Top-down instructions influence the attentional weight on color and shape dimensions during bidimensional search.

Efficient searches are guided by target-distractor distinctiveness: the greater the distinctiveness, the faster the search. Previous research showed that when the target and distractors differ along both color and shape dimensions (i.e., bidimensional search), distinctiveness along individual dimensions combine collinearly to guide the search, following a city-block metric. This result was found when participants expected the target and distractors to differ along both dimensions. In the present study, we used an instruction manipulation to investigate how bidimensional search varies in response to different top-down instructions. Using unidimensional search performance observed in Experiment 1, we predicted bidimensional search performance under three conditions: when participants were instructed to attend to color (Experiment 2), shape (Experiment 3), or both (Experiment 4). Results showed that instructions influenced how distinctiveness along color and shape combine to guide attention: when instructed to search for a target color, participants allocated more attentional weight to the color dimension (and less weight to the shape dimension) compared to when instructed to search for a target shape. Our study presents a novel technique to quantify how top-down instructions change attentional weighting to different features during bidimensional visual searches.