The application of a capacitive probe technique for direct observation of electromechanical processes in the guinea...

The demonstration of evoked mechanical responses of the outer hair cells in the mammalian cochlea by indirect measurements introduces a new range of problems into direct mechanical measurements. Direct and indirect measurements indicate that the frequency spectra of evoked electromechanical responses may extend well into the range of audio frequencies, revealing a need to develop terminology and protocols for distinguishing evoked mechanical responses from the traditional traveling wave when both are apparently superimposed on the motion of the basilar membrane in the normally functioning cochlea. Details are presented of a frequency-modulation capacitive probe technique for measurement of vibrating structures of the guinea pig ear. Considerations include the design of the transducer, calibration, sensitivity, linearity, and sources of noise, as well as the influence of the technique upon the animal preparation, and in particular the issues associated with draining scala tympani for the measurement. Relative advantages and disadvantages of the technique are compared with salient features of other techniques currently available. In view of the apparent complexity of cochlear mechanics some preliminary experiments are required to elucidate some of the key questions about reverse-transduction processes in general. A "simple" first experiment is to test existence of any rectifying or motile response.