C.2 The shape of what we have done

Here is the journey, compressed into a single page.

“Hey Dr. Miles!” begins as a pressure variation in air, on the order of a few milliPascals, with a spectrotemporal structure that encodes the speaker’s mouth movements (movement 2). It propagates to your ear, where it is filtered by your pinna, canal, and head before reaching the eardrum (movement 3). The middle ear’s three ossicles, configured as a mechanical transformer, recover most of the energy that would otherwise be lost to the impedance mismatch between air and cochlear fluid (movement 4). The stapes drives the perilymph, which sets up a traveling wave on the basilar membrane. The wave peaks at a characteristic place determined by frequency — the Greenwood function — sharpened by an active feedback process driven by the outer hair cells, and transduced into electrical signals by the inner hair cells (movement 5). The auditory nerve carries the result as 30,000 channels of spike trains, with tonotopy preserved and phase locking up to a few kilohertz (movement 6). The brainstem extracts ITDs and ILDs in the medial and lateral superior olives, building a spatial code that converges in the inferior colliculus (movement 7). The thalamus relays the result to primary auditory cortex, where spectro-temporal receptive fields decompose the signal into features, belt and parabelt regions group features into auditory objects, and specialized regions of the superior temporal gyrus parse the signal into phonemes and words (movement 8). Higher cortex constructs a meaning from the resulting representation by hierarchical predictive inference, with attention modulating which prediction errors get amplified (movement 9). The listener, at the end of it all, recognizes that they have been called by name.

It is a journey of approximately 35 millimeters of physical distance and 200 milliseconds of physical time, traversing media as different as air, bone, fluid, and neural tissue, encoded in representations as different as pressure waves, mechanical displacements, ion currents, action potentials, and cortical population codes.

It is the journey of every sound you have ever heard.