Visual search for transparency and opacity: attentional guidance by cue combination?

A series of seven experiments explored search for opaque targets among transparent distractors or vice versa. Static stimuli produced very inefficient search. With moving items, search for an opaque target among transparent distractors was quite efficient while search for transparent targets was less efficient (Experiment 1). Transparent and opaque items differed from each other on the basis of motion cues, luminance cues, and figural cues (e.g., junction type). Motion cues were not sufficient to support efficient search (Experiments 2-5). Violations of the luminance rules of transparency disrupt search (Experiments 3 and 4). Experiment 5 shows that search becomes inefficient if X-junctions are removed. Experiments 6 and 7 show that efficient search survives if X-junctions are occluded. It appears that guidance of attention to an opaque target is guidance based on "cue combination" (M. S. Landy, L. T. Maloney, E. B. Johnston, & M. Young, 1995). Several cues must be present to produce a difference between opaque and transparent surfaces that is adequate to guide attention.