Roland Mühler

Amplitude Modulation Following Responses (AMFR) in der audiologischen Diagnostik

ZusammenfassungDerzeit ist die Registrierung von Hirnstammpotenzialen die am weitesten verbreitete Methode zur Bestimmung der Hörschwelle im Rahmen der objektiven audiologischen Diagnostik. Dieses Verfahren ist jedoch mit einer Reihe methodischer Probleme verbunden: mangelnde Frequenzspezifität bei einer breitbandigen Stimulation, geringe Aussagekraft im tieffrequenten Bereich und hohe Subjektivität der Auswertung.Eine Lösung könnte die Registrierung von Amplitude Modulation Following Responses (AMFR) darstellen. Mit ihrer hohen Frequenzspezifität aufgrund ihrer sehr schmalbandigen Stimulation lassen sie Aussagen bis in den niederfrequenten Bereich des Hörens zu.Einen weiteren Vorteil stellt der objektive Nachweis dieser Potenziale auf Basis etablierter statistischer Verfahren dar.Umfangreiche Untersuchungen zum Einfluss der Stimulationsparameter und der Ableitbedingungen haben gezeigt, dass die Registrierung von AMFR als audiologisches Werkzeug erfolgversprechend ist. Im Vordergrund standen bisher Fragen nach der optimalen Modulationsfrequenz, dem Einfluss der Vigilanz auf die Potenzialausbildung und der Nutzung bei der objektiven Bestimmung der Hörschwelle.AbstractThe registration of brainstem potentials currently represents one of the most common methods in objective audiological diagnostics. However, regardless of their use, they are still known to possess important disadvantages, such as low specificity and validity in the lower frequency range due to broadband stimuli, or uncertainties due to the need for subjective evaluation.One potential solution to these problems could involve the registration of amplitude modulation following responses (AMFR).These potentials are being discussed much more regularly within the anglo-american literature due to their known frequencyspecificity within the high frequency range (resulting from a very narrow frequencyband of stimulation), and also their ability to permit assessment of the hearing threshold at lower frequencies.Another additional advantage of AMFR results from the simple statistical verification of its presence.Extensive studies on the influence of both stimulating and recording parameters have also shown that the registration of AMFR could prove to be a very promising audiological tool, with past interest being focussed primarily on the optimal modulation frequency, the influence of vigilance of the generation of potentials, and the precise assessment of an objective threshold.

40-Hz multiple auditory steady-state responses to narrow-band chirps in sedated and anaesthetized infants.

OBJECTIVE The general consensus to date has been that a stimulus repetition rate of 40Hz is not appropriate for the recording of auditory steady-state responses (ASSR) in sedated or anaesthetized infants. The aim of this study was to investigate whether reliable 40-Hz ASSR thresholds can be obtained in sedated infants using narrow-band chirp stimuli in the clinical routine. METHODS 40-Hz auditory brainstem responses (ABR) and 40-Hz ASSR were recorded in 34 infants below the age of 48 months under sedation or under general anaesthesia. ABR were evoked by broad-band chirp stimuli with a flat amplitude spectrum. ASSR were recorded simultaneously from both ears with an adaptive multiple stimulus paradigm using four narrow-band chirps centred at 500, 1000, 2000 and 4000Hz. ABR and ASSR thresholds were evaluated to determine differences between the estimates from the two methods. RESULTS Despite of sedation and anaesthesia, large wave V amplitudes of the chirp evoked 40-Hz ABR were found at levels as low as 10dB above the individual ABR threshold. ASSR thresholds for stimulus repetition rates of 40Hz could be consistently estimated in all 34 infants. Thresholds estimated from the ASSR for the four frequencies of the narrow-band chirps and the threshold derived from the broad-band chirp ABR differ, on average by 3.7dB for the frequency range of 1000-2000Hz and 7.1dB for the frequency range from 2000 to 4000Hz. CONCLUSION In contrast to the general assumption that 40-Hz ASSR are not appropriate for threshold estimation in infants our results demonstrate that multiple ASSR with a stimulus repetition rate of 40Hz can be recorded in sedated and anaesthetized infants using narrow-band chirps. Threshold estimates obtained with 40-Hz ASSR are, on average, slightly higher than those obtained with chirp evoked ABR but allow for a frequency specific characterisation of the hearing ability.

