CHAPTER 4
CHAPTER OUTLINE
Sensation and Perception
Overview: Sensation is a “bottom-up” process of converting physical energy into neural
energy that can be understood by the central nervous system.
It is the detection of stimuli. Perception is a “top-down” process of organizing,
integrating, and interpreting information.
Module 4.1: Vision
1) The Detection of Light: Light visible to the human eye is just a small part of the
electromagnetic spectrum, the continuum of all the frequencies of radiated energy
from the very long wave infrared end to the very short wave end (ultraviolet, X-rays,
and gamma rays).
a) Anatomy of the Eye
i) Light passes through the pupil, an opening in the eye that allows it to enter.
The pupil can change size, depending on level of lighting and the animal’s
overall state of sympathetic nervous system arousal. The iris contains the
muscle tissue that makes the pupil constrict or dilate.
ii) The cornea is a rigid, transparent, very thin layer of cells on the surface of the
eyeball. It focuses light uniformly and sends it back to the lens. The lens is a
flexible structure. It can stretch or contract to be thinner or thicker, which
enables the eye to accommodate and adjust the focus for objects at different
distances.
iii) Light passes through the pupil, strikes the lens, and travels through the
vitreous humor to the retina. The retina is the area at the back of each eyeball
where the visual receptor cells are.
b) Some common disorders of vision:
i) In middle age people develop presbyopia, an inability to focus on nearby
objects due to decreased flexibility of the lenses.
ii) Myopia, or nearsightedness, is an inability to focus on distant objects due to
an elongation of the eyeballs.
iii) Hyperopia, or farsightedness, is difficulty in focusing on near objects due to
eyeballs that are slightly flatter than normal.
iv) Glaucoma is a very serious disorder caused by increasing amounts of pressure
in the eyeballs, and it can damage the optic nerve and cause a progressive loss
of peripheral vision.
v) A cataract is a clouding of the lens that causes blindness. Cataracts can be
treated surgically.
c) The Visual Receptors: There are two types of visual or photoreceptors, the
specialized neurons in the retina. The cones are adapted for color, daytime, and
detailed vision. The rods are adapted for dim light conditions. The proportion of
cones is highest near the fovea or center of the retina; the area of greatest visual
acuity in the human eye.
d) Dark Adaptation: Dark adaptation is the gradual adjustment of vision we
101 experience when entering an area that is dark or very dimly lit. The mechanism is
mediated by the presence and regeneration of retinaldehydes in the visual
receptors. Cones and rods regenerate these chemicals at different rates. The cones
adapt more quickly, but are not nearly as sensitive as adapted rods.
e) The Visual Pathway:
i) Visual receptors initially send signals away from the brain, routing the
messages to bipolar cells, which in turn send them to the ganglion cells that
form the optic nerve, exiting the eye at an area called the blind spot due to the
absence of visual receptors in this location. There are also many smaller blind
spots due to the retina’s blood vessels. The brain “fills these in.”
ii) The signal moves on to the crossover point (the optic chiasm) where half of
the axons of each of the optic nerves are sent to the opposite side of the optic
tract and onto the visual cortex. Some axons send their information to the
cerebral cortex via the midbrain and thalamus, which help to integrate the
visual messages.
iii) Ultimately, each cell in the visual cortex receives input from one part of the
left retina and a corresponding part of the right retina. The alternation between
the pattern in the left retina and the pattern in the right retina is called
binocular rivalry. Because vision happens in the cerebral cortex, a person
with intact eyes and optic nerves may become blind if the visual cortex is
damaged.
2) Color Vision:
a) The Trichromatic Theory: Or the Young-Helmholtz theory, states that vision
depends on the relative response rate of three types of cones. Each type of cone is
sensitive to the wavelengths of light that correspond to: red, green, or blue. Each
wavelength produces a distinct pattern of responding in the three different types
of cones.
b) The Opponent-Process Theory: Trichromatic theory cannot explain all aspects of
color vision. Opponent process theory offers an alternative explanation related to
more complex aspects of color vision. Opponent process theory proposes that we
perceive color in terms of a system of paired opposites: red versus green, yellow
versus blue, and white versus black. Some interesting evidence for this theory is
that we experience negative afterimages: after staring at one color for a long
period of time, we see a hazy shadow of its opposite.
c) The Retinex Theory: Color is a feature that we perceive in relation to the light an
object is reflecting and in comparison to the other objects we are seeing. It is
interesting that objects appear as the same color under a multitude of lighting
conditions. This is color constancy. The theory that best explains these aspects of
color vision is the retinex theory: we perceive color through the cerebral cortex’s
comparison of retinal patterns.
d) The theories of color vision are not viewed as competing, rather, they are all
considered to be correct statements of phenomena that occur at different stages of
visual processing. They all offer correct explanations of various aspects of vision.
