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Perception of object number through an aperture by human infants
INFANT
BEHAVIOR AND DEVELOPMENT
18,359-362
(1995)
BRIEF REPORT
Perception of Object Number Through by Human Infants
an Aperture
MARTHA E. ARTERBERRY University of Minnesota Perception of object number through an aperture by lo- and 12-month-old infants was investigated. Only 12-month-old infants perceived the number of objects. The findings support the conclusion that the perception of object properties over time emerges late in the 1st year. infants
perception
This study investigated the development of the perception of object number over time. Recent work suggests that spatiotemporal integration skills may not emerge until late in the 1st year of life. In studies in which contours of shapes were traced by a point-source light, Rose (1988) and Skouteris, McKenzie, and Day (1992) found that 12-month-olds, but not 6- or IO-month-olds, looked longer at a novel contour than at a familiar contour. These results suggest that 12-month-old infants were able to perceive some aspect of the shape’s contour as it was traced over time, but younger infants could not. Using a stationary aperture and a moving object, Arterberry (1993) investigated the perception of object length in 8-, lo-, 12-, and 24-month-old infants. The only information available for the length of the object was the amount of time it took for the object to pass behind the aperture. Twelve- and 24-montholds provided evidence of discriminating the objects; younger infants did not. In an area where early competence is typically found, this apparent late development of perception of object properties over time is surprising. It is possible that the methods used and/or the properties investigated did not provide an adequate test of younger infants’ aperThe author was supported by a University of Minnesota Graduate School Dissertation Fellowship. Additional support was provided by a NICHHD grant (HD- 16924) awarded to Albert Yonas. The author thanks Kathy Cain, Greg Mattson, Brad Pillow, and Albert Yonas for comments on earlier drafts. Portions of this research were presented at the Society for Research in Child Development, Kansas City, MO, April, 1989, and at the Carnegie Mellon Symposium on Cognition, Pittsburgh, PA, June, 1989. Correspondence and requests for reprints should be sent to Martha E. Atterberry, Department of Psychology, Box 407, Gettysburg College, Gettysburg, PA 17325.
number
aperture
ture-viewing skills. Perceiving an object as it is traced by a point-source light may require attention and memory demands that exceed the abilities of infants younger than 12 months. Moreover, the property of object length may be difficult for infants to perceive, even though the displays contain rich information about the object’s speed and direction of movement if infants have difficulty using duration to determine object length. Rock (198 1) suggested that even adults have difficulty using absolute time in perceiving the length of objects moving behind an aperture. This study tested infants’ perception of object number through a stationary vertical aperture with a task that did not require knowledge of absolute time. Subjects were 46 lo-month-olds (11 females; 24 in Condition 1, 22 in Condition 2; M age = 308 days, range = 293-338) and 52 1Zmontholds (26 females; 22 in Condition 1, 30 in Condition 2; M age = 368 days, range = 354-385). Infants were shown two events. In both, infants viewed identical rabbits through an aperture formed by a gap in two curtains and in full view. The rabbits sat on a stage that moved behind two curtains. Periodically, the curtains were adjusted. In the aperture phase, the curtains produced a small gap (3.6 cm). In thefullview phase, the curtains were opened to produce a large gap (33.0 cm). In the expected event, the number of rabbits remained unchanged from the aperture phase to the fullview phase. In the unexpected event, the number of rabbits either decreased (Condition 1) or increased (Condition 2) from the aperture phase to the full-view phase (Figure 1A and B). Prior to viewing the events, infants were familiarized with the movements of the apparatus without 359
Arterberry
360
the rabbits present. Infants were presented with at least six but no more than nine familiarization trials (M = 6.8) and five test trial pairs. The familiarization phase was terminated after six trials, when infants’ mean looking on two consecutive trials was 50% less than the mean on the first two trials. If this criterion was not met on the ninth trial, this phase ended. Twenty-one lo-month-olds did not complete all live test trial pairs due to fussiness (n = 16), experimenter error (n = 4), and parental interference (n = 1). Twenty-one 12-month-old infants did not complete all 5 test trial pairs due to fussiness (n = 17), experimenter error (n = 2), and parental interference (n = 2). All the infants’ data from completed test trial pairs were included in the analyses. Table 1 contains the mean looking times of the lo- and 12-month-old infants at the expected and unexpected test events. Preliminary analyses for gender differences and differences between infants who did and did not meet the familiarization criterion revealed no significant results, thus, the data were collapsed across gender and familiarization groups. Infants’ mean looking times at the unexpected and expected events were analyzed in a 3 x 2 x 2 analysis of variance for repeated measures, with age (IO, 12 months) and condition (1, 2) as between-subjects factors and event (unexpected, expected) as a within-subjects factor. The analyses revealed only a significant effect for event, F( 1, 94) = 8.72, p < .005. No other main effects or interactions were significant. The main effect for event was due to more looking at the unexpected than at the expected event. Planned comparisons were conducted with each age group to investigate possible age dif-
