Probing the Electrode-Neuron Interface With Focused Cochlear Implant Stimulation

Abstract

Cochlear implants are highly successful neural prostheses for persons with severe or profound hearing loss who gain little benefit from hearing aid amplification. Although implants are capable of providing important spectral and temporal cues for speech perception, performance on speech tests is variable across listeners. Psychophysical measures obtained from individual implant subjects can also be highly variable across implant channels. This review discusses evidence that such variability reflects deviations in the electrode-neuron interface, which refers to an implant channel's ability to effectively stimulate the auditory nerve. It is proposed that focused electrical stimulation is ideally suited to assess channel-to-channel irregularities in the electrode-neuron interface. In implant listeners, it is demonstrated that channels with relatively high thresholds, as measured with the tripolar configuration, exhibit broader psychophysical tuning curves and smaller dynamic ranges than channels with relatively low thresholds. Broader tuning implies that frequency-specific information intended for one population of neurons in the cochlea may activate more distant neurons, and a compressed dynamic range could make it more difficult to resolve intensity-based information, particularly in the presence of competing noise. Degradation of both types of cues would negatively affect speech perception.

Comparisons Between Detection Threshold and Loudness Perception for Individual Cochlear Implant Channels

“…(A direct comparison of measured and simulated loudness growth functions for one of the subjects, S26, appears in Fig. 6 of Bierer, 2010. )…”

“…All channels were stimulated in the partial tripolar (pTP) electrode configuration with various current fractions. As described previously (Jolly et al, 1995; Litvak et al, 2007; Bierer, 2010), the pTP configuration consists of an intracochlear active electrode, with a fraction of the return current divided equally between each of the two nearest flanking electrodes and the remaining current directed to an extracochlear ground electrode. When the current fraction, denoted by σ, is 0, all current is directed to the distant return electrode, which is equivalent to the monopolar (MP) configuration.…”

“…The electrode-neuron interface relates to the effectiveness by which an implant electrode activates the auditory nerve. Several factors can affect the interface, including the viability of the spiral ganglion cell bodies and their processes, the distance between the electrode and the inner wall of the cochlea where the ganglion cells reside, and the degree of bone and tissue growth surrounding the electrode array (see Bierer, 2010 and Pfingst et al, 2011 for reviews). A recent computational model of cochlear electrical stimulation suggests that focused electrode configurations are especially sensitive to factors that degrade the electrode-neuron interface (Goldwyn et al, 2010).…”

Contribution of formant frequency information to vowel perception in steady-state noise by cochlear implant users

“…By optimizing the JND parameters for model fits to each subject's data in the quiet condition, we are controlling for individual factors that affect CI users' discrimination for place of stimulation in the cochlea such as electrode placement, cochlear size, neural survival, etc. (Bierer, 2010;Long et al, 2014). From a modeling perspective it is more parsimonious to minimize the number of degrees of freedom, and assuming an underlying constant JND for place of stimulation allows us to do so.…”

Advances in auditory prostheses

“…Stimulation on a single electrode at a comfortable listening level activates neurons across a cochlear region of several mm, corresponding to ½ to a full octave acoustic equivalent. Two methods for improving the selectivity of neural activation are current field focusing [34–39] and placing the electrodes closer to the nerve. Current focusing uses the overlap of current fields from multiple electrodes to shape the overall current field to make it more localized at the point of stimulation than a single electrode’s field.…”