Abstract
Four experiments explored the mechanisms by which participants achieve efficient conjunction search, by measuring the interference caused by the presence of an irrelevant singleton, unique in either motion direction, colour, or orientation. Experiments 1 (moving vertical target amongst moving horizontal and stationary vertical distractors) and 2 (static vertical target amongst static horizontal and moving vertical distractors) investigated conjunctions of motion and orientation. Experiments 3 and 4 investigated conjunctions of colour and orientation (e.g. red vertical target amongst green vertical and red horizontal distractors). In Experiment 3 all items moved, whereas in Experiment 4 all items (except the motion singleton when present) were stationary. Participants judged whether the target was present or absent. Across all experiments orientation singletons (an item with unique orientation) caused large and robust interference. This orientation interference was strongly modulated by the singleton’s task relevant motion or colour, such that interference was much larger when the singleton shared the target’s motion or colour. The interfering effect of colour was much smaller and was not always statistically significant. Motion singletons interfered very little when the target was defined by colour but created substantial interference when the target was defined by motion. Of particular interest when searching for a stationary target amongst moving and stationary distractors, in Experiment 2, a motion singleton (an item moving in a different direction to all other items) caused substantial interference, even when it had no task relevant features. The results provide evidence for very imprecise control of search by orientation coupled with the selection of subsets of elements based on common colour or motion; sub-set search. However, motion is special in providing very strong bottom-up constraints on search such that distinctive elements within a moving group continue to cause substantial distraction even when they have no target features at all.