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Johan H M Frijns

Publications and source records attributed to Johan H M Frijns.

9 recordsLinked to original sources
PubMed2006

The consequences of neural degeneration regarding optimal cochlear implant position in scala tympani: a model approach.

Cochlear implant research endeavors to optimize the spatial selectivity, threshold and dynamic range with the objective of improving the speech perception performance of the implant user. One of the ways to achieve some of these goals is by electrode design. New cochlear implant electrode designs strive to bring the electrode contacts into close proximity to the nerve fibers in the modiolus: this is done by placing the contacts on the medial side of the array and positioning the implant against the medial wall of scala tympani. The question remains whether this is the optimal position for a cochlea with intact neural fibers and, if so, whether it is also true for a cochlea with degenerated neural fibers. In this study a computational model of the implanted human cochlea is used to investigate the optimal position of the array with respect to threshold, dynamic range and spatial selectivity for a cochlea with intact nerve fibers and for degenerated nerve fibers. In addition, the model is used to evaluate the predictive value of eCAP measurements for obtaining peri-operative information on the neural status. The model predicts improved threshold, dynamic range and spatial selectivity for the peri-modiolar position at the basal end of the cochlea, with minimal influence of neural degeneration. At the apical end of the array (1.5 cochlear turns), the dynamic range and the spatial selectivity are limited due to the occurrence of cross-turn stimulation, with the exception of the condition without neural degeneration and with the electrode array along the lateral wall of scala tympani. The eCAP simulations indicate that a large P(0) peak occurs before the N(1)P(1) complex when the fibers are not degenerated. The absence of this peak might be used as an indicator for neural degeneration.

Action Potentials↗
PubMed2006

Psychophysical assessment of spatial spread of excitation in electrical hearing with single and dual electrode contact maskers.

OBJECTIVE: To evaluate psychophysically the spatial spread of excitation in electrical hearing with a new dual contact masker and to investigate under which conditions it is possible to stimulate fibers in the immediate neighborhood of an electrode contact, which were not excited by neighboring electrode contacts. DESIGN: In this study a psychophysical forward masking paradigm with a dual contact masker was used to avoid off-site listening, the electrical analogue of off-frequency listening. The masker stimulus (300 msec) is presented nonsimultaneously on two electrode contacts, one on the apical side and another on the basal side of the probe contact, followed by a probe stimulus of 20 msec.Unmasked probe thresholds were compared with masked ones at a number of masker-probe distances, whereas growth of masking curves were measured for a fixed masker contact pair. Standard selectivity measurements (single contact masking) and the recovery of forward masking with one masker contact were included for comparison with existing methods. All experiments were carried out with six participants who use the Clarion CII device with a HiFocus I electrode array. RESULTS: For dual contact masking the amount of masking was significantly greater than for single contact masking and the width of the masking patterns was on average 1.1 mm broader than for single contact masking, resulting in a broad region of excitation, with masker-probe overlap for distances greater than 3 mm. Masking widths for dual and single contact masking were highly correlated. Growth of masking curves were highly nonlinear. They showed a strong elevation of the slope that starts for most subjects around the middle of the dynamic range or above. For 4 out of 6 subjects, no probe threshold was found above a masker amplitude of about 400-500 microA. The ratio of the maximum measurable masked probe thresholds and unmasked probe threshold ranged from 1.7 to 2.6 (S4 excluded). Recovery of masking functions follow an exponential decay. Time constants tau for the recovery process ranged from 21.6 msec to 114.9 msec. CONCLUSIONS: With a dual contact masker (1) off-site listening can be avoided, leading to larger estimates of the width of excitation patterns than in single contact masking, (2) it can be estimated for which stimulation level there is complete overlap of excitation patterns of adjacent electrode contacts, (3) it can be shown that stimulation of nerve fibers in the immediate neighborhood of an electrode contact which were not excited by neighboring electrode contacts is only possible if the probe stimulation amplitude is sufficiently high in comparison with amplitudes on neighboring contacts.

Adult↗
PubMed2005

Unraveling the electrically evoked compound action potential.

