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Chapter 4
Sensation and Perception
Sensation and Perception: The Distinction
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Sensation : stimulation of sense organs
Perception: selection, organization, and
interpretation of sensory input
Psychophysics = the study of how physical
stimuli are translated into psychological
experience
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Fig 4.1 - The distinction between sensation and perception. Sensation involves the
stimulation of sensory organs, whereas perception involves the processing and interpretation of
sensory input. The two processes merge at the point where sensory receptors convert physical
energy into neural impulses.
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Psychophysics: Basic Concepts
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Sensation begins with a detectable stimulus
Fechner: the concept of the threshold
– Absolute threshold: detected 50% of the time.
– Just noticeable difference (JND): smallest difference
detectable
• Weber’s law: size of JND proportional to size of initial
stimulus
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Fig 4.2 - The absolute threshold. If absolute thresholds were truly absolute, then
at threshold intensity the probability of detecting a stimulus would jump from 0 to
100%, as graphed here in blue. In reality, the chances of detecting a stimulus
increase gradually with stimulus intensity, as shown in red. Accordingly, an
“absolute” threshold is defined as the intensity level at which the
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probability of detection is 50%.
Psychophysics: Concepts and Issues
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Psychophysical Scaling: Fechner’s Law
Signal-Detection Theory: Sensory processes +
decision processes
Subliminal Perception: Existence vs. practical
effects
Sensory Adaptation: Decline in sensitivity
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Fig 4.3 - Possible outcomes in signal-detection theory. Four outcomes are possible in
attempting to detect the presence of weak signals. The criterion you set for how confident you
want to feel before reporting a signal will affect your responding. For example, if you require
high confidence before reporting a signal, you will minimize false alarms, but you’ll be more
likely to miss some signals.
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Vision: The Stimulus
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Light = electromagnetic radiation
– Amplitude: perception of brightness
– Wavelength: perception of color
– purity: mix of wavelengths
• perception of saturation, or richness of colors.
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Fig 4.5 – Light, the physical stimulus for vision. (a) Light waves vary in amplitude and wavelength. (b)
Within the spectrum of visible light, amplitude (corresponding to physical intensity) affects mainly the
experience of brightness. Wavelength affects mainly the experience of color, and purity is the key determinant
of saturation. (c) If white light (such as sunlight) passes through a prism, the prism separates the light into its
component wavelengths, creating a rainbow of colors. However, visible light is only the
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narrow band of wavelengths to which human eyes happen to be sensitive.
Fig 4.6 - Saturation. Variations in saturation are difficult to describe, but
you can see examples for two colors here.
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The Eye: Converting Light into Neural
Impulses
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The eye: housing and channeling
Components:
– Cornea: where light enters the eye
– Lens: focuses the light rays on the retina
– Iris: colored ring of muscle, constricts or dilates via amount
of light
– Pupil: regulates amount of light
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Fig 4.7 - The human eye. Light passes through the cornea, pupil, and lens and falls on the
light-sensitive surface of the retina, where images of objects are reflected upside down. The
lens adjusts its curvature to focus the images falling on the retina. The pupil regulates the
amount of light passing into the rear chamber of the eye.
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The Retina: An Extension of the CNS
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Retina: absorbs light, processes images, and
sends information to the brain
 Optic disk: where the optic nerve leaves the eye/
blind spot
 Receptor cells:
– Rods: black and white/ low light vision
– Cones: color and daylight vision
• Adaptation: becoming more or less sensitive to light as
needed
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Information processing:
– Receptive fields
– Lateral antagonism
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Fig 4.9 – The retina. The closeup shows the several layers of cells in the retina. The cells
closest to the back of the eye (the rods and cones) are the receptor cells that actually detect
light. The intervening layers of cells receive signals from the rods and cones and form circuits
that begin the process of analyzing incoming information before it is sent to the brain. The
cells feed into many optic fibers, all of which head toward the “hole” in the retina where the
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optic nerve leaves the eye—the point known as the optic disk
(which corresponds to the blind spot).
