Sensation & Perception

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					Sensation & Perception

Psychology 2012 – Spring 2004
Sensation & Perception
Sensation – detection of basic sensory
   Sounds, touch, smells

Perception – interpretation of
sensations in meaningful ways
   Interpreting sounds as music, touch as the
    cat rubbing against you, smells as dinner
Sensation & Perception
Sensory receptors – specialized cells unique
to each sense organ

   They respond to a particular form of sensory

   Transduction – the process of converting different
    forms of energy into electrical impulses that are
    transmitted via neurons to the brain
        Done by the sensory receptors
    Sensory Thresholds
Absolute threshold – the smallest stimulus that can
be detected half the time

Difference threshold – the smallest difference
between 2 stimuli that can be detected half the time
   AKA the just-noticeable difference (jnd)
        Weber’s law – the ability to detect a difference between 2
         stimuli depends on the strength of the original stimulus

Sensory adaptation – sensory receptor cells become
less responsive to a constant stimulus
Wavelength – the
distance from one
wave peak to
         Parts of the Eye
Cornea – clear membrane covering the
visible part of the eye; helps gather and direct
incoming light
Pupil – opening in the middle of the iris that
changes size to let in different amounts of
Iris – colored part of the eye; controls the size
of the pupil
   Dim light = iris widens; more light is let in
   Bright light = iris narrows; less light let in
           Parts of the Eye
Lens – structure located behind the pupil that focuses
light entering the eye
   Accommodation – process by which the lens changes shape
    to focus incoming light on the retina
        Problems with the shape of the eyeball = either
         nearsightedness (long, thin eyeball) or farsightedness (short,
         fat eyeball)

Retina – membrane located in the back of the eye
containing the sensory receptors for vision
   Rods – receptors of the eye that are highly sensitive to light,
    but not color; responsible for peripheral & night vision
   Cones – pointed sensory receptors of the eye that detect
    color vision and fine details
        Most cones are concentrated on the fovea (in the center of the
Visual Processing in the
Bipolar cells – specialized neurons in the
retina that connect with the rods & cones and
the ganglion cells

Ganglion cells – specialized neurons in the
retina that connect to the bipolar cells; axons
of these cells make up the optic nerve
    Each ganglion cell combines, analyzes, and
     encodes information from the rods & cones before
     sending info to the brain
    From the Eye to the
Optic nerve – exits from the back of the eye and
carries information to the visual cortex of the brain
   Made up of a million axons of ganglion cells

Optic chiasm – where the optic nerve fibers from
each eye meet and partly cross over to the opposite
side of the brain
Thalamus – where most of the optic nerve axons go;
processes information about form, color, brightness,
and depth
Visual cortex – information from the cortex goes here,
where it is decoded and interpreted

The perceptual experience of different
wavelengths of light
   Hue – color; different wavelengths
        We see colors from 400 nanometers (violet) to 700 nanometers
        White object – reflecting all colors; black object – absorbs all
         colors; red object – reflects red light

   Brightness – intensity of color
        Amplitude (height) of the light wave

   Saturation – the purity of color (light wave)
        Red = more saturation than pink
         Theories of Color
Trichromatic Theory – the sensation of color is a result of the
cones being especially sensitive to either red (long
wavelengths), green (medium), or blue (short) light
    Color blindness – inability to sense a certain color of light due to a
     deficiency in cone sensitivity

Opponent-Process Theory – the sensation of color is due to
opposing pairs of color receptors (red/green, blue/yellow, and
black/white); when one color in the pair is stimulated, the other
is inhibited
    Afterimages – seeing the other color (e.g., green) in a color pair
     once the original color (e.g., red) is no longer present

Integrated Explanation of Color Vision – cones of the retina
adhere to the trichromatic theory, while the ganglion cells
adhere to the opponent process theory
Pitch – highness or lowness of sound
(frequency – wavelength)
   Hertz – units used to measure frequency

Loudness – determined by intensity
(amplitude - height) of sound wave
   Decibels – units used to measure loudness

Timbre – the complexity of sound
(combination of frequencies)
Outer Ear – part of the ear collecting sound waves
 Pinna, ear canal, & eardrum
 Eardrum – membrane at the end of the outer ear
  that vibrates when hit by sound waves
Middle Ear – amplifies sound waves via the hammer,
the anvil, and the stirrup
Inner ear – part of the ear that changes sound into
neural impulses
  Cochlea – structure that contains sensory
   receptors for sound
       Basilar membrane – membrane in the cochlea containing
        hair cells (receptors for sound)
    Distinguishing Pitch
Frequency theory – the view that the basilar
membrane vibrates at the same frequency as
the sound wave involved
   Works for low frequency sounds

Place theory – the view that different
frequencies cause larger vibrations at
different locations along the basilar
   Works for higher frequency sounds
Olfactory receptors –
where odor molecules are
   Bundles of axons make up
    olfactory nerves

