Developments in young infants' reasoning about occluded objects☆
Introduction
As adults, we often observe situations in which objects become occluded by nearer objects. For example, we might see a child hide behind a couch, a ball roll behind a toy chest, or a spoon fall behind a pot. Our representations of these occlusion situations typically include both the objects that remain visible—the couch, the toy chest, the pot—and the objects that do not—the child, the ball, the spoon. Are infants, like adults, able to represent occluded objects? Piaget (1954) was the first investigator to examine this question. He concluded that it is not until infants are about 8 months of age that they begin to represent the continued existence of occluded objects. This conclusion was based primarily on data from manual search tasks. Piaget found that young infants typically do not search for objects they have observed being hidden: If a toy is covered with a cloth, for example, infants ages 5–7 months make no attempt to lift the cloth and grasp the toy, even though they are usually capable of performing each of these actions.
For the next several decades, researchers generally accepted Piaget's (1954) conclusion that young infants' event representations include only those objects they can directly perceive (for reviews of this early research, see Bremner, 1985; Gratch, 1976; Harris, 1987; Schubert, 1983). This state of affairs began to change during the 1980s, however, when evidence obtained with novel, more sensitive tasks contradicted Piaget's long-standing conclusion (e.g., Baillargeon, 1986, Baillargeon, 1987; Baillargeon & Graber, 1987; Baillargeon, Spelke, & Wasserman, 1985; Hood & Willatts, 1986; Spelke & Kestenbaum, 1986). Today, there is consistent evidence from multiple laboratories that infants ages 2.5 months and older believe that (1) a stationary object continues to exist and retains its location when occluded and (2) a moving object continues to exist and pursues a continuous path when occluded (e.g., Aguiar and Baillargeon, 1999, Aguiar and Baillargeon, 2000; Baillargeon, 1991; Baillargeon & DeVos, 1991; Baillargeon, Graber, DeVos, & Black, 1990; Goubet & Clifton, 1998; Hespos and Baillargeon, 2001a, Hespos and Baillargeon, 2001b; Hespos & Rochat, 1997; Hofstadter & Reznick, 1996; Koechlin, Dehaene, & Mehler, 1998; Lécuyer, 1993; Newcombe, Huttenlocher, & Learmonth, 1999; Rochat & Hespos, 1996; Simon, Hespos, & Rochat, 1995; Spelke, Breinlinger, Macomber, & Jacobson, 1992; Wilcox, 1999; Wilcox & Baillargeon, 1998b; Wilcox, Nadel, & Rosser, 1996; Wynn, 1992).
Such evidence does not mean, of course, that infants as young as 2.5 months of age know all there is to know about occlusion events. Indeed, recent experiments point to two distinct facets of young infants' reasoning about occluded objects that undergo clear developmental change (Aguiar & Baillargeon, 1999; Baillargeon & DeVos, 1991; Spelke & Kestenbaum, 1986; Spelke, Kestenbaum, Simons, & Wein, 1995a). One facet concerns infants' knowledge of the conditions under which objects should and should not be occluded. The other facet involves infants' ability to posit the existence of an additional occluded object to make sense of an event that would otherwise violate their occlusion knowledge. We next review the results of these experiments, which provided the basis for the present research.
