Transcript Sensation and Perception

Sensation and
perception overview
The senses (sensory modalities)
Vision
Audition (hearing)
Olfaction (smell)
Gustation (taste)
Touch (pressure, temperature [warmth and
cold], pain)
Proprioception
Vestibular sense (body’s position in space)
 Kinesthesia (movement and position of parts of the
body in relation to one another)

Sensation and Perception
Sensation and perception are two stages in the
process whereby we construct our internal
representation of the environment.
Our goal as biological organisms is to respond
to, interact with, and adapt to our environments.
Our goal is to extract meaning from the objects
that exist and events that occur around us.
Sensation: physical energy is converted into
neural signals
Perception: signals are selected, organized,
and interpreted
Sensation to attention to perception
Transduction: sensory receptors translate physical
stimulation (light, sound waves, heat, molecules) into
neural signals
2) Neural pathways transmit this information to the
brain
3) Rudimentary neural/mental codes, representing
features in the environment, are constructed in the
brain/mind
4) Features (properties and parts) are combined
(bound) to form objects (wholes); may require
attention
1)
In the case of object recognition, a feature is any property of an object, such
as it’s shape (as determined by it’s edges or boundaries), color, size, and
movement
a)
i.
b)
The letter A has three features: /, \, and –
In the case of categorization, some features of the bird category are feathers,
beaks, lays eggs, builds nests, etc.
Illusory conjunctions
Sensation to attention to perception cont.
5) Objects are matched with prior knowledge and
pattern categorization and recognition occurs
6) Some objects are selected for further
processing (another job for attention)
7) Names of objects are accessed
8) Meanings of objects are accessed
9) Objects are organized into scenes
10) The scene emerges into subjective awareness
(i. e., consciousness); total time = 100 ms
Sensation to attention to perception cont.
Low-level vs. high level processes
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Sensory processes are also known as low-level or peripheral processes
Perceptual processes are also known as high-level or central processes
As information processing progresses, the
representations/codes become more abstract and less
modality-specific.
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For example, if the goal of sensation and perception is to extract
meaning from our world, does it really matter whether you learned
something by reading it (a visual stimulus) or by hearing it (an auditory
stimulus)?
The information processing progression described in steps 1-10
is mostly a bottom-up processing view of sensation and
perception
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In bottom-up processing, the conscious percept is determined primarily
by the information received via sensory receptors
One can also identify top-down processing influences on
sensation and perception
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In top-down processing, memory and general knowledge take part in
determining the conscious percept
See next slide
Sensation to attention to perception cont.
To function effectively, our minds need both
quantitative information and qualitative information
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Quantitative information
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The intensity of a stimulus is coded in the brain by the frequency with
which neurons fire – the higher the intensity, the greater the
frequency of firing
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For example, if someone lightly touches your back, the frequency of
firing would be relatively low
Qualitative information
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The difference between two different stimuli is coded in the brain by
different patterns of neurons firing
 Coarse coding: a single neuron fires when your back is touched
anywhere in a relatively large patch of skin
 Then how do you know precisely where you’re being touched?
 Because each particular spot is defined in terms of a unique set of
several overlapping patches
 Thus, a touch on any particular spot is coded as a unique set (or
pattern) of neurons firing
The cycle of perception:
Sensation and perception are
active processes.
Psychophysics
Psychophysics
Psychophysics
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The study of the correspondence between physical reality
and psychological reality
There is no one-to-one correspondence between physical
reality and psychological reality.
3 examples
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There are physical energies that exist in our environment that
fall outside the normal range of human sensation, but that other
animals can detect (i.e., represent psychologically)
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Some bees can detect ultraviolet radiation
Some snakes can detect infrared radiation
Elephants can detect very low frequency sounds
Dogs can detect very high frequency sounds
Absolute thresholds
Difference thresholds
Psychophysics cont.
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Within the normal range of human sensation, sometimes the
intensity of a stimulus is so faint that we do not report that
we perceive it, even though the physical energy from that
stimulus (e.g., light waves bouncing off an object) is striking
our sensory apparatus (e.g., our retina).
