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you pick up the object, does it appear to grow back to the same size? What might be
even more fun, but a total mess, would be to fill some random objects with tasty and
colorful stuff that delivers a punch of great smells like spaghetti or macaroni and
cheese. Now, imagine you are an infant with these food-filled objects. Red, orange,
and green end up everywhere, like a Jackson Pollock painting, but the awesome part
is that this amateur artwork is edible. Babies tend to love getting their hands in it.
What does red and green mixed together look like? Taste like? Smell like? Does it
feel different? And that splatting noise… Can I make that happen again? Yes, food is
fun to play with!
An experience with baby dinnertime can make one appreciate what we might
have been like while learning about the world. We often take for granted that it has a
lot to offer, and our sensory systems bring some of it into the brain with radar-like
sensitivity. However, our brains have to learn the rules of our world. When an object is
far away, it looks small. As it gets closer, it looks as if it is growing. When it has an
edge, the color of the object seems to change—a darker hue is present and a
shadow is cast. In this chapter, we will examine the psychology of sensation and
perception. These terms are used to describe different stages in the process by which
we acquire information about the world. In sensation, our eyes, ears, and other
sensory receptors absorb raw physical energy. Through the process of transduction,
this raw energy is converted into neural signals that are sent to the brain. In
perception, these signals are then selected, organized, and interpreted, as illustrated
in Figure 3.2. We hope that, while you learn about these fascinating processes, you
will appreciate the incredible complexity in how the world works in collaboration with
our biology and psychology. The result of said collaboration? An infinite combination
of experiences.
sensation. The processes by which our sense organs receive information from
the environment.
transduction. The process by which physical energy is converted into sensory
neural impulses.
perception. The processes by which people select, organize, and interpret
sensations.
Description
Figure 3.2 Processes of Sensation and Perception
Psychologists used to treat sensation and perception as separate. Sensation was
considered a strictly physiological process involving the various sense organs,
receptors, neural pathways, and regions of the brain. Perception was considered a
purely psychological process by which we derive meaning from these sensations. In
this view, the body supplied the raw material and the mind made sense of that
material. We now know, however, that in this continuous stream of events, there is no
bright line dividing sensation and perception. As you will learn, the interaction
between body and mind is seamless, but because different processes are at work,
psychologists still find it useful to make the distinction.
This distinction comes to life in a poignant true story told by neurologist Oliver
Sacks (1995). Virgil, a 50-year-old Oklahoma man, had been blind since the age of 6.
Because Virgil could see light and faint shadows, a local ophthalmologist suggested
that it might be possible to restore his eyesight through surgery. Starting with the right
eye, the doctor removed a thick cataract that blanketed the retina, inserted a new
lens implant, and bandaged the eye for 24 hours. The next day, the bandage was
removed. It was the moment of truth, but Virgil did not cry out with joy or react in any
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other way. Instead, he stared blankly at the surgeon, silent and bewildered. As Sacks
put it, “The dramatic moment stayed vacant, grew longer, sagged.” Was the operation
a success? Could Virgil see? In a manner of speaking, yes. He said he could detect
light, forms, movement, and color, all mixed up in a confusing and cluttered blur. Only
when the doctor started to speak did Virgil realize that he was staring at a face. His
retina was alive and well, but his brain could not make sense of the information.
There was sensation, but no perception.
Even as the weeks passed, Virgil remained disoriented. In the supermarket, he
was overwhelmed, even stressed, by all the visual stimulation—the bright lights, the
shelves lined with cans and jars, the fruits and vegetables, and the people wheeling
carts up and down the aisles. “Everything ran together,” he said. Virgil could not
identify by sight common objects such as chairs and tables that he recognized easily
by touching. He also lacked the ability to perceive depth. He was confused by
shadows, often stopping to step over one. Yet he saw a staircase as a flat surface of
parallel and crossing lines rather than as a three-dimensional solid object. Movement
posed additional problems. He would recognize his dog one moment, but then
wonder if it was the same animal when he saw it from a different angle.
For reasons that are unclear, Virgil suddenly became ill, collapsed, and almost
died. He had a respiratory illness and needed a constant supply of oxygen. By the
time he returned home from the hospital, he had to carry an oxygen tank wherever he
went. Unable to work, Virgil lost his job, his house, and, once again, his eyesight.
