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asgn2m --VISION:
Color Vision
This exercise introduces color vision. It describes the three kinds of cones in the retina, which
create a labeled line (anatomical) code for color. The three kinds of cones respectively respond
most to the "red," the "green," or the "blue" part of the visual spectrum. So each wavelength
(“color”) of light activates its unique combination of "red,” "green,” and "blue.” Therefore, color
coding by the cone receptors corresponds to the Young-Helmholtz trichromatic theory. This
theory was originally based on the fact that any color in the spectrum can be matched by
appropriate fractions of red, green, and blue light.
It then describes how the signals from the cones get converted into a pair of opponent system:
Red-Green and Blue-Yellow. The brain uses this opponent organization throughout. This part of
color coding corresponds to Hering's Opponent process theory. This theory was originally based
on the fact the yellow is also a pure color and on its explanation of afterimages.
When you have finished this exercise you should understand:
! How the three kinds of cones code any color of the spectrum.
! How the opponent neurons in the visual system code colors.
! How the code can explain after-images.
! How yellow is a pure color.
Until the middle of the 20th Century, color
theories were based on psychophysical
evidence from perception of colors and color
mixtures and on anatomical studies of the retina.
Tools for studying the physiology and chemistry
of the nervous system directly were developed
only in the last 50 years.
Figure 1-2m. The opposite ways of mixing colors: mixing
colored lights and mixing paints.
A century of color-mixing experiments provided
much psychophysical evidence about color
vision. These experiments used mixture of light,
not pigment (the coloring material in paint).
These are two different kinds of color mixing,
which obey different rules. You may know about
mixing paint from art class or from experience
with mixing paints. It is subtractive mixing, which
is the opposite of mixing light, which is additive
mixing. Figure 1-2m illustrates these two kinds
of mixtures. Compare the colors reflected back
from the surfaces in the left and right panels.
Paints make color by absorbing (~subtracting)
some wavelengths (wavelength is the main physical
basis for the psychological experience of color) from
white light and reflecting the rest. You see color
only from reflected wavelengths. For example, if
you mix yellow and blue paint, the blue paint
subtracts some of the yellow light that the yellow
paint would reflect, and the yellow paint subtracts
some of the blue light that the blue paint would
reflect. This mixture leaves the intermediate green
reflected the most.
Paint mixture produces less of the reflected
wavelengths for each color of paint. Therefore
paint mixtures are always darker that the two
original paints. Therefore, mixing blue and yellow
paints produces a green paint, which is less bright
than the original yellow or blue paint.
Mixing lights adds wavelengths. Look up close at a
color TV screen. You will see that each point is
made up of three dots of light: red, green, and
blue. At normal viewing distance you cannot make
out the indivi-dual dots. They blend to produce a
single color, which is always brighter than the
individual dots in it.
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Q1. Two slide projectors shine light onto a screen. One projector is covered with a red filter; the other
with a green. Students in a high school art class predict that the overlapping areas of light will be brown,
but it turns out to be a faint pastel. The students probably made the incorrect prediction because they
A. did not observe the colors they got from mixing paint very carefully.
B. knew about subtractive paint mixing but not about additive light mixing.
C. did not know the colors of the spectrum.
D. did not know that colors are created by lights with different wavelengths.
Link to more on the difference between mixing light (additive)
Figure 2-2m. Thomas Young and
and mixing paint (subtractive) and what photopigments are. Link
Hermann von Helmholtz, credited with
to more information about color.
proposing the trichromatic theory of
Nineteenth-century psychophysical experiments on colored light
mixing produced two major theories of color vision. The first,
called the Young-Helmholtz Trichromatic theory (tri = 3, chroma
[as in Kodachrome color film] = color), was based on the facts of
light mixture. Appropriately selected red, green, and blue lights
add up to make white. This led Thomas Young and later
Hermann von Helmholtz to propose that the visual system
contained a red, a green, and a blue system. The summed
activity in each produced the sensation of different colors.
color vision.
Later, Ewald Hering proposed an Opponent Process Theory.
Some things he pointed out to support this idea were:
!
!
!
Yellow is a pure color, like red, green, and blue. A pure color is one that is not a mixture. For
example, orange and violet are not pure colors. They are yellowish red and bluish red, respectively.
Red is a pure color, not a bluish yellow.
Mixtures of complementary("opposite") colored lights produce a neutral (colorless) light (this is
additive light mixture, not subtractive paint mixture). For example, mixing red and green lights and
yellow or blue lights make a neutral, colorless light.
Afterimages appear in the complementary
color to the color of the thing you were looking
Figure 3-2m. Demonstration of afterimages.
at. Stare at a brightly colored form like the red
and green squares in Figure 3-2m for about
20 seconds. Then look at the neutral surface
next to it. The dim, fuzzy afterimage has the
complementary color of the original. I see the
red square's afterimage as pale green and the
green square's afterimage as reddish fuchsia.
Based on such facts, Hering proposed that the
visual system has two balanced opponent pairs of
systems: a yellow-blue system and a red-green
system (and a black-white system, as well, for
lightness).
Match the finding described below with the color theory it fits better
1. Young-Helmholtz Trichromatic (3-color) theory
2. Hering Opponent theory
Q2A. any color in the spectrum can be matched with the proper % of red, green, and blue light
Q2B. yellow is a pure color like red, green and blue, not a blend like violet
Q2C. complementary colors, like red and green, balance each other out
Color Coding by the Cones
In the mid-20th century, researchers managed to
measure properties of individual cones. As the
Young-Helmholtz Trichromatic Theory predicted,
they found three kinds of cones. Each kind of cone
had a different chemical photopigment, which
absorbs the most light at different wavelengths
("colors"). These photopigments are transducers,
because they break down when they absorb light
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and generate graded neural signals. These neural
signals release a neurotransmitter, which activates
the next neurons in the retina. These neural
signals activate the visual pathways to the brain.
! "Blue" cones absorb most (and are activated
most) at about 420 nm (nanometers = 1
billionth meter or 1/40 millionth in.), which is a
blue color.
! "Green" cones absorb most at about 530 nm,
which is a bluish green.
!
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"Red" cones absorb most at about 560 nm,
which is a yellowish green; it is called a "red"
cone because it is the only one that absorbs
much light in the red end of the spectrum, so it
is the primary receptor for red.
More accurate names for the three kinds of cones are
"short," "medium," and "long" wavelength cones, but
blue, green, and red are easier to remember. A fourth
kind of cone, which absorbs most in the very short
wavelengths [purple], has also been reported, but this
discussion ignores it.
Q3. Researchers have found three kinds of cones that differ in
A. the photopigments they contain.
B. the wavelength that they absorb most.
C. size and shape of the light absorbing part.
D. their location on the retina.
E. A and B are both correct.
F. A, B, C, and D are all correct.
Link to more information about Trichromatic theory of color vision.
Coding color: The link between wave length and
absorption by cones, and the link between absorption
by cones and psychological experience of color.
