Academic journal article Perception and Psychophysics

Evidence for a Generic Interceptive Strategy

Academic journal article Perception and Psychophysics

Evidence for a Generic Interceptive Strategy

Article excerpt

In the present work, we first clarify a more precise definition of instantaneous optical angles in control tasks such as interception. We then test how well two interceptive strategies that have been proposed for catching fly balls account for human Frisbee-catching behavior. The first strategy is to maintain the ball's image along a linear optical trajectory (LOT). The second is to keep vertical optical ball velocity decreasing while maintaining constant lateral optical velocity. We found that an LOT accounted for an average of over 96% of the variance in optical Frisbee movement, while maintenance of vertical and lateral optical velocities was random. This work confirms a common interception strategy used across interceptive tasks, extending to complex target trajectories.

With relatively little practice, millions of recreational players become adept at intercepting targets such as baseballs, footballs, and Frisbees, even when the objects travel through trajectories that dramatically change directions in midflight. In the present study, we test whether humans catching Frisbees utilize the same simple viewer-based navigational heuristics that have been established for baseball and cricket fielders catching fly balls (McBeath, Shaffer, & Kaiser, 1995,1996; McLeod, Reed, & Dienes, 2001,2003,2006). When baseball outfielders run to catch fly balls, they use natural, geometrically invariant properties to optically maintain control over the ball. When balls are headed off to the side, fielders select a running path that maintains a linear optical trajectory (LOT) for the ball relative to home plate and the background scenery that guides them to catch the ball. In our previous work, we found evidence supporting the premise that the optical information available to the outfielder can be simply analyzed by examining it as a unified 2-D optical image, the geometry of which is shown in Figure 1, where α and β specify the vertical and lateral optical angles, respectively, between the ball and its initial optical location (home plate). ψ specifies the optical trajectory projection angle, or the observed angle of ball movement in the picture plane relative to the background horizon. In short, an LOT results when the fielder's running speed and direction maintain a rate of change in the horizontal optical angle, β, that matches the rate of change in the vertical optical angle, α. Typically, this results in the outfielder fixating the image of the ball while running along a path so as to actively rotate his vantage at a constant rate. This typically results in the fielder running fastest laterally at the start, getting a little ahead of the ball, and easing up somewhat at the end.

Previously, we have also shown that fielders catching fly balls (McBeath et al., 1995) and dogs catching Frisbees (Shaffer, Krauchunas, Eddy, & McBeath, 2004) simultaneously maintain a constant increase in the tangent of the vertical optical angle, tan a, which serves as a complementary cue to optical linearity. This is also the strategy used when balls are launched in the plane directly toward the fielder (McLeod et al., 2001). Although the LOT does not require a constant increase in tan a, it does have a looser temporal requirement that keeps it monotonically increasing (Dannemiller, Babler, & Babler, 1996). Such navigational strategies as the LOT have been proposed as generic strategies geometrically constrained to ensure collision between a pursuer and its target, including collision of airplanes and boaters tracking another craft, dragonflies and bats chasing prey, and dogs catching Frisbees (Jablonski, 1999; McBeath et al., 1995; Olberg, Worthington, & Venator, 2000; Pollack et al., 1995; Shaffer et al., 2004). Gibson (1979) referred to a subset of these generic strategies as invariants-that is, information in the optic array that remains stable despite other changes in context. These serve as reliable sources of information in navigating through the world. …

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