2.2. Dataset and Preprocessing

During this experiment, EEG data were sampled at 2048 Hz using the standard 64-electrode EEG 10-10 system, as shown in Figure 1a. Thereafter, standard EEG preprocessing techniques were applied. The data were re-referenced to the average of both earlobes, just one earlobe if the other one was too noisy, or another pair of channels if both earlobes were badly recorded or too noisy. In the case of bad channels, these were identified and removed. EEG data were notch-filtered at the line frequency (50 Hz) and its multiples, after which a bandpass filter from 0.2 Hz to 100 Hz was applied. The data were split into epochs, ranging from 0.2 s prestimulus to 1.0 s poststimulus, resulting in 1.2 s epochs. Independent component analysis (ICA) was applied to the epoched data. Any components that represented artifacts were removed through visual inspection of the ICA components.

During the previous steps, some channels were marked as bad channels. Instead of dropping these channels, we chose to interpolate them using their neighboring channels, as the former would result in an inconsistent number of channels across sequences and subjects. The interpolation was performed using the MNE-Python package (All Python packages that were used in this work can be found in Table A1, together with their version number and citation.) [55]. Finally, the data were downsampled to 50 Hz, as this reduced computation time, decreased file read/write time, and saved memory, while generally leading to little or no loss of information [56]. We should however note that, based on the Nyquist theorem, this limits the highest frequency that can be accurately represented to half of the sampling frequency, i.e., 25 Hz. This preprocessing routine ideally resulted in 600 target epochs, 600 distractor epochs, and 300 induction epochs for each of the 42 subjects. However, during preprocessing, some epochs were rejected for various reasons, for example, an excessive number of bad electrodes or too much noise. On average, less than 0.6% of the epochs were rejected per subject.

As mentioned in Section 1, we expected to observe a P3a ERP when subjects saw a target stimulus. The P3a ERP is characterized by a positive voltage deflection between 250 ms and 280 ms after the stimulus, although the exact timing can vary [16,57,58]. As our experiment used visual stimuli, we expected the P3a ERP to be the most pronounced in the parietal-occipital region of the brain [56]. Figure 2 shows the evoked response for one subject, averaged over all parietal-occipital electrodes, as indicated in the figure inset. We observed a clear positive deflection between 200 ms and 300 ms after the stimulus, in line with our expectations.

The evoked response for targets and distractors for one subject. The data were averaged over all electrodes of the parietal-occipital region in the brain, as indicated in the figure inset.