Mechanism of cochlear excitation at low intensities

In order to assess the mechanisms of cochlear activation, the cochlear fluids of one cochlea of a guinea-pig (I) were coupled to those of a cochlea of a second guinea-pig (II) by means of a saline-filled narrow bore tube, the ends of which were placed in the fluids around the opened round windows of both cochleae, thus joining the two cochleae from two different animals into a single, larger, unsealed fluid system. In response to air-conducted sound stimulation of cochlea I, auditory nerve-brainstem evoked responses could be recorded in animal II, not only when the coupling tube was filled with saline, but also when it was filled with ultrasound gel (viscosity 100,000 greater than that of water), when there was a very large hole encompassing a relatively large expanse of the cochlear shell of animal I, and even when animal I was no longer alive. The necessary control experiments were performed. Therefore, it is suggested that at low stimulus intensities, the passive, incoming basilar membrane traveling wave may not activate the cochlea. Instead the fluid pressures (condensation/rarefactions) induced in the cochlear fluids by vibrations of the stapes footplate may be adequate to directly activate the outer hair cells, which then generate an active component of basilar membrane displacement.