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Fig 4.10 - The process of
dark adaptation. The declining
thresholds over time indicate
that your visual sensitivity is
improving, as less and less
light is required for you to be
able to see. Visual sensitivity
improves markedly during the
first 5 to 10 minutes after
entering a dark room, as the
eye’s bright-light receptors (the
cones) rapidly adapt to low light
levels. However, the cones’
adaptation, which is plotted in
purple, soon reaches its limit,
and further improvement
comes from the rods’ adaptation, which is plotted in red.
The rods adapt more slowly
than the cones, but they are
capable of far greater visual
sensitivity in low levels of light.
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The Retina and the Brain: Visual Information
Processing
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Light -> rods and cones -> neural signals ->
bipolar cells -> ganglion cells -> optic nerve ->
optic chiasm -> opposite half brain ->
Main pathway: lateral geniculate nucleus
(thalamus) -> primary visual cortex (occipital lobe)
– magnocellular: where
– parvocellular: what
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Second pathway: superior colliculus ->thalamus ->
primary visual cortex
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Fig 4.13 - Visual pathways through the brain. (a) Input from the right half of the visual field strikes the left side of each retina and is
transmitted to the left hemisphere (shown in red). Input from the left half of the visual field strikes the right side of each retina and is
transmitted to the right hemisphere (shown in green). The nerve fibers from each eye meet at the optic chiasm, where fibers from the
inside half of each retina cross over to the opposite side of the brain. After reaching the optic chiasm, the major visual pathway projects
through the lateral geniculate nucleus in the thalamus and onto the primary visual cortex (shown with solid lines). A second pathway
detours through the superior colliculus and then projects through the thalamus and onto the primary visual cortex (shown with dotted
lines). (b) This inset shows a vertical view of how the optic pathways project through the thalamus and onto the visual
cortex
the back of
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the brain (the two pathways mapped out in diagram (a) are virtually
indistinguishable from this angle).
Fig 4.15 – The what and where pathways from the primary visual cortex. Cortical processing of visual input
is begun in the primary visual cortex. From there, signals are shuttled through the secondary visual cortex and
onward to a variety of other areas in the cortex along a number of pathways. Two prominent pathways are
highlighted here. The magnocellular, or “where pathway,” which processes information about motion and depth,
moves on to areas of the parietal lobe. The parvocellular, or “what pathway,”
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which processes information about color, form, and texture, moves on to areas
of the temporal lobe.
Hubel and Wiesel: Feature Detectors and the
Nobel Prize
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Early 1960’s: Hubel and Wiesel
– Microelectrode recording of axons in primary visual cortex
of animals
– Discovered feature detectors: neurons that respond
selectively to lines, edges, etc.
– Groundbreaking research: Nobel Prize in 1981
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Later research: cells specific to faces in the temporal
lobes of monkeys and humans
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Basics of Color Vision
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Wavelength determines color
– Longer = red / shorter = violet
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Amplitude determines brightness
Purity determines saturation
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Fig 4.16 - The color solid. The color solid shows how color varies along three
perceptual dimensions: brightness (increasing from the bottom to the top of the
solid), hue (changing around the solid’s perimeter), and saturation (increasing
toward the periphery of the solid).
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Fig 4.17 - Subtractive color mixing. Paint pigments selectively reflect specific wavelengths
that give rise to particular colors, as you can see here for blue and yellow, which both also
reflect back a little green. When we mix blue and yellow paint, the mixture absorbs all the colors
that blue and yellow absorbed individually. The mixture is subtractive because more
wavelengths are removed than by each paint alone. The yellow paint in the mixture absorbs the
wavelengths associated with blue and the blue paint in the mixture absorbs the wavelengths
associated with yellow. The only wavelengths left to be reflected back are some of those
associated with green, so the mixture is seen as green.
