Figure 1.
Experimental protocol. At the beginning of each block, subjects were shown for 10 s the set of stimuli they would see in that block. In this example, Block 1 uses color patches for stimuli and has a set size ns = 2; Block n uses shapes and has ns = 6. Each trial included the presentation of a stimulus for 0.5 s, followed by a blue fixation cross until subject pressed 1 of 3 buttons or up to 1.5 s after trial onset. Button press caused the fixation cross to turn white. Feedback was presented for 1 s and came 1.5 s after trial onset. Feedback consisted of the words “correct” or “incorrect” in green and red, respectively. The intertrial interval consisted of a white fixation cross with jittered duration to allow trial-by-trial event-related analysis of fMRI signal. Blocks set sizes varied between one and six and the order was randomized across subjects.