Transcript PsychSci4e_lecppt_ch04x

Gazzaniga • Heatherton • Halpern
Psychological Science
FOURTH EDITION
Chapter 4
Sensation and Perception
©2013 W. W. Norton & Company, Inc.
4.1 How Do We Sense Our Worlds?
Learning Objectives
• Distinguish between sensation and
perception.
• Describe the process of transduction.
• Distinguish between an absolute threshold
and a difference threshold.
• Discuss sensory detection theory.
• Define sensory adaptation.
How Do We Sense Our Worlds?
• Sensation: our sense organs’ detection and
response to external stimulus energy and the
transmission of those responses to the brain
• Perception: the brain’s processing of detected
signals, resulting in internal representations of
the stimuli that form a conscious experience
of the world
• What we sense is the result of how we
perceive
Stimuli Must Be Coded to Be
Understood by the Brain
• Sensory coding: Sensory receptors translate the
physical properties of stimuli into patterns of neural
impulses
• Transduction: a process by which sensory receptors
produce neural impulses when they receive physical or
chemical stimulation
• The brain needs qualitative and quantitative
information about a stimulus
• Sensation and perception result from a symphony of
sensory receptors and the neurons those receptors
communicate with
Psychophysics Measures
the Relationship between
Stimuli and Perception
• Psychologists try to understand the
relationship between the world’s physical
properties and how we sense and perceive
them
• Psychophysics is a subfield that examines our
psychological experiences of physical stimuli
Sensory Thresholds
• Absolute threshold: the minimum intensity of stimulation
that must occur before you experience a sensation
– Example: The absolute threshold for hearing is the
faintest sound a person can detect 50 percent of the
time
• Difference threshold: the minimum amount of change
required for a person to detect a difference (i.e., the “just
noticeable difference”)
• Weber’s law: states that the just noticeable difference
between two stimuli is based on a proportion of the
original stimulus rather than on a fixed amount of
difference
Signal Detection Theory
• Signal detection theory (SDT): states that detecting a
stimulus requires making a judgment about its presence
or absence, based on a subjective interpretation of
ambiguous information
– Example: A radiologist is looking at a CAT scan for the kind of
faint shadow that signals an early-stage cancer. Her judgment
will likely be affected by her knowledge of the patient,
training, experience, motivation, attention and the
knowledge of the consequences of being wrong
• Signal detection research involves a series of trials in
which a stimulus is presented in only some trials. In each
trial, the participant must state whether he or she sensed
the stimulus
Sensory Adaptation
• Sensory adaptation: a decrease in sensitivity
to a constant level of stimulation
• If a stimulus is presented continuously, the
responses of the sensory systems that detect
it tend to diminish over time; when a
continuous stimulus stops, the sensory
systems usually respond strongly as well
4.2 What Are the
Basic Sensory Processes?
• For each of the five major senses — taste,
smell, touch, hearing, and vision — identify
the type of receptor and trace the neural
pathway to the brain
• Distinguish between the neural processes
associated with the experience of immediate
pain and the experience of chronic pain
• Discuss color perception
In Taste, Taste Buds Detect Chemicals
• Gustation: the sense of taste
• Taste buds: sensory organs, mostly on the tongue; come in
the form of tiny, mushroom-shaped structures (papillae)
– Stimulated taste buds send signals to the brain, which then
produces the experience of taste
– Different regions of the tongue are not more sensitive to certain
tastes (Lindemann, 2001)
• Every taste experience is composed of a mixture of five
basic qualities: sweet, sour, salty, bitter, and the relatively
new taste sensation umami (Krulwich, 2007)
• Mothers can pass their eating preferences on to their
offspring
In Smell, the Nasal Cavity
Gathers Odorants
• Olfaction: the sense of smell
• Basic process:
– Odorants pass into the nose and nasal cavity
– Contact a thin layer of tissue embedded with smell
receptors called the olfactory epithelium
– Smell receptors transmit information to the olfactory
bulb, the brain center for smell
– Has the most direct route to the brain
• Smell’s intensity is processed in brain areas also involved
in emotion and memory (Anderson, Christoff et al.,
2003)
In Touch, Sensors In the Skin Detect
Pressure, Temperature, and Pain
• Haptic sense: the sense of touch
• Sense conveys sensations of temperature, pressure, pain, and
where our limbs are in space
• The integration of various signals and higher-level mental
processes produces haptic experiences
• Examples:
– Stroking multiple pressure points can produce a tickling sensation,
