A model for the electrostimulation of the nervus acusticus.

Electrostimulation of the nervus acusticus has been successfully used to achieve speech understanding. A model of nerve excitation due to Fitzhugh [(1961) Biophys. J. 1, 445-446] has been explored to show that the stochastic response of single acoustic mammalian fibers observed by Rose, Brugge, Anderson and Hind [(1967) J. Neurophysiol. 30, 769-793] may be due to the non-linear interaction of sub-threshold electrical nerve activity with the signal. Computations by means of an analogue computer also shows that a phase-locked response with a frequency dependence resembling behaviour observed with electrostimulation is obtained with the model. A dynamic range for single fiber excitation is produced by the non-linear interaction.

New hardware for analog and combined analog and pulsatile sound-encoding strategies.

Development for cochlear implants of primarily analog design focuses in two directions. The first direction is miniaturization. A behind-the-ear (BTE) speech processor has been developed which can replace the body-worn processor for approximately 90% of the users of a MED-EL cochlear implant and works with two 1.4 V hearing aid batteries for between seven and twelve days. Consonant, vowel and sentence testing and patient questioning revealed that the BTE speech processor demonstrates a significant improvement in speech understanding compared to the body-worn processor, and that the patients' device acceptance is superior for the BTE processor. The result of the second direction for our cochlear implant development is the multichannel cochlear implant, CAP, with combined analog and pulsatile stimulation. It aims at complementing the information from the broad-band analog signal by adding spectral information, that is, tonotopic information. This device is capable of simultaneously stimulating one electrode channel with a broad-band analog signal and one of eight electrode channels with a pulsatile signal. The system can also be used for purely analog or for purely pulsatile stimulation. Preliminary results with the first recipient of a CAP cochlear implant system demonstrate that the device works as expected.

Percepts from the Vienna cochlear prosthesis.

This report includes psychophysical data on some of the 13 patients who have been equipped with intracochlear electrodes and the 6 patients who have received an extracochlear electrode. Thresholds and uncomfortable listening levels versus frequency, amplitude difference limens, and gap detection in noise and frequency difference limens have been determined and represent some essential characteristics of prosthetic hearing.

Transcranial Doppler sonography guided by magnetic resonance angiography for improved monitoring of intracranial arteries.

Transcranial Doppler (TCD) sonography combines the advantages of real-time hemodynamic information, cost-effectiveness, and bedside application. However, measurements can be difficult to reproduce because the spatial resolution and the determination of insonation angles are limited. The purpose of this study was to use the high anatomic resolution of three-dimensional (3D) magnetic resonance angiography (MRA) images for the stereotactic guidance of TCD in order to improve the accuracy and reproducibility of TCD examinations. The MRA examinations were performed on a 1.5 T scanner using a 3D flow compensated gradient-echo sequence. A noninvasive stereotactic mask was used for image registration. The MRA data were then transferred to a personal computer. An infrared tracking system registered the position of the head and the ultrasound probe during TCD. This enabled the authors to superimpose a virtual ultrasound beam onto the MRA projections of the intracranial arteries displayed on the monitor of the personal computer. This allows the examiner to easily identify the insonated intracranial artery and displays the insonation angle. In volunteer examinations (n = 10), the accuracy and reproducibility for the localization of specific vessel segments was 2.48 mm for the middle cerebral artery and 2.81 mm for all insonated intracranial arteries (middle cerebral artery, anterior cerebral artery, internal carotid artery, and posterior cerebral artery). Without navigation the reproducibility of vessel segment insonation dropped to 4.7 mm for the middle cerebral artery and to 4.84 mm for all vessels. The authors conclude that 3D MRA, acquired as an initial procedure in patients with intracranial vascular disorders, can be used to provide stereotactic guidance for repeated TCD examinations. This facilitates the reproducible insonation of specific vessel segments.

Geometric approach for coupling enhancement of magnetically coupled coils.

This paper presents a geometric approach for the enhancement of the coupling coefficient between two magnetically coupled coils. It is demonstrated that the coupling coefficient can be considerably enhanced, if the turns of the coils are not concentrated at the circumferences, but distributed across the diameters. For analysis, each of the two coils is assumed to be composed of concentric circular loops. The experimental results are in very good agreement with the theoretical results.

High-efficiency coupling-insensitive transcutaneous power and data transmission via an inductive link.

This paper presents a new approach for transmitting RF power and signal via an inductive link. Such an approach optimizes the power efficiency of the overall transmission scheme comprising the power amplifier plus the inductive link. Power amplification is based on the single ended class E concept. The power amplification stage is self oscillating, the oscillation frequency thus being influenced by the coupling of the coils. The resulting operating frequency offset yields an improved power transmission performance of the circuit since the oscillation frequency tracks the absolute transmission efficiency maximum. A detailed analysis is given. Realization of the described approach requires a minimal number of circuit components. Experimental and theoretical results are in good agreement.

A feedback control system for real-time formant estimation. I--Static and dynamic analysis for sinusoidal input signals.

This paper presents a novel analog scheme suitable for the real-time estimation of formant frequencies. Formant tracking is based on a feedback technique which uses both the amplitude and phase characteristics of two stagger-tuned bandpass filters to give an improved dynamic behavior. The implementation of the system requires a small number of components, and is practical for low-power applications. An analysis of the static and dynamic behavior is given for sinusoidal input signals. The transient response is independent of the amplitude level of the input signal. The system is designed for second formant detection in a cochlear prosthesis system.

A feedback control system for real-time formant estimation. II--Analysis of a hysteresis effect and F2 estimation.

