Effects of display complexity on location and feature inhibition

Inhibition of return refers to the lengthening of reaction times (RTs) to a target when a preceding stimulus has occupied the same location in space. Recently, we observed a robust inhibitory effect for color and shape in moderately complex displays: It is more difficult to detect a target with a particular nonspatial attribute if a stimulus with the same attribute was recently the focus of attention. Such nonspatial inhibitory effects have not generally been found in simpler displays. In the present study, we test how location-based and nonspatial inhibitory effects vary as a function of display complexity (eight, six, four, and two locations). The results demonstrated that (1) location-based inhibition effects were much stronger in more complex displays, whereas the nonspatial inhibition was only slightly stronger in more complex displays; (2) nonspatial inhibitory effects emerged at longer stimulus onset asynchronies than did location-based effects; and (3) nonspatial inhibition appeared only when cues and targets occurred in the same locations, confirming that pure feature repetition does not produce a cost. Taken together, the results are consistent with perceptual processes based on object files that are organized by spatial location. Using somewhat more complex displays than are most commonly employed provides a more sensitive method for observing the role of inhibitory processes in facilitating visual search. In addition, using a relatively wide set of cue-target timing relationships is necessary in order to clearly see how inhibitory effects operate.