Brainamp system

Anesthesia monitoring signals (PPG, ETCO₂, ECG, and cuff‐based blood pressure) were also registered using 3T MRI‐compatible anesthesia monitor (GE Datex‐Ohmeda^™; Aestiva/5 MRI). In some cases, the ETCO₂ data measured from by the anesthesia monitor were corrupted. Thus, we used scanners inbuilt PPG and respiratory bellows data for groups comparisons.
EEG was recorded with an MR‐compatible BrainAmp system (Brain Products, Gilching, Germany) with 32 Ag/AgCl electrodes (including one ECG electrode) placed according to the international 10–20 system. To get low electrode impedances (<5 kΩ), the skin potential was removed with the stick abrasion technique (Vanhatalo et al., 2003). Data sampling rate was 5 kHz and band pass from DC to 250 Hz. Signal quality was tested outside the scanner room by recording 30‐s eyes open and eyes closed. MR‐scanner optical timing pulse and BrainAmp SyncBox were used to ensure that the EEG and fMRI data were in synchrony. The amplifier was placed outside the bore, and cables were stabilized to avoid motion artifacts.

EEG was acquired during the learning phase. The ERPs were recorded from the scalp at 29 standard locations (Fp1/2, Fz, F7/8, F3/4, Fc1/2 Fc5/6, Cz, C3/4, T3/4, Cp1/2, Cp5/6, Pz, P3/4, T5/6, Po1/2, and O1/2) with a BrainAmp system (Brain Products GmbH). In addition, the vertical and horizontal eye-movements were monitored, respectively, using an electrode at the infraorbital ridge of the right eye and at the outer canthi of the eyes. The electrode impedances were maintained below 3 kOhm. The electrophysiological signals were filtered with a bandpass of 0.01–50 Hz (half-amplitude cut-offs) and digitalised at a rate of 250 Hz. The biosignals were referenced online to the electrode in the outer canthus of the right eye and re-referenced off-line to the mean of the activity at the two mastoids. Trials with base-to-peak electro-oculogram amplitude of more than 50 μV, amplifier saturation, or a baseline shift exceeding 200 μV/s were automatically rejected off-line. Stimulus-locked ERPs were averaged for epochs of 1024 ms initiated at 100 ms prior to the stimulus. The mean rejection rate in the final sample of participants analyzed was 24.6 trials ± 16.2.

All electrophysiological data were recorded with a sampling rate of 500 Hz via the BrainVision Recorder software (Version 2.11) using a 32-channel BrainAmp system (Brain Products). We placed 11 gold-cup electrodes (Grass Technologies, Astro-Med) according to the international 10–20 system on the positions: F3, Fz, F4, C3, Cz, C4, P3, Pz, P4, O1, and O2. Linked mastoids were used for offline re-referencing as the data were online referenced against Cz. The position Fpz was used as ground electrode. Additionally, two EMG electrodes were placed on the musculus mentalis for measuring muscle activity during sleep and four EOG electrodes around the eyes to record horizontal and vertical eye movements. Impedances were always kept below 10 kΩ.

We used a Brain-Amp system (Brain Products GmbH, Germany) with two amplifiers to measure two standard single-channel ECG registrations (lead II derivation). One electrode was placed on the upper right chest, one on the left side of the abdomen and the grounding electrode was placed on the right side of the abdomen on both infant and mother. The ECG signal was recorded with a 500 Hz sampling frequency. Interbeat-intervals (IBIs) were then extracted offline using ARTiiFACT (Kaufmann et al., 2011 (link)). The ECG data were visually inspected for (in)correct detections and artifacts by trained research assistants. When ectopic beats or erroneous detections were found, the data were manually corrected (removal of erroneous detection/artifact followed by a cubic spline interpolation; corrections < 1%).

The EEG signals were amplified by a BrainAmp system (Brain Products, Gilching, Germany). Its output was digitized with a resolution of 16 bit and sampled at a rate of 1000 Hz via the lab streaming layer protocol. For data acquisition, we mounted a set of 32 Ag/AgCl electrodes according to the 10/20-system and referenced it to FCz. To capture horizontal eye movements, we placed two additional electrodes on the external canthi of both eyes. We kept the impedances below 20 kΩ with the ground and the reference electrode below 5 kΩ throughout the recording.