Zusammenfassung
Hintergrund: Der Einfluß kontralateral applizierten weißen Rauschens auf transitorisch evozierte
oto-akustische Emissionen (TEOAE) wird von verschiedenen Autoren in Hinblick auf einen
Beitrag cochleärer Efferenzen diskutiert. Methoden: In den vorliegeneden Untersuchungen erfolgte eine ipsilaterale Registrierung der
TEOAE unter Verwendung verschiedener Clickpegel bei gleichzeitiger kontralateraler
Stimulation mit weißem Rauschen verschiedener Pegel. In einer weiteren Meßreihe wurde
das Verhalten der TEOAE nach kontralateraler Stimulation untersucht. Ergebnisse: Für wachsende kontralaterale Stimulation nahm der Pegel der TEOAE ab, gleichzeitig
stieg der Rauschuntergrund an. Die TEOAE erreichten nach Beendigung der kontralateralen
Stimulation den Ausgangszustand innerhalb weniger Minuten, demgegenüber lag auch nach
10 min noch ein erhöhter Rauschuntergrund vor. Schlußfolgerungen: Die Suppression der TEOAE bei gleichzeitigem Ansteigen des Rauschuntergrundes während
kontralateraler Stimulation läßt sich schlüssiger auf eine Änderung der Übertragungseigenschaften
des Mittelohres als auf die Aktivität der cochleären Efferenzen zurückführen. Weitere
Einflüsse, wie Veränderungen des vegetativen Systems, Überhören und Maskierungseffekte
werden diskutiert.
Summary
Background: In recent publications the influence of contralateral white noise on transient evoked
otoacoustic emissions (TEOAE) is discussed with regard on contributions of the efferent
auditory system. Methods: In the present study the effects have been investigated with regards to middle-ear
muscles, efferents and cross hearing. TEOAE to monaural 40-80 dB SPLclicks were recorded
in normal-hearing adults under simultaneous presentation of 20-60dB SPL broadband
noise to the contralateral ear. Control runs were performed before, during a short
break of, and after contralateral stimulation. The control run before contralateral
stimulation was used as a reference. Results: Decrease in TEOAE, and increase in accompanying noise floor, were found to follow
the contralateral stimulation. In particular a 1-3dB decrease was found for contralateral
noise levels of 40 and 60 dB SPL, even though the readings at 60 dB only were statistically
significant (paired-samples t test, p = 0.05). For both TEOAE and noise floor no systematic
dependence on dick intensity was seen. The control runs during temporary break and
after contralateral noise revealed an increase in both TEOAE and noise floor. As a
rule, the TEOAE adapted to the reference within 2-3 min following the cessation of
contralateral stimulation, whereas the increased noise floor level was still noted
after 10 min. Conclusions: Traditionally, suppressing effects of contralateral stimulation on TEOAE have been
attributed to cochlear efferents (CEs). Occasionally, the middle-ear muscle and cross
hearing involvement have been considered as well. Substantially, the present results
and findings of other workers are inconsistent with the basic knowledge of CE functioning:
(i) The decrease in TEOAE under contralateral stimulation is in conflict with an increase
in cochlear microphonics and summating potentials observed during activation of CEs;
(ii) contralateral suppression of TEOAE exhibited no significant dependence on the
test-stimulus level while the CEs are known to be efficient in the range of the low
signal intensities only, and (IM) acoustic activation of the CEs can hardly be expected
to reach levels sufficient to influence the TEOAE mechanism. The present findings,
i.e. decrease in TEOAE and increase in noise floor level, can more reasonably be explained
as being mainly attributable to activation of the middle-ear muscles.
Schlüsselwörter
TEOAE - Kontralaterale Beeinflussung - Mittelohrmuskeln - Efferente Innervation -
Cochleäre Efferenzen - Stapediusreflex
Keywords
TEOAE - Noise floor - Contralateral influence - Cochlear efferents - Olivocochlear
bundle - Middle-ear muscles
