Elsevier

Cognition

Volume 67, Issue 3, 15 July 1998, Pages 255-285
Cognition

Predictive action in infancy: tracking and reaching for moving objects

https://doi.org/10.1016/S0010-0277(98)00029-8Get rights and content

Abstract

Because action plans must anticipate the states of the world which will be obtained when the actions take place, effective actions depend on predictions. The present experiments begin to explore the principles underlying early-developing predictions of object motion, by focusing on 6-month-old infants' head tracking and reaching for moving objects. Infants were presented with an object that moved into reaching space on four trajectories: two linear trajectories that intersected at the center of a display and two trajectories containing a sudden turn at the point of intersection. In two studies, infants' tracking and reaching provided evidence for an extrapolation of the object motion on linear paths, in accord with the principle of inertia. This tendency was remarkably resistant to counter-evidence, for it was observed even after repeated presentations of an object that violated the principle of inertia by spontaneously stopping and then moving in a new direction. In contrast to the present findings, infants fail to extrapolate linear object motion in preferential looking experiments, suggesting that early-developing knowledge of object motion, like mature knowledge, is embedded in multiple systems of representation.

Introduction

Although efforts to articulate the principles governing perceptible object motions challenged scientists for millennia, some sensitivity to constraints on object motion are evident in the simplest creature who tracks or intercepts a moving object. To reach for a moving object, for example, the brain must process incoming information about the object and its motion, plan the object-directed action, and transport the hand to a location in space where the object can be captured. Because these events may unfold over the course of a second or more, reaching must be guided by predictions of the object's future position and motion. The present research begins to investigate the system that generates these predictions.

Predictions of a moving object's future position are possible because object motion is subject to physical constraints. Objects move only on connected and unobstructed paths, and their motions are affected systematically by gravity and by collisions with other objects. In addition, object motion accords with Newton's law of inertia: objects continue in a state of rest or uniform motion unless acted upon by forces. Human adults appear to capitalize on many of these constraints in reaching for objects. Because reaching is a slowly developing skill, however, it is not clear whether mature predictive actions are shaped only by learning about specific patterns of object motion, or whether they rest on more general principles that emerge early in development. In an initial attempt to identify and characterize such core principles, we focus here on the predictive, object-directed actions of humans who have just begun to reach for objects effectively, i.e. 6-month-old infants.

Infants' sensitivity to constraints on object motion has been investigated by studies using preferential looking methods, focusing on infants' tendency to look longer at novel or unnatural events (Baillargeon, 1993; Spelke et al., 1995b). These studies suggest that infants are sensitive to the constraints of cohesion (objects maintain their connectedness as they move), continuity (objects move on connected, unobstructed paths), and contact (objects change their motion on contact with other objects or surfaces) (Spelke and Van de Walle, 1993). When infants are presented with a visible or occluded object that moves toward an obstacle, for example, they look longer at an event in which the object appears to interpenetrate the obstacle and reappears on its far side than at an event in which the object stops on contact with the obstacle (e.g. Baillargeon, 1986; Sitskoorn and Smitsman, 1995; but see also Cohen, 1995).

Similar studies suggest, in contrast, that young infants are not sensitive to inertial properties of object motion. Presented with an object out of reach that moves from view on a straight line, infants under 8 months look no longer if the object reappears at a position displaced far from the line of its visible motion than if it reappears on the original line of motion (Spelke et al., 1994; Spelke et al., 1995a; but see also Baillargeon and Graber, 1987). None of these studies reveals, however, whether infants use constraints on object motion to guide predictive actions such as visual tracking and reaching.

The earliest appearing signs of predictive action have been found in eye tracking (Aslin, 1981; von Hofsten and Rosander, 1996; von Hofsten and Rosander, 1997). Aslin observed that 3-month-old infants' smooth pursuit eye movements sometimes stayed on or were slightly ahead of a target they were tracking (Aslin, 1981). Von Hofsten and Rosander found that the smooth eye movements of 2-month-old infants predicted the smooth changes of a sinusoidal motion. In these studies, however, the infants viewed targets that moved back and forth repetitively. Because the infants who tracked these targets were familiar with their motions, the studies do not reveal how infants track a moving object viewed for the first time, and whether such tracking accords with any constraints on object motion.

