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Major brain surgery is called "major" because it often requires removal of a large portion of brain tissue. Often a tumor or other bit of nastiness will be located somewhere fairly deep under the brain's surface, and in other cases a part or all of a hemisphere must be removed to stop severe epileptic seizures. Either way, the surgeons need to know which hemisphere is responsible for language processing, so they can try to leave it alone and damage the other side. One of the common ways of making this determination is known as the Wada Technique, after its inventor Juhn Wada.

Each hemisphere is served by its own carotid artery, which runs up the neck and supplies blood for that entire side. To determine which hemisphere does language processing, the neurosurgeon injects sodium amobarbital into one or the other carotid. Amobarbital is a powerful sedative/hypnotic barbiturate, and rushes up the artery and through the blood-brain barrier into which ever hemisphere is on that side. That hemisphere temporarily goes dead, leaving the patient with only the other hemisphere remaining conscious. The doctor then asks the patient questions in order to determine if he still has most of his language ability. Usually the Wada test is done on separate occasions for both sides of the brain, to make sure language ability isn't bilateral.

Some of the literature capitalizes this as the WADA Technique. However, since the technique is named after a person and not an acronym, capitalizing the whole word is no doubt a mistake. A precursor to Wada testing was first done by Dr. Wada in Japan in 1949; the first English journal paper explaining use of the technique was in 1960, published with T. Rasmussen.

A little bit off topic, but here are some of the results that have been found with the Wada Technique since 1960. Left hemisphere language dominance is present for 96% of all right-handed people, and the remaining 4% have dominant right hemisphere. Bilaterality of language processing is not found in right-handed people. For left-handers and the ambidextrous, 70% are left dominant, 15% are bilateral, and 15% are right dominant. Note that these numbers were taken from a patient population, and may not be representative of the general population.

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