# Electrophysiological Hyperscanning of Negotiation During Group-Oriented Decision-Making
## 2 Materials and Methods
### EEG Data: Acquisition and Processing  
EEG data were acquired using a 16-channel direct current (DC) amplifier (SYNAMPS system) integrated with the NEUROSCAN 4.2 software. Recordings were performed during both resting-state and negotiation tasks, with electrode placement adhering to the international 10/20 system [48]. A total of 15 Ag/AgCl electrodes (Fp1, Fp2, F3, F4, Fz, Cz, C3, C4, T7, T8, Pz, P3, P4, O1, O2) were positioned on the scalp via an ElectroCap (Compumedics Germany GmbH, Singen, Germany), with the earlobes serving as reference points (Figure 1B). Additionally, two electrooculographic (EOG) electrodes were positioned at the outer canthus of the left eye to monitor ocular artifacts. Prior to data collection, the impedance levels of all electrodes were carefully measured and maintained below 5 k\(\Omega\) to ensure optimal signal integrity. The EEG signal was digitized at a 1000 Hz sampling rate, with a 50 Hz notch filter applied to mitigate electrical interference from power lines. For offline processing, a 0.01-50 Hz infinite impulse response (IIR) bandpass filter with a slope of 48 dB/octave was applied to the data. Continuous EEG recordings were then segmented into two-second epochs, with visual inspection to identify and exclude any artifacts originating from ocular, muscular, or movement-relatedsources. Artifact-free segments were subjected to spectral analysis using fast Fourier transform (FFT) with a Hamming window, achieving a spectral resolution of 0.5 Hz. This procedure facilitated the computation of average power spectral density (PSD) for both resting-state and negotiation conditions. In addition, a rigorous multistep preprocessing pipeline was applied to enhance signal quality. Raw EEG data were re-referenced to linked earlobes and filtered with a 0.01-50 Hz band-pass filter and a 50 Hz notch filter. An Independent Component Analysis (ICA) was employed to identify and remove components linked to ocular and muscular artifacts. Data were then segmented into 2-s epochs, and epochs contaminated by residual artifacts were excluded following combined automated and manual inspection. These procedures ensured high-fidelity signal integrity across participants and dyads, enabling reliable spectral analyses in all frequency bands of interest. Power spectral densities (PSDs) were calculated for the standard EEG frequency bands: delta (0.5-3.5 Hz), theta (4-7.5 Hz), alpha (8-12.5 Hz), beta (13-30 Hz), and gamma (30.5-50 Hz). Signal processing and analysis were conducted using Brain Vision Analyzer 2.0 (Brain Products GmbH, Gilching, Germany). To assess task-induced EEG modulation, the PSD values recorded during the negotiation phase were normalized against the baseline resting-state PSD values. The normalization procedure was performed using the following formula: [Normalized PSD = (PSD\({}_{\text{task}}\) \(-\) PSD\({}_{\text{BL}}\))/PSD\({}_{\text{BL}}\)], where PSD\({}_{\text{task}}\) represents the power measured during the task and PSD\({}_{\text{BL}}\) corresponds to the baseline power. This normalization was applied separately to each frequency band and electrode location for both the negotiator and receiver within each dyad, allowing for the individualized assessment of task-related changes in EEG activity. During the negotiation interaction, the participants were instructed to remain relaxed and maintain a steady posture, minimize motor movements, articulate their speech clearly, and avoid whispering and overlapping with the other participant's speech, adhering to turn-taking conventions. Furthermore, the negotiation exchange was video-recorded, confirming the absence of noticeable changes in facial expressions or posture throughout  
Figure 1: (**A**) The picture displays the timeline of the experimental procedure encompassing the EEG recording during the resting state baseline and the negotiation task. After the negotiation task, participants filled in a scale on the perception of negotiation exchange.