Basilar membrane vibration in the basal turn of the sensitive gerbil cochlea

The basal membrane (BM) velocity responses to pure tones were measured usin g a newly developed laser interferometer microscope that does not require p lacing a reflecting object on the BM. It was demonstrated that the instrume nt is able to measure sub-nanometer vibration from the cochlear partition i n the basal turn of the gerbil. The overall shape of the amplitude spectra shows typical tuning features. The 'best frequencies (BFs) for the BR I loc ations studied were between 14 kHz and 77 kHz, depending on the longitudina l position. For a given BM location, tuning sharpness was input level depen dent, indicated by the Q(10dB), which varied from approximately 3 at low st imulus levers to near 1.5 at high input levels, At frequencies below BF, pa rallel amplitude/frequency curves across stimulus levels indicate a linear growth function. However, at frequencies near BF, the velocity increased li nearly at low levels (<40 dB SPL) and became compressed between 40 and 50 d B SPL, Although the velocity gain for the frequency range below BF was a fu nction of frequency, for a given frequency the gains were approximately con stant across different levels. At frequencies near BF, the velocity gain at low sound pressure level was greater than that at a high sound pressure le vel, indicating a nonlinear negative relationship to stimulus level. The da ta also showed that the BF shifts toward the low frequencies with stimulus intensity increase. The phase spectra showed two important features: (1) at frequencies about half octave below the BF, phase slope is very small, ind icating an extremely short delay; (2) the greatest phase lag occurs at freq uencies near the BF, indicating a significant delay near this frequency ran ge. (C) 2001 Elsevier Science B.V. All rights reserved.