J. Pethe

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.

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.

Near-threshold recordings of amplitude modulation following responses (AMFR) in children of different ages

Amplitude modulation following responses (AMFR) to single near-threshold 40-Hz or 80-Hz amplitudemodulated tones of 1 kHz were recorded in 48 normal children between the ages of 2 months and 14 years. Children under the age of 2 years were tested during natural sleep, and older children were tested awake. The objectives of this study were to find out how the AMFR changes with age and to determine the most effective modulation frequency for objective threshold assessment at different ages. The optimal modulation frequency changed from higher to lower modulation frequencies at about 13 years. In younger children, the 80-Hz AMFR was larger than the 40-Hz AMFR. The 40-Hz response became similar to the adult response by the age of 14 years, while the 80-Hz response had an amplitude greater than half of an adult response by the age of 1 year, changed very little during the investigated period, and reached the amplitude of adult responses at the end of the investigated period. Sumario En 48 nin˜os normales de 2 meses a14 an˜os de edad, se registraron las respuestas consecutivas a modulacio´n de amplitud (AMFR) a tonos u´nicos de 1 kHz casi-umbral de amplitud modulada a 40-80 Hz. Los nin˜os menores a dos an˜os fueron examinados mientras dormi´an naturalmente y los mayores estando despiertos. Los objetivos de este estudio fueron: conocer la forma en que las AMFR cambian con la edad y obtener la frecuencia de modulacio´n ma´s efectiva para la determinacio´n del umbral objetivo a diferentes edades. La frecuencia o´ptima de modulacio´n se modifico´ de alta a baja cerca de los 13 an˜os. En nin˜os menores, la AMFR a 80 Hz fue mayor que la AMFR a 40 Hz. La respuesta a 40 Hz mostro´ similitud a la del adulto a la edad de 14 an˜os, mientras que la respuesta a 80 Hz mostro´ una amplitud mayor que la mitad de la respuesta del adulto al an˜o de edad, cambio´ muy poco durante el tiempo investigado y alcanzo´ la amplitud de la respuesta del adulto al final del periodo de la investigacio´n.

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.

Influence of Electrode Position on Near-Threshold Recording of Auditory Evoked Brainstem Potentials

Use of brainstem potentials in audiology is based on identifying a response in the near-threshold range. Wave V has turned out to be the component of the brainstem potentials that can be detected with a great deal of reliability when assessment of the threshold for the particular stimulus is desirable. As the detection of Wave I can be dispensed within audiologic diagnosis, the question arises as to whether or not a non-cephalic reference electrode compared to the common lateral position of the reference electrode on the ipsilateral mastoid produces a more stable Wave V of increased amplitude. In a group of 20 normal-hearing adults, near-threshold stimulation with clicks was conducted and the brainstem potentials recorded simultaneously with the reference electrode placed in four different locations (ipsilateral mastoid, contralateral mastoid, non-cephalic electrode, and ipsilateral earlobe). At all of the three intensities studied (10 dB nHL, 20 dB nHL and 30 dB nHL), recording with the non-cephalic reference yielded the highest amplitudes, the finding being statistically significant.

Abschätzung der Hörschwelle mit stationären auditorischen evozierten Potentialen: Probleme der digitalen Signalverarbeitung

Zusammenfassung Die objektive Bestimmung des Horvermogens mit stationaren auditorischen evozierten Potentialen (AEP) erfordert die Einhaltung grundlegender Prinzipien der digitalen Signalverarbeitung. In der vorliegenden Arbeit werden die wesentlichen Unterschiede bei der Registrierung transienter und stationarer AEP dargestellt und daraus Bedingungen fur eine optimale Registrierung stationarer AEP im Frequenzbereich abgeleitet. Rohdaten von 40-Hz-Click-AEP und 40-Hz-AMER (Amplitude Modulation Following Response) an 5 bzw. 11 normalhorenden Probanden fur Stimulationspegel von 10 … 80 dB (nHL) wurden offline ausgewertet. Fur einen Frequenzbereich von 30 … 50 Hz wurde die Reststorung als Funktion der analysierten Periodenzahl berechnet. Es konnte gezeigt werden, das dieser schmalbandige Schatzwert der Reststorung unempfindlich gegen tieffrequente EEG-Anteile ist. Fur Meszeiten von 100 s wurden Amplituden und SNR-Schatzwerte als Funktion des Stimulationspegels berechnet.

Resonant Frequency Pattern in Multifrequency Tympanograms: Results in Normally-Hearing Subjects: Patrón de resonancia frecuencial en timpanogramas de multifrecuencia: Resultados en normoyentes

This paper presents experimental data on the evaluation of middle ear resonances by multifrequency tympanometry. Multifrequency tympanograms (MFTs) of 18 normally-hearing subjects were recorded with a frequency resolution of 15 Hz. The fine structure found in the MFT patterns was compared with findings in literature. A first approach for the evaluation of this fine structure was made explaining the great variability of the main ossicular resonance frequencies described in previous publications. The consequence of the present investigation is that the concept of the main ossicular resonance has to be revised critically.