Nuamp

Electroencephalographic recordings were administered using the Neuroscan system (NuAmps, Compumedics Neuroscan, Inc., Charlotte, NC, United States) with a 40-electrode cap in accordance with the International 10–20 system. The stimuli, created in a similar way as the stimuli in the FCDT test, were 1-s 0.25 kHz tones containing an upward frequency change of 0%, 10% or 50% at 0.5 s after the tone onset. Due to time constraints, only one base frequency (0.25 kHz) was used to minimize EEG testing time (approximately 1 h including the time for EEG cap placement and EEG recordings) for the participants. The 0.25 kHz is mapped to the first electrode in Cochlear devices and it is within the range of fundamental frequencies of human voice. CI users were presented with 400 trials for each type of the three stimuli (0%, 10%, and 50% change), with the inter-stimulus-interval at 0.8 s. The order of the stimuli was randomized. During testing, participants were instructed to avoid excessive eye and body movements. Participants read self-selected books or watched movies with closed captioning to keep alert and were asked to ignore the acoustic stimuli.

A 40 channel EEG Quik-Cap (Compumedics Neuroscan, Dresden, Germany) was fastened below the chin of participants with the Cz electrode positioned midway between the nasion and the inion (sagittal plane), and midway between each tragus (coronal plane). Skin was lightly abraded using a sterile blunt needle. Conductive gel was inserted into 12 electrodes, FP1, F3, Fz, F4, C3, Cz, C4, P3, Pz, and P4, according to the international 10–20 system, as well as a reference electrode on the right mastoid and a ground electrode on the right forehead (Ag/AgCl electrodes, Compumedics, Neuroscan). Impedance remained below 5 kΩ. EEG signals were digitised via a 40-channel EEG amplifier (NuAmps, Compumedics Neuroscan, Dresden, Germany) with a sampling rate of 500 Hz, 32 bits accuracy and recorded in SCAN software (Compumedics Neuroscan, Dresden, Germany).

EEG signals were recorded with a 33-channel Quick-Cap connected to a 40-channel NuAmps (NeuroScan Amplifier, Compumedics Inc., Charlotte, NC, USA). The layout of the electrodes followed the International 10–20 system (Figure 2), where A1 and A2 were reference electrodes, the ground channel was at the forehead, and the remaining 30 electrodes were used for recording EEGs. Impedance was kept below 10 kOhm by applying Electro-Gel (Compumedics Inc., Charlotte, NC, USA) to the electrodes. Ocular activity (i.e., electrooculography, EOG) was monitored with two electrodes placed above the left eye and the right side of the right eye, respectively. The recorded EEG and EOG signals were amplified and filtered (0.5–100 Hz), and then digitized with a sampling rate of 500 Hz using the NuAmp amplifier from NeuroScan Inc. Ocular artifacts coming from blinking or eye movements were removed from the EEG signals using the artifact removal software from NeuroScan (Scan4.5). Afterward, the EEG signals were further filtered using a Finite Impulse Response (FIR) filter (0.5–50 Hz). Finally, other possible artifacts caused from generic discontinuities and electromyography were removed using the independent component analysis (ICA) and ADJUST algorithm (Mognon et al., 2011 (link)) provided in the EEGLAB (Infomax ICA).

Participants were seated on a comfortable chair in a sound-treated booth for the experiments. A 40-channel Neuroscan multi-channel EEG system (NuAmps, Compumedics Neuroscan, Inc., Charlotte, NC) was used to record the EEG. Electro-ocular activity (EOG) was monitored so that eye movement artifacts could be identified and rejected during the offline analysis. The continuous EEG data were recorded with a band-pass filter setting from 0.1 to 100 Hz and a sampling rate of 1,000 Hz. The average electrode impedance was lower than 10 kΩ. EEG signals from a total of 1–3 electrodes over the CI coil were not available.
During testing, participants were instructed to avoid excessive eye and body movements. Participants read self-selected magazines to keep alert and were asked to ignore the acoustic stimuli. A total of 400 trials of each of the three types of stimuli (three frequency changes: 0%, 5%, and 50%) were presented. The stimulus conditions were randomized to prevent order effects. The inter-stimulus interval was 800 ms. Each CI ear was tested separately.

EEG and ERP recordings were performed using a standardized methodology and platform (Brain Resource Ltd., Australia). Details of this procedure have been published elsewhere [24] (link), [42] (link).
In summary, participants were seated in a sound and light attenuated room, controlled at an ambient temperature of 22°C. EEG data were acquired from 26 channels: Fp1, Fp2, F7, F3, Fz, F4, F8, FC3, FCz, FC4, T3, C3, Cz, C4, T4, CP3, CPz, CP4, T5, P3, Pz, P4, T6, O1, Oz and O2 (Compumedics Quikcap and NuAmps amplifier; 10–20 electrode international system). Data were offline referenced to averaged mastoids with a ground at Fpz. Horizontal eye movements were recorded with electrodes placed 1.5 cm lateral to the outer canthus of each eye (bipolar). Vertical eye movements were recorded with electrodes placed 3 mm above the middle of the left eyebrow and 1.5 cm below the middle of the left bottom eyelid. Skin resistance was <5 kOhms for all electrodes. A continuous acquisition system was employed and EEG data were EOG corrected offline [43] (link). The sampling rate of all channels was 500 Hz. A high cut-off filter at 100 Hz was employed prior to digitization. P300 latency and amplitude were quantified at Pz.

An electrode cap based on the International 10–20 system was positioned on the subject's head by the experimenter. Figure 1 shows a schematic representation of the electrode sites of the cap. For a detailed review on the EEG setup, please refer to Chan and Davenport's (2008 (link)) review. Before the recording, the subjects were familiarized with the sensations of the paired inspiratory occlusions. Two inspiratory occlusions of 150 ms each, with a 500 ms inter-stimulus-interval, were provided randomly every 2–4 breaths. The onset of occlusion was identified as the start of mouth pressure change (Labchart V7, ADInstruments Inc., Bella Vista, Australia). Approximately 100 paired occlusions were provided during each of the two trials. The trigger box was sending parallel markers to the EEG recording software (Neuroscan 4.5, Compumedics Neuroscan Inc., Charlotte, NC, USA). The continuous EEG was sampled at 1 kHz with a 40-channel EEG system (NuAmps, Compumedics Neuroscan Inc., Charlotte, NC, USA), bandpass filtered from DC to 50 Hz and referenced to the bilateral mastoids. Individual electrode impedance was set below 5 kΩ. After each trial, the subjects were rating their subjective feeling of breathlessness with a visual analog scale (VAS) (0 = not at all breathlessness, and 100 = maximal level of breathlessness).