Parallel processing of two disjunctive targets

We examined the capabilities of preattentive processing by asking subjects to report the relative location of two visual targets, each placed in a separate group of stimuli. In the first experiment, the two targets differed from their distractors on the same feature dimension (shape or color) or on different dimensions, one distinguished by its form and the other by its color. Reaction times (RTs) suggest that the two targets were processed simultaneously (i.e., in parallel). Indeed, the overall processing time was as long when two feature dimensions were involved as the longest processing time for either dimension alone. These results imply that the preattentive processing of several stimuli can occur simultaneously along at least two different feature dimensions. In the second and third experiments, when the target in one group of stimuli was used as the distractors in the other group, RTs increased significantly over RTs in conditions in which the distinctive feature of the targets was unique in the display. In none of these conditions did RTs vary with the number of distractors. These results indicate that although both targets are processed simultaneously when the distinctive features of the targets are unique in the display, two parallel searches (one for each target) must be performed sequentially when there is an overlap between the target in one group of stimuli and distractors in the other. The notion of location as a separate feature dimension is also discussed.