Aphasia

Broca's and Wernicke's Areas We have learnt a lot about which parts of the brain process speech from unfortunate patients who suffered brain injuries such as strokes. In the late 1800s, the French neurologist Paul Broca and the German neurologist Carl Wernicke found that patients with damage to particular parts, typically in the left brain hemisphere, developed difficulties in either producing or understanding speech. The parts of the brains they identified now bear their names, and are shown in the picture here.
In the video on the right you can see an unfortunate patient suffering from Wernicke's aphasia. The patient, a retired dentist, is clearly not deaf and can hear the questions put to him by the interviewer, but the replies he utters are somewhat off topic...