Sensation and Perception

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Sensory and perceptual psychology:
How do our senses work?
How do we perceive the world?
Sensation: the process by which a stimulated receptor (e.g. the eyes or the ears) translates the signal into a neural message which is sent to the brain.

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Sensation and Perception
Sensory and perceptual psychology:

How do our senses work?

How do we perceive the world?
Sensation: the process by which a stimulated receptor
(e.g. the eyes or the ears) translates the signal into a
neural message which is sent to the brain.
Perception: the mental process that elaborates and
assigns meaning to the incoming sensory patterns.
Perception creates an
interpretation of sensation

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Sensation and Perception

Our minds lack direct access to the world(!)

The information we get about the world is always
indirect, filtered through our memories, associations,
emotions, motives, expectations…

The only world we will ever know is the inner reality
of our sensations and perceptions (e.g. phosphenes).

3 attributes common to all the senses:
Sensory adaptation

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Transduction: Changing Stimulation to Sensation

Transduction – the sensory process that converts
physical energy (e.g. light or sound waves) into the
form of neural messages.

The neural impulse carries a code of the sensory
event in a form that can be further processed by the
brain (like a bar code that carries info about the

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Sensory Adaptation

Sensation is critically influenced by

Sensory adaptation – the diminishing
responsiveness of sensory systems to
prolonged stimulation (e.g. cool water,
background music).

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Absolute threshold – the minimum amount of
physical energy that is needed to produce a sensory
experience (the presence or absence of a stimulus is
detected correctly half the time over many trials);

Difference threshold / just noticeable
difference (JND) – the smallest physical
difference between 2 stimuli that can be recognized;

Thresholds may differ in different individuals and
may vary for one person (depending on our mental
alertness and physical condition).

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Weber’s law
The JND is proportional to the intensity of the
stimulus (the JND is always large when the stimulus
intensity is high and always small when the stimulus
intensity is low).

Signal Detection Theory (Green & Swets, 1966)
Sensation depends on the characteristics of the
stimulus, the background stimulation, and the

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Subliminal persuasion

Subliminal = just below the absolute threshold

“Subliminal” advertising – James Vicary (1990s)

“Subliminal” CDs that are sold as remedies for
obesity, shoplifting, smoking, or low self-esteem – do
they work?

Subliminal stimuli can be strong enough to enter our
sensory system (without causing a conscious
awareness of the stimulus), but they have not been
proved to influence our thoughts and behavior (if
they do, it is only by means of expectation bias).

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Check your understanding
The sensory pathways carry information
from the brain to the muscles.
from the sense organs to the brain
from the brain to the sense organs
from the CNS to the autonomic nervous system
Which one refers to the least amount of stimulation
that your perceptual system can detect about half
the time?
the stimulus threshold
the difference threshold
the absolute threshold
the action threshold

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Check your understanding
Which one would involve sensory adaptation?
The water in the swimming pool seems warmer after you’ve
been in it for a while than it did when you first jumped in.
The flavor of a spicy salsa on your taco seems hot by
comparison with the blandness of the sour cream.
You are unaware of a priming stimulus flashed on the screen
at 1/100 of a second.
You prefer the feel of silk to the feel of velvet.

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Check your understanding
When you hear the sound of a tree falling in the
forest, the brain has received nothing but
sound waves from the air.
neural activity in the sensory pathways.
the vibration of the eardrums.
sound waves traveling through the sensory pathways.

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The most complex, the best developed, and the most
important sense for humans;

Helps us detect desired targets, threats, and changes
in our physical environment and to adapt our
behavior accordingly;

The eye has the ability to extract the information
from light waves and transduce them into neural
signals that the brain can process.

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The Eye

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The Eye

The transduction takes place in the retina – the
light-sensitive layer of cells (photoreceptors) at
the back of the eye;

“See in the dark” rods: detect low intensities of light
at night and sense motion;

Cones (concentrated in the very center of the
retina): make the fine distinctions necessary for color

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The Eye

Optic nerve – transports visual information from
the eye to the brain (not light but only patterns of
nerve impulses!)