There are now several published reports that infants ages 2.5–3.5 months believe that an object continues to exist after it becomes occluded (e.g., Aguiar & Baillargeon, 1999; Baillargeon, 1987; Baillargeon & DeVos, 1991; Hespos & Baillargeon, 2001b; Spelke et al., 1992; Wilcox et al., 1996; we return to the possible origins of this belief under Section 9). At the same time, however, there is also evidence that these young infants are rather poor at predicting when an object should be occluded. This evidence comes from two series of experiments, one involving 2.5-months-olds (Aguiar & Baillargeon, 1999) and the other 3- and 3.5-month-olds (Baillargeon & DeVos, 1991). Both series of experiments examined infants' ability to predict whether an object should remain continuously hidden or become temporarily visible when passing behind a screen with an opening in its midsection. Both series of experiments also made use of the violation-of-expectation method (e.g., Baillargeon, 1995, Baillargeon, 1998; Spelke, 1985). In a typical experiment conducted with this method, infants see two test events: One is consistent with the belief or expectation being examined in the experiment (expected event), and the other violates this expectation (unexpected event). Prior to seeing the test events, infants may see habituation events designed to familiarize them with various aspects of the test situation.1 With appropriate controls, reliably longer looking at the unexpected than at the expected event provides evidence that infants (1) possess the expectation under examination, (2) detect the violation in the unexpected event, and (3) are surprised by this violation. Throughout this article, we use the term “surprise” as a shorthand descriptor to denote a state of heightened attention or interest induced by an expectation violation. We make no claims here about the presence or absence of emotional components in this response.2
Experiments with 2.5-month-old infants. In a recent series of experiments (Aguiar & Baillargeon, 1999), 2.5-month-old infants were habituated to a toy mouse (“Minnie Mouse”) that moved back and forth along a track whose center was hidden by a screen (see Fig. 1). The mouse disappeared at one edge of the screen and, after an appropriate interval, reappeared at the other edge. Following habituation, the infants saw two test events. In one (high-window event), a window was created in the screen's upper half; the mouse was shorter than the bottom of the window and so did not become visible when passing behind the screen. In the other event (two-screen event), the entire midsection of the screen was removed, yielding two separate screens. In this event, the mouse should have appeared in the gap between the screens, but it did not in fact do so; the mouse disappeared behind one screen and reappeared from behind the other screen without appearing in the gap between them.
The infants looked reliably longer at the two-screen than at the high-window test event. This result suggested that, when shown the two-screen event, the infants (1) believed that the mouse continued to exist when behind one of the screens, (2) realized that the mouse could not disappear behind one screen and reappear from behind the other screen without traveling the distance between them, and (3) expected the mouse to appear between the screens and were surprised when it failed to do so. This interpretation was supported by the results of a control experiment identical to that just described with one exception: The screen or screens were lowered at the start of each trial to reveal two identical mice. The infants in this control experiment tended to look equally at the two-screen and high-window test events. This negative result suggested that the infants were able to use the information provided at the start of each trial to make sense of the two-screen event: That is, they realized that no mouse appeared in the gap between the screens because one mouse traveled to the left and one to the right of the gap.
In a subsequent experiment (Aguiar & Baillargeon, 1999), 2.5-month-old infants saw test events identical to those just described, except that the two-screen event was modified (see Fig. 2): Only the lower portion of the screen's midsection was removed, creating a low window (low-window event). Because the mouse was shorter than the top of the window, it should have become fully visible—as in the two-screen event—when passing behind the screen. In this experiment, however, the infants tended to look equally at the low- and high-window test events.
Our interpretation of the preceding results—all of which were confirmed in additional experiments conducted with slightly different versions of the events (Aguiar & Baillargeon, 1999)—was that 2.5-month-old infants' knowledge of the conditions under which objects should and should not be occluded is still very limited. Specifically, infants possess only an initial concept centered on a behind/not-behind distinction: They expect objects to be hidden when behind occluders and to be visible otherwise. At this stage, infants have not yet learned to take into account the presence and location of openings in occluders when judging whether objects should be hidden or visible: Objects are expected to be hidden as long as they are behind occluders, whether or not these have openings. Thus, the infants in the experiments just described did not expect the mouse to become visible when passing behind the screen in the low- or high-window test event because in each case the screen constituted a single occluder and the infants' initial concept suggested that the mouse would be hidden when behind this occluder. In the two-screen test event, in contrast, the infants expected the mouse to become visible in the gap between the screens because at that point the mouse did not lie behind any occluder.
Experiments with 3- and 3.5-month-old infants. In another series of experiments (Baillargeon & DeVos, 1991), 3- and 3.5-month-old infants were habituated to a toy carrot that moved back and forth along a track whose center was hidden by a screen (see Fig. 3). On alternate trials, the infants saw a tall and a short carrot slide along the track. Following habituation, the midsection of the screen's upper half was removed to create a high window, and the infants saw two test events. In one (short-carrot event), the short carrot moved along the track; this carrot was shorter than the bottom of the window and so did not become visible when passing behind the screen. In the other event (tall-carrot event), the tall carrot moved along the track; this carrot was taller than the bottom of the window and hence should have become visible when passing behind the screen, but it in fact never appeared in the window.
