Unit 4
Sensation and
Perception
© 2013 Worth Publishers
Psychophysics
Psychophysics is the scientific study of the
relationship between stimuli and the
sensations and perceptions evoked by
these stimuli
Copyright © Allyn and Bacon 2006
Sensation vs. Perception
Sensation
Perception
“The process by which
our sensory receptors
and nervous system
receive and represent
stimulus energies from
our environment.”
“The process of organizing
and interpreting sensory
information, enabling us
to recognize meaningful
objects and events.”
The brain
receives input
from the
sensory organs.
The brain makes
sense out of the
input from
sensory organs.
Transduction
The process that converts physical
energy, (such as light or sound
waves,) into neural messages…the
only language the brain
understands
Receptors
Specialized neurons that are activated
by stimulation and transduce (convert) it
into a nerve impulse
Copyright © Allyn and Bacon 2006
●
Sensory adaptation occurs due
to….
The loss of
responsiveness in
receptor cells after
stimulation has
remained
unchanged for a
You feel the socks on your feet
while
when you put them on…but
not for long…
Signal Detection Theory
Predicts how and when we detect the
presence of a faint stimulus (signal) amid
background stimulation (noise).
Assumes that there is no single absolute threshold.
We detect stuff based on our
experiences, expectations, motivations &
fatigue level.
Absolute Threshold
The smallest amount of stimulation necessary
for a stimulus to be detected
Normal Human Absolute Thresholds
Vision: a candle flame on a clear, dark night = 30 miles
Hearing: A ticking watch in a quiet room = 20 feet
Taste: Tablespoons of sugar dissolved in two gallons of water = One
Smell: Perfume diffused in a small house = one drop (1 part in 500 mil)
Touch: Pressure of the wing of a fly falling on a cheek from .4 inch
Difference Threshold / Just
Noticeable Difference
The smallest difference between two stimuli
that can be detected as a difference
• For Example:
• Turning down stereo volume…you may hear a
differences, your sister with a different
threshold may not
• If you can hear the difference, it exceeds your
threshold…if she can’t, it hasn’t exceeded hers
Weber’s Law
The JND is always
large when the
stimulus intensity
is high, and small
when the stimulus
intensity is low
• For Example: If the TV volume is high, you
have to turn it down a lot to make the
difference noticeable
Weber’s Law In Practice
If you make $5 an hour a 25 cent
hour raise will be noticeable.
But at $10 an hour you may need 50
cents to really realize a difference.
In car sales, after the sale customer
won’t really notice $500 stereo.
After drinking tea with lemon, a
grapefruit will not taste as sour… but
after a roll, it will taste especially
sour.
After holding salty water in mouth, it
will taste less salty, and drinking
fresh water afterwards, it will taste
sweet.
Anatomy of the Eye
Copyright © Allyn and Bacon 2006
Photoreceptors
• Light sensitive cells (neurons), rods and cones, that absorb
Cones
Rods
light energy and respond by
creating neural impulses
RODS
Number
Cells that detect black, white, and
gray;
Necessary
for peripheral
Location
in RetinaVision
Twilight vision
• CONES
120 million
• Retinal cells that detect
colors
6 million
Center
Periphery
• Fine detail
Sensitivity to dim
light
Low
High
Color sensitivity
High
Low
Detail sensitivity
High
Low
• Function in daylight and
well-lit conditions
Parallel Processing
The processing of several
aspects of a problem
simultaneously.
Properties of Light
• Amplitude: the height of the
peaks in a light wave
• determines brightness (larger
amplitude = brighter colors)
• Frequency: the rate at which light
waves move past a given point
• higher frequency = shorter
wavelength
• determines hue (color)(short
wavelength = blue, long = red)
Young-Helmholtz Trichromatic (3 color) Theory
• Theory that says we have three receptors
for
• color: red, green blue
Colors are sensed
by three different
types of cones
sensitive to light in
the red, blue, and
green wavelengths
Explains the earliest stage of color sensation
Opponent-Process Theory
• The red/green cells increase their firing when red is
present and decrease it when green is present.
