Triphasic electrical stimulation pulses with an adjustable phase amplitude ratio (PAR) can reduce stimulus artifacts in electrically evoked compound actions potentials (ECAPs) recording in the cochlea (see companion paper Bahmer and Baumann, submitted for publication). The present study describes the application of triphasic pulses in forward masking paradigms for recording recovery functions. Masking was found to be most effective using equal masker-probe PAR settings. Results were compared with data applying artifact cancellation strategy for biphasic pulses according to Miller et al. (2000). Measurements were accomplished in five subjects (S1-S5) with an equal masker-probe PAR setting, whereby more detailed test series were carried out in one subject (S1). All subjects were users of MED-EL SONATAti100 or PULSARci100 cochlear implants (Innsbruck, Austria). Parameters like asymptote level, absolute refractory period and time constant were determined by fitting exponential functions to the recovery functions. Detailed measurements collected on 11 electrode locations in subject S1 showed similar parameter profiles on basal electrode contacts for both triphasic and Miller artifact cancellation methods, whereas apical/middle electrode contacts differed in part largely. Compared to Miller's artifact cancellation method estimated asymptote levels were lower with triphasic stimulation; the estimated absolute refractory period and time constants were estimated higher on apical electrodes. Results obtained in subjects S2-S5 showed considerable variances and a proper parametrization of the recovery function was possible only very selectively for triphasic pulse stimulation. In these cases, congruencies in estimated asymptote levels and time constants were found when triphasic stimulation and biphasic stimulation according to Miller were compared.
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