5.6 What leaves the auditory nerve

Pulling it all together: the auditory nerve carries an extraordinarily rich representation of the cochleagram. Each of its 30,000 fibers tells the brain about a place along the cochlea, its instantaneous firing rate, the phase of its spikes relative to any phase-locking stimulus, and (when interpreted across the population) the spectrum and dynamics of the input sound. The phrase “Hey Dr. Miles!” arrives at the cochlear nucleus as a 30,000-channel spike-train movie, sampled at sub-millisecond temporal resolution, with the channels arranged tonotopically along an 8 mm axis of the nucleus.

What the brainstem does with this representation — how it extracts spatial location from inter-ear timing differences, how it groups simultaneous spectral components into a single source, how it tracks pitch through harmonic structure — is the subject of movement 7.