The spatial and temporal representation of a tone on the guinea pig basilar membrane
Abstract
In the mammalian cochlea, the basilar membrane's (BM) mechanical responses are amplified, and frequency tuning is sharpened through active feedback from the electromotile outer hair cells (OHCs). To be effective, OHC feedback must be delivered to the correct region of the BM and introduced at the appropriate time in each cycle of BM displacement. To investigate when OHCs contribute to cochlear amplification, a laser-diode interferometer was used to measure tone-evoked BM displacements in the basal turn of the guinea pig cochlea. Measurements were made at multiple sites across the width of the BM, which are tuned to the same characteristic frequency (CF). In response to CF tones, the largest displacements occur in the OHC region and phase lead those measured beneath the outer pillar cells and adjacent to the spiral ligament by about 90°. Postmortem, responses beneath the OHCs are reduced by up to 65 dB, and all regions across the width of the BM move in unison. We suggest that OHCs amplify BM responses to CF tones when the BM is moving at maximum velocity. In regions of the BM where OHCs contribute to its motion, the responses are compressive and nonlinear. We measured the distribution of nonlinear compressive vibrations along the length of the BM in response to a single frequency tone and estimated that OHC amplification is restricted to a 1.25- to 1.40-mm length of BM centered on the CF place.
Footnotes
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↵ * To whom reprint requests should be addressed. E-mail: i.j.russell{at}sussex.ac.uk.
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This paper was presented at the National Academy of Sciences colloquium “Auditory Neuroscience: Development, Transduction, and Integration,” held May 19–21, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA.
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↵ † Karavitaki, K. D. & Mountain, D. C. (1998) Abstr. 21th Midwinter Mtg. Assoc. Res. Otolaryngol. 719; Richter, C. P., Evans, B. N., Hu, X. & Dallos, P. (1998) Abstr. 21th Midwinter Mtg. Assoc. Res. Otolaryngol. 720.
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↵ ‡ Cooper, N. P. (2000) Abstr. 23rd Midwinter Mtg. Assoc. Res. Otolaryngol. 254.
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↵ § Displacement-sensitive self-mixing interferometry does not require the placement of reflective beads on the BM, and we can accurately choose the sites of measurement. There is strong disagreement as to whether beads either remain firmly attached to the BM or are influenced by hydrodynamic forces resulting from movements of the BM (40, 41). A striking example that hydrodynamic forces at the organ of Corti can be quite predominant is seen in the responses of hair cells in the cochleae of mice without attached tectorial membranes that can respond to BM displacement as a consequence of viscous drag acting on their hair bundles [Russell, I. J., Lukashkina, V. A., Kössl, M., Legan, K., Goodyear, R. & Richardson, G. P. (2000) Abstr. 23rd Midwinter Mtg. Assoc. Res. Otolaryngol. 249]. We are not sure whether the Doppler shift in frequency of the laser beam, on which BM velocity and some displacement measurements are based, necessarily reflects movements in the transverse plane of BM motion. We were concerned with this last aspect of measurement when we considered using the Doppler frequency shift that results when light is coupled back into the laser diode, and which is widely used to measure velocity in the plane at right angles to the beam (42). In view of this, we determined early on (unpublished work) that any transverse movements of the BM that might be caused by 10- to 30-nm vibrations of the preparation in the radial plane were not detectable above the 0.4- to 0.8-nm noise floor of our measurement system.
- Abbreviations:
- BM,
- basilar membrane;
- TM,
- tectorial membrane;
- IHC,
- inner hair cell;
- OHC,
- outer hair cell;
- CF,
- characteristic frequency;
- SPL,
- sound pressure level;
- SLAM,
- spiral lamina attachment;
- SLIG,
- spiral ligament attachment
- Copyright © 2000, The National Academy of Sciences
