Abstract
Self-motion through a visual environment creates a pattern of expanding visual motion called optic flow. Heading estimation from optic flow is accurate in rigid environments. But it becomes challenging when other humans introduce independent motion to the scene. Biological motion of human walkers consists of translation through space and associated limb articulation. Heading perception of self-motion towards a group of point-light walkers is possible because the information from limb movement indicates the walkers' translations, and can be used to compensate for their translation in the optic flow. But limb articulation without translation, as if on a treadmill, produces a bias in heading perception. In the present experiments, we determine the path percept of observers in these situations to better understand the interactions between optic flow perception and biological motion perception. We simulated self-motion along a straight path towards a group of eight life-sized point-light walkers, collectively facing to the left or right. Each walker's points combined biological motion and self-motion components. After viewing the stimulus, participants (n = 24) reported their perceived path by adjusting the direction and curvature of a set of two lines, akin to a road. We found that participants consistently drew a curved path even though the simulated movement was always straight. The magnitude of the heading error and the curvature correlated significantly. When we added a ground plane to the scene, perceived curvature was much reduced. We conclude that limb articulation induces perceptual effects that can cause misperceptions of self-motion. We propose that observers deduce the translation walking speed from a walkers' limb articulation. This perceived translation evokes the impression of a rotation of the reference frame for self-motion perception.