ferences in looking at the expected and unexpected events. In light of research on apertureviewing skills by Arterberry (1993) Skouteris et al. (1992), and Rose (1988), it was expected that only the older infants would show a significant increase in looking at the unexpected event. This prediction was confirmed: Only 12-montholds showed a significant increase in looking at the unexpected event, F( 1, 5 1) = 8.67, p < .Ol For lOmonths,F(1,45)=2.06,p>.lO. Twelve-month-old infants provided evidence that they perceived the number of objects that moved behind the aperture. These findings are consistent with suggestions by Arterberry (1993), Rose (1988) and Skouteris et al. (1992) that the ability to perceive object properties over time may emerge late in the 1st year of life. To accomplish this task, infants needed to rely on motion information specifying the speed and direction of the stage, and they needed to have an understanding that objects, when not in sight, retain their physical and spatial properties. In many studies, infants’ early sensitivity to kinetic information has been documented (see Kellman & Hofsten, 1992, and Yonas, Arterben-y, & Granrud, 1987, for reviews). Infants show greater attention to moving objects than to stationary objects, they perceive partially hidden objects undergoing translatory motions as unified, they perceive three-dimensional structure and coherence in kinetic point-light displays, and they perceive depth from motion-carried information. Infants typically accomplish these tasks by 4 to 5 months of age. Moreover, Baillargeon (1993) has found that infants between 4 and 8 months of age have knowledge of the existence of hidden objects and knowledge that hidden objects retain their physical and spatial properties.
TABLE 1 Mean (and Standard Deviation) Lookin Times to the Expected and Unexpected Test Events by lo- an 3 12-Month-Old Infants Decreasea
lncreaseb
Event
Event
Expected
Unexpected
Expected
Unexpected
10 Months
17.88 E!Z (7.15)
19.90 (9.19) 18.44 (7.96)
21.28
12 Months
19.57 (6.60) 19.76 (8.49)
Age
aNumber of objects decreased from two to one in the unexpected event bNumber of objects increased from one to two in the unexpected event.
LY2 (9.04)
361
Perception of Object Number
A
Fi ure 1. Schematic representation of the expected and unexpected test events shown to in9a nts in Condition 1 (A) and in Condition 2 (6). The frames to the left of the arrow show two frames during the small-gap phase of the test trial. The frame to the right of the arrow shows one frame during the large-gap phase of the test trial. The numbers on the rabbits were not present during testing.
At first glance, the findings of this study appear to be in conflict with the above work on infant motion sensitivity. An important difference is the presence of the aperture. The aperture constrains the perceiver such that the rate of scanning of the display is determined by the speed of the objects behind the aperture rather than by the observer. Consequently, the rate in this study may not have been optimal for younger infants, and these infants were unable to revisit parts of the display they had missed because it moved behind the aperture. In addition, the time interval during which integration must take place was longer in this study than in other research on motion-carried information. This longer interval may tax the memory and/or attentional capacities of infants younger than 12 months of age. These findings also appear to conflict with work by Wynn (1992) on infants’ understanding of addition and subtraction. In Wynn’s work, 5-month-old infants also viewed successive events, but it differed from the event used in this study: Wynn presented infants with a full view of the objects before a transformation occurred. For example, in the subtraction condition (2 - 1 = 1 or 2), infants viewed two objects being placed in a case. The occluder was not present, so they had a full view of these objects. Then the occluder was raised, and one object was removed. Following the removal, the occluder dropped, and either one object or
two objects were present. Thus, Wynn provided infants with an initial representation to which changes were made over time (the removal of an object). In the present experiment, infants were required to construct the initial representation over time and then access this representation when the curtains opened to fully reveal the number of objects present. The process of building a representation over time may be more difficult than the process of modifying an existing representation over time. This suggestion is supported by the findings of Craton and Baillargeon (1992) that infants younger than 12 months perceived the width of an object over time when a full view of the object was viewed initially.