Different results were obtained with the 3- and 3.5-month-old infants. The older infants tended to look equally at the tall- and short-carrot habituation events, but looked reliably longer at the tall- than at the short-carrot test event. These results suggested that the infants (1) believed that each carrot continued to exist when behind the screen, (2) realized that each carrot could not disappear at one edge of the screen and reappear at the other edge without traveling the distance between them, (3) recognized that the height of each carrot relative to that of the window determined whether the carrot should appear in the window, and hence (4) expected the tall carrot to appear in the window and were surprised when it failed to do so. This interpretation was supported by the results of a control experiment identical to the initial experiment with one exception: At the start of the testing session, the infants received two pretest trials in which they saw two identical carrots standing motionless on either side of the habituation screen; the infants saw two tall carrots in one trial and two short carrots in the other (Baillargeon & DeVos, 1991). The infants in this control experiment tended to look equally at the tall- and short-carrot test events, suggesting that they were able to use the information provided in the pretest trials to make sense of the tall-carrot event.
The 3-month-old infants in the initial experiment, unlike the 3.5-month-old infants, tended to look equally at the tall- and short-carrot test events. This negative result suggested that the younger infants were not surprised when the tall carrot failed to appear in the window.
Our interpretation of the preceding results was that, by 3.5 months of age, infants have progressed beyond their initial concept of when objects should and should not be occluded: They now consider height information when reasoning about occlusion events. Thus, when watching an object pass behind a screen with a high window, infants expect the object to remain hidden if it is shorter but not taller than the bottom of the window (for similar results with older infants, see Baillargeon & Graber, 1987; Hespos & Baillargeon, 2001a). At 3 months of age, however, infants have not yet identified height as an important occlusion variable; when watching an object pass behind a screen with a high window, infants expect the object to remain hidden irrespective of whether it is shorter or taller than the bottom of the window.
In the present research, we asked whether 3-month-old infants could judge correctly whether an object should become visible when passing behind a screen with a low as opposed to a high window. In such a situation, it is not necessary to encode and compare the relative heights of the object and window to arrive at a correct prediction. From simply knowing that an object is approaching an occluder with an opening extending from its lower edge, one can predict that the object will appear in the opening (as long, of course, as the object and occluder rest on the same horizontal plane; a suspended object might pass above the window). It seemed possible that 3-month-old infants might be able to reason successfully about openings extending from the lower edges of occluders and still fail at reasoning about openings extending from the upper edges of occluders.
In Experiment 1, 3- and 3.5-month-old infants were tested with the same low-window mouse task we had used with 2.5-month-old infants (see Fig. 2; Aguiar & Baillargeon, 1999). As before, the infants were habituated to the mouse moving back and forth behind the screen and then were shown the high- and low-window test events. Given the results presented above, we expected the 3.5-month-old infants to readily detect the violation in the low-window event. The question of interest was whether the 3-month-old infants would also be successful. Negative results would suggest that 3-month-old infants are essentially similar to 2.5-month-old infants in their reasoning about occlusion events and expect any object to remain hidden when passing behind any occluder. On the other hand, positive results would indicate that, by 3 months of age, infants have begun to progress beyond their initial concept of when objects should be occluded: They now expect objects to become visible when passing behind occluders with openings extending from their lower edges.
Spelke and her colleagues (Spelke & Kestenbaum, 1986; Spelke et al., 1995a) have reported evidence that young infants are not only able to represent objects that become occluded: They are also able to posit the existence of additional occluded objects to make sense of events that would otherwise violate their expectations as to when objects should and should not be occluded.
In one experiment (Spelke & Kestenbaum, 1986), 4-month-old infants were habituated to a suspended cylinder that slid back and forth along a track; at the center of the track were two screens separated by a gap. The cylinder disappeared behind one screen and reappeared from behind the other screen without ever appearing in the gap between the screens. Following habituation, the screens were removed, and the infants saw two test events. In one (one-cylinder event), a single cylinder traveled the entire length of the track. In the other event (two-cylinder event), two identical cylinders moved sequentially along the track; the left cylinder had the same trajectory as the cylinder shown to the left of the screens in the habituation event, and the right cylinder had the same trajectory as the cylinder shown to the right of the screens.