The three kinds of cones absorb and are activated by
different but overlapping ranges of light wavelengths
("colors") around their peak absorption wavelength.
These three different absorption curves are the basis
of color coding the retina. Figure 4-2m shows the
relative absorption over the range of visible light for
each kind of cone. The maximum absorption for each
cone is set at 100%. The curve shows the percent of
that maximum at each wavelength.
Figure 4-2m. Light absorbed by the "red," "green,"
and "blue" cones as a function of wavelength. The
Y-axis shows the % of light absorbed (higher %
absorption = stronger stimulation = higher
sensitivity) The Y-axis is the wavelength of light.
Below it are the approximate colors the wavelengths
produce. The vertical blue arrow shows how to read
the absorption for 450 nm (blue) light.
The red curve on the right in Figure 4-2m shows the
"red" cones' light absorption relative to its absorption
at 560nm (its maximum). The green curve in the
middle shows the "green" cone's light absorption
relative to its absorption at 530nm (its maximum).
The blue curve at the left shows the "blue" cones'
light absorption relative to its absorption at 420nm (its
maximum).
The three kinds of cones can code color because
they absorb different percentages of light at each
wave-length. Because percent absorption is
directly related to amount of activation, percent
absorption is a measure of strength of activation.
Every wavelength of light is coded by the
percentages that each kind of cone absorbs.
The four "bullets" that follow are examples of how to
read Figure 4-2m, which shows the relation between
two physical dimensions: wavelength of light on the X
axis and percent absorption on the Y axis. Each kind of
cone has its own curve showing the relation between
these two physical variables for it.
! a 450-nm light (deep blue) activates "Blue" cones
about 90% of maximum, "Green" cones about 60%,
and "Red" cones about 40%.
! a 470-nm light (a greenish blue) activates the
"Blue" cones about 85% of maximum, "Green"
cones about 85%, and the "Red" cones about 50%.
! a 550-nm light (a greenish yellow) activates the
"Blue" cones about 30% of maximum, the "Green"
cones about 95%, and "Red" cones about 95%.
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!
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a 620-nm light (an orangish red) activates the
"Blue" cones at about 10% of maximum, the
"Green" cones about 65%, and "Red" cones about
85%.
Absorption by each kind of cone codes the
psychological dimension of color. Each spectral
colors (a psychological dimension) is coded by a
unique ratio of absorptions in the three cones. For
example:
! deep blue = 90% B, 60% G, 40% R.
! greenish blue = 85% B, 85% G, 50% G.
! greenish yellow = 30% B, 95% G, 95% G.
! orangish red = 10% B, 65% G, 85% G.
Every other spectral color has its own ratio of
absorption by the three kinds of cones.
Using Figure 4-2m above, match the wavelengths below with their approximate percent absorption by
the three different kinds of cones ("Blue," "Green," & "Red" are the names of the three kinds of cones) .
1. 300 nm (ultraviolet) 2. 525 nm (bluish green) 3. 410 nm (blue)
4. 575 nm (greenish yellow) 5. 700 nm (red)
Q4A. "Blue" = 95%; "Green" = 30%; "Red" = 20%
Q4B. "Blue" = 55%; "Green" = 95%; "Red" = 80%
Hint: To figure out alternative A, find 95% on the Y axis (Absorption). Then find where a horizontal line from 95%
meets the "Blue" curve (you will find two places). From that point drop a perpendicular to the X axis. Note where it
crosses the "Green" and "Red" curves. If these crossing points are about .30 and .20 respectively, then you have the
correct place on the X axis, which tells you the matching wavelength. Link to the examples in the preceding text.
Color Coding in the Brain
The activity of visual neurons in the brain is closely
related to monkeys' ability to tell the difference
between different colored lights. Humans and
monkeys detect differences in colors the same
way. Therefore, human color vision is very likely
based on neurons lie those in the monkey brain.
Soon after the three kinds of cones were
discovered, DeValois (1966) and others started to
measure the activity of single neurons in the
monkey's visual system in response to different
wavelengths of light. He also tested whether
monkeys see colors as humans do. He did this
by training monkeys to discriminate between (=
respond differently to) different wavelengths and
intensities of light. He also tested human
volunteers with the same tasks in the same
apparatus.
DeValois showed that the rhesus monkey's color
vision is virtually identical to human color vision.
He compared humans’ and monkeys’ absolute
threshold at different wavelengths and difference
threshold to neighboring wavelengths. The data
from humans and moneys matched almost
perfectly, showing that humans and monkeys see
color the same way. Furthermore, the neural
responses to color measured from the monkey’s
brain fit closely the monkeys’ behavioral
responses to color. These data strongly support
the hypothesis that neurons in human visual
pathways behave like the neurons in the rhesus
monkey's visual pathways. (See also Sandell et
al., 1979)
Q5. Researchers believe that cells in human visual brain areas respond to colors in much the same way
as visual cells in rhesus monkeys respond because
A. many human cells have been measured and found to respond so
B. monkeys and people discriminate (detect) small changes in wavelength in the same way
C. monkeys and people need about the intensity to detect light at all across the spectrum
D. the location of the visual areas of their brains is the same
E. B and C are both correct
F. A, B, C and D are all correct
Neurons in the monkey brain's visual areas
respond to colors as Opponent Process Theory
predicts. Neurons in the monkey's visual pathway
respond to colored light quite differently from the
way cones in the retina do. They can be divided
into two kinds of color-responsive cells:
! Red-Green (R G) opponent cells respond
oppositely to red and green light and do not
respond to blue or yellow light.
! Blue-Yellow (B Y) opponent cells respond
oppositely to blue and yellow light and do not
respond to red or green light.
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Each kind has two, roughly mirror-image
Figure 5-2m. Opponent neurons in the rhesus monkey's
versions. Figure 5-2m illustrates these two
visual pathway
classes of opponent cells. Note that these
opponent cells are active in the absence of any
visual stimulation.
! R- G+ cells: Red light inhibits them,
decreasing their activity below the unstimulated
level. Green light excites them, increases their
activity above unstimulated resting level. Blue
and yellow lights do not affect these neurons.
! R+ G- cells: Red light excites them,
increasing their activity above the unstimulated
level. Green light inhibits them, decreases their
activity below unstimulated resting level. Blue
and yellow lights do not affect these neurons.
The properties of the opponent cells in the visual
! B- Y+ cells: Blue light inhibits them,
brain fit closely with psychophysical observations
decreasing their activity below the unstimulated
that Hering used to infer their existence. For
level. Yellow light excites them, increases their
example, mixtures of red and green light trigger
activity above unstimulated resting level. Red and
little activity in RG opponent cells, and mixtures of
green lights do not affect these neurons.
blue and yellow light have little effect on BY
! B+ Y- cells: Blue light excites them,
opponent cells. These mixtures appear neutral
increasing their activity above the unstimulated
(colorless). Link to more information about
level. Yellow light inhibits them, decreases their
opponent process theory.
activity below unstimulated resting level. Red and
green lights do not affect these neurons.