This paper presents the second part of the analysis of a feedback control system for real-time formant estimation. The system behavior is analyzed for an input signal composed of two sinusoids. If the frequency difference between the two input spectral lines is sufficiently great and the amplitude ratio is within certain limits, a hysteresis effect occurs. Then the system shows a tendency to select one of the two input spectral lines. The existence of the second line has only little influence on the accuracy of the detection of the selected line. From the analysis, conclusions of the system behavior regarding formant estimation can be drawn. A design example for second formant detection is simulated and compared with the results obtained by simulation of a zero-crossing system for F2 estimation and LPC analysis.

Transcutaneous transmission of digital data and energy in a cochlear prosthesis system.

This paper describes the inductive power and data link employed in the CAP Cochlear Prosthesis System (CAP stands for Combined Analog and Pulsative Stimulation Strategy). The inductive link consisting of a parallel-tuned receiver resonant circuit weakly coupled to a series-tuned transmitter resonant circuit, is driven by a self-oscillating class-E-tuned power amplifier. The class-E concept allows coupling-insensitive high-efficiency transcutaneous transmission of power. In the CAP implant, variations of the coil distance within a range of 0 to 9 mm result in changes of the implant supply voltage which are lower than 10%. Within this coil distance range, the mean overall efficiency is 49%. In view of the excellent switching properties of the class-E tuned power oscillator, a practical scheme for data transmission is ASK (Amplitude Shift Keying). To ensure constant energy flow and easy synchronization of the bitstream in the implant, a self-clocking bit format is employed.

Investigations towards an artificial cochlea.

Two versions of multichannel cochlear stimulators have been implanted, one allowing stimulation against a common distributed ground within the cochlea and the other allowing bipolar stimulation. The results obtained indicate that with both versions sufficiently discrete stimulation of the auditory nerve is possible in order to be able to make use of tonotopy for the processing of speech. The bipolar stimulation, however, besides of being characterized by higher thresholds than unipolar stimulation, also leads to a very narrow range of stimulus frequency within which perceived pitch is influenced by stimulus frequency. Surprisingly different psychophysical behaviours are exhibited by the two subjects considered, one being a case of prelingual, the other of acquired deafness. This indicates the marked influence which etiology of deafness might have on design considerations for auditory prostheses.

Designing of and experience with multichannel cochlear implants.

The technology of two different multichannel cochlear prosthetic devices is described. The clinical results after electrical stimulation in five implanted patients are discussed.

[CILAB--a PC-based laboratory speech processor for implementation and evaluation of new stimulation strategies for cochlear implants]. / CILAB--ein PC-basierter Laborsprachprozessor zur Implementierung und Evaluierung neuer Stimulationsstrategien für Cochlea-Implantate.

CILab is a computer-based versatile laboratory system for the implementation and evaluation of innovative stimulation strategies for cochlear implants. In contrast to existing laboratory systems the entire signal processing from the input signal to the creation of the data word for the implant is effected with the aid of a personal computer (PC). This permits rapid implementation of new stimulation strategies or psycho-acoustic tests. Real-time audio processing is also possible by using the CILab as a cochlear implant speech processor. The laboratory system has been employed with success for the evaluation of new strategies and numerous psycho-acoustic tests.

[Movement analysis with microgravitation: theory and practical aspects for the calculation of target movements with the MONIMIR helmet lamp]. / Bewegungsanalyse unter Mikrogravitation: Theorie und Praxis zur Berechnung der Zielbewegung mit der MONIMIR-Helmlampe.

A new optoelectronic system for movement analysis is introduced. This equipment was employed with success during an Austrian-Soviet space flight. The method for the determination of position and orientation is described with the aid of an example, namely the determination of the location of the intersection of a beam of light generated by a helmet-mounted lamp with a screen.

[Transcranial Doppler ultrasonography: stereotactically guided examinations using magnetic resonance angiography]. / Transkranielle Dopplersonographie--stereotaktisch geführte Untersuchungen unter Zuhilfenahme von Magnetresonanzangiographiedaten.

The accurate localization of specific intracranial blood vessels is a major difficulty with transcranial Doppler sonography (TCD). It was the purpose of this study to develop a system enabling stereotactic navigation during a TCD examination on the basis of high-resolution three-dimensional magnetic resonance angiographic (MRA) data. During TCD, the examiner is provided--on a computer screen--with a projected view of the respective intracranial vessel anatomy. With the aid of an optoelectronic localization system, the spatial orientation and localization of the US probe is determined in real time, and correlated with the patient's MRA data using a dedicated stereotactic mask. Subsequently, the US beam and the points of insonation are displayed on the screen overlaid on the vessel anatomy. In this way the examiner gains real time control of the localization of the respective intracranial vessel insonated. Points of insonation can be stored and recalled for follow-up examinations. In addition to the successful verification of the system, it was shown that, in comparison with conventional TCD, stereotactic navigation distinctly improves the reproducibility of repeat TCD examinations.

MRI safety of Med-El C40/C40+ cochlear implants.

Cochlear implants are an accepted means for the treatment of profound bilateral deafness. Magnetic resonance imaging (MRI) is gaining more and more importance as a diagnostic tool. It is therefore very likely that cochlear implant wearers will, at some point in their life, become MRI candidates. This makes a discussion of MRI safety of cochlear implants a most timely subject. With the C40/C40+ system as an example, all the major and potentially troublesome interactions are discussed, as well as the conditions for the safe conduct of MR examinations with cochlear implant wearers. For the C40/C40+ systems, safe MR imaging is possible with 0.2 Tesla, 1.0 Tesla and 1.5 Tesla scanners.