Evidence gathered from a variety of both clinical and experimental cases has demonstrated that the language ability of humans is predominantly lateralized to the left cerebral hemisphere. Initial evidence for this was often provided by clinical cases involving penetrating head injuries or stroke. In such cases, lesions to areas of the brain involved in language produce lasting difficulties in both the production and the comprehension of speech.
The advent of modern surgical techniques has provided a more controlled method of examining brain lateralization for language. The neurologist J. Wada introduced one such method in the late 1940s. This has since become known as the Wada test. During the procedure, Sodium amytal is injected into one of the carotid arteries of an individual while he is counting. As each artery supplies blood to only one of the cerebral hemispheres, hemispheric dominance for language ability can be determined. If the non-dominant hemisphere be involved, the individual continues to count. Conversely, if the dominant hemisphere be involved, the individual becomes confused, loses count, and experiences severe aphasia (difficulty in the production and/or comprehension of speech). Results from this test have indicated that although some people are right-hemisphere dominant for language, the majority of people are left-hemisphere dominant.
Hemispheric deactivation: Wada Test.
Lateralization In Normal Brains