The ability to detect the presence of another organism in visually ambiguous environments (low illumination, high occlusion) is of considerable evolutionary benefit. Human proficiency for the task has been illustrated by the perceptions of stimuli displaying only the motions of the major joints of an otherwise invisible actor, so called point-light displays (Johansson, 1973). Indeed, Mather and colleagues (1992) have demonstrated that rudimentary perceptions are possible based only on the motions of a single point-light representing a human ankle. More recently Troje and colleagues (2006, 2008) have described a 'life detector' that appears tuned expressly to the motions of the ankle of humans and other animals. Specifically, Troje and colleagues propose that it is the apparent interaction of the ankle with the ground, or the 'ballistic-velocity profile', to which the life detector is tuned. Interestingly, when human walkers are inverted the ballistic-velocity profile of the walkers effectively becomes a nonsense in terms of their ability to signal interactions with the ground. Under those conditions perceptions of direction of motion are reversed and robust. This observation poses the question: what is the life detector detecting? Here we present data describing the relative contributions of motion trajectory, variations in velocity, and relative location of the impact point with the ground to perceptions of biological motion.
Authors: Russell Reid, Rick van der Zwan, Anna Brooks, & Duncan Blair
Event: SF08: Speed Papers