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CHAPTER 4
Sensation
OUTLINE
I.
II.
SENSORY SYSTEMS
The first step in some systems occurs when the stimulus is modified by an accessory structure (the
lens of the eye or the outer ear). Transduction, the transformation of incoming energy into neural
activity, takes place at the sensory receptors. Changes in stimuli produce the greatest receptor
response; stimuli that remain at a constant level produce adaptation. Receptors stimulate sensory
neuron activity, which carries messages (except smell) to the thalamus and then to the cerebral
cortex.
A.
The Problem of Coding
Coding is the transformation of an object’s distinguishing physical characteristics into a
pattern of neural activity that precisely identifies those physical characteristics. According to
the doctrine of specific nerve energies, each sensory system will produce codes for only that
one sense. For example, any stimulation of the receptors in the eye, whether from light or
eye pressure, will produce only the sensation of light. Codes for the physical attributes of an
object can take different forms. Temporal codes cause changes in neural activity rates or
timing; spatial codes are represented by the physical location of neural activity.
B.
Linkages: Sensation and Biological Aspects of Psychology
Visual, auditory, and tactile information travels through the thalamus to a primary area of
sensory cortex. The information is contralaterally represented and is mapped out in
topographical representation. The density of neurons in the sense organ determines the
extent of its representation in the brain. Regions of primary sensory cortex are divided into
columns of cells that respond to similar stimuli in the environment. More complex
processing of sensory information occurs in the association cortex.
HEARING
Sound is a repetitive fluctuation in the pressure of a medium such as air. Vibrating objects create
the fluctuations in air pressure that create sound. A repetitive fluctuation or change in pressure is a
wave.
A.
B.
Sound
1. Physical Characteristics of Sound. Waves have three important characteristics:
amplitude, wavelength, and frequency (frequency is described in units called hertz).
2. Psychological Dimensions of Sound. The amplitude of the sound wave determines
loudness; loudness is described in units called decibels. The frequency of the sound
wave determines pitch, how high or low a tone is. Complex waveforms added to the
fundamental frequency that makes up a sound wave produce timbre, a sound’s quality.
The Ear
1. Auditory Accessory Structures. The pinna collects the sound waves, which then strike
the tympanic membrane, which in turn produces a vibration that causes the malleus
(hammer), incus (anvil), and stapes (stirrup) to vibrate.
2. Auditory Transduction. Vibrations pass through the oval window and the fluid in the
cochlea to move the basilar membrane, which stretches along the floor of the cochlea.
C.
D.
Movement of the basilar membrane causes tiny hair cells that touch it to move. Haircell movement causes neuron activity in the auditory nerve, which carries auditory
information to the brain. This process is known as auditory transduction.
3. Deafness. Deafness can be caused by problems with the bones in the inner ear
(conduction deafness) or by problems with the auditory nerve or hair cells (nerve
deafness).
Auditory Pathways, Representations, and Experiences
Information from the right auditory nerve crosses to the left side of the brain and passes
through the thalamus on its way to the primary auditory cortex. Cells in the primary auditory
cortex have preferred frequencies; they respond maximally to sounds of particular
frequencies. A temporal code is used to localize sounds.
Coding Intensity and Frequency
The firing rate of neurons within the auditory nerve increases as sound intensity increases.
Frequency is coded in one of two ways:
1.
2.
Place Theory. Place theory states that hair cells at a particular place on the basilar
membrane respond most to the wave peak associated with a particular frequency of
sound. High frequencies are coded exclusively by the place where the wave peaks.
Frequency Matching. The firing rate of a neuron in the auditory nerve matches the
frequency of a sound wave. The lowest frequencies are coded by frequency matching.
Low to moderate frequencies are coded both by frequency matching and by the place
on the basilar membrane where the traveling wave peaks. The volley theory is another
name for the frequency-matching theory.
III. VISION
A. Light
Visible light is electromagnetic radiation that has a wavelength from about 400 to 750
nanometers. We cannot see electromagnetic radiation or light wavelengths that are outside of
this range. Our sensation depends on light intensity and light wavelength.
B.
Focusing Light
Light waves pass through the cornea, the pupil, and the lens before being focused on the
retina. The iris adjusts the size of the pupil opening. The cornea and the muscles that hold
the lens in place focus the light on the retina through a process called accommodation.
C.
Converting Light into Images
Visual transduction, the conversion of light energy into neural activity, takes place in the
photoreceptors of the retina.
1.
2.
