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APPLIED PHYSICAL BIOPHYSICS AND SCIENCES COMPUTATIONAL BIOLOGY motion, there has not been a systematic study of the motion Human CP Motion Is Substantially Different from the Classic View of of various points across the human CP, or across the OSL in CP Motion. A representative example of the normalized veloc- any species. ity of the human CP, measured at 34 radial locations across the To better understand the mechanics of hearing in humans, we CP tympanic surface, for frequencies below and near the BF, is measured sound-induced motion throughout the CP width in the shown in Fig. 2. The measurements were normalized to the max- base of fresh human temporal bones and compared the results imum velocity of the CP at each frequency (Fig. 2A) or to the with the classic view of cochlear motion. To determine human intracochlear pressure measured in the vestibule (Fig. 2 B–D). CP motion, we viewed the CP scala tympani surface through the The normalized CP responses were independent of the tested and measured transverse motion at many points stimulus levels of 80 to 120 dB sound pressure level (SPL) at the across the CP width with laser Doppler vibrometry. To interpret . these experiments, we examined CP anatomy from histological In response to sound, the human OSL was not stationary (as sections. in classic models), but moved almost as much as the BM. The displacement of the OSL increased linearly with radial distance Results from a pivot point near the modiolus (at a radial location of Human CP Anatomy Differs from the Anatomy in the Base of Labo- −600 µm from the OSL–bridge boundary in Fig. 2A). In the ratory Animals. In the base of most laboratory animals, much of CP bridge region medial to the inner sulcus (IS in Fig. 1C), cochlear anatomy is similar, in particular, the anatomy of the CP. the motion of the bridge was continuous with that of the OSL, The bony plates of the OSL extend laterally (toward the spiral meaning that the velocity continued to increase linearly with the ligament) to come close to, or overlap with, the medial edge of distance from the modiolus. In the more lateral bridge region the BM, approximately in the region of the inner pillars under near the inner sulcus (between the limbus and BM), the motion the inner hair cells. Cochlear anatomy is shown for the base of usually became greater than a simple extension of the OSL guinea pig in Fig. 1A, and additionally for 4 other commonly used pivoting motion (Fig. 2A). The largest transverse CP motion was laboratory animals in SI Appendix, Fig. S1. In the cochlear base generally on the BM near the BM–bridge boundary, close to of laboratory animals such as the guinea pig (Fig. 1A), the spiral where the inner pillars and inner hair cells are located (IP in limbus and its attachment point to the TM rest above the bone of Fig. 2A). Lateral to the CP motion peak, the motion decreased to the OSL. In Fig. 1A, the BM occupies ∼34% and the bony OSL a stationary point where the BM attached to the occupies ∼66% of the CP width. (SL in Fig. 1A). In contrast, in the base of the human cochlea, the BM occu- Fig. 3 shows CP motion in all 6 temporal bones. In all, the OSL pies ∼15% and the bony OSL occupies ∼70% of the CP width and bridge moved in response to sound, including the regions (Fig. 1B). In humans, the bony part of the OSL does not come of the BM–bridge attachment and underneath the TM–limbus close to the BM. Instead, between the lateral edge of the OSL attachment. On average across the 6 temporal bones, the BM bone and the medial edge of the BM, there is nonbony tissue accounted for 27.2 ± 7.7% SD of the total transverse area dis- that forms a third CP region in addition to the OSL and the BM, placement of the CP for frequencies below BF, and 42.7 ± 26.6% which has not been anatomically delineated and which we name SD of the CP area displacement at the BF. This contrasts with the CP bridge (Fig. 1 B and C). In humans, the attachment point classic models in which BM motion is ∼100% of CP area dis- of the TM on the does not rest above the bone of placement. Overall, in all 6 human temporal bones, OSL and the OSL (as in the base of laboratory animals and in cochlear bridge motion accounted for a substantial fraction of the CP area models), but, instead, rests on bridge tissue between the OSL displacement at all tested frequencies. bone and the BM. A magnified view of the CP bridge is shown At frequencies below BF, the whole CP vibrated in phase in Fig. 1C. We define the boundaries of the bridge as the bony with deviations of only a few degrees (pink and purple lines in OSL (medially), the limbus (toward scala vestibuli), the BM (lat- Fig. 2B and SI Appendix, Fig. S3). At frequencies near BF, and erally), and the abutting fluid (toward scala tympani). Within the particularly above BF, the phase in the bridge and BM regions bridge region are connective tissue and auditory nerve fibers. The sometimes lagged the phase in the OSL region (black and green boundary between the bridge, the limbus, and the inner sulcus lines in Fig. 2B and SI Appendix, Fig. S3). These data suggest that, needs further examination and definition. The name “tympanic near and above the BF, there can be phase differences between lip” has been used for the lower border of the inner sulcus (19) the OSL, bridge, and BM. CP phase changed with frequency, as without regard to whether it is above bone or not. The bridge shown by the separation of lines in Fig. 2B; this phase–frequency spans a substantial width. In the cochlear base, the width of the relationship is examined in the next section. bridge was 83 ± 12% SD of the BM width across 6 histologically prepared temporal bones. The bridge is present in all turns of the Similar Tuning Characteristics of the OSL, Bridge, and BM. The human cochlea and has approximately the same width as the BM OSL, CP bridge, and BM all had similar frequency response (both become wider in the apex). tuning (Fig. 2C and SI Appendix, Fig. S3). Across temporal

