Procedure

Subjects sat on a reclining chair in an electrically and acoustically shielded room facing a computer monitor.

The screen was at a distance of approximately 100 cm from the subject’s eyes. A rectangular blue window (11 × 3 cm) was always present at the center of the screen. Stimuli were foveally presented in this window for 500 msec, at a rate of one every 1250 msec (SOA).

The subjects were instructed to avoid blinking while the stimuli were exposed. They were given one practice block before each of the four tasks, which were performed within one session lasting about 1.5 h (not including electrode placement procedures—see below).

The four experiments were presented in fixed order: size task, rhyme task, lexical tasks, and semantic decision.

The fixed order was necessary to reduce the possible interference of a deeper-level process with a more superficial level. However, the order of the three lexical decision tasks was counterbalanced (using a Latin square design) between subjects. In each task, the stimuli were delivered randomly in blocks of 50 items each. The first task included 10 blocks (500 stimuli), and the second task included 6 blocks (284 stimuli); each of the three lexical decision types in the third task was composed of 2 blocks (100 stimuli), and the fourth task was composed of 6 blocks (284 stimuli) (Table 1). Subjects reported the number of target stimuli detected after each block.