Comparative study of the frequency following response-distortion products and distortion product otoacoustic emissions in normal hearing adults

The input/output behavior and the effects of contralateral and ipsilateral acoustic stimulation for the simultaneously recorded DPOAE and the frequency following response distortion products (FFR-DP) were evaluated to determine if these two representations of cochlear nonlinearity share the same cochlear generators. Responses were recorded from normal hearing adults for a stimulus pair of F1: 500 Hz; F2: 610 Hz, presented at levels of 40-70 dB nHL. For the contralateral experiment, the stimulus pair was presented at 50 dB nHL in the presence of three different noises: WBN (50-8000 Hz), NBN centered at F2, and NBN centered at 2F1-F2. For the ipsilateral experiment a suppressor tone (F3) was presented at frequencies close to 2F1-F2, at the geometric mean of F1 and F2, and one octave above that geometric mean. Results revealed differences in the input/output functions of DPOAE and FFR-DP responses, presumably reflecting differences in the levels of contribution from cochlear generators. Results of contralateral stimulation demonstrated differences in the suppression and enhancement patterns for both responses, indicating that both FFR-DP and DPOAE reflects normal active cochlear function, and suggesting differences in the components contributing to the generation of the two responses. Results of ipsilateral suppression demonstrated differences in the suppression and enhancement patterns between FFR-DP and DPOAE, suggesting that FFR-DP reflects more contribution from the cochlear partition at the DP tonotopic place