Panoramic measurements of the apex of the cochlea.
Our understanding of cochlear mechanics is impeded by the lack of truly panoramic data. Sensitive mechanical measurements cover only a narrow cochlear region, mostly in the base. The global spatiotemporal pattern of vibrations along the cochlea cannot be inferred from such local measurements but is often extrapolated beyond the measurement spot under the assumption of scaling invariance. Auditory nerve responses give an alternative window on the entire cochlea, but traditional techniques do not allow recovery of the effective vibration pattern. We developed a new analysis technique to measure cochlear amplitude and phase transfer of fibers with characteristic frequencies <5 kHz. Data from six cats yielded panoramic phase profiles along the apex of the cochlea for an approximately 5 octave range of stimulus frequencies. All profiles accumulated systematic phase lags from base to apex. Phase accumulation was not gradual but showed a two-segment character: a steep segment (slow propagation) around the characteristic position of the stimulus, and a shallow segment (fast propagation) basal to it. The transition between the segments occurred in a narrow region and was smooth. Wavelength near characteristic position decreased from approximately 3.5 to approximately 1 mm for frequencies from 200 to 4000 Hz, corresponding to phase velocities of approximately 0.5 to approximately 5 m/s. The accumulated phase lag between the eardrum and characteristic position varied from approximately 1 cycle at 200 Hz to approximately 2.5 cycle at 4 kHz, invalidating scaling invariance. The generic character of our analysis technique and its success in solving the difficult problem of reconstructing the effective sensory input from neural recordings suggest its wider application as a powerful alternative to customary system analysis techniques.