Object permanence in five-month-old infants.

A new method was devised to test object permanence in young infants. Fivemonth-old infants were habituated to a screen that moved back and forth through a 180-degree arc, in the manner of a drawbridge. After infants reached habituation, a box was centered behind the screen. Infants were shown two test events: a possible event and an impossible event. In the possible event, the screen stopped when it reached the occluded box; in the impossible event, the screen moved through the space occupied by the box. The results indicated that infants looked reliably longer at the impossible than at the possible event. This finding suggested that infants (1) understood that the box continued to exist, in its same location, after it was occluded by the screen, and (2) expected the screen to stop against the occluded box and were surprised, or puzzled, when it failed to do so. A control experiment in which the box was placed next to the screen provided support for this interpretation of the results. Together, the results of these experiments indicate that, contrary to Piaget’s (1954) claims, infants as young as 5 months of age understand that objects continue to exist when occluded. The results also indicate that 5-month-old infants realize that solid objects do not move through the space occupied by other solid objects. *This research was supported by a grant from the National Institute of Health (HD-13248) to ESS. The data analysis was supported by a grant from the National Science Foundation (SES84-08626) to SW. While working on this research, RI3 was supported by fellowships from the Natural Sciences and Engineering Research Council of Canada and the Qu6bec Department of Education. We thank Judy Deloache and Bob Reeve, for their careful reading of the manuscript; Marty Banks, Susan Carey, and Paul Harris, for helpful comments on earlier versions of the manuscript; Wendy Smith Born, Sarah Mangelsdorf, and the members of the Infant Lab at the University of Pennsylvania, for their help with the data collection; and Dawn Iacobucci, for her help with the data analysis. **Reprint requests should be sent to RenCe Baillargeon, Psychology Department, University of Illinois at Urbana-Champaign, 603 East Daniel Street, Champaign, IL 61820, U.S.A. OOlO-0277/85/$5.90 0 Elsevier Sequoia/Printed in The Netherlands 192 R. Baillargeon et al. 1. Background: Piaget’s theory For adults, an object is an entity that exists continuously in time and space: it cannot exist at two separate points in time without having existed during the interval between them, and it cannot appear at two separate points in space without having traveled from one point to the other. Do infants share this conception of objects as temporally and spatially continuous? On the basis of detailed observations of infants’ reactions to object disappearances, Piaget (1954) concluded that they do not. For the young infant, Piaget maintained, each disappearance amounts to an annihilation and each reappearance to a resurrection. An object is not a permanent entity that continuous to exist while out of sight, but an ephemeral entity that is continually made and unmade: “a mere image which reenters the void as soon as it vanishes, and emerges from it for no objective reason” (p. 11). Piaget discerned six stages in the development of the infant’s object concept. He claimed that it is not until infants reach the fourth stage, at about 9 months of age, that they begin to endow objects with permanence, as evidenced by their willingness to search for hidden objects. Piaget observed that prior to stage 4, infants do not search for fully hidden objects. If an attractive toy is covered with a cloth, for example, they make no attempt to lift the cloth and grasp the toy, even though they are capable of performing each of these actions. Beginning in stage 4, however, infants do remove obstacles to retrieve hidden objects. In subsequent stages, infants come to take into account visible (stage 5) and invisible (stage 6) displacements of objects to find objects hidden in successive locations. Why did Piaget select infants’ search for hidden objects as marking the beginning of object permanence? This question is important, because Piaget observed several behaviors prior to stage 4 that are suggestive of object permanence. For example, he noted that as early as stage 1 (O-l month), infants may look at an object, look away from it, and then return to it several seconds later, without any external cue having signaled its continued presence. In addition, Piaget observed that beginning in stage 3 (4-9 months), infants anticipate the future positions of moving objects: if they are tracking an object and temporarily lose sight of it, they look for in further along its trajectory; similarly, if they are holding an object out of sight and accidentally let go it, they stretch their arm to recapture it. Piaget claimed that although these and other behaviors seem to reveal a notion of object permanence, closer analysis indicates “how superficial this interpretation would be and how phenomenalistic the primitive universe remains” (p. 11). Prior to stage 4, Piaget maintained, the infant lacks a concept of physical causality and regards all of reality as being dependent on his Object permanence in infants 193 activity. When he acts upon an object, the infant views the object not as an independent entity but as the extension, or the product, of his action. If the object disappears from view, the infant reproduces or extends his action, because he expects that his action will again produce the object. Proof for Piaget that the infant regards the object as being “at the disposal” of his action is that if his action fails to bring back the object, he does not perform alternative actions to recover it. Beginning in stage 4, however, the infant acts very differently. For example, if a ball rolls behind a cushion and he cannot recapture it by extending his reach, he tries alternative means for recovering it: he lifts the cushion, or pulls it aside, or gropes behind it. According to Piaget, such activities indicate that the infant conceives of the ball, not as a thing at the disposal of a specific action, but as a substantial entity that is located out of sight behind the cushion and that any of several actions may serve to reveal. 