Scala vestibuli prostheses

Cochlear implants for people with severe hearing deficits belong to the most efficient neuroprostheses, although their artificially generated neural patterns miss many details seen in the natural code. Therefore, improvement of the quality of the generated neural pattern is a permanent challenge. Traditionally, a stimulating electrode array is inserted into the scala tympani, a cochlear cavity, which enables simple access for surgery. However, often deep insertion is blocked, e.g., by ossification, and the auditory nerve fibers which are sensitive to low acoustic frequency cannot be stimulated causing severe restrictions in speech understanding. As an alternative, the electrode array can be inserted into the scala vestibuli, the other large cochlear cavity. The mechanisms of this method are barely investigated. Some of the key mechanisms to generate a spiking pattern in the auditory nerve with electric currents are known from nerve fiber recordings in cats. In contrast to cat, human neurons of the auditory nerve have other electrical stimulation characteristics, mainly because of longer dendrites and non-myelinated cell bodies. As ethical reasons prevent such recordings in human, biophysically based computer simulation studies supported our knowledge about differences between feline and human excitation with cochlear implants. The current project is planned to improve the state of the art of such simulation studies in two ways. (i) The first available three-dimensional pathways of human auditory nerve fibers, reconstructed from micro-CTs, are not as simple in shape as assumed in previous studies. Noteworthy, our first investigations show that the pathway irregularities have a considerable impact on the excitation pattern. (ii) Results from the plenty of scala tympani studies are not directly applicable for other electrode placings. This causes a lack of knowledge as not a single simulation study exists for scala vestibuli implants. The goal of the current study is to analyze which auditory nerve fibers excitation features are common and which are different for scala vestibuli versus scala tympani implants. A first surprising result from a single computer experiment showed an essentially lower threshold for the scala vestibuli, although the electrode had a larger distance from the auditory nerve fibers. Detailed analysis of simulation results with healthy and auditory nerve fibers was possible. Moreover, we want to answer open questions for scala vestibuli implants such as: Where is the optimal electrode position for normal and degenerated auditory nerve fibers?

Cochlear implants for people with severe hearing deficits belong to the most efficient neuroprostheses, although their artificially generated neural patterns miss many details seen in the natural code of the 30.000 fibers of the human auditory nerve. Therefore, improvement of the quality of the generated neural pattern is a permanent challenge. Traditionally, a stimulating electrode array is inserted into the scala tympani, a cochlear cavity, which enables simple access for surgery. However, often deep insertion is blocked, e.g., by ossification, and the auditory nerve fibers which are sensitive to low acoustic frequency cannot be stimulated causing severe restrictions in speech understanding. As an alternative, the electrode array can be inserted into the scala vestibuli, the other large cochlear cavity. The mechanisms of this method are barely investigated. Published reports about speech understanding comparing both methods (scala tympani versus scala vestibuli implantation) are contradictory. This problem is based on differences in electrode to fiber distances between users of the implant, their neural cochlear nerve status and their cognitive skills in decoding the neural signals. For a better understanding of both electrode placing methods concerning the different auditory nerve fiber sensitivity to electrical stimulation we used a computational model based on biophysical principles. In contrast to the real situation the electrodes can be placed at the same distance to the nerve fibers for both methods. Calculation of the thresholds for positive and negative pulses for both placements inform on the sensitivity of spike generation. The goal of the current study was to analyze which auditory nerve fibers excitation features are common and which are different for scala vestibuli versus scala tympani implants. We analyzed both common methods for electrode placing close to the nerve fiber ending, that is at the organ of Corti, and close to the soma of the auditory nerve fiber. It was clear that scala tympani electrodes are in a better condition if the electrode is close to the soma because the somas have a rather large distance to the scala vestibuli. Surprisingly, however, for negative current pulses electrode placement close to the fiber ending has a remarkably lower threshold (122 A) for scala vestibuli versus the scala tympani placement (165 A).