Neurofax 1100a digital system

Extraoperative video-ECoG recordings were obtained for 3 to 5 days, using a 192-channel Nihon Kohden Neurofax 1100A Digital System (Nihon Kohden America Inc., Foothill Ranch, CA, USA) at a sampling frequency of 1,000 Hz and an amplifier band pass of 0.08 to 300 Hz. The averaged voltage of ECoG signals derived from the fifth and sixth subdural electrodes of the ECoG amplifier was used as the original reference (Wu et al., 2011 (link)). ECoG signals were then re-montaged to a common average reference. Channels contaminated with artifacts or large interictal epileptiform discharges were excluded from the common average reference to minimize their influence on the results (Fukuda et al., 2008 (link)). Surface electromyography electrodes were placed on the left and right deltoid muscles, and electrooculography electrodes were placed 2.5 cm below and 2.5 cm lateral to the left and right outer canthi. ECoG traces were visually inspected with a time-constant of 0.003 s and a sensitivity of 20 µV/mm; thereby, irregular broadband signals synchronized with facial and ocular muscle activities seen on electrooculography electrodes were treated as artifacts (Otsubo et al., 2008 (link); Jerbi et al., 2009 (link); Kovach et al., 2011 (link); Kojima et al., 2013c (link)). Seizure onset sites were clinically determined (Asano et al., 2009 (link)) and excluded from further analysis.

ECoG data were acquired using a 192-channel Nihon Kohden Neurofax 1100A Digital System, sampled at 1 kHz. Raw electrophysiology data were filtered with 0.1-Hz high-pass and 300-Hz low-pass finite impulse response filters, and 60-Hz line noise harmonics were removed using discrete Fourier transform. Data traces were demeaned, and the study data were epoched into 4,500-ms trials (−1,000 to +3,500 ms from scene onset). Pathological electrodes and electrodes near lesions identified by the clinical teams (Drs. Asano and Auguste) were removed. Electrodes and epochs displaying epileptiform activity or artifactual signal (from poor contact, machine noise, etc.) were then determined based on visual inspection and removed. Each artifact-free electrode was re-referenced to the common average of all artifact-free electrodes. Each study trial was re-inspected after re-referencing and any trials with residual noise were removed. The final data set included 109 occipital electrodes across all participants (mean ± SD, 5 ± 2 electrodes/participant; range, 1–9).

Subdural platinum grid and strip electrodes (10 mm center-to-center distance; 3 mm diameter exposed) were placed as previously described (Asano et al., 2009 (link)). Electrode plates were stitched to adjacent plates or to the edge of the dura mater to prevent electrode plate movement after placement. iEEG recordings were obtained for 3 to 5 days, using a 192-channel Nihon Kohden Neurofax 1100A Digital System (Nihon Kohden America Inc., Foothill Ranch, CA, USA) at a sampling frequency of 1,000 Hz and an amplifier bandpass of 0.016 to 300 Hz. The averaged voltage of iEEG signals derived from the fifth and sixth intracranial electrodes of the iEEG amplifier was used as the original reference (Wu et al., 2011 (link)). iEEG signals were then re-montaged to a common average reference (Sinai et al., 2005 (link); Wu et al., 2011 (link)). Channels within 25 mm of the stimulation site and channels contaminated with artifacts or interictal epileptiform discharges were excluded from the common average reference.