Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_27_библиотеки_им_акад_М_И_Перельмана
.pdf
Description
Figure 3.7 Visual Pathways
Source: Garrett, Brain and Behavior, 5e, 2018, SAGE Publications,
Inc.
The Visual Cortex
Once information from the ganglion cells reaches the visual cortex, it is processed
by feature detectors—neurons that are sensitive only to certain aspects of a visual
image, such as lines or angles. This aspect of vision was first revealed by David
Hubel and Torsten Wiesel (1962), who implanted microelectrodes in the visual
cortexes of cats (and later monkeys). They projected different types of visual stimuli
on a screen, and measured the electrical activity of single cells (illustrated in Figure
3.8). Do different neurons specialize in certain types of information? If neuronal
specialization was the case, then the goal was to map or “decode” the visual cortex.
feature detectors. Neurons in the visual cortex that respond to specific aspects
of a visual stimulus (such as lines or angles).
Description
Figure 3.8 Hubel and Wiesel’s Apparatus
Monica Wierzbicki/Body Scientific Intl.
https://t.me/medicina_free
In a painstaking series of studies, Hubel and Wiesel (1979) discovered that three
types of neurons service the visual cortex and that each type has its own specialists
at work. Simple cells are activated by highly particular images. For example, some
simple cells fire in response to a vertical line in the middle of the screen but not to a
line that is off-center or tilted at a different angle. Other simple cells fire in response to
horizontal lines, wider lines, or lines tilted at a 45-degree angle. The stimulusresponse connection is that specific. Complex cells receive input from many simple
cells. Although complex cells specialize in certain types of images, they react to those
images anywhere in the visual receptive field—center, bottom, side, and so on.
Finally, hypercomplex cells receive input from complex cells and respond to stimulus
patterns. If one simple cell is activated by /, a second by \, and a third by -, the
hypercomplex cell might react to a combination of these features, as in the letter A.
When you consider the complexity of words, faces, landscapes, three-dimensional
objects, skylines, and other images that enrich our lives, it is no wonder that the
visual cortex is packed tightly with more than 100 million neurons.
In 1981, Hubel and Wiesel were awarded a Nobel Prize for their work. In the
years since their discovery of feature detectors in the visual cortex, others have
identified neurons that fire primarily in response to highly specific features such as
color, form, movement, navigation, and the depth of a visual stimulus (Hubel, 1996;
Killian, Jutras, & Buffalo, 2012; Livingstone & Hubel, 1988).
Color Vision
Ruby-red apples. Lush green grass. Although some animals see the world in pale
shades (including the bull, which is supposedly enraged by the sight of a matador’s
bright-red cape), all mammals have some form of color vision (Jacobs, 1993). For us
humans, color is a particularly vital part of the visual experience and is also linked in
interesting ways to emotion. Thus, sadness feels blue, anger makes us see red,
death is mourned in black, and jealousy brings a visit from the green-eyed monster.
Color perception even varies by gender. Abramov and colleagues (2012) and Murray
and colleagues (2012), for example, both found that females are generally better than
males at discriminating between various shades of colors.
In general, individual differences in color discrimination can exist because color is
a property of the viewer, not the object. “The dress that broke the Internet” is a good
example of this. On February 26, 2015, the dress in the accompanying photo stirred
much controversy and was viewed more than 28 million times within 24 hours. Some
viewers perceived the dress as blue and black, while others perceived the dress as
white and gold. The debate became a phenomenon—to the extent that Ellen
DeGeneres posted on Instagram, “From this day on, the world will be divided into two
people: blue and black, or white and gold” (as cited by Weintraub, 2018). Years later,
the debate has been settled—it is a blue and black dress. Neal Adams, an
ophthalmologist, was asked about the optical illusion that caused some people to
perceive it as white and gold. His explanation attributed the difference in perception to
how light strikes the retina. “If light skews in one direction, a color looks blue-black. In
another, it looks yellow-white” (Weintraub, 2018). What can skew the light coming
toward the retina? Issues with the eye, like cataracts and aging of the lens; amberhued glasses; the hue of the light in the store where the photo was taken; and the
screen the image is displayed on can result in this illusion. What about color vision
that isn’t affected by such an illusion? The rose perceived as red is a good example.
