To further understand the response of the human\nbrainstem to electrical stimulation, a series of experiments\ncompared the effect of pulse rate and polarity\non detection thresholds between auditory brainstem\nimplant (ABI) and cochlear implant (CI) patients.\nExperiment 1 showed that for 400-ms pulse trains, ABI\nusers� thresholds dropped by about 2 dB as pulse rate\nwas increased from 71 to 500 pps, but only by an\naverage of 0.6 dB as rate was increased further to 3500\npps. This latter decrease was much smaller than the\n7.7-dB observed for CI users. A similar result was\nobtained for pulse trains with a 40-ms duration.\nFurthermore, experiment 2 showed that the threshold\ndifference between 500- and 3500-pps pulse trains\nremained much smaller for ABI than for CI users,\neven for durations as short as 2 ms, indicating the\neffect of a fast-acting mechanism. Experiment 3\nshowed that ABI users� thresholds were lower for\nalternating-polarity than for fixed-polarity pulse trains,\nand that this difference was greater at 3500 pps than\nat 500 pps, consistent with the effect of pulse rate on\nABI users� thresholds being influenced by charge\ninteractions between successive biphasic pulses. Experiment\n4 compared thresholds and loudness between\ntrains of asymmetric pulses of opposite polarity,\nin monopolar mode, and showed that in both cases\nless current was needed when the anodic, rather than\nthe cathodic, current was concentrated into a short\ntime interval. This finding is similar to that previously\nobserved for CI users and is consistent with ABI users\nbeing more sensitive to anodic than cathodic current.\nWe argue that our results constrain potential explanations\nfor the differences in the perception of\nelectrical stimulation by CI and ABI users, and have\npotential implications for future ABI stimulation\nstrategies.
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