Fast Hearing-Threshold Estimation Using Multiple Auditory Steady-State Responses with Narrow-Band Chirps and Adaptive Stimulus Patterns

This paper describes the estimation of hearing thresholds in normal-hearing and hearing-impaired subjects on the basis of multiple-frequency auditory steady-state responses (ASSRs). The ASSR was measured using two new techniques: (i) adaptive stimulus patterns and (ii) narrow-band chirp stimuli. ASSR thresholds in 16 normal-hearing and 16 hearing-impaired adults were obtained simultaneously at both ears at 500, 1000, 2000, and 4000 Hz, using a multiple-frequency stimulus built up of four one-octave-wide narrow-band chirps with a repetition rate of 40 Hz. A statistical test in the frequency domain was used to detect the response. The recording of the steady-state responses was controlled in eight independent recording channels with an adaptive, semiautomatic algorithm. The average differences between the behavioural hearing thresholds and the ASSR threshold estimate were 10, 8, 13, and 15 dB for test frequencies of 500, 1000, 2000, and 4000 Hz, respectively. The average overall test duration of 18.6 minutes for the threshold estimations at the four frequencies and both ears demonstrates the benefit of an adaptive recording algorithm and the efficiency of optimised narrow-band chirp stimuli.

Sorted averaging—application to auditory event-related responses

To isolate event-related potentials (ERPs) from the biological background noise, averaging of stimulus-locked electroencephalogram (EEG) epochs is needed. Recordings with patients often reveal a noisy background EEG, i.e., a high amplitude and multiple artifacts. Research studies have to deal with a limited number of available epochs. Therefore, averaging and efficient artifact rejection algorithms are needed. This paper focuses on the sorted averaging algorithm which was developed for the recording of auditory brainstem responses. We demonstrate the applicability of sorted averaging on ERPs by comparing three different averaging algorithms on a classical mismatch negativity (MMN) paradigm, recorded at 10 normal hearing volunteers. The resulting estimated signal-to-noise ratio (SNR) of the ERP waveforms was significantly increased compared to established averaging algorithms. Thus, the sorted averaging algorithm provides an improvement of the SNR in recordings with poor SNR (e.g., the MMN) or noisy background EEG (e.g., at Cochlear Implant users).

Amplitude modulation following responses in awake and sleeping humans--a comparison for 40 Hz and 80 Hz modulation frequency.

There have been several studies, which suggest that the amplitude of amplitude modulation following responses (AMFR) is correlated to the state of vigilance, similar to the 40 Hz event-related potentials. The aim of the present study was to compare the dependency of the AMFR-amplitude from the state of sleep for 40 Hz and 80 Hz modulation frequency. Eight normal hearing adults were investigated during natural and drug-induced sleep. The stimuli used were sinusoidally amplitude-modulated tones of 1 kHz carried frequency and 40 or 80 Hz modulation frequency at 60 dB nH stimulation level. For 40 Hz modulation frequency an increase of EEG-activity in the Delta-and Theta-band during periods of sleep correlates significantly with a decreased AMFR-amplitude whereas for 80 Hz no significant relation between stage of sleep and AMFR-amplitude could be found. The results suggest that in audiological use of 40 Hz-AMFR the state of vigilance should be monitored and stabilized at a high level.

Sorted averaging-principle and application to auditory brainstem responses

A new averaging method for evoked potentials, called sorted averaging, is presented. The method requires an ensemble of sweeps stored in memory and is based on the principle of interchangeability of individual sweeps within this ensemble. Sorted averaging is applied by sorting all sweeps according to their estimated background noise and successive averaging of this sorted ensemble, starting with low-noise sweeps. Signal-to-noise power ratio (SNR2) is estimated by calculating the inverse single point variance (Elberling & Don, 1984). This SNR2 estimate increases linearly with the number of sweeps for standard averaging and shows a clear maximum for sorted averaging. Auditory brainstem responses to click stimuli at 70 dB nHL in 20 normal hearing subjects were recorded and 4000 individual sweeps during each run were stored for analysis. In an off-line analysis, SNR2 for standard averaging with 10 microV artefact rejection, for weighted averaging and for sorted averaging were calculated. Sorted averaging was found to yield a significantly higher SNR2.

Development of a Speaker Discrimination Test for Cochlear Implant Users Based on the Oldenburg Logatome Corpus

The purpose of the study was to develop a speaker discrimination test for cochlear implant (CI) users. The speech material was drawn from the Oldenburg Logatome (OLLO) corpus, which contains 150 different logatomes read by 40 German and 10 French native speakers. The prototype test battery included 120 logatome pairs spoken by 5 male and 5 female speakers with balanced representations of the conditions ‘same speaker’ and ‘different speaker’. Ten adult normal-hearing listeners and 12 adult postlingually deafened CI users were included in a study to evaluate the suitability of the test. The mean speaker discrimination score for the CI users was 67.3% correct and for the normal-hearing listeners 92.2% correct. A significant influence of voice gender and fundamental frequency difference on the speaker discrimination score was found in CI users as well as in normal-hearing listeners. Since the test results of the CI users were significantly above chance level and no ceiling effect was observed, we conclude that subsets of the OLLO corpus are very well suited to speaker discrimination experiments in CI users.