REFERENCES Arterberry, M.E. (1993).Development of spatiotemporal integration in infancy. Infant Behavior and Development, 16,343-363. Baillargeon, R. (1993). The object concept revisited: New directions in the investigation of infants’ physical knowledge. In C.E. Granrud (Ed.), Visual perceprim and cognition in infancy. Hillsdale, NJ: Erlbaum. Craton, L.G., & Baillargeon, R. (1992, May). fnfants’spatial and temporal integration abilities. Paper presented at the International Conference on Infant Studies, Miami, FL. Kellman, P.J., & Hofsten, C. van (1992). The world of the moving infant: Perception of motion, stability, and space. In C. Rovee-Collier & L.P. Lipsitt (Eds.),
362
Anterbemy
Advances in infancy research (Vol. 7). Norwood, NJ: Ablex. Rock, I. (198 1). Anorthoscopic perception. Scientific American, 244, 145-153. Rose, S.A. (1988). Shape recognition in infancy: Visual integration of sequential information. Child Development, 59, 1161-l 176. Skouteris, H., McKenzie, B.E., & Day, R. (1992). Integration of sequential information for shape per-
ception by infants: A developmental study. Child Development, 63, I 164-l 176. Yonas, A., Arterberry, M.E., & Granrud, C.E. (1987). Space perception in infancy. In R. Vasta (Ed.), Annals ofchild development. Vol. 4. Greenwich, CT: JAI Press. Wynn, K. (1992). Addition and subtraction by human infants. Nature, 358, 749-750.
BEHAVIOR AND DEVELOPMENT
18,359-362
(1995)
BRIEF REPORT
Perception of Object Number Through by Human Infants
an Aperture
MARTHA E. ARTERBERRY University of Minnesota Perception of object number through an aperture by lo- and 12-month-old infants was investigated. Only 12-month-old infants perceived the number of objects. The findings support the conclusion that the perception of object properties over time emerges late in the 1st year. infants
perception
This study investigated the development of the perception of object number over time. Recent work suggests that spatiotemporal integration skills may not emerge until late in the 1st year of life. In studies in which contours of shapes were traced by a point-source light, Rose (1988) and Skouteris, McKenzie, and Day (1992) found that 12-month-olds, but not 6- or IO-month-olds, looked longer at a novel contour than at a familiar contour. These results suggest that 12-month-old infants were able to perceive some aspect of the shape’s contour as it was traced over time, but younger infants could not. Using a stationary aperture and a moving object, Arterberry (1993) investigated the perception of object length in 8-, lo-, 12-, and 24-month-old infants. The only information available for the length of the object was the amount of time it took for the object to pass behind the aperture. Twelve- and 24-montholds provided evidence of discriminating the objects; younger infants did not. In an area where early competence is typically found, this apparent late development of perception of object properties over time is surprising. It is possible that the methods used and/or the properties investigated did not provide an adequate test of younger infants’ aperThe author was supported by a University of Minnesota Graduate School Dissertation Fellowship. Additional support was provided by a NICHHD grant (HD- 16924) awarded to Albert Yonas. The author thanks Kathy Cain, Greg Mattson, Brad Pillow, and Albert Yonas for comments on earlier drafts. Portions of this research were presented at the Society for Research in Child Development, Kansas City, MO, April, 1989, and at the Carnegie Mellon Symposium on Cognition, Pittsburgh, PA, June, 1989. Correspondence and requests for reprints should be sent to Martha E. Atterberry, Department of Psychology, Box 407, Gettysburg College, Gettysburg, PA 17325.
