SPONTANEOUS OTOACOUSTIC EMISSIONS IN THE BOBTAIL LIZARD .2. INTERACTIONS WITH EXTERNAL TONES

The response of spontaneous otoacoustic emissions to the presentation of external tones was studied in the Australian bobtail lizard. Three basic types of effects were observed: suppression (a reduction in the emission's amplitude), facilitation (an increase in the emission's amp litude) and frequency shifting. The suppressive effect was highly freq uency selective. Iso-suppression tuning curves resembled the rate-thre shold tuning curves of the high-frequency population of VIIIth nerve f ibres in this species. The frequency with the lowest threshold for sup pression corresponded, on average, to the emission's own frequency and did not show any systematic deviation from it. Facilitation of betwee n 2 and 10 dB occurred, but only in response to frequencies within cer tain narrow ranges, and at sound pressure levels below those that supp ressed. The most commonly-observed facilitation range lay between 0.2 and 0.6 octaves above the emission's own frequency and coincided in fr equency with a characteristic notch in the iso-suppression tuning curv e. In the same narrow frequency range, the input/output functions of a mplitude suppression always showed a pronounced increase in slope. The emissions moved their own frequency away from that of an external ton e. The observed shifts were comparatively large (up to -330 Hz) and we re more pronounced in the downward direction.