Sensation and Perception

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					Chapter 4
Basic Principles of Sensation

   Sensation is the process that detects
    stimulation from our bodies and our
Basic Principles of Sensation

   Perception is the process that organizes
    those stimuli into meaningful objects
    and events and interprets them.
Basic Principles of Sensation

Psychophysics is the study of how physical
  stimuli are translated into psychological
Sensory Information Must Be Converted
into Neural Impulses

   Sound, light, etc. cannot travel through our
    nerves to the brain.

       Sensory organs convert their physical
        properties into neural impulses.

       This conversion process is called
Sensory Information Must Be Converted
into Neural Impulses

   Transduction takes place at sensory
       Next, connecting neurons in the sense organs
        send this information to the brain.
       The brain processes these neural impulses into
        what we experience.
       Some stimuli are just as real as those that we can
        transduce, but they are not a part of our sensory
Our Senses Vary in Their Sensitivity

   Absolute threshold: the lowest level of
    intensity of a given stimulus that a
    person can detect half the time

       As people age, their absolute thresholds
        for all senses increase.
Our Senses Vary in Their Sensitivity

   Signal-detection theory contends that
    detection of a stimulus is influenced by
    observers’ expectations.

          How likely is the stimulus to occur?
          How important or rewarding is detecting it?

   So absolute threshold may vary.

   It is usually defined as the intensity of a stimulus
    that can be detected 50% of the time.
             100                                       When stimuli are
                                                        detectable less
of correct
detections    75                                        than 50% of the
                                                        time (below one’s
              50                                        absolute threshold)
                                                        they are
              25                                        “subliminal”
                   Low     Absolute        Medium

                   Intensity of stimulus
Our Senses Vary in Their Sensitivity

   Difference threshold is the smallest difference
    between two stimuli that is detected half
    (50%) of the time.

   It is also called the “just-noticeable
    difference” or jnd.
Our Sensory Receptors Adapt
to Unchanging Stimuli

   Sensory adaptation: the tendency for
    our sensory receptors to have
    decreasing responsiveness to stimuli
    that continue without change.

   Auditory adaptation occurs much more
    slowly than adaptation to odors, tastes,
    and skin sensations.
Sensory Adaptation
We See Only a Narrow Band
of Electromagnetic Radiation
   Wavelengths of visible light range from
    400 to 750 nanometers.
          Shorter wavelengths are experienced as violet.
          Intermediate ones as blue, green, and yellow.
          Longer ones as red.

   Other forms of electromagnetic energy
    that our eyes cannot detect are:
          Radio
          Infrared
          Ultraviolet
          X-ray radiation
Spectrum of
         Physical Properties of
 Short wavelength=high frequency             Great amplitude
(bluish colors, high-pitched sounds)   (bright colors, loud sounds)

   Long wavelength=low frequency             Small amplitude
(reddish colors, low-pitched sounds)    (dull colors, soft sounds)
    Color and Wavelength of Light
   An object appears as a particular color because it
    absorbs certain wavelengths of light and reflects others.

   These wavelengths are simply energy; colors are
    created by our nervous system in response to them.

   Species differ in what they see when looking at the
    same object.

   Our difference threshold for colors is so low that the
    average person can discriminate about 2 million
    different colors.
Color Constancy

The relative constancy of perceived color
 under different conditions of
Major Structures of the Human Eye
Retina’s Reaction to Light
Photoreceptors in the Retina

  Rods (125 million)
       Located at the edges of the retina.
       Are not involved in color vision.
       Function best under low-light conditions.

