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Chapter 4 Sensation and Perception


Chapter 4 Sensation and Perception Sensation and Perception Sensation is the conversion of energy from the environment into a pattern of response by the nervous system. – PowerPoint PPT presentation

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Title: Chapter 4 Sensation and Perception

Chapter 4 Sensation and Perception
Sensation and Perception
  • Sensation is the conversion of energy from the
    environment into a pattern of response by the
    nervous system.
  • Perception is the interpretation of that

Module 4.1
  • Vision

Sensing the World Around Us
  • Stimuli are energies in the environment that
    affect what we do.
  • Receptors are the specialized cells in our bodies
    that convert environmental energies into signals
    for the nervous system.

The Detection of Light
  • Light is the stimulus that the visual system is
    designed to detect.
  • Visible light is just one very small portion of
    the electromagnetic spectrum, which is the
    continuum of all the frequencies of radiated
  • The human eye is designed to detect energy in the
    wavelengths from 400 to 700 nm.

  • Figure 4.2 The lens gets its name from Latin for
    lentil, referring to its shapean appropriate
    choice, as this cross section of the eye shows.
    The names of other parts of the eye also refer to
    their appearance.

The Structure of the Eye
  • The pupil is an adjustable opening in the eye
    through which light enters.
  • The iris is the structure on the surface of the
    eye, surrounding the pupil, and containing the
    muscles that make the pupil dilate or constrict.
  • The iris gives your eye its characteristic color,

The Structure of the Eye
  • The cornea is a rigid, transparent structure on
    the very outer surface of the eyeball. It focuses
    light by directing it through the pupil.
  • When the light goes through the pupil, it is
    directed to the lens.
  • The lens is a flexible structure that can vary in
    thickness, enabling the eye to accommodate,
    adjusting its focus for objects at different

The Structure of the Eye
  • The lens directs the light through a clear,
    jellylike substance called the vitreous humor to
    the back of the eyeball.
  • At the back of the eye is the retina, the
    structure containing the visual receptors.

Common Disorders of Vision
  • Presbyopia develops as humans age because the
    lens decreases in flexibility, resulting in a
    reduced ability to focus on nearby objects.
  • Elongated eyeballs cause myopia, so that the
    person can focus well on nearby objects, but not
    distant ones. This condition is also called
  • Flattened eyeballs cause hyperopia, so that the
    person can focus well on distant objects, but not
    on nearby ones. This is also called

  • Figure 4.3 The flexible, transparent lens changes
    shape so that objects (a) far and (b) near can
    come into focus. The lens bends entering light
    rays so that they fall on the retina. In old age
    the lens becomes rigid, and people find it harder
    to focus on nearby objects

Common Disorders of Vision
  • Glaucoma is a condition caused by increased
    pressure within the eyeball, causing damage to
    the optic nerve and loss of peripheral vision.
  • A cataract is a disorder in which the lens of the
    eye becomes cloudy. This disorder is treated by
    removing and replacing the actual lens with a
    contact lens.

Concept Check
  • What happens if a person with normal vision puts
    on contact lenses designed for a person with

His vision, especially his near vision, will be
The Visual Receptors
  • The retina contains two types of specialized
    neurons, the rods and the cones.
  • Rods far outnumber cones in the human eye.
  • About 5-10 of the visual receptors in the human
    retina are cones.

The Visual Receptors
  • The cones are utilized in color vision, daytime
    vision and detail vision.
  • The rods are adapted for vision in dim light.
  • Species that are active at night have few cones
    and many rods, giving them particularly good
    night vision.

  • TABLE 4.1 Differences between rods and cones

The Visual Receptors
  • The fovea is the center of the human retina, and
    the location of the highest proportion of cones.
  • It is the area of the eye with the greatest
  • Rods are more plentiful in the periphery of the

Concept Check
  • If you see a brightly colored object in the
    periphery of your vision, the colors will not
    seem very bright at all. Why is this?

You have mostly rods in the periphery of your
retina, thus a more limited ability to detect
Dark Adaptation
  • Most humans require one or two minutes to see in
    the dark. This process of gradual improvement is
    called dark adaptation.
  • Exposure to light causes molecules of
    retinaldehydes to be chemically altered and
    stimulate the visual receptors.

Dark Adaptation
  • In conditions of normal daytime light, these
    molecules are depleted and regenerated at about
    the same rate, so the amount available in the
    retina is balanced and level of visual
    sensitivity is constant.
  • In dim light or darkness the receptors regenerate
    these molecules without any subsequent depletion,
    and so the increase in available retinaldehydes
    also enhances dark adaptation.

Dark Adaptation
  • Cones and rods perform this regeneration at
    different rates.
  • Although cones regenerate their retinaldehydes
    more quickly, they are also being more heavily
    used in the daytime.
  • By the time you need enhanced night vision, the
    rods have fully regenerated their supply of
  • The relative abundance of rods, and their
    undisturbed regeneration of the chemical, gives
    them a much higher level of sensitivity to faint

  • Figure 4.8 These graphs show dark adaptation to
    (a) a light you stare at directly, using only
    cones, and (b) a light in your peripheral vision,
    which you see with both cones and rods. (Based on
    E. B. Goldstein, 1989)

Concept Check
  • It is said that dogs and cats can see in the dark
    do you think this is really true?

