Visual search for conjunctions of motion and form: The efficiency of attention to static versus moving items depends on practice

Visual search for some motion-form conjunctions can be performed in paralle l. Yet, if the target is easy to discriminate from the nontargets (target l ine tilted 45 degrees from the vertical), search can be easier for a moving than for a stationary target. Driver and McLeod (1992; Berger & McLeod, 19 96) took this asymmetry to argue that gross aspects of form discrimination are performed within a motion filter that represents only the moving items, whereas discrimination of stationary items (and all fine discrimination) r elies on a static form system. However, recent (unsuccessful) attempts to r eplicate the asymmetry (Muller & Found, 1996; Muller & Maxwell, 1994) sugge sted that it may occur only early during task performance, due to participa nts having difficulty keeping the moving items out of the search for a stat ionary target (but not vice versa). This was confirmed by the present study , which investigated the effects of practice on search among the moving and stationary subset of items. The results suggest that attention to the stat ionary subset is difficult initially because participants cannot efficientl y compensate for the natural bias of the motion filter to pass the moving i tems (rather than filter them out). This ability improves with practice. Th us, there is no fixed limit to performance with stationary targets and, con sequently, no need to assume that any form discrimination is performed with in the motion filter.