Match the response to light with the type of opponent cell
Q6A. excited by yellow light & inhibited by blue light Q6B. excited by green light & inhibited by red light
Q6C. excited by red light & inhibited by blue light
Q6D. unresponsive to blue or yellow light
1. R-G+ 2. either R-G+ or R+G- 3. B-Y+ 4. No such cell exists
The difference in coding between the cone receptors and the brain
neuron response means that the neural signals from the three kinds of
cones must get rearranged when sent to neurons in the brain's visual
system.
Figure 6-2m. Conversion of the
three-color cone code to the
opponent code in the brain.
Figure 6-2m summarizes the way the three kinds of cones probably
connect to opponent neurons in the brain. "Red" cones excite the R G
brain system and inhibit the B Y system; "Green" cones inhibit both
the R G and B Y systems; "Blue" cones excite the B Y system.
Summary of the color system:
1. The retina contains "blue," "green," and "red" cones, which closely
follow the Young-Helmholtz trichromatic theory.
2. The cones' outputs are recombined by red-green and blue-yellow
opponent cells, which follow the Hering opponent process theory.
Q7. Current evidence indicates that ____theory describes color
functions in the cones, and ____ theory describes color coding in
the visual pathways of the brain.
A. opponent process; trichromatic or three color
B. trichromatic or three color; opponent process
C. trichromatic or three color; trichromatic or three color
D. opponent process; opponent process
asgn2m -- VISION: Color Vision
Copyright © 2001 by Gabriel P. Frommer
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p. 6
asgn2n -- HEARING:
The Ear
This exercise describes sound waves, the stimuli for hearing. It then shows how the ear picks up
sound waves and delivers them to the receptors in the inner ear. It covers the following questions:
! What as the properties of sound waves?
! What are the parts of the ear?
! How do the different parts of the ear collect sound waves and deliver them to the sensory receptors
that convert them into neural signals that go to the brain?
This link is to a website that summarizes almost everything about hearing in this and the next exercise, and
then some (on several topics it goes into more detail than is needed for this class ). It has some very good
active illustrations to explain the basic ideas. Some active illustrations use stereo sound, which is helpful but
not absolutely necessary. I suggest you go through this site either before or after doing the next two exercises.
The psychological experience of sound is normally
created by air pressure changes reaching the ear.
The young adult human ear responds to air
pressure changes at frequencies from about 20 Hz
to 20 kHz (Hz = Hertz = cycles per second; kHz =
kilohertz = 1,000 Hz). Many other animal species
hear much higher frequencies, beyond human
hearing. Some bats can hear up to about 200 kHz.
Q1. People cannot hear "silent" dog whistles. Dogs can hear them, but humans can't, because
A. dogs' ears respond to higher frequencies than human ears do
B. the whistles make sounds higher than 20 kHz
C. A and B are both correct
The psychological dimension of pitch is closely (but not
exclusively) related to a physical sound's fundamental
frequency. The fundamental frequency is the lowest of
the group of related frequencies that make up almost all
natural sounds.
Figure 1-2n. Three pure tones and their sum.
The higher the fundamental frequency of a sound wave,
the higher pitched it sounds. For example, the most
important parts of speech sounds are mostly between
about 100 Hz and 1.5 kHz. This range includes the
sound of a deep-voiced man and the high-pitched sound
of a young child speaking. In music, concert A (the A
above middle C) has a fundamental frequency of 440
Hz. The highest note on the piano keyboard has a
fundamental of 4186 Hz. The lowest note on many
organs has a fundamental frequency of 32 Hz, which
The top line of the drawing represents a 1-kHz tone. It has
a listener feels as much as hears. A pure tone has
one complete cycle each millisecond and 1,000 complete
only one frequency of sound waves. It is described
cycles in each second. The second line is also 1 kHz, but it
by a sine wave, a mathematical function based on
is half the intensity (up-and-down size) of the first and is
the circle. A sine wave has a specific frequency
shifted in phase (time relation) relative to the first. This
measured in Hz and power(size, "strength") in
means that the second line has the same number of cycles
decibels (dB) . The top three curves in Figure 1-2n
(or peaks) per second, but its size is half the size of the top
show sine waves that represent three pure tones.
line, and its peak pressure comes slightly later than the
peaks of the top line. The third line represents a 2-kHz tone,
The X axis (vertical) shows sound pressure, which is the
which has exactly twice the peaks per second as the first
power or intensity of the sound wave. The Y axis
two. Its amplitude is the same as the amplitude of the top
(horizontal) shows time in milliseconds (1/1,000 second).
line.
One divided by the duration of one cycle in milliseconds
equals the frequency of the sound in kHz.
Pure tones are rare in nature. Natural sounds are
mixtures of many different sound waves, which can
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be described as the sum of sine waves with different
frequencies and amplitudes. The fourth line in Figure
1-2n shows a mixture of the top three lines. It is a
complex-looking wave, but it is very simple compared
to natural sounds, which have many more than three
pure tones. Link to an animated explanation of sound
frequency and intensity. Use the right arrow button at
the lower right of the screen that appears to go
through the illustrations.
p. 7
Link to a brief description of the difference between
pleasant, musical and harsh, non-musical sounds. Link to
more on the physics of musical scales. On the page that
appears, click on scale. On the page that opens, click on
one or more box beside the different musical notes. Each
one you click on will display the sine wave for that note.
The blue curve shows the sum of the notes you select.
Compare C+E+G, a pleasant-sounding musical chord, with
C+F sharp+G, a harsh chord. To see only the sum, click on
the top box (by "show components" to remove the check
mark. [The X-axis (horizontal) is incorrectly labeled; it
should be milliseconds , not seconds.]
Match each question below with the number in Figure 4-2n at the right
that it matches the best. [At least one alternative must be used more than
once; not all alternatives are necessarily used.]
Q2A. lowest frequency of pure tone
Q2B. lowest intensity of a pure tine
Q2C. sound mixture
Q2D. highest pitch
Hint for A and B Hint for C Hint for D
Figure 2-2n.
The psychological dimension of loudness depends on both the intensity
and the frequency of the sound wave stimulus. The psychological
dimension of loudness is most closely related to the physical amplitude or
power of the sound wave. However, frequency is also important,
because the ear's sensitivity depends on sound frequency. The human
ear is much more sensitive to sounds in the middle range of hearing,
about 1 - 5 kHz, than to lower or higher frequencies. So a low intensity 2
kHz sound sounds much louder than an equally intense sound of 100 Hz or 10 kHz.
Two kHz produces a high-pitched sound, compared to the frequencies people normally use. For example, the most
important parts of speech sounds are mostly between about 100 Hz and 1.5 kHz. In music, concert A (the A above middle
C) has a fundamental (~basic) frequency of 440 Hz. The highest note has a fundamental of 4186 Hz. All musical notes
have many higher frequency harmonics which go well above 5 KHz.
