Short-term overstimulation affects peripheral but not central excitability in an animal model of cochlear implantation.

Objective: Upper current limits (C-levels) are sometimes extremely increased over time since this procedure can enhance speech perception. It should be clarified if a larger amount of electrical stimulation is tolerated by the remaining peripheral and central auditory pathway.Materials and Methods: An animal electrode array was inserted in mechanically deafened guinea pigs. C-levels were adjusted to a mean of approximately 10 CL ('LOS' group), 40 CL ('MOS' group) or 60 CL ('HOS' group) above the electrode specific electrically evoked compound action potential (eCAP) threshold. The stimulation was performed via a sound processor in standardized auditory environment. Implanted and not stimulated animals served as controls.Results: A significant eCAP threshold shift was observed in the 'HOS'-group aftereight hours of stimulation at basal electrodes. Electrically evoked auditory brainstem thresholds were stable over time in all stimulation groups. The ratio between eCAP- and eABR threshold shifts was significantly enhanced in the 'HOS'- group.Conclusion: Even short-time overstimulation reduces the excitability of peripheral but not central auditory structures. The changed relationship between the excitability of spiral ganglion neurons and inferior colliculus neurons seems to indicate an overstimulation. The results are of utmost importance for a safe CI-processor fitting especially in children or non-compliant patients.

Digital Live Imaging of Intraoperative Electrocochleography - First Description of Feasibility and Hearing Preservation During Cochlear Implantation.

INTRODUCTION: Intraoperative electrocochleography (ECochG) during cochlear implantation is a promising tool to preserve residual hearing. However, the time gap between insertion of the electrode and acoustic feedback from the audiologist to the surgeon can cause delay and subsequently irreparable damage to cochlear structures. In this feasibility study, for the first time, real-time visualization of intraoperative ECochG via digital microscope display directly to the surgeon was successfully performed in four patients. MATERIALS AND METHODS: Four patients with residual hearing underwent cochlear implantation. Intraoperative electrocochleography responses were collected and direct visualization during the time of electrode insertion into the surgeon's field of view in the binoculars using augmented real-time digital imaging was realized. The time of electrode insertion was recorded. Hearing preservation was determined by testing postoperative changes in behavioral thresholds. RESULTS: Digital live visualization of intraoperative ECochG using image augmentation in a digital microscope was successfully performed in all cases and enabled direct adaptation of the surgeon's insertion behavior. Mean time of electrode insertion was 129.8 seconds. Postoperative behavioral thresholds were comparable to preoperative taken thresholds. Preservation of residual hearing in the low frequency range was possible. DISCUSSION: This study is the first to describe digital visualization of intraoperative electrocochleography as a new method enabling the surgeon to directly react to changes in amplitude of the cochlea microphonics. Our results show that augmentation of the intraoperative live imaging with electrical potentials could add to hearing preservation during cochlear implantation.

Effect of Contralateral Noise on Speech Intelligibility.

In patients with strong asymmetric hearing loss, standard clinical practice involves testing speech intelligibility in the ear with the higher hearing threshold by simultaneously presenting noise to the other ear. However, psychoacoustic and functional magnetic resonance imaging (fMRI) studies indicate that this approach may be problematic as contralateral noise has a disruptive effect on task processing. Furthermore, fMRI studies have revealed that the effect of contralateral noise on brain activity depends on the lateralization of task processing. The effect of contralateral noise is stronger when task-relevant stimuli are presented ipsilaterally to the hemisphere that is processing the task. In the present study, we tested the effect of four different levels of contralateral noise on speech intelligibility using the Oldenburg sentence test (OLSA). Cortical lateralization of speech processing was assessed upfront by using a visual speech test with fMRI. Contralateral OLSA noise of 65 or 80 dB SPL significantly reduced word intelligibility irrespective of which ear the speech was presented to. In participants with left-lateralized speech processing, 50 dB SPL contralateral OLSA noise led to a significant reduction in speech intelligibility when speech was presented to the left ear, i.e. when speech was presented ipsilaterally to the hemisphere that is mainly processing speech. Thus, contralateral noise, as used in standard clinical practice, not only prevents listeners from using the information in the better-hearing ear but may also have the unintended effect of hampering central processing of speech.

[Loudness optimized registration of compound action potential in cochlear implant recipients]. / Lautheitsoptimierte Messung von Summenaktionspotentialen bei Cochlea Implantat Trägern.

