Searching for two feature singletons in the visual scene: the localized attentional interference effect

The localized attentional interference (LAI) effect was investigated in a visual search task requiring participants to simultaneously monitor two spatially separated features from the same or different dimensions. In Experiment 1, the search type was blocked and targets were defined by fixed feature values in two dimensions (e.g., a yellow item and a circular item). In contrast, in Experiment 2, participants had to look for a color and a form singleton, with the exact feature values varying randomly across trials. In both experiments, reaction times (RTs) were generally slower when two features were CLOSE to, rather than DISTANT from, each other. Moreover, RTs to CLOSE stimuli increased as the search set size increased, while RTs to DISTANT stimuli were unaffected by set size. Experiment 3 also used a singleton search task, but with the two singletons defined either in different dimensions or in the same dimension. A larger interference effect for CLOSE, as compared to DISTANT, stimuli was found for cross-dimension than for intra-dimension targets. These findings suggest that neighboring items, irrespective of whether these items are from the same or different dimensions, interfere with each other in attentional selection, and that searching for two cross-dimension targets may engage a process of dimension switching to effectively solve the ambiguity of each item, especially when these items are close to each other.