Humans can reliably detect very small air pressure changes at the most sensitive frequencies. Bekesy
estimated observers can detect sound waves that move air molecules by less than the diameter of the
hydrogen atom (10-7 mm). Greater sensitivity would be of
little use because the sound of blood flowing through
Figure 3-2n. Curves show the intensity of equally loud
vessels in the head would mask (~cover, drown out) such
tones at different sound frequencies. The grey area is
weak sounds.
audible sounds.
Figure 3-2n plots the absolute threshold of physical
sound pressures [Y axis] as related to physical sound
frequency [X axis] for the "average" observer. The solid
rust colored line separates the grey perceptually audible
and white perceptually inaudible frequencies and
intensities. The dotted lines are equal loudness contours.
Different sound frequencies (physical stimuli) on the
same equal loudness contour sound equally loud
(perceptual experience), even though their intensities
(physical stimuli) can be quite different. For example, a
2kHz-tone at 20 dB sounds about as loud as a 0.1kHz
tone at 50 dB. This fits with the ear's higher sensitivity at
2kHz.
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p. 8
Two stimuli of equal intensity but different frequencies (physical dimensions) can sound quite different in
loudness (perceptual experience). For example, you can easily hear a 2-kHz tone at 40 dB intensity, but a
0.0-Hz tone of the same intensity is just barely audible.
Using Figure 3-2n, match the following with the best alternative. Hint: Each alternative has a pair of points on the
graph. Find both pairs for an alternative, and figure out which match it fits. To find a point, find the frequency on the
X-axis and draw an (imaginary) line straight up. Find the intensity in dB on the y-axis and draw an (imaginary) line straight
across. The place the two lines meet is the location of the point.
Q3A. 1 kHz at 40 dB and 0.1 kHZ (100 Hz) at 63 dB
Q3B. 1 kHz at 40 dB and 0.2 kHz (200 Hz) at 40 dB
Q3C. 2.5 kHz to 5 kHz at -6 dB
Hint for A; Hint for B; Hint for C
1. range of weakest sound detectable by "average" observer
2. two sounds of equal intensity, both audible
3. two sounds of equal loudness
4. two sounds with the same pitch but one softer than the other
Figure 6-2n illustrates the main parts of the ear. The
spiral, snail shell-shaped cochlea is shown
straightened out. The following describes each part
and how it contributes to hearing.
Figure 4-2n. Diagram showing the outer, middle, and
inner ear. The cochlea of the inner ear is spiral-shaped, but
is shown straight to show internal structure and function
more clearly.
The outer ear consists of what is visible from the
outside: the pinna and the ear canal. The pinna is that
flap on the side of the head we call the ear. It funnels
sound waves into the ear canal, the hole in the pinna to
the middle ear. It also helps detect where sound
comes from in the vertical plane.
Cats, horses, and other animals can point their pinnae
to increase sensitivity to sound from one direction over
others. Although the human pinna cannot move, recent
research has shown that the human outer ear does
help locate the source of a sound (Butler, 1998).
The middle ear is an air-filled space separated from
the ear canal by the eardrum or tympanic membrane.
The eardrum is a very thin, delicate layer of skin
across the inner end of the ear canal, like the cover of
a drum. It picks up sound waves from the air in the
ear canal and transfers them to a chain of three tiny
bones called the ossicles. The ossicles carry the
sound vibrations across the air-filled middle ear to the
fluid-filled inner ear.
Riding an elevator, even just a few stories, changes the air
pressure in the outer ear, but the air pressure in the middle
ear does not change. The higher you go, the bigger the pres-
sure difference and the more discomfort you feel. Swallowing relieves this pressure difference and makes your ear
pop. Swallowing opens the Eustachian tubes to the throat to
reestablish balance between inside and outside pressures.
Many of you had very painful middle ear infections. The
pain comes from the infection building up pressure in the
middle ear and pushing out on the ear drum. Before
antibiotics were available, physicians sometimes had to put
a small hole in the ear drum to relieve the pain and prevent
the pressure from rupturing the ear drum.
Match the following functions with their structures
Q4A. set in vibration by air pressure changes
Q4B. carries sound vibrations to the inner ear
Q4C. collects sound pressure like a funnel
1. ossicles 2. ear drum or tympanic membrane 3. pinna (of outer ear)
The cochlea of the inner ear breaks down complex
sound mixtures and converts their parts into neural
signals. The cochlea is a spiral, snail shell-shaped
structure in the skull. The basilar membrane runs
down the middle of the cochlea. It is built to flex at
different places in response to different sound
frequencies above 0.2 kHz. This is how it breaks
down complex sound mixtures into its sine wave
parts (von Bekesy, 1960).
Figure 5-2n is a diagram of the cochlea (shown
straightened out) showing the basilar membrane
running the length of the cochlea with auditory
receptors called hair cells on it. The numbers above
the basilar membrane are the sound frequencies in
kHz that flex the basilar membrane most at that
place.
The diagram shows that the basilar membrane is
organized tonotopically. This means that it flexes
most at neigboring places in response to neighboring
sound frequencies. Low frequencies flex the end of
the basilar membrane that is farthest from the middle
ear. High frequencies flex the basilar membrane the
closest to the round window where sound vibrations
enter the cochlea from the ossicles crossing the
middle ear. Middle frequencies flex the basilar
membrane most in the middle.
Figure 5-n2. Position of maximum flexing on basilar
membrane. When sound activates the inner ear, the basilar
membrane flexes to stimulate the hair cells on it to generate
neural signals. The more intense the sound, the stronger the
hair cells are twisted (and excited).
Arranged along the length of the basilar membrane
are about 40,000 hair cell receptors. When sound
energy flexes the basilar membrane, the hair cells
where it flexes get bent. This generates neural
signals. The more intense the sound, the more the
basilar membrane flexes and the more the hair cells
are bent. When stimulated, the hair cells are excited
and release neurotransmitters at their synapses to
sensory neurons going to the auditory system in the brain.
The basilar membrane-hair cell system is designed to
respond to a wide range of sound intensities, but intense
sounds can damage the hair cells. The more intense the
sound and the longer it lasts, the faster the damage occurs.
This is why people working in places with loud sounds, like
ground crews for jet aircraft, wear hearing protection
devices.
However, many people voluntarily expose themselves to
very loud sounds, which eventually add up to produce
serious hearing loss. One common source is highly
amplified music, especially in enclosed places. Band
members get the most exposure, and some use ear
protection. Those who don't, lose hearing. One person in
this class wrote the following minute paper comment:
"On the topic of hearing, I am a D.J. and am around loud
music all of the time. Sound pressure levels sometime reach
140 db. Also I am into car audio. I have a very loud system.
Over the past 3 years the combined effect of the high spl's
[sound pressure level -- another way of describing sound
intensity] has reduced my ability to detect very high
frequency sounds. Also, I can't hear low intensity sounds
well either."
Match the following functions with their structures
Q5A. flexes at different places depending on the frequency of the sound stimulus
Q5B. what actually generates neural signals in response to sounds
Q5C. auditory part of the inner ear
1. cochlea 2. flexing hair cells on the basilar membrane 3. basilar membrane
Link to in-depth description of the auditory system, with special emphasis on hearing loss and rehabilitation.
