FoundationalConceptsNeuroscience

are in the temporal lobes, and in performing an excision procedure
the surgeon takes great care to avoid damaging areas of the cortex
involved in language. For this reason, it is essential to know whether
the primary language areas in a surgical patient are in the left hemi-
sphere, the right hemisphere, or both.
To make this determination, a presurgical test for determining the
language-dominant cerebral hemisphere was developed in the 1940s
by neurologist Juhn Wada (b. 1924). Wada very carefully injected a
small dose of a sedative-hypnotic barbiturate drug, such as amobarbi-
tal (see Chapter 9), into either the right or left carotid artery. While the
drug is being injected, the patient is asked to count: one, two, three,
four, five, ... Within seconds, the barbiturate infuses the cortex on
one side of the brain and produces widespread inhibition of neuronal
activity. If the drug reaches a part of the brain involved in language
production, in this case speaking, the patient immediately stops
counting and is unable to resume until the effects of the drug begin to
dissipate, several minutes later. However, if the drug has been injected
into the non-language-dominant hemisphere, then as the sedative-
hypnotic infuses the cortex there is a slight hesitation as the effects of
the drug are felt, followed quickly by resumption of counting. Since
its inception in 1949, the Wada test for the lateralization of language
has been administered to thousands of patients as a prelude to brain
surgery. These days it has been largely replaced with noninvasive
brain imaging procedures such as magnetoencephalography and
functional magnetic resonance imaging (see Chapter 17).
From many years of gathering data obtained from stroke lesions,
Wada tests, and, more recently, functional brain imaging, it is possible
to draw some general conclusions about the cerebral lateralization of
language. As expected, most people have left-hemisphere language
dominance. However, there is a relationship between handedness and