CHAPTER 4
SENSATION
& PERCEPTION
Sensation & Perception

Sensation: How energy from the world is
converted into neural impulses our brain can
understand.

Perception: How our brain selects, organizes,
and interprets our sensations.
Psychophysics
 The
study of how the physical world is
interpreted by our brains.
Physical World
Psychological World
Light
Brightness
Sound
Volume
Pressure
Weight
Sugar
Sweet
Sensory Receptors

Specialized cells that
detect stimulus
information and
transmit it to
sensory (afferent)
nerves and the brain.




Classes
A) Photoreceptiondetection of light perceived
as sight
B) Mechanoreceptiondetection of pressure,
vibration, and movement,
perceived as touch, hearing,
and equilibrium.
C) Chemoreceptiondetection of chemical
stimuli, perceived as smell
and taste.
Absolute Threshold

Minimum stimulation
needed to detect a
particular stimulus 50% of
the time.
◦ Examples
Difference Threshold

Minimum difference between
two stimuli required for
detection 50% of the time, also
called just noticeable difference
(JND).
◦ Examples
 Vision: a flame from a single
candle 30 miles away
 Hearing: ticking of a watch 20
feet away
 Touch: an ant walking on your
arm
 You turn the temperature down
from 74 degrees to 73 degrees, and
all of a sudden someone in the
house is cold.
 Your change the volume of the car
by one level and someone
complains that its too loud.
Thresholds

Subliminal Threshold:When
stimuli are below one’s
absolute threshold for
conscious awareness.
Subliminal Threshold
Fechner’s Law
Weber’s Law

The size of a JND is
proportional to the type of
stimulus.
Stimulus
Light
Constant
8%
Weight
2%
Tone
3%

PERCEIVED magnitude of a
JND scales with the number
of JND’s added
◦ Example
 You walk into a dark room
 You turn on one lamp – the
difference between dark and light
is HUGE
 Then you turn on another
lamp… its twice the light, but not
twice as bright
 Then you turn on a third lamp
and barley notice any difference
in lighting
Scaling
Signal-Detection Theory
Our ability to discriminate (tell the difference)
between ‘noises’.
 Detection depends on:

◦
◦
◦
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Person’s experience
Expectations
Motivation
Level of fatigue
Sensory Adaptation

Gradual decline in sensitivity with
constant stimulation
◦ Example:
 Wearing a watch
 The clothes on your back
VISION!
Transduction
 The
transformation of stimulus energy into
neural impulses.
Light

Phototransduction begins with light energy.
◦ Light has three characteristics:
1.


2.


3.
Wavelength (hue/color)
Wider = Red
Narrower = Blue
Amplitude (brightness)
Taller = Brighter
Shorter = Duller
Purity (saturation/richness of color)
The Eye
The Lens
 Accommodation:
The process by
which the eye’s lens
changes shape to
help focus near or
far objects on the
retina.
Retina
Cones
Rods
Long
 100-125 million
 Peripheral vision and
night vision
(Rods=Raccoons)
 Sensitive to dim light
 Outnumber cones in
periphery of retina

Short
 5-6.4 million
 Cones = Color vision
and daylight vision
 Don’t respond well to dim
light
 Provide more sharp and
detailed vision
 Concentrated in center of
retina (fovea only cones)

Photoreceptors
Fovea
Vision Issues
 Nearsightedness:A
condition in which
nearby objects are
seen more clearly than
distant objects.
 Farsightedness:A
condition in which
faraway objects are
seen more clearly than
near objects.
Optic Chiasm
Vision in the Brain

Parallel processing
◦ Your brain now uses the same information to
do multiple things
◦ Example
 determine color & brightness

Feature detectors
◦ Neurons that respond to certain types of
stimuli
 Example
 You have neurons that help you recognize what a face looks
like
Theories of Color Vision
 Trichromatic
theory
◦ The retina contains three receptors that are sensitive
to red, blue and green colors.
Addition of Colors
If three primary colors
(lights) are mixed, the
wavelengths are added
and the color white is
the result.
Subtraction of Colors
If three primary colors
(pigments) are mixed,
subtraction of all
wavelengths occurs and
the color black is the
result.
Color Blindness
Genetic disorder in which people are blind to
green or red colors.
This supports the Trichromatic theory.
Opponent Process Theory
We process primary colors combined in three
pairs of red-green, blue-yellow, and black-white.
AUDITION!
Sound

Acoustical transduction begins with sound energy.
◦ Sound has three characteristics:
1. Frequency (pitch) = wavelength
2. Intensity (loudness) = wave amplitude
3. Quality (timbre) = uniqueness of sound
The Ear
Cochlea
Place Theory

Says sound frequencies
stimulate the basilar
membrane at specific places
resulting in perceived pitch.
Frequency Theory

Says that the speed of
nerve impulses traveling up
the auditory nerve matches
the frequency of a tone,
thus enabling us to know
its pitch
Theories of Audition
Localization of Sounds





We have two ears
Sounds may reach one ear faster than the
other ear
Or sounds may be louder in one ear than
another
This let’s us locate the sound
Our head acts as a “shadow”
Deafness

Conduction Hearing Loss
◦ Caused by damage to the parts of the ear that
connect to the cochlea

Sensorineural Hearing Loss
◦ Caused by damage to the cochlea or to the auditory
nerve
Touch

The sense of touch is a mix of four distinct skin
senses
◦ pressure, warmth, cold, and pain

Extreme heat & extreme cold are THE SAME!
Pain
Pain tells the body that something has gone
wrong.
 CIP (Congenital Insensitivity to Pain)

◦ A rare disease in which the afflicted person feels no
pain.
Gate-Control Theory

