THE POSSIBLE RELATIONSHIP BETWEEN TRANSIENT EVOKED OTOACOUSTIC EMISSIONS AND ORGAN OF CORTI IRREGULARITIES IN THE GUINEA-PIG

Otoacoustic emissions are believed to arise from an active process ass ociated with the outer hair cells in the mammalian organ of Corti. The y have been attributed to the presence of impedance discontinuities on the basilar membrane which might be caused by hair cell irregularitie s. To test this hypothesis we have investigated the possible relations hip between transient evoked otoacoustic emissions (TEOAEs) and anatom ical integrity in the organ of Corti. Click-evoked TEOAEs have been me asured from the ear canals of normal, pigmented guinea pigs using an O todynamics ILO88 analyser. Emissions were present in 18 out of 19 anim als tested and the major frequencies observed were consistently presen t in different measurements over periods of up to ten weeks provided r ecording conditions were satisfactory. The frequency spectra of the TE OAEs resembled those measured in humans but the latencies of the respo nses were considerably shorter. In one acute experiment, the TEOAEs we re shown to be dependent on metabolic energy as they were lost rapidly following termination with an overdose of anaesthetic. In another cas e, evoked emissions of long duration (sustained) at about 1 KHz were o btained from both ears. All cochleae examined showed irregularities, e specially patches of mainly apical outer hair cell loss of differing e xtents. However, there was no evidence that substantial lesions coinci ded consistently with the frequency regions corresponding to the major emissions. Nevertheless, it was noted that the total energy level of emissions was proportional to the total outer hair cell loss, except i n one case, where the outer hair cell loss was substantial and the ene rgy level of TEOAEs was considerably lower. Although there is no clear relationship between TEOAEs of specific frequencies and abnormalities at the corresponding cochloetopic location in the organ of Corti whic h could represent impedance discontinuities, the degree of irregularit y may determine the overall emission level. This finding is consistent with the idea that emissions arise as a result of irregularity produc ing variations in the reflection coefficient.