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

To receive information on the intracochlear magnitude and propagation properties 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 frequency allowed for an assessment of the intracochlear magnitude of the distortions. AVCN neurons with a characteristic frequency (CF) > 1.8 kHz started to respond to the intracochlear distortion stimulus, at primary levels for which the ear-canal level of the corresponding DPOAE was close to the pure-tone threshold of the units. This finding suggests that transmission of sound energy is comparable in the forward and reverse direction, and that mechanical distortions of the cochlea are fully encoded by neurons in the AVCN. For 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 DPOAE, at the threshold of neuronal response. This discrepancy between intracochlear magnitude of cochlear distortions and their acoustical expression may be explained by high-pass filter action of the middle ear during DPOAE re-emission from the cochlea. A contribution to the observed discrepancy of the type of distortion (cubic versus quadratic) used as stimulus, and possible differences in mechanical frequency processing between the apex and base of the gerbil cochlea, are also discussed. The delay of the neuronal response to an intracochlear distortion stimulus was on average 1.1 ms longer than the neuronal delay to an external pure tone of equal frequency and intensity, most likely stemming from the activation delay of the DPOAE generating mechanisms.