Glossary

Terms used in this book.

A reference list of the technical vocabulary used in What is hearing?. Inline occurrences of these terms in the lessons are auto-tooltipped (dotted underline) so you can hover for a quick definition; for a fuller treatment with context, return here.

89 terms from this book.

A

A1: Primary auditory cortex, on Heschl’s gyrus. The cortical entry point of the auditory pathway.
acoustic impedance: The ratio of acoustic pressure to particle velocity in a propagating wave (Z = p/v). For a plane wave in a medium of density ρ and wave speed c, Z = ρc.
amplitude: The magnitude of a wave’s departure from equilibrium. For sound, the size of the pressure fluctuation.
auditory nerve: The ~30,000-fibre bundle carrying spike-train information from the cochlea to the cochlear nucleus in the brainstem.
auditory streaming: The brain’s perceptual organisation of acoustic input into one or more sources (streams). A bistable example is the van Noorden galloping triplet.
auricula: Anatomical name for the pinna.

B

basilar membrane: The membrane separating scala media from scala tympani. Its position-dependent stiffness gives different places different natural frequencies.
Bayes: Bayes’ theorem: P(M|S) = P(S|M)·P(M)/P(S). The brain’s posterior over hypotheses M given sensory data S combines the likelihood with the prior.

C

characteristic frequency: The frequency at which a given place on the basilar membrane (or auditory-nerve fibre, or cortical neuron) responds most strongly.
cochlea: The spiral, fluid-filled organ of the inner ear that performs frequency analysis on incoming sound and transduces it into neural signals.
cochlear amplifier: The active feedback process in the cochlea, driven by outer-hair-cell electromotility, that sharpens basilar-membrane tuning beyond what passive mechanics gives.
cochlear nucleus: The first central auditory structure; receives all auditory-nerve fibres on its own side. Has ventral and dorsal subdivisions.
cochlear synaptopathy: Damage to the synapses between inner hair cells and low-spontaneous-rate auditory-nerve fibres, often invisible on standard audiograms but causing difficulty hearing in noise.

D

decibel: A logarithmic unit of ratio: 20·log10(amplitude ratio) or 10·log10(power ratio). Used for sound pressure level (SPL) and hearing level (HL).
duplex theory: Rayleigh’s theory (1907) that ITDs dominate low-frequency localisation and ILDs dominate high-frequency localisation.

E

ear canal: The tube about 25 mm long running from the pinna to the eardrum. Its closed-tube resonance amplifies frequencies near 3 kHz.
eardrum: The tympanic membrane: a thin sheet at the inner end of the ear canal that vibrates in response to pressure waves and drives the ossicular chain.
electromotility: The voltage-driven length change of outer hair cells, mediated by prestin. Generates the active gain of the cochlear amplifier.
endocochlear potential: The +80 mV potential of the endolymph in scala media relative to perilymph. Powers hair-cell transduction.
endolymph: The fluid in scala media; high in K⁺, low in Na⁺. Held at +80 mV (the endocochlear potential) relative to perilymph.

F

formant: A resonant peak in the spectrum of a vowel. The configuration of mouth and tongue produces F1, F2, F3… which together identify the vowel.
free-energy principle: Friston’s proposal that any self-organising system minimises variational free energy, a bound on negative log-evidence under its generative model.
frequency: The number of oscillation cycles per second, measured in hertz (Hz). For sound, this is what the brain perceives as pitch.

G

Greenwood function: The empirical fit relating cochlear place x to characteristic frequency f: f(x) = A(10^(α(1−x/L)) − K). For humans: A=165.4, α=2.1, K=0.88, L=35 mm.

H

helicotrema: The small opening at the cochlear apex where scala vestibuli and scala tympani communicate, short-circuiting DC pressure imbalance.
Hopf bifurcation: The critical point at which a damped oscillator becomes a self-sustained one. The mammalian cochlea operates near this bifurcation.
HRTF: Head-Related Transfer Function. The full frequency-dependent filter the body applies between a sound source in space and the listener’s eardrum.

