Fig. 1.

Experimental design. Four disks filled with high-contrast dynamic random noise were constantly shown to the subjects. At the beginning of each block, a cue display indicated which of the disks were likely to contain the probe for the following trials (set size 1–4). The probe was a contrast decrement of variable duration, occurring with 50% probability in one of the cued elements. At set size 1, “full attention” could be allocated to the probe, whereas at set sizes 2, 3, and 4, attention had to be “divided” between multiple targets. In 10% of trials (including probe-present trials), a catch stimulus (similar to the probe) was shown at an uncued location. By definition, we assume that it received “minimal attention.” (Set size 4 trials had no uncued location and therefore could not serve as catch trials). At the end of each trial, subjects reported whether the probe was presented among the cued locations, and (using a separate key) whether a catch stimulus had been detected at an uncued location. Probe and catch stimulus duration varied independently between 50 and 1,000 ms. The strength of the contrast decrement could either be 50% (“easy” version) or 30% (“difficult” version).