Neuroscan synamps2

EEG data were recorded from 34 scalp electrodes using a 64-channel Quik-Cap (Neuroscan, VA, USA) and a Neuroscan SynAmps^{2 (link)} acquisition system (Neuroscan, VA, USA) at a sampling rate of 500 Hz. Recording electrodes were positioned according to the International 10–20 placement standard with one ground electrode (AFz) and one reference electrode located between Cz and CPz. Impedances were kept below 15 k $\mathrm{\Omega }$ using Electro-Gel (Electrode-Cap International, OH, USA).
All EEG data were preprocessed offline using custom MATLAB scripts and functions from the open-source EEGLAB (https://sccn.ucsd.edu/eeglab) toolbox. Continuously recorded EEG signals were first bandpass filtered between 1 and 55 Hz using a two-way finite impulse response (FIR) filter (the ‘eegfilt’ function in EEGLAB) and then were re-referenced to average reference. Stereotypical artefacts, including ocular artefacts (EOG) and muscle tension, were removed using an automatic artefact rejection method⁴⁰ based on independent component analysis (ICA).⁴¹ The EEG data were then segmented into non-overlapping 6 s epochs, each spanning 1 s of rest (i.e. baseline) before the beginning of the trial, 2 s of reward cue and 3 s of squeezing. The segmentation resulted in 45 epochs per participant, with 15 epochs per reward value ($1, $10 or $50).

The electroencephalography (EEG) data were recorded by a NeuroScan Synamps‐2 that employed Ag/AgCl electrodes placed at 64 standard locations in accordance with the extended international 10–20 system and a precabled electrode cap. The online reference electrode was placed on the left mastoid during recordings and the EEG was rereferenced offline to the right mastoid for data analysis. All the active electrodes were grounded with GND. The horizontal electrooculogram (HEOG) was employed to measure horizontal eye movements, which were recorded using the voltage difference between the electrodes placed lateral to the external canthi. The vertical electrooculogram (VEOG) was used to detect eye blinks, which were recorded using the voltage difference between two electrodes placed above and below the left eye. The EEG signal was filtered online with a bandpass of 0.05–200 Hz and was sampled at 500 Hz. Of note, the impedances were maintained below 20 kΩ.