which can be pleasant or unpleasant, depending on the mental state
of the person being tickled
– Brain areas involved in touch sensation respond less to selfproduced tactile stimulation than to external tactile stimulation
(Blakemore, Wolpert, & Frith, 1998)
Two Types of Pain
• Pain is part of a warning system that stops you
from continuing activities that may harm you
• Two kinds of nerve fibers have been identified
for pain:
– Fast fibers for sharp, immediate pain; activated by
strong physical pressure and temperature
extremes
– Slow fibers for chronic, dull, steady pain; activated
by chemical changes in tissue when skin is
damaged
In Hearing, the Ear Detects
Sound Waves
• Audition: the sense of sound
• Movements and vibrations of objects cause
the displacement of air molecules, which
produce a sound wave (change in air pressure
that travels through the air)
• A sound wave’s amplitude determines
loudness; its frequency determines pitch
• The ears convert sound waves to brain activity,
which produces the sensation of sound
The Cochlear Implant
• Cochlear implantation has helped people with
severe hearing problems due to the loss of
hair cells in the inner ear
• Works by directly stimulating the auditory
nerve; does not not amplify sound
• When devices are implanted in children born
deaf, the child’s hearing will be quite
functional and he/she will learn to speak
reasonably normally
• The problem of audism
In Vision, the Eye Detects Light Waves
• Most of the scientific study of sensation and
of perception is concerned with vision
• Very little of what we call seeing takes place in
the eyes, but rather as a result of constructive
processes that occur throughout much of the
brain
• Basic structures: cornea, lens, pupil, iris,
retina
Rods and Cones
• The retina has two types of receptor cells:
– Rods: respond at extremely low levels of illumination;
responsible primarily for night vision; found on outer
edges of the retina
– Cones: less sensitive to low levels of light; responsible
primarily for vision under high illumination and for
seeing both color and detail; found throughout the
retina but concentrated at the fovea
• Contain photopigments that initiate the
transduction of light waves into electrical neural
impulses
Transmission From the Eye to the Brain
• A variety of retinal cells perform a series of sophisticated
computations that help the visual system process the
incoming information
• Cells include: bipolar, amacrine, and horizontal cells;
converge on about a million retinal ganglion cells
• Ganglion cells are the first neurons in the visual pathway
with axons, which are gathered into a bundle called the
optic nerve
• At the optic chiasm, axons in the optic nerves cross to the
left and right hemispheres, travel to visual areas of the
thalamus and then to the primary visual cortex in the
occipital lobe
The Color of Light is Determined
by Its Wavelength
• An object appears to be a particular color
because of the wavelengths of light it reflects
• Trichromatic theory: activity in three different
types of cones that are sensitive to different
wavelengths
• Opponent-process theory: Different types of
ganglion cells, working in opposing pairs, create
the perception that R/G, B/Y are opposites
• We categorize color along three dimensions:
hue, saturation, and brightness
Subtractive Color Mixing
• Color can be produced through either the subtractive or
the additive mixture of wavelengths
• Subtractive color mixing: a process of color mixing that
occurs within the stimulus itself; a physical, not
psychological, process
– Mixing paints is one form of subtractive color mixing
because the colors are determined by pigments.
– Wavelengths that a pigment does not absorb are
reflected and enter the eye
• Red, yellow, and blue are the subtractive primary colors
because together these pigments absorb nearly all the
colors of the visible spectrum and when mixed, produce
black
Additive Color Mixing
• Additive color mixing: a process of color
mixing that occurs when different
wavelengths of light interact within the eye’s
receptors; a psychological process
• Additive primary colors are red, green, and
blue because mixing them yields white light
We Have Other Sensory Systems
• Humans, like other animals, have several
internal sensory systems in addition to the five
primary senses
• Kinesthetic sense: perception of the body’s
position in space and movements of our
bodies and our limbs (some include this with
the sense of touch)
• Vestibular sense: perception of balance; uses
information from receptors in the semicircular
canals of the inner ear
4.3 How Does Perception Emerge
from Sensation?
• Identify the primary sensory areas for touch,
hearing, and vision.
• Discuss the gate control theory of pain.
• Explain how the brain localizes sound.
• Distinguish between the “what” and “where”
pathways of the visual system.
• Describe blindsight.
How Does Perception Emerge
from Sensation?
