Application of laser velocimetry to the measurement of basilar membrane vibrations

A commercially available laser Doppler‐shift velocimeter has been coupled to a compound microscope equipped with ultra‐long‐working‐distance objectives for the purpose of measuring basilar membrane vibrations in the chinchilla. The animal preparation is nearly identical to that used in our laboratory for similar measurements using the Mössbauer technique. The vibrometer head is mounted on the third tube of the microscope's trinocular head and its laser beam is focused on high‐refractive‐index glass microbeads (20–50 μm) previously dropped, through the perilymph, on the basilar membrane. Sensitivity of the laser velocimetry system is at least one order of magnitude greater than usually attained using the Mössbauer technique. However, the most important advantage of laser velocimetry vis‐à‐vis the Mössbauer technique is its linearity, which permits undistorted recording of signals over a wide velocity range. Thus, for example, basilar membrane responses to clicks have been measured for stimulus intensities ranging over 80 dB. At low stimulus levels, these responses are as sharply tuned as frequency‐threshold curves of cochlear afferents with comparable characteristic frequency; at higher levels, sharpness of tuning diminishes. [Work supported by NIH.]