The ear is a remarkably sensitive pressure fluctuation detector. In guinea pigs, behavioral measurements indicate a minimum detectable sound pressure of ∼20 μPa at 16 kHz. Such faint sounds produce 0.1-nm basilar membrane displacements, a distance smaller than conformational transitions in ion channels. It seems that noise within the auditory system would swamp such tiny motions, making weak sounds imperceptible. Here we propose a new mechanism contributing to a resolution of this problem and validate it through direct measurement. We hypothesized that vibration at the apical side of hair cells is enhanced compared with that at the commonly measured basilar membrane side. Using in vivo optical coherence tomography, we demonstrated that apical-side vibrations peaked at a higher frequency, had different timing and were enhanced compared with those at the basilar membrane. These effects depend nonlinearly on the stimulus sound pressure level. The timing difference and enhancement of vibrations are important for explaining how the noise problem is circumvented.
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We thank E. de Boer, T. Ren, A. Magnusson and P. Gillespie for critical discussions and reading of the manuscript. This work was supported by US National Institutes of Health, National Institute on Deafness and Other Communication Disorders grants DC00141 (A.L.N.), DC010399 (A.L.N.) and DC010201 (R.K.W.); and Swedish Research Council grant K2008-63X-14061-08-3, the Tysta Skolan Foundation and Hörselskadades Riksförbund (A.F.).
The authors declare no competing financial interests.
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Chen, F., Zha, D., Fridberger, A. et al. A differentially amplified motion in the ear for near-threshold sound detection. Nat Neurosci 14, 770–774 (2011). https://doi.org/10.1038/nn.2827
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