102 e) Color Vision Deficiency: Red-green colorblindness is the most common type of
this disorder. People who have this deficiency experience difficulty distinguishing
red from green and either red or green from yellow. There are three types of
colorblindness, depending on the cone that the afflicted person lacks. People who
have protanopia lack long-wave cones; those with deuteranopia lack mediumwavelength cones; and the rarest variety of all is called yellow-blue
colorblindness, or tritanopia, in which the person lacks short-wave cones.
f) World languages vary in number of color words, and in words for different colors.
English has more words for red than other languages. Some languages have words
referring to more than one color, others lack words for certain colors. The reasons
for these variations are not completely understood.
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Module 4.2: The Nonvisual Senses
1) Hearing:
a) The frequency of a sound wave is the number of cycles it goes through in a
second, designated hertz (Hz). Pitch is the psychological interpretation of
frequency. Loudness depends on the amplitude of sound waves. The waves strike
the tympanic membrane or “eardrum” and cause three tiny bones - the hammer,
anvil and stirrup - to vibrate. These bones work to strengthen the intensity of the
stimulus. The stirrup transmits the intensified signal to the cochlea, a fluid-filled,
snail-shaped organ. Its vibrations cause the hair cells along the basilar membrane
within in it to move and stimulate the axons that begin the auditory nerve.
b) There are two types of hearing loss. Conduction deafness occurs when the bones
connected to the eardrum fail to transmit sound waves properly to the cochlea.
Nerve deafness results from damage to the cochlea, the hair cells, or the auditory
nerve. It is possible to lose ability to detect low amplitudes or select frequencies—
in other words, to develop deficits for hearing soft sounds or high and medium
frequency sounds.
c) Pitch Perception: Pitch detection depends on the frequency of sound waves.
i) At low frequencies, the mechanism is the frequency principle. The basilar
membrane vibrates in synchrony with sound waves, producing action
potentials at the same frequency as the sound.
ii) At higher frequencies, the volley principle and the place principle, or the latter
alone, may be at work. Up to about 4000 Hz groups of hair cells (volleys)
respond to vibrations by producing corresponding action potentials—this is
the volley principle. The place principle, different frequency sound waves
causing vibrations at different places along the basilar membrane, also
operates. Between 100 and 4000 Hz, a combination of the volley principle and
the place principle work to transmit pitch information to the brain. Above
4000 Hz the place principle is the mechanism of perception.
d) Localization of Sounds:
i) The brain’s comparison of the messages coming from each ear localizes the
sound. Loudness and frequency indicate the relative distance of sound
sources, while reverberation can be used to gauge absolute distance.
103 2) The Vestibular Sense:
a) The vestibular sense provides information about the direction of tilt and amount
of acceleration of the head and its position with respect to gravity. This sense
plays a key role in posture and balance.
b) The ears contain the receptors of the vestibular system. There are three
semicircular canals oriented in three directions and filled with a jelly-like
substance.
c) There are also two otolith organs that contain hair cells, and otolith particles that
stimulate the hair cells. The otolith organs send information about the direction of
gravity based on movements in the liquid filling the semicircular canals and
signals from the otolith particles.
3) The Cutaneous Senses:
a) The cutaneous senses are skin senses, comprising what we call touch. It’s a
complex system and is composed of several partly independent senses including
skin pressure, warmth, cold, pain, vibration, movement, and stretch. Collectively,
these senses make up the somatosensory system.
b) Pain:
i) Pain receptors are bare (or free) nerve endings that send signals to the spinal
cord. Pain perception however is a complex interaction of sensation and
emotion. It serves a vital function in promoting survival but prolonged and
intense pain can be unnecessary and disruptive. Controlling pain perception is
an area of great interest in medicine and psychology.
ii) The Gate Theory of Pain: This proposes that pain messages pass through a
gate, presumably in the spinal cord that is able to block the messages. Many
processes either increase or decrease the amount of pain. Mechanisms that
reduce pain include distraction, placebos, and hypnosis. There are also
biological processes to be addressed in the pursuit of effective pain control.
