Effects of Tympanic Membrane Modification on Distortion Product Otoacoustic Emissions in the Cat Ear Canal

Abstract

Acoustic distortion-product (DP) signals recorded in the external ear canal reflect nonlinearities in cochlear mechanics. The level of D P signals is reduced by processes known to damage the organ of Corti, including noise exposure (Kim et al., 1980, Zurek et al., 1982, Siegel et al., 1982, Rosowski et al., 1984, Dolan and Abbas, 1985, Schmiedt, 1986, Wiederhold et al., 1986, Martin et aI., 1987). Thus, DP measurement has been suggested as a diagnostic tool to assess the physiologic state of the organ of Corti (Probst, 1990). Changes in middle-ear stiffness have been shown to alter otoacoustic emissions (e.g., Kemp, 1981). We have noted large variation in the level and threshold for detection of ear-canal DP’s in humans with normal or near-normal hearing. We sec less variability in cats selected for normal-appearing tympanic membranes (TM), whereas cats with scarred or thickened TM’s have lower DP’s which require high primary-tone levels to become detectable. This suggests that an important component of DP variability is related to TM or middle-car pathology. Here we report results of studies in which the TM was modified, either by adding mass or by causing a leak with a small perforation. The aim was to assess the effccts of these modifications on forward and reverse transmission through the middle ear and to determine if such modifications might have disproportionate effects on ear-canal DP signals.