Low-level distortion product otoacoustic emissions in lizards are influenced by spontaneous activity of the inner ear

Distortion product otoacoustic emissions (DPOAEs) from lizards can display a rapid variation in intensity level upon changes in the distortion frequency. The level functions of both cubic distortion products elicited by low-to moderate-level stimuli exhibit pronounced notches across range of the distortion frequencies. To understand the variation in the distortion level, we measured DPOAEs from tokay geckos using primary tones with intensity levels below 50 dB SPL and demonstrated that the level of the distortion signals was strongly correlated with the amplitude spectrum of spontaneous otoacoustic emissions (SOAEs). The DPOAE intensity level was significantly enhanced when the distortion frequency coincided with the center frequency of an SOAE peak. Manipulations of the body temperature further revealed that peaks in the SOAE amplitude spectrum and the DPOAE level functions displayed systematic shifts to lower frequencies of comparable magnitudes upon cooling. Preliminary results from a model of a chain of coupled nonlinear oscillators suggested that an enhanced distortion signal was displayed by the oscillators whose characteristic frequencies were near the distortion frequency.