The subjects were chronically implanted bilaterally in the lateral prefrontal cortex (PFC) with four 8×8 iridium-oxide ‘‘Utah’’ microelectrode arrays (1.0 mm length, 400 µm spacing; Blackrock Microsystems, Salt Lake City, UT), for a total of 256 electrodes (Figure 1C). Arrays were implanted bilaterally, one array in each ventrolateral and dorsolateral PFC. Electrodes in each hemisphere were grounded and referenced to a separate subdural reference wire. LFPs were amplified, low-pass filtered (250 Hz), and recorded at 30 kHz. Spiking activity was amplified, filtered (250–5,000 Hz), and manually thresholded to extract spike waveforms. All threshold-crossing spikes on each electrode were pooled together and analyzed as multi-unit activity (MUA).