The infants looked reliably longer at the one- than at the two-cylinder test event. Further results (Spelke et al., 1995a) established that the infants' preference for the one-cylinder event reliably exceeded that of control infants who were shown only the test events. Spelke (1990) and Spelke and Kestenbaum (1986) took these findings to suggest that the infants in the experimental condition (1) inferred, upon observing that no cylinder appeared between the screens in the habituation event, that two distinct cylinders were involved in the event, and therefore (2) expected to see two cylinders when the screens were removed and were surprised in the one-cylinder event when this expectation was violated. Thus, according to this interpretation, infants as young as 4 months of age can not only represent the existence of objects that become occluded, but can also infer the existence of additional occluded objects. Such a finding is important because it makes clear that young infants' representations of occlusion events are not mere copies of the events, but complex constructions that reflect both their physical knowledge and problem solving processes.
Comparison of the results of Spelke and Kestenbaum (1986) with those of the two-screen mouse experiment described above (see Fig. 1; Aguiar & Baillargeon, 1999) brings to light another developmental change in young infants' reasoning about occlusion events. Recall that the 2.5-month-old infants in that experiment responded with prolonged looking when the mouse failed to appear in the gap between the two screens. This result suggests that the infants did not posit the existence of a second, identical mouse behind the screens. Had the infants generated such an explanation, they would most likely have produced shorter looking times, similar to those of the infants in the control experiment who were shown two mice at the start of each trial.
At what age between 2.5 and 4 months of age do infants begin to posit additional occluded objects to make sense of events that would otherwise violate their occlusion knowledge? We speculated that the present research might help shed light on this question. Evidence that the 3-month-old infants in Experiment 1 looked reliably longer at the low- than at the high-window test event, but that the 3.5-month-old infants did not, would suggest that these older infants were able to produce a two-mouse explanation for the low-window event: That is, they realized that no mouse appeared in the window because no mouse traveled the entire distance behind the screen; instead, two identical mice traveled on opposite sides of the window.3
We were aware that additional evidence would be needed to support this interpretation. Nevertheless, we reasoned that such an interpretation, if valid, would not only help pinpoint the age at which infants can first infer the existence of additional occluded objects, but would also provide converging evidence for Spelke's conclusion that young infants are indeed capable of positing occluded objects (Spelke & Kestenbaum, 1986; Spelke et al., 1995a). Such evidence is important because there is a potential ambiguity in the approach adopted by Spelke and her colleagues. This ambiguity is suggested by the results of the control mouse and control carrot experiments described earlier (Aguiar & Baillargeon, 1999; Baillargeon & DeVos, 1991). The 2.5-month-old infants who saw two mice at the start of each trial readily made use of this hint to generate an explanation for the two-screen test event. Similarly, the 3.5-month-old infants who saw two tall carrots at the start of the experiment were able to use this (subtler) hint to make sense of the tall-carrot test event. Such results raise concerns as to when the 4-month-old infants tested by Spelke and Kestenbaum might have become aware that two cylinders were involved in the habituation event: (1) during the habituation event itself, upon observing that no cylinder appeared between the two screens, or (2) during the two-cylinder test event, upon seeing the two cylinders and realizing that they provided an explanation for the habituation event. In the latter case, one would not be justified in claiming that young infants can posit the existence of an occluded object; one could conclude only that young infants can take advantage of a hint to make sense of a violation event even when they receive the hint after the event.4
How likely were the 3.5-month-old infants in Experiment 1 to infer that two identical mice were used to produce the low-window test event? One reason for being skeptical about such an outcome was that the 3.5-month-old infants in the carrot experiment were not able to generate a two-carrot explanation for the tall-carrot test event; it was only when this explanation was suggested to them that infants' surprise at the event receded (Baillargeon & DeVos, 1991). Furthermore, even 5.5-month-old infants failed to generate a two-object explanation when tested with a task similar to the carrot task (Baillargeon & Graber, 1987).
There was, however, a crucial difference between the carrot task (Baillargeon & DeVos, 1991; Baillargeon & Graber, 1987), on the one hand, and the tasks used in Experiment 1 and in Spelke and Kestenbaum (1986), on the other. The infants in the latter two tasks faced a much more flagrant occlusion violation than did the infants in the carrot task. In these tasks, the entire mouse or cylinder failed to become visible as expected; in the carrot task, in contrast, only the top portion of the tall carrot failed to become visible. It could be that the infants in the carrot task (1) assumed that the tall carrot traveled the distance behind the screen but then (2) were puzzled as to why the top of the carrot did not appear in the window. In the tasks used in Experiment 1 and in Spelke and Kestenbaum (1986), the infants could not assume that the mouse traveled from one end of the screen to the other or that the cylinder traveled from one screen to the other; such tasks could thus be more conducive to infants' production of two-object explanations.
