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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_123_библиотеки_им_акад_М_И_Перельмана
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3
Understanding
Perception and
Cognition
https://t.me/med1917
IN THIS PART ...
Uncover how you perceive the external world by
exploring the senses and sensory systems.
Discover the brain mechanisms involved in learning,
memory, and brain plasticity.
See how the brain uses a hierarchical control system
by looking at the frontal lobes and executive functions.
Recognize how language, thought, and consciousness
involve higher-order brain processes.
Find out what occurs in developmental and mental
disorders and how these disorders can be treated.
https://t.me/med1917
CHAPTER 13 Looking At Vision andHearing 233
Chapter13
Looking At Vision
andHearing
Y
ou see with your eyes, and you hear with your ears, right? Well, yes, but
without the brain, you’d never be able to look at a beautiful sunset or listen
to a favorite song. This chapter takes a closer look at the neural responses
involved in sight and sound.
Vision starts with the optical elements of the eye that focus light onto the retina
at the back of the eye. There, photoreceptor cells capture photons and send infor-
mation for analysis by the visual cortex and other brain areas. The brain extracts
from these neural images dierent aspects of the visual world, such as the color,
movement, and location of edges of objects.
Audition (hearing) begins with sound waves creating signals in the inner ear for
analysis by the auditory cortex and beyond.
IN THIS CHAPTER
»
Seeing the light and converting it to
images
»
Transmitting images via the
thalamus to the occipital lobe
»
Seeing shapes and colors, and
identifying objects and faces
»
Catching a sound wave and sending it
to the brain
»
Considering causes of hearing loss
»
Finding your balance in the inner ear
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234 PART 3 Understanding Perception and Cognition
Imaging and Capturing Light: Vision
The eye evolved tissues that act like the optical elements of a camera. The lens of
the eye acts like a camera’s lens, and the pupil of the eye, which is the opening
through which the light enters, acts like a camera’s aperture.
The cornea is the outermost clear layer at the front of the eye. The cornea does
most of the focusing of light (about 70 percent). The lens is responsible for the
rest of the eye’s focusing. It can change shape to adjust the focus for near and far
objects, a process called accommodation (see Figure13-1).
The image formed by the cornea and lens is projected onto the retina, the neural
lining at the back of the eye. The following sections outline how the retina and
brain convert the optical image to neural images.
Making movies on the retina
The eyes are continually moving. The eyes make large movements, called saccades,
about three to four times a second. Even during so-called xation, in between
saccades, the eye still makes various small movements, called microsaccades, drift,
and tremor.
Eye movement is necessary for you to see. Experiments with an apparatus that
keeps the image from moving on the retina showed that after a few seconds all
vision disappears. The retina adapts to constant light, and only responds to
changes.
FIGURE13-1:
Focusing light
by the eye.
© John Wiley & Sons, Inc.
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CHAPTER 13 Looking At Vision andHearing 235
How do you perceive the world as stationary when the image on the retina moves?
The answer is that the brain associates the eye movements it commands with the
movement of the images on the retina.
The retina is part of the CNS.It develops from the neural tube region called the
diencephalon. It has to be outside the skull because optical imaging through the
skull bones wouldn’t work very well.
Converting through phototransduction
The human eye detects light in the visible spectrum (about 380nm to 740 nm).
The retina has layers of neurons and glial cells that are highly conserved across
vertebrate evolution. Figure13-2 shows a diagram of these layers.
Visible light is electromagnetic energy. Photons are the quantum units of light
that are absorbed by the photoreceptor cells called rods and cones:
»
Rods: Rods work in dim light and are responsible for night vision, called
scotopic vision. Because all rods have the same absorption spectrum, you can’t
have color vision because the signal from the rods contains no information
about the wavelength of the photons absorbed. The human retina has about
120 million rods.
»
Cones: Cones function only in bright daylight, called photopic vision. You have
three dierent cone types called red (long), green (medium), and blue (short)
wavelength cones. Cones are responsible for high acuity and color vision. The
brain deduces wavelength from the ratio of activity of the three dierent cone
types. The human retina has about 6 million cones.
FIGURE13-2:
The neurons and
neural circuit in
the retina.
© John Wiley & Sons, Inc.
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236 PART 3 Understanding Perception and Cognition
Photoreceptors convert light into changes in electrical current through the mem-
brane. This modulates the release of the neurotransmitter glutamate.
Photoreceptors have four main structural parts:
»
Outer segment: The outer segment contains rhodopsin in rods or cone opsins
in cones. These photopigments absorb photons to begin the phototransduc-
tion process. The photopigments are located in membrane disk-like struc-
tures. The outer segment also contains a high concentration of gated
ion channels.
