Zoe (Jing) Xu^1,2 (), Jun-Ming Yu¹, Alejandro Lleras¹, Simona Buetti¹; ¹University of Illinois, Urbana Champaign, ²University of Washington

Visual search targets are not always predictable - sometimes observers know one feature of the target (color of a scarf) but might not be able to anticipate other features (the scarf shape). Here, participants searched for a target that differed from distractors along both color and shape, with one feature dimension being known and one varying randomly across trials. Using a model comparison approach, we quantified the contribution of each feature dimension to attentional guidance. Participants were asked to search using the known dimension. In Experiment 1 (color known), the target was red and had one of four possible shapes. Distractors were orange (color-similar) or pink (color-dissimilar) and had one of two possible shapes. In Experiment 2 (shape known), the target was a house in one of four possible colors. Distractors were triangles (shape-similar) or circles (shape-dissimilar) in one of two possible colors. In both experiments, response times for all target-distractor pairs (e.g., red house target among pink diamonds) were predicted by the weighted combination of the search slopes evaluated in simple color (red among pink) and shape searches (house among diamonds). These weights indicate the extent to which each feature dimension was prioritized in attentional guidance. In Experiment 1, the results showed that the priority given to the unpredictable shape dimension depended on the usefulness of color: the shape weight was 0.85 (color weight=1.15) in the color-dissimilar condition, while the shape weight was 1.6 (color weight=0.4) in the color-similar condition. In Experiment 2, the weight for the unpredictable color dimension was 0.47 (shape weight=1.53) in the shape-dissimilar condition, and the color weight was 1.59 (shape weight=0.41) in the shape-similar condition. These results demonstrate that when the target and distractors are too similar along the known dimension, the visual system increases its reliance on the unpredictable dimension.