We used EEG to record the eFFRs and eACCs. A single trial consisted of a 2.024-s base-rate pulse train immediately followed by a 2.024-s deviant-rate pulse train. The stimulation levels were set to those determined during the behavioral assessment of the stimulation levels. 100 trials of a specific combination were concatenated and presented in one block, without containing any silence between trials, and a trigger was sent at the start of each trial from the programming device to synchronize the stimulation and recording. A minimum of 300 trials per condition was presented to each subject for each condition, with the exact number depending on availability. The number of trials actually analyzed (after artifact rejection) is shown for each subject and condition in Supp. Table I.

EEG was recorded with an 8-channel Biosemi ActiveTwo Hyper-Rate EEG recording system which was specially designed –to our specifications– to limit the distortion of the artifact waveform and therefore potentially shorten the stimulation artifact duration. We hypothesize that this system allows the removal of the stimulation artifacts and enables one to obtain eFFRs free from stimulation artifacts in CI users when stimulating in monopolar mode. The hyper-rate EEG system is based on the Biosemi ActiveTwo EEG-recording system but with a sample rate of 262.144 kHz/channel and with a built-in analog third-order antialiasing filter having a −3 dB point at 50 kHz. The advantage of this system over other EEG systems is that it has both a high dynamic range and a high sampling rate, hence a high bandwidth (Gransier et al., 2020a), which therefore reduces the distortion and reduces the duration of the artifact waveform (Lio et al., 2018). We used Ag/AgCl active recording electrodes to record the EEG. The recording electrodes were placed at the subject's head using an 6-channel cap according a 10–20 system (Jasper, 1957) layout. Six electrodes were placed on the Fz, Fpz, Cz, P9, P10, Iz locations, and two further electrodes were placed on the left (MaL) and right mastoid (MaR). The DRL and CMS electrodes were placed near the central parietal-occipital location of POz. P10 could not be placed in S3 and S5 due to the location of the CI. Offline referencing was used, and the electrodes placed at Fz, Fpz, Cz, and MaR were used as a reference. Electrode combinations with reference electrodes: Fz, Fpz, and Cz are referred to as vertical montage combinations, whereas the electrode combinations (with the exclusion of Fz, Fpz, and Cz) with MaR as the reference electrode are referred to as the horizontal montage. These different reference positions were included for assessing the artifact removal, since artifact topography can vary across CI users (Gransier et al., 2016).

All EEG recordings, at both sites, were obtained in an electrically shielded (Faraday cage) sound booth, where participants sat in a comfortable chair. To minimize tension in the muscles supporting the head, the head and neck were supported by the chair and occasionally by a cushion. To reduce artifacts caused by movements, participants were asked to move as little as possible during the recording. A silent movie with subtitles was played to ensure that the attentional state was similar in all conditions and for all participants.

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