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Vision
The eyes are composed of an optical portion, which focuses the visual image on the
receptor cells, and a neural component, which transforms the visual image into
a pattern of neural discharges.
The Optics of Vision
Light waves are propagated in all directions from every point of a visible object.
Before an accurate image of a point on the object is achieved, these divergent light
waves must pass through an optical system that focuses them back into a point. In the
eye, the image of the object being viewed is focused upon the retina, a thin layer of
neural tissue lining the back of the eyeball. The retina contains the light-sensitive
receptor cells, the rods and cones, as well as several types of neurons. The lens and
cornea of the eye are the optical systems that focus impinging light rays into an
image upon the retina. At a boundary between two substances of different densities,
such as the cornea and the air, light rays are bent so that they travel in a new direction.
The cornea plays a larger quantitative role than the lens in focusing light rays because
the rays are bent more in passing from air into the cornea than they are when passing
into and out of the lens or any other transparent structure of the eye. The surface of
the cornea is curved so that light rays coming from a single point source hit the cornea
at different angles and are bent different amounts, directing the light rays back to a
point after emerging from the lens.
The image is focused on a specialized area known as the fovea centralis, the area of
the retina that gives rise to the greatest visual clarity. The image on the retina is
upside down relative to the original light source, and it is also reversed right to left.
Light rays from objects close to the eye strike the cornea at greater angles and must be
bent more in order to reconverge on the retina. Although, as noted above, the cornea
performs the greater part quantitatively of focusing the visual image on the retina, all
adjustments for distance are made by changes in lens shape. Such changes are part of
the process known as accommodation.
The shape of the lens is controlled by the ciliary muscle and the tension it applies to
the zonular fibers, which attach this smooth muscle to the lens. To focus on distant
objects, the zonular fibers pull the lens into a flattened, oval shape. When their pull is
removed for near vision, the natural elasticity of the lens causes it to become more
spherical. This more spherical shape provides additional bending of the light rays,
which is important to focus near objects on the retina. The ciliary muscle, which is
stimulated by parasympathetic nerves, is circular, like a sphincter, so that it draws
nearer to the lens as it contracts and therefore removes tension on the zonular fibers,
resulting in accommodation for viewing near objects. Accommodation also includes
other mechanisms that move the lens slightly toward the back of the eye, turn the eyes
inward toward the nose (convergence), and constrict the pupil. The sequence of events
for accommodation is reversed when distant objects are viewed.
The cells that make up most of the lens lose their internal membranous organelles
early in life and are thus transparent, but they lack the ability to replicate. The only
lens cells that retain the capacity to divide are on the surface of the lens, and as new
cells are formed, older cells come to lie deeper within the lens.
With increasing age, the central part of the lens becomes denser and stiffer and
acquires a coloration that progresses from yellow to black. Since the lens must be
elastic to assume a more spherical shape during accommodation for near vision,
the increasing stiffness of the lens that occurs with aging makes accommodation for
near vision increasingly difficult. This condition, known as presbyopia, is a normal
part of the aging process and is the reason that people around 45 years of age may
have to begin wearing reading glasses or bifocals for close work.
The changes in lens color that occur with aging are responsible for cataract, which is
an opacity of the lens and one of the most common eye disorders. Early changes in
lens color do not interfere with vision, but is impaired as the process slowly continues.
The opaque lens can be removed surgically. With the aid of an implanted artificial
lens or compensating eyeglasses, effective vision can be restored, although the
ability to accommodate is lost.
Cornea and lens shape and eyeball length determine the point where light rays
reconverge. Defects in vision occur if the eyeball is too long in relation to the
focusing power of the lens. In this case, the images of near objects fall on the retina,
but the images of far objects focus at a point in front of the retina. This is a
nearsighted, or myopic, eye, which is unable to see distant objects clearly.
In contrast, if the eye is too short for the lens, images of distant objects are focused
on the retina but those of near objects are focused behind it. This eye is farsighted, or
hyperopic, and near vision is poor.
Defects in vision also occur where the lens or cornea does not have a smoothly
spherical surface, a condition known as astigmatism. These surface imperfections can
usually be compensated for by eyeglasses.
The lens separates two fluid-filled chambers in the eye, the anterior chamber, which
contains aqueous humor, and the posterior chamber, which contains the more viscous
vitreous humor. These two fluids are colorless and permit the transmission of light
from the front of the eye to the retina. The aqueous humor is formed by special
vascular tissue that overlies the ciliary muscle. In some instances, the aqueous humor
is formed faster than it is removed, which results in increased pressure within the eye.
Glaucoma, the leading cause of irreversible blindness, is a disease in which the axons
of the optic nerve die, but it is often associated with increased pressure within the eye.
