The Vibratory Pattern of the Round Window in Cats

Abstract

The middle ear was removed surgically, and the cochlea was acoustically driven in a tightly closed system built over the oval window niche. Sound pressures were monitored by a probe microphone, its opening being located about 3 mm above the footplate. The round window was coated by a fine bronze powder. Vibration patterns were obtained by time-averaged holography. Initial technical difficulties were presented by the small size of the window (holographic resolution), its complex shape (viewing angle), and the oozing of fluid from its mucus-membrane covering (obscuring the window). Because of the window's small size, a relatively simple pattern had been expected. However, even at low frequencies, the pattern was already complex. This complexity made the evaluation of volume displacement difficult, and also that of inner-ear impedance. Results will be compared to those of an earlier study [Tonndorf et al., Ann. Otol. 75, 752 (1966)], which evaluated inner-ear impedance on the basis of displacement measurements of the round window obtained by a capacitive probe, while the displacement pattern was unknown. [Supported by a number of NIH grants.]