Intertrial target-feature changes do not lead to more distraction by singletons: target uncertainty does.

The presence of an irrelevant singleton disrupts search for a singleton target substantially more when the target feature varies unpredictably (mixed-singleton search) than when it is known in advance (fixed-singleton search). This finding suggests that advance knowledge of the target feature guides singleton search. Pinto et al. [Pinto, Y., Olivers, C. N. L., & Theeuwes, J. (2005). Target uncertainty does not lead to more distraction by singletons: Intertrial priming does. Perception & Psychophysics, 67, 1354-1361] proposed an alternative account, according to which this difference results from inter-trial priming effects. They based their argument on the finding that distractor interference is reduced when the singleton target feature repeats vs. switches from one trial to the next. However, Lamy et al. [Lamy, D., Carmel, T., Egeth, H., & Leber, A. (2006). Effects of search mode and inter-trial priming on singleton search. Perception & Psychophysics, 68, 919-932] reported no such modulation of distractor interference by target-feature repetition. Here, we show that differences in design (blocking conditions of distractor presence in the former study vs. randomly mixing them in the latter) account for this discrepancy. We conclude that the different task demands induced by the blocked distractor-present and distractor-absent conditions rather than distractor presence per se interact with intertrial priming effects. These findings argue against the claim that singleton search relies exclusively on stimulus-driven factors and suggest that preknowledge of the target feature, when available, can guide attention. In addition, the present results challenge the ambiguity hypothesis of intertrial priming, according to which increased competition for attentional selection boosts inter-trial priming effects.