DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS IN THE BOBTAIL LIZARD .1. GENERAL-CHARACTERISTICS

In the ear canal of the bobtail lizard, both emitted harmonics and dis tortion products (DP) were measured, some of which were sensitive to h ypoxia. The minimal levels of primary tones for the production of DP c orresponded closely to the best auditory-nerve fiber thresholds for th is lizard species. DP originating from the two segments of the basilar papilla (which respond to different frequency ranges) differed in a n umber of respects. DP from the higher-frequency segment bore a remarka ble resemblance to the acoustic DP of mammals in many details, in spit e of substantial structural and micromechanical differences between th ese hearing organs, suggesting that it is the characteristics of the h air cells themselves that are the most important factors determining D P generation. Also, DP clearly originated in the high-frequency region of this lizard papilla, where single-nerve fibers do not show two-ton e rate suppression, another nonlinear phenomenon. In some cases, the D P level was clearly affected by the presence of spontaneous otoacousti c 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. T he characteristics of tuning in high-frequency DP showed some remarkab le similarities to the tuning curves of single-nerve fibers, suggestin g that the characteristics of acoustic DP produced by low-level primar y tones were determined by the tuning characteristics of the hearing o rgan at the hair-cell level.