iii) Pain messages cause the release of a neurotransmitter, substance P. At the
same time, other neurons release endorphins. These neurotransmitters,
chemically identical to the opiates that inhibit substance P, decrease pain
sensations.
iv) Ways to Decreases Pain: Paradoxical research findings indicate that inducing
pain can reduce it. Capsaicin, the chemical that makes chili peppers hot,
stimulates receptors that respond to painful heat, causing the release of
substance P at a rate too fast for neurons to resynthesize it. This leads to a
prolonged decrease in pain sensitivity.
c) Phantom Limbs: Individuals who lose a limb through amputation or accident may
experience feelings of touch, tingling, or pain in the part, as if it were still there.
For a long time scientists thought that this was emotionally generated or occurred
through stimulation of the stump of the missing limb. We know now that tissue in
the somatosensory cortex assigned to the now missing limb is still stimulated by
activity in adjacent areas. In fact, the area dedicated to the limb establishes these
connections instead of remaining idle in the brain.
4) The Chemical Senses:
a) Taste:
i) Taste detects chemicals on the tongue and provides information about what
104 we eat and drink. Taste receptors are in the taste buds, located in folds on the
tongue’s surface.
ii) Types of Taste Receptors: Different tastes depend on specific types of
receptors, corresponding to sweet, sour, salty, and bitter. Certain tastes have
been found in non-Western cuisine that cannot be fully described using the
four primary tastes (MSG in some Asian cuisines is an example of this type of
flavor) researchers have proposed another primary taste, umami. There is also
evidence that fat may have separate receptors. Whether there are 4, 5, or 6
types of taste receptors, all people have the same basic set. People in different
parts of the world have characteristic taste preferences due to familiarity, not
differences in sense of taste.
b) Smell:
i) Olfaction is the sense of smell. Olfactory receptors detect the presence of
airborne molecules. They relay corresponding messages to the olfactory bulbs
at the base of the brain. We are not sure how many different types of olfactory
receptors exist—no “primary smells” exist. It is possible that humans have
several hundred types of olfactory receptors, each having a specific channel of
information for a closely related group of chemicals. It may be as highly
specified as the immune system.
ii) Olfaction is important for food selection, and also serves social functions.
Pheromones are odorous chemicals through which animals (particularly other
mammals) identify one another. Humans prefer not to recognize each other by
smell, and take great pains to hide it.
5) Synesthesia: Synesthesia is an unusual condition in which a stimulus of one type,
such as sound, also gives rise to another sense experience, such as color. About 1 in
500 people is reported to experience synesthesia, but the actual rates may be higher.
The causes of this condition are not well understood.
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Module 4.3: The Interpretation of Sensory Information
1) Perception of Minimal Stimuli:
a) Sensory Thresholds and Signal Detection: There is no firm line between when a
stimulus is detectable and when it is not. A complex set of environmental and
individual factors influence the processes of sensation and perception. “Hits”
(correct detections) and “misses” (either incorrect detections or failures to detect)
will occur.
i) A sensory threshold is the intensity at which an individual can accurately
detect a stimulus at least 50% of the time. The absolute threshold is the
sensory threshold at maximum sensitivity.
ii) Signal-detection theory describes the process of making hits, correct
rejections, misses, and false alarms. People’s responses depend on
instructions, strategies, and biases as well as on actual sensory information.
b) Subliminal Perception: The idea that stimuli can be used to influence behavior
even when they are presented so faintly or briefly or along with such strong
distracters that we do not perceive them consciously. Such stimuli (it is claimed)
can dramatically manipulate human behavior, but this is difficult verify, because
105 it is challenging to determine what is “subliminal.”
i) What Subliminal Perception Cannot Do: Controlled scientific evidence does
not support assertions that subliminal stimuli can influence consumer
behavior, or effects of “satanic” messages supposedly embedded in rock
music (“backmasking”). There is no evidence that self-help tapes that contain
subliminal suggestions (to lose weight, etc.) can help alter behavior to achieve
such goals.
ii) What Subliminal Perception Can Do: Subliminal stimuli have subtle effects
on responding. For example, in one study subjects were subliminally “primed”
to recognize objects they previously had not been able to identify. Some
evidence suggests that subliminally presented emotional messages can alter
affect.
