Observations of the Vibration of the Basilar Membrane in Squirrel Monkeys using the Mössbauer Technique

The amplitude and the phase of vibration of the basilar membrane and the bony limbus of the cochlea were measured in living squirrel monkeys using the Mössbauer technique. In the middle ear, the vibration of the malleus (and occasionally the incus) was measured. The Mössbauer technique makes possible the measurement of very small velocities, e.g., 0.2 mm/sec. This sensitivity permits measurement of the motion of the malleus at sound-pressure levels (SPLs) of 90 to 110 dB and measurement of the motion of the basilar membrane at 70 to 120 dB SPL, depending on the frequency. The basilar membrane vibrates nonlinearly for frequencies which produce the largest deflections at the spot on the basilar membrane under observation. The ratio of the displacement of the basilar membrane to that of the malleus was observed to have the following characteristics: (1) As the frequency is increased from a low value, its amplitude increases at 6 dB/oct until just below the maximum ratio where the slope increases to about 24 dB/oct; (2) the maximum ratio was about 24 dB for the SPLs used; (3) for frequencies above that producing the maximum ratio, the drop-off rate was approximately 100 dB/oct; (4) the amplitude ratio did not drop off indefinitely but tended to level off; (5) the motion of the basilar membrane differs from the motion of the malleus by 90° at very low frequencies; (6) for frequencies below that producing the maximum ratio, the phase differences between the motion of the basilar membrane and that of the malleus is a linear function of frequency; (7) near the frequency corresponding to the maximum ratio, the phase difference decreases at a faster rate; and (8) the phase difference approaches a constant value (7π 8π or 9π) for high frequencies. Anatomical constraints allowed only a small portion of the basal turn to be studied (6.5–7.5 kHz produced maximum deflection of the basilar membrane in this region).