Gravitational acceleration as a cue for absolute size and distance ?

1066 People need to process the absolute distance and size of objects in order to act in the environment. For example, in order to catch and grasp a thrown ball successfully, a person must place his/her hands in the appropriate spatial position and in an appropriate conformation. To do so, he/she must use an appropriate combination of absolute variables (size of the ball, its distance, velocity, and temporal range). In the present paper, we address the question of how perceivers might pick up absolute distance and size information in such contexts. The cues that an observer might employ to estimate absolute distance (and from that, to estimate absolute size) can be divided into those that are strictly monocular and those that are binocular as well. In principle, at least three monocular cues can be used to estimate absolute distance: accommodation, convergence, and familiar size. However, none of these is very useful in general viewing situations: Human observers are not very sensitive to accommodation (Fisher & Ciuffreda, 1988), convergence is only useful for objects at close range (von Hofsten, 1990), and familiar size requires knowledge of the object’s actual size. Binocular information is also potentially useful. According to the conventional wisdom, stereopsis only specifies relative distance (i.e., depth); but it can actually specify absolute depth as well, if both horizontal and vertical disparities are employed efficiently (see, e.g., Gillam & Lawergren, 1983; LonguetHiggins & Mayhew, 1982). The evidence that human observers use such information accurately, however, is mixed (e.g., Cumming, Johnston, & Parker, 1991; Rogers & Bradshaw, 1993). Similarly, for certain object/observer motions, one can in principle estimate absolute distance from a combination of binocular disparity and motion parallax (e.g., Kellman & Kaiser, 1995), but there is no empirical evidence that human observers can actually use this information. Saxberg (1987a) and Watson, Banks, von Hofsten, and Royden (1992) suggested a novel monocular cue from which observers could estimate the absolute size and/or distance of objects whose motions are affected by gravity. Such cases include projectiles in flight, pendulum motion, fluid wave motion, and others. Saxberg, for example, showed that one can compute the distance, Z, to an object whose motion is governed by its initial velocity and gravity only. With perspective projection onto a plane parallel to the gravitational vector, The authors thank Geoffrey P. Bingham, Myron L. Braunstein, James A. Crowell, W. Farrell Edwards, and Walt Johnson for helpful discussion. John C. Baird, Hal A. Sedgwick, and one anonymous reviewer provided valuable comments on an earlier version of this paper. The research was supported by NASA RTOP 538-04-11 and by NSF Research Grant DBS-9309820 and AFOSR Grant F49620 to M.S.B. Portions of this research were presented in St. Louis at the annual meeting of the Psychonomic Society (November 1994). H. H. is with the Zentrum für interdisziplinaire Forschung at Bielefeld; M.S.B. is affiliated with both the School of Optometry and the Department of Psychology at Berkeley. Correspondence may be addressed to M. K. Kaiser, Mail Stop 262-2, NASA Ames Research Center, Moffett Field, CA 94035 (e-mail: moose@eos. arc.nasa.gov). Gravitational acceleration as a cue for absolute size and distance?