Seventeen subjects (6 female, mean 27.7 years, range 20–40) participated in three runs each of a visual oddball paradigm. The 375 (125 per run) total stimuli were presented for 200 ms each with a 2–3 s uniformly-distributed variable inter-trial interval (ITI) and probability of target ⅕. The first two stimuli of each run were constrained to be standards. The target and standard stimuli were, respectively, a large red circle and a small green circle on isoluminant gray backgrounds (3.45° and 1.15° visual angles). The larger target stimuli boosted EEG discriminator performance, and did not confound our final results, since we interpreted only the variability within the target class. Because our study focused on task-related attentional states, subjects were asked to respond to target stimuli, using a button press with the right index finger on an MR-compatible button response pad. Stimuli were presented to subjects using E-Prime software (PST, Pittsburgh, PA) and a VisuaStim Digital System (Resonance Technology, Northridge, CA) 600×800 goggle display. All subjects gave informed consent following the protocol of the Columbia University Institutional Review Board.
