Goal-relevant events need not be rare to boost memory for concurrent images.

In the attentional boost effect, memory for images presented at the same time as unrelated targets (e.g., an orange square) is enhanced relative to images presented at the same time as distractors (e.g., a blue square). One difficulty in understanding the nature of this enhancement is that, in most experiments demonstrating the attentional boost effect, targets have been less common than distractors. As a result, the memory enhancement associated with target detection may have been driven by differences in the relative frequencies of targets and distractors. In four experiments, participants encoded images into memory at the same time that they monitored a second, unrelated stimulus stream for targets. In some conditions, targets were as common as distractors (1:1 ratio); in others, targets were rare (1:6 ratio). The attentional boost effect was present when the target and distractor frequencies were equated, ruling out oddball and distinctiveness effects as explanations. These effects were observed when targets required a buttonpress and when they were covertly counted. Memory enhancements were not observed for images presented at the same time as rare distractor stimuli. We concluded that selectively attending to events that require an overt or covert response enhances the processing of concurrent information.