Some of the earliest evidence for hemispheric specialization came from research on language.
In the 1860s, a French surgeon named Paul Broca studied a patient who had lost the ability to speak.
The patient had suffered a stroke that damaged the left hemisphere’s frontal lobe, but did not affect the right hemisphere’s frontal lobe.
Broca noted that other patients who had suffered comparable damage to the right frontal lobe did not lose their ability to speak.
So Broca proposed that language was a special ability controlled by the left hemisphere.
Because Broca’s hypothesis turned out to be correct, we now refer to the speech-producing region of the left frontal lobe as Broca’s area.
The German physician Karl Wernicke was able to trace other language problems to left hemisphere damage.
He studied patients who could still speak, but who couldn’t comprehend anything spoken to them—in fact, they didn’t seem to comprehend their own speech.
He discovered that people with this type of aphasia tended to have brain damage not in Broca’s area, but in a region at the back of the left hemisphere’s temporal lobe.
This part of the brain is now called Wernicke’s area.
More recent research shows that in approximately 90% of adults the production of language, as well as language comprehension, is controlled primarily by the left hemisphere.
In the other 10% – mainly left-handed people – language abilities are controlled by the right hemisphere or shared across both hemispheres.