Biomedical subjects

C M Zierhofer

Publications and source records attributed to C M Zierhofer.

7 recordsLinked to original sources

PubMed2001

Analysis of a linear model for electrical stimulation of axons--critical remarks on the "activating function concept".

A comprehensive description of a linear model of an axon of infinite length exposed to an external voltage is presented. The steady-state transmembrane potential is derived as a function proportional to the convolution product of the second spatial difference sn of the external potential (the "activating function") and the impulse response psin of a spatial low-pass filter. The impulse response psin represents the influence of the axon and is fully characterized by the axon's length constant lambda. A closed-form solution of the cable equation can be given in the spatial Fourier domain. Due to a "spectral acceleration effect", the overall transmembrane potential approximates the steady-state considerably faster than an exponential with the axon's membrane time constant tau. The effect is increasingly pronounced, the smaller the distance between the electrode and the axon. Regarding myelinated fibers and practically relevant electrode/axon distances and pulse widths, the transmembrane potential at the end of a stimulation pulse can be substantially better approximated by the steady-state condition than by the initial response as claimed by the "activating function concept." Quantitative limits for the range of validity of the activating function concept are derived.

Axons↗

PubMed1999

Influence of automatic gain control parameter settings on speech understanding of cochlear implant users employing the continuous interleaved sampling strategy.

OBJECTIVE: The purpose of the present study was to gather data on the influence of compression ratio and attack and release times of slow-acting front-end automatic gain control (AGC) systems on speech understanding of cochlear implant users in various listening situations. The data should allow evaluation of the usefulness of front-end AGC in body-worn speech processors. DESIGN: Subjects were 12 experienced postlingually deafened adult users of the Med-El Combi-40 multichannel cochlear implant. Six different front-end configurations, including a linear setting, the standard AGC of the Med-El Combi-40 processor, and four slow-acting dual front-end AGCs (use of two instead of one level detector for improved transient handling), were evaluated in two experiments. In experiment 1, tests were performed at 55, 70 and 85 dB SPL, roughly corresponding to soft, medium and loud speech. Experiment 2 was intended to evaluate the quality of transient handling of the six configurations. In this experiment an intense transient ("chink") at 100 dB SPL was spliced onto the beginning of each sentence (presented at 85 dB SPL). RESULTS: At 55 dB SPL subjects performed significantly more poorly with the linear setting than with the AGC settings, but no differences in performance could be found for the AGC settings. At 70 dB SPL subjects showed the poorest performance with the high compression ratio dual front-end AGCs. No differences in performance of the six front-end configurations were found at 85 dB SPL. In the presence of intense chinks, performance of the standard AGC dropped significantly. CONCLUSIONS: The results indicate that slow-acting front-end AGC can be used effectively in speech processors for cochlear implants to expand the range of input levels that are audible for the cochlear implant user, without any need to adjust a processor control, and that incorporation of an additional fast-acting AGC component can improve performance under conditions where intense transients occur.

Adult↗

PubMed1997

The advanced Combi 40+ cochlear implant.

A 12-channel cochlear implant (CI) for high-rate pulsatile stimulation strategies is presented. Symmetric biphasic current pulses can be generated up to a maximum pulse repetition rate of 18.18 kpulses/second. The stimulation pulse amplitude can be selected within 1.5 microA-1.5 mA. Data and power are transcutaneously transferred using a single radiofrequency (RF) channel. A fully digital data transfer format is employed at an overall data rate of 600 kBit/second. The implant contains a single mixed analog/digital CMOS-ASIC (Application Specific Integrated Description) for data synchronization and stimulus generation. Stimulation signals are applied via a monopolar intracochlear multichannel electrode. Output capacitors for each channel are employed for safety reasons. A self-calibrating back telemetry system is included for estimating the channel impedances and field distribution along the electrode array. Dimensions of the ceramic package of the implant are only 33.50 x 23.40 x 3.95 mm3.

Cochlear Implantation↗

PubMed1996

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.

Equipment Design↗

PubMed1993

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.

Cochlear Implants↗

PubMed1993

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.

Computer Simulation↗

PubMed1990

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.

Amplifiers, Electronic↗