Absolute Threshold
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The smallest amount of stimulation that can be accurately
detected 50% of the time is the absolute threshold.
“Limen” is the German word for theshold.
Subliminal means “below the limen”.
Subliminal perception: Does stimulation that falls below the
absolute threshold influence behavior?
Psychophysics cont.
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Within the normal range of human sensation, sometimes a change in the
intensity of a stimulus occurs, but we do not report that the stimulus has
changed.
Difference Thresholds
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The lowest level of stimulation required to sense a change in stimulation
Weber’s Law
 In order to perceive a change in stimulation, that change
must be greater as the magnitude of the stimulation
becomes greater
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The ratio of the change in stimulation (delta I) to the magnitude of the
stimulation (I) is a constant (k)
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delta I/I = k
For judging the weight of objects, k = 1/50
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Thus, if an object weighs 50 grams, an increase of 1 gram will be
noticeable.
However, if an object weighs 100 grams, the weight will need to
increase by 2 grams in order for you to report that you’ve
noticed a difference.
Psychophysics cont.
Signal-Detection Theory
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Reporting that a stimulus is
present is jointly determined
by the signal and the subject’s
response criterion.
Separates subject’s sensitivity
from subject’s propensity to
guess.
Hits – false alarms.
Yea sayer
Yes
No
On
89%
11%
Off
41%
59%
Nay sayer
Stimulus
Response
Yes
No
On
Hit
Miss
Off
False
alarm
Correct
rejection
On
Yes
65%
No
35%
Off
8%
92%
Sensory modalities
Taste (Gustation)
Photograph of tongue
surface (top), magnified 75
times
500 to 10,000 taste buds line
the tongue and mouth
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Taste receptors (~ 50) are
down inside each “bud”
Children have more taste
buds than adults
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Older adults have less intense
taste sensations than younger
adults
Taste preferences begin in the
womb
Supertasters have more
taste buds, but are often
more picky eaters
Smell (Olfaction)
Touch (Haptic sense)
Skin is the largest organ in the body (18 sq ft)
Pressure, temperature, and pain
Different receptors in the skin transduce different
types of physical information that, in the end, is routed
to the somatosensory cortex (by way of the thalamus)
Sensitivity to Touch
The Heat Grill
The brain interprets
simultaneous firing of
both receptor types in
interesting ways.
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When a person grasps
two braided water
pipes – one with cold
water running through
it and one with warm
water – the sensation
is “very hot” or “wet”.
Pain
Pain in the skin
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When cells are damaged, they release substance P, that
stimulates free nerve endings
Two types of pain sense neuronal axons
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Myelinated L-fibers transmit all types of information very
quickly to the spinal cord
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Adaptive because this type of pain causes us to recoil quickly
Unmyelinated S-fibers transmit pain information
relatively slowly to the spinal cord
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Adaptive because this type of pain forces us to allow injured tissue to
recuperate
Gate control theory
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L-fibers inhibit neural transmission in S-fibers
This is why rubbing the skin near an injury can lessen
the pain.
Hearing (Audition)
Sound
Sound waves are vibrations of molecules in the air
 Frequency (perceived as pitch)
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What is white noise?
Amplitude (determines loudness)
Measured in decibels (dB), which is a logarithmic
scale
 An increase of 10 dB corresponds to a change in
sound power of 10 times the comparison stimulus
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Complexity (determines timbre or tonal quality)
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Determines the difference between middle C played
by a trumpet vs. a flute
The Human Ear
Auditory transduction
When sound waves hit
the fluid of the inner ear,
the fluid causes the
basilar membrane to
move up and down,
activating electrical
potentials in the hair cells
(the receptor cells for
hearing).
