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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
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
from above, reveals the brain
neural networks within the two
hemispheres, and the corpus (a) (b)
callosum neural bridge between
them.
Copyright © Bedford, Freeman & Worth Publishers.
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.
Left Right
Figure 1.4-20 visual field visual field To appreciate these findings, we need to focus
The information highway for a minute on the peculiar nature of our visual wir-
from eye to brain ing, illustrated in Figure 1.4-20 Note that each eye

.
receives sensory information from the entire visual
field. But in each eye, information from the left half
of your field of vision goes to your right hemisphere,
and information from the right half of your visual field
goes to your left hemisphere, which usually controls
speech. Information received by either hemisphere
is quickly transmitted to the other across the corpus
callosum. In a person with a severed corpus callosum,
this information sharing does not take place.
Knowing these facts, Sperry and Gazzaniga
could send information to a patient’s left or right
Optic hemisphere. As the person stared at a spot, the
nerves
researchers flashed a stimulus to its right or left. They
could do this with you, too, but in your intact brain,
the hemisphere receiving the information would
instantly pass the news to the other side. Because the
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 the right hemisphere) and ART in the right field (which
by cutting the fibers (mainly
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

03_myersAPpsychology4e_28116_ch01_002_163.indd 82 15/12/23 9:23 AM