On the relationship between cognitive load and the efficiency of distractor rejection in visual search: The case of motion-form conjunctions

Search for a target defined by a conjunction of movement and shape (a moving X amongst moving O and static Xs) is performed efficiently, with static distractors contributing little to observed RT. How search may be restricted to the moving items, whilst static items are ignored is not fully understood. Whether, passive bottom-up, or active top-down control processes are recruited is unknown. The current study addressed this question by asking participants to search for a motion–shape conjunction target whilst holding a low (1 digit) or a high (6 digit) verbal memory load. In Experiment 1 the number of distractors with the target motion (moving Os), shape (static Xs), or neither (static O) was systematically varied (4 or 8 items). Target localisation time was most sensitive to the number of moving objects, less sensitive to the number of target shaped items, and insensitive to the number of items with no target features. A memory load of six items slowed responding, but the effect of increasing each type of distractor remained unchanged. Experiment 2 compared conjunction (moving X target amongst moving O and static X distractors) and feature (moving X target amongst moving O and static O distractors) search. Both searches were slowed to the same extent under a high memory load but search slope did not change. The results are consistent with the idea that sustained distractor rejection in search for motion–form conjunctions is largely insensitive to manipulations of cognitive load.