Perceiving event structure in brief actions.

Event segmentation is a fundamental component of human perception and cognition. The field of event cognition studies how people decide where distinct events occur in incoming sensory data, how these "event boundaries" alter decision-making and memory processes, how events reveal themselves in neural activity, and how events may be represented within perception itself. The latter point is critical - the representation of events in the first place is filtered through perception. But what counts as a minimal event that is perceptible to humans? And to what extent is the perceptual representation of minimal events driven by physical properties within stimuli (e.g., sudden changes of a tennis ball's direction when one player strikes it) versus the semantic structure of events (e.g., "step one" versus "step two" of a tennis serve)? Here, across seven preregistered experiments, we explore the perceptual representation of event structure within single brief actions, and dissociate the roles of visual features and semantic structures in the perceptual segmentation of minimal events. First, participants produced boundary labels by segmenting videos of brief physical actions (e.g., kicking a ball). Then, separate groups of observers were asked to visually detect subtle disruptions in the video clips, unaware that the disruptions systematically occurred at boundary versus non-boundary timepoints. The results consistently showed an interfering effect of event boundaries on the detection of disruptions, suggesting a spontaneous perceptual representation of action structure even in very brief single actions. Moreover, boundary effects were strongest when stimuli were presented in recognizable forms versus distorted forms that only preserved lower-level features. Thus, automatic and rapid perceptual segmentation of single actions that only last several seconds may be driven by both sensory cues and our internal models of the world.