Andreas Bahmer

Recording and analysis of electrically evoked compound action potentials (ECAPs) with MED-EL cochlear implants and different artifact reduction strategies in Matlab

Electrically evoked compound action potentials (ECAPs) are used in auditory research to evaluate the response of the auditory nerve to electrical stimulation. Animal preparations are typically used for the recording. With the introduction of a new generation of cochlear implants, however it is possible to record the response of the auditory nerve to electrical stimulation in humans as well, which is used in the clinic to test whether the implant works properly and whether the auditory nerve is responsive. Currently, ECAPs are used to estimate thresholds for speech processor programs. In addition, ECAPs recordings allow new research to be addressed, e.g., to evaluate enhanced electrical stimulation patterns. Research platforms are required to test user-defined stimuli and algorithms for the ECAPs analysis. Clinical fitting software that records ECAPs is not flexible enough for this purpose. To enable a larger group of scientists to pursue research in this field, we introduce a flexible setup that allows to change stimulation and recording parameters. ECAP recording and analysis software was developed in Matlab (The Mathworks, Inc.) for standard PC, using a National instruments (PCI-6533, National Instruments, Austin, TX) card and a Research Interface Box 2 (RIB2, Department of Ion Physics and Applied Physics at the University of Innsbruck, Innsbruck, Austria) for MED-EL cochlear implants. ECAP recordings of a human subject with three different artifact reduction methods (alternating, Miller modified masker-probe, triphasic pulses) are presented and compared.

Recording of electrically evoked auditory brainstem responses (E-ABR) with an integrated stimulus generator in Matlab

Electrical auditory brainstem responses (E-ABRs) of subjects with cochlear implants are used for monitoring the physiologic responses of early signal processing of the auditory system. Additionally, E-ABR measurements allow the diagnosis of retro-cochlear diseases. Therefore, E-ABR should be available in every cochlear implant center as a diagnostic tool. In this paper, we introduce a low-cost setup designed to perform an E-ABR as well as a conventional ABR for research purposes. The distributable form was developed with Matlab and the Matlab Compiler (The Mathworks Inc.). For the ABR, only a PC with a soundcard, conventional system headphones, and an EEG pre-amplifier are necessary; for E-ABR, in addition, an interface to the cochlea implant is required. For our purposes, we implemented an interface for the Combi 40+/Pulsar implant (MED-EL, Innsbruck).

Recording of electrically evoked auditory brainstem responses after electrical stimulation with biphasic, triphasic and precision triphasic pulses

Biphasic electrical pulses are the standard stimulation pulses in current cochlear implants. In auditory brainstem recordings biphasic pulses generate a significant artifact that disrupts brainstem responses, which are magnitudes smaller. Triphasic pulses may minimize artifacts by restoring the neural membrane to its resting potential faster than biphasic pulses and make auditory brainstem responses detection easier. We compared biphasic pulses with triphasic and precision triphasic pulses to evoke brainstem responses in human subjects. For this purpose, electrically evoked brainstem response audiometry was performed in 10 (11 ears) cochlear implant patients. Artifacts and brainstem responses evoked by bi- and triphasic stimulation were analyzed. Artifact amplitude and decay time were related to pulse pattern shape, but application of averaging and alternation reduced the deterioration of electrically evoked brainstem responses independent of pulse pattern shape. Contrary to our expectations, biphasic pulses showed a higher detectability in comparison to triphasic pulse stimulation at the same stimulation amplitude.

Application of triphasic pulses with adjustable phase amplitude ratio (PAR) for cochlear ECAP recording: I. amplitude growth functions.

