Comparison of the acoustic and neural distortion product at 2f1-f2 in normal-hearing adults

Input/output functions of the simultaneously recorded acoustic distortion product otoacoustic emissions (DPOAE) and neural frequency following-response distortion products (FFR-DP) at 2f1-f2 were evaluated to determine if these two representations of cochlear nonlinearity exhibit similar response behavior, which would suggest shared cochlear generators. Responses were recorded from normal-hearing adults for a tone burst stimulus pair (F1: 500 Hz; F2: 612 Hz) at 40-70 dB nHL. DPOAE responses were recorded from the ear canal, and FFR responses were recorded differentially from scalp electrodes, representing a vertical configuration. The input/output function for FFR-DP revealed a compressive saturating nonlinearity, whereas the DPOAE input/output function exhibited a linear growth at higher intensities following a compressive behavior at moderate levels. Results appear to suggest that cochlear generators may be contributing differentially to the acoustic and the neural distortion products. Also, FFR-DP responses appeared more identifiable and less variable, particularly at lower stimulus levels, than the corresponding DPOAE. These findings may point to a potential benefit of applying FFR testing to complement DPOAE in evaluating cochlear function at low frequencies.