Spontaneous cellular vibrations in the guinea-pig cochlea.

Mechanical vibrations of Hensen cells were measured with a laser-heterodyne interferometer in the third turn of the guinea-pig temporal-bone preparation without the application of an external stimulus. Smoothed periodograms (spectral-density estimates vs frequency) were constructed from the velocity vs time waveforms recorded from individual cells. For some cells, several peaks appear in the periodograms at levels as high as 10 dB above the noise floor, indicating the presence of spontaneous vibrations. The frequencies at which the peaks are located differ in different preparations, indicating that the observed peaks are not caused by the presence of ambient noise or ambient vibrations. It is demonstrated that smoothed-periodogram analysis is superior to fast-Fourier-transform analysis for discerning these spontaneous spectral components. The frequency tuning curves of cells from which spontaneous vibrations were measured (determined by applying an external stimulus to the ear) have single principal peaks. When the spontaneous spectral features are present, their frequencies lie, for the most part, within the principal-peak region of the tuning curve. We propose that these spontaneous vibrations originate at the outer hair cells and are the source of spontaneous otoacoustic emissions in the ear.