"A perfectly bilateral machine or organism could not perform left-right discriminations, which much include scanning in a preferred (e.g. left-to-right) direction and distinguishing between such letters as d and b, p and q and such words as saw and was, such organisms could only make mirror-image responses to mirror-image stimuli..." (J.L. Bradshaw, 1983)
The right hemisphere is biased toward global processing and the left for local processing. Robert Sperry listed the following specializations for the right hemisphere: rhythm, spatial awareness, Gestalt (the whole picture), imagination, daydreaming, color, dimension. This supposedly describes the creative right brain of many left-handed artists. The left hemisphere, which controls the right hand - typically the dominant hand for writing the name - and which has a higher ratio of grey to white matter than the right hemisphere, specializes in verbal-linguistic and analytic functions.
Many techniques have been developed in the field of left-right brain research, but the Wada test must be one of the most dramatic and conclusive experiments. During a surgical procedure where the large arteries leading to the brain are exposed, injection of drugs into the bloodstream will selectively anesthetize one hemisphere. The Wada test is not performed very often, but it might be used before major brain surgery to determine which hemisphere is dominant for language.
From Human cerebral asymmetry, J.L. Bradshaw writes,
"Wada (1949) first developed the procedure known as the Wada test: an injection of the barbiturate sodium amytal (amobarbital) into one (left or right) of the common carotid arteries that supplies its ipsilateral hemisphere. A temporary loss of function is produced on the affected side of the brain: a flattening of the EEG, along with hemiparesis, hemianesthesia, and hemianopsia. It is in effect a reversible hemispherectomy, permitting tests of higher mental functions in the other hemisphere. If the drug is injected into the dominant hemisphere, there is usually total and abrupt cessation of speech. In view of the slight but definite risk accompanying the insertion of the needle or catheter into the carotid artery, the Wada test is employed only when absolutely necessary, and consequently such patients usually have more or less severe existing abnormalities of cerebral function. Moreover, since the unilateral suppression lasts only five to ten minutes and the experience itself is probably disturbing to the patient, usually all that can be determined is which hemisphere is specialized for speech production. Typically, the upraised contralateral arm and leg fall to the bed with a flaccid paralysis a few seconds after the injection. If the injected hemisphere is nondominant for speech, there is an abrupt and more or less total cessation of speech shortly after the injection; it lasts until recovery from the hemiparesis is well advanced. The patient makes characteristic dysphasic responses (perseveration, misnaming, mixing up the sequence of numbers and of days of the week, etc.) for several minutes before speech returns to normal. The Wada test has shown that dextrals have clear left hemisphere language dominance in over 90 percent of the cases, and sinistrals in 70 percent of cases. The effects of unilateral ECT are similar to those of unilaterally injected sodium amytal." (J.L. Bradshaw, 1983)
Depression of the left hemisphere with the Wada technique is associated with a depressive catastrophic reaction and for most people the traumatic loss of speech. Depression of the right hemisphere with sodium amytal leads to a euphoric or manic response.
Bradshaw, J. L. and N. C. Nettleton (1983). Human cerebral asymmetry. Prentice-Hall, New Jersey.