C.3 What we have not done

Honest scope statement: the essay has not touched a great many things that would belong in a more complete treatment of hearing.

It has not discussed speech production — the other half of the speech communication chain. Vocal folds, formants generated by vocal-tract shape, the articulatory machinery of the lips and tongue and palate, the motor control of breathing for sustained vocalization. All of it deserves its own essay.

It has not discussed the cerebellum, which plays an important role in fine temporal processing of sound, in motor entrainment to rhythm, and in (possibly) phoneme classification.

It has not discussed auditory imagery — the mental experience of hearing music or speech when none is acoustically present — or its close relatives, earworms and musical hallucinations, which can tell us a great deal about the generative side of the predictive-coding framework.

It has not discussed tinnitus — the perception of sound in the absence of a corresponding acoustic stimulus — which probably represents pathological gain in cortical or subcortical auditory networks and is a window onto how the system normally avoids hearing nothing.

It has not discussed music as a domain in its own right — pitch perception, rhythm, harmony, expectation and resolution, the cross-cultural patterns and individual differences that make musicology one of the richest meeting grounds of acoustics, neuroscience, and culture.

It has not discussed non-human hearing beyond the briefest mention of barn owls — echolocation in bats and cetaceans, infrasound in elephants, the song-learning circuits of songbirds, the auditory specializations of every species that lives by listening.

It has not discussed the developmental origins of any of this — how the cochlea forms, how cortical specializations for speech and music emerge over the first decade of life, how auditory deprivation in critical periods reshapes the system, how cochlear implants restore hearing to deafened ears by bypassing the hair cells entirely.

It has not, finally, discussed the clinical dimension in any sustained way — audiology as a practice, hearing aids and cochlear implants as engineering, the patient experience of progressive hearing loss, the relationships between hearing and cognition and dementia.

I am aware of all of these. I have written this essay in a particular shape, a single thread from molecules to meaning, and that shape has demanded that other shapes be left for other essays.