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Some of the earliest evidence for hemispheric specialization came from research on language.

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In the 1860s, a French surgeon named Paul Broca studied a patient who had lost the ability to speak.

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The patient had suffered a stroke that damaged the left hemisphere’s frontal lobe, but did not affect the right hemisphere’s frontal lobe.

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Broca noted that other patients who had suffered comparable damage to the right frontal lobe did not lose their ability to speak.

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So Broca proposed that language was a special ability controlled by the left hemisphere.

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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.

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The German physician Karl Wernicke was able to trace other language problems to left hemisphere damage.

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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.

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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.

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This part of the brain is now called Wernicke’s area.

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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.

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In the other 10% – mainly left-handed people – language abilities are controlled by the right hemisphere or shared across both hemispheres.