Panoramic Measurements of the Apex of the Cochlea

Abstract

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 Ͻ5 kHz. Data from six cats yielded panoramic phase profiles along the apex of the cochlea for an ϳ5 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 ϳ3.5 to ϳ1 mm for frequencies from 200 to 4000 Hz, corresponding to phase velocities of ϳ0.5 to ϳ5 m/s. The accumulated phase lag between the eardrum and characteristic position varied from ϳ1 cycle at 200 Hz to ϳ2.5 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.

Questioning cochlear amplification

“…2. It reproduces panoramic measurements of cochlear wave propagation compiled from the phase of single nerve-fiber responses in single cat ears [10,19]. These spatial phase profiles show sharp bends at a location just basal to the peak region of the wave, which connect the shallow basal end of the phase profile (fast propagation) to the steeper portion near the peak (slow propagation).…”

Frequency selectivity without resonance in a fluid waveguide

“…The deceleration produces a densification (focusing) of the energy that creates the peaking. This approach was motivated by neural data revealing a steep deceleration of cochlear waves near their peak (5,11). Rather than building an elaborate biophysical model, the aim was to find the simplest possible fluid waveguide exhibiting steep deceleration and peaking.…”

Reverse Correlation Analysis of Auditory-Nerve Fiber Responses to Broadband Noise in a Bird, the Barn Owl

“…In mammals, it has been shown that the phase curves were concave upward for CFG1 kHz and downward above (van der Heijden and Joris 2006;Temchin et al 2011;Versteegh et al 2011). As shown in Figure 10B, the concavity is related to the sign of the IF slope: A positive slope yields an upward concavity.…”