Distractor rejection in visual search breaks down with more than a single distractor feature.

Previous research has established that interference from irrelevant-but-salient color distractors occurs when observers search for variable-shape singletons among uniform shapes (singleton detection), but not when they look for a specific shape among mixed shapes (feature search). In the current study, we replicated this finding, and additionally, we introduced a random variation of color. Instead of always presenting the same target and distractor colors, colors were swapped randomly from trial to trial. With random color variation, interference from color distractors was generally stronger, and in particular, it also occurred in feature search mode, suggesting that observers were unable to focus on the relevant dimension (shape) even under the most favorable conditions. A second experiment showed that interference in feature search mode occurred with variable distractor color when target color was fixed, but not with variable target color when distractor color was fixed. Overall, the results question the view that interference in feature search is absent because observers monitor a single feature map (i.e., shape). Rather, random variation of an irrelevant feature induces participants to also monitor the irrelevant feature, which results in interference. Thus, complete distractor rejection is limited to situations with a single distractor feature.