Although these animals have much better vision in
dim light than we do, there must be some light
present for the rods to function.
Concept Check
  • In the daytime which predominates, the fovea or
    periphery of the eye?

Unless you walk into a dark room, you will be
using the fovea, because cones are the
receptors for daytime (well-lighted) vision.
The Visual Pathway
  • The visual receptors send their impulses away
    from the brain, toward the center of the eye.
  • First the bipolar cells gather the impulses from
    the rods and cones.
  • Then the bipolar cells make synaptic contacts
    with ganglion cells.

  • Figure 4.7 Because so many rods converge their
    input into the next layer of the visual system,
    known as bipolar cells, even a small amount of
    light falling on the rods can stimulate the
    bipolar cells. Thus, the periphery of the retina,
    with many rods, has good perception of faint
    light. However, because bipolars in the periphery
    get input from so many receptors, they have only
    imprecise information about the location and
    shape of objects.

The Visual Pathway
  • The axons of the ganglion cells join together to
    form the optic nerve, which makes a U-turn and
    exits the eye.
  • There are no photoreceptors at the point at which
    the nerve leaves the eye. This is called the
    blind spot.
  • You are not aware of your blind spot because
    information from the retina of each eye fills
    in the blind spot in the other eye. This
    integration occurs in the visual cortex.

The Visual Pathway
  • At the optic chiasm, half of each optic nerve
    crosses to go to the opposite side of the brain.
  • At this point the axons begin to separate,
    sending information to a number of locations in
    the brain.
  • The greatest number of axons goes to the
    occipital lobe via the thalamus.

  • Figure 4.9 Axons from cells in the retina depart
    the eye at the blind spot and form the optic
    nerve. In humans about half the axons in the
    optic nerve cross to the opposite side of the
    brain at the optic chiasm. Some optic nerve axons
    carry information to the midbrain others carry
    it to the thalamus, which relays information to
    the cerebral cortex.

The Visual Pathway
  • The information from each retina is integrated in
    the visual cortex.
  • Each cell in the cortex receives input from both
    the left and the right retinas.
  • When the retinas are focused on the same point in
    space, the input from each side is easily
    integrated because the message is from each is
    almost the same.

The Visual Pathway
  • If the images conflict with each other, cortical
    cells will be alternately stimulated and
    inhibited as they try to integrate the
  • The alternation between seeing the conflicting
    information from each retina is called binocular

  • Figure 4.11 To produce binocular rivalry, move
    your eyes toward the page until the two circles
    seem to merge. You will alternate between seeing
    red lines and green lines.

The Visual Pathway
  • The brain activity of the visual cortex is
    crucial for the sense of vision.
  • People with intact eyes but a damaged visual
    cortex lose the ability to imagine visual imagery.

Color Vision
  • Different wavelengths of electromagnetic energy
    correspond to different colors of light.
  • There are three kinds of cones that respond to
    different wavelengths.
  • Cells in the visual path process the information
    from these cones in terms of opposites.

Color Vision
  • The three types of information are
  • Red vs. green
  • Yellow vs. blue
  • White vs. black
  • The cells in the cerebral cortex integrate the
    input from the parts of the visual field to
    create a color experience for the objects that we

Color Vision
  • The Young-Helmholtz theory
  • This is also known as the trichromatic theory.
  • It proposes that our receptors respond to three
    primary colors.
  • Color vision depends on the relative rate of
    response by the three types of cones.

Color Vision
  • Each type of cone is most sensitive to a specific
    range of electromagnetic wavelengths.
  • Short wavelengths are seen as blue.
  • Medium wavelengths are seen as green.
  • Long wavelengths are seen as red.

  • Figure 4.13 Sensitivity of three types of cones
    to different wavelengths of light. (Based on data
    of Bowmaker Dartnall, 1980)

Color Vision
  • Each wavelength induces different levels of
    activity in each type of cone.
  • For example, light that stimulates the medium and
    long wavelength cones about equally will be
    perceived as yellow.
  • Light that excites all three types equally is
    perceived as white.

Color Vision
  • The Opponent-Process Theory
  • Trichromatic theory does not account for some of
    the more complicated aspects of color perception.
  • People experience four colors as primary red,
    green, blue and yellow.
  • People also report seeing colored after-images
    after staring at an object of one color. If you
    stare at a red object, you tend to see a green
    after-image when you stop staring.

Color Vision
  • The Opponent-Process Theory
  • Because of these facts, Ewald Hering proposed
    that we perceive color not in terms of separate
    categories but rather in a system of paired
  • Red vs. green
  • Yellow vs. blue
  • White vs. black

Color Vision
  • The Opponent-Process Theory
  • The negative after-images that we experience
    after staring at objects are results of the
    alternating stimulation and inhibition of neurons
    in the visual system.
  • A bipolar neuron that responds strongly to yellow
    will be inhibited by blue.
  • After youve stared at a yellow object, your
    fatigued bipolar cell will behave as if its been
    inhibited, and yield a sensation of blue.

