Constructing a three-dimensional electrical model of a living cochlear implant user's cochlea.

BACKGROUND: Hearing performance varies greatly among users of cochlear implants. Current three-dimensional cochlear models that predict the electrical fields inside a stimulated cochlea and their effect on neural excitation are generally based on a generic human or guinea pig cochlear shape that does not take inter-user morphological variations into account. This precludes prediction of user-specific performance. AIMS: The aim of this study is to develop a model of the implanted cochlea of a specific living human individual and to assess if the inclusion of morphological variations in cochlear models affects predicted outcomes significantly. METHODS: Five three-dimensional electric volume conduction models of the implanted cochleae of individual living users were constructed from standard CT scan data. These models were embedded in head models that include monopolar return electrodes in accurate anatomic positions. Potential distributions and neural excitation patterns were predicted for each of the models. RESULTS: Modeled potential distributions and neural excitation profiles (threshold amplitudes, center frequencies, and bandwidths) are affected by user-specific cochlear morphology and electrode placement within the cochlea. CONCLUSIONS: This work suggests that the use of user-specific models is indicated when more detailed analysis is required than what is available from generic models. Copyright © 2015 John Wiley & Sons, Ltd.

The effect of the resistive properties of bone on neural excitation and electric fields in cochlear implant models.

The resistivity of bone is the most variable of all the tissues in the human body, ranging from 312 Ω cm to 84,745 Ω cm. Volume conduction models of cochlear implants have generally used a resistivity value of 641 Ω cm for the bone surrounding the cochlea. This study investigated the effect that bone resistivity has on modelled neural thresholds and intracochlear potentials using user-specific volume conduction models of implanted cochleae applying monopolar stimulation. The complexity of the description of the head volume enveloping the cochlea was varied between a simple infinite bone volume and a detailed skull containing a brain volume, scalp and accurate return electrode position. It was found that, depending on the structure of the head model and implementation of the return electrode, different bone resistivity values are necessary to match model predictions to data from literature. Modelled forward-masked spatial tuning curve (fmSTC) widths and slopes and intracochlear electric field profile length constants were obtained for a range of bone resistivity values for the various head models. The predictions were compared to measurements found in literature. It was concluded that, depending on the head model, a bone resistivity value between 3500 Ω cm and 10,500 Ω cm allows prediction of neural and electrical responses that match measured data. A general recommendation is made to use a resistivity value of approximately 10,000 Ω cm for bone volumes in conduction models of the implanted cochlea when neural excitation is predicted and a value of approximately 6500 Ω cm when predicting electric fields inside the cochlear duct.

Free field frequency discrimination abilities of cochlear implant users.

Poor music perception abilities of cochlear implant users may be attributed to limited pitch resolution afforded by the implant system. We investigated (i) what the typical frequency discrimination thresholds of cochlear implant users would be in free field listening conditions and (ii) whether frequency discrimination behaviour would be influenced by the position of the reference frequency relative to the frequency response of filters selected from the user's map. Frequency discrimination thresholds were determined according to an adaptive two-alternative forced choice (2AFC) method, using pure tones delivered in free field conditions. Results showed that finer frequency resolution than previously thought could be available to cochlear implant users. Results are interpreted in terms of intermediate pitch percepts possibly created by near-simultaneous activation of adjacent electrodes, resulting in overlapping neural populations to be stimulated. The findings may contribute to strategies aiming to improve music perception abilities of cochlear implant users.

Initial results from a model of ephaptic excitation in the electrically excited peripheral auditory nervous system.

This article reports on a study performed to investigate the occurrence and effect of ephaptic excitation in electrical stimulation of the auditory system. The objective of the study was to quantify the influence of ephaptic excitation on nerve stimulation and determine whether it is a necessary factor in neuromodeling. It was shown with a simple model that ephaptic excitation could be important at stimulus intensities close to threshold. The results show that the contribution of ephaptic excitation is significant up to at least 6-7dB above threshold. Cochlear implant patients normally have a small dynamic range (average of 7dB), indicating that the ephaptic effect might be important in models of the implanted cochlea.

A model of frequency discrimination with optimal processing of auditory nerve spike intervals.

This paper investigates phase-lock coding of frequency in the auditory system. One objective with the current model was to construct an optimal central estimation mechanism able to extract frequency directly from spike trains. The model bases estimates of the stimulus frequency on inter-spike intervals of spike trains phase-locked to a pure tone stimulus. Phase-locking is the tendency of spikes to cluster around multiples of the stimulus period. It is assumed that these clusters have Gaussian distributions with variance that depends on the amount of phase-locking. Inter-spike intervals are then noisy measurements of the actual period of the stimulus waveform. The problem of estimating frequency from inter-spike intervals can be solved optimally with a Kalman filter. It is shown that the number of inter-spike intervals observed in the stimulus interval determines frequency discrimination at low frequencies, while the variance of spike clusters dominates at higher frequencies. Timing information in spike intervals is sufficient to account for human frequency discrimination performance up to 5000 Hz. When spikes are available on each stimulus cycle, the model can accurately predict frequency discrimination thresholds as a function of frequency, intensity and duration.