The auditory system evolved from the balance system, which tells animals the direction in which their body is
accelerating. The body experiences acceleration from two sources:
! The force of gravity, which continuously pulls you toward the center of the earth
! The forces produced by moving
Because they stand on two legs, humans must quickly and accurately adjust posture to keep from falling. The
balance system in the head, together with muscle and joint senses and vision, provide the necessary
information for this automatic adjustment. For a brief description of the balance system, click HERE.
asgn2n -- HEARING: The Ear
Copyright © 2001 by Gabriel P. Frommer
asgn2o -- HEARING:
Perception of Pitch; Sound Location
This exercise describes how the auditory system codes sound. The auditory system codes two basic
features of sounds: that different frequencies (~ "pitches") that make up most sounds and the location
from which a sound is coming. It outlines two theories of pitch perception, Place and Pattern, and
shows how Place Theory represents pitch exclusively above about 2,000 Hz and Pattern Theory
represents pitch exclusively below about 200 Hz. Both operate between 200 Hz and 2,000 Hz. It then
describes the way animals (including people) can locate the source of a sound and the way some bats
use the same kind of information to identify location, size, and motion of flying insects.
The exercise covers the following questions:
! What is the function of the basilar membrane of the inner ear in coding information about sound
frequency?
! How does Place Theory explain how sound frequency is coded?
! How does Pattern Theory explain how sound frequency is coded?
! How does having two ears provide a code for locating where a sound is coming from?
Perception of Pitch
Natural sounds are complex mixtures of different
frequencies. The difference between natural sounds
is the result of the mixture of sound frequencies that
make up sounds. To perceive complex sounds, the
auditory system first breaks down the complex
mixtures of frequencies that make up natural sounds.
Then the auditory system codes the frequencies that
make up sounds. Finally the auditory system extracts
from this information the features that represent what
the sound is.
A basic task for hearing researchers is to understand
how the auditory system does this breakdown and
reconstruction process. Researchers are much
farther along in understanding the breakdown and
coding process that they are in understanding how
the pieces are put together into a perception of the
sound.
Q1. Sounds are made up of mixtures of different sound frequencies. The ear first
A. adds together the sound frequencies from a sound source.
B. breaks down the sound into the frequencies in it.
C. divides sounds into places and patterns of frequencies.
D. codes sounds by the number of frequencies they contain.
E. C and D are both correct
Two theories, Place theory and Pattern Theory,
describe how the ear codes the simple pure tones
that make up complex sounds. The physiologist and
physicist, Hermann von Helmholtz proposed a place
theory about 150 years ago. He argued that different
sound frequencies activated the basilar membrane at
different places. He proposed that the basilar
membrane had little resonators arranged along its
length. Each resonator reacts selectively to the sound
frequency that matches its own natural resonating
frequency.
For example, one resonator is tuned to respond to 1
kHz (1,000 Hz). Its neighbor on one side is tuned to
1.1 kHz; its neighbor on the other side is tuned to 0.9
kHz. When a resonator is activated, it in turn activates
its own hair cell receptor to generate neural signals
(transduction). The nerve fiber attached to that hair
cell carries that neural signal to the brain, which can
tell what frequency was stimulating the ear by which
neuron was activated. This code tells the brain what
pure sounds made up the complex mixture.
Modern research (von Bekesy, 1960) shows that Helmholtz
was correct about the coding principle of place on basilar
membrane right, but he was not correct about the actual
mechanism of detecting different frequencies. Selective
stimulation of different places on the basilar membrane for
different sound frequencies has a different mechanism:
"traveling" waves that peak at the place on the basilar membrane that matches the sound frequency that produces it.
Pattern theory com pares the basilar m em brane to a
m icrophone, which responds to all audible
frequencies. Just as a m icrophone converts sounds
into tem poral (tim e) patterns of electrical signals, the
hair cells on the basilar m em brane convert sound
waves into tem poral patterns of nerve im pulses. The
tim e pattern of im pulses m atches the tim e pattern of
the air pressure changes that form the sound. This
theory also turns out to be correct, but only at lower
frequencies.
asgn2o
p. 11
Mark each item with the theory with which it goes better.
1. Place Theory
2. Pattern Theory
3. both theories
4. neither theory
Q2A. where the basilar m em brane in the cochlea bends
Q2B. how m any pieces a pure tone breaks into
Q2C. codes the frequencies in com plex sounds
Q2D. codes the tim e pattern of the sound wave in the tim e pattern of im pulses in auditory nerve
For m any years these two theories com peted for
acceptance. Research eventually showed that place
theory operates at higher frequencies, and pattern
theory operates at lower frequencies. Both operate (in
humans) between about 200 Hz and 2 kHz. This
frequency range carries m uch of the sound
inform ation that is especially im portant, like speech
and fundam ental tones in the m iddle of the m usical
scale (concert A or m iddle A on the piano keyboard,
440 Hz fundam ental frequency). Place theory operates
exclusively above about 2 kHz, and pattern theory
operates exclusively below about 200 Hz.
Q3. The best current evidence about place and pattern theories of pitch perception indicates that
A. place theory operates exclusively above 2 kHz.
B. pattern theory operates exclusively below about 200 Hz.
C. both theories operate between about 200 Hz and 2 kHz.
D. pattern theory is correct, and place theory is wrong
E. place theory is correct, and pattern theory is wrong
F. A, B, and C are all correct
To understand Place theory, recall the structure of the
cochlea (inner ear) shown in Figure 1-2o. Note that the
basilar m em brane running the length of the cochlea,
and the hair cell receptors distributed along its length.
Place theory is a form of labeled line (anatom ical)
coding, because it states that sound frequency is coded
as place on the basilar membrane. The basilar
m em brane bends m ost at different places depending on
the frequency of the stim ulating sound. The num bers
above the basilar m em brane in Figure 1-2o show the
location of m axim um bending for different frequencies.
(Note that octaves are evenly spaced.) So the basilar
membrane codes the frequency of sound into place on
basilar membrane. Bending a place on the basilar
m em brane activates the hair cells at that place. Thus
each frequency activates (m ost) a sm all set of hair
cells.
Figure 2-2o shows the bending on the basilar
m em brane when the ear is stim ulated by a 0.8 kHz
tone. The basilar m em brane bends m ost at one place,
and the hair cells at that place and their connecting
afferent nerve fibers to the brain are activated. Other
frequencies bend the basilar m em brane m ost at
different positions and activate a
Figure. 3-2o. Geory
different set of hair cells and their
von Bekesy.
afferent nerve fibers. In sum m ary,
each sound frequency is
represented by a place on the
basilar membrane, the hair cells at
that place, and the sensory nerve
fibers from those hair cells.
Figure 1-2o. Basilar membrane in the cochlea showing
the place of maximum bending for different sound
frequencies. This is the basis of the Place Theory of
coding sound pitch.
