Spontaneous otoacoustic emissions in the bobtail lizard. I: General characteristics

Spontaneous otoacoustic emissions in the external ear canal of the bobtail lizard were identified on the basis of their consistent presence, their temperature- and hypoxia-dependence, and their suppressibility by external tones. They were found in 86% of ears investigated, and each ear generated on average 10 emissions. Their sound-pressure levels lay between -10 and 9.3 dB SPL, and their centre frequencies between 0.93 and 4.61 kHz at 30 degrees C body temperature. Previous studies have shown that these frequencies are processed in the basal basilar-papillar segment by hair-cell areas that are strictly bidirectionally oriented and are covered by tectorial sallets. In contrast, no spontaneous otoacoustic emissions were found in the frequency range known to be processed by the apical, low-frequency segment of the basilar papilla. The mean frequency distance between emissions varied systematically across the frequency range in a way consistent with the hypothesis that they are generated by anatomically-defined groups of hair cells and their tectorial sallets. The 3dB-bandwidth of the emissions depended on their amplitude above the noise, but was at least 9 Hz.