• With the exception of olfaction, all sensory
information is relayed from the thalamus to
cortical and other areas of the brain
• Information is projected separately from the
thalamus to primary sensory areas of the
cerebral cortex
• In these areas the perceptual process begins
in earnest
In Touch, the Brain Integrates
Sensory Information from
Different Regions of the Body
• Touch information from the thalamus is projected to
the primary somatosensory cortex
• In the 1940s, Wilder Penfield discovered that
electrical stimulation of the primary somatosensory
cortex could evoke the sensation of touch in different
regions of the body (Penfield & Jasper, 1954)
• The most sensitive regions of the body, such as lips
and fingers, have a greater amount of cortex devoted
to them
Gate Control Theory
• Pain is a complex experience that depends on
biological, psychological, and cultural factors
• Melzack’s gate control theory of pain: Pain
receptors must be activated and a neural
“gate” in the spinal cord must allow the
signals through to the brain
• Pain signals transmitted by small-diameter
nerve fibers can be blocked at the level of the
spinal cord by the firing of larger sensory
nerve fibers
Controlling Pain
• Drug treatments (ibuprofen, acetaminophen,
Novocain, anesthetics)
• Cognitive states (distraction, positive mood,
relaxation) can close the pain gate
• Some mental processes, such as worrying about or
focusing on the painful stimulus, seem to open pain
gates
– Well-rested research participants rated the same level of a
painful stimulus as less painful than did participants who were
fearful, anxious, or depressed (Loggia, Mogil, & Bushnell, 2008;
Sullivan et al., 2001)
In Hearing, the Brain Integrates
Sensory Information from the Ears
• Auditory neurons in the thalamus extend their
axons to the primary auditory cortex
• Neurons in the primary auditory cortex code
the frequency (or pitch) of auditory stimuli
• To locate the origin of a sound (auditory
localization), the brain integrates the different
sensory information coming from each of our
two ears
In Vision, the Brain Processes Sensory
Information from the Eyes
• The study of perception has focused to a large
extent on the primary visual cortex and the
multiple areas in which the retinal image is
processed
• According to some estimates, up to half of the
cerebral cortex may participate in visual
perception
What Versus Where
• Visual areas beyond the primary visual cortex
form two parallel processing streams, or
pathways
• Ventral stream appears to be specialized for
the perception and recognition of objects
• Dorsal stream seems to be specialized for
spatial perception (determining where an
object is)
• These two processing streams are therefore
known as the “what” stream and the “where”
stream
Blindsight
• Blindsight: a condition in which people who
are blind have some visual capacities in the
absence of any visual awareness
• Example: A person might not be able to see
anything on his or her left. However, when a
stimulus is presented in this blind field, the
patient can respond unconsciously to that
stimulus
4.4 What Factors Influence
Visual Perception?
• Describe the Gestalt principles of perceptual
organization.
• Identify the brain regions associated with
facial perception.
• Identify cues for depth perception.
• Explain how the visual system perceives
motion.
• Discuss how perceptual constancy is achieved.
Object Perception Requires
Construction
• Perceptual psychologists believe that illusions
reveal the mechanisms that help our visual
systems determine the sizes and distances of
objects in the visual environment
• Researchers rely on these tricks to reveal
automatic perceptual systems that, in most
circumstances, result in accurate perception
Gestalt Principles of
Perceptual Organization
• The German word Gestalt means “shape” or
“form.” As used in psychology, Gestalt means
“organized whole.”
• Gestalt psychology postulated a series of laws
to explain how our brains group the perceived
features of a visual scene into organized
wholes
Figure and Ground
• Among the most basic organizing principles is
distinguishing between figure and ground
• A classic illustration of this is the reversible
figure illusion, in which figure and ground
switch back and forth (ambiguous)
• In identifying what is “figure,” the brain
assigns the rest of the scene to the
background
Proximity and Similarity
• Two of the most important Gestalt principles
concern proximity and similarity
• Principle of proximity: The closer two figures
are to each other, the more likely we are to
group them and see them as part of the same
object
• Principle of similarity: We tend to group
figures according to how closely they
resemble each other
The “Best” Forms
• Good continuation: the tendency to interpret
intersecting lines as continuous rather than as
changing direction radically
• Closure: the tendency to complete figures that
have gaps
• Illusory contours: We sometimes perceive
contours and cues to depth even though they
do not exist
Bottom-Up and Top-Down
Information Processing
• How do we assemble the information about parts
into a perception of a whole object?