With the advent of eCAP recording tools such as NRT and NRI for cochlear implants, neural monitoring has become widely used to ascertain the integrity of the neural/electrode interface as well as for assisting in the setting of program levels. The basic concepts of eCAP recordings are deduced from the acoustical equivalent of the electrocochleogram. There are, however, indications that under electrical stimulation some of these do not hold, like the unitary response concept (i.e., the principle that every fiber produces the same contribution to the eCAP). Computer modeling has proven to be a valuable tool for gaining insight into the functioning of electrical stimulation. In this study the extension of a three-dimensional human cochlea, incorporating back-measuring capabilities, is described. Using this new model, the contribution of single fiber action potentials (SFAPs) to the measured eCAP is investigated. The model predicts that contrary to common belief--the compound action potential as measured by the cochlear implant system does not necessarily reflect the propagated action potential along the auditory nerve.

Action Potentials↗
PubMed2004

A new method for dealing with the stimulus artefact in electrically evoked compound action potential measurements.

OBJECTIVE: Residual charge on recording electrodes leads to elevated potentials after the end of the stimulus, which can easily overload the electrically evoked compound action potential (eCAP) recording systems (neural response imaging or neural response telemetry). A new method for dealing with this problem was tested in a series of animal experiments. MATERIAL AND METHODS: We developed an amplifier with a compensation circuit that reduces the effect of the residual charge by electrical subtraction at the input. Using this amplifier we compared different artefact rejection protocols simultaneously in chronically implanted guinea pigs. A new, systematic nomenclature for the various forward masking schemes, based on the number of frames involved, is proposed. RESULTS: Proper adjustment of the compensation circuit reduces the overload time from > 200 micros to < 30 micros, but the compensation signals influence the final output signal considerably. To eliminate this deliberately introduced, reproducible artefact, an additional artefact rejection scheme is necessary. With alternating polarity (AP) and forward masking paradigms we could reliably record the N1 peak. Forward masking responses reveal shorter latencies for cathodic-first biphasic stimuli than for anodic-first pulses. The average of these two closely resembles the response obtained with the AP paradigm. CONCLUSIONS: It is worthwhile implementing the electrical compensation method proposed herein in clinical neural response imaging or neural response telemetry systems, as it represents a more robust way of assessing the eCAP.

Action Potentials↗
PubMed2004

Speech recognition with a cochlear implant using triphasic charge-balanced pulses.

OBJECTIVE: Typically, symmetrical charge-balanced biphasic current pulses are used in cochlear implants to ensure biological safety. Theoretically, monophasic pulses are more effective, but potentially noxious, stimuli. In this study we charge-balanced such monophasic pulses during selected non-stimulated intervals, effectively leading to triphasic pulses with a 4:1 amplitude ratio between the cathodic and anodic phases. Apart from ensuring safety, this is also expected to reduce power consumption and channel interaction. MATERIAL AND METHODS: Seven experienced Clarion CII cochlear implant users with a multichannel (12-16 channels) monopolar continuous interleaved sampling (CIS) strategy participated in the study. Three different CIS strategies were fitted using the Clarion Research Interface (CRI-2). The reference was an implementation of each subject's own CIS program. The two strategies tested used triphasic pulses on the same channels, one with half-wave rectification (TP-HWR) and one without rectification (TP-NoR) at the input. Directly after fitting (i.e. without any training), speech perception (phoneme score on consonant-vowel-consonant words) was measured in silence (sound-only) and in speech-shaped background noise with signal:noise ratios (SNRs) of +5 and 0 dB. RESULTS: Speech perception with the reference via the CRI-2 was equal to that of the free-field condition with the subjects' own speech processor. With the TP-NoR strategy, speech perception improved significantly (from 89% to 93%) in silence and in the 0-dB SNR condition (from 43% to 49%). With a SNR of +5 dB, performance was stable at approximately 66%. With the TP-HWR strategy, performance increased significantly in the 0- and +5-dB SNR conditions, to 55% and 74%, respectively. Power consumption was reduced in both strategies, to 30% and 36% for TP-HWR and TP-NoR, respectively. CONCLUSION: The new triphasic strategies are most promising, with respect to both their improved speech perception and reduced power requirements. The optimal parameters will have to be identified following long-term use.

Adult↗
PubMed2004

Clinical ototoxicity of teicoplanin.

