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Biomedical subjects

J H Frijns

Publications and source records attributed to J H Frijns.

17 recordsLinked to original sources
PubMed2002

[Information provision for the biomedical sciences in the Netherlands faces an uncertain future].

The availability of biomedical information in the Netherlands is largely guaranteed by the libraries of the eight medical faculties and related university hospitals together with the Netherlands Institute for Scientific Information Services (Dutch acronym: NIWI). The acquisition of biomedical journals has always been coordinated at a national level. Up until now the NIWI has played a central role in filling in the gaps left by the other collections. It also functions as a national archive and a main document delivery centre. The NIWI receives financial support from the Dutch government to perform these tasks. Recently, NIWI announced that it intends to terminate its acquisition and document delivery service for the biomedical sciences. The remaining eight libraries foresee a number of negative effects if no alternatives to the present NIWI service are developed. We are of the opinion that this situation necessitates a national, biomedical information strategy.

Computers↗
PubMed2001

The importance of human cochlear anatomy for the results of modiolus-hugging multichannel cochlear implants.

HYPOTHESIS: The fact that the anatomy of the basal turn of the human cochlea, especially, is essentially different from that of other species is likely to influence the outcome of cochlear implantation. BACKGROUND: Multichannel cochlear implants give better speech understanding than single-channel devices. They are intended to make use of the tonotopic organization of the cochlea by selectively stimulating subpopulations of the auditory nerve. At higher stimulus levels and with monopolar stimulation, excitation of nerve fibers from other turns may interfere with this concept, especially with modiolus-hugging electrodes. METHODS: A three-dimensional spiraling computer model of the human cochlea, based on histologic data, was used to test the spatial selectivity and the dynamic range before cross-turn stimulation takes place for the Clarion HiFocus implant with and without a positioner. The results were compared with a similar model of the guinea pig cochlea. RESULTS: In humans (in contrast to the guinea pig), a well-designed modiolus-hugging electrode yielded reduced current thresholds and high spatial selectivity without reduction of the useful dynamic range. The apical turn of the human cochlea, however, is largely comparable in this respect with the guinea pig cochlea, where cross-turn stimulation reduces the dynamic range substantially. CONCLUSION: The clinical success of cochlear implantation in humans and the favorable results with modiolus-hugging devices depend on the anatomy of the human cochlea.

Acoustic Stimulation↗
PubMed2000

Field patterns in a 3D tapered spiral model of the electrically stimulated cochlea.

Despite the fact that cochlear implants are widely and successfully used in clinical practice, relatively little is known to date about the electric field patterns they set up in the cochlea. Based upon the available measurements and modelling results, the scala tympani is usually considered to be a preferential current pathway that acts like a leaky transmission line. Therefore, most authors assume the current thresholds to decay exponentially along the length of the scala tympani. Here we present potential distributions calculated with a fully three-dimensional, spiralling volume conduction model of the guinea pig cochlea, and try to identify its preferential current pathways. The relatively well conducting scala tympani turns out to be the main one indeed, but the exponential decay (J approximately e(-z)) of current is only a good description of the far-field behaviour. In the vicinity of the electrodes, i.e. near the fibres that are most easily excited, higher current densities are found, that are best described by a spherical spread of the current (J approximately 1/R(2)). The results are compared with those obtained with a variant of our previous, rotationally symmetric, model and with measurements in the literature. The implications of the findings are discussed in the light of simulated neural responses.

Cochlea↗
PubMed2000

Improving the accuracy of the boundary element method by the use of second-order interpolation functions.

The boundary element method (BEM) is a widely used method to solve biomedical electromagnetic volume conduction problems. The commonly used formulation of this method uses constant interpolation functions for the potential and flat triangular surface elements. Linear interpolation for the potential on a flat triangular mesh turned out to yield a better accuracy. In this paper, we introduce quadratic interpolation functions for the potential and quadratically curved surface elements, resulting from second-order spatial interpolation. Theoretically, this results in an accuracy that is inversely proportional to the third power of element size. The method is tested on a four concentric sphere geometry, representative for electroencephalogram modeling, and compared to previous solutions of this problem in literature. In addition, a cylindrical test configuration is used. We conclude that the use of quadratic interpolation functions for the potential and of quadratically curved surface elements in BEM results in a significant increase in accuracy and in some cases even a reduction of the computation time with the same number of nodes involved in the calculations.

