Examining the Factors that Contribute to Non-Monotonic Growth of the  Otoacoustic Emission in Humans

Cubic distortion product otoacoustic emission input-output functions in humans show a complex pattern of growth. To further investigate the growth of the $$ otoacoustic emission, magnitude and phase input-output functions were obtained from human subjects using a range of stimulus levels, frequencies, and frequency ratios. Three factors related to cochlear nonlinearity may produce non-monotonic input-output functions: a two-component interaction, an operating point shift, and two-tone suppression. To complement data interpretation, a local model of distortion product otoacoustic emission generation was fit to the magnitude spectrum of the averaged ear canal sound pressure recording to quantify operating point shift. Results obtained are consistent with non-monotonic growth occurring primarily as a result of two-tone suppression and/or a two-component interaction. These two mechanisms are expected to operate at different stimulus levels, with different signature magnitude and phase patterns, and are unlikely to overlap in producing non-monotonic growth. An operating point shift was suggested in three cases. These results support multiple factors contributing to the complexity of growth of the $$ otoacoustic emission in humans and highlight the importance of looking at phase in addition to magnitude when interpreting distortion product otoacoustic emission growth.