Figure 2-2o. Basilar membrane bending at one specific
location in response to sound at 0.8 kHz. This bending
stimulates the hair cell receptors at this location, which
generate neural signals on the auditory nerve fibers they
connect to.
Georg Von Bekesy (Figure 3-2o)
won the Nobel Prize in 1961 for showing that this is what the basilar membrane actually
does (Bekesy, 1960). He measured the basilar membrane's movement in response to
different sound frequencies and found that lower frequencies (above 200 Hz) bend the
asgn2o
p. 12
basilar membrane most at its far end, away from the
middle ear. Middle frequencies bend the middle part
of the basilar membrane most. High frequencies bend
the basilar membrane most at the end of the basilar
membrane nearest the input from the middle ear.
Place theory works at frequencies above about 200
Hz (concert A [A above middle C] = 440 Hz) and
operates exclusively above about 2 kHz.
Q4. Place theory predicts that damage to the hair cells about half way down the basilar will produce loss of
perception of _____. [Hint: to answer this you must take the range of human hearing and relate it to how the
basilar membrane codes sound frequency.]
A. all sound frequencies
B. all frequencies above about 3-4 kHz
C. all frequencies below about 3-4 kHz
D. all frequencies except at about 3-4 kHz
E. sound frequencies at about 3-4 kHz
The coding of sound frequency in the brain preserves
the place code generated by the basilar membrane.
The basilar membrane is "mapped" in the auditory
areas of the brain as illustrated in Figure 4-2o. As
described above, when a part of the basilar
membrane bends, the bending activates the hair cells
at that place on the basilar membrane. These hair
cells connect to nerve pathways leading to the
auditory areas of the brain. The nerve pathways
preserve information about their places of origin on
the basilar membrane.
! Hair cells at the far end of the basilar membrane
away from the middle ear connect to the front end
of the auditory cortex
! Hair cells at the end of the basilar membrane
near the middle ear connect to the other end of
the auditory cortex.
! Hair cells in the middle of the basilar membrane
connect to the middle of the auditory cortex.
The far end of the basilar membrane (away from the
middle ear) bends most in response to lower
frequencies, so lower frequencies activate the front
end of the auditory cortex. The near end of the basilar
membrane bends most in response to the highest
frequencies humans can hear, so these frequencies
activate the back end of the auditory cortex. The
Figure 4-2o. Map of basilar membrane on primary auditory
cortex. Sensory neurons from neighboring places on the
basilar membrane connect (through several synapses) to
neighboring places on the primary auditory cortex.
middle of the basilar membrane bends most to middle
frequencies. So these frequencies activate the middle
of the auditory cortex. This is a labeled line or
anatomical code; sound frequency is coded by the
anatomical location (or place) that is activated in the
auditory cortex.
Q5. Neighboring sound frequencies activate nerve cells on neighboring strips on the auditory cortex. They can
do this because __.
A. neighboring areas on the auditory cortex receives connections from hair cell receptors on neighboring areas
of the basilar membrane
B. neighboring areas of the basilar membrane are activated by neighboring sound frequencies
C. neighboring areas of the basilar membrane are connected to each other, allowing neighboring frequencies
to overlap on the auditory cortex
D. A and B are both correct
Recall that pattern theory describes coding at
frequencies, below about 2 kHz. Only this kind of
code operates below about 200 Hz. According to
pattern theory, the temporal pattern (pattern in time)
of the impulses of the auditory nerve fibers matches
the temporal pattern of the sound waves. Figure 5-2o
on the next page shows that the pattern of nerve
impulses in the auditory nerve matches the pattern of
sound waves (above the median pressure).
Each vertical line in the second trace labeled "neuron
firing" is an all-or-nothing nerve impulse. The third
trace represents the moment-to-moment rate of
all-or-nothing impulses. The rate of responding (or
firing) of neurons follows the sound pressure waves
because each wave bends the basilar membrane.
asgn2o
Bending the basilar membrane makes the hair cells on it
generate neural impulses in the auditory nerve to the
brain. So each sound wave will trigger a burst of
impulses. The bigger the wave, the more impulses it
triggers. The more frequently sound waves come, the
more frequently bursts of impulses travel up the nerve
fibers to the brain.
p. 13
Figure 5-2o. Temporal (~time) pattern of impulses in
auditory neurons codes sound frequency by reflecting
temporal pattern of sound pressure changes.
When researchers measured the pattern of impulses in
the auditory nerve, they found that at lower frequencies
(below 2 kHz) the nerve impulses lock onto the pattern
of sound waves and match the temporal (time) pattern of
the sound pressure changes, just as pattern theory
predicts. For example, a 200-Hz tone produces sound
pressure waves every 5 milliseconds. So the pressure
waves trigger impulses in the auditory nerve at 5millisecond intervals. The brain somehow uses this time
pattern of impulses to produce the perception of (low) pitch.
Q6. According to pattern theory, auditory nerve cells (as a group) respond to a 333-Hz tone with impulses
about every ___ milliseconds. [Hint: this tone produces 333 sound pressure waves every second; what is the
time interval in milliseconds between successive sound pressure waves?]
A. 333
B. 3
C. 333/3 =111
D. not enough information provided
Link to a more advanced discussion of pitch perception. Link to
Figure 6-2o. Sound pressure waves from
many sources about hearing.
one side reach the ear on that side sooner
and stronger that the ear on the other side.
Perceiving Where a Sound Comes From
When you hear a sound, you can usually tell quite accurately where
it is coming from. You hear your name called and turn to look
toward where the sound seemed to come from. You can tell where
to look because your brain unconsciously and automatically
compares the time and intensity of the sound waves reaching the
two ears. Your brain can do this because of the properties of sound.
Figure 6-2o illustrates this for sound coming from the right.
Sound travels through air at about 300 m/sec (1,000 ft/sec).
Because your two ears are about .3m (1ft.) apart, sound from your
right travels about .3m farther to reach your left ear than your right.
This means that the sound takes 1 millisecond longer to reach your
left ear.
Furthermore, your head is in the way. Much of the sound energy
from sound pressure waves on your right (especially at higher
frequencies) does not reach your left ear. This means that the sound
in your left ear is weaker than in your right ear.
Q7. [Mark EACH item True (T) or False (F)] Maury starts to walk across the street but stops when he hears a
car coming from his left. He can tell where the car is because its sound
T F A. reaches his right ear before his left
T F B. is stronger in his right ear than in his left
T F C. reaches his left ear before his right
T F D. is stronger in his left ear than in his right
T F E. has a lower pitch in his left ear than in his right
T F F. has a higher pitch in his left ear than in his right
People can use sound to find their way around in the
dark. Blind people who use canes tap the cane on the
ground. The tap does two things: it helps sense the
ground by touch, and it produces sounds that reflect
well off surfaces, like walls and trees. Blind people
learn to use these sound cues to guide themselves.
asgn2p
p. 14
Sighted people have the same skill, but they may not
use it much. You may have noticed that you can "feel"
when you are approaching a solid wall in the dark. It
feels like a gentle pressure or tension on your face.