Concept Check
  • A bipolar cell is stimulated by red wavelengths.
    You stare at a red object. What will happen when
    you stop staring?

You will see a negative afterimage in green.
Concept Check
  • Why do negative after-images that you see seem to
    move around?

Because the image is in your eye, not from any
object at which you are gazing.
Color Vision
  • The Retinex Theory
  • The trichromatic and opponent-process theory
    dont account for our experience of color
  • Color constancy is the tendency of an object to
    appear nearly the same color even though we see
    it in a variety of lighting conditions.

Color Vision
  • The Retinex Theory
  • Edwin Land proposed that we perceive color
    because the cerebral cortex compares various
    retinal patterns (thus the name retina cortex
  • By comparing different patterns of light from
    different areas of the retina, cortical cells
    synthesize a color perception for each area.

Color Vision
  • The Retinex Theory
  • The fact that certain types of brain damage
    disrupt color constancy, causing for example an
    object to look orange under one level or type of
    lighting, and red, green, yellow or even white
    under other conditions, is considered to be
    strong evidence for the Retinex theory.

Color Vision
  • Colorblindness
  • Total inability to distinguish colors is very
    rare except as a result of brain damage.
  • About 4 of all people are partly colorblind.

Color Vision
  • Colorblindness
  • Colorblindness can result from the absence of one
    of the three types of cones.
  • Colorblindness can also result when one of the
    three types of cones is less responsive than the
    other two. The color that stimulates that type of
    cone is seen as almost gray.

Color Vision
  • Colorblindness
  • Red-green colorblindness is the most common type.
  • There are two forms protanopia, in which the
    afflicted person lacks long-wavelength cones, and
    deuteranopia, in which the person lacks
    medium-wavelength cones.
  • Yellow-blue colorblindness (known as trianopia)
    is very rare.

How We See
  • Before animals could see color, there was no
  • What you see is in your brain. Not an exact
    representation of the world around you, but a
    construction and interpretation of many stimuli.
  • Sensation seems simple, but it is perhaps one of
    the most challenging areas of this science.

Module 4.2
  • The Nonvisual Senses

  • The ear is designed to detect and transmit sound
    waves to the brain.
  • Sound waves are vibrations in the air or other
  • Sound waves vary according to frequency and
  • Frequency is measured by the number of vibrations
    or cycles of the sound wave per second, referred
    to as hertz (Hz.)

  • Figure 4.20 The period (time) between the peaks
    of a sound wave determines the frequency of the
    sound we experience frequencies as different
    pitches. The vertical range, or amplitude, of a
    wave determines the sounds intensity and

  • The perception of frequency is referred to as
  • We perceive a high-frequency sound wave as
    high-pitched, and a low-frequency wave as
  • Amplitude is intensity of sound waves and is
    perceived as loudness.
  • Pitch and loudness are psychological experiences,
    and the perception of these qualities does not
    solely depend on frequency and amplitude.

  • The Ear
  • The ear is a complex organ. It converts weak
    sound waves into waves of pressure that can be
    transported by sensory neurons and interpreted by
    the brain.
  • The cochlea is the location of the hearing
  • It is a spiral-shaped organ with canals
    containing fluid.

  • The Ear
  • Sound waves strike the tympanic membrane, or
  • The vibrations of the eardrum cause three very
    tiny bones, the malleus, the incus, and the
    stapes, (literally the hammer, anvil and stirrup)
    to work to make the sound waves become stronger
  • The stirrup causes the cochlea to vibrate.
  • This vibration displaces hair cells along the
    basilar membrane within the cochlea.

  • Figure 4.21 When sound waves strike the eardrum
    (a), they cause it to vibrate. The eardrum is
    connected to three tiny bonesthe hammer, anvil,
    and stirrupthat convert the sound wave into a
    series of strong vibrations in the fluid-filled
    cochlea (b). Those vibrations displace the hair
    cells along the basilar membrane in the cochlea,
    which is aptly named after the Greek word for
    snail. Here the dimensions of the cochlea have
    been changed to make the general principles clear.

  • The Ear
  • The hair cells are connected to neurons of the
    auditory nerve.
  • The auditory nerve transmits the impulses from
    the cochlea to the cerebral cortex.

  • Hearing Loss
  • There are two common forms of deafness.
  • Conduction deafness results when the three
    special bones in the ear fail to transmit sound
    waves properly to the cochlea.
  • Nerve deafness results from damage to the
    structures that receive and transmit the impulses
    - the cochlea, hair cells or auditory nerve.

  • Pitch Perception
  • Adult humans can hear sound waves approximately
    between 15 and 20,000 Hz.
  • How we hear pitch depends in part on the
    frequency to which we are listening.

  • Pitch Perception
  • At low frequency (up to about 100 Hz), we hear by
    the workings of the frequency principle.
  • Sound waves passing through the fluid in the
    cochlea cause all the hair cells to vibrate,
    producing action potentials that are synchronized
    with the sound waves.

  • Pitch Perception
  • At about 100-4000 Hz, we hear by the workings of
    the volley principle.
  • Fewer hair cells can fire at this pace, but those
    that do respond in groups, called volleys, and
    produce action potentials.
  • Volleys are the chief mechanism for transmitting
    most speech and music to the brain.

