Transcript Sensation and Perception - Columbus State University

Chapter 3
Sensation and Perception
Sensation vs. Perception
• Sensation: result of energy (light)
stimulating a sense organ (eye)
• Perception: the process of creating
meaningful patterns (a word) from raw
sensory information
• Vision: is considered by many to be our
most important sense
Sensory Thresholds
• Psychophysics: study of our ability to detect
sensory stimuli (e.g., light)
• absolute threshold: minimum intensity of a
stimulus (light) one can detect 50% of the
times it is presented (e.g., 50 out of 100 trials)
• difference threshold: minimum CHANGE in
stimulus intensity that one can detect 50% of
the times it is changed (e.g., 50 out of 100 trials)
– Also called the “Just Noticable Difference” or
“jnd”
Weber’s Law
• The jnd for any sense is a constant
fraction or proportion of the
stimulation being judged.
• For judging weight:
– 2 pounds must be added to a 10 lb. to detect the
change (2/10 = 20%)
– to detect a change in a 100 lb. weight, 20 lbs.
must be added (20%)
Subliminal Perception
• definition: sensing and perceiving
information without being aware of it
• can it effect us?:
– Maybe, in minor ways (buying more popcorn)
– No, not in major ways (going against our
values)
– Sorry, we cannot study subliminally
Does subliminal advertising work?
• The goal of using subliminal
advertising is to increase the likelihood
that you will buy a particular product.
• Subliminal advertising CANNOT
reliably or significantly change an
individual’s behavior.
• Most advertising relies on repetition,
which is different and DOES work.
Visual System
Light: The Stimulus for Vision
Light: that part of the electromagnetic
spectrum we can detect with our eyes
Cross Section of the Eye
Parts of the Eye
• Cornea: clear protective layer on the
front of eye
• Iris: colored part of eye, opens and
closes to let in more or less light
• Pupil: opening created by iris through
which light passes
• Lens: changes shape to focus on
objects at different distances
More about the Lens
• Accommodation: tiny muscles change
the shape of the lens to focus on far or
near objects
– far: muscles are relaxed making lens flatter
– near: muscles are tensed making lens thicker
• Presbyopia: normal loss of ability to
accommodate that comes with age
Near and Far Sightedness
• Caused by shape of eye and/or power of lens
• Farsightedness: is less common
– eye too short and/or lens too weak
– light focuses behind retinal
– correct with “convex” lens to add power
• Nearsightedness: is more common
– eye is too long and/or lens is too powerful
– light focuses in front of retina
– correct with “concave” lens to reduce power
The Retina and its Parts
• Retina: inner layer on back of eye that
contains “light-sensitive” rods and cones
• Optic Nerve: bundle of axons running
from retina to visual (occipital) cortex
• Blind Spot: spot on the retina where optic
nerve exits eye, there are no receptors
(rods or cones) there
• Fovea: center of the retina where “acuity”
(ability to see fine detail) is greatest
Cells in the Retina
Cells in the Retina
• Receptor cells: rods and cones, fire in
response to light
• bipolar cells: carry signals from
receptors to ganglion cells
• Ganglion cells: axons of ganglion cells
form the optic nerve
• Wiring: each cone has its own bipolar
and ganglion cell while several rods
share one bipolar and ganglion cell
Distribution of Cells in the Retina
Fovea
Periphery
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Rods
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located mainly in periphery
responsible for night vision
detail not detected
see black, white, and gray
several rods share 1 bipolar and 1
ganglion cell
• rod vision lacks detail, but, by
combining their efforts, groups of
rods allow us to see in low light
Cones
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located mainly in fovea
work best in bright light
enable us to see fine detail
responsible for color vision
each cone has its own bipolar and
ganglion cell
• this allows us to see detail but
bright light is needed
Rods see in low light but lack detail
A
B
C
Brain sees one blurry dot
Cones see detail but require bright light
A
B
Brain sees A and B
Visual Pathways to the Brain
Optic Chiasm: Where optic nerves (actually 1/2
of each retina) “cross over” from left to right
Dark Adaptation (not in book)
• dark adaptation: increased sensitivity
of rods and cones in darkness
—e.g., entering a darkened room
• Cones: adapt for 10 minutes bet never
become very sensitive
• Rods: continue adapting for 30 minutes
and become much more sensitive
Rods vs. Cones (not in book)
Cones
Rods
amount of
light
needed to
see
5
10
15
20
Minutes in darkness
25
30
Dark Adaptation Experiment
• Keep left eye closed or covered for at
least 10 minutes, 15 or 20 is better.
