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Figure 1.4-19
The corpus callosum
This large band of neural
fibers connects the two brain
hemispheres. (a) To photograph
this half-brain, a surgeon
separated the hemispheres
by cutting through the corpus Dr. Patric Hagmann/CHUV, UNIL, Lausanne, Switzerland
callosum (see the blue arrow)
and lower brain regions. (b) This
high-resolution diffusion spectrum Martin M. Rotker/Science Source
image, showing a top-facing brain
from above, reveals the brain
neural networks within the two
hemispheres, and the corpus (a) (b)
callosum neural bridge between
them.

messages between them ( Figure 1.4-19 ). The neurosurgeons knew that psychologists Roger

Sperry, Ronald Myers, and Michael Gazzaniga had divided cats’ and monkeys’ brains in this
manner, with no serious ill effects.
So, the surgeons operated. The result? The seizures all but disappeared. The patients
with these split brains were surprisingly healthy, with their personality and intellect
hardly affected. Waking from surgery, one even joked that he had a “splitting headache”
( Gazzaniga, 1967 ). By sharing their experiences, these patients have greatly expanded our
understanding of interactions between the intact
brain’s two hemispheres.
TEACH 1.4-9 Figure 1.4-20 visual field visual field To appreciate these findings, we need to focus
Left
Right
The information highway for a minute on the peculiar nature of our visual wir-
Teaching Tip from eye to brain ing, illustrated in Figure 1.4-20 Note that each eye

.

Remind students that visual informa- receives sensory information from the entire visual
field. But in each eye, information from the left half
tion directed to each side of the brain of your field of vision goes to your right hemisphere,
comes from visual fields, not from and information from the right half of your visual field
each eye. The left eye doesn’t send goes to your left hemisphere, which usually controls
speech. Information received by either hemisphere
information to the right hemisphere, is quickly transmitted to the other across the corpus
and vice versa—the right halves of callosum. In a person with a severed corpus callosum,
each eye send information to the right this information sharing does not take place.
Knowing these facts, Sperry and Gazzaniga
hemisphere, and vice versa. could send information to a patient’s left or right
Optic
Left visual field Right half of each nerves hemisphere. As the person stared at a spot, the
eye Right hemisphere researchers flashed a stimulus to its right or left. They
could do this with you, too, but in your intact brain,
Right visual field Left half of each Copyright © Bedford, Freeman & Worth Publishers. the hemisphere receiving the information would
instantly pass the news to the other side. Because the
eye Left hemisphere Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
Optic split-brain surgery had cut the communication lines
chiasm
Speech between the hemispheres, the researchers could, with
these patients, quiz each hemisphere separately.
In an early experiment, Gazzaniga (1967) asked
split-brain patients to stare at a dot as he flashed
split brain a condition resulting HE•ART on a screen ( Figure 1.4-21 ). Thus, HE

from surgery that separates appeared in their left visual field (which transmits to
the brain’s two hemispheres
by cutting the fibers (mainly the right hemisphere) and ART in the right field (which
those of the corpus callosum) Visual area Corpus Visual area transmits to the left hemisphere). When he then asked
connecting them. of left callosum of right them to say what they had seen, the patients reported
hemisphere hemisphere that they had seen ART. But when asked to point with
82 Unit 1 Biological Bases of Behavior
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82 Unit 1 Biological Bases of Behavior

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