Spatial Buildup of Cochlear Compression Revisited

In 2013 we published an experimental study (Versteegh and Van der Heijden, doi.org/10.1007/s10162-013-0393-0) that introduced a traveling-wave perspective of cochlear nonlinearity, the so-called spatial buildup of compression and suppression. We recorded basilar membrane (BM) motion in the base of the gerbil cochlea, and obtained experimental evidence for the spatial buildup by independently varying the frequencies of the suppressor and the probe. The study concluded that the nonlinear response at any point in the cochlea can only be properly understood in terms of a spatial buildup of compression. Traveling waves start out as linear responses to the stapes drive and gradually accumulate compressive nonlinearity after entering their peak region, ~½ octave short of their best place. Likewise, the nonlinear effects of intensity on vibration phase can only be understood in terms of local changes in propagation velocity of the wave. Once viewed through this lens, a large body of published data on the effects of compression and suppression, often featuring complex patterns and seemingly contradicting trends, was shown to fit in a unifying framework. Despite the emphasis on spatial buildup, the data underling the 2013 study were obtained from single BM locations, due to the need for reflective microbeads with laser Doppler vibrometry. The spatial aspect was only explored by proxy, i.e., by varying the stimulus frequency and exploiting the tonotopy of the cochlear response (“scaling”). Optical coherence tomography (OCT) vibrometry has liberated us from this limitation, and in the current work we actually traced the spatial buildup of compression. In the BM and OHC region of the 13-24-kHz region of the gerbil cochlea, we recorded responses to equal-amplitude tone pairs centered at 15 kHz, spaced by 20 Hz. The resulting beating pattern affords a dynamic way of obtaining I/O curves. Spectral analysis of the responses yielded an accurate quantitative description of the compressive growth. We observed that the amount of compression (the difference in gain between the peak and dip episodes of the beat) increased gradually and systematically along the course of the slowly propagating wave, which confirms the spatial buildup claimed in our 2013 study. With increasing instantaneous intensity, the spatial rate of compression increased systematically over the nonlinear region. Overall, the conclusions of the 2013 study were confirmed, emphasizing the tight relation between cochlear compression and the wave character of cochlear vibrations.