• Then, close right eye and open left
• Switch back and forth a few times
• Do you see a difference?
• You should if you kept your left eye
closed long enough
Color Vision
3 Properties of Color
• 1. hue: the actual color (red, green, etc.)
– determined by “wavelength” of light
• 2. saturation: how pure and vivid the
color is
– single wavelength (laser) > fully saturated light
– many wavelengths (light bulb) > desaturated light
• 3. brightness: intensity of the light
– low light results in loss of color
– extreme brightness seems to lighten color
Light of a Single wavelength
wavelength
intensity
2 Types of Color Mixing
• subtractive color mixing: mixing
pigments (like paints)
– more pigments = more wavelengths absorbed,
paint gets darker (until black)
• additive color mixing: mixing lights of
differing wavelengths
– more wavelengths = more light, surface gets
lighter (until white)
Primary Colors
• Primary Colors: sets of 3 colors that
can be mixed to produce any other
color
• For Visual System: set of interest is
“Red Green and Blue”
Complementary Colors
(additive mixing only)
• Pair of colors that when mixed cancel
out to produce “gray” and not another
color
• Two pair of complementary colors for
the visual system
– 1. Red - Green
– 2. Blue - Yellow
Theories of Color Vision
Trichromatic
and
Opponent - Process
Trichromatic Theory
• There are three types of cones: red,
green, and blue.
• The colors we see result from the
combined signals from the three types
of cones.
Flag Afterimage Stimulus
Afterimages
• Afterimage: visual image that
remains after the light stimulus is
removed (after flash from camera)
• Complementary Afterimages:
afterimage appearing in
complementary colors, create a
“problem” for trichromatic theory
Opponent-Process Theory
• we have pairs of opposing color
cells higher in the visual system
– red-green
– yellow-blue
– (also black-white)
• this provides an explanation for
complementary afterimages
Explaining Complementary
Afterimages
• white normally stimulates the red and
green cells equally
• exposure to green fatigues the green
cell while the red cell rests
• exposure to white NOW causes red
receptor to respond but green receptor
is “tired”
• we see red instead of white
Both Theories are Correct
• We do have three types of cones
(Trichromatic) in the retina
• We also have pairs of color cells
(opponent process) higher in the visual
system
Colorblindness
• trichromats: people with normal color
vision
• dichromats: can’t distinguish either
– red fom green or
– yellow from blue
• monochromats: people who are totally
colorblind, more severe
Hearing
3 Properties of Sound
Sound travels in waves as does light
• 1. Pitch: determined by “frequency,” the
number of cycles per second of a sound
wave
• 2. Loudness: determined by “amplitude”
(height) of the sound wave
• 3. Timbre: determined by “complexity
and shape” of the sound wave, gives each
sound its unique quality
Units of Measurement for Sound
• hertz (Hz): to measure pitch
(frequency), number of cycles per
second of the sound wave
– a baby’s cry is at about 3,000 Hz
• decibel (db): to measure loudness
(amplitude) of the sound wave
– I’m speaking at about 70 db
Decibel Levels for Common Sounds
The Ear
© Prentice Hall, 1999
Structure of the Ear
• 1. Outer Ear:
– Pinna: directs sound waves into the auditory
canal
– Auditory Canal: conducts sound to the eardrum
– Eardrum: thin membrane that vibrates in
response to sound, and transfers sound energy
to bones of the middle ear
Structure of the Ear
• 2. Middle Ear: three tiny bones
“amplify sound” and transfer sound
energy to the inner ear
– Hammer
– Anvil
– Stirrup
Structure of the Ear
• 3. Inner Ear: where sound energy is
registered
– Cochlea: snail shaped fluid-filled structure
– Oval window: thin membrane, transfers
vibrations from stirrup to fluid of cochlea
– Basilar membrane: runs the length of the
cochlea
– Organ of Corti: rests on basilar membrane,
contains “receptor” cells
– Round window: absorbs energy and equalizes
pressure in the cochlea
Receptor Cells on the Organ of Corti
• Recetor (hair) cells have hair-like fibers
• Waves in fluid bend the fibers making
the cells “fire”