When sunlight shines on a red rose, only the long red rays in the spectrum are
reflected into our eyes. All other wavelengths are absorbed in the flower’s surface. (If
no wavelengths were absorbed, then the rose would appear white.) Ironically, then,
the rose holds everything but red. Most people can discriminate among 200 different
colors and thousands of different shades. How do we do it? There are two major
theories of color vision: the trichromatic theory and the opponent-process theory
(Jacobs, 2014; Kaiser & Boynton, 1996; Lennie, 2000).
https://t.me/medicina_free
What colors of fabric do you see when you look at this dress? For a few days in
2015, people were divided over that question. The division highlighted an important
lesson in perception—color is created in our brains.
picture alliance / Contributor / Getty
Early in the 19th century, physiologists Thomas Young (1802) and Hermann von
Helmholtz (1852) argued that the human eye is receptive to three primary colors—
red, blue, and green—and that all other colors are derived from combinations of
these primaries. By recording the neural responses of individual cones to different
wavelengths of light, 20th-century researchers later confirmed the Young-Helmholtz
trichromatic theory (Schnapf, Kraft, & Baylor, 1987; Wald, 1964). Specifically, there
are three types of cones, each having a different photochemical that produces a
particular response to light. One type fires most when struck by short wavelengths, so
it picks up the color blue. The second type is most sensitive to the middle
wavelengths, for the color green. The third type is most sensitive to long
wavelengths, for the color red. In short, blue cones, green cones, and red cones
serve as the building blocks for color vision. The different combinations of cones
produce other colors in the eye’s “palette.” Activate both red and green cones, for
example, and you will see the color yellow. Activate all three types of cones, and
white is produced (illustrated in Figure 3.9).
trichromatic theory. A theory of color vision stating that the retina contains three
types of color receptors—for red, blue, and green—and that these combine to
produce all other colors.
https://t.me/medicina_free
Description
Figure 3.9 Trichromatic Theory
German physiologist Ewald Hering (1878) was not completely satisfied with the
trichromatic theory. As he saw it, yellow was a primary color, not a derivative of red
and green. He also noticed that certain color combinations just do not seem to exist.
A mix of red and blue gives rise to varying shades of purple, but what is reddish
green? Another puzzling phenomenon that didn’t fit was the occurrence of negative
afterimages, sensations that persist after prolonged exposure to a stimulus.
afterimage. A visual sensation that persists after prolonged exposure to and
removal of a stimulus.
TRY THIS!
Afterimages
To observe the formation of an afterimage, TRY THIS: At the center of the
accompanying image is a small white dot. Stare at it for 60 seconds with minimal
blinking, and then look at a white sheet of paper or a white wall. According to the
opponent-process theory, a negative afterimage should appear that converts into the
face of a celebrity. If an afterimage doesn’t appear right away, blink and look again.
https://t.me/medicina_free
Source: Via Wikimedia Commons, Afterimage by Dimitri Parant,
concentrate 30 seconds on the white dot and close your eyes 10 seconds.
2009, Flickr.
Putting the pieces together, Hering proposed the opponent-process theory of
color vision. According to this theory, there are three types of visual receptors, and
each is sensitive to a pair of complementary or “opponent” colors. One type reacts to
the colors blue and yellow, a second type detects red and green, and a third type
detects variations in brightness ranging from black to white. The color wheel in Figure
3.10 illustrates how these primary colors and their “companions” line up on nearly
opposite sides of the circle. Within each pair of red-green, blue-yellow, and blackwhite receptors, some parts fire more to one color whereas other parts react to its
opposite. That is why we never see bluish yellow or reddish green, but we might see
bluish green and reddish yellow. While seeing one color at a specific spot on the
retina, you cannot also see its opposite on the same spot (Conway, 2002, 2013;
Jacobs, 2014).
opponent-process theory. The theory that color vision is derived from three pairs
of opposing receptors. The opponent colors are blue and yellow, red and green, and
black and white.
https://t.me/medicina_free
Description
Figure 3.10 The Color Wheel
Source: Garrett, Brain and Behavior, 5e, 2018, SAGE Publications, Inc
The opponent-process theory can explain two aspects of color vision that its
predecessor theory could not. First, it explains afterimages. Think about what you
saw when you completed the Try This! exercise. Staring at the blue areas of
Beyoncé’s altered image causes the blue-seeing cells to fire. Then, when the blue
color is removed from view, these parts of the cells become temporarily fatigued,
leaving only the yellow parts to fire normally (Vimal, Pokorny, & Smith, 1987). This
process tips the neural balance to yellow, which produces a brief “rebound” effect
(staring at green and black triggers a similar rebounding of red and white,
respectively).
