84 Optic Flow

OPTIC FLOW IS THE PATTERN of motion present at the eye of a moving observer. Such flow patterns contain information about self-motion, moving objects, and the threedimensional (3D) layout of the environment, and could potentially be exploited to control locomotion. The term was first used by Gibson (1950) to generalize Helmholtz's notion of motion parallax from single objects to the continuous environment surrounding the observer. He developed the concept during World War II while working on methods for pilot testing and training, after he concluded that the classical depth cues were inadequate to explain a pilot's practical ability to land an airplane. But little research was done on the subject until the I 980s, when its relevance to robot and vehicle guidance was recognized and computer animation made it amenable to study. Optic flow is a key example of Gibson's (1979) ecological approach to perception and action. This functionalist approach emphasizes that the task of vision is to guide successful behavior in the natural environment, and stresses the importance of higher-order optical information that specifies complex properties of that environment and its relationship to the observer. The nervous system is viewed as providing causal support for the detection of informational variables and the modulation of action variables. Optic flow offers a case study of a higher-order variable that has been formally analyzed and empirically investigated in some detail. In what follows, we will trace optic flow through the cycle of perception and action. The past 20 years have seen intensive research on the topic, with parallel developments in the domains of human perception, primate neurophysiology, and computational vision. This chapter aims to integrate these perspectives to address both mechanistic questions about how optic flow patterns are extracted by the visual cortex and functional questions about how this information is used to perceive selfmotion and control behavior. (For related reviews see Lappe et aI., 1999; vVarren, 1998; regarding structure from motion, see Todd, 1995.)