The cones are sensitive to electromagnetic waves of
certain lengths (which is why we cannot see radio
waves, Xrays, or microwaves).

Trichromatic theory – the idea that colors are
sensed by three different types of cones sensitive to
light in the red, blue, and green wavelengths → then
colors are processed in complementary pairs (red &
green, yellow & blue) – opponent-process theory.

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Electromagnetic Waves

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Color Blindness

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Color Blindness

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The Ear

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Provides us with the ability to locate objects in space;
especially important in orienting us towards distant
events (which we cannot see);

Sounds are carried through air (so for e.g. in space there
are no sounds!)

Two physical properties of any sound wave that
determine how it will be sensed: frequency (the number
of vibrations or cycles the wave completes in a given
amount of time: Hz) and amplitude (the physical
strength of the sound wave: bells or decibels – units of
sound pressure or energy).

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Frequency → pitch (high/low; Hz); as with light,
our sensitivity to sound spans only a limited range of
the sound waves that occur in nature;

Amplitude → intensity (quiet/loud; dB);

Mixture of tones → timbre (e.g. distinguishing the
voice of a friend from that of a stranger).
Remember: Sound is not a physical phenomenon
but a purely psychological sensation!

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Intensity of common sounds

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Human Senses
Sense Organ
Light waves
Rods and cones
of retina
motion, textures
Sound waves
Hair cells of the
Pitch, loudness,
Skin senses
Nerve endings
in the skin
Touch, warmth,
Hair cells of
Odors (musky,
flowery, burnt,
minty, etc.)

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Human Senses
Soluble substances
Taste buds of
the tongue
Flavors (sweet,
sour, salty,
Many intense or
extreme stimuli:
mechanical stimuli
Net of pain
fibers all over
the body
Hair cells of
the basilar
Acute pain,
chronic pain
Body position,
movement, balance
Hair cells in
connected to
joints, and
Position of
body parts in

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Human Senses

Each sense gives us information about a different
aspect of our internal or external environment;

Each sense operates on similar principles – each
transduces physical stimuli into neural activity;

Each sense is more sensitive to change than to
constant stimulation;

Each sense has a specialized region in the brain
devoted to it.

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Position and Movement

Vestibular sense: the body position sense that
orients us with respect to gravity (straight, leaning,
reclining, upside down); how we are moving or how
our motion is changing;

Disorders of the vestibular sense → extreme
dizziness and disorientation.

Kinesthetic sense: keeps track of body parts
relative to each other (crossing your legs, clapping
your hands); usually automatic and effortless .

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Unlike the other senses, smell signals are not relayed
through the thalamus → this means that smell evolved
earlier than all the other senses;

Smell has an intimate connection with memory;

Smell still remains a
major factor in survival
because it helps us detect
and avoid potential
sources of danger
(e.g. decaying food).

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Taste cooperates with smell (e.g. if you cannot smell
food, it seems tasteless);

Individuals vary in their sensitivity to taste
sensations (depends on the density of taste buds on
the tongue);

Infants have a heightened sensitivity to taste which
diminishes with age;

Taste receptors can be easily damaged by alcohol,
smoke, acids, or hot foods (but can also be renewed;
total loss of taste is extremely rare ).

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Central role of touch in human

The skin’s sensitivity to stimulation varies
tremendously throughout the body (e.g.
fingertips: 10 x more accurate than the back;
the most sensitive parts: face, tongue, hands
→ effective eating, speaking, and grasping).

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The Skin Senses

Pain: an essential defense signal (warns us of
potential harm, helps us to survive in hostile
environments and to get treatment);

Melzack & Wall: the gate-control theory;

The effectiveness of placebo drugs: the expectation
of pain relief is enough to cause the brain to release
painkilling endorphins;

The threshold of pain varies enormously from person
to person.

