Journal of Experimental Psychology: Human Perception and Performance 1989, Vol. 15, No. 3, 507-528 Copyright 1989 by the American Psychological Association, Inc. 0096-1523/89/S00.75 Hefting for a Maximum Distance Throw: A Smart Perceptual Mechanism Geoffrey P. Bingham Haskins Laboratories, New Haven, Connecticut and The Center for the Ecological Study of Perception and Action University of Connecticut R. C. Schmidt and Lawrence D. Rqsenblum The Center for the Ecological Study of Perception and Action University of Connecticut and Haskins Laboratories, New Haven, Connecticut Objects for throwing to a maximum distance were selected by hefting objects varying in size and weight. Preferred weights increased with size reproducing size-weight illusion scaling between weight and volume. In maximum distance throws, preferred objects were thrown the farthest. Throwing was related to hefting as a smart perceptual mechanism. Two strategies for conveying high kinetic energy to projectiles were investigated by studying the kinematics of hefting light, preferred, and heavy objects. Changes in tendon lengths occurring when objects of varying size were grasped corresponded to changes in stiffness at the wrist. Hefting with preferred objects produced an invariant phase between the wrist and elbow. This result corresponded to an optimal relation at peak kinetic energy for the hefting. A paradigm for the study of perceptual properties was compared to size-weight illusion methodology. A task familiar to many from childhood is that of standing on a beach, in a field, or on a cliff and selecting, by hefting, the stone that can be thrown the farthest distance. Like the perfect skipping stone, the optimal throwing stone evokes an ardent glow of confidence in one's ability to discover and use this appealingly simple, yet distinct tool. What is the optimal throwing stone? Assuming a spherical shape and a fairly homogeneous mass distribution, the relevant object properties are size and weight. What is the appropriate configuration of size and weight and how is it determined? Are people truly able to select from objects varying in size and weight those optimal for throwing to a maximum distance? If so, how? The human perception-action system has been described as a system that temporarily assembles smart, special purpose, deterministic machines over relevant physical properties of the organism and the environment to perform specific tasks (Bingham, 1988b; Fowler & Turvey, 1978; Kugler & Turvey, 1987; Saltzman & Kelso, 1987; Solomon & Turvey, 1988). Geoffrey P. Bingham was supported while performing this work by a National Institute of Health Individual Fellowship Award (AM- 07412). He also wishes to acknowledge the help and advice of Roy Davis of the Department of Engineering and Computer Science at Trinity College; Bruce Kay of the Department of Mechanical Engi- neering at the Massachusetts Institute of Technology; Kevin Munhall of the Department of Psychology at the University of Western On- tario; and Elliot Saltzman, Bruno Repp, and Len Katz of Haskins Laboratories. Also, we appreciated the constructive criticism of Jim Cutting, Sverker Runeson, and an anonymous reviewer. Thanks to Ed Wiley and Dick Sharkany at Haskins for providing technical assistance. Correspondence concerning this article should be addressed to Geoffrey P. Bingham, who is now at the Department of Psychology, Indiana University, Bloomington, Indiana 47405. According to this approach, a stone for throwing would be a component of a softly assembled throwing machine. For optimal performance, the projectile's properties must be scaled to the remaining components of this task-specific de- vice according to its dynamical organization. How might the optimal configuration of projectile mass and size, as deter- mined by the dynamics of throwing, be perceived through hefting? Runeson (1977a) suggested that perceptual mecha- nisms are "smart," meaning that they take advantage of peculiar, task-specific circumstances in the interests of effi- ciency and reliability in task performance. Taking advantage of task-specific circumstances often may te the only means of achieving successful performance (Bingham, 1988b). Heft- ing shares both anatomy and certain kinematic, and by im- plication dynamic, properties with throwing. These common aspects could provide the circumstantial basis for smart per- ceptual organization. If hefting and throwing exhibit similar dynamical organization, then hefting could contain informa- tion about the dynamics of throwing. In particular, hefting with an object might provide information about that object as a potential component of a throwing machine. The required information would be, in part, about the mass of the potential projectile. Mass is a dynamic property (Bingham, 1988a). Perceptual information about dynamics must be mapped through the kinematics of actions and events to spatial- temporal patterns that can be detected by perceptual systems (Bingham, 1987a, 1987b, 1988a, 1988b; Runeson, 1977b; Runeson & Frykholm, 1983). Information about the mass- related properties of events resides in resulting patterns of motion. For instance, Runeson and Frykholm (1983) dem- onstrated that the amount of weight being lifted by a person can be judged accurately given only visual apprehension of the pattern of lifting motions. Bingham (1987a) showed that 507