number
aperture
ture-viewing skills. Perceiving an object as it is traced by a point-source light may require attention and memory demands that exceed the abilities of infants younger than 12 months. Moreover, the property of object length may be difficult for infants to perceive, even though the displays contain rich information about the object’s speed and direction of movement if infants have difficulty using duration to determine object length. Rock (198 1) suggested that even adults have difficulty using absolute time in perceiving the length of objects moving behind an aperture. This study tested infants’ perception of object number through a stationary vertical aperture with a task that did not require knowledge of absolute time. Subjects were 46 lo-month-olds (11 females; 24 in Condition 1, 22 in Condition 2; M age = 308 days, range = 293-338) and 52 1Zmontholds (26 females; 22 in Condition 1, 30 in Condition 2; M age = 368 days, range = 354-385). Infants were shown two events. In both, infants viewed identical rabbits through an aperture formed by a gap in two curtains and in full view. The rabbits sat on a stage that moved behind two curtains. Periodically, the curtains were adjusted. In the aperture phase, the curtains produced a small gap (3.6 cm). In thefullview phase, the curtains were opened to produce a large gap (33.0 cm). In the expected event, the number of rabbits remained unchanged from the aperture phase to the fullview phase. In the unexpected event, the number of rabbits either decreased (Condition 1) or increased (Condition 2) from the aperture phase to the full-view phase (Figure 1A and B). Prior to viewing the events, infants were familiarized with the movements of the apparatus without 359
Arterberry
360
the rabbits present. Infants were presented with at least six but no more than nine familiarization trials (M = 6.8) and five test trial pairs. The familiarization phase was terminated after six trials, when infants’ mean looking on two consecutive trials was 50% less than the mean on the first two trials. If this criterion was not met on the ninth trial, this phase ended. Twenty-one lo-month-olds did not complete all live test trial pairs due to fussiness (n = 16), experimenter error (n = 4), and parental interference (n = 1). Twenty-one 12-month-old infants did not complete all 5 test trial pairs due to fussiness (n = 17), experimenter error (n = 2), and parental interference (n = 2). All the infants’ data from completed test trial pairs were included in the analyses. Table 1 contains the mean looking times of the lo- and 12-month-old infants at the expected and unexpected test events. Preliminary analyses for gender differences and differences between infants who did and did not meet the familiarization criterion revealed no significant results, thus, the data were collapsed across gender and familiarization groups. Infants’ mean looking times at the unexpected and expected events were analyzed in a 3 x 2 x 2 analysis of variance for repeated measures, with age (IO, 12 months) and condition (1, 2) as between-subjects factors and event (unexpected, expected) as a within-subjects factor. The analyses revealed only a significant effect for event, F( 1, 94) = 8.72, p < .005. No other main effects or interactions were significant. The main effect for event was due to more looking at the unexpected than at the expected event. Planned comparisons were conducted with each age group to investigate possible age dif-
ferences in looking at the expected and unexpected events. In light of research on apertureviewing skills by Arterberry (1993) Skouteris et al. (1992), and Rose (1988), it was expected that only the older infants would show a significant increase in looking at the unexpected event. This prediction was confirmed: Only 12-montholds showed a significant increase in looking at the unexpected event, F( 1, 5 1) = 8.67, p < .Ol For lOmonths,F(1,45)=2.06,p>.lO. Twelve-month-old infants provided evidence that they perceived the number of objects that moved behind the aperture. These findings are consistent with suggestions by Arterberry (1993), Rose (1988) and Skouteris et al. (1992) that the ability to perceive object properties over time may emerge late in the 1st year of life. To accomplish this task, infants needed to rely on motion information specifying the speed and direction of the stage, and they needed to have an understanding that objects, when not in sight, retain their physical and spatial properties. In many studies, infants’ early sensitivity to kinetic information has been documented (see Kellman & Hofsten, 1992, and Yonas, Arterben-y, & Granrud, 1987, for reviews). Infants show greater attention to moving objects than to stationary objects, they perceive partially hidden objects undergoing translatory motions as unified, they perceive three-dimensional structure and coherence in kinetic point-light displays, and they perceive depth from motion-carried information. Infants typically accomplish these tasks by 4 to 5 months of age. Moreover, Baillargeon (1993) has found that infants between 4 and 8 months of age have knowledge of the existence of hidden objects and knowledge that hidden objects retain their physical and spatial properties.
