Visual search for feature singletons: multiple mechanisms produce sequence effects in visual search.

Selection of a feature singleton target in visual search tasks, e.g., a red target among green distractors, is very fast--as if the target "popped out" of the display. Interestingly, reaction times (RTs) sometimes even decrease with an increase in the number of distractors (while keeping the presentation area fixed), i.e., there is a negative RT/display density relationship. Furthermore, repeating--versus changing--target-defining properties across trials also speeds up RTs. The present study investigated how display density influences two similar but dissociable types of such intertrial effects, namely (a) priming of pop-out (PoP), observed when the target-defining dimension is fixed, e.g., color, and only the features of the target and distractors, e.g., red and green, vary across trials and (b) the dimension-repetition effect (DRE), observed when both the features and dimensions of the target vary, e.g., from red circle (color) to blue square (shape target) among blue circles. Experiment 1 examined PoP magnitude with sparse (three-item) versus dense (36-item) displays in conditions in which the distractors' color either (a) varied, i.e., red target, green distractors versus green target, red distractors, or (b) it was fixed (blue). Significant PoP was observed only for sparse distractors conditions. Experiment 2 investigated the DRE magnitude across display densities with distractors always being fixed: Significant DREs of comparable magnitude were observed with both sparse and dense displays. This dissociation between the PoP and DREs suggests, first, the existence of multiple mechanisms of intertrial effects and, second, that PoP is specific to low target-distractor signal-to-noise ratios when the target fails to pop out.