Summary: Supplementary Methods
Subjects. We tested three subjects who had been surgically implanted with bilateral depth electrode
arrays into prefrontal cortex. Intracranial recordings can be ethically obtained in cases of medically
intractable epilepsy, where they are used to precisely localize regions of seizure onset. The clinical team
determined the placement of these electrode arrays so as to best localize epileptogenic regions. All
subjects had normal range personality and intelligence and were able to perform the task well. Our
research protocol was approved by the institutional review board at Huntington Memorial Hospital,
Pasadena, CA, and informed consent was obtained from each the subjects.
Each subject had a single electrode array implanted into vPF in each hemisphere, orthogonally placed
on a lateral to medial trajectory through inferior frontal gyrus. All three subjects had similar placements
(Supplementary Figure 1).
Intracranial recordings. We utilized the two most lateral electrode contacts on each array for a total of
four electrodes per subject (two per hemisphere) and 12 electrodes across all three subjects. iEEG
signal was recorded using standard clinical depth electrode arrays with 1 mm platinum contacts and an
inter-electrode spacing of 8 mm (Spencer electrodes; Ad-Tech Medical, Racine, WI). The signal was
amplified, sampled at 200 Hz, and band-pass filtered between 0.370 Hz (Grass-Telefactor apparatus,
West Warwick, RI). Recordings were acquired using a sub-galeal reference and then re-referenced to a
common average reference (CAR). Choice of reference did not affect the results.
Although intracranial depth recordings mainly reflect local processing, it is possible that distant
sources may also contribute to the observed activity. To address this we computed the bipolar derivation