A Unified Fielder Theory for Interception of Moving Objects Either above or below the Horizon

Article excerpt

A unified fielder theory is presented that explains how humans navigate to intercept targets that approach from either above or below the horizon. Despite vastly different physical forces affecting airborne and ground-based moving targets, a common set of invariant perception-action principles appears to guide pursuers. When intercepting airborne projectiles, fielders keep the target image rising at a constant optical speed in a vertical image plane and moving in a constant optical direction in an image plane that remains perpendicular to gaze direction. We confirm that fielders use the same strategies to intercept grounders. Fielders maintained a cotangent of gaze angle that decreases linearly with time (accounting for 98.7% of variance in ball speed) and a linear optical trajectory along an image plane that remains perpendicular to gaze direction (accounting for 98.2% of variance in ball position). The universality of maintaining optical speed and direction for both airborne and ground-based targets supports the theory that these mechanisms are domain independent.

Past research into the perceptual strategy used by humans, animals, and robots to navigate to a projectile destination has established that interception can be achieved using only a small number of simple perception-action control heuristics. Here, we examine whether or not the same control heuristics apply when navigating to intercept in the physically vastly different case of targets moving along the ground. If so, this simple strategy can extend to interception from virtually any direction in many domains, both biological and mechanical.

When a fielder runs to catch a ball, a driver navigates to avoid a collision, or a pedestrian intercepts a fellow walker, they each utilize unconscious spatial-orienting principles and other automatic perceptual control mechanisms. At the same time, parallel higher level cognitive processes, such as map building, object discrimination, and conscious analysis of visual streams, are also being performed. Nevertheless, past research on pursuer ability to predict projectile destinations on the basis of knowledge of physics has not been substantiated (Saxberg, 1987; Shaffer & McBeath, 2002, 2005). For tasks such as interception of moving objects, humans are guided primarily by rapid, unconscious, dynamic, visual-motor control strategies rather than by strategies based on higher level reasoning (Milner & Goodale, 1995). In this study, we tested whether or not interception of ground balls is consistent with usage of only a few simple, perceptually invariant principles.

When catching a moving target, such as a projectile approaching from above the horizon, humans are surprisingly accurate at navigating to the destination, even under varying conditions caused by wind, projectile spin, and other factors. Similarly, when a moving target descends to approach from below die horizon, fielders appear to seamlessly continue their navigational strategy to successfully intercept along the ground. We present evidence to support the parallel use of two general functional heuristics of interceptive action based entirely on control of perceptually invariant principles. This study is the first to utilize high-precision, marker-based, motion-capture technology to unambiguously measure head, body, and target positions during high-speed interception tasks. This type of perceptual modeling is relevant not only for human and animal navigational behavior (Shaffer, Krauchunas, Eddy, & McBeath, 2004), but also for applied areas such as teleoperation, flight training (Beall & Loomis, 1997), and mobile robot design (Sugar & McBeath, 200 Ia, 200 Ib; Sugar, McBeath, Suluh, & Mundhra, 2006).

Perception-Action Control Mechanisms

Previous research supports the use of navigational mechanisms that guide fly ball interception by maintaining geometric perceptual invariants between pursuer and target projectile. …