No strong evidence of ototoxicity of teicoplanin can be found in the literature, possibly because of conservative definitions of ototoxicity. We performed audiometry over time to compare the ototoxicity of teicoplanin with that of cloxacillin as a non-ototoxic standard. The data were analyzed with a linear mixed-effects model. The hearing thresholds of 12 patients who were treated with teicoplanin for severe staphylococcal infections showed a slight but significant increase over time, whereas the thresholds of 5 patients treated with cloxacillin decreased significantly during treatment. This improvement in hearing with cloxacillin may be attributed to improvement of the clinical condition. This outcome implies that previous reports that suggest a lack of ototoxicity of teicoplanin potentially underestimate the risk and should be interpreted accordingly.

Anti-Bacterial Agents↗
PubMed2003

Optimizing the number of electrodes with high-rate stimulation of the clarion CII cochlear implant.

OBJECTIVE: This blind crossover study evaluates the effect of the number of electrodes of the Clarion CII cochlear implant on speech perception in silence and in noise using a "high-rate" continuous interleaved sampling (CIS) strategy. MATERIAL AND METHODS: Nine users of this implant with 3-11 months of experience of an 8-channel CIS strategy [833 pulses per second (pps)/channel, 75 micros/phase] were fitted in a random order with 8-, 12- and 16-channel CIS strategies (+/- 1,400 pps/channel, 21 micros/phase). After 1 month of exclusive use of each strategy the performance was tested with consonant-vowel-consonant words in silence (sound only) and in speech-shaped background noise with signal-to-noise ratios (SNRs) of + 10, + 5, 0 and -5 dB. RESULTS: With "high-rate" strategies most patients' speech understanding in noise improved, although the optimum number of electrodes was highly variable. Generally, faster performers benefited from more active electrodes, whilst slower performers deteriorated. If each patient's optimal strategy was determined by a weighted sum of the test results at +10, + 5 and 0 dB SNR, the average phoneme score improved from 57% to 72% at a SNR of + 5 dB, and from 46% to 56% at a SNR of 0 dB. The average phoneme score in silence was approximately 85% for all strategies. CONCLUSION: We conclude that speech perception (especially in noise) can improve significantly with "high-rate" speech processing strategies, provided that the optimum number of electrodes is determined for each patient individually.

Acoustic Stimulation↗
PubMed2002

Initial evaluation of the Clarion CII cochlear implant: speech perception and neural response imaging.

OBJECTIVE: To evaluate the new Clarion CII cochlear implant with the perimodiolar HiFocus electrode array, including both speech perception outcomes and the device's capabilities of measuring the electrically evoked compound action potential (eCAP) of the auditory nerve (Neural Response Imaging, NRI). DESIGN: The speech perception scores on CVC words without lip reading were monitored prospectively for the 10 postlingually deaf patients implanted with the Clarion CII device in the period July 2000 until May 2001 in the Leiden University Medical Center. Preoperative and postoperative NRI recordings were made, applying various combinations of monopolar stimulating and recording electrodes with the alternating polarity paradigm available in the test bench software. RESULTS: Nine patients preferred the CIS, one the PPS strategy, none the SAS strategy. With their favorite strategy they acquired significant open set speech understanding within a few weeks, resulting in an average CVC phoneme score of 84% (word score 66%) at the end of the study (follow-up 3 to 11 mo). In speech-shaped noise, the average phoneme recognition threshold (PRT) was reached at a signal to noise ratio just below 0 dB. The NRI recordings had clear N1 and P1 peaks if there was at least one contact between the stimulating and recording electrodes, necessitating just 15 sweeps for a reliable recording. We observed considerable inter-patient and inter-electrode variability, but for a given situation NRI input/output curves were stable over time. More apical contacts generally elicited larger eCAPs. Response amplitudes tended to peak at recording sites around apical and basal stimulating electrodes, suggesting a limited spread of excitation. Preliminary recordings with the forward masking paradigm were consistent with the ones with the alternating polarity scheme. CONCLUSIONS: The Clarion CII is a promising cochlear implant with which our first 10 patients have obtained excellent speech perception results. The NRI system yields high quality signals with a limited number of sweeps at a high sampling rate.

Acoustic Stimulation↗