Extensive tests found no response to light whatsoever—and no electrical activity in
the visual cortex. He was totally blind. But all was not lost. As Sacks (1995) put it,
“Now, at last, Virgil is allowed to not see, allowed to escape from the glaring,
confusing world of sight and space, and to return to his own true being, the intimate,
concentrated world of the other senses that had been his home for almost fifty years”
(p. 152).
It is the study of sensation that will start the chapter and allow us to examine the
physiology of vision, hearing, smell, taste, touch, and other sensory systems. Then
we will examine the psychological processes of perception that enable us to
comprehend and interpret this raw material. As we will see, the world “out there”
comes to us through an interaction of physical energy, the body, and the mind.
MEASURING THE SENSORY EXPERIENCE
LEARNING OBJECTIVES
Distinguish between a stimulus and a sensation; recognize the interplay between
the two that inspired some of the first psychologists to measure them.
Explain how physical energy becomes a psychological experience.
Determine the minimum amount of stimulation needed to register on our
senses.
Differentiate between measurements for the smallest amount of stimulation
detectable and the smallest change in stimulation detectable.
Light, vibration, odor-filled molecules, cold winds, warm breezes, and the collision
of bodies. The first generation of psychologists, including Wilhelm Wundt, raised the
most basic of questions: Whatever the stimulus, how does physical energy become a
psychological experience? How much light is necessary to see? How can you hear a
pin drop or detect minute variations in pitch well enough to tune a musical
instrument? How different must two wines be for a wine taster to tell them apart?
Inspired by the work of Gustav Fechner (1860), questions of this nature gave birth to
psychology’s first subfield, psychophysics: the study of the relationship between
physical stimuli and subjective sensations. The key to psychophysics is
measurement. Because sensation is subjective, it cannot be measured using
objective instruments the way you assess height, weight, or time. There are no
yardsticks, or scales, or stopwatches—only subjects and their self-reports. New
procedures thus had to be devised to maximize the accuracy of these reports
(Gescheider, 1997). Much of the material discussed in this chapter was derived from
these psychophysical procedures.
psychophysics. The study of the relationship between physical stimulation and
subjective sensations.
Absolute Thresholds
What is the minimum amount of light that we can see, the weakest vibration that
we can hear, or the faintest odor that we can smell? How much sugar needs to be
added to a food for us to taste more sweetness? What is the slightest amount of skin
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pressure, as in a tickle, that we can feel? Just how sensitive are our sensory
systems? Researchers interested in a sensation begin by trying to determine an
absolute threshold, the minimum level of stimulation that an organism can detect.
absolute threshold. The smallest amount of stimulation that can be detected.
Absolute threshold can be derived in different ways. One method is simply to ask
a subject to adjust the intensity of a stimulus until it is barely detectable. A second
method is to gradually increase the intensity level and ask subjects from one trial to
the next if they detect the stimulus. A third method is to vary the stimulus presentation
randomly, again checking with the subject on each trial. Over the years, research has
shown that absolute thresholds are not “absolute.” There is no single point on the
intensity scale at which people suddenly detect a stimulus. Rather, detection rates
increase gradually. Psychophysics researchers thus define absolute threshold as the
point at which a stimulus can be detected 50 percent of the time. Defined in this way,
some of our absolute thresholds are highly impressive, as summarized in Table 3.1.
Table 3.1
Signal-Detection Theory
Imagine you are a subject in a classical psychophysics experiment. You are sitting
in a darkened room staring at a blank wall, and the experimenter presents a series of
flashes varying in brightness. Did you see it? What about the next one, and the one
after that? On some trials, the flashes are clear, well above threshold, so you say yes.
But on other trials, you are just not sure. With the experimenter waiting for a
response, what do you say? Confronted with this dilemma, some subjects prefer to
say yes (when in doubt, go for it). Others, more cautious, say no (unless it is clear, do
not go out on a limb). These tendencies to respond yes or no in uncertain situations
are individual response biases—and they have little to do with sensation. The
problem for the researcher, then, is that a subject’s responses are influenced not only
by the strength of the signal but also by background factors such as the subject’s
personality, motivation, and expectations.
