*2.2. Procedure*

Participants performed a modified Eriksen flanker task in which arrows appeared on a personal computer display with congruen<sup>t</sup> (e.g., →→→→→) and incongruent (e.g., →→←→→) conditions [47]. They were instructed to respond to the central arrow target, while ignoring the adjacent arrows, by pressing one of two buttons indicating the direction of the middle arrow (i.e., right versus left). The stimuli remained on the screen for 250 msec, with an interval of 1500 msec between consecutive trials.

Each participant was seated 65 centimeters directly in front of the computer monitor and told to place equal emphasis on speed and accuracy in responding. Following 40 practice trials, each subject completed eight blocks of 64 trials for a total of 512 trials. Performance feedback was provided after every block to yield an error rate of approximately 10%, with encouragemen<sup>t</sup> to focus on speed if there were fewer than four errors or to focus on accuracy if there were more than 10 errors [48].

## *2.3. Electrophysiological Methods*

The electroencephalogram (EEG) was recorded from 64 Ag/AgCl scalp electrodes embedded in a nylon mesh cap, two mastoid electrodes, and two vertical and two horizontal electro-oculographic electrodes using the BioSemi ActiveTwo system (Amsterdam, The Netherlands). Data were digitized at 512 Hz, referenced to a ground formed from a common mode sense active electrode and driven right leg passive electrode (http://www.biosemi.com/ faq/ cms&drl.htm), and re-referenced o ffline to the average of the two mastoid electrodes. Data were bandpass filtered 0.1–30 Hz using zero-phase shift filters. EEG data were screened using automated algorithms that rejected epochs in which the absolute voltage exceeded 500 μV and epochs containing peak to peak activity greater than 500 μV within 200 msec, with a 100 msec moving window, for midline channels (Fz, FCz, Cz, CPz, and Pz). Ocular movement artifacts were then corrected using a regression-based algorithm [49]. After ocular correction, individual trials were rejected if they contained absolute amplitudes greater than 100 μV, a change greater than 50 μV measured from one data point to the next point, or a maximum voltage di fference less than 0.5 μV within a trial in any of the midline electrodes.
