Search

Abstract: Cochlear implants work on the principle of direct electrical stimulation of the auditory nerve. It has advantages to replace their traditional wired electrodes by a high-density thin-film multielectrode stimulation array which is relatively small in dimensions, stable, resistant to electrolysis, and offers batch fabrication capabilities with good electrical and mechanical properties. In this paper a development review of cochlear implants (CI''s) along with the fabrication of stiff silicon probes using silicon micromachining is described. The initial design for a new microelectrode array is described with microfabrication steps for stiff silicon probes which is a step towards a future flexible CI-electrodes. These stiff probes were fabricated in order to study the problems involved in fabrication and the behaviour of the probes when inserted perpendicular to the auditory nerve. The probes were fabricated with lengths ranging from 3.5mm to 10.5mm with thickness between 60 to 70 microns. [Copyright &y& Elsevier]

A recent study showed that 25 patients with the CII cochlear implant system (HiFocus I electrode) with partially inserted positioner obtained a significantly better speech perception than a demographically identical group (20 patients) with the electrode alone. CT scans in the positioner group showed basally a perimodiolar, and apically a lateral electrode position. A computational model of the human cochlea also predicted that a perimodiolar position of the electrode array is best for basal contacts, while a lateral position is preferable for more apical ones. This study reports the concept and initial testing of a new, one-piece, electrode design, intended to yield a similar position of the contacts and suitable for cochleae of all sizes with minimal insertion trauma. Prototype electrodes were inserted in fresh human temporal bones and the position of the contacts was verified with multislice and high-resolution CT scans prior to a careful dissection, documenting the insertion trauma. This showed that the new electrode is able to attain the desired position with minimal damage to the intracochlear structures. It is concluded that the new electrode meets its design criteria, and is worth a clinical evaluation as it promises good speech perception results without the negative effects reported for the positioner. [Copyright &y& Elsevier]

Electrically evoked compound action potentials (eCAPs) are measurements of the auditory nerve's response to electrical stimulation. ECAP amplitudes during pulse trains can exhibit temporal alternations. The magnitude of this alternation tends to diminish over time during the stimulus. How this pattern relates to the temporal behavior of nerve fibers is not known. We hypothesized that the stochasticity, refractoriness, adaptation of the threshold and spike-times influence pulse-train eCAP responses. Thirty thousand auditory nerve fibers were modeled in a three-dimensional cochlear model incorporating pulse-shape effects, pulse-history effects, and stochasticity in the individual neural responses. ECAPs in response to pulse trains of different rates and amplitudes were modeled for fibers with different stochastic properties (by variation of the relative spread) and different temporal properties (by variation of the refractory periods, adaptation and latency). The model predicts alternation of peak amplitudes similar to available human data. In addition, the peak alternation was affected by changing the refractoriness, adaptation, and relative spread of auditory nerve fibers. As these parameters are related to factors such as the duration of deafness and neural survival, this study suggests that the eCAP pattern in response to pulse trains could be used to assess the underlying temporal and stochastic behavior of the auditory nerve. As these properties affect the nerve's response to pulse trains, they are of uttermost importance to sound perception with cochlear implants. • Responses of thirty thousand auditory nerve fibers were modeled in a three-dimensional cochlear model. • The cochlear and auditory nerve model incorporates pulse-shape, temporal, and stochastic effects. • Electrically evoked compound action potentials (eCAPs) responses to pulse trains were predicted. • The typical temporal alternation pattern is related to stochastic, refractory and adaptive behavior. • The eCAP pattern in response to pulse trains can be used to assess the state of the nerve. [ABSTRACT FROM AUTHOR]

Introduction. The objective was to identify the influence of the cochlear anatomy on the electrode-to-neural interface in cochlear implants. Methods. A three-dimensional spiralling computer model of the implanted inner ear of man and guinea pig,1 including a new clinical perimodular electrode array (Clarion Hi-Focus with Positioner) was used to study the spatial selectivity and dynamic range of the device. At higher stimulus levels it was expected that excitation of nerve fibres from more apical turns would interfere with the concept of making use of the tonotopic organization of the cochlea. Results. In humans (in contrast with the guinea pig), a well-designed perimodular electrode yields lower current thresholds and a higher spatial selectivity than conventional electrodes, while preserving a good dynamic range. The apical turn of the human cochlea, however, is largely similar in this respect to the guinea pig cochlea, in which cross-turn stimulation reduces the dynamic range substantially. Conclusion. The clinical success of cochlear implantation in man and the favourable results with perimodular devices depend upon the anatomy of the human cochlea. [ABSTRACT FROM AUTHOR]