Evidence for this possibility came from a preliminary experiment with 5.5-month-old infants (reported in Baillargeon, 1994b). The infants saw habituation and test events similar to those in Experiment 1, except that toy rabbits were used instead of toy mice. The infants tended to look equally at the low- and high-window test events. This negative finding contrasted with the positive results obtained in the carrot experiments (Baillargeon & DeVos, 1991; Baillargeon & Graber, 1987) and supported Spelke's conclusion (Spelke & Kestenbaum, 1986; Spelke et al., 1995a) that young infants are able, under some conditions at least, to posit the existence of occluded objects.
Experiment 1 thus examined whether 3.5-month-old infants (1) would show a reliable preference for the low- over the high-window test event, suggesting that they were surprised by, and could not spontaneously generate an explanation for, the mouse's failure to appear in the low window, or (2), like the 5.5-month-olds in the preliminary experiment just described (Baillargeon, 1994b), would show no preference for the low-window event, suggesting that they readily inferred that two identical mice were involved in the event.
Section snippets
Experiment 1
In Experiment 1, 3- and 3.5-month-old infants were tested using the low-window mouse task described earlier (Aguiar & Baillargeon, 1999; see Fig. 2). The infants were first habituated to a toy mouse moving back and forth behind a screen. Next, the infants saw test events similar to the habituation event, except that the screen had a window in its upper (high-window event) or lower (low-window event) midsection. The mouse's visible trajectory was exactly the same in all of the habituation and
Experiment 2
Our interpretation of the responses of the 3.5-month-old infants in Experiments 1 and 1A was that they showed little surprise at the low-window test event because they were readily able to generate a two-mouse explanation for the event. Because the screen was never lowered, the infants had no information contradicting the notion that two mice might be present in the apparatus. A two-mouse explanation was thus entirely consistent with the information available to the infants.
In Experiment 2,
Method
Participants. Participants were 10 healthy term infants, 5 male and 5 female, ranging in age from 101 to 126 days (M=108.3 days). An additional 11 infants were tested but eliminated, 8 because of fussiness, 1 because the infant looked for 90 s on three test trials, 1 because of inattentiveness, and 1 because the primary observer had difficulty following the direction of the infant's gaze.
Apparatus and events. The apparatus and events used in Experiment 3 were identical to those in Experiment 2,
Method
Participants. Participants were 12 healthy term infants, 6 male and 6 female, ranging in age from 102 to 121 days (M=110.3 days). An additional 6 infants were tested but eliminated, 2 because of fussiness, 2 because they looked for 90 s on three test trials, 1 because the infant failed to track the mouse along its entire trajectory in at least one of the habituation trials, and 1 because the primary observer had difficulty following the direction of the infant's gaze.
Apparatus and events. The
Experiment 5
The 3-month-old infants in Experiment 1 looked reliably longer at the low- than at the high-window test event. This positive result contrasted with two negative results discussed in Section 1: (1) the negative result obtained with the 2.5-month-old infants tested with habituation and test events similar to those in Experiment 1 (Aguiar & Baillargeon, 1999), and (2) the negative result obtained with the 3- but not the 3.5-month-old infants in the carrot experiment (Baillargeon & DeVos, 1991; see
Experiment 6
The positive responses of the 3-month-old infants in Experiment 1 contrasted not only with the negative responses obtained with 2.5- and 3-month-old infants in earlier experiments (Aguiar & Baillargeon, 1999; Baillargeon & DeVos, 1991), but also with the negative responses of the 3.5-month-old infants in Experiments 1 and 1A. As suggested earlier, our interpretation of this discrepancy was that, unlike these older infants, the younger infants were not able to spontaneously generate a two-mouse
Method
Participants. Participants were 10 healthy term infants, 5 male and 5 female, ranging in age from 93 to 100 days (M=95.9 days). An additional 8 infants were tested but eliminated, 7 because of fussiness and 1 because the infant looked for 90 s on three test trials.