  Cones (7 million)
       Located near the center of the retina (the
       Require bright light to be activated.
       Play a key role in color vision.
Receptors in the Human Eye

                   Cones       Rods
Number             6 million   120 million
Location in        Center      Periphery
Sensitivity in     Low         High
dim light

Color sensitive?   Yes         No
Sound Is the Stimulus for Hearing

   Soundwaves (pressure) are created
    when an object vibrates.
       Wave speed or frequency corresponds to
       Amplitude (wave height) corresponds to
        loudness of a sound.
       Most sounds are a combination of many
        different waves of different frequencies.
           This sound complexity is caled timbre.
Auditory System: Three major
parts of the Ear

    Outer ear:
        The pinna is the most visible part of the outer
        The auditory canal is funnel shaped.
        The eardrum is at the end of the auditory canal
         and it vibrates in sequence with sound waves.
Audition- The Ear
Auditory System: Three major
parts of the Ear

    Middle ear:
      The ossicles are three tiny

       interconnected bones—the
       hammer, anvil, and stirrup —that
       move and amplify sound waves
       before sending them to the inner
Auditory System: Three major
parts of the Ear
   Inner Ear
       innermost part of ear, containing the
        cochlea, semicircular canals and
        vestibular sacs
   Cochlea
       coiled, bony, fluid-filled tube in the
        inner ear through which sound waves
        trigger nerve impulses
Sound Localization

     Sound localization: the ability to locate objects
      in space solely on the basis of the sounds they
          Because the ears are only 6 inches apart, the time
           lag between the sound reaching both ears is very
          Even such small time lags provide the auditory
           system with sufficient information to locate the
Sound Localization
Pitch Perception: Place Theory

    Place theory: contends that we hear
     different pitches because different sound
     waves trigger hair cells on different places
     of the cochlea’s basilar membrane.
Pitch Perception: Frequency Theory

       Frequency theory: contends that pitch is
        determined by the frequency with which
        the basilar membrane vibrates.
Pitch Perception

    Place theory best explains high-frequency
     sounds, while frequency theory best
     explains low-frequency sounds. Mid-
     frequency sounds are best explained by
     volley theory, a revision of frequency
The Intensity of Some Common
Smell and Taste: The Chemical Senses

     Olfaction: the sense of smell

         The stimuli are airborne molecules

         Olfactory receptor cells are at the top of the nasal

         These cells transmit information to the olfactory bulb
          at the base of the brain.

         The olfactory bulb processes this information and
          sends it to the primary olfactory cortex.

   Olfactory sensitivity is determined by
    the number of receptors in the

   Odors can evoke memories and feelings
    associated with past events.
    Smell and Taste: The Chemical
   Gustation: the sense of taste

        Gustation occurs when a substance makes contact
         with special receptor cells in the mouth, called taste

        Most taste buds are located on the tongue, but
         some are in the throat and on the roof of the mouth.
    Gustation: the sense of taste

   When taste cells absorb chemicals dissolved in
    saliva, they trigger neural impulses, transmitted
    to one of two brain areas:

       First: information first sent to the thalamus and then
        to the primary gustatory cortex, where taste
        identification occurs

       Second: information sent to the limbic system,
        which allows a quick response to a taste prior to
        conscious identification of it (example, spitting out
        sour milk)
The Five Primary Tastes
        The Skin Senses Pressure,
        Temperature, & Pain
   Our skin is our largest sensory organ.
   Sense of touch is actually a combination of
    three skin senses:

       Pressure: physical pressure on the skin

       Temperature: The skin contains two kinds of
        temperature receptors, one sensitive to warm
        and the other to cold.
    Pain: The Body’s Warning System
   Pain is induced through tissue damage or intense
    stimulation of sensory receptors.

   Gate-control theory proposes that small-diameter nerve
    fibers (S-fibers) and large-diameter nerve fibers (L-
    fibers) open and close “gateways” for pain in the spinal

   Pain gateways can be closed—thus preventing pain
    messages from reaching the brain—by a class of
    substances known as endorphins.
     The Proprioceptive Senses: Body
     Movement and Location
   Kinesthetic sense: provides information
    about the movement and location of body
    parts with respect to one another
          This information comes from proprioceptors
           (receptors in muscles, joints, and ligaments.)

   Vestibular sense: provides information on
    the position of the body in space by sensing
    gravity and motion (inner ear).

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