A Guinea Pig BHuman medial lateral radial transverse C Limbus TM

Limbus

TM

OSL BM OSL Bridge BM

Fig. 1. Cross-sectional anatomy of guinea pig and human CPs. Colored bars show the radial extents of the OSL, the BM, and, in humans, the soft tissue “bridge” between the OSL and BM. (A) Guinea pig 10-kHz place (5.5 mm from the base) and (B) human 9-kHz place (6 mm from the base). (C) Magnified view showing the bridge region outlined with a dashed blue line. Other abbreviations are inner sulcus (IS) and spiral ligament (SL).

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APPLIED PHYSICAL BIOPHYSICS AND SCIENCES COMPUTATIONAL BIOLOGY motions in the base of larger mammals, or mammals more closely basal to the BF region, the sound pressure is close to uniform in a related to humans (e.g., primates), are similar to humans remains transverse section across the cochlear scalae and would affect the to be determined (for related work, see refs. 16, 24, and 25). OSL, bridge, and BM regions similarly. However, in the short- Further, the sharper cochlear tuning found in humans compared wave region near the BF, experiments and models of cochlear with laboratory animals (25–27) may be related to our findings, mechanics suggest that the sound pressure spread away from the but this remains to be determined. BM may be spatially limited (12, 35), which may cause the OSL Our CP motion measurements were made in the base of the and bridge regions to move differently than the BM. cochlea, which is tuned to high frequencies. Our anatomical Another possible limitation of our measurements is that CP study on histological sections from 21 human temporal bones tissue properties may change substantially after death. Although shows that the bridge is present in all specimens. Also, the BM we have seen no indication of this from our measurements over and bridge width are approximately the same and increase from hours if the CP was kept moist, time effects cannot be ruled out, base to apex. Additionally, the human OSL movement reported because our earliest measurements were 47 h postmortem. Many by Stenfelt et al. (18) was from the 2-kHz cochlear region, animal studies of BM motion before and shortly after death have which shows that human OSL movement is not restricted to the shown a large initial motion decrease near BF (and a change in cochlear base. Considering these observations, the overall pat- BF) from the loss of cochlear amplification, but, over the next tern of OSL and bridge motion we have seen in the base is likely few hours, only small changes occurred. Over longer periods present throughout the human cochlea. This does not rule out after death, BM stiffness decreased (36, 37) and low-frequency there being differences in cochlear motions between base and BM motion increased (38), but, at 16 h postmortem, the shape of apex as has been found in laboratory animals (5, 6, 8). the cross-sectional motion profile was not affected (30). In mice, TM material properties and TM traveling waves were similar at Comparison to Other Studies. Previous studies of human CP 1 h versus 48 h postmortem, and are similar to those of post- anatomy have described aspects of the CP bridge, but these stud- mortem human TMs (39). These experiments used cold, moist ies were not focused on structural relevance to dynamics and (never frozen) storage, as was done in our present study. Human did not clearly delineate the CP bridge from other structures cochlear input impedance from intraoperative measurements in (19, 28, 29). The first reported quantitative measurements of live cochleas was similar to that in cadaveric temporal bones, OSL motion in a human temporal bone was by Stenfelt et al. which suggests that overall CP properties did not grossly change (18). In 1 specimen, they measured at points near the modio- after death (40). Our measurements of CP motion in passive lus (OSL1), near the lateral edge of the OSL (OSL2), and on human cochleas are the freshest yet made, but may not be the the BM. However, they did not distinguish a CP bridge region same as just after death. and made no mention of how they decided where their measure- ment points were relative to the detailed anatomy (18). Stenfelt Modeling Human CP Motion. We found that the low-frequency et al.’s measurements were near the 