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Sensation & Perception
Sensation Detect physical energy & encode into neural
signal
Perception Select, organize & interpret sensations Psychophysics– the study of the relationships
between sensory experiences and the physical stimuli that cause them
Sensation
Bottom-Up Processing Start w/ sense receptors and works
up to the brain’s integration
Top-Down Processing Starts w/ higher-level mental
processes that construct perceptions drawing on our experience & expectations
Top-Down Processing
1. Please read the following paragraph,
2. Raise your hand as you finish reading it.
Here it is:
Top-Down Processing
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Sensation - Thresholds
Gustav Fechner Sensory thresholds October 22, 1850: “Just barely
noticeable”
Ernst Weber Concept of “JND”
Difference Threshold
Weber’s Law- to perceive as different, 2 stimuli must differ by a constant minimum percentage (JND); larger or stronger the stimulus, larger the change required to notice
A JND is simply the amount of change in an objects size, shape, color or brightness that is recognizable 50% of the time
Sensation - Thresholds
Absolute Threshold minimum stimulation needed to detect a
particular stimulus 50% of the time
Difference Threshold Min. difference between 2 stimuli
required for detection 50% of time Just Noticeable Difference (JND)
Absolute Threshold
-min. stimulation needed to detect a stimulus 50% of time
Subliminal- When stimuli are
below one’s absolute threshold for conscious awareness
0
25
50
75
100
Low Absolutethreshold
Medium
Intensity of stimulus
Percentageof correctdetections
Subliminal stimuli
Examples of Absolute ThresholdSenseDetection Threshold
Light A candle flame at 30 mls on dark, clear night
Sound Tick of mechanical watch at 20 feet
Taste 1 teaspoon of sugar in 2 gallons of water
Smell 1 drop of perfume in a 3 bedroom apartment
Touch The wing of a bee falling on cheek from 1 cm
Sensation - Thresholds
Sensory adaptation- diminished sensitivity as a consequence of constant stimulation
-With any sense
e.g. Rhodopsin Photopigment in eyesDark/light adaption
Signal-Detection Theory Studies the motivation, sensitivity,
and decision making in detecting the presence or absence of a stimulus
Stroop Effect- namecolor of words, not what the word is
Video
The science behind vision
Structure of the Eye
The eye works like a camera, using a lens to focus light onto a photo-sensitive surface at the back of a sealed structure.Presbyopia
Light rays
1.Cornea
2.Pupil
Blind spot
Opticnerve
5.Retina
Fovea (pointof central focus)
4.Lens
3.Iris
Organization of Retina
Toopticnerve
Ganglioncell
Amacrinecell
Bipolarcell
Horizontalcell
Cone Rod
Light
Light
Cross section of retina shown vastly magnifiedin the diagram to the right
Photochemical is located hereBack of the eye
Retina’s Reaction to LightOptic nerve
carries neural impulses from the eye to the brain
Blind Spot point at which the optic nerve leaves
the eye, creating a “blind spot” because there are no receptor cells located there
Blind spotTest
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Blind spot
Fovea
60 40 20 0 20 40 60
180
140
100
60
20
0
180
140
100
60
20
0
60 40 20 0 20 40 60Distance on retina from fovea (degrees)
Fovea
Blind spot
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Distance on retina from fovea (degrees)
Fovea
Blind spot
Blind spot
Retina – Photoreceptor part Rods
Location: peripheral of retina Detect: black, white & gray; movementFunction best in: twilight or low light
ConesLocation: near center of retina Detect: fine detail and color visionFunction best in: daylight or well-lit
conditions
Th
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san
ds
of
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squ
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Blind spot
Fovea
60 40 20 0 20 40 60
180
140
100
60
20
0
180
140
100
60
20
0
60 40 20 0 20 40 60Distance on retina from fovea (degrees)
Fovea
Blind spot
Th
ou
san
ds
of
con
es p
er
squ
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Distance on retina from fovea (degrees)
Fovea
Blind spot
Rods Cones
Distribution of Rods and Cones
Sensation and Hearing
Sensation and Hearing
How do we hear?