• The yellow/blue cells have an increased response to
yellow and a decreased response to blue.
• Another type of cell increases its response rate for
white light and decreases in the absence of light.
Copyright © Allyn and Bacon 2006
How Sound Waves Become Auditory
Sensations
Copyright © Allyn and Bacon 2006
Sounds have only three sensory qualities:
Pitch – Loudness Pitch
Sensory characteristic of sound
produced by the frequency of the
sound wave
Loudness
High Frequency
Sensory characteristic of sound
produced by the amplitude
(intensity) of the sound wave…More
intense sound waves produce louder
sounds
High Amplitude
3. The cochlea vibrates and jostles fluid in the its tube.
16,000 Hair Cells!!
Like wheat in a wheat field bending to varying
levels of wind.
Most Hearing Damage is a result of damage to
these hair cells
Think of Carpet?
Loud noises v Damage to Hair Cells
The jostling causes
bends
hair cellsto that
line the cochlea’s
Ringing
is Equivalent
Bleeding!
surface
4.
• This triggers impulses in the adjacent auditory nerve (the nerve that sends
messages to auditory cortex of temporal lobe)
Place Theory •
Different hairs vibrate in the
cochlea when they different
pitches.
• So some hairs vibrate when they
hear high and other vibrate when
they hear low pitches.
Frequency Theory
• All the hairs vibrate
but at different
speeds.
Position and Movement
Vestibular Sense
Sense of body
orientation
withtheir vestibular senses
Children
develop
respect to by
gravity
swinging, spinning, twirling, and
rolling around on the ground.
Tells how our bodies, especially the head, are postured.
Children
gravitate
toward these
(Straight, leaning,
upside
down,…)
movements
Also, how we
are movingnaturally.
and how our motion is
changing Vestibular movement is in fact considered
critical to
brain
Closely associated
with
thedevelopment.
inner ear and is carried to
the brain on a branch of the auditory nerve
Copyright © Allyn and Bacon 2006
Kinesthetic Sense
• Sense of body position and movement of
body parts relative to each other
Proprioception is another term that is
often used interchangeably with
Kinesthesia
Figure 4.23
FIGURE 4.23 The skin senses include touch, pressure, pain, cold, and warmth. This drawing
shows different forms the skin receptors can take. The only clearly specialized receptor is the
Pacinian corpuscle, which is highly sensitive to pressure. Free nerve endings are receptors for
pain and any of the other sensations. For reasons that are not clear, cold is sensed near the
surface of the skin, and warmth is sensed deeper (Carlson, 1998).
Figure 4.21
FIGURE 4.21 (a) Olfactory nerve fibers respond to gaseous molecules. Receptor cells are shown
in cross section to the left. (b) Olfactory receptors are located in the upper nasal cavity. (c) On
the right, an extreme close-up of an olfactory receptor shows fibers that sense gaseous
molecules of various shapes.
Figure 4.22
FIGURE 4.22 Receptors for taste: (a) The tongue is covered with small protrusions called papillae. (b) Most taste
buds are found around the top edges of the tongue (shaded area). However, some are located elsewhere,
including under the tongue. Stimulation of the central part of the tongue causes no taste sensations. All four
primary taste sensations occur anywhere that taste buds exist. (c) An enlarged drawing shows that taste buds
are located near the base of papillae. (d) Detail of a taste bud. These receptors also occur in other parts of the
digestive system, such as the lining of the mouth. (Fig. 4.22). As food is chewed, it dissolves and enters the taste
buds, where it sets off nerve impulses.Much like smell, sweet and bitter tastes appear to be based on a lockandkey match between molecules and intricately shaped receptors. Saltiness and sourness, however, are
triggered by a direct flow of charged atoms into the tips of taste cells (Lindemann, 2001).
