Sensation and Perception
• Now that we have looked at what a stimuli is,
we need to understand what a sensation is,
and how our brain perceives it.
Sensation vs. perception
• Sensation is the stimulation of sense organs.
• Perception is the selection, organization, and
interpretation of sensory input
• It is the organization of sensory input into
something meaningful.
Psychophysics
• Psychophysics – the study of how physical
stimuli are translated into psychological
experience.
A new way of looking at threshold…
• Threshold can also be explained as the
dividing point between energy levels that do
and do not have a detectible effect.
• Absolute threshold – the minimum amount of
stimulation that an organism can detect.
JND’s
• JND – Just Noticeable Difference
– the smallest difference in the
amount of stimuli that an
organism can detect. The size of
the JND is constantly proportional
to the initial stimuli. – Weber’s
Law
• The perceived magnitude of the
experience is proportional to the
number of JND’s that the
originating experience is above the
absolute threshold.
Signal detection
• Signal Detection Theory states that the
detection of a stimuli involves decision making
processes as well sensory processes.
• These can be conscious or unconscious
• This can be affected by a variety of factors. Eg.
Noise.
• This is all important for perception without
awareness. – Subliminal advertising.
Sensory Adaptation
• Sensory adaptation –
the gradual decline in
sensitivity to a
prolonged stimulus.
• Possibly an evolutionary
development: we need
to know about the
changes rather than the
constants in our
environment.
Subliminal advertising
One to One Fallacy
• Sensory adaptation, Weber’s Law, SignalDetection Theory, and JND’s all show that
there is no one to one correspondence
between sensory input and sensory
experience.
The 5 Senses – The Eye
• The most sophisticated sensory organ that an
organism can possess is the eye:
• It can see over vast distances in almost real
time.
• It can detect minute changes and motion and
can self-adjust quickly to varying levels of
JND’s.
The stimulus: Light
• Light is a form of electromagnetic radiation that has
three varying properties:
– Wavelength
– Amplitude
– Purity
• These properties are all sensed in different ways by
the eye:
– Hue
– Brightness
– Saturation
Light interacts with the eye
• Light enters the eye through
the cornea and is modified
by the lens.
• The lens focuses light on the
receptor surface at the back
of the eye called the retina.
• The pupil is the opening in
the iris, or the colored
muscle surrounding it. It
closes and opens to regulate
the amount of light entering
the eye.
Rods and cones
• Light passes through the cornea,
pupil, and lens to fall on the retina
– the light sensitive layer of
receptors that line the back of the
eye cavity.
• Light is detected by two different
kinds of cells – rods and cones
• Rods – more sensitive to light and
dark and peripheral vision
• Cones – more sensitive to daylight
vision and color. Cones are
concentrated in the center of the
retina - fovea
Eye as a processing organ
• The layer of intervening
cells between the light
source and the rods and
cones actually processes
the image as inverted
before sending the
information through the
optic disk to the brain.
Color vision
Color is a psychological interpretation, not a
property of light itself.
Humans can distinguish roughly 1 million
different colors created by subtractive or
additive mixing.
Green exhibits the most variation in perception
Color vision
• Light perception is a mix of two different
theories:
• Trichromatic theory – there are three
different types of cones each excited to a
different wavelength. – red, green, blue.
• Opponent-Process theory – receptors in the
eye have antagonistic responses to three
PAIRS of colors. – red/green, blue/yellow,
black/white.
Opponent-process Theory
• When the three types of cones are stimulated,
it has an inhibitory effect on the opposite
color on the spectrum. This is what produces
an afterimage – a visual image that persists
after the stimuli is removed.
Color processing in the retina
• Cells in the retina
respond in opposite
ways to the same
wavelength.
• Eg. The same cell will be
excited by green and
inhibited by red, and
there are other cells
that are excited by red
and inhibited by green.
Perception
• As the eye sends signals to the brain, these
are processed with implicit assumptions about
the reality presented that go BEYOND what is
only seen
• Impossible figures – objects that can be
represented in two dimensional pictures but
cannot exist in three dimensional space.
• Perceptual constancy – the tendency to
experience a stable perception in the face of
continually changing sensory input.
• Size, shape, and brightness
Depth perception
• Involves interpretation of cues that indicate
how near or far away objects are.
• There are two different sets of cues that can
help us judge distance.
– Monocular cues
– Binocular cues
Binocular cues
Clues about distance based on the differing view
from the two eyes.
– Retinal disparity – objects within 25 ft. of the
viewer project images to slightly different
locations on the right and left retina. The brain will
interpret these two differing images as depth.
– Convergence – sensing the eyes converging
toward each other as they focus on closer objects.
Monocular cues
• Cues about distance based on the image in
each eye alone.
• Motion parallax – images of objects at
different distances move across the retina at
different rates.
Monocular cues - Pictorial depth
• Cues about distance that can be given in a flat
picture frame.
• Linear perspective – parallel lines converge as
they move away from the viewer
• Texture gradients – details become clearer as
objects near, become smoother as objects are
far away.
