet, salty, sour, & bitter

The Skin & Body Senses
Skin senses provide information about our
physical interaction with objects in the
environment
The body senses keep us informed about our
position & orientation in space


Skin = largest and heaviest sense organ
Pacinian corpuscle – receptor beneath the skin
involved in sense of touch

Touch & temperature – more sensory receptors in certain
areas of the skin (hands, faces, lips) than others (back,
arms, & legs)
The Skin & Body Senses
Pain – the sensation of discomfort or suffering that
occurs in varying degrees of intensity

Free nerve endings in the skin, muscles or internal organs
carry messages of pain to the spinal cord, releasing
“substance P”

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Substance P – neurotransmitter that is involved in transmitting
pain messages to the brain
Depending on how the brain interprets pain, it can be
intensified or reduced

Gate-control theory – sensation of pain is controlled by a series
of “gates” in spinal cord.

Depending on how the brain interprets pain, these gates are
opened (more pain sensation) or closed (less pain sensation)
 Psychological factors increasing pain: anxiety, fear,
hopelessness
 Psychological factors decreasing pain: positive emotions,
distraction, feelings of control
The Skin & Body Senses

Psychological factors also influence the release of
endorphins, which can:

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Inhibit pain messages in the brain, and
Inhibit the release of substance P in the spinal cord
Psychological factors can affect muscle tension,
blood flow, arousal, & heart rate

All of these can affect the experience of pain
The Skin & Body Senses
Movement, Position, & Balance

Kinesthetic sense – sensation of body parts in relation to one
another

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Proprioceptors – sensory receptors in the joints, the inner ear and
the muscles
 Constantly communicate information to the brain about
changes in body position and muscle tension
Vestibular sense – sense of balance through responding to
changes in gravity, motion, and body position
 Semicircular canals and vestibular sacs located in the ear
 Filled with fluid and hairlike receptor cells that sense
changes in body position or gravity
 Vision also affects this sense
 When vision is at odds with the semicircular canals
and vestibular sacs, the result can be dissiness,
disorientation, & nausea
Perception
The organization and interpretation of raw
sensory data

Bottom-up processing – analysis that moves from
recognizing the parts of a stimulus to the whole
stimulus

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Data-driven processing
Used most during ambiguous situations
Top-down processing – analysis that moves from
the whole stimulus to the parts

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Conceptually driven processing
Drawing on our experiences to arrive at meaningful
perceptions
The Perception of Shape
Figure-ground relationship – a principle of
perceptual organization that states that we
automatically separate a stimulus into:
1.
The feature that stands out (the figure) from:

2.
Figure – the main element of a scene

Usually stands out clearly
Its less distinct background (the ground)

Ground – the background
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Usually is more fuzzy and less clear than the figure
The Perception of Shape
– What is it?
Perceptual grouping – when perceiving
a form, we tend to group different
elements of it together

We actively organize elements to try to
produce well-defined, whole objects
The Perception of Shape
Law of Pragnanz (simplicity) – when
several perceptual organizations are
possible, we interpret objects in the
simplest ways

Example – three squares
Depth Perception – How
far away is it?
The ability to perceive distance of an object
and its three dimensional qualities

Monocular Cues – only require one eye to notice
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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
Overlap – when one object blocks another object, the
partially blocked object is viewed as being farther away
Aerial perspective – hazy or slightly blurred objects
appear to be farther away
Depth Perception
Monocular Cues

Texture gradient – crisp and distinct objects
appear closer than fuzzy and blended objects
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Linear perspective – parallel lines seem to meet in
the distance; the closer together the lines appear
to be, the greater the perception of distance

Motion parallax – when moving, nearby objects
seem to move by faster than more distant objects
Depth Perception
Binocular cues – require both eyes to notice

Convergence – the degree to which eye muscles
rotate to focus on an object

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The more the eyes rotate inward, the closer the object is
perceived to be
Binocular disparity – when retinal image is very
different between the two eyes, the object is
interpreted as being very close

When retinal image is very similar, object is interpreted
as being further away
Convergence
Repeat
The Perception of Motion –
Where is it going?
Neural pathways doing a combination
of the following
1.
2.
3.
Sensing eye muscle activity
Sensing the changing retinal image
Contrasting the moving object with its
stationary background
The Perception of
Motion
Illusions of motion
1.
Induced motion – our tendency to perceive an
object moving and a background staying still
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2.
Even when a background is actually moving
Karl Duncker – the dot and the rectangular frame
Stroboscopic motion – the illusion that two or
more flashing lights are actually one moving light

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If the flashing of the second light is within 1/10 of a
second
The brain’s visual system perceives motion, even
though there is no movement across the retina
Perceptual Constancies
Tendency to perceive familiar objects as unchanging
despite actual changes in sensory input

Size constancy – the perception that an object remains the
same size despite a changed retinal image

Shape constancy – the tendency to perceive objects as
having a similar shape despite the images cast on the retina
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Brightness constancy – the tendency to perceive the
brightness of an object as the same despite changes in
lighting conditions
Perceptual Illusions
Stimuli that tend to make us misperceive the
true characteristics of objects or images

The Muller-Lyer illusion – the misperception of
identical length of two lines due to arrows pointing
in different directions

The moon illusion – the misperception that the
moon is larger when it is closer to the horizon
Illusions
Repeat
Experience &
Perception
Perceptual set – the influence of prior
assumptions and expectations on
perceptual interpretations

Observers often interpret ambiguous
stimuli in terms of what their expectations
are

Circular clouds = UFOs, rocks in a river = the
Lock Ness Monster
Experience &
Perception
Our perceptions are a combination of
the following:
Cues that indicate distance, form,
movement, etc.
 Our expectations, learning experiences,
and cultural experiences