Enter signal-detection theory. Based on the assumption that performance is
determined jointly by the strength of a signal and the subject’s response criterion (that
is, the subject’s willingness to say yes rather than no), signal-detection theory gave
rise to a more sophisticated method. On some trials, a weak stimulus is presented.
On others, no stimulus is presented. By comparing a subject’s “hit” versus “miss” rate
on stimulus trials to the subject’s tendency to commit “false alarms” by saying yes in
blank trials, a researcher can mathematically separate the subject’s detection
performance from the response bias (Green & Swets, 1966; Wickens, 2001;
Wickens, Hollands, Banbury, & Parasuraman, 2016). The method of establishing
absolute thresholds was based on the assumption that a threshold is determined
solely by the stimulus. But signal-detection theory recognizes that response biases
are also at work. This approach provides the psychologist with a valuable tool for
analyzing why air traffic controllers are so quick to detect danger signals on the radar
screen (Wickens et al., 2009), why overeager witnesses identify innocent suspects in
police lineups (Wixted & Mickes, 2014), or why doctors tend to over-diagnose certain
diseases from available test results (Lynn & Barrett, 2014; Swets, 1996). Signaldetection theory has also been used to evaluate how clinical psychologists make the
prediction that someone will suffer from a psychological disorder (Bardeen, Stevens,
Clark, Lahti, & Cropsey, 2015), attempt suicide, or erupt in violence (McFall & Treat,
1999).
signal-detection theory. The theory that detecting a stimulus is jointly
determined by the signal and the subject’s response criterion.
Difference Thresholds
Sensory capacities are measured not only by our ability to detect low levels of
stimulation but also by the extent to which we can detect subtle differences. This
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ability is determined by asking subjects to compare the brightness of two light bulbs,
the loudness of two tones, the weight of two blocks, and so on. Given one stimulus,
subjects are asked to adjust the level of another stimulus so that the two are the
same. Or subjects are given the two stimuli and asked to report whether they are the
same or different. Either way, it is possible to pinpoint the smallest change in
stimulation that subjects can detect 50 percent of the time. This point is called the
difference threshold, or just noticeable difference (JND).
just noticeable difference (JND). The smallest amount of change in a stimulus
that can be detected.
Tuning a piano requires a heightened ability to detect subtle differences among
tones.
iStock.com/syolacan
While measuring difference thresholds, Ernst Weber (1834) quickly noticed that
JNDs increase with the size or intensity of the stimulus—and that the magnitude of a
JND is a constant proportion of the original stimulus. In other words, as the stimulus
increases in magnitude, a greater change is needed before it can be detected. This
general principle is known as Weber’s law. To illustrate, the JND for weight is 1/50,
or 2 percent. In other words, if you lift a 50-ounce object, and then a 51-ounce object,
you will probably notice that the second one is heavier than the first. However, you
would not feel a difference between one object that weighs 50 pounds and another
that weighs 50 pounds, 1 ounce. Again, there is an absolute difference of 1 ounce;
but a JND of 2 percent means that if your reference point is a 50-pound object, you
would not detect a difference unless the second object is equal to or greater than 51
pounds. Except at the extremes, Weber’s law provides a good estimate of our
difference thresholds. It can also be applied to other senses—though each has a
different threshold. For example, the JND is 10 percent for loudness. Consider, for
example, the last time you turned up the volume. Imagine the manufacturer of your
headphones did not know about the JND. As a result, turning up the volume one
notch meant increasing the volume 1 percent. Would you notice the change?
Unfortunately, you likely would not. It would take turning up the volume by 10 notches
for you to notice a slight increase. Thus, those headphones would probably be
returned not just by you but by many other consumers who believed the headphones
did not work properly.
Weber’s law. The principle that the just noticeable difference of a stimulus is a
constant proportion despite variations in intensity.
SENSATION
LEARNING OBJECTIVES
Imagine what life would be like for one day without sensation.
List the steps that occur when the human visual system converts light into
meaningful color images.
Describe the structures in the auditory system that convert vibrating air
molecules into meaningful sounds.
Compare and contrast the chemical senses of smell to taste.
Examine how those who have synesthesia experience the world.