  • Figure 4.22 The auditory system responds
    differently to low-, medium-, and high-frequency
    tones. (a) At low frequencies hair cells at many
    points along the basilar membrane produce
    impulses in synchrony with the sound waves. (b)
    At medium frequencies different cells produce
    impulses in synchrony with different sound waves,
    but the group as a whole still produces one or
    more impulses for each wave. (c) At high
    frequencies only one point along the basilar
    membrane vibrates hair cells at other locations
    remain still.

  • Pitch Perception
  • Beyond 4000 Hz, we hear by the workings of the
    place principle.
  • The place principle states that the location of
    the hair cells stimulated by the sound waves
    depends on their frequency.
  • The highest frequency sounds vibrate hair cells
    near the stirrup.
  • Between 100 and 4000 Hz, we are hearing due to
    the combined effects of the volley and place

Concept Check
  • You are listening to your mother on the
    telephone. Which principle(s) of hearing are
    operating to help you hear her?

Volley and place
  • Localization of sounds
  • How does the auditory system determine the source
    of a sound?
  • To estimate the approximate location of origin of
    a sound, the auditory system compares the
    messages received by the two ears.
  • The sound waves will arrive at the closer ear
    slightly sooner (if coming from the right, it
    arrives at the right ear just a little before it
    arrives at the left ear.)

  • Figure 4.23 The stereophonic hearing of our ears
    enables us to determine where a sound is coming
    from. The ear located closest to the sound will
    receive the sound waves first. A change of less
    than one ten-thousandth of 1 second can alter our
    perception of the location of a sound source.

  • Localization of sounds
  • The distance of a sound can be estimated based on
    loudness and pitch.
  • A sound that is growing louder is interpreted as
  • A higher frequency sound is interpreted as nearer
    than a low frequency sound a sound that is
    increasing in pitch is interpreted as
  • The only cue for absolute distance is the amount
    of reverberation experienced by the listener.

Concept Check
  • If a person who uses hearing aids in both ears is
    only wearing one in the right ear, what will be
    the effect on sound localization?

Sounds may be interpreted as coming from the
right even when they arent.
Concept Check
  • Why is it hard to tell whether a sound originates
    in front or behind you?

Because the sounds arrive in both ears at the
same time, there is no basis for comparison of
the source of the sound.
The Vestibular Sense
  • What we generally call balance is the vestibular
  • The vestibule is a structure in the inner ear on
    each side of the head.
  • Changes in the position of the vestibule cause
    receptors to be stimulated.
  • These receptors tell the brain the direction of
    tilt, amount of acceleration and position of the
    head with respect to gravity.
  • The vestibular sense plays a crucial role in
    maintaining balance and posture.

The Vestibular Sense
  • The structure of the vestibular system
  • Three semicircular canals are oriented in three
  • These canals contain a jellylike substance and
    are lined with hair cells.
  • Acceleration causes the jellylike substance to
    move the hair cells, stimulating them.

The Vestibular Sense
  • The structure of the vestibular system
  • Hair cells are also contained in two otolith
  • The otoliths are calcium carbonate particles.
  • These particles stimulate different sets of hair
    cells, depending on which way the head tilts.
  • They are telling your brain which way is up.

  • Figure 4.24 (a) Location of and (b) structures of
    the vestibule. (c) Moving your head or body
    displaces hair cells that report the tilt of your
    head and the direction and acceleration of

The Cutaneous Senses
  • Touch is actually considered to be several
    independent senses
  • Pressure
  • Warmth and Cold
  • Pain
  • Vibration
  • Movement and Stretch of Skin
  • These sensations depend on several different
    kinds of receptors.

The Cutaneous Senses
  • These are most noticeable in our skin, but we do
    have the same receptors in our internal organs,
    allowing us to feel internal pain, pressure, and
    temperature changes.
  • Therefore we also refer to these senses as
    comprising the somatosensory system.

  • Figure 4.25 Cutaneous sensation is the product of
    many kinds of receptors, each sensitive to a
    particular kind of information.

The Cutaneous Senses
  • The primary somatosensory cortex
  • In certain areas, such as the fingertips and
    lips, there are proportionally many more
    cutaneous receptors.
  • These areas also are allotted more tissue in the
    parietal lobes of the human cerebral cortex.
  • Most humans with no impairment in these areas are
    very good at identifying familiar objects by
    touch alone.

The Cutaneous Senses
  • Pain
  • Pain receptors are simple nerve endings that
    travel to the spinal cord.
  • The perception of pain is a complex mixture of
    sensation and perception that is in part mediated
    by emotion.
  • Two different areas of the brain govern sensory
    and emotional interpretations.
  • This is one reason that at least some people can
    be distracted or use self-hypnosis to manage
    reactions to pain.

The Cutaneous Senses
  • The gate theory of pain
  • Just seeking treatment or believing that one has
    been treated can result in a reduction of
  • The effectiveness of placebos in reducing the
    experience of pain has been well supported by
    experimental studies.
  • A variety of processes can increase or decrease
    pain to injured areas of the body.

