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Current cochlear implant systems use fast pulsatile stimulation to deliver the temporal modulations of speech and to, potentially, improve the neural representation of such modulations by restoring the independence of neural firing. The realization of these benefits may vary with other pulse rate-dependent temporal interactions that occur at the neural membrane, e.g., per(i)stimulatory adaptation and its post-stimulatory or forward masking effects. This study attempted to characterize adaptation and recovery of the electrically evoked compound action potential (ECAP) using probe pulses delivered within and following brief (100 ms) high-rate masker (1800 pps) pulse trains at various current levels in adults and children.
With this stimulus paradigm, the ECAP amplitude typically achieved a steady state during the course of pulse train stimulation. The ECAP amplitude at steady state was, on average, a similar proportion (50-70%) of the amplitude at onset for various stimulus levels and in both age groups. However, long-term adaptation effects, evidenced by the decrease in onset ECAP amplitude, were greater in adults particularly at lower levels in the ECAP dynamic range. Instances of alternation in ECAP amplitude were seen at stimulus levels that were higher in the ECAP dynamic range.
The forward masking effects of pulse train stimulation were quantified by the ECAP amplitude in response to a subsequent probe pulse normalized by the response to the same pulse presented alone. Pulse train forward masking increased with the level of the masker pulse train and decreased with the level of the probe stimulus. The recovery of the ECAP for probes that were lower in level than the masker pulse train was incomplete at 600 ms after masker offset, consistent with long-term cumulative effects observed in the response to the probe alone. Masker pulse trains that are lower in level than the probe pulse produced proportionally small decrements in the ECAP amplitude with complete recovery within 250 ms of pulse train offset particularly in adults. ECAP recovery of a probe preceded by a masker pulse train of equal level followed a monotonic or non-monotonic pattern consistent with a hypothesis of both adaptation and facilitation occurring with pulse train stimulation. The various patterns of recovery may attest to the occurrence of more than a single process in the same subset of nerve fibers or in different fibers. We hypothesize that the variations in the recovery patterns may be attributable to individual differences in the status of the auditory nerve and possibly, the variations in temporal interactions across the spatial domain at different stimulus levels.
Finally, the probe-evoked ECAP amplitude at steady state in children and briefly, e.g., 20 ms, after pulse train offset in both age groups could be predicted by the ECAP amplitude in response to the same probe pulse when preceded at a brief interval (1.2 or 2 ms) by a single masker pulse of the same level as the masker pulse train.
Further investigation may reveal if the observed differences in neural responsiveness to pulsatile stimulation, among individuals account for differences in psychophysical measures, including speech perception and whether there may be an "optimal" neural output that could be evoked by an individually "optimized" signal.