2-kHz BF place, while ours cross-sectional motion of the CP can be described by a composite were near the 9- to 15-kHz BF places. These factors prevent us beam model with 2 elements: a rigid rod hinged near the modi- from being able to make a detailed comparison of their results olus (emulating the OSL and the medial half of the CP bridge) and ours. However, Stenfelt et al.’s measurements are consistent and a flexible beam with constant bending stiffness, simply sup- with ours in that the magnitude of motion of the OSL was com- ported at both ends (emulating the lateral part of the CP bridge parable to that of the BM, and that both BM and OSL phase and the BM). This model, described in SI Appendix, captures the patterns were appropriate for these structures to be carrying overall properties of our CP motion measurements: 1) The OSL traveling waves. moves as a stiff plate hinged near the modiolus, 2) the motion Several studies in animals have reported measurements of is large in the bridge region under the attachment of the TM to sound-induced motion at different radial positions across the the limbus and where the BM attaches to the bridge, and 3) the BM and sometimes from the OSL (reviewed in the Introduc- maximum CP motion occurs near the BM–bridge connection. tion). Reports from most animal experiments found minimal BM There are 2 other modeling studies that are relevant in motion at its attachment to the OSL and the largest transverse that their CPs included both a rigid part and a flexible part motion in the region from the outer pillars to the center of the [Kapuria et al. (41) and Taber and Steele (42); see SI Appendix ]. BM, with only small differences in the phase seen at different BM In these models, the volume displacement of the near-rigid locations (5, 30, 31). One exception is the studies of Nilsen and region was large compared with that of the flexible BM region, Russell (32), who made measurements with a self-mixing laser but the near-rigid region had relatively little influence on the and reported that the largest motions were near the outer pillars, shape and BF of the traveling wave, except for delaying the but they also reported large phase differences (up to 180◦) for phase. These results may imply that the presence of a large fluid small variations in radial position. The different results of Nilsen volume displacement by the OSL and bridge in humans may and Russell can be accounted for by the possibility that their have little effect on BF characteristics set by the BM. How- method included motion of deep structures of the OoC, rather ever, these models did not include cochlear micromechanics, than just the surface motion of the BM. For a more detailed in particular, the mechanical drives to hair cell stereocilia, where discussion, see ref. 33. major effects due to a mobile OSL and bridge might be expected.

Implications of Measurements from a Fresh Cadaveric, Passive Prepa- Implications of the Human CP Motion Profile. To understand how ration. Passive-cochlea data are important for understanding CP motion may relate to the motion of structures within the human CP motion at high sound levels, or when active processes OoC, we consider simplified cross-sectional diagrams of the cease to work (e.g., in sensorineural hearing loss), and as an anatomy and low-frequency motion of the CP and OoC (Fig. 4). important foundation to understand CP motion with cochlear The classic view of BM and OoC motion is shown in Fig. 4A. amplification. As the BM moves up (toward scala vestibuli), the OoC rotates Most animal studies have found no differences in the shape counterclockwise about the bottom of the inner pillars, with BM of the radial profile of BM transverse motion between active motion greatest around the outer pillars, OHCs, and the center and passive cochleas (30), but whether this holds for humans of the BM. The resulting rotation of the TM about a station- is unknown. In an active cochlea, outer hair cells (OHCs) pro- ary origin at the attachment to the limbus produces little radial duce cochlear amplification by adding energy to the traveling motion of the TM at the TM attachment to OHC stereocilia. It wave (34). In the long-wave region of the traveling wave, which is has been hypothesized that the TM has substantial radial motion

