Color needs luminance for visual selection during scene search

When searching visual scenes, in addition to high-level scene semantics, we use low-level visual information from objects' defining features, such as color and luminance contrasts. What is the relative influence of color and luminance? Basic perceptual research suggests that we are not very sensitive to peripheral color. Yet color is thought to be an important basic feature guiding visual search. Previous gaze-contingent research shows that targets can be identified faster in color than in black-and-white scenes, therefore the availability of color in the visual periphery indeed helps visual search. However, color contrasts and luminance contrasts usually covary at object boundaries. Here we study the isolated roles of color and luminance during object-in-scene search by presenting either only color or only luminance contrasts in peripheral vision, using a gaze-contingent moving-window display and varying the amount and type of peripheral preview. We found that peripheral target selection was more efficient with luminance contrasts, whereas color was hardly used beyond the parafovea. We conclude that color contrasts in peripheral vision can only be efficiently used in scene search when they are correlated with luminance contrasts.