Scientists have learned a lot about where and how brains process language by studying people who have lost their ability to produce or understand language after an accident or illness. Modern methods of neuroscience such as functional magnetic resonance imaging have also shown us which areas of the brains get activated when we are processing language. By stimulating certain brain areas electrically, we have also found many areas that are involved in language processing.
Wernicke's and Broca's Areas
Some of the earliest observations about language lateralization--that certain areas of the brain process language--came from studying patients who had had lesions. This is how both Broca's and Wernicke's areas were discovered. Paul Broca demonstrated in 1861 that his patient, who had had a lesion in the left hemisphere of the brain, had lost his ability to speak. The area in which he had the lesion, a structure in the left frontal lobe, later became known as Broca's area. Later, it was discover that as a result of a lesion on the left temporal hemisphere, a patient lost his ability to understand speech. This area was named Wernicke's area after the doctor who first reported his findings.
Left Hemisphere
After the findings of Broca and Wernicke, researchers concluded that a damage to the left hemisphere typically resulted in aphasia, a condition in which language functions are severely impaired. It has been estimated that over 95 percent of right-handed and approximately 70 percent of the left-handed people are processing language in their left hemisphere. This has been shown, for example, by Drs. Gazzaniga and Hutsler in their book "Hemispheric Specialization."
Split-Brain Patients
Further evidence for hemispheric specialization was presented by Dr. Gazzaniga in the journal Neuron in 1995. He investigated patients for whom the connections between the right and left hemisphere had been cut to prevent severe epilepsy seizures from traveling from one hemisphere to another. In these so-called split-brain patients, both hemispheres were working perfectly but were not able to communicate with one another. Dr. Gazzaniga found out that when he projected an image to the right visual field, which is controlled by the left hemisphere, the patients could describe what they saw. But when the same image was displayed to the left visual field, which is controlled by the right hemisphere, the patients could not say anything. Dr. Gazzaniga concluded that the left brain is dominant for language and speech.
Right Hemisphere
Although the left hemisphere is the main language area, the right hemisphere still performs some language-related activities such as auditory comprehension. It is also involved in metaphors, jokes and sarcasm. This was shown by Drs. Kolb and Whishaw in their book "Fundamentals of Human Neuropsychology."
Planum Temporale
Geschwind and Levitsky reported in their classic study that was published in the journal Science in 1968 that an area called planum temporal, located in the superior temporal plane and part of the Wernicke's area, was larger in size on the left hemisphere as compared to the right side. This finding provided the first neuroanatomical evidence for the left hemisphere's specialization for language. Later, Dr. Chi and colleagues showed in a study published in 1977 that this difference in size is already in place in fetuses. Interestingly, it has been found that in children suffering from dyslexia, a language-related disability, this difference in size oftentimes cannot be seen.
References
- "Hemispheric Specialization"; Michael Gazzaniga, Jeffrey J. Hutsler; 1999
- "Neuron" Journal; Principles of human brain organization derived from split-brain studies; Gazzaniga; vol. 14, 217-228, 1995.
- Science; Human brain: left-right asymmetries in thetemporal speech region.;Geschwind, Levitsky; vol. 161, 186--87, 1968.
- Archives of Neurology Journal; asymmetries of speechareas; vol. 34, 346--48, 1977
- "Fundamentals of Human Neuropsychology"; Bryan Kolb, Ian Q. Whishaw; 1990