Algorithms↗
PubMed2000

Perceptual characteristics of adductor spasmodic dysphonia.

The evaluation of perceptual symptoms is one of the mainstays in the diagnostic workup of adductor spasmodic dysphonia (ADSD). The most widely used perceptual rating system is the GRBAS scale. However, with this system, not all audible features of ADSD can be described. Therefore, we extended the GRBAS system by 6 parameters. The aim of this study was assessment of the perceptual characteristics of ADSD with the "extended" GRBAS system and evaluation of its reproducibility. Seventy-seven patients were scored independently by 3 experienced observers quantifying the voice sound profile. Cluster analysis distinguished 4 different voice types. Intraobserver and interobserver agreement among 8 less-experienced observers appeared fair-to-good. This study demonstrates that the "extended" GRBAS system is suitable for perceptual characterization of ADSD. On the grounds of the fair-to-good interobserver variability, it is appropriate for clinical use. Moreover, 4 different voice clusters of ADSD were identified, with significant differences in voice and demographic parameters.

Adult↗
PubMed1998

[The cochlea: modern physiologic insights into sensorineural hearing loss].

The cochlea is organized tonotopically: each frequency between 20 Hz and 20 kHz has its own location on the basilar membrane. In addition the cochlea has a large dynamic range (sound intensity). Active movements of the outer hair cells of the sensory organ of Corti play a crucial part in determining hearing acuity and frequency selectivity. Long-known properties such as combination tones and more recent observations like spontaneous and evoked otoacoustic emissions (the cochlea as sound source) fit in the same concept of a cochlea acting as a non-linear fine-tuned amplifier. Most sensorineural losses in people with diminished hearing are caused by injury to the hair cells, e.g. by medicines. Damage of outer hair cells impairs the cochlear amplifier, leading to loss of sensitivity to soft sounds and loss of frequency selectivity. Damage to the inner hair cells has much the same effect as conductive hearing loss. Tinnitus is not caused by otoacoustic emissions, but probably by denervation hypersensitivity following loss of cochlear input.

Cochlea↗
PubMed1997

Transmitter release in inner hair cell synapses: a model analysis of spontaneous and driven rate properties of cochlear nerve fibres.

The inner hair cell (IHC) synapse is one of the stages of cochlear processing that determine the relation between sound pressure level and spike rate in auditory nerve fibres. Transmitter released in the non-stimulated condition is held responsible for the wide range of spontaneous spike rates (SR) observed in these fibres. Properties of stimulated spike activity in auditory nerve fibres, including rate threshold and operating range of a fibre, are known to systematically vary with SR. This paper presents a model analysis of the relation between IHC transmembrane potential and transmitter release rate as becoming manifest in these spontaneous and driven rate properties. A previously developed computational model is used to identify those transfer properties of its synapse section which lead to reproduction of the variation of rate thresholds, shapes of rate-intensity functions and maximal driven rate with SR known from the literature. First a simple additive release model, in which driven transmitter release depends linearly on IHC potential, is elaborated. Its results lead to the hypothesis that the true release function is non-linear and variable across synapses generating different SR. An exponential release function is then introduced, with parameters varying across SR in a physiologically dictated way. This approach leads to adequate reproduction of the variation in rate thresholds and rate-intensity functions with SR. Finally, the model is applied in an inverse way to directly estimate the release function from given rate-intensity functions. The conclusion of both forward and inverse model analyses is that transmitter release is a non-linear function of IHC potential which, by the systematic variation of its parameters across SR, effectively leads to the physiological variation in dynamic range across fibres of different SR. Possible relations of these results with ultrastructural morphology and basic physiology of IHC synapses are discussed.

Acoustic Stimulation↗
PubMed1996

Spatial selectivity in a rotationally symmetric model of the electrically stimulated cochlea.

A rotationally symmetric model of electrical stimulation of the guinea pig cochlea with active neural elements is used to study the influence of temporal stimulus parameters and electrode configurations on the spatial selectivity of electrical stimulation by cochlear implants. The width of the excitation patterns is determined with respect to the position of the stimulating electrode pairs in the cochlea. Computed O10 AB values are compared against single fibre data from the cat cochlear nerve as measured by Van den Honert and Stypulkowsky (1987). It turns out that the use of charge-balanced asymmetric rather than symmetric biphasic pulses approximately doubles the number of independent channels that can be applied in a cochlear implant with longitudinal bipolar electrodes, like a configuration with radial electrode pairs using symmetric biphasic pulse stimulation will also do. Finally, the influence on Selectivity of the physiological variation in diameter of the cochlear nerve fibres and of a possible loss of their peripheral processes is studied.

