Electrical impedance measurements of cochlear structures using the four-electrode reflection-coefficient technique

In individuals with severe-to-profound hearing loss, cochlear implants (CIs) bypass normal inner ear function by applying electrical current directly into the cochlea, thereby stimulating surviving auditory nerve fibers. Although cochlear implants are able to restore some auditory sensation, they are far from providing normal hearing. It has been estimated that up to 75% of the current injected via a CI is shunted along scala tympani and is not available to stimulate auditory neurons. The path of the injected current and the consequent population of stimulated spiral ganglion cells are dependent upon the positions of the electrode contacts within the cochlea and the impedances of cochlear structures. However, characterization of the current path remains one of the most critical, yet least understood, aspects of cochlear implantation. In particular, the impedances of cochlear structures, including the modiolus, are either unknown or based upon estimates derived from circuit models. Impedance values for many cochlear structures have never been measured. By combining the hemicochlea preparation, a cochlea cut in half along its mid-modiolar plane, and the four-electrode reflection-coefficient technique, impedances can be measured for cochlear tissues in a cochlear cross section including the modiolus. Advantages and disadvantages of the method are discussed in detail and electrical impedance measurements obtained in the gerbil hemicochlea are presented. The resistivity values for the cochlear wall in Ωcm are, 528 (range: 432-708) for scala media 3rd turn, 502 (range: 421-616) for scala tympani 3rd turn and scala vestibuli 2nd turn, 627 (range: 531-759) for scala media 2nd turn, 434 (range: 353-555) for scala tympani 2nd turn and scala vestibuli basal turn, 434 (range: 373-514) for scala media basal turn, and 590 (range: 546-643) for scala tympani basal turn. The resistivity was 455 Ωcm (range: 426-487) for the modiolus.