  • Figure 4.26 Pain messages from the skin are
    relayed from spinal cord cells to the brain.
    According to the gate theory of pain, those
    spinal cord cells serve as a gate that can block
    or enhance the signal. The proposed neural
    circuitry is simplified in this diagram. Green
    lines indicate axons with excitatory inputs red
    lines indicate axons with inhibitory inputs.

The Cutaneous Senses
  • The gate theory of pain
  • On the basis of these observations, Metzack and
    Wall (1965) proposed the gate theory of pain.
  • This is the theory that pain messages must pass
    through a gate, thought to be in the spinal
  • This gate can block the messages.

The Cutaneous Senses
  • Neurotransmitters and pain
  • Substance P is a neurotransmitter that the
    nervous system releases for intense pains.
  • Reactions to painful stimuli are reduced in
    animals that lack substance P.

  • Figure 4.27 Substance P is the neurotransmitter
    most responsible for pain sensations. Endorphins
    are neurotransmitters that block the release of
    substance P, thereby decreasing pain sensations.
    Opiates decrease pain by mimicking the effects of

The Cutaneous Senses
  • Neurotransmitters and pain
  • Endorphins, which are chemically identical to
    opiates, are released by the nervous system in
    response to the release of substance P.
  • They effectively weaken pain sensations.
  • Endorphin release can also be induced by sensory
    experiences such as listening to music or sexual

The Cutaneous Senses
  • Neurotransmitters and pain
  • Capsaicin is the chemical that is present in hot
  • It stimulates receptors that respond to painful
  • It causes the release of substance P and depletes
    supply of it in the nervous system.
  • Creams containing capsaicin can be used to
    relieve muscle pain.

Phantom Limbs
  • A fascinating phenomenon in neuroscience now
    under study is the experience of phantom limbs
  • In phantom limb phenomenon, an amputee feels a
    missing body as if it were still there
  • These were once thought to be an emotional
    reaction or mere irritation of the stump of the
    missing limb

Phantom Limbs
  • The sensations are now understood to be produced
    by activity in the neurons of areas in the
    somatosensory cortex adjacent to the area once
    belonging to the missing limb
  • For example, the neurons of the face area are
    adjacent to the hand area of the somatosensory
  • These face neurons may occasionally produce a
    feeling of a phantom hand by stimulating the area
    that once registered only sensations from the hand

The Chemical Senses
  • Taste and smell are jointly referred to as the
    chemical senses. Many invertebrates rely almost
    entirely on these senses other mammals use them
    much more heavily than do humans.

The Chemical Senses
  • Taste
  • The sense of taste detects chemicals on the
  • Its major function is to control and motivate our
    eating and drinking.
  • The taste buds are located in the folds on the
    surface of the tongue. They contain the vast
    majority of human taste receptors.

  • Figure 4.29 (a) The tongue is a powerful muscle
    used for speaking and eating. Taste buds, which
    react to chemicals dissolved in saliva, are
    located along the edge of the tongue in adult
    humans but are more widely distributed in
    children. (b) A cross section through part of the
    surface of the tongue showing taste buds. (c) A
    cross section of one taste bud. Each taste bud
    has about 50 receptor cells within it.

The Chemical Senses
  • Taste receptors
  • Traditionally the view from Western medicine has
    held that there are four primary tastes sweet,
    sour, salty and bitter.
  • The flavor of monosodium glutamate (MSG), a
    common ingredient in Asian cooking, may represent
    a fifth.
  • Researchers are using the word umami for this
    fifth type of taste receptor.

The Chemical Senses
  • Olfaction
  • Olfaction is another term for the sense of smell.
  • The receptors for smell are located in the mucous
    membranes in the rear air passages of the nose.
  • They detect the presence of airborne molecules of

  • Figure 4.31 Olfaction, like any other sensory
    system, converts physical energy into a complex
    pattern of brain activity.

The Chemical Senses
  • Olfaction
  • We are aware now that there are at least hundreds
    of types of olfactory receptors (contrast this
    with the number of types of visual receptors.)
  • Other mammals have far many more receptors than
    humans do.
  • Each type of olfactory receptor is extremely
    specialized for one small group of closely
    related chemicals.

  • Figure 4.30 The olfactory receptor cells lining
    the nasal cavity send information to the
    olfactory bulb in the brain. There are at least
    100 types of receptors with specialized responses
    to airborne chemicals.

The Chemical Senses
  • Olfaction
  • Smell is vital for food selection.
  • Neurons in the prefrontal cortex receive both
    taste and olfactory input, and combine them to
    produce the perception of flavor.
  • The olfactory tract also bypasses the relay
    system in the thalamus.
  • It travels to the olfactory bulb, a structure in
    the base of the brain that is directly in contact
    with the limbic system

The Chemical Senses
  • Olfaction
  • Especially in nonhuman mammals, olfaction plays a
    vital social role.
  • These animals rely heavily on pheromones,
    chemicals that they release into the environment.
  • Pheromones are important for sexual
    communication, acting upon the vomeronasal organ
    to send messages to other individuals regarding
    fertility and sexual receptivity.

The Chemical Senses
  • Olfaction
  • Humans prefer not to rely upon the social
    influences of pheromones and olfaction.
  • But there is some evidence that they play a role
  • In one study, it was shown that female college
    students who room together tend to have
    synchronized menstrual cycles.

