Why can a stroke take away speech but leave understanding?

Because language is not one job done in one place. Aphasia, a loss of language, is most often caused by stroke: about a quarter of people who have an acute stroke develop it. Depending on where the damage falls, a person may understand everything but struggle to get words out, or speak smoothly but without meaning. Two regions of the left brain explain much of this: Broca's area, in the frontal lobe, is linked to producing speech, and Wernicke's area plays a major role in understanding spoken and written language.

The first clue came from a man known as "Tan". In 1861 the surgeon Paul Broca visited a patient, Louis Victor Leborgne, who had a 21-year progressive loss of speech and paralysis but no loss of comprehension or mental function. He was nicknamed "Tan" because he could not clearly speak any word other than "tan". When Leborgne died, Broca found a lesion in the left frontal lobe. A second patient could speak only five simple, meaningful words, and by 1865 Broca had published autopsy findings from twelve patients.

In 1874 the German neurologist Carl Wernicke proposed the mirror image: damage to the left posterior superior temporal gyrus disrupts the link between how a spoken word sounds and how it is said. People with damage to Wernicke's area usually speak with normal flow and rhythm, yet their utterances lack meaningful content, and, unlike in Broca's aphasia, they are usually unaware of their deficits. So a stroke in one place can take away fluent speech, and in another can take away sense, with each leaving the other partly standing. Newer work adds a caveat: language comprehension relies on a distributed network of brain regions rather than a simple division between the two.