Visual Search for Singleton Targets Redundantly Defined in Two Feature Dimensions: Coactive Processing of Color-Motion Targets?

In 2 visual search experiments, the role of feature contrast/saliency signals in generating detection responses to singleton feature targets in visual search was investigated using the redundant-target paradigm. Experiment 1 showed that coactive integration of dimensional signals is not restricted to targets defined on the color and orientation dimensions; rather, targets involving any of the combinations of color, orientation, and motion, are integrated coactively, as evidenced by violations of Miller's (1982) race model inequality. Experiment 2 replicated the findings of Experiment 1 for color-motion targets, with the target items' luminance adjusted, individually for each observer, to that of the distractors. The evidence for coactive processing of motion (saliency) with color and, respectively, orientation (saliency) signals suggests that, at variance with a recent suggestion by Li (2002; Koene & Zhaoping, 2007), signal integration in feature search tasks occurs at a stage following initial feature coding in primary visual cortex (V1), even though feature contrast computations in V1 may well contribute to saliency coding. In sum, the results suggest that detection responses were based on an integrated, overall-saliency representation indicating the presence of an odd-one-out item in the display, consistent with the dimension-weighting account of visual search (Müller et al., 1995, 2003).