Proposes that our spinal cord contains
“gates” that either block pain or allow it to
be sensed.
Taste
•
•
A chemical sense
Originally, it was believed the tongue had 4
taste receptors:
• sweet, salty, sour, and bitter
•
Receptors for a fifth taste have been
discovered
• “Umami”
Smell
A chemical sense
 Also known as Olfaction
 Odors enter the nasal cavity to stimulate
Olfactory receptors

Flavor

Combination of taste and smell
◦ Example:
 Strawberries are sweet
 But smell is what distinguishes a strawberry from a
blueberry
Kinesthesis

Knowing where our body parts are
located at all times
Vestibular Sense
Our sense of balance
 Located in the inner ear
 Monitors head and body position

PERCEPTION
BOTTOM-UP PROCESSING
 Sensory
receptors register information
about the external environment and send
it up to the brain for interpretation.
◦ A progression from individual elements
to the whole.
 More accurate, slower
 Feature analysis
TOP-DOWN PROCESSING
Launched by cognitive processing at the
brain’s higher levels, that allows the
organism to sense what is happening and
to apply the framework to the
information from the world.
◦ A progression from the whole to the
elements.
 Faster, error prone
 Gestalt principles
 Perceptual set

Principles of Perception
•
Distal stimuli
• The world outside the body
•
Proximal stimuli
• The energy received by our senses
Selective Attention
We can only pay attention to one thing at
a time, but that doesn’t mean we can’t
constantly change our perceptions of the
world around us.
 This is what allows us to see reversible
figures.

PERCEPTUAL SET

A predisposition or readiness to perceive
something in a particular way.
◦ expectancy
GESTALT PSYCHOLOGY
School of thought interested in how
people naturally organize their
perceptions according to certain patterns.
 Emphasizes that the whole is greater
than the sum of its parts.
 Max Wertheimer

GESTALT PRINCIPLES
Used to explain how we perceive objects.
 Figure-ground
 Proximity
 Closure
 Similarity
 Simplicity
 Continuity

FIGURE AND GROUND
PROXIMITY
CLOSURE
SIMILARITY
SIMPLICITY
CONTINUITY
VISUAL PERCEPTION

Shape

Depth

Motion

Constancy
SHAPE PERCEPTION

The ability to perceive the form of an
object.
◦ Outlines and contours
DEPTH PERCEPTION

The ability to perceive objects in 3-D.
◦ Enables us to judge distances
Binocular cues: from 2 eyes
 Monocular cues: from 1 eye

VISUAL CLIFF



Studied by Eleanor Gibson & Richard
Walk (1959)
Suggests that human infants of
crawling age have depth perception
newborn animals also show depth
perception
BINOCULAR CUES
 Depth
cues that depend on the
combination of the images in the left and
right eyes and on the way the two eyes
work together.
 You need both eyes to experience this
type of depth perception
• Retinal disparity: difference between
the images in the two eyes.
• Convergence: cues to depth and
distance from the eye muscles.
MONOCULAR CUES
Depth cues available from the image in one
eye, right or left.
 You only need one eye to experience this type
of depth perception
 Pictorial depth cues: clues about distance
that can be given in a flat picture.

MONOCULAR CUES
• Familiar size- standard size
◦ Relative size- larger=closer; smaller=further
• Height in field/Relative height- lower=closer;
higher=further
• Linear perspective- parallel lines get closer as they
recede
• Overlap/Interposition- closer conceals further
• Shading/Shadowing- light and dark create 3-D
impressions
• Texture gradient- course and distinct=closer; fine and
indistinct=further
• Relative clarity- clear=closer; hazy=further
FAMILIAR & RELATIVE SIZE
RELATIVE HEIGHT
LINEAR PERSPECTIVE
INTERPOSITION
SHADING
TEXTURE GRADIENT
RELATIVE CLARITY
MOTION PERCEPTION



The ability to detect movement.
• Real vs. perceived
Apparent movement: the perception that a
stationary object is moving.
Objects traveling towards us grow in size and
objects moving away from us shrink in size.
• The same is true when the observer moves
to or away from an object.
MOTION PERCEPTION
• Motion parallax/Relative motion
• Phi phenomenon
• Stroboscopic effect
MOTION PARALLAX
Images of objects at different distances are moving across
the retina at different rates.
• As we move, stationary objects seem to move as well
• Objects that are closer appear to move faster; objects
that are farther away appear to move slower
• Objects above a fixation point seem to move in the same
direction (with us), objects below a fixation point seem
to move in the opposite direction (past us)
• Depth cue
• Example: Cows vs. light posts while driving on the Turnpike
to Orlando

MOTION PARALLAX
PHI PHENOMENON
A
series of lights flashed in rapid
succession will appear to be one moving
light.
• Creates an illusion of movement
 Example: Neon signs use this principle to
create motion perception
STROBOSCOPIC EFFECT
A series of still pictures presented in
rapid succession will appear to be moving.
• Creates an illusion of movement
 Also called the strobe effect
 Example: movies and flip books

PERCEPTUAL CONSTANCY
The recognition that objects are constant and
unchanging even though sensory input about
them is changing.
 We perceive objects as remaining the same,
despite the images they cast on our retina.

◦ Size constancy: object remains the same size
◦ Shape constancy: object remains the same shape
◦ Color constancy: object remains the same color
SIZE CONSTANCY
SHAPE CONSTANCY
COLOR CONSTANCY
OPTICAL ILLUSIONS
An apparent inexplicable discrepancy between
the appearance of a visual stimulus and its
physical reality.
 Famous illusions:

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Muller-Lyer
Ponzo
Zollner
Hermann grid
Ames room
Impossible figures
MULLER-LYER ILLUSION
PONZO ILLUSION
ZOLLNER
HERMANN GRID ILLUSION
AMES ROOM
IMPOSSIBLE FIGURES