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In school, we are taught that there are five senses: vision, hearing, taste, smell,
and touch. This simple notion can be traced to the writings of Aristotle (384–322
BCE). Even today, people who believe in “extrasensory” perception, or ESP, call it the
“sixth sense.” In fact, we have more than five sensory modalities. Vision has two
subsystems, one for daylight and one for nighttime conditions. The chemical senses
of taste and smell are easily distinguished, but touch is really a mixture of several
skin senses—including pressure, pain, warmth, and cold. We also have a keen sense
of balance and of the position and movement of our body parts. Combined, these
various systems bring in a steady stream of information from the world around us.
Vision
Humans, with some exceptions, are visual creatures. How many times have you
said, “Show me,” “I’ll believe it when I see it,” “Out of sight, out of mind,” or “My eyes
are playing tricks on me”? Like other aspects of human anatomy, our visual system is
a highly adapted product of evolution. The earliest forms of life could “see” in the sea
through faint patches of membrane that were sensitive to light. They could tell
brightness from dark and even turn toward the light source. In contrast, other features
—such as shapes, textures, motion, and color—could be detected later, only by more
advanced forms of life (Land & Fernald, 1992).
Light
For every sensory system, physical energy is the source of stimulation. The
stimulus input for vision is light—a form of energy known as electromagnetic radiation
that travels through empty space in oscillating waves. As illustrated in Figure 3.3,
what we see as light comes from a narrow band in the spectrum of electromagnetic
radiation. All matter gives off electromagnetic radiation of different wavelengths (a
wavelength is measured by the distance between waves). The sun and other stars
give off radiation that includes light. So do fires and electric lamps. Visible
wavelengths range from about 380 to 760 nanometers (a nanometer is one billionth
of a meter). Thus, some waves (such as X-rays, ultraviolet rays, and gamma rays)
are too short for us to see and fall below our visible range. Others (such as infrared
rays, TV signals, radio waves, and radar) are too long for us to see, so they exceed
our visible range. Other organisms have sensory capabilities that are different from
ours. For example, most insects can see shorter wavelengths in the ultraviolet
spectrum, and most fish and reptiles can see longer wavelengths in the infrared
spectrum.
Description
Figure 3.3 The Electromagnetic Spectrum
The length of a light wave determines its hue, or perceived color. To the human
eye, white light is made up of all visible wavelengths combined. Short wavelengths
look bluish, medium wavelengths look greenish, and long wavelengths look reddish.
The picturesque colors of the visible spectrum can be seen in a rainbow or in the
spectrum of colors produced when white light passes through a glass prism. A
second property of light is its intensity, or amplitude, as measured by the height of the
peaks in the wave. As wavelength determines color, amplitude determines
brightness. The higher the amplitude, the brighter the light appears to be. A third
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physical property of light is its purity, as measured by the number of wavelengths that
make up the light. Purity influences the saturation, or richness, of colors. The fewer
wavelengths there are in a light (the purer it is), the richer or more saturated is the
color. A pure red light made up of only a narrow band of wavelengths would give off a
rich fire-engine or tomato-like color. In contrast, white light—which contains all visible
wavelengths—is completely unsaturated and lacking in color.
The Visual System
The Eye
The fantastic journey of neural impulses through the human visual system begins
with the eye—an extension of the brain and the most exposed part of the central
nervous system. Lying in a protective bony socket within the skull, the eye converts,
or transduces, light waves into electrochemical neural impulses. The major structures
of the human eye are presented in Figure 3.4.
Description
Figure 3.4 Structures of the Human Eye
Light waves provide the stimulus input for vision, but what is seen depends on the
capabilities of the visual system that is in place. Accordingly, different species see the
world in different ways. Eagles can spot a tiny field mouse moving in the grass a mile
away. Owls can see at night, in low levels of illumination. Cows and sheep have their
eyes on the sides of the head, enabling them to spot predators sneaking up from
behind. Each species has evolved visual systems uniquely suited to its way of life
(Archer, Djamgoz, Loew, Partridge, & Vallerga, 1999; Nieder, 2012).
Light rays from the outside world first pass through the cornea, a clear, curved
membrane, or “window.” The cornea bends light so that it is sharply focused within
the eye. Abnormalities in the shape of the cornea cause astigmatism, usually
experienced as a selective blurring of parts of the image at a particular orientation,
such as horizontal. Next comes the ring-shaped iris, which gives the eye its color.