Sound waves are transformed into vibrations in the ear, and the strength of those vibrations are coded by sensory neurons
We hear when sound waves interact with the structures of the ear
Sound Waves
Frequency of a sound wave is related the pitch of a sound (high – low)
Amplitude of a sound wave is related to loudness of a sound (loud – soft) Measured in decibels Humans can hear to about 140 decibels
(above 110 and below 80 can damage)
Major Structures of the Ear Outer Ear - acts as a funnel to direct sound
waves towards inner structures
Middle Ear - consists of three small bones (or ossicles) that amplify the sound
Inner Ear - contains the structures that actually transduce sound into neural response
Basiliar Membrane
Outer ear Middle ear Inner ear
A sound causesthe basilarmembrane to waveup and down.
Basilarmembrane
Hair cells
Tectorialmembrane
Round window
Eardrum
Oval window
Cochlea,partiallyuncoiled
Stirrup
AnvilHammer
Soundwaves
Auditorycanal
Coding of Sounds
The pattern of vibration along the Basilar Membrane depends on the Frequency of the sound wave
Basilarmembrane
Direction of traveling wave
Pain
Gate-Control Theory theory that the spinal cord
contains a neurological “gate” that blocks pain signals or allows them to pass on to the brain
(Melzack & Wall, 1962, 1965)
Gate Control Theory of Pain
Small fibers sends pain information up the spinal cord ('opens the gate').
Large fibers transmit non-pain sensation & inhibit the flow of pain ('closes the gate').
This is one reason that rubbing a region of soreness helps to reduce pain.
Small fiber = pain Large fiber = pressure
Color Theory Color blindness Perception
Vision - wavelength is related to color
Short wavelength=high frequency(bluish colors)
Long wavelength=low frequency(reddish colors)
Great amplitude(bright colors)
Small amplitude(dull colors)
Wavelength (Hue)
Different wavelengths of light resultin different colors.
400 nm 700 nmLong wavelengthsShort wavelengths
Violet Indigo Blue Green Yellow Orange Red
Theories of Color VisionYoung- Helmholtz trichromatic theory: Based on behavioral experiments, Helmholtz
suggested that the retina has 3 types of receptors that are sensitive to red, blue and
green colors.
Subtractive Color MixingIf three primary
colors (pigments) are mixed,
subtraction of all wavelengths occurs and the color black
is the result.
Additive Color MixingIf three primary colors (lights) are mixed, the wavelengths are added and the color
white is the result.
Photoreceptors
RedCones
GreenCones
Longwave
Mediumwave
Shortwave
People who are “colorblind” lack functioning red- or green-sensitive cones.
BlueCones
Color Blindness
Color deficient vision
People who suffer red-green blindness have trouble perceiving the number within the design
Color BlindnessGenetic disorder in which people are blind
to green or red colors. Also tends to be gender based towards males. This supports the Trichromatic theory.
Ishihara Test
Opponent Process TheoryHering proposed that we process four
primary colors combined in pairs of red-green, blue-yellow, and black-white.
Opponent Colors
Gaze at the middle of the flag for about 30Seconds. When it disappears, stare at the dot and report
whether or not you see Britain's flag.
Color Constancy
Color of an object remains the same under different illuminations. However, when context
changes the color of an object may look different.
Perception
I. Selective Attention
II. Perceptual Illusions
III. Perceptual Organization
IV. Monocular vs Binocular Cues
V. Perception of Motion
Perception
The process of selecting, organizing, and interpreting sensory information, which enables us to recognize meaningful objects and events.
I. Selective Attention
Perceptions about objects change from moment to moment. We can perceive different forms of
the Necker cube; however, we can only pay attention to one aspect of the object at a time.
