6.7 What leaves the brainstem

By the time the spike trains from the auditory nerve have passed through the cochlear nucleus, the superior olive, and the inferior colliculus, the auditory pathway has done three things:

1. Preserved spectral information. The tonotopic map of the cochlea is mapped onto each brainstem nucleus, all the way up.
2. Extracted spatial information. ITDs and ILDs are converted into population codes for azimuth and elevation.
3. Begun to construct auditory objects. Some IC neurons already show selectivity for complex stimuli (frequency sweeps, amplitude modulations, even species-specific vocalizations) that go beyond pure-tone tuning.

The next stop is the medial geniculate body of the thalamus, and from there, the primary auditory cortex. Up there, the auditory pathway transitions from sub-cortical processing — fast, automatic, evolutionarily ancient — to cortical processing, where the perceptual organization of sound begins. That is the subject of movement 8.