TABLE 1 Mean (and Standard Deviation) Lookin Times to the Expected and Unexpected Test Events by lo- an 3 12-Month-Old Infants Decreasea
lncreaseb
Event
Event
Expected
Unexpected
Expected
Unexpected
10 Months
17.88 E!Z (7.15)
19.90 (9.19) 18.44 (7.96)
21.28
12 Months
19.57 (6.60) 19.76 (8.49)
Age
aNumber of objects decreased from two to one in the unexpected event bNumber of objects increased from one to two in the unexpected event.
LY2 (9.04)
361
Perception of Object Number
A
Fi ure 1. Schematic representation of the expected and unexpected test events shown to in9a nts in Condition 1 (A) and in Condition 2 (6). The frames to the left of the arrow show two frames during the small-gap phase of the test trial. The frame to the right of the arrow shows one frame during the large-gap phase of the test trial. The numbers on the rabbits were not present during testing.
At first glance, the findings of this study appear to be in conflict with the above work on infant motion sensitivity. An important difference is the presence of the aperture. The aperture constrains the perceiver such that the rate of scanning of the display is determined by the speed of the objects behind the aperture rather than by the observer. Consequently, the rate in this study may not have been optimal for younger infants, and these infants were unable to revisit parts of the display they had missed because it moved behind the aperture. In addition, the time interval during which integration must take place was longer in this study than in other research on motion-carried information. This longer interval may tax the memory and/or attentional capacities of infants younger than 12 months of age. These findings also appear to conflict with work by Wynn (1992) on infants’ understanding of addition and subtraction. In Wynn’s work, 5-month-old infants also viewed successive events, but it differed from the event used in this study: Wynn presented infants with a full view of the objects before a transformation occurred. For example, in the subtraction condition (2 - 1 = 1 or 2), infants viewed two objects being placed in a case. The occluder was not present, so they had a full view of these objects. Then the occluder was raised, and one object was removed. Following the removal, the occluder dropped, and either one object or
two objects were present. Thus, Wynn provided infants with an initial representation to which changes were made over time (the removal of an object). In the present experiment, infants were required to construct the initial representation over time and then access this representation when the curtains opened to fully reveal the number of objects present. The process of building a representation over time may be more difficult than the process of modifying an existing representation over time. This suggestion is supported by the findings of Craton and Baillargeon (1992) that infants younger than 12 months perceived the width of an object over time when a full view of the object was viewed initially.
REFERENCES Arterberry, M.E. (1993).Development of spatiotemporal integration in infancy. Infant Behavior and Development, 16,343-363. Baillargeon, R. (1993). The object concept revisited: New directions in the investigation of infants’ physical knowledge. In C.E. Granrud (Ed.), Visual perceprim and cognition in infancy. Hillsdale, NJ: Erlbaum. Craton, L.G., & Baillargeon, R. (1992, May). fnfants’spatial and temporal integration abilities. Paper presented at the International Conference on Infant Studies, Miami, FL. Kellman, P.J., & Hofsten, C. van (1992). The world of the moving infant: Perception of motion, stability, and space. In C. Rovee-Collier & L.P. Lipsitt (Eds.),
362
Anterbemy
Advances in infancy research (Vol. 7). Norwood, NJ: Ablex. Rock, I. (198 1). Anorthoscopic perception. Scientific American, 244, 145-153. Rose, S.A. (1988). Shape recognition in infancy: Visual integration of sequential information. Child Development, 59, 1161-l 176. Skouteris, H., McKenzie, B.E., & Day, R. (1992). Integration of sequential information for shape per-
ception by infants: A developmental study. Child Development, 63, I 164-l 176. Yonas, A., Arterberry, M.E., & Granrud, C.E. (1987). Space perception in infancy. In R. Vasta (Ed.), Annals ofchild development. Vol. 4. Greenwich, CT: JAI Press. Wynn, K. (1992). Addition and subtraction by human infants. Nature, 358, 749-750.