gms | German Medical Science

The improvement of speech recognition of people with profound sensory hearing loss is done by direct stimulation of the hearing nerve with a Cochlear Implant (CI). Because some of those patients have residual hearing abilities, especially at low frequencies the simultaneous acoustic stimulation at those low frequencies was proposed (electro-acoustic stimulation, EAS) [Ref. 1]. We show, that is possible to enhance the Basilar Membrane (BM) displacement at places with intact sensory cells when a cochlear implant electrode fixes areas of the BM with destroyed sensory cells.

To study the influence of implanted CI electrodes on the wave propagation in the cochlea we use a slightly modified Finite Element model of the cochlea we developed before [Ref. 2].

The upper part of the electrode in the Scala tympani is in direct contact with the Scala tympani layer of the BM. In contrast to this the lower part of the electrode touches the bony structure at the inner spiral limbus and therefore the BM is able to vibrate when acoustic signals are transmitted to the cochlea.

An electrode fixes the BM between 8,5 mm and 17,7 mm along a length of 9,2 mm in the cochlea. All degrees of freedom (displacements and rotations) are set to zero in this area.

When the stapes footplate (not shown) is stimulated by a signal of amplitude 1 Pa and f = 2000 Hz in the healthy cochlea the BM is displaced. If the BM is fixed by the CI electrode the displacements in the vicinity of the fixation are larger up to 6 dB.

The systematic fixation of the BM by the CI electrode at specific places causes the BM to displace with higher amplitudes in the vicinity of the fixed section. Therefore hopefully healthy sensory cells are displaced with higher amplitudes. This should improve speech recognition of patients with simultaneous electro-acoustic stimulation.