Sensory Systems
  • The world that is sensed by a cat, a snail, or a
    bat is very different that the world that is
    sensed by you and me.
  • The function of our senses is to give us the
    information that we need most to survive and
    thrive in our environment.

Module 4.3
  • The Interpretation of Sensory Information

Perception of Minimal Stimuli
  • Thresholds
  • Early psychological researchers thought it would
    be relatively simple to determine the weakest
    possible stimuli that humans could detect.
  • They were wrong.

  • Figure 4.33 Typical results of an experiment to
    measure a sensory threshold. There is no sharp
    boundary between stimuli that you can perceive
    and stimuli that you cannot perceive.

Perception of Minimal Stimuli
  • Thresholds
  • It was soon discovered that no sharp line exists
    between stimuli that a person can detect and
    those that they cannot.
  • Therefore, a sensory threshold was defined as
    intensity at which a given individual can detect
    a stimulus 50 of the time.
  • There are no guarantees however that an
    individual will report all the stimuli above the
    threshold, or fail to report all those below it.

Perception of Minimal Stimuli
  • Thresholds
  • The environment (i.e. lighting conditions) will
    also influence the individuals thresholds.
  • The absolute threshold has been defined as the
    sensory threshold at the time of maximum
    sensitivity that is, when conditions would allow
    for the best possible receptivity to the stimulus.

  • Figure 4.34 People can make two kinds of correct
    judgments (green backgrounds) and two kinds of
    errors (red backgrounds). Someone who too readily
    reports the stimulus present would get many hits,
    but also many false alarms.

Perception of Minimal Stimuli
  • Signal detection theory
  • When trying to detect relatively weak stimuli,
    people can be correct and incorrect in two
    different ways, respectively.
  • A hit is a correct detection of an actual
  • A correct rejection occurs when no stimulus is
    presented and no detection is claimed.
  • A miss is an incorrect rejection when a stimulus
    actually is presented.
  • A false alarm is an incorrect detection when no
    stimulus is presented.

Perception of Minimal Stimuli
  • Signal detection theory
  • Signal-detection theory is the study of peoples
    tendencies to make hits, correct rejections,
    false alarms, and misses.
  • Several factors work together to influence the
    rates of these outcomes.
  • The response in each trial does depend on what
    the persons senses are conveying.
  • But an individuals responses may also depend on
    their willingness to take a risk of an incorrect
    response, and on the emotions that a particular
    stimulus might evoke.

  • Figure 4.35a Results of an experiment to measure
    a sensory threshold using two different sets of
    instructions (with first version of instructions.)

  • Figure 4.35b Results of an experiment to measure
    a sensory threshold using two different sets of
    instructions (with second version of

Perception of Minimal Stimuli
  • Subliminal Perception
  • The concept of subliminal perception is well
    known to the general public.
  • Subliminal perception is the idea that a stimulus
    can influence behavior even when it is so weak or
    brief that we do not perceive it consciously.
  • There is concern that subliminal perception can
    powerfully manipulate human behavior.

Perception of Minimal Stimuli
  • What does subliminal mean?
  • When the term subliminal is used, it refers to
    the quality of being below the (sensory)
  • Scientists use it to indicate that the stimulus
    was not consciously detected in a given
  • Because the only way to know if a stimulus has
    been detected is to ask, it is very difficult to
    interpret the results of research on subliminal

Perception of Minimal Stimuli
  • What subliminal perception cannot do
  • Claims that subliminal stimuli in advertisements
    can make people buy things are unsupportable.
  • This claim has been tested repeatedly and no
    evidence has been found.
  • Advertisements in American culture have little
    need of subliminal stimuli. They are overtly and
    effectively manipulative.

Perception of Minimal Stimuli
  • What subliminal perception cannot do
  • Messages in music (recorded backwards or
    superimposed) cannot make people do anything,
    evil or otherwise.
  • This claim has also been repeatedly tested under
    controlled conditions.
  • No one listening to the messages can discern
    these messages.
  • No ones behavior has been changed after
    listening to music containing messages.

Perception of Minimal Stimuli
  • What subliminal perception cannot do
  • Subliminal audiotapes just dont work
  • Claims that addictions can be overcome,
    self-esteem can be improved, and general
    self-improvement can be achieved through the use
    of subliminal audiotapes are also unsupported.
  • Any results achieved through the use of these
    tapes can be attributed to the placebo effect or
    to the individual users motivation to improve.

Perception of Minimal Stimuli
  • What subliminal perception can do
  • Some subtle effects on subsequent perception and
    emotion have been supported
  • Priming individuals to see an object in
    subsequent presentations has been achieved
    through repeated presentations (Bar Biederman,
  • Emotional states can be influenced by subliminal
    presentation of messages that may be perceived as
    emotionally loaded (Masling et al., 1991)

Perception of Minimal Stimuli
  • Subliminal perception
  • The fact that subliminal perception can influence
    behavior at all is interesting.
  • But the effects overall are much smaller than
    people hope or fear.