Taste
Taste buds –
Receptors for taste
(primarily on the
upper side of the
tongue)
Copyright © Allyn and Bacon 2006
What is the Relationship
Between Perception
and Sensation?
Perception brings meaning
to sensation, so perception
produces an interpretation of
the external world, not a
perfect representation of it
Copyright © Allyn and Bacon 2006
The Gestalt Laws of Perceptual
Grouping
• Max Wertheimer (1923)
These laws suggest how our
brains prefer to group
stimulus elements together to
form a percept
Perceptual Set
Readiness to detect a particular stimulus in a
given context.
We tend to perceive what we expect to
perceive.
Copyright © Allyn and Bacon 2006
Making sense of the world
What am I
seeing?
Bottom-up
processing:
taking sensory
information and
then assembling
and integrating it
Top-down
processing:
using models,
ideas, and
expectations to
interpret sensory
information
Is that
something I’ve
seen before?
• Gibson and Walk
found that all
species tested can
perceive and avoid a
sharp drop by the
time they take up
independent
locomotion (About
6 months in
humans)
Binocular cues
Information processed
into both eyes.
Both of your eyes help
strengthen your
perception in depth.
Monocular cues
Depth
information
perceived by
the use of
only one eye.
Although both types of cue are presented separately in this resource, in
reality they all work together
Retinal disparity
Human eyes are spaced apart,
separated by the nose. The average
distance of this spacing is 7cms.
This means that the brain receives two
slightly different pictures of the same
scene
The Gestalt Laws of Perceptual Grouping
Similarity
Proximity
Continuity
Common fate
Prägnanz
• Prägnanz
• We tend to order our experience in a
manner that is regular, orderly,
symmetric, and simple
• This is what makes things such as
proofreading a text so difficult
Reality is reduced to its simplest form
Law of Pragnanz and the ‘face’ on
Mars
• People mentally
process visual
information that…
• is symmetrical
• contains the
simplest shapes
possible
• contains the
fewest number of
shapes
Monocular Cues
Information about depth
that relies on the input
of one eye
 Relative Size
The smaller image of two
objects of the same size
appears more distant
 Interposition
Closer object blocks
distant object
 Relative Clarity
Hazy object seen as more
distant
 Texture
coarse = close
fine = distant
Relative Height
• Higher objects are farther
away
Relative Motion
As we move, objects at different
distances change their relative
positions in our visual image,
with those closest moving
most
Linear Perspective
The converging of parallel lines
indicates greater distance, as
in looking down a road
Light and Shadow
Dimmer objects seem more
distant.
Relative size
This photo is an example of
relative size. Even though the
beer bottle in the front of the
picture is the same size as the one
in the back of the picture, the one
in front looks much bigger
because it’s colser
Relative Height
In this scene the furthest objects, the
trees, are also the highest. We know
the boats are nearer to us than the
trees because they are lower down
The same effect works in real life as well as
paintings
The cloth of the couch is much
more detailed the closer it is to the
camera. The further away, the
harder it is to make out the
texture’s pattern
and this
Interposition or overlap
We assume the tree is in front
of the house because it
obscures our view of it. It is
superimposed on the front of
the house, it overlaps it.
Likewise we know the
tutor is nearer to us than
the blackboard because
she obscures (overlaps)
a portion of it
Relative Motion
When an observer passes through a scene it moves through the
observers field of vision at different speeds
Perceptual Constancy
Ability to recognize the same object under
different conditions, such as changes in light,
distance, or location
A dog is running at you – you
don’t perceive it as growing
larger
Copyright © Allyn and Bacon 2006
• Shape Consistency:
Explains why we do not see
people morphing in shape as
they walk past us and we see
them from a different
perspective.
• Color Consistency:
Explains why a shirt will look
the same shade of blue in dim
light or sunlight Copyright © Allyn and Bacon 2006