Figure 1.

Experimental design. Visual stimuli were presented in one of the four quadrants at a peripheral location and probed under two conditions: attended and unattended. A schematic outline of one scan run is shown. In the unattended condition (frame #2), subjects performed a demanding letter-counting task at fixation, counting the number of target letters among distracters, while ignoring the peripheral stimuli. In the attended condition (frame #5), subjects were instructed by a brief occurrence of a cue at fixation (frame #4) to covertly direct attention to one of the four peripheral locations and to count the occurrences of a target stimulus in the location closest to fixation. Attended and unattended presentations were interleaved with blank fixation periods (frames #1, #3, and #6) during which subjects performed the letter-counting task. Blocks of attended and unattended presentations alternated with blank periods.