The amount of light entering the eye is controlled by muscles in the ring like,
pigmented tissue known as the iris, the color being of no importance as long as the
tissue is sufficiently opaque to prevent the passage of light.
The hole in the center of the iris through which light enters the eye is the pupil.
The iris is composed of smooth muscle, which is innervated by autonomic nerves.
Stimulation of sympathetic nerves to the iris enlarges the pupil by causing the radially
arranged muscle fibers to contract. Stimulation of parasympathetic fibers to the iris
makes the pupil smaller by causing the sphincter muscle fibers, which circle around
the pupil, to contract.
These neurally induced changes occur in response to light-sensitive reflexes. Bright
light causes a decrease in the diameter of the pupil, which reduces the amount of light
entering the eye and restricts the light to the central part of the lens for more accurate
vision. Conversely, the iris enlarges in dim light, when maximal illumination is
needed. Changes also occur as a result of emotion or pain.
Photoreceptor Cells
The photoreceptor cells in the retina are called rods and cones because of the shapes
of their light-sensitive tips. Note that the light-sensitive portion of the photoreceptor
cells—the tips of the rods and cones—faces away from the incoming light, and the
light must pass through all the cell layers of the retina before reaching the
photoreceptors and stimulating them. A pigmented layer (the choroid), which lies
behind the retina, absorbs light and prevents its reflection back to the rods and cones,
which would cause the visual image to be blurred.
The rods are extremely sensitive and respond to very low levels of illumination,
whereas the cones are considerably less sensitive and respond only when
the light is brighter than, for example, twilight. The photoreceptors contain molecules
called photopigments, which absorb light. There are four different photopigments in
the retina, one (rhodopsin) in the rods and one in each of the three cone types. Each
photopigment contains an opsin and a chromophore.
Opsin is a collective term for a group of integral membrane proteins, one of which
surrounds and binds a chromophore molecule. The chromophore, which is the actual
light-sensitive part of the photopigment, is the same in each of the four photopigments
and is retinal, a derivative of vitamin A. The opsin differs in each of the four
photopigments.
Since each type of opsin binds to the chromophore in a different way and filters light
differently, each of the four photopigments absorbs light most effectively at a
different part of the visible spectrum. For example, one photopigment absorbs
wavelengths in the range of red light best, whereas another absorbs green light best.
Within the photoreceptor cells, the photopigments lie in specialized membranes that
are arranged in highly ordered stacks, or discs, parallel to the surface of the retina.
The repeated layers of membranes in each photoreceptor may contain over a billion
molecules of photopigment, providing an effective trap for light.
Light activates retinal, causing it to change shape. This change triggers a cascade of
biochemical events lead to hyperpolarization of the photoreceptor cell’s plasma
membrane and, thereby, decreased release of neurotransmitter (glutamate) from the
cell. Note that in the case of photoreceptors the response of the cell a stimulus (light)
is a hyperpolarizing receptor potential and a decrease in neurotransmitter release. The
decrease in neurotransmitter then causes the bipolar cells, which synapse with the
photoreceptor cell, to undergo a hyperpolarization in membrane potential.
After its activation by light, retinal changes back to its resting shape by several
mechanisms that do not depend on light but are enzyme mediated. Thus, in the dark,
retinal has its resting shape, the photoreceptor cell is partially depolarized, and more
neurotransmitter is being released. When one steps back from a place of bright
sunlight into a darkened room, dark adaptation, a temporary “blindness,” takes place.
In the low levels of illumination of the darkened room, vision can only be supplied by
the rods, which have greater sensitivity than the cones. During the exposure to bright
light, however, the rods’ rhodopsin has been completely activated. It cannot respond
fully again until it is restored to its resting state, a process requiring some tens of
minutes. Dark adaptation occurs, in part, as enzymes regenerate the initial form of
rhodopsin, which can respond to light.
Neural Pathways of Vision
The neural pathways of vision begin with the rods and cones. These photoreceptors
communicate by way of electrical synapses with each other and with second order
neurons, the bipolar cell. The bipolar cells synapse (still within the retina) both upon
neurons that pass information horizontally from one part of the retina to another and
upon the ganglion cells. Via these latter synapses, the ganglion cells are caused to
respond differentially to the various characteristics of visual images, such as color,
intensity, form, and movement. Thus, a great deal of information processing takes
place at this early stage of the sensory pathway.
The distinct characteristics of the visual image are transmitted through the visual
system along multiple, parallel pathways by two types of ganglion cells, each type
concerned with different aspects of the visual stimulus. Parallel processing of
information continues all the way to and within the cerebral cortex, to the highest
stages of visual neural networks.
Ganglion cells are the first cells in the visual system to respond to activation by
producing action potentials, whereas the rods and cones and almost all other retinal
neurons produce only graded potentials.