This study describes the use of triphasic electrical stimulation pulses with an adjustable phase amplitude ratio (PAR) for the reduction of electrical stimulus artifacts. It is hypothesized that the setting of a certain PAR can facilitate a nearly artifact-free recording of electrically evoked compound action potentials (ECAP) in the cochlea. Artifact reduction with triphasic pulses using single epochs is expected to prevent latency or polarity effects, which are seen in standard forward masking or alternating polarity strategies. Although the application of a third phase is already implemented in implants manufactured by MED-EL (Zierhofer, 2003) and Cochlear (Sydney, Nucleus 5 System; van Dijk et al. (2007)) for the reduction of stimulation artifacts generated with these stimulators in ECAP measurements, an elaborate systematic evaluation of PAR for artifact reduction has not yet been conducted (compare evaluation for one subject Schoesser et al. (2001)). In the present paper, the effect of PAR variation on human ECAP recording and the feasibility of amplitude growth function recording with triphasic pulses and an optimized PAR are evaluated. Measurements were accomplished in five subjects, whereby more detailed test series were carried out in one subject. All subjects were implanted with devices from the company MED-EL, Innsbruck. A comparison of PAR optimized triphasic pulses was carried out against two other measurement techniques (biphasic alternating polarity stimulation and biphasic stimulation according to Miller) for apical, middle, and basal electrodes. ECAP thresholds were estimated by means of amplitude growth functions. However, recording of ECAP with triphasic pulses showed drawbacks: additional artifacts depending on stimulation and/or recording parameters are introduced, the ratio between the additional artifact and improved detectability of neural responses is dependent on PAR, and response thresholds obtained with triphasic pulses--although similar in shape--are in most cases substantially higher compared to thresholds measured with the Miller method. Higher thresholds most probably occur because the triphasic pulse patterns seem to less effectively stimulate neural structures compared to biphasic pulses since measured response thresholds are higher. For certain electrode groups threshold profiles obtained with triphasic pulses were found to be similar compared to stimulation with biphasic pulses.

Effects of electrical pulse polarity shape on intra cochlear neural responses in humans: Triphasic pulses with cathodic second phase

Charge balanced pulses are used in modern cochlear implants to avoid direct current (DC) stimulation that may damage neural tissues. In this context the effect of electrical pulse shape and polarity is still a matter of debate and the most effective pulse shape needs to be determined (Bahmer et al., 2010a; Undurraga et al., 2010; Wieringen et al., 2008; Macherey et al., 2008). Therefore, we conducted electrophysiological measurements, namely electrical compound action potentials (ECAPs) to assess response strength elicited by various pulse shapes and polarities in five cochlear implant recipients (SonataTI100/PulsarCI100 devices, MED-EL Innsbruck). ECAP response strength depending on pulse shape was compared with individual psychophysical thresholds. Results indicated the weakest response amplitude and highest thresholds for symmetric triphasic pulse shapes (with cathodic second phase), and the strongest response amplitude and lowest thresholds for biphasic pulses with anodic first phase. Biphasic pulses with cathodic first phase generated intermediate response amplitude and thresholds.

New parallel stimulation strategies revisited: effect of synchronous multi electrode stimulation on rate discrimination in cochlear implant users.

Abstract Objectives Most cochlear implants implement stimulation strategies which apply sequential electrical pulses to encode acoustic signals such as speech, noise, and sounds via electrical stimulation of the auditory nerve. Parallel stimulation of adjacent electrodes has been employed in recent cochlear implant (CI) systems in an endeavor to improve coding of pitch information (e.g. FS4-p fine structure with parallel signal processing MED-EL, Innsbruck, Austria; VCIS, AB Corp., Sylmar, CA, USA). We investigated whether parallel stimulation of three adjacent electrodes enhances rate pitch perception compared with single electrode stimulation. Methods Most comfortable loudness (MCLs) levels were assessed in single and multi electrode condition in 12 subjects (15 ears, PULSARci100/SONATAti100 implant, MED-EL). Rate pitch discrimination was determined by means of an adaptive procedure (two-interval two-alternative forced choice, 2I2AFC) at individual MCL in the single- and multi-electrode condition at base frequencies of 100, 200, 283, 400, and 566 pulses per second (pps) (single electrode condition: electrode 5, multi electrode condition: electrode 4, 5, 6; PULSARci100/SONATAti100 implant: 12 electrode contacts; 1, most apical; 12, most basal). Results To achieve MCL in the multi-electrode condition significantly higher stimulation current compared with single stimulation was required. No significant difference between single- and multi-electrode condition just noticeable differences in rate discrimination (JNDR) group was found. In contrast, a pairwise comparison of individual results in a subgroup recruited out of successfully completed runs at high base rates showed statistically an improved rate discrimination in 17 of 24 runs in the multi-electrode condition. Therefore, a potential effect of parallel stimulation on rate discrimination is conceivable. Discussion The results in a subgroup of this study indicate that, compared with single-electrode stimulation, synchronous multi-electrode stimulation of three adjacent electrodes shows improvement rate discrimination in 17 of 24 test runs (binomial and χ2 test, P = 0.05) but did not result in statistically better JNDRs (best averaged improvement 19.8% at base rate 400 pps).