This feeling is an illusion. It is actually based on
sound reflections from the wall.
Some animals use sound much more than humans to
find their way through their environment. Owls can
hunt in complete darkness, using the sounds that
mice make as they run through the grass to swoop
down and catch them. These sounds reach the owl's
two ears at slightly different times and with slightly
different intensities. (The owl's right ear is lower than
the left, so the owl can tell up and down as well.)
Some species of bats use sound in place of vision in
a different way. They use sonar (an auditory version
of radar) to avoid obstacles and catch insects in flight.
They make very intense, very high frequency sounds
and detect the echoes. These kinds of bats have
extra large pinnae (outer ears) to collect sound better.
In addition, the auditory system in their brain is
specializing to extract detailed information about the
world from the echoes that lets the sound echos
replace vision. These bats detect location, size,
direction of motion, and even surface texture of
objects. Link to information about bats, including their
hearing abilities.
Q8. Bats can catch flying insect in complete darkness because
A. their ears are much more sensitive to very weak sound.
B. they hear the beat of the insect's wings.
C. their brains extract location, motion, & size from echoes of the sounds the bats themselves make.
D. they emit (send out) light pulses to see in the dark.
E. they have very sensitive eyes that see in complete darkness.
Asgn2o -- Coding Sensory Information
Copyright © 1999 by Gabriel P. Frommer
asgn2p -- TOUCH AND PAIN
This exercise describes the labeled line (or anatomical) model of touch perception. It then shows that
this theory applied to pain does not explain why pain is not perfectly tied to injury. It describes one
extension, Gate Control Theory, that seems quite successful in explaining many features of pain that do
not fit simply into the basic labeled line model.
It covers the following topics:
! How labeled line theory explains perception of touch qualities (touch, vibration, pressure, warmth,
pain, etc.).
! How pain does not fit a simple version labeled line theory, because injury and pain can be unlinked.
! How Gate Control Theory attempts to explain the fact that pain and injury can be unlinked.
The somatosensory (soma = body) or "touch" sense
codes information about the many different kinds of
stimuli that people can detect from the skin, muscles,
and joints. Therefore, the somatosensory sense is
really a combination of several different sensory
qualities or kinds of sensory experiences, including,
among others, light touch, vibration, pressure, hair
movement, joint position, warmth, cooling, and at
least two different types of pain.
Each of these sensory qualities has one (or more)
type of sensory receptor. Each kind of receptor is
selectively tuned to respond to one (or sometimes a
few) of these kinds of stimuli. That is, each kind of
receptor responds selectively to its own adequate
stimuli. This is another example of labeled line or
anatomical coding.
Some receptors are very selective. They respond
only to vibration, only to change in pressure, only to
warming, only to (potential) injury, etc., as adequate
stimuli. Others are less selective, responding to, for
example, light touch, as well as warming and
damaging stimuli, or some other combination of
somatosensory stimuli. Each kind of receptor is part
of its own kind of sensory (afferent) neuron, which in
turn connect to its own kinds of neurons in the
somatosensory (~touch) system in the brain.
Experience of different sensory qualities appears to
depend on activity in the different kinds of
asgn2p
somatosensory neurons in the brain.Figure 1-2p shows four
different kinds of stimuli activating four different sensory
systems, each activated by its own adequate stimulus (there
are several more kinds, which are not shown). These different
kinds of neurons reach different, though closely related, places
in the cerebral touch system. Sensory fibers connect to many
other parts of the brain and spinal cord, but these areas appear
to be less important for touch perception.
Q1. What is the code that lets you perceive a touch on your
skin as different from warming your skin? Warmth and
touch ____
A. feel different B. activate pathways that reach warmth and
touch parts of the somatosensory cortex
C. activate different receptors D. B and C are both correct
E. A, B, and C are all correct
The neurons in the touch system can be divided into
two kinds based on the size of their axons (nerve
fibers) and cell bodies. One kind has larger diameter
(thicker) axons (nerve fibers), up to about 25 :m
(1/1,000 inch). The other kind uses smaller diameter
axons. These two kinds of axons have different
properties.
Larger diameter fibers serve the cue function for
touch. This means they tell what kind of stimulus is
acting and especially where on the body it is acting.
! They have smaller receptive fields, which lets
them signal more accurately where on the body
the stimulation is.
! They respond to what people ordinarily call touch.
" Gentle stimuli, like vibration, light touch
(including hair movement), and pressure,
activate larger diameter nerve fibers
" They carry information from only one of
several different types of somatosensory
receptors.
! They produce experiences that are usually
neutral or pleasant.
! They conduct all-or-nothing impulses fast, up to
about 100 m/sec (325 ft/sec).
! They are more sensitive to direct electrical
stimulation (all cells respond to electrical stimulation) than are smaller diameter nerve fibers.
p. 15
Figure 1-2p. Labeled line theory of touch
“qualities” in the somatosensory system.
Smaller diameter fibers usually serve the motivating
quality of touch.
! They often have very large receptive fields.
Because they respond to stimuli over a wide
area, they cannot signal where a stimulus is very
accurately.
! They respond to the full range of stimuli to the
body, warning, cooling, itch, and different kinds of
painful stimulation, as well as different kinds of
touch.
" Some respond to more than one kind of
stimulation. For example, some respond to
touch and cooling or warning, others to touch,
warming, and damaging stimuli, etc.
! They can produce very unpleasant sensory
experiences, even the ones responding to light
touch. Try very gently stroking the very edge of
your lip with your finger, or better, a fine brush.
After several strokes, most people report that it
feels very unpleasant, and they try to "wipe out"
that sensation by rubbing the area.
! They conduct impulses more slowly, as slow as 1
m/sec (3 1/3 ft/sec) or less.
! They are less sensitive to direct electrical
stimulation than are larger diameter nerve fibers.
Match the following properties to the axon size they go with:
1. large diameter axons (nerve fibers) in peripheral nerves
2. smaller diameter axons
Q2A. higher sensitivity to electrical stimulation
Q2B. trigger unpleasant experience, like pain
Q2C. can produce unpleasant sensation in response to repeated light touch
Q2D. some respond to more than one kind of stimulus
The dominant theory of somatosensory perception is
based on several well-established facts, which can be
summarized as follows: When one kind of neuron in
the brain becomes active (for example, neurons
connected to pressure receptors), the brain can tell
what kind of stimulus is affecting the skin (in the
example, pressure) by which kind of neuron is active
and where on the body it comes from.
This is an example of labeled line or anatomical
coding, because the type of neuron that is active
codes the stimulus that (normally) activates it. The
asgn2p
p. 16
neurons that respond to different kinds of stimulation
are separate and connect to different places in the
somatosensory (~touch) areas of the brain. So, each
neuron going to the touch areas of the brain has a
label on it, indicating what it is sensitive to. When it
becomes active the brain can recognize what the
stimulus is from the label on the active neuron.
