Action Modes and Laws of Control for the Visual Guidance of Action

SUMMARY Visual information provides a crucial source of constraint on the action system, so that functional movements are coordinated with the local environment. In visually guiding an action, the actor chooses a perceptually specified affordance to be realized, a corresponding mode of action. and appropriate laws of control by which to visually regulate the action. The affordance problem concerns Iww an organism perceives what cu:tions a given situation affords, and selects an appropriate affordance to realize. An affordance, such as a passable opening, is determined by the fit between the material properties of the environment and those of the organism, yielding optimal points at which action is most efficient, and critical points at whu:h a phase transition to a new mode of action occurs. It is proposed that for each affordanu there exists a corresponding action mode that will realize it, whose free parameters are regulated by a unique set of laws of control. The control problem concerns how the dynamic force-related parameters of the action system are modulated by the kinematic uariables of optical information to yield a precisely adapted mouement. It is proposed that laws of control can be derived from ecological optics and physics that relate changes in optical flow to the changes in force required to achieue a desired action. To demonstrate the generality of this argument. it is worked out in detail for the case of uisual flight control in insects, and then applied to human action in studies of bipedal stair climbing, walking through narrow apertures, and the visual control ofstep length during running.