Temporal coding
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The entire basilar
membrane vibrates slowly
for low frequencies and
more rapidly for high
frequencies
Place theory
Auditory Localization
Sounds from different
directions are not
identical as they arrive at
left and right ears
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Loudness
Timing
Phase
The brain calculates a
sound’s location by
using these differences
Visual sensation
The Electromagnetic Spectrum
Structures of the Human Eye
The Retina
Receptive
Fields I
-- a place or
spot in your
visual
environment for
which an
individual
neuron is
“responsible”
-- allows for the
detection of
edges
Receptive
Fields II
-- the cell fires
more when light
illuminates the
center and less
when the center is
dark
-- the cell fires
more when the
periphery is dark
and less when the
periphery is
illuminated
Hermann Grid
Color Vision
Trichromatic theory:
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T. Young (1802) & H. von Helmholtz (1852) both
proposed that the eye detects 3 primary colors: red,
blue, & green
There are three types of cones, each of which is most
maximally sensitive to a different wavelength
All other colors can be derived by combining these
three
Color Vision cont.
Opponent process
theory
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Color opponent
cells in the LGN are
activated by one
color and inhibited
by the other: blackwhite, blue-yellow,
red-green
Color “blindness”
Afterimages
Proprioceptive senses
Vestibular sense: position of the body in space
Vestibular sacs and semicircular canals are located
in the inner ear, above the cochlea
 Vestibular sacs sense gravity and the position of the
head in space
 Semicircular canals sense acceleration as the head
moves
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Kinesthesia: movement and position of muscles
and limbs relative to each other
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Receptors located in joints, tendons, and muscles.
Extrasensory Perception
Extrasensory Perception (ESP):
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The ability to perceive something without ordinary sensory
information
Three types of ESP:
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Telepathy – Mind-to-mind communication
Clairvoyance – Perception of remote events
Precognition – Ability to see future events
57% of Americans believe in ESP
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CBS News (2002)
But ESP has not been scientifically demonstrated
Parapsychology
J. B. Rhine conducted many experiments on ESP using stimuli such as these
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Guess the symbol on the card
In a 25-card deck, with 5 different cards, participants correctly identified 7.1 (5 correct
guesses would be expected by chance)
Rhine believed that his evidence supported the existence of ESP, but his findings
were flawed
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Experimental procedures were faulty
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Cards had been handled by the participants and were marked in various subtle ways
Experimental findings could not be replicated
Perception
The topographic nature of
primary perceptual processing
Neurons in the primary sensory areas of the
cerebral cortex have topographic organizational
structures.
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Neurons in the primary auditory cortex have a
tonotopic organization.
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Different parts of the primary auditory cortex are
differentially sensitive to different frequencies, just
like the basilar membrane.
Neurons in the somatosensory cortex have a
somatotopic organization.
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Connected areas of the body are represented next to
each other in the cortex (see next slide).
Somatopic organization in cortex
Visual Perception
The organization of features into objects and
scenes, the location of those objects in space, and
the interpretation (beginning with classification) of
those objects.
Retinotopic maps in the brain
Macaque monkeys were
injected with a glucoselinked radioactive substance
After viewing an image, the
monkeys were sacrificed and
their striate cortex was flat
mounted
Then the radiation was
measured
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Those cells firing the most will
use the most glucose and will
show the most radiation
Hierarchical processing of features
Simple cells: respond to lines of a particular orientation
Complex cells: may respond to lines that terminate or to
corners; may respond to lines anywhere in their receptive field
Hypercomplex cells: respond to a combination of features
Visual Pathways
Form perception (or perceptual
organization)
We must be able to segregate the stimulation
the visual system receives into separate
objects.
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“Parsing” our visual world.
Gestalt maxim
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The whole is greater than the sum of it’s parts.
Figure-ground
People inherently distinguish between figure (the
object they are viewing) and ground (background)
 Ambiguous or reversible figures
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Ambiguous (reversible) figures
Ambiguous (reversible) figures
Ambiguous (reversible) figures
Ambiguous (reversible) figures
Similarity
Objects that are of a
similar color, size, or
shape are usually
perceived as part of a
pattern
Proximity
When objects are
close to one another,
we tend to perceive
them together rather
than separately
Good continuation
Items that continue a
pattern or direction
tend to be grouped
together as part of a
pattern
Closure
We are inclined to overlook incompleteness in
sensory information and
to perceive a whole object
even where none really
exists.