Photoreceptors. The photoreceptors contain photopigments that break down in reaction
to light and cause changes in photoreceptor membrane potentials. Dark adaptation, the
gradually increasing ability to see in the dark, occurs as new photopigments are
synthesized. Rods, one type of photoreceptor, contain the photopigment rhodopsin and
are more sensitive to light but cannot discriminate between colors. Cones, the other
type of photoreceptor, contain three varieties of iodopsin, which can detect color and
are more active in bright light. Cones are highly concentrated in the fovea, where
acuity is greatest.
Interactions in the Retina. (Examination of Figure 4.10 in your text will greatly
facilitate your understanding of this section.) Activity in the eye actually enhances the
visual image. Light energy stimulates photoreceptor cells, which stimulate bipolar
cells. Many photoreceptors converge on one bipolar cell. Photoreceptors also stimulate
interneurons, which signal (usually in an inhibitory manner) surrounding bipolar cells.
As a result, the photoreceptor that receives the greatest light energy stimulus will
D.
E.
F.
G.
stimulate one bipolar cell and, through lateral inhibition (involving interneurons),
decrease the stimulation of surrounding bipolar cells. As a result, the brain receives
messages of light contrasts or comparisons from two bipolar cells that represent
neighboring points in the visual field.
3. Ganglion Cells and Their Receptive Fields. Bipolar cells stimulate ganglion cells,
whose axons form the optic nerve that extends into the brain. Most ganglion cells have
center-surround receptive fields. Stimulation of ganglion cells enhances the sensation
of variations, such as edges and small spots of light or dark.
Visual Pathways
The axons of all the ganglion cells combine to form the optic nerve, which travels to the
brain. There are no photoreceptors at the point where the optic nerve leaves the eye, creating
a blind spot. At the optic chiasm, fibers carrying information about the right side of the
visual field cross over to the left side of the brain (see Figure 4.21 in your text). Beyond the
optic chiasm the fibers synapse in the lateral geniculate nucleus (LGN); from the LGN,
fibers go to the primary visual cortex, located in the occipital lobe. The cells in the cortex,
arranged in columns, form topographical maps of the visual world. Larger areas of the
cortex are devoted to areas of the retina that have many photoreceptors.
Visual Representations
1. Parallel Processing of Visual Properties. The LGN is organized into multiple layers of
neurons; each layer contains a complete map of the retina. Each also responds to
different aspects of visual stimuli: form, color, movement, and distance. There is no
place in the cortex at which all of this information merges. Instead, each layer
processes information in parallel (simultaneously) and communicates with the other
layers.
2. Hierarchical Processing of Visual Information. Cells in the cortex that respond to
specific features of objects are called feature detectors. Complex feature detectors are
built out of simpler feature detectors.
Seeing Color
1. Wavelengths and Color Sensation. Hue, saturation, and brightness are three
psychological aspects of color sensation. Additive color mixing, the mixing of
different wavelengths of light, always produces a lighter color. In subtractive color
mixing, continued combinations of color (as in paints) will produce black.
2. The Trichromatic Theory of Color Vision. According to the trichromatic theory, there
are three types of cones, each of which responds best to a different color (red, green, or
blue) or wavelength.
3. The Opponent-Process Theory of Color Vision. The opponent-process theory states
that the visual elements sensitive to color occur in three pairs and that the members of
each pair inhibit each other. Each element signals one or the other color in a pair, but
never both. The three element pairs, which contain complementary colors, are redgreen, blue-yellow, and black-white.
4. A Synthesis and an Update. Together, the two theories of color vision can explain color
vision. The existence of three types of cones that pick up information about red, green,
and blue color conforms to trichromatic theory. And the center-surround receptive
fields of the ganglion cells are color-coded in pairs, which correspond to the threeelement pairs of the opponent-process theory.
5. Colorblindness. People who are colorblind discriminate fewer colors than other people.
Their cones lack the color-sensitive pigments for red, green, or blue.
Interaction of the Senses: Synesthesia
Various dimensions of vision interact, and vision can also interact with hearing and other
senses in a process known as synesthesia. For example, some people experience certain
letters, numbers, or sounds as being in certain colors.
IV. THE CHEMICAL SENSES: SMELL AND TASTE
A. Olfaction
Olfaction, or smell, occurs when airborne chemicals are detected by receptor cells in the
upper part of the nose. Axons from the nose travel to the olfactory bulb, and from there
axons spread to many areas of the brain, especially to the amygdala. Chemicals called
pheromones can shape the behavior and physiology of animals. For instance, mammals lick
the pheromones of others of their kind and pass them into a specialized portion of the
olfactory system called the vomeronasal organ. Although no solid evidence exists to suggest
the action of pheromones in humans, it is known that people learn to associate smells with
sexual activity or certain people.
V.
B.
