Sensation and Perception

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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.

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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.

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Psychophysics

• Psychophysics the study of how physical stimuli
  are translated into psychological experience.

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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.

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JNDs

• 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.
  Webers Law
• The perceived magnitude of the experience is
  proportional to the number of JNDs that the
  originating experience is above the absolute
  threshold.

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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.

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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.

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Subliminal advertising
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One to One Fallacy

• Sensory adaptation, Webers Law, Signal-Detection
  Theory, and JNDs all show that there is no one
  to one correspondence between sensory input and
  sensory experience.

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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 JNDs.

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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

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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.

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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

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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.

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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

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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.

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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.

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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.

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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

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• Impossible figures objects that can be
  represented in two dimensional pictures but
  cannot exist in three dimensional space.

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• Perceptual constancy the tendency to experience
  a stable perception in the face of continually
  changing sensory input.
• Size, shape, and brightness

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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

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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.

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Monocular cues

• Cues about distance based on the image in each
  eye alone.

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• Motion parallax images of objects at different
  distances move across the retina at different
  rates.

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Monocular cues - Pictorial depth

• Cues about distance that can be given in a flat
  picture frame.

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• Linear perspective parallel lines converge as
  they move away from the viewer

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• Texture gradients details become clearer as
  objects near, become smoother as objects are far
  away.

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• Interposition an object that comes between the
  viewer and another object, it must be closer.

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• Relative size closer objects appear larger. 

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• Height in plane distant objects appear higher
  in picture.

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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.

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Feature Analysis

• When we are faced with a stimuli, our brain keys
  on certain feature detectors to analyze its
  content and make a decision about what that
  object is and means.

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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

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• Figure and ground What part of the image is
  grouped into the foreground or the background
  will determine how an image is perceived.

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• Proximity Things that are nearer to each other
  are perceived to be grouped.

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• Similarity People tend to group stimuli that are
  similar.

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• Continuity People tend to follow whatever
  direction their eye is led. People connect points
  that form a line or a smooth curve.

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• Simplicity People tend to organize complex
  visual images in the simplest way possible.

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• Closure People tend to group elements in a way
  that creates a sense of closure or completeness.

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Which principle applies?
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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

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Experiment

• What makes a symbol into a letter?
• Define a particular letter.

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Kinesthesis and Vestibular Senses

• The sensation that allows an organism to orient
  itself in space and informs about its own
  movement.

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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
  heads rotation.

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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.

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False Motion

• The reverse effect can take place as motion of
  the eyes can affect the perceived motion of the
  vestibular senses.

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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.

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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.

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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.

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Taste

• Taste is vital to the organism for providing
  information about substances that may or may not
  be ingested.

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• 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.

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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.

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Smell

• Smell is one of the three distant senses
  (hearing, sight)

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Olfaction

• Smell (olfaction) occurs when chemicals in the
  environment excite receptors located at the top
  of the nasal cavity olfactory epitheluim

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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.

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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.

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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.

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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.

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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.

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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

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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.

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• 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.

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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.

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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.

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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.

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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.

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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.

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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.

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• 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

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• 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.

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Misleading cues

• Impossible figures objects that can be
  represented in two dimensional pictures but
  cannot exist in three dimensional space.

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• Perceptual constancy the tendency to experience
  a stable perception in the face of continually
  changing sensory input.

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Describe the difference between pre-Renaissance
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.

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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.

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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.

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What perceptual principles are operating in
Picassos 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!

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How many different images can you find in
Salvatore Dalis 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.