Perception and Recognition of Patterns
  • Brightness contrast
  • There are interesting fundamental questions to
    answer in the area of perception
  • How does your brain decide how bright an object
  • The apparent brightness of an object that you are
    looking at can be increased or decreased by the
    objects around it.
  • This phenomenon is called brightness contrast.

Perception and Recognition of Patterns
  • Face recognition
  • There are several interesting processes involved
    in face recognition
  • To some extent, we use unusual characteristics to
    recognize faces.
  • Most people recognize faces as a synthesized
    whole configuration of features.
  • There seems to be a module in the brain devoted
    to face recognition. If this area is damaged, it
    is possible to lose the ability to recognize
  • Children who have been diagnosed with autism also
    are much poorer than average at face recognition.

The Feature-Detector Approach
  • One explanation for how we analyze complex
    stimuli suggests that we break them down into
    component parts
  • We have feature detectors, specialized neurons
    that respond to the presence of certain simple
    features, such as angles and lines.
  • For example, one feature detector might be
    stimulated only by the presence of vertical
    lines, or 90? angles.
  • Feature detectors are essential for the first
    stages of analysis, but perception of complex
    stimuli requires other processes as well.

The Feature-Detector Approach
  • Hubel Wiesels experiments
  • Important evidence for the existence of feature
    detectors comes from the Nobel Prize winning
    research of Hubel and Wiesel (1981).
  • They inserted thin electrodes into cells of the
    visual cortex in monkeys and cats and recorded
    activity of those cells when different light
    patterns were shown to the animals.

  • Figure 4.40 Hubel and Wiesel implanted electrodes
    to record the activity of neurons in the
    occipital cortex of a cat. Then they compared the
    responses evoked by various patterns of light and
    darkness on the retina. In most cases a neuron
    responded vigorously when a portion of the retina
    saw a bar of light oriented at a
  • particular angle. When the angle of the bar
    changed, that cell became silent but another cell

The Feature-Detector Approach
  • Hubel Wiesels experiments
  • The researchers were able to identify cells that
    fired only in the presence of vertical bars of
    light, and others that only fired for horizontal
  • In later experiments, they found cells that only
    fired in response to movement in particular

The Feature-Detector Approach
  • The waterfall illusion experienced by humans is
    evidence that humans do indeed have feature
  • In this illusion, a person first stares at a
    waterfall for one minute or more.
  • If the person then looks at cliffs immediately
    after staring at the waterfall, the cliffs will
    appear to flow upward.
  • This suggests that the cells that detect downward
    motion have become fatigued from the act of
    staring at the waterfall.

The Feature-Detector Approach
  • Do feature detectors explain perception?
  • Scientists believe that feature detectors are
    just a first step in a series of complex
    processes that create perception.
  • Simple visual illusions such as the Necker cube
    suggest that we must also actively impose meaning
    on images that we see.
  • There is a branch of psychology that specializes
    in explaining how humans arrive at the integrated
    whole images and make meaningful interpretations
    of the visual world.

Gestalt Psychology
  • Gestalt psychology focuses on the human ability
    to perceive overall patterns.
  • The word Gestalt has no true English equivalent,
    but is close to synonymous with pattern or
  • According to Gestalt psychologists, visual
    perception is an active creation, not merely the
    adding up of lines and movement.

Gestalt Psychology
  • Principles of Gestalt Psychology
  • When looking at an image, we make a distinction
    between figure and ground.

Gestalt Psychology
  • Principles of Gestalt psychology
  • This is a picture of a reversible figure a
    stimulus that can be perceived in more than one
    way. When we decide which side is the front of
    the object, then we will see it as a stable
    image. We are imposing order on an array, not
    just adding up small features.

Gestalt Psychology
  • Principles of Gestalt psychology
  • The principle of proximity states that humans
    tend to perceive objects close together as
    belonging to a group.
  • The principle of similarity states that we
    perceive objects that resemble each other as
    forming a group.

Gestalt Psychology
  • Principles of Gestalt psychology
  • We may perceive continuation, and fill in gaps in
    lines, or closure of familiar figures.
  • We tend to perceive a good figure, one that is
    simple and symmetrical.
  • Gestalt visual principles have analogs in the
    perception of sound.

  • Figure 4.46 Gestalt principles of (a) proximity,
    (b) similarity, (c) continuation, (d) closure,
    and (e) good figure.

Perception of Movement and Depth
  • Visual constancy
  • Visual constancy is our tendency to perceive
    objects as keeping their size, shape and color
    even though the image that strikes our retina
    changes from moment to moment.
  • So an automobile that is driving away looks like
    it is moving away, not merely shrinking, even
    though the image on our two retinas is growing

  • Figure 4.50 (a) Shape constancy We perceive all
    three doors as rectangles. (b) Size constancy We
    perceive all three hands as equal in size.

Perception of Movement
  • Motionblindness can result from damage to a small
    area of the temporal lobe.
  • This fact is further evidence that the visual
    system analyzes different aspects of an image via
    different pathways in the brain.

Perception of Movement
  • How do we distinguish between our own movement
    and the movement of objects?
  • The vestibular system works to keep the visual
    system informed of the movements of your head.
  • We see motion when an object is moving relative
    to the background.
  • When an object is stationary and the background
    is moving, we may experience induced movement, a
    visual illusion in which we incorrectly perceive
    the object as moving.

