Representation of acoustic signals in the human cochlea in presence of a cochlear implant electrode

BACKGROUND In subjects with remaining low frequency hearing, combined electric-acoustic stimulation (EAS) of the auditory system is a new therapeutic perspective. Intracochlear introduction of a cochlear implant electrode, however, may alter the biomechanical properties of the inner ear and thus affect perception of acoustic stimuli. STUDY DESIGN Based on histological observations of morphologic changes after cochlear implantation in cadaveric and post mortem studies the effects of basilar membrane (BM) stiffening in the ascending basal and middle turns of the cochlea due to close contact of the BM with the electrode were simulated in a 3D-computational finite element model of the inner ear. To verify our simulated results, pre- and postoperative pure-tone audiograms of 13 subjects with substantial residual hearing, who underwent cochlear implantation, were evaluated. RESULTS In the scenario of partial BM-fixation, acoustic energy of middle (2 kHz) and high (6 kHz) frequency was focused basally and apically to the fixed section, increasing BM displacement amplitudes up to 6 dB at a stimulation level of 94 dB (SPL). Lower frequencies were not affected by fixation in the basal and middle turn of the cochlea. In implanted subjects, a small but significant decrease of thresholds was observed at 1.5 kHz, a place in tonotopy adjacent to the tip region of the implanted electrode. CONCLUSION Our model suggests that stiffening of the basilar membrane adjacent to an implanted electrode into the basal and middle cochlear turn did not affect BM movement in the low frequency area. Focussing of acoustic energy may increase perception in regions adjacent to the fixed section. Observations in implanted subjects were concordant with our model predictions. High frequencies, however, should not be amplified in patients using EAS to avoid disturbances in discrimination due to tonotopically incorrect frequency representation.