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 amplitude) and frequency shifting. The suppressive effect was highly frequency selective. Iso-suppression tuning curves resembled the rate-threshold tuning curves of the high-frequency population of VIIIth nerve fibres in this species. The frequency with the lowest threshold for suppression corresponded, on average, to the emission's own frequency and did not show any systematic deviation from it. Facilitation of between 2 and 10 dB occurred, but only in response to frequencies within certain narrow ranges, and at sound pressure levels below those that suppressed. The most commonly-observed facilitation range lay between 0.2 and 0.6 octaves above the emission's own frequency and coincided in frequency with a characteristic notch in the iso-suppression tuning curve. In the same narrow frequency range, the input/output functions of amplitude suppression always showed a pronounced increase in slope. The emissions moved their own frequency away from that of an external tone. The observed shifts were comparatively large (up to -330 Hz) and were more pronounced in the downward direction.