2.3. EEG Recording and Analysis

Two 16-channel EEG systems were used for the EEG signal recording (V-AMP: Brain Products, München; LiveAmp: Brain Products, GmbH, Gliching, Germany). Specifically, the electrodes were placed on the individual’s scalp with the use of two ElectroCaps at the following positions: F3, F1, Fz, F2, F4, T7, T8, C3, Cz, C4, P3, P1, P2, P4, O1, and O2 (Figure 2). Moreover, for the V-AMP system, an electrooculography (EOG) electrode has been positioned on the external canthi. A 5 kΩ electrode impedance was monitored before data collection for each individual. We used 1000 Hz for data sampling, with a 0.01–200 Hz input filter and a 50 Hz notch filter. Acquired data were filtered offline using a 0.5–40 Hz bandpass filter. Moreover, to reduce problems associated with signal-noise, a common offline average reference was calculated [64]. Concerning signal evaluation, portions of data containing artifacts were deleted, and an algorithm that uses a regression analysis in combination with the artifact average was utilized for ocular and motor artifacts correction. The EEG data were finally extracted in the frequency band: delta (0.5–4 Hz), theta (4–8 Hz), alpha (8–12 Hz), and beta (14–20 Hz) [65]. The mean EEG power for each channel and each frequency band was calculated by averaging data related only to the gesture reproduction phase using a 4-s segment. Finally, to obtain inter-brain connectivity, the partial correlation coefficient Πij was computed by normalizing the inverse of the covariance matrix: Γ = Σ − 1:

The figure shows the EEG channels location in the following positions: F3, F1, Fz, F2, F4, T7, C3, Cz, C4, T8, P3, P1, P2, P4, O1, and O2.

These measures represent the covariance of two signals, which allows the calculation of the partial correlation coefficients between two series of data in response to specific conditions (the experimental conditions).