'Target Merging' in Multiple Object Tracking
This page contains some sample demonstrations from the following paper:
Scholl, B. J., Pylyshyn, Z. W., & Feldman, J. (2001). What is a visual object? Evidence from target merging in multiple object tracking. Cognition, 80(1/2), 159-177.
The demonstrations are provided as Quicktime movies, which can be downloaded or viewed directly in most web-browsers. These movies are a bit large and choppy, but they should be sufficient to illustrate the basic conditions. If the movies seem too choppy or if the lines are not drawn smoothly, try downloading the movies and playing them off your local hard drive.  
The multiple object tracking task allows us to explore various issues related to object-based attention in an attentionally demanding situation. One of the most crucial tasks relating to any cognitive or perceptual process, for instance, is to determine the nature of the basic units over which that process operates. We have used MOT to examine what can 'count' as an independently attended visual object in the first place. We investigated this question by using a technique we call target merging: we alter tracking displays so that distinct target and distractor locations appeared perceptually to be parts of the same object, by merging pairs of items (one target with one distractor) in various ways. The data show that target merging makes the tracking task far more difficult, to varying degrees depending on exactly how the items are merged with regard to connectedness, part structure, and other types of perceptual grouping. Tracking thus appears to be an 'object-based' process, and these various factors mediate what counts as an 'object' of visual attention. Below are just a few of the many conditions we tested. (These studies are also described in Scholl, 2001, Cognition.)  
Tracking boxes (baseline) (516 KB)  
Tracking ends of lines (528 KB)  
Tracking ends of 'rubber bands' (with occlusion) (1.1 MB)  
Tracking ends of dumbbells (860 KB)