People typically find their way through woods and buildings without harm or injury. That is, while traveling at reasonable speed, we can determine our heading and avoid obstacles without doing damage to them or to ourselves. Constraints of physics and human reaction time dictate that we must be able to determine our heading to within one degree of visual angle, or about half the width of one's thumb head at arm's length. Otherwise, we risk injury in our daily movements. Perceptual scientists generally agree that visual information provides the usual means for locomotor guidance and that this information lies in the relative motions of stationary objects around a moving observer. There is disagreement, however, on how best to characterize these relative motions to determine exactly what information is used. This research will test a theory positing a recursive two-step process. According to the theory, people first search out objects to look at and follow them (using pursuit eye movements) while they are moving forward and second monitor the relative motions of objects nearer and farther than the one they are looking at, but in its vicinity. In general, nearer objects move faster than, and in the opposite direction from, farther objects and near objects move in a direction opposite to one's heading. Thus, to be able to determine one's heading, one simply gazes at an object anywhere in the field of view and observes the direction of the fastest movements around the point of gaze. If these move right, the heading is usually to the left; if these move left, the heading is usually to the right. This research will test the theory in a number of ways, using computer- generated displays, which will simulate various types of environments and various types of observer movements through them. In particular, the research will explore conditions in which people make errors in determining their heading and the reasons for those errors. An understanding of these errors will have implications for air-traffic safety, highway safety, and the safety of visually-impaired pedestrians.