Olfactory bulb – where
sensation of smell is
registered; in the olfactory
cortex at the front of the
     Smell Sensitivity
Sense of smell varies among animals
Dogs have 200 million olfactory rods,
spread out in a much bigger nose
Humans differ greatly in ability to detect
The most sensitive people are 20 times
more sensitive than the least
Taste buds – sensory
receptors for taste
 Located on the tongue
  and inside the mouth and
 Sweet, salty, sour, &
The Skin & Body Senses
Skin senses provide information about our
physical interaction with objects in the
The body senses keep us informed about our
position & orientation in space
    Skin = largest and heaviest sense organ
    Pacinian corpuscle – receptor beneath the skin
     involved in sense of touch
         Touch & temperature – more sensory receptors in certain
          areas of the skin (hands, faces, lips) than others (back,
          arms, & legs)
The Skin & Body Senses
Pain – the sensation of discomfort or suffering that
occurs in varying degrees of intensity
    Free nerve endings in the skin, muscles or internal organs
     carry messages of pain to the spinal cord, releasing
     “substance P”
         Substance P – neurotransmitter that is involved in transmitting
          pain messages to the brain
    Depending on how the brain interprets pain, it can be
     intensified or reduced
         Gate-control theory – sensation of pain is controlled by a series
          of “gates” in spinal cord.
               Depending on how the brain interprets pain, these gates are
                opened (more pain sensation) or closed (less pain sensation)
                   Psychological factors increasing pain: anxiety, fear,
                   Psychological factors decreasing pain: positive emotions,
                    distraction, feelings of control
The Skin & Body Senses

    Psychological factors also influence the release of
     endorphins, which can:
         Inhibit pain messages in the brain, and
         Inhibit the release of substance P in the spinal cord
    Psychological factors can affect muscle tension,
     blood flow, arousal, & heart rate
         All of these can affect the experience of pain
The Skin & Body Senses
Movement, Position, & Balance
      Kinesthetic sense – sensation of body parts in relation to one
            Proprioceptors – sensory receptors in the joints, the inner ear and
             the muscles
                 Constantly communicate information to the brain about
                  changes in body position and muscle tension
      Vestibular sense – sense of balance through responding to
       changes in gravity, motion, and body position
          Semicircular canals and vestibular sacs located in the ear

              Filled with fluid and hairlike receptor cells that sense
                changes in body position or gravity
          Vision also affects this sense

              When vision is at odds with the semicircular canals
                and vestibular sacs, the result can be dissiness,
                disorientation, & nausea
The organization and interpretation of raw
sensory data
   Bottom-up processing – analysis that moves from
    recognizing the parts of a stimulus to the whole
        Data-driven processing
        Used most during ambiguous situations

   Top-down processing – analysis that moves from
    the whole stimulus to the parts
        Conceptually driven processing
        Drawing on our experiences to arrive at meaningful
The Perception of Shape
 Figure-ground relationship – a principle of
 perceptual organization that states that we
 automatically separate a stimulus into:
  1.   The feature that stands out (the figure) from:
          Figure – the main element of a scene
              Usually stands out clearly
  2.   Its less distinct background (the ground)
          Ground – the background
              Usually is more fuzzy and less clear than the figure
The Perception of Shape
     – What is it?
Perceptual grouping – when perceiving
a form, we tend to group different
elements of it together
    We actively organize elements to try to
     produce well-defined, whole objects
The Perception of Shape
Law of Pragnanz (simplicity) – when
several perceptual organizations are
possible, we interpret objects in the
simplest ways
    Example – three squares
Depth Perception – How
    far away is it?
The ability to perceive distance of an object
and its three dimensional qualities
   Monocular Cues – only require one eye to notice
        Relative size – if two or more objects are assumed to be
         similar in size, the object that appears larger is perceived
         as being closer

        Overlap – when one object blocks another object, the
         partially blocked object is viewed as being farther away

        Aerial perspective – hazy or slightly blurred objects
         appear to be farther away
      Depth Perception
Monocular Cues
   Texture gradient – crisp and distinct objects
    appear closer than fuzzy and blended objects

   Linear perspective – parallel lines seem to meet in
    the distance; the closer together the lines appear
    to be, the greater the perception of distance

   Motion parallax – when moving, nearby objects
    seem to move by faster than more distant objects
         Depth Perception
Binocular cues – require both eyes to notice
   Convergence – the degree to which eye muscles
    rotate to focus on an object
        The more the eyes rotate inward, the closer the object is
         perceived to be

   Binocular disparity – when retinal image is very
    different between the two eyes, the object is
    interpreted as being very close
        When retinal image is very similar, object is interpreted
         as being further away

The Perception of Motion –
    Where is it going?
  Neural pathways doing a combination
  of the following
 1.   Sensing eye muscle activity
 2.   Sensing the changing retinal image
 3.   Contrasting the moving object with its
      stationary background
         The Perception of
 Illusions of motion
1.   Induced motion – our tendency to perceive an
     object moving and a background staying still
        Even when a background is actually moving
        Karl Duncker – the dot and the rectangular frame
2.   Stroboscopic motion – the illusion that two or
     more flashing lights are actually one moving light
        If the flashing of the second light is within 1/10 of a
        The brain’s visual system perceives motion, even
         though there is no movement across the retina
Perceptual Constancies
Tendency to perceive familiar objects as unchanging
despite actual changes in sensory input
   Size constancy – the perception that an object remains the
    same size despite a changed retinal image

   Shape constancy – the tendency to perceive objects as
    having a similar shape despite the images cast on the retina

   Brightness constancy – the tendency to perceive the
    brightness of an object as the same despite changes in
    lighting conditions
    Perceptual Illusions
Stimuli that tend to make us misperceive the
true characteristics of objects or images
   The Muller-Lyer illusion – the misperception of
    identical length of two lines due to arrows pointing
    in different directions

   The moon illusion – the misperception that the
    moon is larger when it is closer to the horizon

             Experience &
Perceptual set – the influence of prior
assumptions and expectations on
perceptual interpretations
   Observers often interpret ambiguous
    stimuli in terms of what their expectations
        Circular clouds = UFOs, rocks in a river = the
         Lock Ness Monster
       Experience &
Our perceptions are a combination of
the following:
 Cues that indicate distance, form,
  movement, etc.
 Our expectations, learning experiences,
  and cultural experiences

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