Distortion-product otoacoustic emissions in the bobtail lizard. I: General characteristics

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Abstract
In the ear canal of the bobtail lizard, both emitted harmonics and distortion products (DP) were measured, some of which were sensitive to hypoxia. The minimal levels of primary tones for the production of DP corresponded closely to the best auditory-nerve fiber thresholds for this lizard species. DP originating from the two segments of the basilar papilla (which respond to different frequency ranges) differed in a number of respects. DP from the higher-frequency segment bore a remarkable resemblance to the acoustic DP of mammals in many details, in spite of substantial structural and micromechanical differences between these hearing organs, suggesting that it is the characteristics of the hair cells themselves that are the most important factors determining DP generation. Also, DP clearly originated in the high-frequency region of this lizard papilla, where single-nerve fibers do not show two-tone rate suppression, another nonlinear phenomenon. In some cases, the DP level was clearly affected by the presence of spontaneous otoacoustic emissions. The frequency selectivity of the DP 2 f1−f2 and 2 f2−f1 in the bobtail lizard was also studied, by measuring thresholds of DP detection while varying the frequency ratio of the primary tones. The characteristics of tuning in high-frequency DP showed some remarkable similarities to the tuning curves of single-nerve fibers, suggesting that the characteristics of acoustic DP produced by low-level primary tones were determined by the tuning characteristics of the hearing organ at the hair-cell level.

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