HEADING THROUGH A CROWD 1 Heading Through A

The ability to navigate through crowds of moving people accurately, ef9iciently and without causing collisions is essential for our day-to-day lives. Vision provides key information about one’s own self-motion, as well as the motions of other people in the crowd. These two types of information (optic 9low and biological motion) have each been investigated extensively; however, surprisingly little research has been dedicated to investigating how they are processed when presented concurrently. Here we show that patterns of biological motion have a negative impact on visual heading estimation when people within the crowd move their limbs but do not move through the scene. Conversely, limb motion facilitates heading estimation when walkers move independently through the scene. Interestingly, this facilitation occurs for crowds containing both regular and perturbed depictions of humans, suggesting that it is likely caused by low-level motion cues inherent in the biological motion of other people. HEADING THROUGH A CROWD 3 Heading Through A Crowd The pattern of radial optic Xlow produced on the retina by movement through the world is a rich source of information about our environment and can be used as a purely visual means for guiding locomotion (Gibson, 1950). When the physical environment is entirely rigid, estimates of the direction in which one is heading can be derived by analyzing the motion vectors in the optic Xlow pattern (Bruss & Horn, 1983; Longuet-Higgins & Prazdny, 1980). Humans are able to estimate their heading from rigid scenes with a degree of accuracy adequate for safe navigation (Cutting, Springer, Braren, & Johnson, 1992; Warren & Hannon, 1988; Warren, Morris, & Kalish, 1988). Rigid scenes are frequently used in experimental settings; however, real world encounters often encompass non-rigid environments. Natural scenes are regularly populated by externally moving objects and people, which pose potential problems for heading estimation. Though the computation of heading is fairly robust and remains accurate even at relatively low signal to noise ratios (van den Berg, 1992; Warren, Blackwell, Kurtz, Hatsopoulos, & Kalish, 1991), independent object movement in a scene disturbs the global pattern of optic Xlow and causes inaccuracies in heading detection (Andersen & Saidpour, 2002; Layton & Fajen, 2015, 2016; Royden & Hildreth, 1996; Warren & Saunders, 1995). In crowds, this is exacerbated by the fact that people not only translate through space, but also move their limbs while doing so. Though limb motion adds further noise to the optic Xlow Xield, it also conveys information about the nature of a person’s actions (Dittrich, 1993; Johansson, 1973), movement (Cutting, Moore, & Morrison, 1988; Graf et al., HEADING THROUGH A CROWD 4 2007; Masselink & Lappe, 2015; Thurman & Lu, 2015) and intentions (Blakemore & Decety, 2001; Diaz, Fajen, & Phillips, 2012). This information can be ascertained from the patterns of motion produced by the movement of the major joints, which is commonly referred to as biological motion (Blake & Shiffrar, 2007). In theory, the visual system could take advantage of the additional information provided by biological motion in order to compensate for walker motion during self-motion, thus reducing the impact of other moving people on heading computation. For example, if biological motion analysis were to provide the motion direction and speed of each walker, this common motion could theoretically be subtracted from the Xlow velocities to remove some of the nonrigid motion. Riddell and Lappe (2017) recently studied self-motion towards a single, approaching walker and found that observers could distinguish the motion produced by the walker from the optic Xlow produced by observer movement, but confounded the two sources of motion in heading perception. In the present study we examined the ability to estimate heading from displays depicting linear self-motion through crowds of point-light walkers in four experiments focussing on the role of the human Xigure, limb movement, walker movement, and stability with regard to the environment.