To understand speech or other auditory information, the brain has to interpret the pattern of nerve impulses sent to it by the ear. As you have seen in earlier parts of this guide, the cochlea of the inner ear carries out a frequency analysis of incoming sounds, which is picked up by the cochlear hair cells and sent to the brain. This establishes a pattern of activity across the nerve fibres of the auditory nerve which is a lot like the spectrogram you saw earlier.
The brain then seems to maintain this spectrographic, or tonotopic representation throughout the early parts of the auditory pathway. This is shown here schematically using drawings of auditory brainstem nuclei based on studies on cats and rodents (left) and best frequency maps from the auditory cortex of a ferret (right) and of a human (below). The principle is the same in the brains of all mammals, including yours. The brain thus initially processes sounds frequency band by frequency band. But to understand speech it needs to recognize sound patterns that can span many frequency bands at once. How it does that is still largely unknown, but we do know a little bit about which brain areas are likely to be involved.
|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...|
Neurosurgeons needing to remove parts of diseased brain tissue sometimes zap a small area of the brain before they remove it to deactivate it temporarily and test whether that brain area is essential for a particular function. The circles on this picture of a brain show areas tested in this manner. The filled circles mark areas which were found to be essential for understanding speech.