EEG pre-processing
EEG data (.bdf files) were processed offline using the eeglab 2022.1 toolbox (Delorme & Makeig, 2004) in Matlab 2022b. All raw data, processed data, and codes for pre-processing and analysis are available on Open Science Framework (Project 43alldata in SPN catalogue, https://osf.io/2sncj/). Data was re-referenced to scalp average, downsampled to 256 Hz, low pass filtered at 25Hz, and segmented in -0.5 to + 1 second epochs with -200 to 0 ms pre-stimulus baseline (resulting .set files in the 01_original folders). Channels were identified for interpolation with a semi-automated routine and zeroed during cleaning with Independent Components Analysis (ICA, Jung, 2000, .set files in the 02_ICA folders). ICA components were then removed from the data with the Adjust toolbox, and interpolated channels were then re-introduced (.set files in the 03_Pruned folders). EEG Data was then re-referenced to the scalp average again. Any trial where amplitude exceeded +/- 100 microvolts at any electrode were removed (.set files in the 04_AmpEx folders). Finally, data were separated into conditions (05_epochs folders) and compiled in the SET Data.mat structure. We then averaged over remaining trials for each subject and condition (resulting files inProject 43 folder in the SPN catalogue, https://osf.io/2sncj/).
On average, 5.77 ICA components were removed from each participant (min = 1, max = 23). ICA component removal rates were similar for Oddball alcohol (mean = 6.5, min = 1, max = 15), Oddball placebo (mean = 6.15, min = 2, max = 23) Regularity alcohol (mean = 6.15, min = 2, max = 14) and Regularity placebo (mean = 4.15, min = 1, max =12).
Trial exclusion rate was around 4-5% per condition in the Oddball task, and around 8% per condition in the Regularity task. Trial exclusion rate was 31% in the worst participant and condition. Therefore, all SPN waves were based on at least averaging over at least 69 trials per participant. One participant from the Oddball task was replaced because trial exclusion rate was over 50% (following the pre-registered criteria).