I

ILD: Interaural Level Difference. The intensity difference between the two ears, dominant at frequencies whose wavelength is smaller than the head.
impedance: In acoustics, the ratio of pressure to particle velocity. A measure of how strongly a medium resists being moved by a wave.
incus: The second middle-ear ossicle (the “anvil”); pivots between malleus and stapes.
inferior colliculus: A midbrain nucleus and obligatory waystation for ascending auditory information. Where ITDs, ILDs and other features converge into a spatial map.
inner hair cell: The cochlea’s actual transducer. ~3,500 of them along the basilar membrane, each contacting 10–30 auditory-nerve fibres.
interaural level difference: The intensity difference between the two ears, dominant at frequencies whose wavelength is smaller than the head.
interaural time difference: The arrival-time difference for a sound between the two ears. Humans resolve differences as small as 10 μs.
ITD: Interaural Time Difference. The arrival-time difference for a sound between the two ears. Humans resolve differences as small as 10 μs.

J

Jeffress model: Lloyd Jeffress (1948) proposed delay-line coincidence detectors as the neural mechanism for ITD measurement; still the canonical picture for the MSO and bird homologue.

L

lateral lemniscus: The white-matter tract carrying the ascending auditory pathway from cochlear nucleus and superior olive to the inferior colliculus.
lateral superior olive: LSO. Brainstem nucleus that computes interaural level differences from excitatory ipsilateral and inhibitory contralateral inputs.
likelihood: In Bayesian inference, P(S|M): how probable the observed sensory input is given that hypothesis M is true.
LSO: Lateral superior olive. Brainstem nucleus that computes interaural level differences.

M

malleus: The first middle-ear ossicle (the “hammer”); attached to the eardrum on one side and the incus on the other.
McGurk effect: A multisensory illusion in which the visual articulation of one syllable and the audio track of another produce a third percept.
medial geniculate body: MGB. The thalamic relay for auditory information, sitting between the inferior colliculus and the auditory cortex.
medial superior olive: MSO. Brainstem nucleus that computes interaural time differences using coincidence detection.
MET channel: Mechanoelectrical Transduction channel. A mechanically-gated ion channel on stereocilia that opens when the bundle is deflected, depolarising the hair cell.
MSO: Medial superior olive. Brainstem nucleus that computes interaural time differences.

O

organ of Corti: The sensory structure on the basilar membrane containing inner and outer hair cells, supporting cells, and the tectorial membrane.
ossicles: The three smallest bones in the body — malleus, incus, stapes — that transmit motion from the eardrum to the oval window of the cochlea.
outer hair cell: Active mechanical element of the organ of Corti. ~12,000 of them; they shorten and lengthen with voltage, sharpening basilar-membrane tuning.
oval window: A membrane-covered opening at the base of scala vestibuli, where the stapes footplate drives the cochlear fluid.

P

perilymph: The fluid in scala vestibuli and scala tympani; ionic composition resembles extracellular fluid (high Na⁺, low K⁺).
phase locking: The tendency of an auditory-nerve fibre to fire at a preferred phase of a low-frequency stimulus cycle, up to ~5 kHz.
phonemic restoration: A perceptual effect in which a phoneme replaced by noise is heard as intact, restored by context-driven priors.
pinna: The visible outer flap of the ear. Its folds and ridges encode elevation by introducing spectral notches that depend on the sound source’s vertical angle.
place coding: Encoding of frequency by which location on the basilar membrane (or fibre in the auditory nerve) is most active.
plane wave: A wave whose phase fronts are infinite parallel planes; idealisation of a wave from a distant source, valid locally near the listener.
posterior: In Bayesian inference, P(M|S): the probability of hypothesis M given the sensory input S. This is what the brain perceives.
predictive coding: A theory of cortical processing in which higher layers predict the activity of lower layers, and only the prediction errors propagate upward.
prestin: The membrane protein in outer hair cells that changes conformation with voltage, producing the cell’s electromotility.
primary auditory cortex: A1. The first cortical processing stage for sound, on Heschl’s gyrus of the superior temporal plane. Tonotopically organised.
prior: In Bayesian inference, P(M): the probability of a hypothesis before any sensory input. Context, expectation, learned experience shape this.