Necker Cube
Demonstration video
II. Perceptual Illusions
Illusions provide good examples in understanding how perception is
organized. Studying faulty perception is as important as studying other
perceptual phenomena.
Line AB is longer than line BC.
Tall Arch
In this picture, the vertical
dimension of the arch looks
longer than the horizontal
dimension. However, both
are equal.
48
Illusion of a Worm
The figure on the right gives the illusion of a blue hazy “worm” when it is nothing else but blue lines identical to the figure on the left.
© 1981, by perm
ission of Christoph R
edies and L
othar Spillmann and Pion L
imited, L
ondon
49
3-D Illusion
It takes a great deal of effort to perceive this figure in two dimensions.
Reprinted w
ith kind permission of E
lsevier Science-NL
. Adapted from
H
offman, D
. & R
ichards, W. Parts of recognition. C
ognition, 63, 29-78
Perception
Which line is bigger? A or B? Ponzo illusion
Which line is longer? A-B or C-D?
A
BD
C
52
III. Perceptional OrganizationWhen vision competes with our
other senses, vision usually wins – a phenomena called visual
capture.
How do we form meaningful perceptions from sensory
information?
Perceptional Organization
Given a cluster of sensations, we organize them into a gestalt
Gestalt: form or whole (German word)
Perception
Shape constancy
More shape constancy:
Organization of the visual field into objects (figures) that stand out from their
surroundings (ground).
Form Perception
Tim
e Savings S
uggestion, © 2003 R
oger Sheperd.
57
Grouping
After distinguishing the figure from the ground, our perception needs to organize the figure into a meaningful form using
grouping rules.
58
Grouping & Reality
Although grouping principles usually help us construct reality, they may occasionally lead us
astray.
Both photos by W
alter Wick. R
eprinted from G
AM
ES
Magazine. .©
1983 PCS G
ames L
imited Partnership
59
Depth Perception
Visual Cliff
Depth perception enables us to judge distances. Gibson and Walk (1960) suggested that human infants (crawling age) have depth perception. Even newborn animals show depth perception.
Inne
rvis
ions
IV. Monocular vs Binocular Cues
61
Binocular CuesRetinal disparity: Images from the two eyes differ.
Try looking at your two index fingers when pointing them towards each other half an inch apart and about 5 inches directly in front of your eyes. You will see a “finger sausage” as shown in the inset.
Binocular Cues
Convergence: Neuromuscular cues. When two eyes move inward (towards the nose)
to see near objects and outward (away from the nose) to see faraway objects.
Monocular Cues
Relative Size: If two objects are similar in size, we perceive the one that casts a smaller retinal image to be farther away.
Monocular CuesInterposition: Objects that occlude
(block) other objects tend to be perceived as closer.
Monocular Cues
Relative Clarity: Because light from distant objects passes through more light than closer objects, we perceive hazy objects to be farther away than those objects that appear sharp and clear.
Monocular Cues
Texture Gradient: Indistinct (fine) texture signals an increasing
distance.
Monocular Cues
Relative Height: We perceive objects that are higher in our field of vision to be farther away than those that are lower.
Monocular CuesRelative motion: Objects closer to a
fixation point move faster and in opposing direction to those objects that are farther away from a fixation point, moving slower
and in the same direction.
Monocular CuesLinear Perspective: Parallel lines, such as railroad tracks, appear to converge in the distance. The more the lines converge, the
greater their perceived distance.
Monocular CuesLight and Shadow: Nearby objects reflect more light into our eyes than more distant
objects. Given two identical objects, the dimmer one appears to be farther away.
V. Perception of Motion
Motion PerceptionMotion Perception: Objects traveling
towards us grow in size and those moving away shrink in size. The same is true when the observer moves to or from an object.
73
Apparent Motion
Phi Phenomenon: When lights flash at a certain speed they tend to present illusions of motion. Neon signs use this principle to
create motion perception.
Two lights flashing one after the other.One light jumping from one point to another: Illusion of motion.
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