Participants were seated comfortably in a dimly lit sound-attenuated temperature-regulated booth (Industrial Acoustics, Bronx, NY). They were instructed to keep their eyes closed and remain relaxed, but to not fall asleep. EEG data were collected in the awake, eyes-closed condition for 4.25 minutes. Each participant wore a fitted electrode cap (Electro-Cap International, Eaton, OH) using the 19-channel montage as specified according to the 10–20 international system (Supplementary Figure 1). The nose was used as a reference, and a forehead electrode served as the ground electrode. Both vertical and horizontal eye movements were monitored with electrodes that were placed supraorbitally and at the outer canthus of the left eye to perform ocular artifact correction. Electrode impedances were maintained below 5 kΩ. Electrical activity was amplified 100,000 times by Sensorium (Charlotte, VT) EPA-2 electrophysiology amplifiers with either a bandpass between 0.02 and 50 Hz and digitized on a Concurrent (Atlanta, GA) 5550 computer at a sampling rate of 256 Hz or a band pass between 0.02 Hz and 100.0 Hz on a Neuroscan system (Version 4.1 to 4.5) (Neurosoft, Inc., El Paso,
