Matching biological motion at extreme distances.

The goal of the current paper was to determine the maximum distance at which an actor could be placed so that an observer would still be able to interpret their behavior. Although we know a great deal about the limits of action perception, particularly through studies of biological motion processing, this question of distance has not been previously documented. We began by reviewing the sizes of point-light figures used in 100 previous studies of biological motion. We found that with an average figure height of 6.6° [corrected] visual angle, actors were effectively 15 m from the observer, assuming average physical height of 1.75 m. No previous studies had explicitly examined extreme distances. Here, we introduce a new matching task in which we systematically varied the apparent distance of point-light figures relative to a fixed viewing position by manipulating size. Our results suggest that a variety of human actions could potentially be interpreted up to 1000 m away, a distance at which a human figure would subtend only 0.1° visual angle in height. Dynamic figures could be interpreted at further distances than static figures (Experiment 1), and upright figures were similarly processed more efficiently than inverted figures (Experiment 2). We discuss these findings in the context of the processing mechanisms thought to underlie action perception and suggest that the ability to match actions at extreme distance is another example of the robust nature of biological motion processing.