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believe that a split-brain patient’s mind resembles a river that has branched into separate
streams, each unaware of its influence on the other (Pinto et al., 2017). (Reading these reports,
can you imagine a patient enjoying a solitary game of “rock, paper, scissors” — left versus right
hand?)
When the “two minds” are at odds, the left hemisphere does mental gymnastics to
rationalize reactions it does not understand. If a patient follows an order (“Walk”) sent
to the right hemisphere, a strange thing happens. The left hemisphere, unaware of the
order, doesn’t know why the patient begins walking. But if asked, the patient doesn’t reply,
“I don’t know.” Instead, the left hemisphere improvises — “I’m going into the house to get a
Coke.” Gazzaniga (2006), who described these patients as “the most fascinating people on
earth,” realized that the conscious left hemisphere resembles an “interpreter” that instantly
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
constructs explanations. The brain, he concluded, often runs on autopilot; it acts first and
then explains itself.

Copyright © Bedford, Freeman & Worth Publishers.

®
AP Science Practice Check Your Understanding

Examine the Concept Apply the Concept
▶ ▶Explain what is meant by split brain. ▶ ▶(a) If we flash a red light to the right hemisphere of a person
▶ ▶Explain the classic split-brain studies. with a split brain, and flash a green light to the left hemisphere,
will each hemisphere observe its own color? (b) Will the person
be aware that the colors differ? (c) What will the person verbally
report seeing?
▶ ▶Can you put yourself in the shoes of a patient with a split
brain? What would it be like to have knowledge which you were
unaware of and couldn’t verbally report but were nevertheless
able to act on?
Answers to the Examine the Concept questions can be found in Appendix C at the end of the book.

Right–Left Differences in the Intact Brain

So, what about the 99.99+ percent of us with undivided brains? Does each of our hemi-
spheres also perform distinct functions? The short answer is Yes. When a person performs
a perceptual task, a brain scan often reveals increased activity (brain waves, blood flow, and
glucose consumption) in the right hemisphere. When the person speaks or does a math
calculation, activity usually increases in the left hemisphere.
A dramatic demonstration of hemispheric specialization happens before some types of
brain surgery. To locate the patient’s language centers, the surgeon injects a sedative into
the neck artery feeding blood to the left hemisphere, which usually controls speech. Before
the injection, the patient is lying down, arms in the air, chatting with the doctor. Can you
predict what happens when the drug puts the left hemisphere to sleep? Within seconds,
the person’s right arm falls limp. If the left hemisphere is controlling language, the patient
will be speechless until the drug wears off. If the drug is injected into the artery to the right
hemisphere, the left arm will fall limp, but the person will still be able to speak.
To the brain, language is language, whether spoken or signed. (See Module 3.5 for
more on how and where the brain processes language.) Just as hearing people usually use
the left hemisphere to process spoken language, deaf people use the left hemisphere to
process sign language (Corina et al., 1992; Hickok et al., 2001). Thus, a left hemisphere
stroke disrupts a deaf person’s signing, much as it would disrupt a hearing person’s
speaking (Corina, 1998).

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