Chapter 4 Sensation and Perception

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Chapter 4Sensation and Perception
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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
  information.

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Module 4.1

• Vision

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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.

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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
  energy.
• The human eye is designed to detect energy in the
  wavelengths from 400 to 700 nm.

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• 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.

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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,
  too.

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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
  distances.

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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.

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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
  nearsightedness.
• Flattened eyeballs cause hyperopia, so that the
  person can focus well on distant objects, but not
  on nearby ones. This is also called
  farsightedness.

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• 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

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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.

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Concept Check

• What happens if a person with normal vision puts
  on contact lenses designed for a person with
  myopia?

His vision, especially his near vision, will be
blurry.
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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.

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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.

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• TABLE 4.1 Differences between rods and cones

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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
  acuity.
• Rods are more plentiful in the periphery of the
  retina.

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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
color.
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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.

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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.

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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
  retinaldehydes.
• The relative abundance of rods, and their
  undisturbed regeneration of the chemical, gives
  them a much higher level of sensitivity to faint
  light

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• 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)

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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.
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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.
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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.

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• 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.

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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.

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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.

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• 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.

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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.

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The Visual Pathway

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

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• 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.

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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.

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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.

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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
  see.

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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.

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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.

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

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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.

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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.

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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
  opposites.
• Red vs. green
• Yellow vs. blue
• White vs. black

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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.

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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.
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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.
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Color Vision

• The Retinex Theory
• The trichromatic and opponent-process theory
  dont account for our experience of color
  constancy.
• 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.

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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
  retinex.)
• By comparing different patterns of light from
  different areas of the retina, cortical cells
  synthesize a color perception for each area.

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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.

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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.

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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.

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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.

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How We See

• Before animals could see color, there was no
  color.
• 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.

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Module 4.2

• The Nonvisual Senses

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Hearing

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

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• 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
  loudness.

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Hearing

• The perception of frequency is referred to as
  pitch.
• We perceive a high-frequency sound wave as
  high-pitched, and a low-frequency wave as
  low-pitched.
• 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.

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Hearing

• 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
  receptors.
• It is a spiral-shaped organ with canals
  containing fluid.

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Hearing

• The Ear
• Sound waves strike the tympanic membrane, or
  eardrum.
• 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
  signals.
• The stirrup causes the cochlea to vibrate.
• This vibration displaces hair cells along the
  basilar membrane within the cochlea.

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• 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.

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Hearing

• 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.

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Hearing

• 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.

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Hearing

• 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.

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Hearing

• 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.

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Hearing

• 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.

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• 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.

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Hearing

• 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
  principles.

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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
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Hearing

• 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.)

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• 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.

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Hearing

• 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
  approaching.
• A higher frequency sound is interpreted as nearer
  than a low frequency sound a sound that is
  increasing in pitch is interpreted as
  approaching.
• The only cue for absolute distance is the amount
  of reverberation experienced by the listener.

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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.
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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.
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The Vestibular Sense

• What we generally call balance is the vestibular
  sense.
• 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.

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The Vestibular Sense

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

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The Vestibular Sense

• The structure of the vestibular system
• Hair cells are also contained in two otolith
  organs.
• 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.

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• 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
  movement.

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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.

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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.

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

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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.

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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.

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The Cutaneous Senses

• The gate theory of pain
• Just seeking treatment or believing that one has
  been treated can result in a reduction of
  symptoms.
• 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.

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• 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.

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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
  cord.
• This gate can block the messages.

84
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.

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• 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
  endorphins.

86
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
  activity.

87
The Cutaneous Senses

• Neurotransmitters and pain
• Capsaicin is the chemical that is present in hot
  peppers.
• It stimulates receptors that respond to painful
  heat.
• 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.

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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

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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
  cortex
• These face neurons may occasionally produce a
  feeling of a phantom hand by stimulating the area
  that once registered only sensations from the hand

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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.

91
The Chemical Senses

• Taste
• The sense of taste detects chemicals on the
  tongue.
• 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.

92

• 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.

93
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.

94
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
  chemicals.

95

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

96
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.

97

• 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.

98
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

99
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.

100
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
  anyway.
• In one study, it was shown that female college
  students who room together tend to have
  synchronized menstrual cycles.

101
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.

102
Module 4.3

• The Interpretation of Sensory Information

103
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.

104

• 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.

105
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.

106
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.

107

• 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.

108
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
  stimulus.
• 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.

109
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.

110

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

111

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

112
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.

113
Perception of Minimal Stimuli

• What does subliminal mean?
• When the term subliminal is used, it refers to
  the quality of being below the (sensory)
  threshold.
• Scientists use it to indicate that the stimulus
  was not consciously detected in a given
  presentation.
• 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
  stimuli.

114
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.

115
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.

116
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.

117
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,
  1998)
• Emotional states can be influenced by subliminal
  presentation of messages that may be perceived as
  emotionally loaded (Masling et al., 1991)

118
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.

119
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
  is?
• 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.

120
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
  faces.
• Children who have been diagnosed with autism also
  are much poorer than average at face recognition.

121
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.

122
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.

123

• 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
  responded.

124
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
  bars.
• In later experiments, they found cells that only
  fired in response to movement in particular
  directions.

125
The Feature-Detector Approach

• The waterfall illusion experienced by humans is
  evidence that humans do indeed have feature
  detectors.
• 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.

126
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.

127
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
  configuration.
• According to Gestalt psychologists, visual
  perception is an active creation, not merely the
  adding up of lines and movement.

128
Gestalt Psychology

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

129
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.

130
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.

131
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.

132

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

133
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
  smaller.

134

• 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.

135
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.

136
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.

137
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.

138
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.)

139
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
  object.

140

• 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.

141
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.

142
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.

143
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.

144
Optical Illusions

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

145
Optical Illusions

• When we misjudge distance, we misjudge size as
  well.
• For example, the Ames room illusion causes us to
  misjudge the heights of people standing in it
  using a powerfully misleading set of background
  cues.
• 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.

146

• 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)

147

• 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.

148

• 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.

149
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
  principle.

150

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

151
Optical Illusions

• Vision plays a prominent role in some auditory
  illusions.
• 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.

152
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.

153
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.

154
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
  large.
• 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.

155
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.