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CP implications in functional differences the consequential of highly measurements the CP shows human 4B) our (Fig. of extrapolation simple the versus a ME tptooysaf n ieRvc o ehia support; and technical 1 Garyfallia for Fig. and design measurements; Ravicz anatomical helping Mike with for and helping Pagonis for staff, Idoff Otopathology Cornelia (MEE) Ear ACKNOWLEDGMENTS. in provided the histological are from data Appendix from motion specimens SI All quantified animal institution. our several was at and Laboratory Anatomy pres- Otopathology specimens 48). sound to human record (35, delivered of to were vestibule used preparations kHz were the sensors 24 in pressure and optic sure Hz Fiber reflected 100 canal. tissue’s ear between the the laser tones using a CP, pure used the Acoustic Measurements CP. on light. the directly over focused film scala vibrometer thin in Doppler cadav- a fluid to The y) viewed membrane. drained was 60.2 its was CP = removing tympani The after mean postmortem. window h old, round 108 y the to through 78 47 to specimens, bone (53 temporal human eric 6 from were measurements OSL ohermcaisi h rgno motn ai properties basic important of origin the is mechanics Cochlear classic the between OoC the of top the at difference major One r tlatprl opnae ydfeetT motion TM different by compensated partly least at are . Limbus moving etakDaeJns h ascuet y and Eye Massachusetts the Jones, Diane thank We Bridge i.S2 Fig. Appendix, SI moving motion maximum IAppendix SI TM BM We S2. and S1 Figs. Appendix, SI NSLts Articles Latest PNAS nbif h Pmotion CP the brief, In . ugssta the that suggests | f6 of 5 vs.

APPLIED PHYSICAL BIOPHYSICS AND SCIENCES COMPUTATIONAL BIOLOGY thank Dennis Freeman and Sunil Puria for discussions and manuscript com- and Other Communication Disorders Grant R01DC013303, fellowships from ments, and 2 anonymous reviewers for helpful suggestions. This work was the German National Academic Foundation and the American Otological supported by National Institutes of Health/National Institute on Deafness Society, and an Amelia Peabody scholarship from MEE.

