The neural pulses are routed from the left and right cochleae up to the highest level, which is the primary auditory cortex in the brain. As usual, hierarchical processing occurs as the signals are combined through neural structures. This enables multiple frequencies and phase shifts to be analyzed. An early structure called the superior olive receives signals from both ears so that differences in amplitude and phase can be processed. This will become important in Section 11.3 for determining the location of an audio source. At the highest level, the primary auditory cortex is mapped out tonotopically (locations are based on frequency), much in the same way as topographic mapping of the visual cortex.