Research by von Hofsten (1980); von Hofsten (1983)provides evidence that infants' object-directed reaching is predictive. As soon as infants begin catching stationary objects successfully (at about 18 weeks), they also begin to catch moving objects. Infants catch an object that approaches them by initiating arm and hand movements before the object is within reaching distance, aiming ahead of the object's current position toward a place where the paths of the object and the hand can intersect. In the experiment by von Hofsten (1980), aiming was accurate on the first attempted reach for a fast moving object (30 cm/s). These findings indicate that young infants can reach predictively without trial and error learning over the course of the experimental session. Nevertheless, the target objects in this study always traveled on the same circular path (albeit at variable speeds and distances and from variable starting positions), and infants tended to watch an object moving in its circular path before attempting their first reach. As subjects' reaches were well timed even the first time that they viewed an object moving at a given distance and velocity, infants evidently predicted that an object moving at a certain speed would maintain that speed. We do not know, however, how predictive reaching is guided by other spatio-temporal constraints on object motion.

In summary, studies of tracking and reaching for moving objects show that infants predict the future position of an object, but they reveal little about the nature or limits of infants' predictions. Systematic study of the principles guiding predictive actions requires manipulation of the spatial as well as the temporal properties of object motion. The present studies begin this effort.

In two experiments, we investigated how 6-month-old infants track and reach for visible objects whose motions either accord with, or appear to violate, the principle of inertia. In Experiment 1, infants viewed an object that either moved in a straight line at a constant speed (`constant linear motion') or underwent an abrupt stopping and turning at the midpoint of its trajectory in the absence of any visible cause (`interrupted non-linear motion'). The same kinds of motion paths were shown in Experiment 2, with the exception that objects moving on straight paths also stopped abruptly at the intersection before continuing (`interrupted linear motion'). Linear and non-linear motions starting from the upper left or upper right corners of the display, and intersecting at the midpoint of the display, were presented with equal frequency in each experiment, and infants' head tracking and reaching for the objects were observed. If tracking and reaching accord with the principle of inertia, then infants were expected to track the object more smoothly and to reach for it more effectively on the linear paths of motion. They were also expected to err systematically in their tracking and reaching for the turning object, by directing their head and hands away from its actual position toward a position that a linearly moving object would occupy.

Section snippets

Experiment 1

In the first study, infants were presented with four different paths of object motion on a nearly vertical, flat surface (Fig. 1). For two of the paths, the object started at the upper left corner of the surface and moved diagonally toward the center; for the other two paths, the object started at the upper right corner of the surface and moved diagonally toward the center. At the point of intersection of these paths, straight above and out of reach of the subject, the object either continued

Experiment 2

The infants in Experiment 2 were presented two linear and two non-linear paths of motion, as in Experiment 1. Each of the four paths was presented in a block of six to nine trials, in order to maximize any short-term learning effects that might be present in the infants' reaching and head tracking. Both kinds of motion were stopped for 100 ms at the intersection point so as to equate for the noises that accompanied the linear and the abruptly turned motions and to evaluate the effect of

General discussion

The present findings provide evidence that 6-month-old infants act prospectively on moving objects by extrapolating object motion on linear paths. The clearest evidence for these extrapolations came from observations of the convergent reaching behavior of the contralateral hand on trials. In the present studies, infants aimed for the area 10–20 cm contralateral to the starting position of the object motion. The reaching movements of the contralateral hand converged consistently on this region

Acknowledgements

We would like to thank the enthusiastic parents and energetic infants who made this research possible, and Dan Simons, Richard Darlington, Yuko Munakata, Randy O'Reilly, and all the members of the Cornell data analysis seminar for their criticisms and suggestions concerning the analyses. We also thank Frank Keil and the attendees of the Cornell Developmental Lab Group (Justin Barrett, James Beale, Linda Hermer, Deborah King, Nico Larco, Daniel Levin, Carole Lunney, Julia Noland, Marie

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