Gustation
Papillae contain receptors for taste, which can detect only sweet, sour, bitter, salty, umami,
and astringent flavors. Each taste bud in the papillae responds best to one or two of these
categories but weakly to the other categories. This is how gustation, our sense of taste,
operates.
C.
Smell, Taste, and Flavor
Flavor is a combination of smell and taste. The temperature and texture of food, along with
human nutritional states, can enhance flavor.
SOMATIC SENSES AND THE VESTIBULAR SYSTEM
The somatic senses, also known as the somatosensory systems, include the skin senses of touch,
temperature, and pain. They also include kinesthesia, the sense that tells the brain where the parts
of the body are relative to one another. The vestibular system tells the brain about the position of
the head in space.
A.
Touch and Temperature
Survival would be extremely difficult without a sense of touch.
1.
2.
3.
4.
B.
Stimulus and Receptors for Touch. There are many types of receptors in or somewhere
near the skin that respond to mechanical deformation of the skin. The fingers, which
have many receptors for touch, are the primary sensory apparatus for exploring by
touch.
Adaptation of Touch Receptors. We are most sensitive to changes in touch. When in
constant contact with a stimulus, the touch receptors show adaptation by decreasing the
rate at which they fire.
Coding and Representation of Touch Information. Information about the weight and
vibration of a stimulus is coded by the number of nerves stimulated and the frequency
at which individual nerves fire. Location is coded by the organization of the
information, which is topographical and contralateralized; it tells the brain where you
have been touched.
Temperature. Some skin receptors are sensitive to warmth and cold. Many receptors
that respond to temperature also respond to touch.
Pain
1. Pain as an Information Sense. Painful stimuli cause the release of chemicals, causing
pain nerves to fire. A-delta fibers and C fibers carry information about different types
of pain to the spinal cord, where they synapse with neurons that carry the pain signals
to the thalamus and other brain areas.
2.
C.
Emotional Aspects of Pain. Specific pathways carry an emotional component of the
painful stimulus to areas of the hindbrain and reticular formation, activating emotional
responses. Our expectations of the onset and intensity of pain affect our evaluation of
it.
3. Modulation of Pain: The Gate Control Theory. According to the gate control theory,
pain can be blocked in several ways at the spinal cord. Other sensory information,
aside from pain, can compete and take over pain pathways. The brain can produce
analgesia by sending signals down the spinal cord.
4. Natural Analgesics. Serotonin and endorphins are naturally occurring substances that
block synapses in fibers carrying pain signals. The body releases endorphins in painful
situations: when experiencing labor pains during childbirth; when eating very hot,
spicy food; and when people believe they are receiving a painkiller.
Thinking Critically: Does Acupuncture Relieve Pain?
What am I being asked to believe or accept?
Acupuncturists assert that twirling a needle in the skin can relieve pain caused by just about
any stimulus.
What evidence is available to support the assertion?
Well-controlled studies are surprisingly rare overall, and the results are contradictory.
However, acupuncture does stimulate the release of endorphins, which are naturally
occurring pain relievers.
Are there alternative ways of interpreting the evidence?
Endorphins can be activated by placebos, thus suggesting that acupuncture may not have an
entirely physical basis for pain relief.
What additional evidence would help to evaluate the alternatives?
Researchers need to study the relationship between internal painkilling systems and external
methods for stimulating them. In addition, an understanding of the types of pain for which
acupuncture is most effective may lead to a better understanding of the possible physical
effects of acupuncture.
What conclusions are reasonable?
Acupuncture does relieve certain types of pain, but it is not a panacea.
D.
Proprioception: Sensing Body Position
We know where we are and what each part of our body is doing through information
provided by the proprioceptive sensory system.
1.
E.
Vestibular Sense. Often called balance, the vestibular sense tells the brain about the
head and body’s position in space as well as about general movement. Fluid and
otoliths within the vestibular sacs, and fluid within the semicircular canals located near
the inner ear, shift as our position changes; the resulting movement of hair cells
stimulates neurons. The vestibular system has connections to the cerebellum, parts of
the autonomic nervous system, and the eyes.
2. Kinesthesia. The ability to know where the parts of your body are with respect to one
another is called kinesthesia. It operates through information sent from joint and
muscle-fiber receptors to the thalamus and then to the somatosensory cortex and the
cerebellum.
Focus on Research Methods: The Case of the Mysterious Spells
Linda was studied by Jozsef Janszky, a neurologist, for recurring spells that began with
orgasmic sensations, followed by a staring, unresponsive state. Using EEG and MRI
technology, Janszky found that Linda had abnormal tissue in her right temporal lobe which
made seizures likely to occur there. After removing the tissue surgically, her seizures in that
area stopped. It may be that the right temporal cortex is important in creating the sensory
experience of orgasm.