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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.

Tonotopy in Brainstem Tonotopy in Ferret Cortex

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.