First, she has tried to determine which visual properties are detected preattentively. Second, she has formulated a hypothesis about how the human visual system performs preattentive processing. Triesman ran experiments using target and boundary detection to classify preattentive features. For target detection, subjects had to determine whether a target element was present or absent in a field of background distractor elements. Boundary detection involved placing a group of target elements with a unique visual feature within a set of distractors to see if the boundary could be preattentively detected.
Researchers test for preattentive target detection by varying the number of distractors in a scene.
If search time is relatively constant and below some chosen threshold, independent of the number of distractors, the search is said to be preattentive. Similarly, for boundary detection, if users can classify the boundary within some fixed exposure duration, the feature used to define the boundary is said to be preattentive. The human visual system cannot decide to change where the eye is looking within this time frame. It is important to note that some of these features are asymmetric. For example, a sloped line in a sea of vertical lines can be detected preattentively.