The iris is a muscle that is controlled by the autonomic nervous system. Its function is
to regulate the size of the pupil—the small, round hole in the iris through which light
passes. The iris causes the pupil to dilate (enlarge) under dim viewing conditions to
let in more light and to contract (shrink) under brightness to let in less light.
cornea. The clear outer membrane that bends light so that it is sharply focused in
the eye.
iris. The ring of muscle tissue that gives eyes their color and controls the size of
the pupil.
pupil. The small round hole in the iris of the eye through which light passes.
Behind the pupil, light continues through the lens, another transparent structure
whose function is to fine-tune the focusing of the light. The lens brings an image into
focus by changing its shape, in a process called accommodation. Specifically, the
lens becomes more rounded for focusing on nearby objects and flatter for more
distant objects (the cornea, which has a fixed shape, cannot make these adjustments
for different distances). With age, the lens loses much of its elasticity and keeps the
flatter shape appropriate for viewing at a distance—called presbyopia (Garner &
Garner, 2016). As a result, many middle-aged people start to need corrective vision
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devices or procedures such as glasses or intraocular lens implants (Mesa &
Monteiro, 2018).
lens. A transparent structure in the eye that focuses light on the retina.
accommodation. In Piaget’s theory, the process of modifying existing cognitive
structures in response to new information.
Filling the central part of the eyeball is a clear jellylike substance called the
vitreous humor. Light passes through this fluid before it reaches the retina. The retina
is a multilayered screen of cells that lines the back inside surface of the eyeball
(illustrated in Figure 3.5). It is one of the most fascinating tissues in the body—both
because of its function, which is to transform patterns of light into images that the
brain can use, and because of its structure, which illustrates many basic principles of
neural organization. In an odd twist of nature, the image projected on the retina is
upside down. That is, light from the top part of the visual field stimulates
photoreceptor cells in the bottom part of the retina, and vice versa.
retina. The rear, multilayered part of the eye where rods and cones convert light
into neural impulses.
Description
Figure 3.5 The Retina
The retina has aptly been called an extension of the brain (Gregory, 1998). It has
several relatively transparent layers and contains 130 million photoreceptor cells that
convert light energy into neural activity. The layer closest to the back of the eyeball is
lined with two specialized types of nerve cells called rods and cones. Rods are long,
thin, cylindrical cells that are highly sensitive to light. They are concentrated in the
sides of the retina and are active for black-and-white vision in dim light. Under
impossibly ideal conditions, rods have the capacity to detect the light produced by
one ten-billionth of a watt. Cones are shorter, thicker, more tapered cells that are
sensitive to color under high levels of illumination. Cones are densely clustered in the
center of the fovea, the pinhead-size center of the retina. Unlike the rest of the retina,
the fovea contains only cones, and the ratio of rods to cones increases in the outer
edges of the retina. Therefore, cones are sprinkled throughout the retina; by contrast,
the fovea contains a cluster of cones but has no rods.
rods. Rod-shaped photoreceptor cells in the retina that are highly sensitive to
light.
cones. Cone-shaped photoreceptor cells in the retina that are sensitive to color.
fovea. The center of the retina, where cones are clustered.
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In owls and other nocturnal (active only at night) animals, the retina contains only
rods. Thus, they can see at night, but their vision is in black and white. In chipmunks,
pigeons, and other diurnal (active only during the day) animals, the retina contains
only cones. Thus, they are virtually blind at night. In animals that are active both day
and night, the retina has a mixture of rods and cones. For example, the human retina
has about 120 million rods and 7 million cones, enabling us to see colors under
normal lighting and to make out forms under low levels of illumination. Individuals
whose retinas contain no rods suffer from night blindness; those without cones lack
all color vision.
This photo shows how rods and cones, magnified approximately 14,000 times,
line the back wall of the retina.
Omikron / Science Source
Often, we need to adjust to radical changes in illumination. You step inside a
darkened movie theater on a sunny day. As you start walking down the aisle,
however, you have to put your arms out and inch forward slowly, stumbling around.