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Theories of Color Vision
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Trichromatic theory - Young and Helmholtz
– Receptors for red, green, blue – color mixing
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Opponent Process theory – Hering
– 3 pairs of antagonistic colors
– red/green, blue/yellow, black/white
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Current perspective: both theories necessary
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Fig 4.19 – The color circle and complementary colors. Colors opposite each other on this
color circle are complements, or “opposites.” Additively mixing complementary colors produces
gray. Opponent process principles help explain this effect as well as the other peculiarities of
complementary colors noted in the text.
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Fig 4.21 – Reconciling theories of color vision. Contemporary texplanations of color
vision include aspects of both the trichromatic and opponent process theories. As predicted
by trichromatic theory, there are three types of receptors for color--cones sensitive to short,
medium, and long wavelengths. However, these cones are organized into receptive fields
that excite or inhibit the firing of higher-level visual cells in the retina, thalamus, and cortex.
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antagonistic ways to blue versus yellow, red versus green, and black
versus white.
Perceiving Forms, Patterns, and Objects
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Reversible figures
Perceptual sets
Inattentional blindness
Feature detection theory - bottom-up processing.
Form perception - top-down processing
Subjective contours
Gestalt psychologists: the whole is more than the
sum of its parts
– Reversible figures and perceptual sets demonstrate that the
same visual stimulus can result in very different perceptions
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Fig 4.24 – Feature analysis in form perception. One vigorously debated theory of form
perception is that the brain has cells that respond to specific aspects or features of stimuli,
such as lines and angles. Neurons functioning as higher-level analyzers then respond to
input from these “feature detectors.” The more input each analyzer receives, the more active
it becomes. Finally, other neurons weigh signals from these analyzers and make a “decision”
about the stimulus. In this way perception of a form is arrived at by assembling elements from
the bottom up.
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Fig 4.25 – Bottom-up versus top-down processing. As explained in these
diagrams, bottom-up processing progresses from individual elements to whole
elements, whereas top-down processing progresses from the whole to the
individual elements.
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Principles of Perception
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Gestalt principles of form perception:
– figure-ground, proximity, similarity, continuity, closure, and
simplicity
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Recent research:
– Distal (stimuli outside the body) vs. proximal (stimulus
energies impinging on sensory receptors) stimuli.
– Perceptual hypotheses
• Context
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Fig 4.27 – The
principle of figure and
ground. Whether you
see two faces or a vase
depends on which part
of this drawing you see
as figure and which as
background. Although
this reversible drawing
allows you to switch
back and forth between
two ways of organizing
your perception, you
can’t perceive the
drawing both ways at
once.
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Fig 4.28 – Gestalt principles
of perceptual organization.
Gestalt principles help explain
how people subjectively
organize perception. (a)
Proximity: These dots might
well be organized in vertical
columns rather than horizontal
rows, but because of proximity
(the dots are closer together
horizontally), they tend to be
perceived in rows. (b) Closure:
Even though the figures are
incomplete, you fill in the blanks
and see a circle and a dog. (c)
Similarity: Because of similarity
of color, you see dots organized
into the number 2 instead of a
random array. If you did not
group similar elements, you
wouldn’t see the number 2 here.
(d) Simplicity: You could view
this as a complicated 11-sided
figure, but given the preference
for simplicity, you are more
likely to see it as a rectangle
and a triangle. (e) Continuity:
You tend to group these dots in
a way that produces a smooth
path rather than an abrupt shift
in direction.
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Fig 4. 29 – Distal and proximal stimuli. Proximal stimuli are often distorted, shifting
representations of distal stimuli in the real world. If you look directly down at a small, square
piece of paper on a desk (a), the distal stimulus (the paper) and the proximal stimulus (the image
projected on your retina) will both be square. But as you move the paper away on the desktop,
as shown in (b) and (c), the square distal stimulus projects an increasingly trapezoidal image on
your retina, making the proximal stimulus more and more distorted. Nevertheless, you continue
to perceive a square.
