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The Conceptual Basis for the Design of an Auditory Prosthetic Device

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The cochlea is designed to receive informational quanta (elementary signals) defined over four signal parameters by the bounding conditions: f 0 t 0 = Δf Δt = ½, where f 0 is signal mid-frequency, fa is signal mid-period, Δf is signal bandwidth, and Δt is signal duration. Four parameter elementary signal detection and ieception permits the encoding of signal amplitude modulation, besides simple signal frequency. Essentially, the cochlea samples both frequency and also amplitude modulation.

Physically, the sampling of amplitude modulation is accomplished by the hydromechanical arrangements of force flows due to the curvature of the cochlea (a Cornu spiral). The first type of cell in the cochlea, the inner hair cell, is stimulated by the hydromechanical force due to a travelling wave set up by the frequency components of the signal at a place determined by the exponentially changing elastic component of the basilar membrane. The hydromechanical force due to a travelling wave set up by amplitude modulated components of the signal is the stimulus to the second type of cell in the cochlea: the outer hair cell.

Due to the curvature of the cochlea, an amplitude modulated signal, such as the simplest, an elementary signal, will elicit two travelling waves, which will stimulate the two types of hair cells. A sinusoidal signal will elicit only one travelling wave which will stimulate the inner hair cells and the outer hair cells less than maximally.

In this paper, the theoretical background for the fluid mechanical analysis of elementary signals is presented and explained in terms of statistical hydromechanics and functional calculus. This analysis is completely general, and defines the conceptual problem for any auditory prosthetic device which aspires to approximate normal auditory processing. Because the electrodes of an electrical stimulation device must stimulate the same places stimulated hydromechanically by travelling waves to achieve analogous reception of the same stimulus, the analysis offered here is immediately relevant to the design of a stimulation prosthetic device.

Zusammenfassung

Die Cochlea ist zum Empfang von Informationsquanten (Elementarsignale) bestimmt, welche über vier Signalparameter durch folgende Randbedingungen definiert sind: f 0 t 0 = Δf Δt = 1/2, wobei f 0 die Signalmittenfrequenz, t 0 die entsprechende Periode, Δf die Bandbreite des Signals und Δt die Signaldauer ist. Der Empfang und die Wahrnehmung von Elementarsignalen auf der Grundlage von vier Parametern gestattet die Kodierung der Amplitudenmodulation des Signals neben der einfachen Signalfrequenz. Im wesentlichen tastet die Cochlea sowohl die Frequenz als auch die Amplitudenmodulation ab.

Auf Grund der Krümmung der Cochlea werden durch ein amplitudenmoduliertes Signal wie — im einfachsten Fall — ein Elementarsignal zwei Wanderwellen entstehen, welche die zwei Arten von Haarzellen stimulieren. Ein sinusförmiges Signal wird nur eine Wanderwelle auslösen, welche die inneren Haarzellen stimuliert.

In dieser Arbeit wird der theoretische Hintergrund für die hydromechanische Analyse von Elementarsignalen vorgestellt und mit den Begriffen der statistischen Hydromechanik und der mathematischen Analysis beschrieben. Diese Analyse ist völlig allgemein und definiert die begrifflichen Probleme für jegliche Hörprothese, welche den normalen Hörprozeß näherungsweise nachbilden soll. Um gleichartige Wahrnehmung des gleichen Reizes zu erreichen, müssen die Elektroden eines elektrischen Stimulators die gleichen Stellen reizen wie die, welche von Wanderwellen hydromechanisch stimuliert werden. Aus diesem Grund ist die hier vorgestellte Analyse von unmittelbarer Bedeutung für den Bau einer Stimulationsprothese.

Sommaire

La cochlee est destinée à recevoir des quanta d'information (signaux élémentaires) définis selon quatre paramètres de signal liés par les conditions: f 0 t 0 = Δf Δt = 1/2, où f 0 est la fréquence médiane du signal, t 0 sa période médiane, Δf sa largeur de bande et Δt sa durée. La détection et la réception des signaux élémentaires à quatre paramètres permettent le codage de la modulation d'amplitude d'un signal quelconque en plus de la fréquence d'un signal simple. Le rôle essentiel de la cochlée est d'enregistrer ensemble la fréquence et la modulation d'amplitude.

Du fait de la courbure de la cochlée, un signal modulé en amplitude, comme du reste le signal élémentaire le plus simple, y excitera deux ondes progressives qui stimuleront les deux types de cellules ciliées. Mais un signal sinusoïdal n'excitera, par contre, qu'une seule onde progressive, celle qui viendra stimuler les cellules ciliées internes.

Le présent article expose les fondements théoriques d'une analyse des signaux élémentaires de la mécanique des fluides et les explicite par les moyens de l'hydromécanique statistique et du calcul fonctionnel. Cette analyse est d'une généralité totale et définit le problème conceptuel qui se pose pour n'importe quel projet de prothèse auditive dont l'objectif est d'approcher au mieux le processus de l'audition normale. Dans un appareil à stimulation électrique, les électrodes devront stimuler les mêmes places que les ondes progressives hydromécaniques correspondant au même stimulus, de sorte que l'analyse présentée ici s'applique immédiatement aux projets de prothèses à stimulation cochléaire.
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Document Type: Research Article

Publication date: February 1, 1983

More about this publication?
  • Acta Acustica united with Acustica, published together with the European Acoustics Association (EAA), is an international, peer-reviewed journal on acoustics. It publishes original articles on all subjects in the field of acoustics, such as general linear acoustics, nonlinear acoustics, macrosonics, flow acoustics, atmospheric sound, underwater sound, ultrasonics, physical acoustics, structural acoustics, noise control, active control, environmental noise, building acoustics, room acoustics, acoustic materials, acoustic signal processing, computational and numerical acoustics, hearing, audiology and psychoacoustics, speech, musical acoustics, electroacoustics, auditory quality of systems. It reports on original scientific research in acoustics and on engineering applications. The journal considers scientific papers, technical and applied papers, book reviews, short communications, doctoral thesis abstracts, etc. In irregular intervals also special issues and review articles are published.
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