The axons of the ganglion cells form the output from the retina—the optic nerve,
cranial nerve II. The two optic nerves meet at the base of the brain to form the optic
chiasm, where some of the fibers cross to the opposite side of the brain, providing
both cerebral hemispheres with input from each eye.
Optic nerve fibers project to several structures in the brain, the largest number passing
to the thalamus, where the information from the different ganglion cell types is kept
distinct. In addition to the input from the retina, many neurons of the lateral geniculate
nucleus also receive input from the brainstem reticular formation and input relayed
back from the visual cortex. These nonretinal inputs can control the transmission of
information from the retina to the visual cortex and may be involved in the ability to
shift attention between vision and the other sensory modalities. The lateral geniculate
nucleus sends action potentials to the visual cortex, the primary visual area of the
cerebral cortex. Different aspects of visual information are carried in parallel
pathways and are processed simultaneously in a number of independent ways in
different parts of the cerebral cortex before they are reintegrated to produce the
conscious sensation of sight and the perceptions associated with it. The cells of the
visual pathways are organized to handle information about line, contrast, movement,
and color. They do not, however, form a picture in the brain. Rather, they form a
spatial and temporal pattern of electrical activity. We mentioned that a substantial
number of fibers of the visual pathway project to regions of the brain other than the
visual cortex. For example, visual information is transmitted to the suprachiasmatic
nucleus, which lies just above the optic chiasm and functions as a “biological clock.”
Information about diurnal cycles of light intensity is used to entrain this neuronal
clock. Other visual information is passed to the brainstem and cerebellum, where it is
used in the coordination of eye and head movements, fixation of gaze, and change in
pupil size.
Color Vision
The colors we perceive are related to the wavelengths of light that are reflected,
absorbed, or transmitted by the pigments in the objects of our visual world. For
example, an object appears red because shorter wavelengths, which would be
perceived as blue, are absorbed by the object, while the longer wavelengths,
perceived as red, are reflected from the object to excite the photopigment of the retina
most sensitive to red.
Light perceived as white is a mixture of all wavelengths, and black is the absence of
all light. Color vision begins with activation of the photopigments in the cone receptor
cells. Human retinas have three kinds of cones, which contain red-, green-, or bluesensitive photopigments. As their names imply, these pigments absorb and hence
respond optimally to light of different wavelengths. Because the red pigment is
actually more sensitive to the wavelengths that correspond to yellow, this pigment is
sometimes called the yellow photopigment.
Although each type of cone is excited most effectively by light of one particular
wavelength, it responds to other wavelengths as well. Thus, for any given wavelength,
the three cone types are excited to different degrees. For example, in response to light
of 531-nm wavelengths, the green cones respond maximally, the red cones less, and
the blue cones not at all. Our sensation of color depends upon the relative outputs of
these three types of cone cells and their comparison by higher-order cells in the visual
system. The pathways for color vision follow those described in.
Ganglion cells of one type respond to a broad band of wavelengths. In other words,
they receive input from all three types of cones, and they signal not specific color but
general brightness. Ganglion cells of a second type code specific colors. These latter
cells are also called opponent color cells because they have an excitatory input from
one type of cone receptor and an inhibitory input from another. The cell gives a weak
response when stimulated with a white light because the light contains both blue and
red wavelengths. Other more complicated patterns also exist. The output from these
cells is recorded by multiple—and as yet unclear—strategies in visual centers of the
brain. At high light intensities, as in daylight vision, most people—92 percent of the
male population and over 99 percent of the female population—have normal color
vision. People with the most common kind of color blindness—a better term is color
deficiency—either lack the red or green cone pigments entirely or have them in an
abnormal form; as a result, they have trouble perceiving red versus green.
Eye Movement
The cones are most concentrated in the fovea centralis, and images focused there are
seen with the greatest acuity. In order to focus the most important point in the visual
image (the fixation point) on the fovea and keep it there, the eyeball must be able to
move. Six skeletal muscles attached to the outside of each eyeball control its
movement. These muscles perform two basic movements, fast and slow. The fast
movements, called saccades, are small, jerking movements that rapidly bring the eye
from one fixation point to another to allow search of the visual field. In addition,
saccades move the visual image over the receptors, thereby preventing adaptation.
Saccades also occur during certain periods of sleep when the eyes are closed, and may
be associated with “watching” the visual imagery of dreams. Slow eye movements are
involved both in tracking visual objects as they move through the visual field and
during compensation for movements of the head. The control centers for these
compensating movements obtain their information about head movement from the
vestibular system, which will be described shortly. Control systems for the other slow
movements of the eyes require the continuous feedback of visual information about
the moving object.