Subjective Contour
Geon Theory
Geons
(geometric
icons) are
simple 3D
component
shapes
An “alphabet” of
36 are stored in
memory
Geons are
combined to
identify essential
contours of
objects
Face perception
In human beings, facial expression and the perception
of faces are an important mode of communication
A specific part of the brain is dedicated to face
perception
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The fusiform gyrus in the right hemisphere
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Located on the ventral portion of the right temporal lobe
Damage to the fusiform gyrus results in prosopagnosia – an
inability to recognize faces
Thatcher illusion
Same race effect in face recognition
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Face memory is better for faces from one’s own race
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And this is more true of whites, than blacks.
There is also more activation in the fusiform gyrus when
perceiving a same race face than when perceiving a
different race face.
Depth Perception
How far away is a particular object?
Binocular cues
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Information integrated from both eyes
Monocular cues
Information from a single eye
 Artists working in two-dimensional media rely on
monocular depth cues to represent a 3-D world
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Binocular Cues
Oculomotor cues
Our eyes converge (turn inward towards the nose)
to focus on objects that are closer and diverge to
focus on objects that are farther away
 Receptors in the oculomotor muscles send this
information to the brain
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Retinal disparity
The difference between images captured in the two
retinas
 This difference diminishes as depth increases
 Binocular cells in visual cortex are retinal disparity
detectors, firing to specific retinal disparities
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Monocular Cues
Relative size
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When looking at two objects known to be of similar
size, the object that looks smaller (because it
projects a smaller image on the retina) will be
perceived as farther away
Texture gradient
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At close distances, patterns appear coarse and less
closely packed, while at farther distances they
appear finer and more densely packed
Linear perspective
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Parallel lines converge at a distance
Monocular Cues cont.
Interposition
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When one object blocks our view of another object,
the obstructed object is perceived as more distant
Elevation
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As objects in our plane of vision that are below the
horizon get closer to the horizon (and, if above the
horizon, they get farther from the horizon), they are
perceived as more distant
Motion parallax
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When a person is moving, nearby objects speed by
whereas far objects move by more slowly
Perceptual Constancy
The perception of objects stays constant despite
changes in sensory stimulation.
Size constancy
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Even though two objects of the same size but at
different depths (distances) create differently sized
retinal images, we still perceive them as the same size.
Color constancy
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The color of an object remains constant despite
changes in illumination.
Shape constancy
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Despite the fact that the same object produces a
different retinal image nearly every time we encounter it,
we still recognize it as that object.
Size perception (and size constancy)
depends on depth perception
Ames Box (Ames
Room)
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When one child stands in
a near corner and another
(of similar height) stands
in a far corner, the room
creates the illusion that
they are both equidistant
from the viewer
Therefore perceptual
processing does not
adjust for distance and we
interpret the small retinal
image as coming from a
smaller person and the
larger retinal image as
coming from a large
person.
The Ponzo Illusion
The lines project the
same size retinal image
because they are the
same size. However,
linear perspective cues
tell us that the top line
is farther away.
However, in order for
something farther away
to project the same
size retinal image, it
must be larger. So, we
perceive the top line as
larger.
The moon illusion
The moon produces
retinal images of the
same size no matter
where it is in the sky
However, we perceive it
as closer when it is not on
the horizon
In order for it to produce
the same size retinal
image and be closer, it
must be smaller
So, we perceive it as
smaller.
The Muller-Lyer Illusion
Again, the lines project the same size retinal image because they are
the same size. However, linear perspective cues tell us that the righthand line is farther away. Therefore, in order for something that is
farther away to project the same-size retinal image, it must be
larger! So we perceive it as larger.
Shape Constancy
Even though these images cast shadows of different
shapes, we still see the quarter as round
Two systems for motion perception