Brainamp

We placed 64 active electrodes (actiCAP, Brain Products GmbH, Gilching, Germany) on the scalp according to the 10–10 system. The reference and ground electrodes were positioned at the right mastoid and AFz, respectively. Six additional active electrodes were placed at the superior, inferior and outer canthi of the right and left eyes to record electrooculographic (EOG) activity. Supplementary Fig. 1 visualizes the locations of all 70 electrodes. After the electrodes were placed on the scalp, their 3D positions and the positions of anatomical landmarks (nasion, left-/right preauricular points) were recorded with an ultrasound based digitizer (ELPOS, Zebris Medical GmbH, Isny, Germany). EEG and EOG signals were recorded at 1 ​kHz (BrainAmp, Brain Products GmbH, Gilching, Germany). The paradigm was implemented in Python 2.7 based on the simulation and neuroscience application (SNAP) platform (Kothe, 2012 ) and the 3D engine Panda3D (Goslin and Mine, 2004 (link)). The screen position signals of the visual stimuli (cursor, target) were recorded at 60 ​Hz. All signals (EEG, EOG and stimuli) were recorded using the lab streaming layer (LSL) protocol (Kothe et al., 2019 ) and synchronized offline using a photodiode signal, which captured an impulse on the screen at the start of each trial. All signals were then resampled to 200 ​Hz.

The EEG was recorded with a 64-channel recording system (Brainamp with actiCAP, Brain Products Gmbh, Munich) according to the 10–10 system montage^{66 (link)}. Scalp impedance was kept below 10 kΩ. FCz served as the reference electrode, AFz as the ground electrode. Horizontal and vertical electrooculographic signals were recorded with two additional electrodes at the left and right outer canthi and one electrode at the left infraorbital. The EEG was online band-pass filtered between 0.1 and 100 Hz, and the data digitized with a sampling rate of 500 Hz.

Wakefulness and sleep during scanning were confirmed by polysomnography as described above using an MR-compatible polysomnographic recording system (BrainAmp, BrainCap and BrainAmp ExG MR devices and electrodes; Brain Products GmbH, Gilching, Germany). Impedances of all electrodes were kept below 10 kΩ. The bioelectric data were synchronized with the scanner clock, sampled at 5 kHz (filtered between 0.01 and 200 Hz) and referenced to the vertex (Cz).

For each participant, we collected 64-channel EEG data with a BrainAmp (Brain Products, Gilching, Germany) amplifier for at least 15 minutes at a 1000 Hz sampling rate. The impedance of all the electrodes was kept below 5 kΩ. Scalp electrodes were positioned according to the international 10–20 system. All electrode sites were referenced online to FCz.
EEG data were analysed offline using the EEGLAB toolbox and processed using an average reference. After offline referencing, the EEG signal was high-pass filtered at 0.1 Hz and low-pass filtered at 30 Hz and then segmented into 3-second-long epochs. Each epoch was baseline corrected relative to the mean voltage over the entire epoch. The artifact-free periods were submitted to independent component analysis (ICA) using the runica function, and the bad channels were interpolated using the planar gradiometers incorporated in EEGLAB.

Figure 3 illustrates the locations of the EEG channels and fNIRS optodes which were integrated in a standard EEG cap according to the extended international 10-05 system. Scalp EEG potentials were recorded (BrainAmp, Brain Products GmbH, Munich, Germany) from 32 positions with Ag/AgCl electrodes (actiCAP, Brain Products GmbH, Germany). The left mastoid was used as common reference and EEG was grounded to Cz. All impedances were kept below 20 kΩ at the onset of each session. EEG data was digitized at 1 kHz, high-pass filtered with a time constant of 10 sec and stored for off-line data analysis using the »BrainVision Recorder« Software (Brain Products, Munich, Germany). All EEG data analyses were performed with custom written or adapted scripts in MATLAB^®.

The EEG was recorded with Ag-AgCl electrodes from 27 standard positions of the extended 10-20-system (Fp1, Fp2, F7, F3, Fz, F4, F8, FC5, FC1, FC2, FC6, T7, C3, Cz, C4, T8, CP5, CP1, CP2, CP6, P7, P3, Pz, P4, P8, O1, and O2) and from the left and right mastoids (M1 and M2). All electrodes were referenced to the tip of the nose. The vertical electrooculogram (EOG) was recorded between Fp1 and an infraorbitally placed electrode and the horizontal EOG between the outer canthi of the two eyes. Impedances of all electrodes were kept below 10 kΩ. EEG and EOG were filtered online with a bandpass of 0.1–250 Hz and sampled with a digitization rate of 500 Hz (BrainAmp, Brain Products, Gilching, Germany). Time was recorded for each button press.