gms | German Medical Science

In previous reports we described the adaptation properties of the auditory periphery in the rat in response to repetitive acoustic stimulation. To achieve this purpose, auditory near-field evoked potentials were recorded from an electrode chronically implanted in the ventral cochlear nucleus (VCN). In our first reports, adaptation properties had been established for auditory nerve by considering the first one (N₁) of two negative deflections of these potentials, this wave being widely accepted to represent the summed activity of primary auditory neurons. Based on the general opinion that the second negative deflection (N₂) is generated by secondary auditory neurons located in the cochlear nucleus, the adaptive behavior of this second peak was now also quantified in order to assess its properties of adaptation as a function of repetition rate and intensity of repetitive acoustic stimulation. Stimuli were 250-ms trains of repetitive clicks (100 μsec) presented at rates between 100 and 1'000 pulses per second and intensities ranging from 5 to 70 dB SPL. The adaptation time course for the N₂ peak displayed the same three stages previously established for N₁: an initial rapid adaptation, followed by a short-term adaptation and, finally, a steady state. Likewise, the effects of varying stimulus parameters were, except for minor discrepancies, similar to those observed for N₁: adaptation became more pronounced and occurred faster as stimulus intensity and / or repetition rate were increased. We conclude that adaptation patterns in the first two auditory relays are very similar. Our results suggest that an electrical stimulus paradigm developed for cochlear implants, such as the one established by Loquet et al. [Ref. 1], may also be usable in terms of direct artificial (electrical) stimulation of the brainstem at the level of the VCN in order to reproduce natural acoustic adaptation. However, only a detailed empirical study would allow to definitely validate the applicability of such a coding strategy in the cochlear nucleus or, if necessary, to further refine the paradigm.