Q3. According to anatomical coding (labeled line) theory, you can tell the difference between touch, pressure,
and pain because
A. the anatomy of the receptors for the three kinds of stimuli looks different under the microscope.
B. the somatosensory (~ touch) area is just behind the frontal lobe in the middle of the brain.
C. the stimuli activate different neurons which reach different labeled areas of the brain's touch system.
D. the pattern of activity each stimulus generates gives that activity its special label.
E. A and D are both correct.
The labeled line coding theory of pain states that pain
reactions occur when the nerve cells selectively
responsive to noxious (damaging or potentially
damaging) stimuli are activated. These sensory nerve
fibers connect to the brain's pain system and activate
it to produce pain experience.
This theory is incomplete for several reasons. One
reason is the variable relation between injury and
pain: damage and pain experience don't always go
together. Under some conditions, injury can occur
without pain. Under other conditions, injury can have
healed, but the person feels intense pain.
Many reports describe examples of severe injury that
is painless. Soldiers report that having an arm blown
away can feel like a hard pat. Athletes sometimes do
not notice injuries until after a game is over. As many
as 40% of severely injured, conscious accident
victims do not request pain medication when they
arrive at the emergency room. Clearly such injuries
must activate the pain system, but the victims often
do not report pain. (For some examples of failing to
notice pain from severe injuries that people in past
semesters have reported, click HERE.)
On the other hand, intense pain can occur long after
injury has healed. One example is a condition called
causalgia (meaning burning pain), which sometimes
develops after a penetrating injury, like a bullet, near
a peripheral nerve. Long after the wound has healed,
In people suffering from this disorder, repeated gentle
stimulation, especially near the area affected by the
nerve damage, can often trigger intense pain. For a
description of a form of this disorder, called Reflex
Sympathetic Dystrophy Syndrome, click HERE. A few
patients who lose an arm or a leg develop intense
pain in their phantom limbs. (Adults still feel a
distorted version of the limb long after it was
amputated). Phantom limb pain is more likely to occur
if the arm or leg had hurt before amputation.
Q4. A simple labeled line theory of coding pain states that activation of receptors sensitive to noxious
(unpleasant) stimuli will activate pathways that reach the "pain area" of the brain. However, severe injury
can be painless and intense pain can develop without noxious stimuli. Why do these facts raise a problem
for such a theory? [The preceding material tells you enough to let you figure out the answer.]
A. the brain must have several pain areas in it, so the pain signals can get to too many places
B. pain is located in the body and not in the brain
C. according to the theory, severe injury should consistently activate the pain system, and the pain system
should not turn on without noxious stimuli
D. some people must have the pain system incorrectly connected to the somatosensory receptors
asgn2p
Gate control theory attempts to include these
facts about the imperfect relation between
injury and pain (Melzack & Wall, 1965). Gate
Control theory states that signals from pain
receptors must pass through an active filter or
"gate" in the spinal cord to get to the pain
systems of the brain.
p. 17
Figure 2-2p. The basic features of Gate Control theory.
Figure 2-2p summarizes Gate Control Theory.
Activity in smaller diameter sensory nerve
fibers (shown in red), especially ones activated
by noxious (~unpleasant) stimuli, tend to "open
the gate" and send signals into the brain's pain
system. Activity in thicker sensory nerve fibers
(shown in blue), which respond to touch, pressure, or vibration, can close this gate. Closing
the gate blocks signals from the pain sensory
fibers, which prevents the signals from reaching
the pain system in the brain. Activity in the brain
can also close this gate by activating inhibitory
pathways descending from the brain to the gate
(shown in fuchsia).
Match the effects below with that actions that are most likely to produce them.
Q5A. decreases pain by activating large diameter fibers from skin to close the spinal gate
Q5B. increase pain by decreasing connections that close the spinal gate
Q5C. decreases pain by activating fibers from brain that close the spinal gate.
1. applying a vibrator near painful area
2. stress
3. damaging thicker sensory afferent nerve fibers so they fail to conduct all-or-nothing impulses
The fact that activity in thicker fibers activated by
touch and vibration can "close the gate" explains
some things people do to control pain and predicts
new methods for pain control. For example, it
explains why rubbing an injury can decrease its pain.
Rubbing activates the thicker fibers that react to
touch, vibration, and pressure, so rubbing activates
the systems that shut the gate.
Soon after Melzak and Wall proposed Gate Control
Theory, Wall applied it to controlling intractable
(uncontrollable) pain. He proposed using weak
electrical stimulation to stimulate selectively thicker
diameter nerve fibers. Thicker axons respond more
easily to electrical stimulation, and they "close the
grate" between sensory afferent nerve fibers.
Therefore, weak electrical stimulation of peripheral
nerves should be useful for pain control.
This technique often turns out to be successful.
Electrically stimulating through electrodes placed
around sensory nerves from an injured area (or on
the skin over that nerve) relieves many kinds of pain.
Even electrical stimulation through electrodes
carefully placed on the skin surface close to sensory
nerves can relieve pain. This technique is now used
after some forms of surgery.
Q6. A recent article (Barnhill et al., 1996) showed that simply pressing for ten seconds on an injection site
decreases pain of the injection by about 1/3. According to Gate Control Theory, pressing works because
the pressure
A. reduces blood flow, so lack of oxygen deadens nerves.
B. selectively blocks small diameter nerve fibers in peripheral nerves
C. selectively stimulates large diameter nerve fibers in peripheral nerves
D. inhibits transmission through the synapses from pain-signaling nerve fibers to pathways to the brain's pain
system
E. C and D are both correct
F. A, B and D are both correct
asgn2p
p. 18
Gate Control theory can also explain (partly) why
psychological states, such as intense excitement or
stress, can block pain. Such states appear to activate
the descending nerve fibers from the brain. These
fibers close the gate in the spinal cord, which inhibits
signals from the small diameter sensory fibers. Thus,
they do not activate the pathway to the pain system in
the central nervous system.
Neurons that "shut the gate" use endorphins as their
chemical neurotransmitter. Opiates (morphine and
related pain relievers) mimic the action of endorphins,
which explains part of morphine's analgesic (pain
relieving) effects. Asgn2k on adaptation and
inhibitory processes also describes descending
control, using the pain system as its example.
Q7. A placebo is a physiologically inactive substance that can successfully relieve pain, apparently because
people expect them to do so. Gate Control Theory explains (part of) the placebo effect in pain because the
placebo may
A. activate the descending nerve fibers in the brain that prevent signals from passing through the spinal filter to
the pain system
B. activate the large diameter peripheral nerves that prevent signals from passing through the spinal filter to
the pain system
C. inhibit the small diameter peripheral nerves that send signals through the spinal filter to the pain system
D. all of the above are correct
Link to an article about the development of Gate Control Theory.
Click HERE or HERE (then scroll down) or HERE for articles on pain and pain management.
Sources of information about other senses: Olfaction (smell).
asgn2p -- Touch and Pain
Copyright © 2002 by Gabriel P. Frommer