10Coding of intensity & loudness

TCTwo codes for intensity

Loudness is carried by two neural codes working together. The first is firing rate: as a sound grows louder, each auditory-nerve fibre fires faster, up to saturation (Module 9). The second is recruitment of fibres — not to be confused with the clinical sign of the same name — meaning that louder sounds excite a wider population: more fibres at the centre of the excitation, and a spreading skirt of fibres tuned to neighbouring places. Total loudness reflects the summed activity across the whole responding population.[2012]

This is why no single fibre's saturation limits our hearing: even after the most sensitive fibres have maxed out, rising level keeps recruiting higher-threshold fibres and widening the active region, so the population code keeps growing.

TCHow loudness grows

Perceived loudness does not grow in proportion to sound pressure or even to decibels; across most of the range it grows roughly as a power functionof intensity (Stevens' law), so that a 10 dB increase corresponds approximately to a doubling of loudness. Loudness also sums across frequency: a sound spread over many frequency bands is louder than the same energy in one band, because it engages more of the cochlea. These facts make loudness a property of the whole pattern of neural activity, not of any single channel.[2013]

CThe dynamic range

The usable dynamic range of normal hearing — from the faintest audible sound to the loudest tolerable one — exceeds 100 dB. Speech occupies only about a 30 dBwindow within it (from soft high-frequency consonants to loud low-frequency vowels). In sensorineural hearing loss the range shrinks from both ends: the threshold rises while the uncomfortable level changes little, so the whole of speech must be packed into a much smaller window — the core problem a hearing aid's compression, or an implant's loudness mapping, has to solve.[2009]

FTRecruitment in hearing loss

When outer hair cells are lost, loudness growth becomes abnormal: thresholds rise, but once a sound is audible its loudness climbs unusually fast toward discomfort — clinical recruitment (introduced in Module 7). Compare the three regimes below: the compressive, wide-range normal curve; the steep, squeezed recruiting curve of cochlear loss; and the very narrow electric window.

FTElectric loudness coding

A cochlear implant codes loudness mainly by the amplitude of the current it delivers (and, to a lesser extent, by pulse rate and the number of electrodes active). But the range between a barely-audible current and an uncomfortably-loud one — between threshold (T) and the comfortable maximum (C) — is strikingly small, and loudness grows steeply, even expansively, across it. Electric hearing therefore reverses the cochlea's compression: where the normal ear squeezes a wide input into a comfortable loudness, the implant must do that squeezing in the processor, because the nerve's electrical loudness window is so narrow.[2009]

TThe physiology behind T and C

Everything in this module reappears in the programming booth. Setting T and C levelsis precisely the act of measuring the electrical dynamic range of this module; the processor's input dynamic range and compression are how the wide acoustic range is mapped into it; and the electrical stapedius reflex is an objective read-out of where comfortable loudness sits. The objective-measures chapter (Module 9 there) is, in this sense, applied loudness physiology.[2009]

Intensity is one of the nerve's two great dimensions. The other is frequency — and how the nerve codes pitch, by place and by timing, is the next module.