After a few minutes, you can see again. This experience illustrates dark adaptation,
the process by which eyes become more sensitive to light in a dark environment. It
takes about 30 minutes for you to fully adapt to the dark—at which point the eyes
become 10,000 times more sensitive. It also takes time to adjust to bright light. That
is why, when you leave a movie theater during the day, everything seems so “washed
out” that you have to squint at first to keep out the glare. This is an instance of light
adaptation, the process by which our eyes become less sensitive to light under high
levels of illumination.
dark adaptation. A process of adjustment by which the eyes become more
sensitive to light in a dark environment.
light adaptation. The process of adjustment by which the eyes become less
sensitive to light in a bright environment.
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Bipolar cells activate ganglion cells, which send the information from the eyeball
to the brain.
Science Picture Co / Alamy Stock Photo
When light strikes the rods and cones, it sparks a chain of events within a network
of interconnected neurons that results in vision. Rods and cones contain
photopigments, chemicals that break down in response to light, thus triggering neural
impulses. These impulses activate bipolar cells, which, in turn, activate nearby
ganglion cells. The axons of the ganglion cells form the optic nerve, a pathway that
carries visual information from each eyeball to the brain. The area where the optic
nerve enters the eye has no rods or cones, only axons. So each eye has a blind
spot. We don’t usually notice it because our eyes are always moving, but it’s possible
to find the blind spot through a simple exercise (illustrated in Figure 3.6).
optic nerve. The pathway that carries visual information from the eyeball to the
brain.
blind spot. A part of the retina through which the optic nerve passes. Lacking
rods and cones, this spot is not responsive to light.
Description
Figure 3.6 The Blind Spot
Psychologists used to think that electrical impulses were delivered from the retina
to the brain, as on an assembly line. It was as if there was a simple division of labor,
whereby the retinal neurons “received” sensory information and passed it along on a
conveyer belt to the visual cortex for perceptual “processing.” We now know that the
mechanisms of vision are more complex. Ultimately, signals from 130 million rods and
cones are funneled through a mere 1 million axons in the optic nerve. This pattern of
processing means that the bipolar and ganglion cells must be integrating and
compressing signals from multiple receptors, and the number of bipolar and ganglion
cells interacting with each rod and cone has an impact on visual detail detection.
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As an analogy for this type of processing, consider having a conversation with a
large group versus an individual. When attempting to converse with a large group of
people at one time, you can get the general idea of what the people are saying, but
much information can be lost. This is like the rods’ relationship with bipolar and
ganglion cells. Now, when you have a conversation with one or two people, much
more information is heard and, in turn, less information is lost. This is akin to the
cones’ relationship with bipolar and ganglion cells. This is also why our cones are so
much better at sending detailed visual information than our rods—fewer organisms
are trying to have a conversation at once! The retina is a “smart” optical instrument.
Not only does it receive light, but it also processes visual information (Casanova &
Ptito, 2001; Hoffman, 1998; Palmer, 1999).
Any ganglion cell that represents a cluster of neighboring rods and cones receives
input from a sizable portion of the retina. This region is called a receptive field. By
recording the activity of individual ganglion cells, researchers have found many
different types of receptive fields (Alitto & Usrey, 2015; Kuffler, 1953). The most
common are circular “center-surround” fields in which light falling in the center has
the opposite effect of light in the surrounding area. Some cells are activated by light in
the center and inhibited by light in the surrounding area (center-on cells). Others work
the opposite way; when light hits the center of the cell, it is inhibited but activated by
light that hits the surrounding area (center-off cells). This arrangement makes the
human eye particularly attuned to brightness-and-darkness contrasts in the visual
field—contrasts that indicate corners, borders, and edges.
receptive field. An area of the retina in which stimulation triggers a response in a
cell within the visual system.
Visual Pathways
Axon fibers of ganglion cells form the optic nerve, which is the first part of the
visual pathway that links each eyeball to the brain. The two optic nerves meet at the
optic chiasm, where axons from the inside half of each eye cross over to the opposite
half of the brain. This arrangement means that the left visual field of both eyes is
projected to the right side of the brain, and the right visual field of both eyes is
projected to the left side of the brain. After reaching the optic chiasm, the nerve fibers
travel along two tracts, through the thalamus—the relay station where sensory
signals are directed—to appropriate areas of the visual cortex, located in the back of
the brain (illustrated in Figure 3.7). The visual cortex is the main informationprocessing center for visual information.
visual cortex. Located in the back of the brain, it is the main informationprocessing center for visual information.
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