Abstract
Mechanical vibrations of Hensen cells were measured with a laser-heterodyne interferometer in the guinea-pig temporal-bone preparation without the application of an external acoustic stimulus. Smoothed periodograms (spectral-density estimates v. frequency) were constructed from the velocity v. time waveforms recorded from individual cells. Several peaks were seen 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 were located differed in different preparations, indicating that the observed peaks were not caused by the presence of ambient noise or ambient vibrations. Furthermore, vibrations were seen only in fresh preparations. The tuning curves of cells from which spontaneous vibrations were measured (determined by applying an external stimulus to the ear) had single principal peaks. Several peaks in the periodogram were found to be located within the principal-peak region of the tuning curve. The spontaneous response does not arise from noise filtered through the tuning curve which would have a single peak. We propose that these spontaneous vibrations originate at the outer hair cells and are the source of spontaneous otoacoustic emissions in the ear.