7 The central auditory nervous system
7.1 The ascending auditory pathway
Up till now we have dealt with the anatomy of the auditory periphery and how the basic attributes of sound are coded within the auditory periphery. A great deal of additional processing takes place in the neural centres that lie in the auditory brainstem and cerebral cortex. Because localisation and other binaural perceptions depend on the interaction of information arriving at the two ears, we need to study the central auditory centres, since auditory nerves from the two cochleae interact only at the brainstem and cerebral cortex. This section deals with the structure and function of the central auditory nervous system (CANS).
Within the brainstem almost all fibres of the auditory nerve synapse on cells of the cochlear nucleus. The relationship between the cochlear nucleus and higher auditory centres is shown in Figure 27. This figure is highly schematic and simplified and shows only the main tracts and nuclei of the CANS, although other nuclei exist.
Once they leave the cochlear nucleus, most of the axons of the cochlear nucleus cells cross over to the opposite side (contralateral side) of the brain (Figure 27). This means that most of the auditory information processed by each half of the brain comes from the ear on the other side of the head. This is in contrast to that found in the visual system, where ganglion cell fibres either cross or stay on the same side of the brain in equal proportions. Both crossed and uncrossed fibres from the cochlear nuclei synapse in the area of the brainstem called the superior olivary complex. This is the first place in the ascending pathway to receive information from both ears. Neural impulses are transmitted from the superior olivary complex to the inferior colliculus through and/or around the lateral lemniscus (some fibres synapse in the lateral lemniscus but most travel through it to the inferior colliculus), from there to the medial geniculate body and finally to the auditory cortex. The location of the auditory cortex on the surface of the brain is shown in Figure 28.