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

The basal membrane (BM) velocity responses to pure tones were measured using a newly developed laser interferometer microscope that does not require placing a reflecting object on the BM. It was demonstrated that the instrument is able to measure sub-nanometer vibration from the cochlear partition in the basal turn of the gerbil. The overall shape of the amplitude spectra shows typical tuning features. The 'best' frequencies (BFs) for the BM locations studied were between 14 kHz and 27 kHz, depending on the longitudinal position. For a given BM location, tuning sharpness was input level dependent, indicated by the Q(10dB), which varied from approximately 3 at low stimulus levels to near 1.5 at high input levels. At frequencies below BF, parallel amplitude/frequency curves across stimulus levels indicate a linear growth function. However, at frequencies near BF, the velocity increased linearly at low levels (<40 dB SPL) and became compressed between 40 and 50 dB SPL. Although the velocity gain for the frequency range below BF was a function of frequency, for a given frequency the gains were approximately constant across different levels. At frequencies near BF, the velocity gain at low sound pressure level was greater than that at a high sound pressure level, indicating a nonlinear negative relationship to stimulus level. The data 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, indicating an extremely short delay; (2) the greatest phase lag occurs at frequencies near the BF, indicating a significant delay near this frequency range.