Visual search for conjunctions of motion and form: display density and asymmetry reversal

In visual search for motion-form conjunctions, search rates have been reported to be faster for moving than for stationary targets if the target-nontarget discrimination is easy (45 degrees target line tilt from vertical), but this asymmetry is reversed if the discrimination is difficult (9 degrees tilt) (J. Driver & P. McLeod, 1992). Driver and McLeod proposed that gross aspects of form discrimination are performed within a motion filter that represents only the moving items, whereas fine discriminations rely on a stationary form system that is poor at filtering by motion. However, H. J. Müller and J. Maxwell (1994) failed to observe the asymmetry reversal, possibly because they used lower density displays. The study reported in this article also did not yield an effect due to varying display density. This lends support to the notion of a unitary form system, with the role of the motion filter being limited to guiding the search to the moving items or, if required by the task, the stationary items.