Parallel search

Performance on visual search tasks is often measured by the time it takes to complete the search. Psychologists then examine the effects of varying the nature of the difference between target and distractors, and the number of distractors. When the target differs from the distractors on only a single feature (such as tilt), the search time involved in making a decision whether or not a target is present is about the same whatever the number of distractors, and whether or not there is a target in the array (Treisman & Gormican, 1988). In positive trials the target is present in the display, whereas in negative trials the target is absent in the display. This pattern of performance is described as parallel search, [parallel search a visual search task in which the time to find the target is independent of the number of items in the stimulus array because the items are all processed at the same time (in parallel)] as items from all over the display are analysed separately and simultaneously. As well as tilt, stimulus dimensions on which target/distractor differences allow parallel search include luminance (Gilchrist et al., 1997), colour (Treisman & Gelade, 1980), size (Humphreys et al., 1994), curvature (Wolfe et al., 1992) and motion (McLeod et al., 1988). This list of features is very similar to those that give after-effects and govern grouping and segmentation.