We simultaneously and continuously recorded EEG using a custom-built MR-compatible EEG system (Goldman et al., 2009; Sajda et al., 2010), with differential amplifier and bipolar EEG cap. The caps are configured with 36 Ag/AgCl electrodes including left and right mastoids, arranged as 43 bipolar pairs (Supplementary Figure 1). Bipolar pair leads are twisted to minimize inductive pickup from the magnetic gradient pulses and subject head motion in the main magnetic field. This oversampling of electrodes ensures data from a complete set of electrodes even in instances when discarding noisy channels is necessary. To enable removal of gradient artifacts in our offline preprocessing, we synchronized the 1-kHz-sampled EEG with the scanner clock by sending a transistor-transistor logic (TTL) pulse to a field-programmable gate array (FPGA) card (National Instruments, Austin, TX) at the start of each of 170 functional image acquisitions. All electrode impedances were kept below 20 kΩ, including 10 kΩ resistors built into each electrode for subject safety. A comprehensive description of the hardware, along with the preprocessing and analysis methods described throughout the remainder of Section 2, can be found inSajda et al. (2010).
