Neuronal response to cochlear distortion products in the anteroventral cochlear nucleus of the gerbil

To receive information on the intracochlear magnitude and propagation prope rties of cochlear distortion products, the neuronal response of AVCN cells to distortion stimuli and the acoustical correlates in the ear canal (DPOAE ) were measured for frequencies between 0.7 and 45.3 kHz. Comparison of the growth of neuronal response to a distortion stimulus and the neuronal rate -versus-level function for an externally applied pure tone of equal frequen cy allowed for an assessment of the intracochlear magnitude of the distorti ons. AVCN neurons with a characteristic frequency (CF) >1.8 kHz started to respond to the intracochlear distortion stimulus, at primary levels for whi ch the ear-canal level of the corresponding DPOAE was close to the pure-ton e threshold of the units. This finding suggests that transmission of sound energy is comparable in the forward and reverse direction, and that mechani cal distortions of the cochlea are fully encoded by neurons in the AVCN. Fo r neurons with a CF <1.8 kHz, the intracochlear magnitude of the distortion stimulus appeared to be about 15 to 30 dB higher than the corresponding DP OAE, at the threshold of neuronal response. This discrepancy between intrac ochlear magnitude of cochlear distortions and their acoustical expression m ay be explained by high-pass filter action of the middle ear during DPOAE r e-emission from the cochlea. A contribution to the observed discrepancy of the type of distortion (cubic versus quadratic) used as stimulus, and possi ble differences in mechanical frequency processing between the apex and bas e of the gerbil cochlea, are also discussed. The delay of the neuronal resp onse to an intracochlear distortion stimulus was on average 1.1 ms longer t han the neuronal delay to an external pure tone of equal frequency and inte nsity, most likely stemming from the activation delay of the DPOAE generati ng mechanisms. (C) 1999 Acoustical Society of America. [S0001-4966(99)01301 -6].