Cochlear mechanics: new insights from vibrometry and optical coherence tomography
Section snippets
Early and recent techniques of cochlear vibrometry
The cellular component of the cochlea's sensory tissue, termed the organ of Corti (OoC), is a long narrow strip, bounded by two acellular structures, the basilar and tectorial membranes (BM and TM) and surrounded by fluid chambers. A sound stimulus enters the cochlea at the basal end, and makes its way to frequency-dependent locations by means of a fluid-mechanical traveling wave. The leading actors of hearing are the hair cells (HC), whose stereocilia ‘hair’ extend from the apical (top)
OCT vibrometry in the cochlea
OCT imaging uses a low-coherence infrared light source that can penetrate biological tissue, allowing imaging and interferometric motion measurements at depths of several millimeters. The axial imaging resolution is determined by the bandwidth of the light source (larger bandwidth = better resolution), with resolution values in the micrometer range. Lateral resolution is determined by the lens numerical aperture, as in standard microscopy [26]. In the first applications of OCT to cochlear
Physiological advances
There are two significant benefits of OCT-vibrometry over classic heterodyne vibrometry: the ability to measure different layers within the OoC and the ability to measure through the bone of the cochlear capsule.
The ability to measure within the OoC has exposed significant and unanticipated OoC motions. Motions at the RL and in the OHC/Deiters cell regions exhibit higher amplitudes than at the BM, and enhanced and nonlinear responses in these regions extend to sub-CF frequencies [37, 38, 39, 40
Future
The development and application of more intense and broader bandwidth light sources will improve axial resolution and vibrometry signal:noise [56]. Axial resolution is also improved by employing a shorter wavelength light source [26]. Lateral resolution can be improved by using a higher numerical aperture objective lens [57,28]. However, this comes at the expense of reduced axial working range and distance and will not be practical for some physiological measurements. OCT systems have been
Funding
The authors are supported by an NIH/NIDCD grant to EO and the Emil Capita Foundation.
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
•• of outstanding interest
Acknowledgements
We thank Marcel van der Heijden and coauthors for the data shown in Figure 2 of [37] and John Oghalai and coauthors for the use of Figure 3 in [45••].
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