PRESSURE-INDUCED BASILAR-MEMBRANE POSITION SHIFTS AND THE STIMULUS-EVOKED POTENTIALS IN THE LOW-FREQUENCY REGION OF THE GUINEA-PIG COCHLEA

We have used the guinea pig isolated temporal bone preparation to inve stigate changes in the nonlinear properties of the tone-evoked cochlea r potentials during reversible step displacements of the basilar membr ane towards either the scala tympani or the scala vestibuli. The posit ion shifts were produced by changing the hydrostatic pressure in the s cala tympani. The pressures involved were calculated from measurements of the fluid flow through the system, and the cochlear DC impedance c alculated (1.5 x 10(11) kg m(-4) s(-1), n = 10). Confocal microscopic visualization of the organ of Corti showed that pressure increases in the scala tympani caused alterations oi the position of the reticular lamina and stereocilia bundles. For low pressures, there was a sigmoid al relation between the DC pressure applied to the scala tympani (and thus the position shift of the organ of Corti) and the amplitude of th e summating potential. The cochlear microphonic potential also showed a pronounced dependence on the applied pressure: pressure changes alte red the amplitude of the fundamental as well as its harmonics. In addi tion, the sound pressure level at which the responses began to saturat e was increased, implying a transition towards a linear behaviour. An increase of the phase lag of the cochlear microphonic potential was se en when the basilar membrane was shifted towards the scala vestibuli. We have also measured the intracochlear DC pressure using piezoresisti ve pressure transducers. The results are discussed in terms of changes in the non-linear properties of cochlear transduction. In addition, t he implications of these results for the pathophysiology and diagnosis of Meniere's disease are discussed.