»
Inner segment: The inner segment contains mitochondria for energy and
machinery for transporting rhodopsin or cone opsins made in the cell body.
»
Cell body: The cell body contains the nucleus and the sites where proteins are
manufactured.
»
Synaptic terminal: The synaptic terminal is where glutamate is released onto
bipolar and horizontal cells.
Light and the transduction cascade
When a photoreceptor absorbs a photon of light, a cascade of events leads to a
change in glutamate release to other neurons in the retina:
1. Rhodopsin is the light receptor, the photopigment in rods. There are similar
cone opsin receptors in cones. Opsins are G protein-coupled receptors that in
IN THE DARK: CURRENTS AND
SYNAPTICRELEASE
Inside photoreceptor cells in the dark, the second messenger cyclic guanosine mono-
phosphate (cGMP) is continuously synthesized by an enzyme. cGMP binds to and opens
a channel called the cyclic nucleotide gated (CNG) channel that conducts an inward
sodium current. High cGMP levels open many CNG sodium channels in the dark. Leak
potassium channels in the inner segment continuously produce a balancing outward
potassium current. The dark current is the simultaneous entry of sodium in the outer
segment and exit of potassium in the inner segment.
The dark current leads to a membrane potential in the dark of approximately –40 mV.At
this membrane potential, voltage-gated calcium channels at the presynaptic region are
open that stimulates high release of the neurotransmitter glutamate in the dark.
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CHAPTER 13 Looking At Vision andHearing 237
the dark (inactivated state) have the molecule 11-cis retinal bound to them.
When rhodopsin or cone opsins absorb a photon, this induces a transition of
the bound 11-cis retinal to all-trans retinal, called an isomerization. This changes
the conformation of the bound retinal and this activates the rhodopsin protein
and its signaling cascade.
2. The activated (all-trans retinal) rhodopsin stimulates a G protein in the outer
segment called transducin, which causes transducin to dissociate from
guanosine diphosphate (GDP) and bind guanosine triphosphate (GTP). The
GTP-bound transducin activates the enzyme phosphodiesterase (PDE) that
breaks down the cGMP into 5’-GMP.
3. cGMP promotes the opening of CNG sodium channels in the outer segment.
Therefore, the reduction in the cGMP levels (following the photon absorption)
closes the CNG sodium channels.
4. Closure of the CNG sodium channels decreases the sodium current leading to
hyperpolarization of the membrane potential toward the potassium equilib-
rium potential, E
K
.
5. Hyperpolarization of the photoreceptor causes the presynaptic terminal, called
the pedicle to release less glutamate. Photoreceptors don’t re action poten-
tials. Instead, the absorption of photons continuously modulates the mem-
brane potential and release of glutamate.
6. Photoreceptor cells synapse on two types of nearby cells, called bipolar and
horizontal cells, which we discuss in the next section.
In vertebrates, photoreceptors hyperpolarize to light. However, some inverte-
brates, like squids, have photoreceptors that depolarize to light using dierent
photochemistry.
Photoreceptor adaptation
One form of light adaptation is the switch from rods in dim light to cones in bright
light. Both rods and cones themselves adapt to light through mechanisms in their
phototransduction cascade. These adaptational processes allow the photorecep-
tors to generate relatively large signals for changes in light level (contrast),
instead of communicating only the absolute light level.
Photoreceptor distribution in the retina
The photoreceptor distribution (see Figure 13-3) in the retina is highly
non-uniform.
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238 PART 3 Understanding Perception and Cognition
Cone photoreceptors are smaller and concentrated in the central retina (called the
macula) compared with the periphery. Within the macula is a region called the
fovea (the central one degree of the visual eld) where cone photoreceptors are
most densely packed. Rod photoreceptors are more evenly distributed, though still
higher in the macula than the periphery, except for the fovea, where they’re com-
pletely absent. The macula and fovea within it are responsible for central vision.
The fovea and the cones densely packed within it provide the highest level of
visual acuity.
Retinal pigment epithelial cells are nonneural cells located on the outermost region
of the retina, next to photoreceptor cells. The retinal pigment epithelium func-
tions in light absorption, epithelial transport, spatial ion buering, phagocytosis,
secretion, and immune modulation. We discuss dysfunction of the retinal pigment
epithelial cells in the section “Losing vision: Blindness” later in this chapter.
Joining the Nervous System:
Photoreceptor Output
Photoreceptors release glutamate from their synaptic terminals at a synapse
called the ribbon synapse to communicate with bipolar and horizontal cells (see
Figure13-2). These synapses are in a layer called the outer plexiform layer (plex-
iform is a technical word for synaptic). Here we discuss the ow of information
from photoreceptor cells to bipolar cells and horizontal cells.
