ANALYSES OF MOSSBAUER MECHANICAL MEASUREMENTS INDICATE THAT THE COCHLEA IS MECHANICALLY ACTIVE

Using the results of Mossbauer measurements, mechanical activity in the cochlea was tested for by comparing the measured basilar membrane (BM) transverse velocity amplitude with that calculated for a lossless mechanically passive system, derived from the measured BM velocity phase. If the cochlea is considered to be a lossless mechanically passive system, then the transverse velocity amplitude can be calculated from the group velocity and the relative variation of stiffness along the BM. The group velocity can be derived from the Mossbauer phase measurements, and the relative variation of stiffness along the BM can be derived from the frequency map of the cochlea. Making some general assumptions, the actual transverse velocity amplitudes are then compared from the Mossbauer amplitude data with those derived from the Mossbauer phase data, to determine if there is a significant transverse velocity gain. This operation was performed on several sets of Mossbauer data. From Mossbauer data showing sharp tuning, differences were found of up to 40 dB between the actual transverse velocity amplitude and the calculated lossless passive transverse velocity amplitude derived from the phase data. Examination of the assumptions made during the calculation of the lossless passive transverse velocity amplitude showed that none could account for a 40-dB transverse velocity gain. Thus, it is concluded that this transverse velocity gain can only be accounted for by the contribution to the amplitude of the transverse BM velocity by mechanically active elements along the cochlear duct. From Mossbauer data showing much less sharp tuning, it was found that the actual transverse velocity amplitude was approximately equal to the calculated lossless passive transverse velocity amplitude, basal to the characteristic place. This result is attributed to the disabling of the mechanically active elements along the cochlear duct. Apical to the characteristic place, the actual transverse velocity amplitude is shown to behave in a manner that suggests that the effective damping increases markedly in this region. This increase in effective damping is not necessarily due to an increase in viscous damping but could be due to any mechanism removing energy from the BM traveling wave. As an example, this paper discusses how this effective damping could be accounted for the transfer of energy to another mode of vibration at the characteristic place.