[The value of basic research as applied to cochlear implants]. / Die bydrae van basiese navorsing in kliniese toepassings met verwysing na kogleêre inplantings.

This article discusses the value of basic research as applied to cochlear implants. The article is aimed at clinicians and audiologists who are working in the field of cochlear implants or who are interested in this field. The article also gives a more general introduction to modelling for researchers in the clinical environment. It provides an entry point to cochlear implant research and reviews the application of basic research to new developments in cochlear implants. It is shown what has been achieved so far and which problems still exist. The role of multidisciplinary research teams to solve these problems is discussed. Experimental research and modelling co-operate to solve problems and make new discoveries. The importance of modelling as a tool for basic research is emphasized.

What do cochlear implants teach us about the encoding of frequency in the auditory system?

This article explores the coding of frequency information in the auditory system from the viewpoint of what has been learnt from cochlear implants. Cochlear implants may provide a window on central auditory nervous system function by creating the possibility to separate place and temporal information. An existing model of frequency discrimination in the acoustically stimulated auditory system is extended to include electrical stimulation. To be able to predict frequency difference limens for acoustic stimulation, an important assumption is that one spike per stimulus cycle is available, which may be provided by the existence of a volley principle. It is shown that to predict frequency difference limens for electrical stimulation of the auditory system, it must also be assumed that electrical stimulation causes desynchronization at a central auditory nervous system integration centre. With these assumptions, the model predicts the degradation in frequency discrimination that occurs for electrical stimulation. Finally, it is shown that cochlear implants have not yet proven conclusively that either rate-place coding or temporal coding is predominant in the auditory system.

A model of frequency coding in the central auditory nervous system.

A phenomenological model for neural coding in the central auditory system is presented. This model is based on average rate-place codes and the hypothesis is that the rate-place code present in the population of low spontaneous rate nerve fibres is adequate to account for frequency discrimination thresholds across the entire audible frequency range. The activity of a population of nerve fibres in response to an input pure tone is calculated and a neural spike train pattern is generated. An optimal central observer estimates the input frequency from the spike train pattern. The model output is the frequency difference limen at the specific input frequency, determined from the estimated input frequency. It is shown that a rate-place code can account for psychoacoustically observed frequency difference limens. The model also supports the hypothesis that a human listener does not make full use of all the information relevant to frequency that is available in auditory nerve spike trains.

Gap detection as a measure of electrode interaction in cochlear implants.

Gap detection thresholds were measured as an indication of the amount of interaction between electrodes in a cochlear implant. The hypothesis in this study was as follows: when the two stimuli that bound the gap stimulate the same electrode, and thus the same neural population, the gap detection threshold will be short. As two stimuli are presented to two electrodes that are more widely separated, the amount of neural overlap of the two stimuli decreases, the stimuli sound more dissimilar, and the gap thresholds increase. Gap detection thresholds can thus be used to infer the amount of overlap in neural populations stimulated by two electrodes. Three users of the Nucleus cochlear implant participated in this study. Gap detection thresholds were measured as a function of the distance between the two electrode pairs and as a function of the spacing between the two electrodes of a bipolar pair (i.e., using different modes of stimulation). The results indicate that measuring gap detection thresholds may provide an estimate of the amount of electrode interaction. Gap detection thresholds were a function of the physical separation of the electrode pairs used for the two stimuli that bound the gap. Lower gap thresholds were observed when the two electrode pairs were closely spaced, and gap thresholds increased as the separation increased, resulting in a "psychophysical tuning curve" as a function of electrode separation. The sharpness of tuning varied across subjects, and for the three subjects in this study, the tuning was generally sharper for the subjects with better speech recognition. The data also indicate that increasing the separation between active and reference electrodes has limited effect on spatial selectivity (or tuning) as measured perceptually.

Place pitch discrimination and speech recognition in cochlear implant users.

The considerable variability in speech perception performance among cochlear implant patients makes it difficult to compare the effectiveness of different speech processing strategies. One result is that optimal individualized processor parameter setting is not always achieved. This paper investigates the relationship between place pitch discrimination ability and speech perception to establish whether pitch ranking could be used as an aid in better patient-specific fitting of processors. Three subjects participated in this study. Place pitch discrimination ability was measured and this information was used to design new channel to electrode allocations for each subject. Several allocations were evaluated with speech tests with consonant, vowel and sentence material. It is shown that there is correlation between the perceptual pitch distance between electrodes and speech perception performance. The results indicate that pitch ranking ability might be used both as an indicator of the speech perception potential of an implant user and in the choice of better electrode configurations.

The masking property of the auditory system: the masking of speech signals.

The masking property of the auditory system is well known in the context of two-tone masking. For complex (speech) signals, the effects of masking are less well known. This paper explores the masking of speech signals, by calculating which parts of the speech signal is inaudible because of masking. The theory for the masking of one tone by another is expanded, to establish an equation for the masking threshold. This masking threshold takes into account the masking of each frequency component on all other frequency components. Speech is then synthesized in which the supposedly inaudible parts of the speech signal are discarded, and the effects are evaluated in a very simple psychoacoustic experiment. It is shown that the information below the masking threshold is indeed redundant.