Francis A. Spelman

Orthogonal-plane fluorescence optical sectioning: Three-dimensional imaging of macroscopic biological specimens

An imaging technique called orthogonal‐plane fluorescence optical sectioning (OPFOS) was developed to image the internal architecture of the cochlea. Expressions for the three‐dimensional point spread function and the axial and lateral resolution are derived. Methodologies for tissue preparation and for construction, alignment, calibration and characterization of an OPFOS apparatus are presented. The instrument described produced focused, high‐resolution images of optical sections of an intact, excised guineapig cochlea. The lateral and axial resolutions of the images were 10 and 26 μm, respectively, within a 1·5‐mm field of view.

Quadrupolar stimulation for cochlear prostheses: modeling and experimental data

Cochlear implants are electrically driven in monopolar, bipolar, or common ground mode. Ideally, a quadrupolar mode is created with three colinear electrodes, where the outer poles are half the inverse polarity value of the center electrode. The resulting field is highly focused. Models of point sources show that the quadrupolar paradigm offers a greater choice of parameters to shape the field. Simulation with a lumped-parameter model of the cochlea confirms the focusing action of the quadrupole in the layers of the inner ear. Field measurements in saline solution and in the scala tympani of guinea pigs show that focusing occurs with the quadrupolar mode. It is conceivable that quadrupolar stimulation will affect the pitch place coding, reduce channel interaction and limit facial or tactile stimulation induced by current spread.

Current Density Profiles of Surface Mounted and Recessed Electrodes for Neural Prostheses

A Greens function approach has been used to solve Lapalces equation for the quasi-static fields of a recessed, disk electrode. The resulting integral equation was solved numerically using the moment method. An analysis of the error in the approximate solution shows that it must be less than 7 percent for the cases studied. The calculations indicate that a recessed electrode has a more uniform current density profile than a surface mounted electrode. This is true both at the electrode surface, and at the electrode carrier¿tissue junction. The significance of this finding is discussed as is its application to electrochemical, histopathological, and physiological studies of neural prostheses. The clinical use of recessed electrodes in cochlear implants is recommended.

The past, present, and future of cochlear prostheses

The author discusses the accomplishments and challenges in treating sensorineurnal deafness through electrical stimulation.

In vitro measurement and characterization of current density profiles produced by nonrecessed, simple recessed, and radially varying recessed stimulating electrodes

Potential fields induced by nonrecessed, simple recessed and radially varying recessed electrode designs were measured in vitro. Comparison of experimental results with theoretical analyses substantiated the experimental measurement technique and emphasized the importance of considering both nonuniform charge injection and surface electrochemistry when designing implantable stimulating electrodes. Radially varying recesses produced uniform charge injection at the electrode surface and at the aperture-tissue interface. In general, the radially varying recessed electrodes provided a combination of uniform charge injection and flexibility in design and fabrication that warrants their incorporation into all appropriate planar stimulating electrode designs.<<ETX>>

Multi-electrode cochlear implant and method of manufacturing the same

A multi-electrode cochlear implant is taught in which approximately twenty or more insulated metal wires are wound around a flexible tube. These wires are held in place with a further layer of dielectric insulating material. The insulation is selectively removed with a laser beam to form electrodes. Two or more layers or valences of wires can be used, with the inner layer of wires terminating distal to the outer layers to provide a stepwise approximation of the tapering of the scala tympani. A core of shape memory material may be introduced into the tube, so that the implant will retain an effective shape after implantation.

Lumped-parameter model for in vivo cochlear stimulation

A lumped-parameter model that simulates the in vivo electrical properties of a guinea pig cochlea implanted with a multielectrode stimulating array is presented. A basic model of the low-frequency electroanatomy in a normally functioning guinea pig cochlea is developed by adding critical membrane capacitances to D. Strelioffs resistive network model (1973). The basic model of normal cochlear tissues is modified to account for anatomical and physiological differences between a normal and implanted cochlea, resulting in an impedance model of an implanted cochlea. Simulating the results of in vivo cochlear stimulation verifies the accuracy with which the modified cochlear model represents electrical properties within an electrically stimulated cochlea. Generalized simulations using this model suggest a straightforward phasing scheme capable of achieving sharply focused, channel-independent multielectrode cochlear stimulation.<<ETX>>

Psychophysical evaluation of cochlear prostheses in a monkey model.

Functional aspects of cochlear prostheses implanted in the scala tympani were tested in monkeys trained to perform a simple reaction-time task. Thresholds for detection of electrical stimulation and dynamic ranges were tested for a wide range of frequencies of sinusoidal stimulation and for biphasic rectangular pulses of various durations and repetition rates. The results are comparable with available data from implanted human patients and extend these findings, exploring various aspects of electrical stimulation in greater detail.

Three-dimensional reconstruction of the cochlea from two-dimensional images of optical sections

This paper describes a methodology for three-dimensional (3D) computer-aided reconstruction of the guinea pig cochlea using orthogonal-plane fluorescence optical sectioning. Specimens are sectioned optically, allowing them to remain intact during observation. Equations to correct the data for specimen translation and rotation are developed and 3D reconstructions of the scala tympani, round window membrane, and cochlear aqueduct are presented. The error associated with the reconstruction is estimated to be < 19 microns.

Cochlear Electrode Arrays: Past, Present and Future

Cochlear implants are very successful devices: more than 60000 people use them throughout the world. Key to the success of these prostheses is the development of electrode arrays that place contacts close to the target neurons, survive for decades in the tissues of the inner ear, and that provide reliable and repeatable excitation to the cells of the auditory nerve. This article describes the early electrode arrays and their development into the arrays that are used presently in clinical cochlear prostheses. While integrated circuit techniques were proposed and tested in the laboratory two decades ago, the present clinical devices still are hand built and made of wire-based technologies. Current approaches that seek to automate the construction of cochlear electrode arrays are described and discussed.