• Axons (afferent) from these cells form
the “auditory nerve”
• There are also “efferent” axons that
adjust sensitivity of the recptor cells
Hair Cell in the Organ of Corti
Theories of Hearing
• Loudness: the louder the sound, the
more receptors fire
• Pitch (frequency) 3 theories
– place theory:
– frequency theory
– volley theory (principle)
Place Theory
• different frequencies cause vibrations
at different locations (places) along
basilar membrane
• higher frequencies at base, lower
frequencies at top
Frequency Theory
• The entire basilar membrane vibrates
at the same frequency as the incoming
sound
• What is the problem here? (hint, we
hear frequencies close to 20,000 Hz)
• It takes a neuron several (3)
milliseconds to fire and reset
– at best we could hear at about 333 Hz
Volley Theory (Principle)
• Receptor cells fire in groups: first one,
then another, then a third
• Resetting: the first group of cells to fire
are resetting while the second and third
groups fire and so on
Which Treory is Correct?
All seem to be correct
• Place theory: operates at lower
frequencies
• Frequency Theory: operates for the
higher frequencies but utilizing the
“volley principle”
Vestibular Sense
Information about balance and body position i
• 3 fluid filled structures work just like like
the cochlea
– Semi-circular canals: body rotation
– Utricle: horizontal movement
– Saccule: Vertical movement/gravity
• Motion Sickness: caused by disagreement
between eyes and vestibular sense
Perceptual Constancies (visual)
• Even though the light stimulus on the
retina changes, EXPERIENCE helps us
perceive a “constant” environment
• Size constancy
• Shape constance
• Color constnacy
Perceptual Constancies (cont.)
• Size - The farther away an object is, the
smaller the image is on the retina. Yet we
know it is still the same size.
• Shape - A partially open door casts a
“trapezoid” on the retina. Yet, we know
it is a rectangle.
• Color - Different lighting dramatically
alters color. Yet, we know the color of an
object does not change.
Depth Perception (seeing in 3D)
• monocular cues: depth cues
requiring only one eye (learned)
• binocular cues: depth cues
requiring both eyes (inate/inborn)
Monocular Cues
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superposition
• texture gradient
linear perspective • shadowing
aerial perspective • motion parallax
elevation
1. Superposition
• One object, by partially blocking
another, is perceived as being closer.
2. Linear Perspective
• Two parallel lines appear to converge
as they move away.
3. Aerial Perspective
• More distant objects have less detail
than closer objects because the
atmosphere bends and distorts light
waves
• Imagine the New York city skyline
4. Elevation
The higher an object is, the farther away
it appears.
5. Texture Gradient
• Objects seen at greater distances
appear to be smoother and less
textured.
• Imagine looking out over a field of
crops.
6. Shadowing
• Shadows often appear on the parts of
objects that are more distant.
7. Motion Parallax
• Has to do with relationship between
movement and distance
• When riding in train or car
– near objects rush past you
– distant objects seem to move with you
Binocular Cues
• 1. Retinal Disparity: each retina sees a
slightly different image because of
separation
• 2. convergence: as eyes turn inward to
focus on close object, muscles send
signals to brain
Perceiving Movement
(Illusions of Movement)
• The autokinetic illusion
• stroboscopic motion
• the phi phenomenon
Autokinetic Illusion
• A stationary spot of light in a dark
room appears to move
• Occurs because we lack of a “frame of
reference”
• Our eyes constantly make small
involuntary movements which we are
normally unaware of
Stroboscopic Motion
• Apparent motion resulting from seeing
gradually changing images in rapid
succession (e.g., a movie or cartoon)
• Once a certain speed is reached, the
eye cannot separate the images and
continuous motion is perceived
Phi Phenomenon
• Apparent movement caused by
flashing lights in sequence (e.g., movie
theatre marquis)
• One light flashes, then a second, and a
third. With the right combination of
distance and speed, we perceive one
moving light