Animals↗
PubMed1995

Potential distributions and neural excitation patterns in a rotationally symmetric model of the electrically stimulated cochlea.

In spite of many satisfactory results, the clinical outcome of cochlear implantation is poorly predictable and further insight into the fundamentals of electrical nerve stimulation in this complex geometry is necessary. For this purpose we developed a rotationally symmetric volume conductor model of the implanted cochlea, using the Boundary Element Method (BEM). This configuration mimics the cochlear anatomy more closely than previous, unrolled models. The calculated potential distribution in the cochlea due to stimulating electrodes is combined with a multiple non-linear node model of auditory nerve fibres, which we recently developed. The combined model is used to compute excitation profiles of the auditory nerve for a variety of stimulus levels and electrode positions. The model predicts that the excitation threshold, the spatial selectivity and the dynamic range depend on the exact position of the electrode in the scala tympani. These results are in good agreement with recently published electrical ABR data. It is shown that the use of actively modelled nerve fibres is essential to obtain correct predictions for the biphasic stimuli typically used in cochlear implants and that unrolling the cochlear duct as done in previous models leads to erroneous predictions regarding modiolar stimulation.

Animals↗
PubMed1994

A model of myelinated nerve fibres for electrical prosthesis design.

Starting with the spatially extended non-linear node model (Reilly et al., 1985), which incorporates Frankenhaeuser-Huxley non-linearities at each of several nodes in a row, a model is developed to describe many aspects of the behaviour of mammalian nerve fibres in a quantitative way. By taking into account the effects of temperature and by introducing a realistic nerve morphology, a good fit is obtained between the shape, duration and conduction velocity of simulated and in vivo action potentials in mammals. The resulting model correctly predicts the influence of physiological variations of body temperature on various aspects of nerve behaviour. It is shown that the absolute refractory period predicted by the model is within physiological ranges. Both in vivo and in the model, the spike amplitude and the spike conduction velocity are reduced in the relative refractory period. It is concluded that single-node models (although widely used) cannot replace this multiple nonlinear node model, as the stimulus repetition rates that can be followed by the simulated nerve fibre are limited by impulse conduction properties, rather than by the frequency following behaviour of a single node.

Action Potentials↗
PubMed1994

Ultrasound assessment of cervical dynamics during the first stage of labor.

OBJECTIVE: Assessment of cervical dynamics during the first stage of labor by a combination of ultrasound cervimetry and intrauterine tocography. METHODS: Sixty-two parturients were divided into four groups: nulliparous women in spontaneous (n = 9) or oxytocin-induced labor (n = 26), parous women in spontaneous (n = 11) or oxytocin-induced labor (n = 16). Intrauterine pressure and cervical dilatation were continuously recorded and assessed by off-line computer analysis. RESULTS: All women with spontaneous labor showed cervical responses to uterine contractions at the beginning of the recording. The first cervical response to a uterine contraction occurred at a significantly smaller dilatation in parous than in nulliparous women with induced labor (2.9 and 3.6 cm, respectively). Also, acceleration of cervical dilatation occurred at less dilatation in parous than in nulliparous women (3.4 cm and 4.8 cm, respectively), and myometrial work per cm of cervical dilatation was less in parous than in nulliparous parturients. CONCLUSIONS: The results indicate significant differences between cervical dilatation patterns in nulliparous and parous women, which may be due to structural cervical changes caused by labor and parturition. The labor patterns found were different from those originally described by Friedman (Friedman EA. Graphic analysis of labor. Am J Obstet Gynecol 1954; 68: 1568-1575), as no deceleration phases were detected. Ultrasound cervimetry is a valuable technique for the study of cervical dynamics during labor.

Adolescent↗
PubMed1994

A quantitative approach to modeling mammalian myelinated nerve fibers for electrical prosthesis design.

