2.4 Electrophysiological Recording and Data Processing
The EEG was recorded from the scalp using either 33 or 64 channels from the Active-Two BioSemi system (BioSemi, Inc., Amsterdam, the Netherlands) using a modified 10-20 electrode placement system (American Electroencephalographic Society, 1994). Additional electrodes were placed on the left and right earlobes and two bipolar electrooculograms (EOG) electrodes were placed at the supra- and infraorbital regions of the left eye to measure vertical eye movements as well as on the left and right outer canthi to measure the horizontal eye movements. All EEG data were collected using a sampling rate of 1024Hz.
The EEG data were analyzed offline using the software BrainVision Analyzer2 (Brain Products GmbH, Munich, Germany). The data were referenced to the averaged signals of bilateral earlobes and then filtered with a bandpass filter of 0.1-30 Hz with 24 dB/oct. The continuous data were then segmented into 1200 ms epochs, spanning from 200 ms before stimulus onset to 1000 ms after stimulus onset. Segments were then baseline-corrected based on the average voltage of -200 to 0 ms of stimulus onset, and eye movement artifacts were removed via a regression approach based on the vertical EOG (VEOG) channel (Segalowitz, 1996) then baseline-corrected again using the period of -200 to 0 ms of stimulus onset. Segments containing voltage greater than ± 100 µV in the midline (e.g. Fz, FCz, Cz, Pz) and VEOG channels were rejected. The segments were then averaged to create averaged ERPs for each participant. The number of segments in the averaged ERPs ranged from 220 to 473. The averaged ERPs obtained for each participant were scored using a custom-built peak-picking computer program (Gavin, 2013) written in MATLAB (Mathworks, Natick, MA). This program performs automatic scoring and visual inspection of ERP components, and, when necessary, allows for manual marking of components to adjust for any values that were scored on a slope instead of a peak. In a previous study (Lin, et al., 2020), for each participant, the averaged ERP components were measured from baseline-to-peak using the following time windows: P1 between 0 and 100 ms; N1 between 70 and 150 ms; P2 between 110 and 240 ms; N2 between 170 and 350 ms; and P3 between 320 and 575 ms. The N3 was not measured. All components were measured at the site Cz. These data were used as the input data for ST measurement for this current study.