The auditory nerve (AN), originating in the spiral ganglion in the cochlea, carries the output signals of the cochlea's IHCs into the same-side (ipsilateral) cochlear nucleus (CN) in the brainstem, just a few centimeters away, as shown in Figure 20.1. To a large extent, the inputs and outputs of the CN tell us what information the brain is getting from the ear, and what the important first steps are in processing that information.

Observations of physiological behavior of the CN support our emphasis on the use of fine temporal structure in sound representation, as in the Fletcher (1930) “space– time pattern theory” and the Wever and Bray (1930b) “volley theory” that we reviewed in Chapter 2. The CN's inputs and outputs are events, synchronized to sound waveform structure in a very precise and robust way in some parts of the CN, but less synchronized in other parts, serving different subsequent processing pathways.

In the CN, pathways diverge in support of a number of processing functions in parallel, variously specialized for binaural processing, for periodicity detection, and for other monaural feature extraction. The tonotopic organization of the auditory nerve is maintained as a spatial dimension, and various subsequent brain areas use another spatial dimension to organize the extracted features into what we call auditory images, of various sorts.