Interindividual variation of sensitivity to frequency modulation: its relation with click-evoked and distortion product otoacoustic emissions.

The frequency modulation detection limen (FMDL) with a low modulation rate has been used as a measure of the listener's sensitivity to the temporal fine structure of a stimulus, which is represented by the pattern of neural phase locking at the auditory periphery. An alternative to the phase locking cue, the excitation pattern cue, has been suggested to contribute to frequency modulation (FM) detection. If the excitation pattern cue has a significant contribution to low-rate FM detection, the functionality of cochlear mechanics underlying the excitation pattern should be reflected in low-rate FMDLs. This study explored the relationship between cochlear mechanics and low-rate FMDLs by evaluating physiological measures of cochlear functions, namely distortion product otoacoustic emissions (DPOAEs) and click-evoked otoacoustic emissions (CEOAEs). DPOAEs and CEOAEs reflect nonlinear cochlear gain. CEOAEs have been considered also to reflect the degree of irregularity, such as spatial variations in number or geometry of outer hair cells, on the basilar membrane. The irregularity profile could affect the reliability of the phase locking cue, thereby influencing the FMDLs. The features extracted from DPOAEs and CEOAEs, when combined, could account for more than 30 % of the inter-listener variation of low-rate FMDLs. This implies that both cochlear gain and irregularity on the basilar membrane have some influence on sensitivity to low-rate FM: the loss of cochlear gain or broader tuning might influence the excitation pattern cue, and the irregularity on the basilar membrane might disturb the ability to use the phase locking cue.