Abstract
We investigated the effects of electric multi-pulse stimulation on electrically evoked auditory brainstem responses (eABRs). Multi-pulses with a high burst rate of 10,000 pps were assembled from pulses of 45-μs phase duration. Conditions of 1, 2, 4, 8, and 16 pulses were investigated. Psychophysical thresholds (THRs) and most comfortable levels (MCLs) in multi-pulse conditions were measured. Psychophysical temporal integration functions (slopes of THRs/MCLs as a function of number of pulses) were −1.30 and −0.93 dB/doubling of the number of pulses, which correspond to the doubling of pulse duration. A total of 15 eABR conditions with different numbers of pulses and amplitudes were measured. The morphology of eABRs to multi-pulse stimuli did not differ from those to conventional single pulses. eABR wave eV amplitudes and latencies were analyzed extensively. At a fixed stimulation amplitude, an increasing number of pulses caused increasing wave eV amplitudes up to a certain, subject-dependent number of pulses. Then, amplitudes either saturated or even decreased. This contradicted the conventional amplitude growth functions and also contradicted psychophysical results. We showed that destructive interference could be a possible reason for such a finding, where peaks and troughs of responses to the first pulses were suppressed by those of successive pulses in the train. This study provides data on psychophysical THRs and MCLs and corresponding eABR responses for stimulation with single-pulse and multi-pulse stimuli with increasing duration. Therefore, it provides insights how pulse trains integrate at the level of the brainstem.
Highlights
Cochlear implants (CI) can restore hearing and speech understanding to people with severe to profound hearing loss to a surprisingly high degree by electrical stimulation of the residual auditory nerves (ANs)
Since evoked auditory brainstem responses (eABRs) wave eIII was corrupted by the multi-pulse stimulation artifact especially in measuring conditions with larger number of pulses, we focused on wave eV amplitudes and latencies
Often even large artifacts decay rapidly, such that they do not interfere with the eABR waves and blanking of the artifact-contaminated region is sufficient (Tykocinski et al, 1995; Truy et al, 1998)
Summary
Cochlear implants (CI) can restore hearing and speech understanding to people with severe to profound hearing loss to a surprisingly high degree by electrical stimulation of the residual auditory nerves (ANs). Middlebrooks (2004) and Zhou et al (2012) attributed the steeper MPI slopes at rates above 1000 pps to a residual partial depolarization mechanism, where initial subthreshold pulses partially depolarize a single AN or a group of ANs and further pulses, accruing within a 1-ms time window, increase the chance of firing an action potential, lowering the THR level. Similar to human results, Litvak et al (2003) found a sustained discharge rate in cat ANFs in response to a 5000-pps pulse train They claimed that, since eABRs to High-Frequency Multi-Pulse Stimulation no correlation between simultaneous measurements of pairs of ANF activities was found, the 5000-pps pulse rate desynchronized the auditory nerve activities, which is, again, evidence that high stimulation rates could improve neural representation to electric stimuli. We evaluated the contribution of nerve responses to each pulse or to a few consecutive pulses in multi-pulse stimulation to estimate the post-stimulus time histogram (PSTH) of the nerve
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