2. Tests of Piaget’s theory In recent years, Piaget’s (1954) description of the sequences of changes in infants’ search behavior has been tested by numerous investigators and has been accepted with few modifications (see Gratch, 1975, 1976; Harris, 1985; Schuberth, 1983, for reviews). Nevertheless, Piaget’s interpretation of this sequence has been questioned. A number of authors (e.g., Bower, 1974; Diamond and Goldman-Rakic, 1983) have suggested that young infants’ failure to search for hidden objects stems not from a lack of object permanence but from an inability to perform coordinated actions. Perhaps ironically, support for this hypothesis comes from Piaget’s (1952) own work on the development of action. Piaget found that the capacity to act in a coordinated manner develops very slowly over the course of infancy. He noted that a major milestone is achieved at about 9 months of age, when infants begin to coordinate separate actions into means-ends sequences. In these sequences, infants perform one action in order to create the conditions under which they will be able to perform a second, independent action. Since Piaget’s (1954) search task requires infants to coordinate two separate actions (one upon the occluder and one upon the object), young infants could fail this task because they are generally unable to perform such an action sequence. A number of studies, notably by Bower (1967, 1972, 1974; Bower, Broughton and Moore, 1971; Bower and,Wishart, 1972), have attempted to investigate young infants’ conception of an object using methods that do not require coordinated sequences of actions. Bower’s studies have yielded four findings that seem to provide evidence for object permanence in infants well 194 R. Baillargeon et al. below 9 months. First, 7-week-old infants were found to discriminate between disappearances that signaled the continued existence of an object (e.g., gradual occlusion), and disappearances that did not (e.g., gradual dissolution or sudden implosion) (Bower, 1967). Second, 2-month-old infants were found to anticipate the reappearance of an object that stopped behind a screen, “looking to that half of the movement path the object would have reached had it not stopped” (Bower et al., 1971, p. 183). Third, 5-month-old infants were found to show disruptions in their tracking when an object was altered while passing behind a screen: they tended to look back at the screen, as though in search of the original object (Bower, 1974; Bower et al., 1971). Finally, 5-month-old infants were found to reach for an object that had been “hidden” by darkening the room (Bower and Wishart, 1972). Although suggestive, Bower’s findings do not provide conclusive evidence for object permanence in young infants. First, methodological problems cast doubts upon the validity of the results (Gratch, 1976; Harris, 1985). Second, the results are open to alternative interpretations that do not implicate object permanence. In particular, most of the results could be explained by Piagetian theory in terms of the extension of an ongoing action or the reproduction of a previous action. When infants reach for an object in the dark, they could simply be extending an action initiated before the lights were extinguished. Similarly, when infants anticipate the reappearance of an object, they could be extending a tracking motion begun prior to the object’s disappearance. Finally, when infants’look back at a screen, after a novel object has emerged from behind it, they could be repeating a prior action of looking in that direction, with the expectation that this action will again produce the original object. The first finding cited above cannot be explained in terms of the extension or the reproduction of an action, but it, too, is open to other interpretations. One interpretation, mentioned by Bower et al. (1971), apparently has its source in Piaget: “Piaget (personal communication) has rightly objected that the methods used were insufficient to demonstrate that the infants were responding to objects as such, rather than to perceptual configurations which contained the object as an undifferentiated element” (p. 182). Another interpretation is that infants discriminate between permanence and impermanence sequences on the basis of superficial expectations about the way objects typically disappear, rather than on the basis of a belief in object permanence. In their daily environment, infants often see objects occlude one another but they rarely, if ever, see objects implode from view or dissolve into the air. Hence, infants could respond differently to occlusions than to implosions or dissolutions because occlusions are the only type of disappearance that is familiar to them. Object permanence in infants 195 3. The present experiment Because of the difficulties associated with Piaget’s and Bower’s tasks, we sought a new means of testing object permanence in young infants. Like Bower, we chose not to rely on manual search as our index of object permanence. However, we tried to find an index that could not depend on (1) the extension or reproduction of an action, or (2) knowledge about superficial properties of object disappearances. The method we devised was rather indirect. It focused on infants’ understanding of the principle that a solid object cannot move through the space occupied by another solid object (“solidity principle”). Infants’ understanding of this principle was tested in a situation involving a visible object and an occluded object. If infants were surprised when the visible object appeared to move through the space occupied by the occluded object, it would suggest that they took account of the existence and the location of the occluded object. In other words, evidence that infants applied the solidity principle would also provide evidence that they possessed object permanence. In the experiment, a box was placed on a surface behind a wooden screen. The screen initially lay flat, so that the box was clearly visible. The screen was then raised, in the manner of a drawbridge, thus hiding the box from view. Infants were shown two test events: a possible event and an impossible event. In the possible event, the screen moved until it reached the occluded box, stopped, and then returned to its initial position (see Figure 1A). In the impossible event, the screen moved until it reached the occluded box-and then kept on going as though the box were no longer there! The screen completed a full 180-degree arc before it reversed direction and returned to its initial position, revealing the box standing intact in the same location as before (see Figure 1B). To adults, the possible event is consistent with the solidity principle: the screen stops when it encounters the box. The impossible event, in contrast, violates the principle: the screen appears to move freely through the space occupied by the box. Note that adults would not perceive the event as impossible if they did not believe that the box continued to exist, in its same location, after it was occluded by the screen. To test infants’ perception of these events, we used a habituation paradigm. Infants were habituated to the screen moving back and forth through a 180-degree arc, with no box present. After infants reached habituation, the box was placed behind the screen, and infants were shown the possible and impossible events. Our reasoning was as follows. If infants understood that (1) the box continued to exist, in its same location, after it was occluded by the screen, and (2) the screen could not move through the space occupied by the box, then they should perceive the impossible event to be 196 R. Baillargeon et al. Figure 1. Schematic representation of the possible and impossible test events used in the principal experiment.