Tool Module: Using Wada’s Test To Identify the Dominant Hemisphere for Language

Wada’s test was first applied to humans in the 1960s by Dr. Juhn Wada at the Montreal Neurological Institute. This procedure consists in injecting a short-acting anaesthetic (a barbiturate such as sodium amobarbital—also known as amytal—for example) into one of the carotid arteries. The anaesthetic then spreads preferentially into the blood vessels of the brain hemisphere that is ipsilateral with respect to the injection. After 10 minutes, because the sensorimotor functions on each side of the body are controlled by the opposite side of the brain, the limbs on the side of the body that is contralateral with respect to the injection become paralyzed, and somatic sensation is suppressed.

Once a subject has been anaesthetized in this way, researchers can test the language capacities of the anaesthetized hemisphere by asking him or her questions. If the hemisphere that has been anaesthetized is the subject’s dominant hemisphere for speech, the subject will be unable to speak and will have troubles in understanding speech until the effects of the anaesthetic have worn off. But if the anaesthetized hemisphere is not the subject’s dominant hemisphere for speech, then he or she will be able to answer questions normally.

Wada’s test is most often used as part of the preparations for neurosurgery on epileptic patients, to determine which of the patient’s brain hemispheres controls language. Thus, when the surgeons are removing the epileptic focus, they can make every effort to spare the language areas in this hemisphere, given the disastrous consequences that loss of language would have for the patient. (The disadvantage of Wada’s test is that it must be conducted quickly, both because the effects of amytal last only a short time—about 10 to 15 minutes—and because it can often cause the patient to fall asleep.)

The many Wada tests that have been performed on epilepsy patients indicate that the vast majority of right-handed persons and a large percentage of left-handed persons have their major language functions in the left hemisphere. Thus, some left-handers have their language functions in the left hemisphere but their dominant motor areas in the right, which makes it hard to imagine a close relationship between the lateralization of these two functions. Manual lateralization (handedness) appears to be, in the same way as language lateralization but independently of it, an example of the advantage for the brain in concentrating a functional specialization on one side of the brain so as to optimize the connections among the neural circuits involved.

Following a Wada test, and just before removing an epileptic focus or a brain tumour, the neurosurgeon can determine the boundaries of the language areas more precisely by applying weak local electrical stimuli directly to the cortex and thus producing “brain maps” during the actual operation. This technique was developed in the 1940s and 1950s by Wilder Penfield and his colleagues at the Montreal Neurological Institute. In general, their brain maps confirmed the findings obtained by other methods, for example, that a large area of the left perisylvian cortex clearly plays a role in the understanding and production of language.

These methods also revealed how greatly the locations of the language areas vary from one person to another. This variability is so great that the relationship between these locations and those shown in medical manuals is often highly approximate.