Verschooten, Desloovere and Joris (2018) recorded electrocochleographic mass potentials in normal-hearing human volunteers and concluded that human cochlear frequency tuning is unusually sharp (compound-action-potential Q10Q_{10} roughly 1.6 times that of cat and chinchilla and 1.3 times that of macaque, measured with identical paradigms), while the upper frequency limit of auditory-nerve phase locking to temporal fine structure is at best similar to, and more likely lower than, that of common laboratory species. The open problem is to establish the peripheral coding limits of the human cochlea and their place among mammals: (i) determine whether human tuning sharpness is genuinely exceptional across mammals and which anatomical or evolutionary factors (cochlear length, octave range, allometric scaling) account for it; (ii) replace the predicted human single-fiber Q10Q_{10} values, which rest on conversion functions derived in cat, chinchilla and macaque and cannot be validated by direct recordings in awake humans, with validated human tuning estimates; and (iii) determine the actual human upper frequency limit of phase locking to temporal fine structure, for which expert estimates still span roughly 1.5-10 kHz.

Background

The cochlea decomposes sound into frequency bands and encodes the temporal waveform within each band. These two operations bound what the auditory nerve (AN, the fiber bundle carrying all auditory information from the ear to the brain) can transmit, and therefore bound every account of human hearing based on place coding (information in which fibers are active, via cochlear place and firing rate) or temporal coding (information in the timing of spikes). Frequency tuning is quantified by the quality factor Q10Q_{10}: a fiber's characteristic frequency (the frequency it responds to best) divided by the bandwidth 10 dB above its best sensitivity, with higher values meaning sharper tuning. Phase locking is the ability of AN fibers to time their spikes to a fixed phase of the stimulus waveform — its 'temporal fine structure' (TFS), the fast cycle-by-cycle pressure fluctuations, as opposed to the slower sound envelope; in mammals phase locking degrades above a species-dependent upper frequency limit (about 5 kHz in cat; Johnson 1980). Behavioral use of TFS is uncontested for binaural sensitivity below about 1.3 kHz (Brughera, Dunai and Hartmann 2013), but its monaural use for pitch and speech perception at higher frequencies is debated.

In animals both properties are measured from single AN fibers, which is infeasible in humans. Sharp human tuning was claimed from behavioral masking experiments (Oxenham and Shera 2003), from stimulus-frequency otoacoustic emissions — faint sounds generated by the cochlea itself, from which cochlear filtering can be inferred (Shera, Guinan and Oxenham 2002) — and from compound action potentials (CAP, the summed electrical response of many AN fibers, recordable with an electrode placed through the eardrum onto the cochlear bony wall) in a few clinical subjects (Harrison, Aran and Erre 1981). These claims were contradicted by reanalyses of basilar-membrane vibration measurements (the basilar membrane is the cochlea's vibrating sensory partition, on which different frequencies peak at different positions; Ruggero and Temchin 2005) and by suppression tuning of spontaneous otoacoustic emissions (Manley and van Dijk 2016), while single-fiber data in Old-World monkeys showed primate tuning sharper than cat (Joris et al. 2011).

Verschooten, Desloovere and Joris (2018) obtained the first systematic electrophysiological measurements of both properties in normal-hearing humans, using transtympanic electrocochleography in awake, locally anesthetized volunteers (technique consolidated in Verschooten and Joris 2022). Tuning was measured with a notched-noise forward-masking CAP paradigm (a masker noise containing a spectral gap around the probe frequency): a masker's frequency-specific reduction of a probe-tone CAP traces out a tuning curve (the method and its animal validation are given in Verschooten, Robles and Joris 2015 and Verschooten and Joris 2012). The phase-locking limit was estimated from the high-frequency decline of the auditory neurophonic, a mass potential reflecting summed phase-locked AN spikes (its use for extracting phase-locking limits was validated in species with known single-fiber limits by Verschooten, Robles and Joris 2015). Human CAP-Q10Q_{10} exceeded values obtained with the identical paradigm in cat and chinchilla by roughly a factor of 1.6 and in macaque by roughly 1.3; applying conversion functions (the ratio of single-fiber to CAP Q10Q_{10} measured in cat, chinchilla and macaque) predicts human single-fiber Q10Q_{10} slightly above macaque. The phase-locking estimates instead placed the human limit below that of monkey and cat. The authors concluded that our species excels in sharpness of frequency tuning but not in temporal coding of fine structure, and explicitly left open: whether such sharp tuning is exceptional among mammals and what anatomical or evolutionary factors — cochlear length, the number of octaves packaged along it, allometric scaling (cf. the phylogenetic analysis of disproportionate cochlear length in genus Homo by Braga et al. 2015) — account for it; the fact that human single-fiber Q10Q_{10} rests on cross-species conversion functions that cannot be validated directly; and how central auditory structures extract TFS versus place-rate information given this peripheral trade-off. The methodological caveats they acknowledge (lower signal-to-noise ratio, higher stimulus levels, bony-wall rather than round-window electrode geometry, possible spatial summation along the much longer human auditory nerve, and limited frequency coverage of about 2-6 kHz) constrain any future resolution.

Problem Statement

The problem is to establish the peripheral coding limits of the human cochlea — the two properties that jointly bound every place- or temporal-coding account of human hearing — and their comparative status among mammals. Concretely:

  1. Comparative status and basis of tuning sharpness. Determine whether human frequency tuning (cochlear filtering, quantified by CAP-based and single-fiber-equivalent Q10Q_{10} as a function of frequency) is genuinely exceptional among mammals, and identify the anatomical or evolutionary factors — for example cochlear length, the number of octaves packaged along the cochlea, and allometric scaling — that account for the observed human-versus-animal differences (roughly 1.6 times cat and chinchilla, 1.3 times macaque, measured with identical paradigms).

  2. Validated human tuning estimates. The predicted human single-fiber Q10Q_{10} values rest on conversion functions (ratios of single-fiber to CAP Q10Q_{10}) measured in cat, chinchilla and macaque. Either validate these conversions for the human ear, or provide an alternative in-vivo quantification of human cochlear filtering that does not depend on cross-species conversion — direct single-fiber recording in awake humans not being feasible.

  3. The human phase-locking limit. Determine the actual upper frequency limit of auditory-nerve phase locking to temporal fine structure in humans, for which current estimates span roughly 1.5 to 10 kHz: reconcile the electrophysiological estimate that the human limit is at best similar to, and more likely lower than, that of cat (about 5 kHz) with psychophysical claims of temporal-fine-structure use at high frequencies.

These three components serve one overarching objective — replacing contested, conversion-dependent and model-dependent limits with established human peripheral coding limits — and jointly decide the reappraisal of place-rate versus temporal coding schemes for human pitch and speech perception that motivated the source study. How central auditory decoders exploit the resulting trade-off is the downstream consequence of these limits and is not itself part of the completion boundary.

References

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