FIGURE13-3:
Distribution of
rods and cones in
the retina.
© John Wiley & Sons, Inc.
https://t.me/med1917
CHAPTER 13 Looking At Vision andHearing 239
Converting light patterns to contrast:
Bipolar and horizontal cells
Bipolar cells transmit the photoreceptor signal to retinal ganglion cells (called
ganglion cells from here on). Horizontal cells modulate the signal from photore-
ceptors to bipolar cells. They are inhibitory neurons that integrate signals they
receive from surrounding photoreceptors and provide inhibitory feedback to cen-
tral photoreceptors and bipolar cells they connect to.
Bipolar cells may have inputs from just one or a few photoreceptor cells (as occurs
within the fovea) or from many photoreceptors (outside the fovea).
Horizontal cells compute an average light intensity received by surrounding pho-
toreceptors and subtract that level from the center photoreceptors going to each
bipolar cell by releasing inhibitory neurotransmitter back on to photoreceptors.
This process is called lateral inhibition. The result is more glutamate is released for
parts of the image darker than the average and less glutamate is released for parts
of the image brighter. This receptive eld organization enhances contrast.
Making symmetry with On and
O bipolar cells
Two types of bipolar cells respond in opposite ways to the glutamate released by
photoreceptors, as shown in Figure13-2. The bipolar cell types are
»
On bipolar cells: They’re depolarized by light. On bipolar cells express metabo-
tropic receptors (see Chapter5) for glutamate that are inhibitory (called mGluR6)
because they open nonselective cation channels. In the dark, glutamate levels are
high and the mGluR6 receptors inhibit bipolar cells, so they’re hyperpolarized.
Light in the center and the decrease of glutamate released by photoreceptors
leads to less inhibition (disinhibition) of the On bipolar cells, so they become
depolarized (excited) by light. Bipolar cells also transmit to ganglion and amacrine
cells using glutamate. In the dark, On bipolar cells release little glutamate to their
targets but become depolarized by light and release more glutamate.
»
O bipolar cells: They’re hyperpolarized by light because they express an
ionotropic receptor for glutamate, called the AMPA receptor, which is excitatory
(see Chapter5). In the dark, high glutamate release activates AMPA receptors,
which depolarize O bipolar cells that then release more glutamate to their targets.
With light, reduced glutamate release from photoreceptors leads to hyperpolariza-
tion of the O bipolar cells, so they release less glutamate onto their target cells.
The On-O parallel channels are maintained through ganglion cells and on to the
thalamus.
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240 PART 3 Understanding Perception and Cognition
Projecting with retinal ganglion cells
Bipolar cells project to ganglion cells and amacrine cells. Ganglion cells are the
output cells from the retina, the only neurons in the retina that send axons to the
brain. Ganglion cells project to at least 15 areas of the brain.
On and O bipolar cells are connected to matching ganglion cells called On-center
(On) and O-center (O) ganglion cells. Inside the fovea there’s one to one trans-
mission from bipolar cells to ganglion cells, but outside the fovea there’s conver-
gence of many bipolar cells onto each ganglion cell. The four main types of
ganglion cells are as follows:
»
Parvocellular (P)
»
Magnocellular (M)
»
Koniocellular (K)
»
Intrinsically photosensitive ganglion cells
Modulating bipolar cell input to ganglion
cells: amacrine cells
Bipolar cells also transmit to amacrine cells. Most amacrine cells are inhibitory
neurons that function in lateral inhibition by sending feedback signals to ganglion
cells. In addition, amacrine cells can communicate with other amacrine cells and
ganglion cells. A specic class of amacrine cell (called AII) receives synaptic inputs
from rod bipolar cells, which send signals to cone bipolar cells that synapse on
ganglion cells, forming an essential link in the rod to ganglion cell pathway.
PARVOCELLULAR CELLS: COLOR AND SHAPE
The most abundant ganglion cells in mammalian retina have brisk-sustained
responses, meaning that they have high action potential ring rates that are
modulated by small changes in light intensity. In primate retina, these are called
parvocellular (P) cells. These ganglion cells have small cell bodies and axons that
connect to the parvocellular layers in the lateral geniculate nucleus (LGN) of the
thalamus. They code for color, shape, and ne detail. The fovea has specialized
parvocellular ganglion cells, called midget ganglion cells. Here, a single cone pho-
toreceptor connects to one cone bipolar cell, which connects to one ganglion cell
(though lateral interactions with horizontal and amacrine cells still occur). This
produces the high acuity in the human fovea.
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