1 Introduction In our daily life, we see a number of objects at the same time. These `objects' may remain still; however, frequently a number of them may move around, and their motions are usually independent of each other. Despite their changes across space and time, we rarely get confused and are able to determine which object went where. This ability implies that our visual cognitive processes hold a representation that persists dynamically across space and time. Numerous studies suggest the existence of object representa- tions that can track changes in the location of an object over time [eg object files (Kahneman and Treisman 1984; Kahneman et al 1992)]. However, the manner in which dynamic information (such as the motion of objects) contributes to maintaining such representations is still an open question. Stone (1998, 1999) suggested that information on both form and motion (in his case, rotation of the object) is integrated and represented in visual memory. Newell et al (2004) demonstrated that the trajectory (path) of an object is an effective cue in object categorisation tasks, and that motion information is as perceptually salient as other features such as colour or shape. Newell et al (2004) argued that motion information is incorporated into visual memory along with other object information. However, the role that motion information itself plays in dynamic visual processing has not yet been resolved. Further, since only one object was presented in the experi- ment of Newell et al (2004), it is unclear whether this type of motion information is actually stored and used in natural viewing situations. Along these lines, we have explored the role of object motion information, in this case the trajectory, in the con- struction and maintenance of object representations. Pylyshyn and Storm (1988) tested the ability to maintain access to multiple objects using an experimental paradigm referred to as multiple object tracking (MOT). In an MOT task, participants track several moving targets among a set of moving distrac- tors. In most cases, the objects (targets and distractors) are identical with regard to shape Grouping and trajectory storage in multiple object tracking: Impairments due to common item motions Perception, 2006, volume 35, pages 483 ^ 495 Mutsumi Suganumaô, Kazuhiko Yokosawa Department of Psychology, Graduate School of Humanities and Sociology, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan (ô also JSPS Research Fellow); e-mail: mutsumi1008@hotmail.com, yokosawa@L.u-tokyo.ac.jp Received 13 January 2004, in revised form 22 June 2005 Abstract. In our natural viewing, we notice that objects change their locations across space and time. However, there has been relatively little consideration of the role of motion information in the construction and maintenance of object representations. We investigated this question in the context of the multiple object tracking (MOT) paradigm, wherein observers must keep track of target objects as they move randomly amid featurally identical distractors. In three experi- ments, we observed impairments in tracking ability when the motions of the target and distractor items shared particular properties. Specifically, we observed impairments when the target and distractor items were in a chasing relationship or moved in a uniform direction. Surprisingly, tracking ability was impaired by these manipulations even when observers failed to notice them. Our results suggest that differentiable trajectory information is an important factor in successful performance of MOT tasks. More generally, these results suggest that various types of common motion can serve as cues to form more global object representations even in the absence of other grouping cues. DOI:10.1068/p5487