Apparatus, events, and procedure. The apparatus, events, and procedure used in Experiment 7 were identical to those in Experiment 3. Five infants failed to satisfy the habituation criterion within 9 trials; the other 5 infants took an
General discussion
The present results reveal two separate developments in young infants' reasoning about occluded objects. The first concerns infants' knowledge of the conditions under which objects should and should not be occluded. The results of Experiments 1, 5, and 7, together with prior results by Aguiar and Baillargeon (1999) and Baillargeon and DeVos (1991), make clear that 3-month-old infants have progressed beyond their initial concept of when objects should be occluded, even though they have not yet
References (65)
- et al.
2.5-month-old infants' reasoning about when objects should and should not be occluded
Cognitive Psychology
(1999) Representing the existence and the location of hidden objects: Object permanence in 6- and 8-month-old infants
Cognition
(1986)Reasoning about the height and location of a hidden object in 4.5- and 6.5-month-old infants
Cognition
(1991)- et al.
Location memory in 8-month-old infants in a non-search AB task: further evidence
Cognitive Development
(1989) - et al.
Where's the rabbit? 5.5-month-old infants' representation of the height of a hidden object
Cognitive Development
(1987) - et al.
Why do young infants fail to search for hidden objects?
Cognition
(1990) - et al.
Object permanence in 5-month-old infants
Cognition
(1985) Object tracking and search in infancy: A review of data and a theoretical evaluation
Developmental Review
(1985)Who put the cog in infant cognition? Is rich interpretation too costly?
Infant Behavior and Development
(1998)- et al.
Dynamic mental representation in infancy
Cognition
(1997)
Object representation, identity, and the paradox of early permanence: steps toward a new framework
Infant Behavior and Development
Intuitions about support in 4.5-month-old infants
Cognition
Infants' coding of location in continuous space
Infant Behavior and Development
Tracking and anticipation of invisible spatial transformations by 4- to 8-month-old infants
Cognitive Development
Do infants understand simple arithmetic? A replication of Wynn (1992)
Cognitive Development
Principles of object perception
Cognitive Science
Initial knowledge: Six suggestions
Cognition
Object individuation: Infants' use of shape, size, pattern, and color
Cognition
Object individuation in infancy: The use of featural information in reasoning about occlusion events
Cognitive Psychology
Location memory in healthy preterm and fullterm infants
Infant Behavior and Development
8.5-month-old infants' reasoning about containment events
Child Development
Perseveration and problem solving in infancy
Perception of object properties over time
Object permanence in 3.5- and 4.5-month- old infants
Developmental Psychology
How do infants learn about the physical world?
Current Directions in Psychological Science
Physical reasoning in young infants: Seeking explanations for unexpected events
British Journal of Developmental Psychology
Physical reasoning in infancy
Infants' understanding of the physical world
Young infants' expectations about hidden objects: A reply to three challenges (article with peer commentaries and response)
Developmental Science
Reply to Bogartz, Shinskey, and Schilling; Schilling; and Cashon and Cohen
Infancy
Cited by (112)
The influence of language input on 3-year-olds' learning about novel social categories
2022, Acta PsychologicaFive-month-old infants attribute inferences based on general knowledge to agents
2021, Journal of Experimental Child PsychologyExecutive functions
2020, Handbook of Clinical Neurology
- ☆
This research was supported by grants to the first author from the Natural Sciences and Engineering Research Council of Canada, The University of Waterloo, and CAPES-Brasilia/Brasil (BEX-2688); and by a grant to the second author from the National Institute of Child Health and Human Development (HD-21104). We thank Susan Carey, Dov Cohen, Cynthia Fisher, Kristine Onishi, Philippe Rochat, and Jason Sullivan for many helpful comments and suggestions; Yuyan Luo and Karen Menard for their help with the statistical analyses; and Laura Brueckner, Deepa Block, Beth Cullum, Laura Glaser, Susan Hespos, Gavin Huntley-Fenner, Lisa Kaufman, Laura Kotovsky, Melsie Minna, Helen Raschke, Teresa Wilcox, and the undergraduate assistants in the Infant Cognition Laboratory at the University of Illinois for their help with the data collection. We also thank the parents who kindly agreed to have their infants participate in the research.