Zur Abhängigkeit der amplitude modulation following response von der Vigilanz

ZusammenfassungHintergrund und Fragestellung. Die “amplitude modulation following responses” (AMFR) erlauben aufgrund der schmalbandigen Anregung der Kochlea eine gute Einschätzung des Verlaufs der Hörschwelle. Eine diagnostische Nutzung der AMFR setzt aber die Kenntnis der Abhängigkeit dieser Potentiale vom Vigilanzniveau des Untersuchten voraus.In den vereinzelten Untersuchungen, die sich in der Literatur zu dieser Problematik finden, wurde qualitativ die Amplitude der AMFR von wachen mit der Amplitude von schlafenden Probanden verglichen. Eine quantitative Bestimmung des Vigilanzniveaus durch die Registrierung physiologischer Parameter erfolgte nicht. Im Rahmen der vorliegenden Arbeit sollte die für die klinische Anwendung der AMFR wichtige Abhängigkeit der AMFR-Amplitude vom Vigilanzniveau des Untersuchten bestimmt werden. Patienten und Methodik. Dazu wurde an 8 erwachsenen Normalpersonen die Korrelation zwischen der Amplitude der AMFR und der EEG-Amplitude im Delta- und im Theta-Band bestimmt, da diese Frequenzen besonders durch das Vigilanzniveau beeinflusst werden. Als Stimulus diente ein 1-kHz-Sinuston, dessen Amplitude mit 40 Hz oder 80 Hz sinusförmig moduliert wurde. Die Untersuchungen erfolgten an den gleichen Probanden im natürlichen Schlaf und nach Gabe von Diazepam. Ergebnisse. Sowohl im natürlichen Schlaf als auch nach Gabe von Diazepam zeigte sich bei 40 Hz Modulationsfrequenz ein deutlicher Zusammenhang zwischen der AMFR-Amplitude und der Amplitude des EEG. Eine gesteigerte EEG-Amplitude bewirkte eine signifikante Verringerung der AMFR-Amplitude. Dagegen konnte bei 80 Hz Modulationsfrequenz keine Abhängigkeit der AMFR-Amplitude von der EEG-Amplitude nachgewiesen werden. Unter allen Untersuchungsbedingungen zeigten die mit 80 Hz Modulationsfrequenz ausgelösten Potentiale eine geringere Amplitude als bei Stimulation mit 40 Hz. Schlussfolgerungen. Die Ergebnisse der vorliegenden Studie legen nahe, dass bei der audiologischen Nutzung der 40-Hz-AMFR an Erwachsenen für ein konstant hohes Vigilanzniveau zu sorgen ist. Obwohl die 80-Hz-AMFR sich als unabhängig von der Vigilanz erwiesen, spricht ihre deutlich geringere Amplitude gegen eine audiologische Nutzung an Erwachsenen.AbstractBackground and objective. Amplitude modulation following responses (AMFR) allows good estimation of the hearing threshold due to the very narrow band excitation of the cochlea. Audiological use of AMFR requires knowledge of the relationship of these responses to the state of vigilance. The few studies published compared only qualitatively the amplitude of AMFR recorded in awake subjects to that recorded in sleeping subjects. A quantitative determination of the level of vigilance on the basis of recorded physiological parameters has not yet been carried out. In the present study, the relationship between the amplitude of AMFR and the level of vigilance was investigated quantitatively. Patients/methods. In eight adults with normal hearing, the relationship between the AMFR amplitude and EEG amplitude in the delta- and theta-band was determined. The amplitude in both frequency bands was used to indicate the state of vigilance. The subjects were studied during natural and drug-induced sleep. A 1-kHz carrier tone with a sinusoidally modulated amplitude of 40 Hz or 80 Hz was used as stimulus. Results. At 40-Hz modulation frequency, the AMFR amplitude correlates with the EEG amplitude both in natural and drug-induced sleep. An increase in EEG activity is paralleled by a significant reduction of AMFR amplitude. At 80-Hz modulation frequency, no relationship between AMFR amplitude and EEG activity could be detected. Under all conditions, the amplitudes of AMFR evoked by a modulation frequency of 80 Hz were significantly lower than those evoked by 40 Hz. Conclusions. These results suggest that for an audiological use of the 40-Hz AMFR the state of vigilance should be stabilised at a constantly high level. In spite of the lower influence of vigilance on the 80-Hz AMFR, this response appears less ideal for threshold estimation in adults due to the significantly smaller amplitudes.