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Fig 4.30 – A famous
reversible figure.
What do you see?
Consult the text to
learn what the two
possible
interpretations of this
figure are.
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Fig 4.31 - The Necker cube. The tinted surface of this reversible
figure can become either the front or the back of the cube.
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Fig 4.33 – Context effects. The context in which a stimulus is seen can affect your
perceptual hypotheses.
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Depth and Distance Perception
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Binocular cues – clues from both eyes together
– retinal disparity
– convergence
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Monocular cues – clues from a single eye
– motion parallax
– accommodation
– pictorial depth cues
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Stability in the Perceptual World: Perceptual
Constancies
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Perceptual constancies – stable perceptions amid
changing stimuli
–
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–
–
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Size
Shape
Brightness
Hue
Location in space
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Optical Illusions: The Power of Misleading
Cues
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Optical Illusions - discrepancy between visual
appearance and physical reality
Famous optical illusions: Muller-Lyer Illusion,
Ponzo Illusion, Poggendorf Illusion, Upside-Down
T Illusion, Zollner Illusion, the Ames Room, and
Impossible Figures
Cultural differences: Perceptual hypotheses at work
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Fig 4.38 - The MüllerLyer illusion. Go
ahead, measure them:
the two vertical lines
are of equal length.
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Fig 4.39 - Explaining the Müller-Lyer illusion. The figure on the left seems to be
closer, since it looks like an outside corner, thrust toward you, whereas the figure
on the right looks like an inside corner thrust away from you. Given retinal images
of the same length, you assume that the “closer” line is shorter.
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Fig 4.43 – Three classic impossible figures. The figures are impossible, yet they
clearly exist—on the page. What makes them impossible is that they appear to be
three-dimensional representations yet are drawn in a way that frustrates mental
attempts to “assemble” their features into possible objects. It’s difficult to see the
drawings simply as lines lying in a plane—even though this perceptual hypothesis
is the only one that resolves the contradiction.
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Hearing: The Auditory System
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Stimulus = sound waves (vibrations of molecules
traveling in air)
– Amplitude (loudness)
– Wavelength (pitch)
– Purity (timbre)
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Wavelength described in terms of frequency:
measured in cycles per second (Hz)
– Frequency increase = pitch increase
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Fig 4.45 – Sound, the physical stimulus for hearing. (a) Like light, sound
travels in waves— in this case, waves of air pressure. A smooth curve would
represent a pure tone, such as that produced by a tuning fork. Most sounds,
however, are complex. For example, the wave shown here is for middle C
played on a piano. The sound wave for the same note played on a violin would
have the same wavelength (or frequency) as this one, but the “wrinkles” in the
wave would be different, corresponding to the differences in timbre between
the two sounds. (b) The table shows the main relations between objective
aspects of sound and subjective perceptions.
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The Ear: Three Divisions
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External ear (pinna): collects sound.
Middle ear: the ossicles (hammer, anvil, stirrup)
Inner ear: the cochlea
– a fluid-filled, coiled tunnel
– contains the hair cells, the auditory receptors
– lined up on the basilar membrane
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Fig 4.47 – The human
ear. Converting sound
pressure to information
processed by the
nervous system involves
a complex relay of
stimuli: Waves of air
pressure create
vibrations in the
eardrum, which in turn
cause oscillations in the
tiny bones in the inner
ear (the hammer, anvil,
and stirrup). As they are
relayed from one bone
to the next, the
oscillations are
magnified and then
transformed into
pressure waves moving
through a liquid medium
in the cochlea. These
waves cause the basilar
membrane to oscillate,
stimulating the hair cells
that are the actual
auditory receptors (see
Figure 4.48).
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Fig 4.48 – The basilar
membrane. The figure
shows the cochlea
unwound and cut open
to reveal the basilar
membrane, which is
covered with thousands
of hair cells (the
auditory receptors).