• Interposition – an object that comes between
the viewer and another object, it must be
closer.
• Relative size – closer objects appear larger.
• Height in plane – distant objects appear
higher in picture.
Perception of form and shape
• The same visual input can produce radically
different perceptions.
• Perceptual set – a readiness to perceive a
stimuli in a particular way.
• This readiness causes us to assemble specific
elements in an image into a more complex
form – feature analysis.
Feature Analysis
• When we are faced with a stimuli, our brain
keys on certain feature detectors to analyze
it’s content and make a decision about what
that object is and means.
Gestalt Analysis
Gestalt Analysis – the presentation of the entire
set of stimuli as a whole leads to the
organization of its individual parts.
• There are several principles of Gestalt
psychology and perception:
Figure and ground: What part of the image is
grouped into the foreground or the background
will determine how an image is perceived.
Proximity: Things that are nearer to each other
are perceived to be grouped.
Similarity: People tend to group stimuli that are
similar.
Continuity: People tend to follow whatever
direction their eye is led. People connect points
that form a line or a smooth curve.
Simplicity: People tend to organize complex
visual images in the simplest way possible.
Closure: People tend to group elements in a way
that creates a sense of closure or completeness.
Which principle applies?
Your homework
• 1) find an image on the internet
• 2) print it
• 3) explain the Gestalt perceptive elements at
work in it
• 4)write those explanations down
• 5) staple the picture to the explanation, put
your name on it and hand it in FOR
TOMORROW
• 6) it will be marked out of 5 based on your
accuracy and explanation
Experiment
• What makes a symbol into a letter?
• Define a particular letter.
Kinesthesis and Vestibular Senses
• The sensation that allows an organism to
orient itself in space and informs about its
own movement.
Skeletal movement and orientation
• Skeletal movement is sensed through kinesthesis, the
feedback we get from the muscles, tendons, and
joints as they move.
• Orientation is sensed through receptors in the inner
ear of each side of the head called the semicircular
canals.
• Three canals contain a fluid that rotates as the head
moves, this motion causes tiny hairs in the vestibules
to move.
• This provides information about the extent of the
head’s rotation.
Vision stabilization
• This sense plays a vital role in stabilizing
vision.
• As the head and body move through space,
the vestibular senses from each side of the
head relay their information directly to the
muscles that control each eye.
• The motion of the head is cancelled by an
equal and opposite motion of the eye.
False Motion
• The reverse effect can take place as motion of
the eyes can affect the perceived motion of
the vestibular senses.
The Skin
• Researchers believe that there are four
distinct skin sensations: pressure, warmth,
cold, and pain.
• In this case, there are different receptors for
each kind of sense quality.
Multiple specialized receptors
• Variations in pressure sensations are produced
by different receptors in the skin.
• Some are wrapped around the base of hair
follicles and sense movements of the hair
• Others are capsules that are easily bent by
slight deformations of the skin.
• Other capsules respond to vibrations, steady
indentation of the skin, and others to sudden
movement across the skin.
Mystery of pain
• Less is known about the sensation of
temperature and pain.
• Some of these experiences are triggered by
free nerve endings with no specialized
structures attached to them.
• There is little known about the nature of pain.
Research has shown that it results from an
intense triggering of a given receptor.
Taste
• Taste is vital to the organism for providing
information about substances that may or
may not be ingested.
• In humans, taste receptors
sense chemicals dissolved in
water on the tongue. These
receptors are grouped into
taste buds on the tongue and
elsewhere in the mouth.
• Taste sensations can be
divided into four basic
qualities: bitter, sour, sweet,
and salty.
• All other tastes are a
combination of these four.
Specific receptors
• Nerve fibers are specifically targeted to
respond to similar chemical compounds. Some
fibers respond best to salts, others to sugars,
etc.
• Relatively few substances stimulate one type
of nerve only so most taste sensations are a
pattern of varying levels of each taste specific
fiber.
Smell
• Smell is one of the three distant senses
(hearing, sight)
Olfaction
• Smell (olfaction) occurs when chemicals in the
environment excite receptors located at the
top of the nasal cavity – olfactory epitheluim
The nature of smell
• Several sensory categories exist for smell:
fragrant, spicy, and putrid.
• However, it remains unknown how chemicals
can trigger these specific sensations.
• It is believed that it is a pattern sensation
rather than sense-specific nerves that is
responsible.
Smell + Taste
• Smell occurs within the mouth as well – flavor
• Flavor depends largely on smell rather than
taste.
• When our sense of smell is temporarily
impaired, our sense of taste goes with it.
Smell as a distant sense
• Studies have shown that smell has an
important effect on behaviour.
• It can be used to warn an organism of danger
• It can be used to sell things to consumers
• It can be used to identify people.
Pheromones
• Pheromones are chemical substances released
by organisms that communicate information
and influence behaviour of other members of
the same species.
