Abstract #905, Date Wednesday, Jan 30 2002 1:00PM - 12:00PM Session W5 Cochlear Mechanics
Three Dimensional, in vivo Measurements of the Tectorial Membrane's Vibratory Responses to Sound
Wei Dong, Nigel P. Cooper
Previous studies of tectorial membrane (TM) motion in response to sound have had mixed results. One study showed the TM to move in simple, almost rectilinear paths which were oriented almost perpendicular to the reticular lamina, while two others have claimed that the TM moves in a more complex manner, with the major axes of its elliptical orbits lying almost parallel to the reticular lamina at certain frequencies. All of these studies have been performed in vitro, and it is not clear which set of observations is more likely to reflect the behavior of the TM in vivo. The aim of the present study is to ascertain how the TM moves in vivo. Reflective microbeads were deposited on the TM in the apical turn of the guinea-pig cochlea. The guinea-pigs were deeply anesthetized, and were killed humanely at the end of the in vivo measurements to permit comparisons of in vivo and post-mortem data. The sound-evoked vibrations of the beads were recorded from multiple viewing angles using a displacement-sensitive heterodyne laser interferometer. Three dimensional (3D) reconstructions of the TM's motion were then made using a modified version of the technique described by Decraemer et al. (1994). The acoustic stimuli were short tone pips ranging from 50Hz to 1kHz and from 60 to 80 dB SPL. The transfer functions for the TM resembled those of high-order low-pass filters. In general, the amplitudes of the transfer functions changed significantly with the viewing angle, but the phases did not. The basic patterns of the transfer functions changed very little for several hours post-mortem. The 3D reconstructions showed that: (1) below the transfer function's cutoff frequency of ~600Hz, the TM vibrates around thin elliptical pathways whose major axes are directed more perpendicular to the reticular lamina than parallel to it. (2) above the cutoff frequency, the motion appears more complex (i.e. less rectilinear). Supported by Defeating Deafness and the Royal Society.