– Bottom-up processing: Data are relayed in the brain
from lower to higher levels of processing
– Top-down processing: Information at higher levels
of mental processing can influence lower, “earlier”
levels in the processing hierarchy
• The flight crew of New Zealand Flight 901 failed to
notice the 12,000-foot volcano looming in front of
them because the pilots saw what they expected to
see
Face Perception
• The visual system is sensitive to faces:
– We can more readily discern information about a person’s
mood, attentiveness, sex, race, and age by looking at a
person’s face than by listening to them talk, watching them
walk, or studying their clothing (Bruce & Young, 1986)
– Whites are much better at recognizing white faces than at
recognizing black faces (Brigham & Malpass, 1985)
• Prosopagnosia: deficits in the ability to recognize faces
• Cortical regions, even specific neurons, seem to be
specialized to perceive faces and are sensitive to facial
expression and gaze direction
Depth Perception is Important
for Locating Objects
• How are we able to construct a threedimensional mental representation of the visual
world from two-dimensional retinal input?
• Binocular depth cues: available from both eyes
together and contribute to bottom-up
processing
• Monocular depth cues: available from each eye
alone and provide organizational information
for top-down processing
Binocular Depth Perception
• Binocular disparity (or retinal disparity): This cue is
caused by the distance between humans’ two eyes
– The brain uses the disparity between these two
retinal images to compute distances to nearby
objects
– Stereoscopic vision: the ability to determine an
object’s depth based on that object’s projections to
each eye
• Convergence: When eye muscles turn the eyes inward,
the brain knows how much the eyes are converging
and uses this information to perceive distance
Monocular Depth Perception
• We can perceive depth with one eye because
of monocular depth cues
• Pictorial depth cues:
– Occlusion
– Relative size
– Familiar size
– Linear perspective
– Texture gradient
– Position relative to horizon
Motion Cues for Depth Perception
• Motion parallax: The brain uses cues from the
relative movements of objects that are at
various distances from the observer
– When you watch the scenery from a moving car,
near objects such as mailboxes seem to pass
quickly, far objects more slowly, whereas objects
farther away appear to match your speed
– Objects at an intermediate distance (a house) move
opposite the direction of closer ones (a mailbox),
whereas distant objects (a mountain) move in the
same direction relative to the intermediate-distance
object
Size Perception Depends
on Distance Perception
• The size of an object’s retinal image depends
on that object’s distance from the observer
• To determine an object’s size the visual system
needs to know how far away it is
• Depth cues can fool us into seeing depth when
it is not there; a lack of depth cues can fool us
into not seeing depth when it is there
Ames Boxes
• Ames boxes: first crafted in the 1940s by
Adelbert Ames, a painter turned scientist
• Ames boxes’ rooms play with linear
perspective and other distance cues to create
size illusions
The Ponzo Illusion
• Classic example of a size/distance illusion
• Explained: Monocular depth cues make the
two-dimensional figure seem threedimensional (Rock, 1984)
• This illusion shows how much we rely on
depth perception to gauge size; the brain uses
depth cues even when depth is absent
Motion Perception
Has Internal and External Cues
• How does the brain know what is moving?
• After receiving damage to secondary visual
areas of her brain — areas critical for motion
perception—M.P., a German woman, saw the
world as a series of snapshots rather than as a
moving image (Zihl, von Cramon, & Mai, 1983)
• Neurons specialized for detecting movement
fire when movement occurs
Motion Aftereffects
• Waterfall effect: If you stare at a waterfall and then
turn away, the scenery you are now looking at will
seem to move upward
• Explained:
– The visual cortex has neurons that respond to
movement in a given direction
– When you stare at a moving stimulus long enough,
these direction-specific neurons adapt to the motion
and become fatigued
– When the stimulus is removed, other motion
detectors that respond to all other directions are
more active than the fatigued motion detectors
Compensation for
Head and Eye Movement
• When you see what appears to be a moving object, how
do you know whether the object is moving, you are
moving, or your eyes are moving?
• Explained:
– The brain calculates an object’s perceived movements by
monitoring the movement of the eyes, and perhaps also of the
head, as they track a moving object
– Motion detectors track an image’s motion across the retina
Stroboscopic Motion Perception
• Stroboscopic movement: a perceptual illusion that
occurs when two or more slightly different images
are presented in rapid succession
• Max Wertheimer conducted experiments in 1912
by flashing, at different intervals, two vertical lines
placed close together
– When the interval was about 60 milliseconds, the line
appeared to jump from one place to another
– At slightly longer intervals, the line appeared to move
continuously — a phenomenon called phi movement
Perceptual Constancies
Are Based on Ratio Relationships
• How does the brain know that a person is 6 feet tall
when the retinal image of that person changes size?
• Perceptual constancy: The brain correctly perceives
objects as constant despite sensory data that could lead
it to think otherwise
Size
Color
Shape
Lightness
• The brain computes a ratio based on relative magnitude
rather than on sensations’ absolute magnitude; perceptual
systems are tuned to detect changes from baseline
conditions, not just to respond to sensory inputs