Background Postoperative measurements of compound action potentials are not always possible due to the insufficient acceptance of the CI-recipients. This study investigated the impact of different parameters on the acceptance of the measurements. Methods Compound action potentials of 16 CI recipients were measured with different pulse-widths. Recipients performed a loudness rating at the potential thresholds with the different sequences. Results Compound action potentials obtained with higher pulse-widths were rated softer than those obtained with smaller pulse-widths. Conclusions Compound action potentials measured with higher pulse-widths generate a gap between loudest acceptable presentation level and potential threshold. This gap contributes to a higher acceptance of postoperative measurements.

Facilitation and refractoriness of the electrically evoked compound action potential.

In this study we aim to resolve the contributions of facilitation and refractoriness at very short pulse intervals. Measurements of the refractory properties of the electrically evoked compound action potential (ECAP) of the auditory nerve in cochlear implant (CI) users at inter pulse intervals below 300 µs are influenced by facilitation and recovery effects. ECAPs were recorded using masker pulses with a wide range of current levels relative to the probe pulse levels, for three suprathreshold probe levels and pulse intervals from 13 to 200 µs. Evoked potentials were measured for 21 CI patients by using the masked response extraction artifact cancellation procedure. During analysis of the measurements the stimulation current was not used as absolute value, but in relation to the patient's individual ECAP threshold. This enabled a more general approach to describe facilitation as a probe level independent effect. Maximum facilitation was found for all tested inter pulse intervals at masker levels near patient's individual ECAP threshold, independent from probe level. For short inter pulse intervals an increased N1P1 amplitude was measured for subthreshold masker levels down to 120 CL below patient's individual ECAP threshold in contrast to the recreated state. ECAPs recorded with inter pulse intervals up to 200 µs are influenced by facilitation and recovery. Facilitation effects are most pronounced for masker levels at or below ECAP threshold, while recovery effects increase with higher masker levels above ECAP threshold. The local maximum of the ECAP amplitude for masker levels around ECAP threshold can be explained by the mutual influence of maximum facilitation and minimal refractoriness.

[Results of Speech Audiometry Testing in Case of Transcranial CROS Depend on Duration of Deafness]. / Audiologische Ergebnisse bei transkranieller CROS-Versorgung in Abhängigkeit von der Ertaubungsdauer.

In single sided deafness, treatment with transcranial CROS makes pseudo stereophonic listening possible. This leads to improved speech understanding in noise. However, several reports show large variations between the individual results of the benefit by speech audiometry in noise. One possible factor is the duration and changes in the auditory pathway between the onset of deafness to CROS supply. 18 patients with single sided deafness have tested a transcranial CROS supply. The speech intelligibility thresholds were evaluated in 2 spatial hearing situations each with CROS and unaided. When speech was arriving at 45° from the deaf ear and noise at 45° from the hearing ear, a significant improvement of speech reception threshold for 50% intelligibility was detected by CROS supply compared the medians. Patients with longer duration of deafness showed higher benefit by an average of -4.0 dB, in contrast to patients with short duration of deafness. In the reversed situation, that speech on the hearing ear and noise on the deaf ear was a significant deterioration observed, an average of 3.1 dB at longer deafened patients. In addition, a highly significant correlation was found with the individual changes in the speech recognition threshold between the two situations. The duration of deafness is an essential factor in the individual benefit estimate for treatment with transcranial CROS supply. The time frame for audimetrically proven changes in the auditory pathway as a result of unilateral auditory deprivation can be estimated to about 1-3 years. Patients with short duration of deafness have low or no benefit from transcranial CROS supply.

Detection of binaural interaction in free-field evoked auditory brainstem responses by time-scale representations.

The so called ß-wave of the binaural interaction component (BIC) in auditory brainstem responses (ABR) has been shown to be an objective measure for binaural interaction (BI). This component is the arithmetical difference between the sum of the monaurally evoked ABRs and the binaurally evoked ABR. Unfortunately, these neural responses are known to be very fragile and as a result the calculated BIC. An additional issue is, that the findings of this measurement are predominantly needed in people with hearing loss who may use hearing devices like hearing aids (HA) or cochlear implants (CI), thus they are not able to use headphones (like in conventional ABR measurements) during the detection of possible BI. This is a crucial problem, because it is known that factors like the interaural time delay (ITD) between the receiving ears are responsible for solving tasks like sound source localization or sound source separation, but specially designed measurements to coordinate the fitting of HAs or CIs with respect to BI are still missing. In this paper, we introduce a new measurement setup that is able to detect BI depending on different ITDs in free-field evoked responses by using the more reliable instantaneous phase in the time-scale representation. With this pilot study we are able to demonstrate a decreasing BI with an increasing ITD using the wavelet phase synchronization stability analysis in ten normal hearing subjects.

