Target uncertainty does not lead to more distraction by singletons: intertrial priming does.

In two experiments, we examined why a singleton distractor has a stronger interfering effect in visual search when the target identity is uncertain. When participants searched for a shape, a color singleton distractor had a larger slowing effect in a mixed block, in which the target shape could change from trial to trial, than in a pure block, in which the target shape remained the same. Importantly, this increased singleton distractor effect could be traced back entirely to intertrial priming, since the increased costs occurred only on trials in which the target and the singleton distractor swapped identity (Experiment 1, allowing for priming between targets and singleton distractors) or on trials in which the target alone changed identity while the singleton distractor remained constant (Experiment 2, allowing for priming between targets only). This suggests that target uncertainty itself does not lead to strategic changes in the attentional selection of singletons. Instead, selection is affected by relatively automatic priming mechanisms that may be enhanced by competition for attention.