Abstract
To further understand the response of the human brainstem to electrical stimulation, a series of experiments compared the effect of pulse rate and polarity on detection thresholds between auditory brainstem implant (ABI) and cochlear implant (CI) patients. Experiment 1 showed that for 400-ms pulse trains, ABI users’ thresholds dropped by about 2 dB as pulse rate was increased from 71 to 500 pps, but only by an average of 0.6 dB as rate was increased further to 3500 pps. This latter decrease was much smaller than the 7.7-dB observed for CI users. A similar result was obtained for pulse trains with a 40-ms duration. Furthermore, experiment 2 showed that the threshold difference between 500- and 3500-pps pulse trains remained much smaller for ABI than for CI users, even for durations as short as 2 ms, indicating the effect of a fast-acting mechanism. Experiment 3 showed that ABI users’ thresholds were lower for alternating-polarity than for fixed-polarity pulse trains, and that this difference was greater at 3500 pps than at 500 pps, consistent with the effect of pulse rate on ABI users’ thresholds being influenced by charge interactions between successive biphasic pulses. Experiment 4 compared thresholds and loudness between trains of asymmetric pulses of opposite polarity, in monopolar mode, and showed that in both cases less current was needed when the anodic, rather than the cathodic, current was concentrated into a short time interval. This finding is similar to that previously observed for CI users and is consistent with ABI users being more sensitive to anodic than cathodic current. We argue that our results constrain potential explanations for the differences in the perception of electrical stimulation by CI and ABI users, and have potential implications for future ABI stimulation strategies.
Highlights
Auditory brainstem implants (ABIs) can restore hearing to deaf patients for whom a cochlear implant (CI) is unsuitable
We argue that our results constrain potential explanations for the differences in the perception of electrical stimulation by CI and ABI users, and have potential implications for future ABI stimulation strategies
As discussed above, one proposed reason for why thresholds drop with increasing rate in CI users, and so it was important to determine the extent to which such interactions occur for ABI users
Summary
Auditory brainstem implants (ABIs) can restore hearing to deaf patients for whom a cochlear implant (CI) is unsuitable. As discussed above, one proposed reason for why thresholds drop with increasing rate in CI users, and so it was important to determine the extent to which such interactions occur for ABI users These three experiments show that (a) the small effect of pulse rate on ABI users’ thresholds occurs even for very short-duration stimuli and cannot be due to long-term adaptation, (b) shortterm adaptation with a time constant below 10 ms may contribute to the difference, and (c) at high pulse rates, successive pulses do interact at the level of the cell membrane, and that these interactions are at least as large for ABI as for CI users. These two durations were chosen so as to test the possible effect of adaptation having a time constant longer than about 40 ms, such as the short-term adaptation reported for CI stimulation of the cat auditory nerve by Zhang et al (2007)
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