Cochlear implants encode speech information by stimulating the auditory nerve with amplitude-modulated pulse trains. A computer model of the auditory nerve's response to electrical stimulation can be used to evaluate different approaches to improving CI patients' perception. In this paper a computationally efficient stochastic and adaptive auditory nerve model was used to investigate full nerve responses to amplitude-modulated electrical pulse trains. The model was validated for nerve responses to AM pulse trains via comparison with animal data. The influence of different parameters, such as adaptation and stochasticity, on long-term adaptation and modulation-following behavior was investigated. Responses to pulse trains with different pulse amplitudes, amplitude modulation frequencies, and modulation depths were modeled. Rate responses as well as period histograms, Vector Strength and the fundamental frequency were characterized in different time bins. The response alterations, including frequency following behavior, observed over the stimulus duration were similar to those seen in animal experiments. The tested model can be used to predict complete nerve responses to arbitrary input, and thus to different sound coding strategies. [ABSTRACT FROM AUTHOR]

Cochlear implants (CIs) rehabilitate hearing impairment through direct electrical stimulation of the auditory nerve. New stimulation strategies can be evaluated using computational models. In this study, a computationally efficient model that accurately predicts auditory nerve responses to CI pulse train input was developed. A three-dimensional volume conduction and active nerve model developed at Leiden University Medical Center was extended with stochasticity, adaptation, and accommodation. This complete model includes spatial and temporal characteristics of both the cochlea and the auditory nerve. The model was validated by comparison with experimentally measured single fiber action potential responses to pulse trains published in the literature. The effects of pulse rate and pulse amplitude on spiking patterns were investigated. The modeled neural responses to CI stimulation were very similar to the single fiber action potential measurements in animal experiments. The model's responses to pulse train stimulation with respect to spatial location were also investigated. Adaptation was stronger at the borders of the stimulated area than in the center. By combining spatial details with long-term temporal components and a broad implementation of stochasticity a comprehensive model was developed that was validated for long duration electric stimulation of a wide range of pulse rates and amplitudes. The model can be used to evaluate auditory nerve responses to cochlear implant sound coding strategies. [ABSTRACT FROM AUTHOR]

Abstract: The common assumption that the electrically evoked compound action potential (eCAP) has a linear relationship with the number of excited nerve fibres is derived from the acoustical unitary response concept. This study tests the validity of this hypothesis for electrical stimulation. Five guinea pigs were implanted with the tip of a human HiFocus electrode. eCAPs were measured with the forward masking paradigm, using anodic- and cathodic-leading biphasic current pulses and the inter-pulse interval was varied. Masker and probe amplitudes were varied either individually or simultaneously. Surprisingly, at high levels decreasing eCAP amplitudes were measured with increasing stimulus current. In search for an explanation, the experimental conditions were implemented in our 3D computational model of the implanted guinea pig cochlea to perform a functional comparison. In the final experiment, with fixed inter-pulse interval (IPI) and anodic-leading pulses, increasing stimulus currents showed growing numbers of excited nerve fibres and decreasing eCAP amplitudes at high levels, again. While simulating the relative contribution of single fibres to the overall eCAP, an explanation for this could be found in a waveform change in the modelled single fibre action potentials at high levels. We conclude that highly stimulated nerve fibres have another contribution to the eCAP response than lower stimulated fibres, which leads to a reduction of the eCAP amplitude at high levels. [Copyright &y& Elsevier]

• Power law and exponential adaptation comparably predict responses to short duration. • Power law adaptation better predicts responses to long duration stimulation. • Long duration responses can be approximated with a sum of exponents. • Power law adaptation can optimize signal coding in cochlear implants. Despite the introduction of many new sound-coding strategies speech perception outcomes in cochlear implant listeners have leveled off. Computer models may help speed up the evaluation of new sound-coding strategies, but most existing models of auditory nerve responses to electrical stimulation include limited temporal detail, as the effects of longer stimulation, such as adaptation, are not well-studied. Measured neural responses to stimulation with both short (400 ms) and long (10 min) duration high-rate (5kpps) pulse trains were compared in terms of spike rate and vector strength (VS) with model outcomes obtained with different forms of adaptation. A previously published model combining biophysical and phenomenological approaches was adjusted with adaptation modeled as a single decaying exponent, multiple exponents and a power law. For long duration data, power law adaptation by far outperforms the single exponent model, especially when it is optimized per fiber. For short duration data, all tested models performed comparably well, with slightly better performance of the single exponent model for VS and of the power law model for the spike rates. The power law parameter sets obtained when fitted to the long duration data also yielded adequate predictions for short duration stimulation, and vice versa. The power law function can be approximated with multiple exponents, which is physiologically more viable. The number of required exponents depends on the duration of simulation; the 400 ms data was well-replicated by two exponents (23 and 212 ms), whereas the 10-minute data required at least seven exponents (ranging from 4 ms to 600 s). Adaptation of the auditory nerve to high-rate electrical stimulation can best be described by a power-law or a sum of exponents. This gives an adequate fit for both short and long duration stimuli, such as CI speech segments. [ABSTRACT FROM AUTHOR]