The Number of Expected Targets Modulates Access to Working Memory: A New Unified Account of Lag-1 Sparing and Distractor Intrusions

In rapid serial visual presentation (RSVP) tasks, failures of attentional selectivity are frequently observed when a single target is followed by a potentially reportable distractor (distractor intrusions). However, in tasks with two targets, accuracy for both targets is typically high when they are presented in immediate succession (lag-1 sparing). To account for this apparent contradiction, we tested whether expectations about the number of targets affects the number of items encoded in working memory (WM). Colored target digits were embedded among gray letters and digits. The first target was followed either by a gray digit, or a second target (another colored digit). To manipulate expectations, the ratio of one-target and two-targets trials (75% to 25% or vice versa) was varied between blocks. Participants were much more likely to report seeing two targets when two targets were expected. Analogous results were obtained in an additional experiment where two successive colored digits appeared on all trials, and participants were instructed to either report both or only the first digit. ERP markers of attentional allocation (N2pc) and WM storage processes (contralateral delayed activity, CDA) were larger when two targets were expected, regardless of the actual number of targets. These results show that the number of expected targets modulates the activation of sensory representations during attentional episodes, which affects the probability that they are subsequently encoded in WM. These findings suggest that a single mechanism can account both for lag-1 sparing and distractor intrusions in RSVP tasks. They also provide new evidence for strategic top-down control over WM encoding. Public Significance StatementWhen targets and distractors are presented in rapid succession at the same location, temporal attention plays an important role in whether observers can correctly report these targets. Our results suggest that expectations about the number of targets play an important role in modulating temporal attention. They show that expecting more targets increases the amount of information that can gain access to working memory.