1. E. ter Kuile, Die Uebertragung der Energie von der Grundmembran auf die 25. C. J. Sumner et al., Mammalian behavior and physiology converge to confirm sharper Haarzellen. Archiv fuer die Gesammte Physiologie des Menschen und der Thiere 79, cochlear tuning in humans. Proc. Natl. Acad. Sci. U.S.A. 115, 11322–11326 (2018). 146–157 (1900). 26. C. A. Shera, J. J. Guinan, A. J. Oxenham, Revised estimates of human cochlear tuning 2. NP Cooper, A. Vavakou, M. V. D. Heijden, Vibration hotspots reveal longitudinal fun- from otoacoustic and behavioral measurements. Proc. Natl. Acad. Sci. U.S.A. 99, 3318– neling of sound-evoked motion in the mammalian cochlea. Nat. Commun. 9, 3054 3323 (2002). (2018). 27. S. Raufer, S. Verhulst, Otoacoustic emission estimates of human basilar mem- 3. H. Y. Lee et al., Two-dimensional cochlear micromechanics measured in vivo demon- brane impulse response duration and cochlear filter tuning. Hearing Res. 342:150– strate radial tuning within the mouse . J. Neurosci. 36, 8160–8173 160.(2016). (2016). 28. A. Corti, Recherches sur l’organe de l’ou¨ıe des mammiferes (Zeitschrift fuer 4. T. Ren, W. He, W. Kemp, Reticular lamina and vibrations in living wissenschaftliche Zoologie, Akademische Verlagsgesellschaft Geest und Portig, 1854), mouse cochleae. Proc. Natl. Acad. Sci. U.S.A. 113, 9910–9915 (2016). vol. 3. 5. R. L Warren et al., Minimal basilar membrane motion in low-frequency hearing. Proc. 29. K. Neubert, Die Basilarmembran des Menschen und ihr Verankerungssystem. Z. fur¨ Natl. Acad. Sci. U.S.A. 113, E4304–E4310 (2016). Anatomie Entwicklungsgeschichte 114:540–590 (1950). 6. A. Recio-Spinoso, J. S Oghalai, Mechanical tuning and amplification within the apex 30. N. P. Cooper, “Radial variation in the vibrations of the cochlear partition” in Recent of the Guinea pig cochlea. J. Physiol. 595, 4549–4561 (2017). Developments In Auditory Mechanics, H. Wada T. Takasaka, K. Ikeda, K. Ohyama, 7. H. Y. Lee et al., Noninvasive in vivo imaging reveals differences between tectorial Eds. (World Scientific, 2000), pp. 109–115. membrane and basilar membrane traveling waves in the mouse cochlea. Proc. Natl. 31. W. S. Rhode, A. Recio, Study of mechanical motions in the basal region of the Acad. Sci. U.S.A. 112, 3128–3133 (2015). chinchilla cochlea. J. Acoust. Soc. Am. 107, 3317–3332 (2000). 8. W. Dong et al., Organ of Corti vibration within the intact gerbil cochlea measured 32. K. E. Nilsen, I. J. Russell, The spatial and temporal representation of a tone on the by volumetric optical coherence tomography and vibrometry. J. Neurophysiol. 120, Guinea pig basilar membrane. Proc. Natl. Acad. Sci. U.S.A. 97, 11751–11758 (2000). 2847–2857 (2018). 33. J. J. Guinan, Olivocochlear efferents: Their action, effects, measurement and uses, and 9. S. Narayan, A. N. Temchin, A. Recio, M. Ruggero, Frequency tuning of basilar mem- the impact of the new conception of cochlear mechanical responses. Hearing Res. 362, brane and auditory nerve fibers in the same cochleae. Science 282, 1882–1884 38–47 (2018). (1998). 34. E. de Boer, A. L. Nuttall, The “inverse problem” solved for a three-dimensional model 10. C. R. Steele, J. B. de Monvel, S. Puria, A multiscale model of the organ of Corti. J. of the cochlea. III. Brushing-up the solution method. J. Acoust. Soc. Am. 105, 3410– Mech. Mater. Struct. 4, 755–778 (2009). 3420 (1999). 11. J. Meaud, K. Grosh, The effect of and basilar membrane 35. E. S. Olson, Intracochlear pressure measurements related to cochlear tuning. J. longitudinal coupling in cochlear mechanics. J. Acoust. Soc. Am. 127, 1411–1421 Acoust. Soc. Am. 110, 349–367 (2001). (2010). 36. R. C. Naidu, D. C. Mountain, Measurements of the stiffness map challenge a basic 12. G. Zweig, Nonlinear cochlear mechanics. J. Acoust. Soc. Am. 139, 2561–2578 tenet of cochlear theories. Hearing Res. 124, 124–131 (1998). (2016). 