Q

quality factor: Q. A dimensionless measure of resonance sharpness. Equal to the peak amplitude (relative to DC) of a damped driven oscillator.

R

rate coding: Encoding of stimulus intensity (or frequency above the phase-locking limit) by the firing rate of a neuron.
reflection: When a wave hits a boundary between two media, part of its energy turns back into the first medium. The reflection coefficient R = (Z2 − Z1)/(Z1 + Z2).
resonance: The condition where a driving frequency matches a system’s natural frequency, producing maximum response amplitude.
reverberation: The collection of reflections that follows a direct sound in a room, gradually decaying. Characterised by reverberation time, T60.
ribbon synapse: A specialised presynaptic structure in inner hair cells (and a few other sensory cells) that releases vesicles continuously at high rates with sub-millisecond precision.
round window: A flexible membrane at the base of scala tympani that bulges out to give the incompressible cochlear fluid somewhere to go.

S

scala media: The middle cochlear chamber, filled with endolymph; contains the organ of Corti.
scala tympani: The lower of the three cochlear chambers, filled with perilymph; terminates at the round window.
scala vestibuli: The upper of the three cochlear chambers, filled with perilymph; the stapes pushes into it at the oval window.
Shepard tone: An auditory illusion of a continuously rising (or falling) pitch, created by stacking octaves with a fading amplitude envelope.
speed of sound: The propagation speed of small-amplitude pressure disturbances. ≈343 m/s in air at room temperature, ≈1480 m/s in water.
spiral ganglion: The collection of cell bodies whose axons form the auditory nerve. Each spiral-ganglion neuron contacts one or a few inner hair cells.
stapedius reflex: A bilateral reflex that contracts the stapedius muscle in response to loud sounds, attenuating ossicular transmission at low frequencies by 10–15 dB.
stapes: The third middle-ear ossicle (the “stirrup”); its footplate seats in the oval window and drives the cochlear fluid.
stereocilia: Actin-filled rod-like protrusions on the apical surface of hair cells, arranged in a graded array with tip links connecting neighbours.
STRF: Spectro-Temporal Receptive Field. The 2-D function (frequency × time) characterising what stimulus features drive a cortical neuron to fire.
superior olivary complex: A brainstem region where information from both ears first meets. Contains the MSO (ITDs) and LSO (ILDs).
superior temporal gyrus: STG. A region of temporal cortex extending forward from A1; contains regions specialised for processing the phonemic structure of speech.

T

tectorial membrane: A gelatinous strip overhanging the organ of Corti. Couples basilar-membrane motion to the stereocilia of the hair cells.
thalamus: A central diencephalic structure that relays most sensory information to cortex. The medial geniculate body is its auditory subdivision.
tip link: A molecular thread connecting the top of one stereocilium to the side of its taller neighbour. Stretching opens the MET channel.
tonotopy: Spatial organisation in which neighbouring elements respond to neighbouring frequencies. Preserved from cochlea through brainstem to cortex.
traveling wave: A wave that propagates while maintaining its shape (in a uniform medium) or that varies its shape as it moves (in a non-uniform one like the cochlea).
tympanic membrane: The eardrum: a thin sheet that vibrates in response to pressure waves and drives the ossicular chain.

V

vector strength: A measure of phase-locking strength: |⟨exp(iφ)⟩| averaged over many spikes. 1 = perfect, 0 = random.

W

wave equation: A second-order partial differential equation describing how a disturbance propagates. For pressure in air: ∂²p/∂t² = c²∇²p.
WKB approximation: A method for solving wave equations with slowly-varying coefficients. Gives a solution of the form A(x)·exp(i·∫κ(x)dx) for a position-dependent wavenumber.