A simulation of chopper neurons in the cochlear nucleus with wideband input from onset neurons

The unique temporal and spectral properties of chopper neurons in the cochlear nucleus cannot be fully explained by current popular models. A new model of sustained chopper neurons was therefore suggested based on the assumption that chopper neurons receive input both from onset neurons and the auditory nerve (Bahmer and Langner in Biol Cybern 95:4, 2006). As a result of the interaction of broadband input from onset neurons and narrowband input from the auditory nerve, the chopper neurons in our model are characterized by a remarkable combination of sharp frequency tuning to pure tones and faithful periodicity coding. Our simulations show that the width of the spectral integration of the onset neuron is crucial for both the precision of periodicity coding and their resolution of single components of sinusoidally amplitude-modulated sine waves. One may hypothesize, therefore, that it would be an advantage if the hearing system were able to adapt the spectral integration of onset neurons to varying stimulus conditions.

A case of bilateral cochlear implantation in single-sided untreated acoustic neurinoma

Acoustic neurinoma affects the acousticofacial nerve and therefore in many cases is a contraindication for cochlear implantation and an indication for brainstem implant. Nevertheless benefit in these patients has been shown after tumour removal and cochlear implantation. The first case of bilateral cochlear implantation in a patient with single-sided untreated acoustic neurinoma is described here. In a 49-year-old woman with progressive hearing loss during the last 12 years we preoperatively diagnosed an acoustic neurinoma of the left side. After cochlear implantation of the right side she was sequentially implanted on the affected side as well. Before surgery radiological control of the tumour for signs of growth was performed and the patient was thoroughly informed of the situation and possible therapies and dangers. Speech discrimination scores obtained in the second implanted ear came up to the performance of the first implant after 6 months.

Application of triphasic pulses with adjustable phase amplitude ratio (PAR) for cochlear ECAP recording: II. Recovery functions

Triphasic electrical stimulation pulses with an adjustable phase amplitude ratio (PAR) can reduce stimulus artifacts in electrically evoked compound actions potentials (ECAPs) recording in the cochlea (see companion paper Bahmer and Baumann, submitted for publication). The present study describes the application of triphasic pulses in forward masking paradigms for recording recovery functions. Masking was found to be most effective using equal masker-probe PAR settings. Results were compared with data applying artifact cancellation strategy for biphasic pulses according to Miller et al. (2000). Measurements were accomplished in five subjects (S1-S5) with an equal masker-probe PAR setting, whereby more detailed test series were carried out in one subject (S1). All subjects were users of MED-EL SONATAti100 or PULSARci100 cochlear implants (Innsbruck, Austria). Parameters like asymptote level, absolute refractory period and time constant were determined by fitting exponential functions to the recovery functions. Detailed measurements collected on 11 electrode locations in subject S1 showed similar parameter profiles on basal electrode contacts for both triphasic and Miller artifact cancellation methods, whereas apical/middle electrode contacts differed in part largely. Compared to Millers artifact cancellation method estimated asymptote levels were lower with triphasic stimulation; the estimated absolute refractory period and time constants were estimated higher on apical electrodes. Results obtained in subjects S2-S5 showed considerable variances and a proper parametrization of the recovery function was possible only very selectively for triphasic pulse stimulation. In these cases, congruencies in estimated asymptote levels and time constants were found when triphasic stimulation and biphasic stimulation according to Miller were compared.

Recording and online analysis of auditory steady state responses (ASSR) in Matlab.

Auditory steady state responses (ASSR) are a current research focus because of their potential use as a diagnostic tool. Research platforms are required to test user defined stimuli and algorithms for the analysis of electrophysiologic responses. Commercially available ASSR devices are not adequately flexible. To enable a larger group of scientists to pursue ASSR research, we introduce a cost-efficient and flexible ASSR setup. ASSR recording and online analysis software in Matlab (The Mathworks, Inc.) was developed for a standard PC equipped with an external professional sound card, audiometric headphones, and an EEG biosignal preamplifier.