Computer-based holography and middle-ear function

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The tympanic membrane (TM) is the initial structure involved in the middle-ear's acoustic-mechanical transformation of environmental sounds to sound within the inner ear. There is good evidence that the form of the TM helps define the frequency range to which the ear is sensitive, including correlations between the sensitivity and range of hearing and the size and shape of the TM. While we know some basic facts about the workings of the healthy TM in a limited frequency range, there are many issues that are unresolved including the effect of TM shape on function, the effect of large differences in TM mechanical properties across species as well as how the TM functions at higher frequencies. There are also questions regarding the workings of the pathologic TM, e.g. How do perforations affect the motion of the entire TM? and Do grafted membranes work like normal TMs? The work proposed here will apply a real-time fiber-optic electro- holographic system based on fast computer-based video-processing to the study of the sound-induced displacement of the TM in normal ears of several animal species (including humans) as well as in ears with induced pathologies and reconstructions. The optical measurement system produces a continuously updated display of time-averaged holograms (up to 500 frames a second) of the displacement of the surface of the TM. This display allows observations of iso-displacement contours of the entire TM surface (with a resolution of 50-200 nm) while the amplitude and/or frequency of the stimulus sound are continuously varied. Such observations lead to easy identification of the critical frequencies and the level dependence of TM displacement patterns. A second version of the system (stroboscopic holography) allows measurement with resolutions of 1-10 nm of the magnitude and phase of the displacement of the entire membrane surface. A third version (dual-wavelength holography) allows measurement of the static shape of the TM with 1-10 nm resolution. The applications of these optical techniques include the identification and quantification of wave travel on the surface of the normal TM, investigations of inter-specific differences in TM motion and TM mechanical parameters, and tests of hypotheses concerning the sensitivity of membrane displacement patterns to ossicular disorders and TM perforations as well as the motion of various TM graft configurations in surgically reconstructed human ears.