Tracking Objects vs. Substances
Here are some demonstrations of the various conditions discussed in the following paper:
vanMarle, K., & Scholl, B. J. (2003). Attentive tracking of objects vs. substances. Psychological Science, 14(5), 498-504.
These demonstrations are provided as Quicktime movies, which can be downloaded or viewed directly in most web-browsers. (To download a free Quicktime player, go here.) These movies are a bit large and choppy, and the spatiotemporal parameters are slightly different than those used in the experiments, but they should be sufficient to illustrate the basic effects.  
Recent research in vision science, infant cognition, and word-learning all suggest a special role for the processing of individual discrete objects. But what counts as an object? Answers to this question often depend on contrasting object-based processing with the processing of spatial areas, or unbound visual features. In infant cognition and word-learning, though, another salient contrast has been between rigid cohesive objects and nonsolid substances. Whereas objects may move from one location to another, a nonsolid substance must pour from one location to another. Here we explore whether attentive tracking processes are sensitive to dynamic information of this type. We used a multiple-object tracking paradigm in this study: For each display, you should attempt to track the four items which are briefly flashed at the beginning of the movie. (Our subjects watched these movies, then had to use the computer mouse to indicate the 4 targets at the end of each trial.)
Experiment 1a: Objects vs. Substances
Our basic contrast in Experiment 1 showed that subjects could do very well at tracking 4 in 8 distinct, rigid, cohesive objects, as in this Object condition movie, but were drastically impaired when trying to track objects which moved in a substance-like manner, essentially pouring from one location to another, as in this Substance condition movie. These conditions were tested between subjects, and each subject saw trials wherein the items moved through the identical trajectories, at the same speeds, etc.
Experiment 1b: Non-overlapping Objects vs. Substances
A further control experiment demonstrated that the selective impairment for tracking substances in Experiment 1a was not due simply to the fact that the 'streams' in the Substance tended to overlap more often than the localized items in the Objects (even though the objects experienced a greater proportional overlap). Even when the objects and substances did not overlap during their haphazard motions, observers were still able to track objects, as in this Non-overlapping Object condition movie, much more successfully than substances, as in this Non-overlapping Substance condition movie. Again, these conditions were tested between subjects, and each subject saw trials wherein the items moved through the identical trajectories, at the same speeds, etc. [Note: For some reason this non-overlapping Substance movie evidently plays in an especially 'choppy' fashion on some browsers. If you download the movie and play it locally, the choppiness should go away.]
Experiment 2: Rigidity vs. Cohesion vs. Extension
Rigidity: The 'Morphing' Condition
Because the substance-like motion used in Experiment 1 violated several conceptually-independent constraints on objecthood, we attempted in Experiment 2 to determine the nature of the critical difference. The culprit turned out not to be the violation of rigidity, per se, since subjects performed well in our 'Morphing' condition, when they had to track objects which still moved as coherent entities but constantly 'mophed' between different shapes, constantly shifting their local boundaries -- as in this Morphing condition movie.
Cohesion: The 'Groups' Condition
The culprit also turned out not to be the violation of cohesion, per se, since subjects performed well in our 'Groups' condition, when the items moved from position to position as a group of four small squares which underwent some relative motion while moving, like a compact swarm of insects -- as in this Groups condition movie.
Extension: The 'Slinky' Condition
The factor which appeared to be critically responsible for the impairment in attentively tracking substances turned out to be a particular type of repeated nonrigid motion: extending from a local image area to a long extended feature cluster, then shrinking back into a local object. (Note that this type of transformation essentially renders 'the' location of the item ambiguous. It is as if you were asked to point to a slithering snake: Would you point to its head? To its middle? What if its middle kept changing as it stretched and shrunk?) We demonstrated this with a final 'Slinky' condition, wherein the objects extended and contracted from position to position in the same manner as the Substance condition, but in doing so they always kept a single unbroken boundary -- as in this Slinky condition movie.
As described in our paper, these results have implications for the understanding of the object/substance distinction in other areas of cognitive science -- such as infant cognition and word learning. Beyond these links to other research areas, these results contribute directly to the study of what counts as an 'object' of dynamic object-based attention. Previous studies of object-based attention have focused on contrasts with spatial areas or unbound visual features. Our results represent a new third contrast: attentional tracking is primarily based on discrete objects, rather than locations, features -- or substances.