Second, opponent-process theory can explain color deficiency, usually a genetic
disorder. In actuality, only about 1 in 100,000 people are color “blind,” seeing the
world in only black, white, and shades of gray (Nathans, 1989). Rather, color-deficient
people tend to confuse certain colors. The most common problem, particularly among
2 percent of men (Álvaro, Moreira, Lillo, & Franklin, 2015), is red-green color
deficiency, which is an inability to distinguish between red and green because both
appear gray (illustrated in Figure 3.11). Though very rare, a second form of color
deficiency is the inability to distinguish between—you guessed it—blue and yellow.
Figure 3.11 Test of Color Deficiency
Album / Alamy Stock Photo
PRISMA ARCHIVO / Alamy Stock Photo
For many years, researchers debated the relative merits of the trichromatic and
opponent-process theories of color vision. As often happens in either-or debates, it
https://t.me/medicina_free
now appears that both theories are correct—and that the way people sense color is a
two-step process. According to this view, the human retina contains red, blue, and
green cones, as suggested by the trichromatic theory. But in the thalamus—where
these signals are sent en route to the visual cortex—single-cell recordings reveal that
the neurons operate in accordance with the opponent-process theory. That is, some
cells are excited by red and inhibited by green, and vice versa. Other cells react to
blue and yellow. Color vision is complex and still not fully understood (Conway, 2013;
Jacobs, 2014; Simunovic, 2016).
This is how these colored displays look to persons with (a) red-green color
deficiency calledprotanopia, (b) a second form of red-green color deficiency
calleddeuteranopia, and (c) blue-yellow color deficiency called tritanopia.
LEARNING CHECK
The Eyes Have It
Below are parts of the visual system, listed alphabetically. Rearrange them in the
order in which they process visual input, starting with the first part of the eye to
receive light and ending with the part of the brain that processes visual information.
(Answer: cornea, iris, pupil, lens, vitreous humor, retina, optic nerve, optic chiasm,
thalamus, visual cortex.)
Hearing
If you had to lose your sense of either sight or sound, which would you choose? It
is easy to take the sounds of everyday life for granted—and, indeed, we often say
that “silence is golden.” But auditory sensations surround us and inform us: the
chatter of voices, music throbbing from stereo speakers, the trickling of water over
pebbles, the crunching of potato chips, the hum of a fluorescent lamp, the crack of a
wooden bat against a baseball, the clinking of champagne glasses on New Year’s
Eve, the screeching of brakes, and the chime of an incoming text message. In fact,
we are so dependent on noise that U.S. regulators are requiring electric cars to make
sounds when traveling at speeds over 18.6 miles per hour (Matousek, 2018). If you
aren’t sure why this is so important, think about trying to cross the street with nothing
but your auditory senses to rely on. But what is sound, and how do we hear it?
Sound Waves
Every sensation is born of energy. For vision, or seeing, the stimulus is light. For
audition, or hearing, the stimulus is sound. As in light, sound travels in waves.
Physically, sound is vibration, a pattern of rapid wavelike movement of air molecules.
First, something has to move—an engine, vocal cords, violin strings, or clapping
hands. The movement jolts the surrounding molecules of air, and these collide with
other air molecules. Like the ocean, sound ripples in waves that ebb and flow in all
directions. It loses energy from one ripple to the next, however, which is the reason
sound fades at a distance. Sound travels through air at 750 miles per hour—much
https://t.me/medicina_free
slower than the speed of light, which is 186,000 miles per second. That is why, in
thunderstorms, you see lightning before you hear the accompanying thunder.
audition. The sense of hearing.
Like light, sound waves can be distinguished by three major properties, as
summarized in Table 3.2. The first is wavelength, or frequency. As molecules of air
push outward from a source, they expand and compress in cycles. Frequency,
measured by the number of cycles completed per second, is expressed as hertz (Hz).
One cycle per second equals 1 Hz. Subjectively, the frequency of a sound wave
determines its pitch (the highness or lowness of a sound). The higher the frequency,
the higher the pitch. In May 2018, pitch became a popular Google search thanks to
the “Yanny vs. Laurel debate.” A viral tweet by YouTube personality Cloe Feldman
played an audio track that divided listeners into two camps; people heard either
“Yanny” or “Laurel” (Gutman, 2018). To demonstrate that all listeners could hear both
names with a simple adjustment to pitch, software developer Steve Pomeroy created
an audio file posted on SoundCloud (Gutman, 2018). When the pitch dropped by 30
percent, listeners most likely heard “Laurel.” When the pitch raised by 30 percent, the
listeners most likely heard “Yanny.” Thus, the apparatus we use to play an audio file
can make all the difference. This is why sound engineers and headphone settings
have so much influence on what we hear.