37. L. U. E. Kohlloeffel, A study of basilar membrane vibrations. III. The basilar membrane 13. E. de Boer, A. L. Nuttall, C. A. Shera, Wave propagation patterns in a “classical” three- frequency response curve in the living Guinea pig. Acustica 27, 82–89 (1972). dimensional model of the cochlea. J. Acoust. Soc. Am. 121, 352–362 (2007). 38. W. S. Rhode, “An investigation of postmortem cochlear mechanics using the 14. G. von Bekesy, Experiments in Hearing (Acoustical Society of America, 1960). Mossbauer¨ effect” in Basic Mechanisms of Hearing, A. R. Møller, Ed. (Academic Press, 15. L. U. E. Kohlloeffel, “Problems in aural sound conduction” in Mechanics of Hearing- New York, 1973), pp. 49–67. Proceedings of the IUTAM/ICA Symposium, E. de Boer, M. Viergever, Eds. (Martinus 39. S. Farrahi, R. Ghaffari, J. Sellon, H. Nakajima, D. Freeman, Tectorial membrane trav- Nijhoff Publishers, 1983), pp. 211–217. eling waves underlie sharp auditory tuning in humans. Biophysical J. 111, 921–924 16. S. Rhode, Observations of the vibration of the basilar membrane in squirrel monkeys (2016). using the mossbauer technique. J. Acoust. Soc. Am. 49, 1281–1231 (1971). 40. W. Chien et al., Measurements of velocity in live human . Hearing Res. 249, 17. W. S. Rhode, Basilar membrane mechanics in the 6–9kHz region of sensitive chinchilla 54–61 (2009). cochleae. J. Acoust. Soc. Am. 121, 2792–2804 (2007). 41. S. Kapuria, C. R. Steele, S. Puria, Unraveling the mystery of hearing in gerbil and other 18. S. Stenfelt, S. Puria, N. Hato, R. L. Goode, Basilar membrane and rodents with an arch-beam model of the basilar membrane. Scientific Rep 7, 228 motion in human cadavers with air and bone conduction stimuli. Hearing Res 181, (2017). 131–143 (2003). 42. A. Taber, C. R. Steele, Cochlear model including three-dimensional fluid and four 19. S. N. Merchant, J. Nadol, Schuknecht’s Pathology of the Ear, S. N. Merchant, J. Nadol, modes of partition flexibility. J. Acoust. Soc. Am. 70, 426–436 (1981). Eds. (PMPH, ed. 3, 2010). 43. J. B. Allen, Cochlear micromechanics-A physical model of transduction. J. Acoust. Soc. 20. N. P Cooper, W. S. Rhode, Basilar membrane mechanics in the hook region of cat Am. 68, 1660–1670 (1980). and Guinea-pig cochleae: Sharp tuning and nonlinearity in the absence of baseline 44. J. J. Zwislocki, Theory of cochlear mechanics. Hearing Res. 2, 171–182 (1980). position shifts. Hearing Res. 63, 163–190 (1992). 45. R. Ghaffari, A. J. Aranyosi, D. M. Freeman, Longitudinally propagating traveling 21. W. Dong, E. S. Olson, In vivo impedance of the gerbil cochlear partition at auditory waves of the mammalian tectorial membrane. Proc. Natl. Acad. Sci. U.S.A. 104, frequencies. Biophysical J 97, 1233–1243 (2009). 16510–16515 (2007). 22. A. Recio, N. C. Rich, S. S. Narayan, M. A. Ruggero, Basilar-membrane responses to 46. M. Nowotny, A. W. Gummer, Nanomechanics of the subtectorial space caused by elec- clicks at the base of the chinchilla cochlea. J. Acoust. Soc. Am. 103, 1972–1989 tromechanics of cochlear outer hair cells. Proc. Natl. Acad. Sci. U.S.A. 103, 2120–2125 (1998). (2006). 23. T. Gundersen, O. Skarstein, T. Sikkeland, A study of the vibration of the basilar mem- 47. A. W. Gummer, W. Hemmert, H. P. Zenner, Resonant tectorial membrane motion in brane in human temporal bone preparations by the use of the mossbauer effect. Acta the : Its crucial role in frequency tuning. Proc. Natl. Acad. Sci. U.S.A. 93, Oto-Laryngologica 86:225–232 (1978). 8727–8732 (1996). 24. P. X. Joris et al., Frequency selectivity in Old-World monkeys corroborates sharp 48. H. H. Nakajima et al., Differential intracochlear sound pressure measurements in cochlear tuning in humans. Proc. Natl. Acad. Sci. U.S.A. 108, 17516–17520 (2011). normal human temporal bones. J. Assoc. Res. Otolaryngol. 10, 23–36 (2009).

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