Pressure waves in the
fluid filling the cochlea
cause oscillations to
travel in waves down
the basilar membrane,
stimulating the hair
cells to .re. Although
the entire membrane
vibrates, as predicted
by frequency theory,
the point along the
membrane where the
wave peaks depends
on the frequency of the
sound stimulus, as
suggested by place
theory.
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The Auditory Pathway
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Sound waves vibrate bones of the middle ear
 Stirrup hits against the oval window of cochlea
 Sets the fluid inside in motion
 Hair cells are stimulated with the movement of the
basilar membrane
 Physical stimulation converted into neural impulses
 Sent through the thalamus to the auditory cortex
(temporal lobes)
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Theories of Hearing: Place or Frequency?
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Hermann von Helmholtz (1863)
– Place theory
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Other researchers (Rutherford, 1886)
– Frequency theory
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Georg von Bekesy (1947)
– Traveling wave theory
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Auditory Localization: Where Did that Sound
Come From?
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Two cues critical:
Intensity (loudness)
Timing of sounds arriving at each ear
– Head as “shadow” or partial sound barrier
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Timing differences as small as 1/100,000 of a second
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Fig 4.49 – Cues in
auditory localization. A
sound coming from the left
reaches the left ear sooner
than the right. When the
sound reaches the right
ear, it is also less intense
because it has traveled a
greater distance and
because it is in the sound
shadow produced by the
listener’s head. These cues
are used to localize the
sources of sound in space.
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The Chemical Senses: Taste
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Taste (gustation)
Physical stimulus: soluble chemical substances
– Receptor cells found in taste buds
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Pathway: taste buds -> neural impulse ->
thalamus -> cortex
– Four primary tastes: sweet, sour, bitter, and salty
– Taste: learned and social processes
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Fig 4.50 – The tongue and taste. (a) Taste buds line the trenches around tiny bumps on the tongue
called papillae (see the inset). (b) There are three types of papillae, which are distributed on the
tongue as shown here. The taste buds found in each type of papillae show slightly different
sensitivities to the four basic tastes, as mapped out in the graph at the top. This, sensitivity to the
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primary tastes varies across the tongue, but these variations are small, and all
four primary tastes can de detected wherever there are taste receptors.
The Chemical Senses: Smell
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Smell (Olfaction)
Physical stimuli: substances carried in the air
– dissolved in fluid, the mucus in the nose
– Olfactory receptors = olfactory cilia
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Pathway: Olfactory cilia -> neural impulse ->
olfactory nerve -> olfactory bulb (brain)
– Does not go through thalamus
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Fig 4.52 – The
olfactory system.
Odor molecules
travel through the
nasal passages and
stimulate olfactory
cilia. An enlargement
of these hairlike
olfactory receptors is
shown in the inset.
The olfactory nerves
transmit neural
impulses through the
olfactory bulb to the
brain.
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Skin Senses: Touch
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Physical stimuli = mechanical, thermal, and chemical energy
impinging on the skin.
Pathway: Sensory receptors -> the spinal column ->
brainstem -> cross to opposite side of brain -> thalamus
-> somatosensory (parietal lobe)
Temperature: free nerve endings in the skin
Pain receptors: also free nerve endings
– Two pain pathways: fast vs. slow
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Fig 4.54 - The two
pathways for pain
signals. Pain signals
are sent from
receptors to the brain
along the two
pathways depicted
here. The fast
pathway, shown in
red, and the slow
pathway, shown in
black, depend on
different types of
nerve fibers and are
routed through
different parts of the
thalamus. The gate
control mechanism
posited by Melzack
and Wall (1965)
apparently depends
on descending signals
originating in an area
of the midbrain (the
pathway shown in
green).
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Other Senses: Kinesthetic and Vestibular
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Kinesthesis - knowing the position of the various
parts of the body
– Receptors in joints/muscles
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Vestibular - equilibrium/balance
– Semicircular canals
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