EEG was acquired on three EEG acquisition systems (with two different amplifiers) during the course of the study. The first two were a 32-channel system (N = 7 subjects) and a 40-channel system (N = 81 subjects), both which used the NuAmps single-ended, 32/40-channel amplifier and Scan 4.2 Acquisition software (Compumedics Neuroscan Inc., El Paso, USA). The third was a 64-channel system (N = 8 subjects) which used the SynAmps2 amplifier and Scan 4.3 acquisition software (Compumedics Neuroscan Inc., El Paso, USA). Electrode sites Fz, FCz, Cz, CPz and Pz, which were the main electrode sites analyzed in this study, were common to all three systems. A right ear reference electrode was used for all recordings. The ground electrode was placed 4 cm above the nasion for the 32-channel and 40-channel caps, and 8 cm above the nasion for the 64-channel caps. Electrode site impedances were kept below 10 kΩ for all recordings. A vertical electro-oculogram (EOG) was recorded from a bipolar electrode site pair placed above and below the left eye for use in offline reduction of ocular artifacts. The EEG and EOG channels were sampled at 250 Hz and stored for offline analysis.