This paper presents an upgraded cable model of mammalian myelinated nerve fibers in an extracellularly applied field. The kinetics of the nodes is based upon voltage clamp data in rat motor fibers at 37 degrees C, while the resting membrane potential is computed with the Goldman equation. The resulting spike shape, conduction velocity, strength/duration behavior, and absolute and relative refractory period are in good quantitative agreement with published experimental data in mammals at normal body temperature and at 20 degrees C. Results at intermediate temperatures however, suggest that the widely used concept of a constant Q10 for the rate constants is invalid. In addition, the model generates realistic abortive spikes towards the end of the absolute refractory period and it can describe the consequences of repetitive firing. The results stress the advantages of a multiple nonlinear node model even if only time aspects of nerve behavior are under study. It turned out, that the model presented here describes in vivo neural properties relevant for electrical prosthesis design better than previous models in literature.

Animals↗
PubMed1992

Does intervention improve the natural course of glomus tumors? A series of 108 patients seen in a 32-year period.

To acquire more insight into the results of treatment versus the "natural" course of glomus tumors, we studied the clinical data of 108 patients, in 58 of whom the disease was hereditary. During a period of 32 years (1956 to 1988), 175 tumors were diagnosed: 52 glomus jugulotympanic tumors, 32 vagal body tumors, and 91 carotid body tumors. The results of radical surgical treatment were disappointing for tumors located at the skull base, ie, nonradical in 59% (n = 23) of the cases, but very good for the carotid body tumors, for which 96% (n = 68) radical excision was achieved. Moreover, surgery at the level of the skull base dramatically increased morbidity, since it frequently induced cranial nerve palsy. During the follow-up period (maximal observation time 32 years, mean 13.5 years) none of the patients died of residual or recurrent tumor or developed distant metastases, irrespective of the mode and outcome of treatment. When these results are combined with the results of pedigree analysis, a realistic approximation of the "natural" course of the disease for both hereditary and nonfamilial tumors can be made. The results raise the question of whether this natural behavior is really improved by intervention. We conclude that removal of carotid body tumors and solitary vagal body tumors should be considered in order to prevent future morbidity. However, for skull base and bilateral glomus tumors a more conservative monitored "wait and see" policy can be sensible and should be considered in any proposal for treatment of head and neck paragangliomas.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1991

Assessment of cervical dilatation during labor: a review.

Various techniques for assessment of cervical dilatation during labor or 'cervimetry' have been described, including digital, (electro)mechanical, electromagnetic and ultrasound cervimetry. In this paper the validity and usefulness of instrumental cervimetry is assessed on the basis of the available literature. Cervimetry using ultrasound transducers allows continuous and reliable recording of cervical dilatation during labor with little discomfort to the patient. In comparison with other instrumental techniques for measurement of cervical dilation ultrasound cervimetry seems to be the most promising method available.

Cervix Uteri↗
PubMed1986

A central spectrum theory of binaural processing. The binaural edge pitch revisited.

Recently, Klein and Hartmann [J. Acoust. Soc. Am. 70, 51-61 (1981)] investigated a new dichotic pitch, called the binaural edge pitch (BEP). They used computer-generated periodic noise signals to generate BEP. In the study presented here, the BEP is investigated in order to evaluate the predictions of the central spectrum theory with regard to this stimulus. Pitch-matching experiments using a nonperiodic BEP stimulus, produced by means of a modulation technique, led to the conclusion that the strongest pitch sensation in the BEP has the character of a weak fluctuating pure tones in noise, which corresponds to a frequency, equal or almost equal to the frequency of the phase transition. This result fits in with the predictions of the central spectrum theory, which, for instance, does not need the assumption of central lateral inhibition for explaining this pitch. Furthermore, it is shown that this theory can also predict the results obtained in lateralization measurements and BMLD measurements using BEP stimuli as well as related stimuli. The results are compared with the data obtained by Klein and Hartmann.

Auditory Threshold↗
PubMedDate not supplied

A multi-channel simultaneous data acquisition and waveform generator system designed for medical applications.

To meet the needs of our research programmes on auditory prostheses for the totally deaf, a 15-channel data acquisition and waveform generator system with flexible triggering, pacing and linking was developed. It allows both synchronous and asynchronous operation at high speed. Each channel of the system, which is controlled by a simple personal computer, has an on-board microcontroller, a 512 kWord signal memory, a voltage input and both voltage and current outputs. The system includes a master pacer and trigger unit with elaborate hardware and software triggering options. This paper describes the system hardware and the software used to control it. Finally, some of its application are demonstrated. The flexibility of the system makes it widely applicable in the field of biomedical engineering.

Animals↗