This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

The Wada Test

The two hemispheres usually cooperate, but they do not necessarily contribute equally to every task. Language usually involves the left hemisphere more than the right. Spatial tasks (such as reading a map) usually involve the right more than the left. When one hemisphere is more involved in an activity than the other, this is called hemispheric specialization or lateralization (LAT-er-al-iz-A-shun) from the word "lateral" meaning "side."

What is lateralization? What is the Wada test?

Some of the best evidence relating to hemispheric specialization comes from the Wada test—a procedure carried out before brain surgery. In the Wada test each hemisphere is put to sleep at different times by injecting anesthetic into a major artery that leads only to that hemisphere. While one hemisphere is anesthetized, the other remains awake and conscious. Once half the brain is anesthetized, doctors can interview the other half (if the patient is capable of talking) or give instructions to perform simple tasks. This helps to determine what skills or abilities might be confined to one hemisphere. Typically, each hemisphere is tested on a different day before surgery.

The Wada test reveals that 95% of right-handers and 70% of left-handers speak fluently only when the left hemisphere is awake and not anesthetized. 15% of left-handers can speak using their right hemispheres, when the left hemisphere is asleep, but they cannot speak when only the left hemisphere is awake. Another 15% of left-handers can speak when either side is awake.

How does the Wada test confirm lateralization of emotion?

One of the interesting findings from the Wada test is related to emotion. When the left hemisphere is put to sleep, leaving the right active, patients typically became sad and worried. When the right hemisphere is anesthetized, leaving the left awake, patients typically are happy and unconcerned about upcoming brain surgery.

Write to Dr. Dewey at

Don't see what you need? Psych Web has over 1,000 pages, so it may be elsewhere on the site. Do a site-specific Google search using the box below.

Custom Search

Copyright © 2007-2011 Russ Dewey