Energy Flux in the Cochlea: Evidence Against Power Amplification of the Traveling Wave

Traveling waves in the inner ear exhibit an amplitude peak that shifts with frequency. The peaking is commonly believed to rely on motile processes that amplify the wave by inserting energy. We recorded the vibrations at adjacent positions on the basilar membrane in sensitive gerbil cochleae and tested the putative power amplification in two ways. First, we determined the energy flux of the traveling wave at its peak and compared it to the acoustic power entering the ear, thereby obtaining the net cochlear power gain. For soft sounds, the energy flux at the peak was 1 ± 0.6 dB less than the middle ear input power. For more intense sounds, increasingly smaller fractions of the acoustic power actually reached the peak region. Thus, we found no net power amplification of soft sounds and a strong net attenuation of intense sounds. Second, we analyzed local wave propagation on the basilar membrane. We found that the waves slowed down abruptly when approaching their peak, causing an energy densification that quantitatively matched the amplitude peaking, similar to the growth of sea waves approaching the beach. Thus, we found no local power amplification of soft sounds and strong local attenuation of intense sounds. The most parsimonious interpretation of these findings is that cochlear sensitivity is not realized by amplifying acoustic energy, but by spatially focusing it, and that dynamic compression is realized by adjusting the amount of dissipation to sound intensity

The spatial buildup of compression and suppression in the mammalian cochlea

We recorded responses of the gerbil basilar membrane (BM) to wideband tone complexes. The intensity of one component was varied and the effects on the amplitude and phase of the others were assessed. This suppression paradigm enabled us to vary probe frequency and suppressor frequency independently, allowing the use of simple scaling arguments to analyze the spatial buildup of the nonlinear interaction between traveling waves. Most suppressors had the same effects on probe amplitude and phase as did wideband intensity increments. The main exception were suppressors above the characteristic frequency (CF) of the recording location, for which the frequency range of most affected probes was not constant, but shifted upward with suppressor frequency. BM displacement reliably predicted the effectiveness of low-side suppressors, but not high-side suppressors. We found "anti-suppression" of probes well below CF, i.e., suppressor-induced enhancement of probe response amplitude. Large (>1 cycle) phase effects occurred for above-CF probes. Phase shifts varied nonmonotonically, but systematically, with suppressor level, probe frequency, and suppressor frequency, reconciling apparent discrepancies in the literature. The analysis of spatial buildup revealed an accumulation of local effects on the propagation of the traveling wave, with larger BM displacement reducing the local forward gain. The propagation speed of the wave was also affected. With larger BM displacement, the basal portion of the wave slowed down, while the apical part sped up. This framework of spatial buildup of local effects unifies the widely different effects of overall intensity, low-side suppressors, and high-side suppressors on BM responses