2) Perception and the Recognition of Patterns: How do we know what we are seeing? It
should be an easy question to answer but it is not. For example, the phenomenon of
brightness contrast shows that an object’s brightness is related to that of the objects
around it, not just its own apparent brightness.
a) Facial recognition is complicated. Complex patterns like faces are easy to
identify, but challenging to reconstruct, owing to the amount of detail. Evidence
suggests that recognition of faces is fundamentally different from recognizing
other objects. Many psychologists believe face recognition has its own “brain
module.”
b) The Feature-Detector Approach: One theory states that pattern recognition
operates via feature detectors in the visual cortex - specialized neurons in the
visual cortex that respond to the presence of certain simple features, such as lines
and angles. Feature detectors account for preliminary steps in complex visual
processing.
c) Gestalt Psychology: We possess an ability to perceive overall patterns. Gestalt
psychologists believe that the perception of a given stimulus cannot be broken
into component parts. Visual perception is an active process that is aided by
organizational principles such as figure and ground (distinguishing the object
from the background), proximity (the tendency to perceive objects close together
as a group), similarity (the tendency to perceive like objects as a group),
continuation (tendency to fill in gaps), and closure (tendency to imagine the rest
of an incomplete figure). Generally, we tend to perceive a good figure—a
“simple, symmetrical figure.” Gestalt principles apply to hearing as well as vision.
d) Feature Detectors and Gestalt Psychology: These two approaches to perception
are not in opposition to each other. Perception involves processes of assembling
individual pieces of information (this requires the ability to detect and identify
features), as well as organizing and reorganizing the information (the processes
described by Gestalt psychology).
3) Perceiving Movement and Depth: Objects that recede from us become smaller, yet we
see motion and not shrinking. This demonstrates visual constancy, the tendency to
perceive objects as keeping their shape, size, and color, even though the information
from the stimulus that reaches our retinas changes.
a) Perception of Movement: How do we distinguish between our own movement
and that of objects? The vestibular system informs visual areas of the brain about
106 head movements and helps in distinguishing our movements from those of things
we see. We perceive motion when an object moves relative to its background.
i) Our perceptions can also be incorrect. The phenomenon of induced movement
occurs when we perceive an object as moving against a background when in
reality the background is moving and the object is stationary (clouds move
across the moon on a clear night; we may perceive the moon moving and the
clouds staying still) - apparent movement.
ii) We also incorrectly perceive motion due to the illusion of movement created
by a rapid succession of stationary images called stroboscopic movement.—
this is how Bart Simpson, Stewie Griffin, and other cartoon characters come
to life.
iii) Moving neon signs are a good example of the phi phenomenon. This is the
illusion of movement that occurs when stationary lights placed very close
together blink on and off at different intervals.
b) Depth Perception: This is perception of distance. It depends on a number of
factors. Some are called binocular cues because they depend on the action of both
eyes.
i) Retinal disparity - the difference in the apparent position of an object as seen
by the left and right retinas, is one of these binocular cues. The amount of the
discrepancy is the gauge for the distance. The other is the convergence of our
two eyes, which is important for seeing close-up. It’s the degree to which the
eyeballs turn in to allow the retinas to focus on what we are viewing.
ii) Monocular cues allow us to judge depth and distance effectively with just one
eye. These are the same cues used by skilled painter to create the illusion of
depth on the two-dimensional surface of a canvas. These include object size,
linear perspective, detail, interposition, texture gradient, shadows, and
accommodation. A depth cue we use motion parallax—we judge a fast
moving object as closer than a slow moving object (think about a car whizzing
by you on the street and an airplane cruising above you in the sky.)
4) Optical Illusions: These are educational if amusing misinterpretations of visual
stimuli. Depth perception and size perception are related, therefore misjudgment of
distance and of size often go together. Two-dimensional drawings offer cues that lead
to erroneous depth perception. (In general, the fewer cues we have for size and
distance, the more likely we are to misjudge these features.) The brain frequently and
readily interprets two-dimensional drawings as if they are three-dimensional objects.
a) Auditory illusions are based on similar principles—if we incorrectly estimate the
distance to a sound source, we may misjudge the intensity of the sound.
b) The Moon Illusion: The moon illusion refers to the perception that the moon
appears about 30% larger when it is close to the horizon than when it is higher in
the sky. The explanation lies in the observer, not in the characteristics of the light
rays. Explanations for the illusion involve size comparisons between the moon
and surrounding objects, and the perceived distance of the moon when it is on the
horizon versus when it is overhead. It is a complex illusion.
107