Heading backward: Perceived direction of movement in contracting and expanding optical flow fields

Two goals were pursued in an investigation of possible visual sources for directionality judgments of ego-motion. First, J. J. Gibson’s (1950) global radial outflow hypothesis was contrasted with a simple extrapolation strategy. Second, backing-direction judgments capitalizing on the informational equivalence of global radial outflow created during forward ego-motion and global radial inflow during backward ego-motion were explored. In comparing the accuracy of heading and backing judgments, new insights about global flow and extrapolation strategies were found. Consistent with the hypothesis of an extrapolation strategy, Experiment 1 demonstrated that backing judgments were more accurate than heading judgments when linear observer motion was simulated by means of a point-light flow field. In this case, accuracy was higher with two-point-light displays (extrapolation) than with more complex displays (global flow). Experiment 2 showed that in cases where extrapolation was not possible, such as circular motion, no advantage of backing judgments could be found and judgments were generally less accurate. We conclude that, whenever possible, observers use extrapolation to determine their heading/backing. Only when global flow is the only good source of information do they rely on it.