Surface segregation influences pre-attentive search in depth

While searching for an object in a cluttered scene, in some situations, the visual system adopts a pre-attentive parallel search, where the time taken is independent of the number of items in the scene. In others, the search is serial, time taken being a function of the set size. We show that detecting the number of targets (2, 3 or 4) that differ in depth from background items is a parallel process, but only when they are all in the same surface in depth. The search is serial if the targets are in different depth planes, but parallel even if the targets were on a surface tilted in depth, showing that surface segregation influences a parallel, apparently pre-attentive, stage.