On the frequency spectrum of Amplitude Modulation Following Responses

Objective detection of Amplitude Modulation Following Responses (AMFR) is based on statistics applied after signal transformation from the time to the frequency domain by means of Discrete Fourier Transformation. In theory the frequency resolution of such transformation depends only on the analysed time window. In practise frequency resolution is also limited by the error caused by minimal difference between the clocks used for stimulus generation and Analogue/Digital-conversion. Small differences in clock frequencies may cause a spread of energy to neighbouring bins. In order to avoid this error we derived the sample clock for the A/D-conversion from the stimulator clock. By means of this technique the frequency structure of the AMFR was investigated. It is shown that if technical induced errors are excluded, the energy of the AMFR-response is limited to a very narrow frequency band. No physiologically induced disturbances of the phase locking of the AMFR to the modulation frequency could be observed. Additionally it is demonstrated that an increase of frequency resolution leads to an improved signal to noise ratio similar to the increase of averages in the time domain.

Objektive audiologische Diagnostik im Kindesalter

ZusammenfassungDie objektiven, auf der Messung von otoakustischen Emissionen (OAE) und akustisch evozierten Potenzialen (AEP) beruhenden Hörprüfmethoden sind unentbehrlicher Bestandteil der Pädaudiologie in Bezug auf die frühe Erkennung und Versorgung angeborener kindlicher Hörstörungen. Ihr korrekter und effizienter Einsatz erfordert Kenntnisse, die über die Grundausstattung vieler Anwender hinausgehen und einer kontinuierlichen Aktualisierung bedürfen. Im vorliegenden Übersichtsartikel werden viele der für die Sicherheit im Umgang mit den Verfahren und den Ergebnissen relevanten Aspekte angesprochen. Die Darstellung orientiert sich nicht an den Methoden, sondern am zentralen Problem des Praxisalltags, nämlich der qualitativen und quantitativen Beschreibung einer Hörstörung in Bezug auf ihr Ausmaß, den betroffenen Frequenzbereich und ihre Identität hinsichtlich Art und Ort der Läsion. Die Sicherheit der Diagnose kann durch Beachtung weniger und einfacher Regeln optimiert werden. Zentrale Bedeutung haben hierbei die miteinander verzahnten Parameter Reststörung, Signal/Rausch-Verhältnis und Reproduzierbarkeit, auf deren korrekte Handhabung ausführlich eingegangen wird. Weitere Gegenstände des Artikels sind die Erkennung von Fallstricken, die Beschreibung neuer Entwicklungen wie Chirp und „auditory steady-state responses“ (ASSR) und schließlich die Betrachtung des für die Audiometrie in den ersten Lebensjahren außerordentlich wichtigen Aspekts der Reifung.AbstractObjective methods based on the measurement of otoacoustic emissions (OAE) and auditory evoked potentials (AEP) are indispensable in pediatric audiology especially for the early detection and therapy of congenital hearing impairment. The correct and efficient use requires knowledge and skills which are beyond the basic equipment of many users and require continuous updating. In the present review many aspects relevant for the safe handling of the methods and interpretation of the results are addressed. The presentation does not focus on the methods themselves but on the core problem of the practical daily routine, namely the qualitative and quantitative description of hearing loss in terms of its extent, the frequency range affected and the identity with respect to the type and site of the lesion. The certainty of the diagnosis can be optimized by observing few and simple rules. Central importance is attached to the thorough discussion of the interrelated parameters residual noise, signal-to-noise ratio and reproducibility in order to promote their correct use. Further subjects of this article are the recognition of pitfalls, the description of new developments, such as chirp and auditory steady-state responses (ASSR) and finally the consideration of the extraordinarily important aspect of maturation.Objective methods based on the measurement of otoacoustic emissions (OAE) and auditory evoked potentials (AEP) are indispensable in pediatric audiology especially for the early detection and therapy of congenital hearing impairment. The correct and efficient use requires knowledge and skills which are beyond the basic equipment of many users and require continuous updating. In the present review many aspects relevant for the safe handling of the methods and interpretation of the results are addressed. The presentation does not focus on the methods themselves but on the core problem of the practical daily routine, namely the qualitative and quantitative description of hearing loss in terms of its extent, the frequency range affected and the identity with respect to the type and site of the lesion. The certainty of the diagnosis can be optimized by observing few and simple rules. Central importance is attached to the thorough discussion of the interrelated parameters residual noise, signal-to-noise ratio and reproducibility in order to promote their correct use. Further subjects of this article are the recognition of pitfalls, the description of new developments, such as chirp and auditory steady-state responses (ASSR) and finally the consideration of the extraordinarily important aspect of maturation.