EEG Data Acquisition and Preprocessing

To collect EEG data, 128-channel HydroCel “Sensor Nets” (Electrical Geodesics, Eugene, OR) were used. Signals were 50 Hz low-pass and 0.1 Hz high-pass filtered and digitized at 500 Hz. At the end of the EEG session, three major fiducials (nasion, left and right periauricular points) and three-dimensional (3D) locations of all electrodes were digitized using a 3Space Fastrack 3D digitizer (Polhemus, Colchester, VT). These 3D locations were used to co-register the electrodes to the T1-weighted anatomical MRI scans. EEG raw data and artifact rejection, including blink rejection and eye movement artifact detection, were evaluated offline based on a sample-by-sample thresholding procedure using the software package designed by the Norcia research group (Ales, Appelbaum, Cottereau, & Norcia, 2013 (link); Cottereau, Ales, & Norcia, 2015 (link)). Noisy sensors were replaced by the average of the six nearest spatial neighbors. The EEG was re-referenced to the common average of all the sensors after noisy sensors were substituted.