Visual search for oriented lines: the role of angular relations between targets and distractors.

Subjects can perform parallel visual search for an item of unique orientation when it is presented on a background texture composed of lines of two other orientations. A number of cues can be used to speed this search. It helps if the target has a unique categorical status (e.g., it is the only 'steep' item) (Wolfe et al., J. Exp. Psychol. Hum. Percept. Perf. 18, 34-49 1992). It helps if the two background orientations are symmetrical about a vertical axis (Wolfe and Friedman-Hill, Psychol. Sci. 3, 194-198, 1992). The experiments presented here show that it helps if the angles formed by the target with one of the distractor types are smaller than the angles formed by the two distractor orientations. These results illustrate that stimulus similarity is a complex concept even for a simple feature like orientation.