Probing Cochlear Tuning in the Marmoset

The biomechanical challenges our ears face are immense, as they must detect very fast (0.01-200 kHz), very small (sub-angstrom) motions in warm, salty water. Nonetheless, their performance is remarkable: A dynamic range spanning over 12 orders of magnitude of energy, the lower end of which providing sensitivity below the thermal noise floor. Furthermore, the ear exhibits a striking manifestation: It not only detects sound but also generates and subsequently emits it as well. These very faint sounds, known as otoacoustic emissions (OAEs), can be detected in the ear canal using a sensitive microphone. Thought to be a by-product of an underlying amplification mechanism, they provide a valuable probe into the biophysics of the cochlea, where study is otherwise extremely difficult (given that the inner ear is completely encased in the hardest bone in the body). It has been demonstrated that a certain type of evoked emission (stimulus-frequency otoacoustic emissions, SFOAEs) can be used to objectively estimate the frequency selectivity of the cochlea. Comparative data suggests humans may be unique in this regard, with relatively sharp tuning. The goal of this study was to help establish how tuning may be correlated with basilar-membrane (BM) length. Because inter-species comparisons can be complicated by phylogenetic differences, we sought to minimize these confounds by measuring SFOAE delays in the marmoset (Callithrix jacchus), a New World primate with a relatively short BM (~14 mm) and good high frequency hearing. These data suggest a correlation between SFOAE delay and BM length among primates, although the comparison with cat demonstrates that BM length cannot, by itself, explain delay differences across species. If SFOAE delays provide a reliable measure of cochlear tuning as proposed, the data suggest that tuning is sharper in marmoset than in cats below 8 kHz, encompassing a frequency range relevant for the monkey’s vocalizations.