Binaural integration of periodically alternating speech following cochlear implantation in subjects with profound sensorineural unilateral hearing loss.

In cochlear implant (CI) recipients with unilateral hearing loss (UHL) and normal hearing (NH) in the contralateral ear, the central auditory system receives signals of different auditory modalities, i.e. electrically via the CI ear as well as acoustically via the NH ear. The present study investigates binaural integration of bimodal stimulation in the central auditory system of 10 CI subjects with UHL by applying a modified version of the Rapidly Alternating Speech Perception (RASP) test to characterise speech recognition ability under monotic and dichotic listening arrangements. Subsequently, the results for each monotic and dichotic test condition were compared to quantify the binaural benefit from CI usage. The study results demonstrate significantly improved speech recognition under dichotic compared to monotic listening conditions, providing evidence that there is binaural integration of acoustically and electrically transmitted speech segments in the central nervous system at brainstem and cortical levels. In contrast to more commonly used tests of binaural integration, such as localisation, the RASP test provides the clinical option to investigate binaural integration involving structures at the cortical level.

Melody identification for cochlear implant users and normal hearers using expanded pitch contours.

Music perception is considered unsatisfactory for most cochlear implant (CI) users. Usually, rhythm identification is adequate while pitch and melody recognition are rather limited. The aim of this study was to investigate whether insufficient contour information in the low-frequency range is one cause that contributes to the poor melody recognition results in CI users. For this purpose, the recognition of familiar melodies was tested with three differently expanded pitch contours. Ten cochlear implant subjects and five normal-hearing (NH) volunteers were investigated. Each subject chose ten out of a possible set of 23 well-known nursery songs without verbal cues. The songs were played in the original version and with three different pitch-contour expansions. All versions were tested with and without rhythm and in random order. CI subjects exhibited best results when melodies were presented with expanded pitch contours, although no clear preference for a specific contour modification was observed. Normal-hearing subjects exhibited poorer results for expanded pitch contours, especially when testing without rhythm. Both NH and CI-user groups exhibited large inter-individual differences, and melody recognition with rhythm was always better than melody recognition without rhythm. Insufficient contour information in the low-frequency range is confirmed as one contributing cause for the poor melody recognition results in CI users. Therefore, other efforts to improve low-frequency pitch discrimination, e.g., a more sophisticated design of the electrode array, a focus of the electrical stimulation pattern or an improved signal processing scheme could potentially improve melody recognition as well.

An "endosteal electrode" for cochlear implantation in cases with residual hearing? Feasibility study: preliminary temporal bone experiments.

BACKGROUND: Over the years, an increasing number of patients with some degree of residual hearing have received cochlear implants. In these cases, the marginal benefit provided by hearing aids alone is not sufficient; however, as experience has already shown, when hearing aids are used in combination with a cochlear implant, more benefit may be obtained. As a prerequisite, this requires that residual hair cell function must remain intact postoperatively. One of the European pioneers of cochlear implants, Ernst Lehnhardt, questioned whether residual hearing might better be preserved if the implanted electrode permits the fluid-filled inner ear space to remain intact. Subsequently, he proposed insertion of a very flat electrode array design into the extraluminal space between the spiral ligament and the bony cochlear wall (endosteum). OBJECTIVE: Our study aimed to determine whether it is feasible to insert an endosteal electrode model intracochlearly but extraluminarily, anatomically, and ultimately surgically and to determine the impact on surrounding intracochlear structures. METHODS: Insertion of two silicon models of an endosteal electrode were carried out in 15 human temporal bones. Histologic examination of the temporal bones after electrode insertion was performed on both fresh and fixed specimens to determine whether the desired anatomic site of insertion was achieved. In combination with light reflected and electron microscopic techniques, the extent to which the surrounding structures were impacted was also examined. RESULTS: Successful insertion of the prototype silicon endosteal electrodes was performed intracochlearly and extraluminarly in 11 of the 15 temporal bone specimens, confirming the anatomic feasibility of insertion into the crevice between the spiral ligament and endosteum. CONCLUSIONS: On the basis of the anatomy of the human temporal bone, insertion of an "endosteal electrode" is feasible. Subsequently, in vivo animal studies are needed to determine the physical effects of insertion of an endosteal electrode design prototype upon the functionality of the surrounding intracochlear structures and in particularly the ability to preserve hearing function.