Perception of Movement
  • Stroboscopic movement is an illusion of movement
    created by a rapid succession of stationary
    images. Animation and motion pictures work by
    stroboscopic movement.
  • The phi effect, in which your brain creates
    motion from rows of adjacent lights blinking on
    and off sequentially, is exploited by many a
    nightclub and motel owner.

Depth Perception
  • Our retinas are two-dimensional surfaces, but
    they give us very good depth perception our
    ability to perceive distance.
  • There are several factors involved in creating
    our depth perception.
  • Some are binocular cues (depending on both eyes)
    and others are monocular (needing only one eye.)

Depth Perception
  • Binocular cues
  • One important contributor is retinal disparity,
    which is the difference in apparent position of
    an object seen by each retina.
  • This discrepancy allows us to gauge distance.
  • Convergence is the degree to which our eyes must
    turn in to allow us to focus on a very close

  • Figure 4.53 Convergence of the eyes as a cue to
    distance. The more this viewer must converge her
    eyes toward each other in order to focus on an
    object, the closer the object must be.

Depth Perception
  • Monocular cues
  • Monocular cues allow a person to judge depth and
    distance accurately using only one eye.
  • Object size can be used if we already have an
    idea of the approximate size of the objects.
  • Linear perspective is used when parallel lines
    are drawn so that they converge as they approach
    the horizon.
  • Detail generally objects that are closer can be
    seen in greater detail than those that are
    farther away.

Depth Perception
  • Monocular cues
  • Interposition nearby objects will obstruct
    objects that are farther away.
  • Texture gradient refers to the fact that clusters
    of objects will seem more densely packed the
    farther away the clusters are.
  • Shadows give clues to distance depending on size
    and position.

Depth Perception
  • Monocular cues
  • Accommodation, as you will recall, is how the
    lens changes shape to focus on objects, growing
    thinner to focus on nearby objects and thicker to
    focus on close things.
  • Motion parallax is the principle that close
    objects will pass by faster than distant objects.

Optical Illusions
  • An optical illusion is a misinterpretation of a
    visual stimulus.
  • Psychologists are attempting to find a
    parsimonious explanation for these
  • Many can be explained by considering the
    relationship between size perception and depth

Optical Illusions
  • When we misjudge distance, we misjudge size as
  • For example, the Ames room illusion causes us to
    misjudge the heights of people standing in it
    using a powerfully misleading set of background
  • We see an immensely tall and a very short person,
    but once we remove all the misleading cues, we
    realize that they are people of similar height
    standing at different distances in relation to us.

  • FIGURE 4.59b The Ames room is a study in
    deceptive perception, designed to be viewed
    through a peephole with one eye. (b) This diagram
    shows the positions of the people in the Ames
    room and demonstrates how the illusion of
    distance is created. (Wilson et al., 1964)

  • Figure 4.57 The trade-off between size and
    distance A given image on the retina can
    indicate either a small, close object or a large,
    distant object.

  • Figure 4.61 Many optical illusions depend on
    misjudging distances. The jar on the right seems
    larger because the context makes it appear
    farther away.

Optical Illusions
  • Even a two-dimensional drawing can contain cues
    that lead to the erroneous perception of depth.
  • The drawings of M.C. Escher work by this

  • Figure 4.60 These two-dimensional drawings puzzle
    us because we try to interpret them as
    three-dimensional objects.

Optical Illusions
  • Vision plays a prominent role in some auditory
  • Visual capture effect is the tendency to identify
    a sound as coming from a visually prominent
    source rather than its actual source. The
    inaccurate judgment of sounds distance leads us
    also to misjudge its intensity.
  • Ventriloquism works using this auditory illusion.

Optical Illusions
  • Cross-cultural influences
  • It is thought that how an individual sees the
    Muller-Lyer illusion is partly influenced by
    cultural and other factors.
  • The illusion is stronger for city dwellers and
    for children.
  • This suggests that experience with buildings and
    with drawings of objects may have some impact on
    interpretation of two-dimensional images.

Optical Illusions
  • The moon illusion
  • To most people, the moon appears to be about 30
    larger when it is close to the horizon.
  • Measuring it with navigational equipment will
    prove to you that it is in fact the same size.
  • It is hard to explain exactly why this illusion
    occurs, but it probably is influenced by our
    tendency to use background cues for reference in
    judging size.

Optical Illusions
  • The moon illusion
  • When the moon is at the horizon, we can compare
    it to the other familiar objects and the
    interposed terrain, so we judge it to be very
  • When it is high in the sky, we have no basis to
    gauge its distance at all. We unconsciously judge
    the horizon moon to be more distant, and
    therefore larger.
  • This latter explanation fits with the general
    notion that optical illusions are a product of
    misjudgments of size and distance.

Visual Illusions and Perception
  • The moon illusion and all that we are learning
    about visual perception and misperception
    reinforce an important point.
  • What you are seeing is not out there its in
    your brain.
  • Vision is usually an accurate if complex
    reconstruction of the world around us, but we can
    be very, very mistaken about what we think we see.