Quantitative in vivo measurements of inner ear tissue resistivities: I. In vitro characterization.

An in vivo resistivity measurement system, based on the four-electrode reflection-coefficient technique that nondestructively measures the complex resistivity of cochlear tissues, is described. Details of the theory and instrumentation used for noninvasive measurement of resistivity are presented. In vitro experiments both characterize the accuracy of the proposed resistivity measurement system and establish general criteria for ensuring that a particular theoretical model accurately represents the experimentally measured geometry. Two idealized geometries (two-layer planar and two-layer spherical) are measured experimentally; error analyses using experimental results describe the maximum error with which the experimental system noninvasively estimates resistivity from experimental reflection coefficient measurements. The precise accuracy of a noninvasive resistivity estimate depends on both the variability for experimentally measuring the reflection coefficient of a particular geometry and the average value of the measured reflection coefficient. For example, two-point measurements of an in vitro two-layer planar interface allow noninvasive estimation of complex resistivity with total errors of less than 1%. In addition to characterizing accuracy of resistivity estimates for different in vitro geometries, two general criteria were established: 1) any inhomogeneity within 13.3 times the average interelectrode separation (i.e., within 1662.5 microns for an interelectrode separation of 125 microns) from the microelectrode array must be included in the geometry of the theoretical model and 2) all inhomogeneous boundaries with a radius of curvature greater than 100 times the average interelectrode separation (i.e., greater than 12.5 mm for an average electrode separation of 125 microns) are accurately represented by a planar geometric model.(ABSTRACT TRUNCATED AT 250 WORDS)