Abstract
The currency of visual experience is frequently not static scenes, but dynamic events. And perhaps the most central topic in the study of event perception is *event segmentation* -- how the visual system carves a continuous stream of input into discrete temporal units. A different tradition has tended to focus on particular types of events, the most famous example of which may be *causal launching*: a disc (A) moves until it reaches another stationary disc (B), at which point A stops and B starts moving in the same direction. Since these two well-studied topics (event segmentation and causal perception) have never been integrated, we asked a simple question: how many events are there in causal launching? Just one (the impact)? Or two (A’s motion and B’s motion)? We explored this using spatial memory, predicting that memory for intermediate moments within a single event representation should be worse than memory for moments at event boundaries. Observers watched asynchronous animations in which each of six discs started and stopped moving at different times, and (in different experiments) simply indicated each disc’s initial and final position. The discs came in pairs, and in some cases A launched B. To ensure that the results reflect perceived causality, other trials involved the same component motions but with spatiotemporal gaps between them (which eliminate perceived launching). The critical locations were the two intermediate ones (A’s final position and B’s initial position), and spatial memory was indeed worse for launching displays (perhaps because these locations occurred in the middle of a single ongoing event) compared to displays with spatiotemporal gaps (perhaps because these same locations now occurred at the perceived event boundary between A’s motion and B’s motion). This suggests that causal perception leads the two distinct motions to be represented as a single visual event.