ECoG Data Collection and Direct Cortical Stimulation

All procedures in this study were approved by the Institutional Review Board of the University of North Carolina at Chapel Hill (IRB Number 13–2710), and written informed consent was obtained from all the participants. The participants underwent temporary implantation of subdural electrodes for presurgical localization of seizure focus in the Epilepsy Monitoring Unit at the UNC Neurosciences Hospital. Electrode grids were implanted over the different cortical regions as described in Table 1. S2 Fig denotes the location of electrodes for the three patients.

The electrodes were made of platinum-iridium alloy, were 4 mm in diameter (2.5 mm exposed), and were embedded in silicone (Ad-Tech Medical, Racine, Wisconsin, United States). The inter-electrode distance in each grid was 10 mm. Four electrodes in a separate set placed far from the recording grids were chosen to be reference electrodes. ECoG signals were recorded using a 128-channel Aura LTM 64 acquisition system and the corresponding TWin software (Grass Technologies, Warwick, Rhode Island, United States) at 800 Hz sampling rate.

Electrical stimulation consisted of a train of biphasic pulses 2 mA in amplitude and 200 μs in duration, with a pulse every 100 ms (10 Hz) generated by a S12x cortical stimulator (Grass Technologies, Warwick, Rhode Island, United States) and applied between a pair of adjacent electrodes for 5 s (Electrodes marked Blue in S2 Fig). The stimulations were spaced at about 15 s between trials to enable safety monitoring for the occurrence of after-discharges.