Figure 1.

Experimental design. a, Subjects saw four dots (black) and retained their locations in working memory (indicated here by white squares for illustration only). After a variable interval, a cue line ran through the vicinity of one or two of the dots. Participants were asked to select in the mind the location of the dot that was closest to the cue line and maintain it as a specific target item. The number of cue presentations varied from two up to five. At the end of a trial subjects judged whether a probe matched the dot-position that had been cued by the immediately preceding line and received a feedback. A short response window (0.9 s) ensured that each cue was actively attended and processed (for details, see Materials and Methods). b, The sequence of cues determined the demands for updating: During the high-selection/update-condition (+S+U), the cue line ran through the vicinity of two of the encoded dots requiring the selection of the closer dot position. In addition, the cue indicated a different target dot as on the immediately preceding cue presentation, thus needing an update of the focus of attention. In the high selection/no-update-condition (+S−U), the cue was ambiguous too but indicated the same target as the preceding cue, hence not requiring any change of the focus of attention. In the low-selection/update-condition (−S+U), the cue line ran through the vicinity of only one dot and required an update of the focus of attention. In the low-selection/no-update-condition (−S−U), a cue unambiguously denoted the same target as in the preceding presentation. ISI, Interstimulus interval; ITI, intertrial interval.