•
•
Females will secrete a substance that signals their sexual receptiveness
Rats who have been shocked in a cage will secrete a substance that will cause the
next rat placed in the same cage to respond with anxiety.
• Studies have also shown that menstrual synchrony is
triggered by pheromones of females living together.
• Also, female sensitivity to pheomones fluctuates
over the duration of the menstrual cycle and will
peak during ovulation.
Hearing
• Hearing is produced by variations in the air
pressure around the head.
• These variations have two properties that are
sensed by the ear.
• 1)Wavelength – the distance between two
incoming waves.
• 2) Amplitude – the height of the incoming
waves.
• These two properties can be sensed by the
brain as: loudness and pitch.
Complex waves
• Most waves encountered in the evironment
are a combination of several other waves.
• The brain is able to separate the various sound
patterns into their component parts
Sound
• Sound waves are collected by the outer ear and funneled to a
membrane called the ear drum.
• Waves are then transferred across a second chamber where
they trigger movement in a series of small bones called the
ossicles.
• Vibrations of this membrane cause changes in
the fluid contained in the cochlea, a snail
shaped chamber lined with tiny hairs.
• These fluid changes trigger movement in the
hairs that are sensed by receptors and passed
along the auditory nerve to the brain.
Sensory interaction
• This pattern of stimulation illustrates a
principle of sensation in which the response
by a sensory system to a stimulus rarely
depends on that stimulus alone, but is also
affected by other stimuli that are occurring or
may have just occurred recently – Sensory
interaction.
Sensory Coding
• With all the different senses that we have
looked at, the final problem becomes
• how does the body encode neural impulses
into complex sensations?
• There must be some property of the neural
impulses that differentiate one sensation from
another across all the different sense
receptors.
Stimulus intensity
• There is one code for intensity. We have seen
that this can be communicated in two ways:
• The number of neurons firing.
• The rate of each neuron firing.
Stimulus quality
• There is another code for quality.
• There are two theories that try to explain this:
• Specificity theory – different sense qualities are
triggered by different nerves being stimulated, each
one specific for a different quality.
• Across-fiber pattern theory – different sense
qualities are triggered by a pattern of activation
across numerous nerves. Different sensations are
actually different patterns of nerves firing in specific
ways.
Perceptual Cues
• One primary example of perceptual cues at
work is depth perception - involves
interpretation of cues that indicate how near
or far away objects are.
There are two different sets of cues that
can help us judge distance.
• Binocular cues – clues about distance based on
the differing view from the two eyes.
• Retinal disparity – objects within 25 ft. of the viewer
project images to slightly different locations on the
right and left retina. The brain will interpret these
two differing images as depth
• Convergence – sensing the eyes converging toward
each other as they focus on closer objects.
• Monocular cues – cues about distance based on the
image in each eye alone
• Motion parallax – images of objects at
different distances move across the retina at
different rates.
• Pictorial depth cues – cues about distance that
can be given in a flat picture frame.
• Linear perspective – parallel lines converge as
they move away from the viewer
• Texture gradients – details become clearer as objects
near, become smoother as objects are far away.
• Interposition – an object that comes between the
viewer and another object, it must be closer.
• Relative size – closer objects appear larger.
• Height in plane – distant objects appear higher in
picture.
Misleading cues…
• Impossible figures –
objects that can be
represented in two
dimensional pictures
but cannot exist in three
dimensional space.
• Perceptual constancy – the tendency to
experience a stable perception in the face of
continually changing sensory input.
Describe the difference between preRenaissance painters and post-Renaissance
painters. How did understanding of
perception cues change?
• Pre-Renaissance painters did not have a grasp of
pictorial depth cues. Their renderings are awkward
and flat. Renaissance painters mastered these cues,
especially linear perspective, height in plane, and
interposition. Instead of just relating the story of an
event, they sought to re-create the reality or the
illusion of the event happening.
How did the Impressionists use principles of
perception to go beyond reality?
• Impressionists used separate spots of pure colors
that would blur together at a distance. He used
complementary colors to create the impression of a
scene after it has been experienced rather than
recreating the scene exactly at it was.
Using what we have already learned about sight
sensation, describe how Pointillism
operates.
• Seurat used small points of pure colors in an additive way.
From a distance, these colours would be combines by the
brain into stimuli of a single color. This shows an awareness of
texture gradient. Also, continuity and closure Gestalt principles
operate to create objects out of groupings of dots.
What perceptual principles are operating in
Picasso’s work? How does this affect the
way you like it?
• Cubists applied the principles
of feature analysis to their
paintings, They reduced
objects to their geometric
component shapes and
reassembled them on a flat
plane. This triggers responses
in the viewer using closure,
continuity, similarity, and
proximity. It is seen by the
eye but understood by the
mind – freaky!
How many different images can you find in Salvatore Dali’s The
Hallucinogenic Toreador? Describe the effect this has on
you as the viewer.
•
•
•
•
Bullfighter in the Venus de Milos,
a bull in the shapes below to the left,
a man waving a cape in the air.
This gives the painting a dream like quality.