Waveguard

Continuous EEG data were collected using a compact 32-channel system (eego™sports 32, ANT Neuro^®, Enschede, The Netherlands). A gel-based electrode cap with sintered Ag/AgCl electrodes was used (Waveguard, ANT Neuro^®, 10–20 system). The online reference was placed at the CPz electrode. Signal was acquired with eego sports acquisition software connected to a 24 bits amplifier at a sampling rate of 500 Hz. Impedances for all electrodes were kept below 20 kΩ. EEG signals were recorded across 30 channels: Fp1, Fpz, Fp2, F7, F3, Fz, F4, F8, FC5, FC1, FC2, FC6, T7, C3, Cz, C4, T8, CP5, CP1, CP2, CP6, P7, P3, Pz, P4, P8, POz, O1, Oz, and O2 excluding the mastoids electrodes (M1 and M2). The starting and ending points of each block composing the acquisition (Rest, Task 1, Task 2, Task 3) were manually labeled using the acquisition software.

Eyes-closed resting-state EEGs were recorded using an EEG-1200 Nihon Kohden (Neurofax 1200, Tokyo, Japan) system. We used 30 Ag/AgCl sintered ring electrodes (Waveguard; ANT-Neuro, Enschede, Netherlands). Data were sampled at 500 Hz, and recorded with a low-pass filter at 0·5 Hz and a high-pass filter at 60 Hz. The recordings lasted 5 min and were carried out twice for each participant.
For processing, raw EEG data were bandpass filtered at 1–30 Hz. EEG fragments contaminated with muscle artifacts and eye blink noise were excluded by visual inspection. After removing the contaminated fragments, we used an artifact detector in LORETA-KEY software (http://www.uzh.ch/keyinst/loreta.htm) ^{30 (link)} and distinguished possible artifacts autonomously. We adopted 2-min epochs with as few artifacts as possible in each session.
Current densities were calculated from the EEG data using standardized low-resolution brain electromagnetic tomography (sLORETA)^{30 (link)}. The region of interest (ROI) for the rACC (x = 0, y = 45, z = 0 in the Montreal Neurological Institute space) was selected based on a previous study investigating neurophysiological mechanisms in patients with MDD²² . The ROI for the DLPFC is defined by the location under the stimulation site, based on the 10–10 EEG coordinates (x = –52, y = 26, z = 28)^{31 (link)}. We analysed the alpha band between 8.5 and 13 Hz.

EEG data were recorded with a NicOne EEG amplifier (Cardinal Healthcare/Natus, USA) or a Cognitrace amplifier (ANT B.V., Enschede, The Netherlands). EEG caps (sintered Ag/AgCl electrodes; Waveguard, ANT-Neuro, Germany) had 19 or 28 scalp electrodes positioned according to the International 10–20 standard. The same 19 EEG channels (Fp1, Fp2, F7, F3, Fz, F4, F8, T7, C3, Cz, C4, T8, P7, P3, Pz, P4, P8, O1, O2) across all recordings were selected for analyses. Further details regarding EEG acquisition in newborns can be found elsewhere³⁸ .

EEG was acquired from 64 scalp positions using NeurOne amplifiers (NeurOne Tesla, Mega Electronics Ltd, Kuopio, Finland) with a 24-bit-resolution, Input Impedance >1 GΩ and a 128-channels wet-gel electrode EEG cap (WaveGuard, ANT Neuro, Hengelo, Netherlands). The 64 scalp positions followed the 10–20 system. The Ag/AgCl electrodes were filled with conductive electrode gel, using a blunt needle to abrase superficial layers of the skin, i.e., stratum corneum. Cz was the reference electrode. The ground electrode was localized between Fz and AFz. The signal was sampled at 5,000 Hz. An anti-aliasing hardware filter was applied at 1,250 Hz. The amplifier was localized in a distance of 3 m and powered through a battery pack. The subjects’ scalp below the electrodes was treated to obtain impedance values below 5 kΩ. Therefore, subjects were instructed to wash their hair thoroughly before the experiment. For all subjects, the mean number of electrodes with impedances between 5 and 10 kΩ was 6 electrodes, maximum 14 electrodes, minimum 0 electrodes. Average impedance of these “high impedance” channels (impedance >5 kΩ) was at 5.7 kΩ.

Scalp EEG was measured at conceptional age of 42.1 ± 0.9 weeks in AED and 42.2 ± 0.9 weeks in HC groups (or 2.0 ± 1.0 and 2.0 ± 1.1 weeks from birth, respectively; mean ± SD for all) with no significant difference (P = 0.51, Wilcoxon rank-sum test). The EEG recording was performed during daytime sleep with a NicOne EEG amplifier (Cardinal Healthcare/Natus, USA), using initial sampling frequency (Fs) of 250 Hz or 500 Hz. The neonatal Waveguard caps had 20–32 sintered Ag/AgCl electrodes (ANT-Neuro, Germany) placed according to the 10–20 international system. For further analysis, the same 19 channels were selected from all subjects: Fp1, Fp2, F7, F3, Fz, F4, F8, T7, C3, Cz, C4, T8, P7, P3, Pz, P4, P8, O1, and O2.
EEG epochs for the network analyses were selected from both neonatal sleep (vigilance) states, AS and QS. They were identified visually using the standard combination of electrophysiological and behavioral measures (André et al. 2010 (link)), including polygraphic channels (submental electromyogram, electrocardiogram, electrooculogram, and respiration sensor). EEG signal during AS is known to exhibit continuous fluctuations, while polygraphic channels show irregular respiration and occasional eye movements. Conversely, EEG signal during QS is characteristically discontinuous, while respiration is markedly regular.