Table 3.2
Humans can hear frequencies ranging from about 20 Hz to 20,000 Hz—in music,
the equivalent of almost 10 octaves. Homing pigeons and elephants can hear lower
frequencies. Bats, dogs, and dolphins hear at higher frequencies (dogs can hear at
50,000 Hz, which is why a “silent” dog whistle is not silent to a dog). Most of the
sounds we need to hear, and certainly those we enjoy hearing, are well within this
range. The lowest note on a piano is 27.5 Hz, the highest note is 4,180 Hz, and the
voices of conversation range from 200 to 800 Hz. When all frequencies of the sound
spectrum are combined, they produce a hissing sound. This hissing is called white
noise—named by analogy to the white light that results from the combination of all
wavelengths in the visible light spectrum.
white noise. A hissing sound that results from a combination of all frequencies of
the sound spectrum.
The second property of sound is amplitude. Amplitude refers to the intensity, or
height, of each sound wave. In physical terms, the amplitude of a wave determines its
loudness. The greater the amplitude, the louder the sound. We may not be able to
hear a pin drop, but our ears are responsive to a remarkably wide range of
amplitudes. For variations within this range, amplitude is measured in decibels (dB).
Figure 3.12 provides examples of the loudness of various sounds at different dB
levels. You will see that dB levels over 120 are painful and can cause permanent
damage to the ears (Kryter, 1994).
https://t.me/medicina_free
Description
Figure 3.12 Common Sounds and the Amounts of Noise They Produce, in
Decibels
A third property of sound is purity, or complexity. Strike a tuning fork, and you will
produce something rare: a pure tone consisting of a single frequency of vibration. In
reality, most sounds are complex mixtures of waves of different frequencies. Speech,
music, a ringing bell, and a breaking window are familiar examples (Bregman, 1990;
Krumhansl, 1991). The complexity of a sound determines its timbre, or tonal quality.
Play the same note at the same loudness on a piano, trumpet, saxophone, tuba, and
violin, and what you will hear are differences in timbre.
The Auditory System
Philosophers like to ponder the age-old question, “If a tree falls in a forest, but no
one is around to hear it, does it make a sound?” This really is a profound question.
We know that the fall of a tree sends waves of molecules blasting through the air, but
we also know that without an auditory system to catch these molecules—well, you
make the call. As in vision, hearing requires that energy be detected, converted into
neural impulses, and relayed to the brain. As shown in Figure 3.13, this complex
process begins in the three-part (outer, middle, and inner) structure of the human ear.
https://t.me/medicina_free
Description
Figure 3.13 The Human Ear
Sound waves are collected in the outer ear, beginning with the fleshy pinna. Some
animals, such as dogs, cats, and deer, can wiggle this structure like a radar dish to
maximize the reception of sound (humans cannot). Research shows that the folds of
the pinna enable people to pinpoint the location of sounds—for example, whether
they come from above us or below, in front or behind. The sound waves are then
funneled through the auditory canal to the eardrum, a tightly stretched membrane that
separates the outer and middle portions of the ear. The eardrum vibrates back and
forth to the waves, thereby setting into motion a series of tiny connecting bones in the
middle ear—the hammer, the anvil, and the stirrup (for you trivia buffs, these are the
three smallest bones in the body). This middle-ear activity amplifies sound by a factor
of 30. The last of these bones, the stirrup, then vibrates against a soft inner-ear
membrane called the oval window. This vibration is transmitted to the fluid that fills
the canals of the cochlea, a snail-shape tube—and the resulting motion presses up
against the basilar membrane, which brushes up against an array of 16,000 sensitive
hair cells. These hair cells bend, exciting fibers in the auditory nerve—a bundle of
axons that link to auditory centers of the brain. Also in the inner ear are semicircular
canals, which, as we will see later, play a critical role in balance (Hudspeth, 2000;
Phillips et al., 2015).
To summarize, the “plumbing” and “wiring” that turn sound waves into meaningful
input are fairly intricate. Sound waves collected in the outer ear are transmitted into a
salt-watery fluid and then transformed into electrical impulses in the inner ear. From
the auditory nerve, signals then cross to the other side of the brain. Next, they get
routed to the thalamus, where the medial geniculate body transforms the signals en
route to the auditory cortex (Bartlett, 2013) to assist with frequency discrimination.
Once there, the signals are processed by cells that specialize in high, middle, or low
frequencies of sound (Leaver & Rauschecker, 2016).
https://t.me/medicina_free
Соседние файлы в папке Библиотека им академика М.И. Перельмана