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Bringing meaning to sensation, producing an
interpretation of the world (by means of
associations, memories, emotions, motives…);

Feature detectors: specialized groups of cells in
our brains that are
dedicated to the detection
of specific features,
e.g. those of the human

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Bottom-Up and Top-Down Processing
Receptors send
info to the cortex →
basic analysis
(Is it moving?
What color is it?
Is it loud, sweet,
painful, wet, hot…?)

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Bottom-Up and Top-Down Processing
The cerebral cortex invokes
the perceiver’s goals, past experience,
motivations, knowlegde,
expectations, memory
(Will it satisfy my hunger?
Is she liberal or conservative?
Will that help me get my degree?)

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Perceptual Constancies
The ability to see an object as being the same shape
from different angles or distances → helps us to
identify and track objects in a changing world.

Color constancy (in different shades of light)

Size constancy (at different distances; depth

Shape constancy (from different angles)

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Shape Constancy

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Perceptual Ambiguity and Distortion

Illusions: when our mind deceives us by
interpreting a stimulus pattern in a manner that is
demonstrably incorrect;

Illusions are another piece of evidence proving that
there is a discrepancy between our percepts and
external reality.

Illusions operate at the most basic sensory levels and
cannot be overcome by conscious knowledge (e.g. the
black and white Hermann Grid).

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The Hermann Grid

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The Hermann Grid

As you stare at the center of the grid, dark, fuzzy
spots appear at the intersections of the white bars.
But when you focus on an intersection, the spot

The firing of cells which are sensitive to light-dark
boundaries inhibits the activity of adjacent cells that
would otherwise detect the white grid lines.

Even though you know there are no gray spots, this
knowledge cannot overcome the illusion, which
operates at a more basic, sensory level.

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Mr Angry and Mr Calm
Which face is angry, and which is calm? When you
look from a distance, does anything change?

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Perceptual Ambiguity and Distortion
Ambiguous figures:
Once you have seen both conflicting
meanings, your perception will shift back
and forth between them (the shifting of
perceptual control between the right and
left hemispheres of the brain).

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Vase or Faces?

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The Necker Cube: Above or Below?

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Old Woman or Young Lady?

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The Müller-Lyer figures
Which of the two horizontal lines is longer?

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The Müller-Lyer figures

One possible explanation: we unconsciously
interpret the figures as three-dimensional objects
and see the arrowheads as the inside and outside
corners of a building or a room, which project toward
or away from us → we judge the outside corner to be
closer (and shorter) and the inside corner to be
farther (and longer).

People in the Zulu culture who live among curves
rather than lines and square corners see the lines as
equal in length.

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Ebbinghaus Illusion
Which central circle is bigger?

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Perceptual Organization: the Gestalt Theory

Our brains are innately programmed to perceive not
just stimuli but patterns in stimulation (Gestalt) –
not just parts but wholes (which are more than the
sums of their parts).

Figure and ground: our attention is drawn by the
figure; everything else becomes the ground (in
music, in food, in visual images).

Closure: we see incomplete figures as wholes by
supplying the missing segments.

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The Gestalt Theory: Closure

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The Gestalt Theory: Perceptual Grouping

Law of similarity: we group together those things
that have a similar look (or sound, or feel, etc.).

Law of proximity: we group together those things
that are near each other.

Law of continuity: we prefer smoothly connected
and continuous figures to disjointed ones.

Law of Prägnanz (meaningfulness): we tend to
perceive the simplest pattern possible – the percept
requiring the least effort.

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The Gestalt Theory: Perceptual Grouping

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The Nature of Depth Perception
Eleanor Gibson and Richard Walk, the visual cliff:
apprehension about depth appears only when the
infant learns to crawl.

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Context and Expectations
Once we identify the context, we form expectations
about what persons, objects, and events we are likely
to experience (Biederman, 1989).
Context may fool us into misperceiving some stimuli,
but it is extremely useful in identifying ambiguous
stimuli (in a dimly lit room or in the case of hard-to-
read handwriting).

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Perceptual Illusion?

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Perceptual Illusion?

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After more than a century of study, the
details of sensation are pretty well
known, but perception still has its
uncertain spots. One thing is certain:
perception is not a duplicate of reality.

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