Fig. 1.

Experimental procedure. One second to 2 s after pressing the space bar, the search array appears for 30–200 ms depending on the randomly chosen SOA for this trial. Observers report the presence or absence of the target stimulus T among distracting L letters. After the variable SOA (from 30–450 ms relative to search array onset), two probe letters appear randomly for 80 ms at two of the four search array locations. For probe onset SOAs greater than 200 ms, an additional empty screen is presented between the search task and the probe detection task (the fixation point is maintained). In other words, if the SOA was shorter than 200 ms, the interstimulus interval (ISI) was zero; otherwise, the ISI was greater than zero. Masks follow probe stimuli for 200 ms. After mask offset, observers first report the presence or absence of the T among L’s, and then the identity of the two probe stimuli by selecting letters from a list, using the computer mouse. A trial ends when observers click on the end button.