Abstract
Recent work suggests that the units of visual short-term memory (VSTM) are integrated object representations. The primary evidence for this comes from sequential change detection studies in static displays. Here we study how VSTM operates in dynamic displays, wherein the identities of objects must be maintained over time and motion. On each trial, participants viewed 2, 4, or 6 colored moving shapes, each of which disappeared at an invisible occluder, and reappeared 900 ms later at the occluder's other edge. In the Occlusion condition, objects disappeared and reappeared via deletion and accretion along a single edge. In the No Occlusion condition, objects simply disappeared and reappeared instantaneously. Subjects had to detect color changes that occurred on half of the trials. Performance was compared (within subjects) to a Static condition, wherein objects disappeared and reappeared at the same location. Overall performance was equivalent in all conditions, but the interactions with set size proved interesting: There was no difference between conditions with 2 objects, due to a ceiling effect. With 6, performance was better in the Static condition. Most intriguingly, performance with 4 objects was actually *better* with dynamic stimuli in the No Occlusion condition. These results suggest that VSTM storage may occur differently for displays of 4 or fewer objects: up to four objects can be simultaneously attended, with motion then serving as a cue to help divide attention. Efficient VSTM encoding thus occurs in terms of integrated object representations. In contrast, you cannot divide attention over 6 objects in the first place; thus the motion cue cannot aid VSTM encoding, and is only an added distraction. Most generally, these results demonstrate that change detection can be used to explore the factors which aid or constrain VSTM in dynamic displays, and further experiments investigate how manipulations of object persistence influence VSTM encoding.
Supported by an NSF graduate research fellowship to JIF and NSF #BCS-0132444 to BJS.