- Email: [email protected]
Does reaching in the dark for unseen objects reflect representation in infants?
DOES REACHING IN THE DARK FOR UNSEEN OBJECTS REFLECT REPRESENTATION IN INFANTS? Rachel K. Clifton Eve E. Perris Daniel D. McCall University of Massachusetts
Five groups of 7-month-old infants had varying amounts of experience with a sounding object in the light before being presented with it in the dark. Neither visual nor auditory experience was necessary for accurate reaching in the dark, suggesting that infants represented the unseen object.
representation
cognitive development
A wealth of evidence confirms early reports (Bower & Wishart, 1972; Wishart, Bower, & Dunkeld, 1978) that infants will reach out and grasp objects they cannot see, when presented to them in the dark. This behavior can be seen from the onset of reaching around 16 weeks of age. Clifton, Muir, Ashmead, and Clarkson (1993) tested infants longitudinally prior to the onset of reaching (8 to 10 weeks of age) through the age of onset, and about one month beyond onset. They found that infants reached in the dark for sounding or glowing objects just as early as they reached for objects in the
object permanence
reaching
light. Hood and Willatts (1986) reported that 5-month-olds would reach in the dark for the remembered position of an object seen in the light. In more elaborate situations, 6-to-7month olds have been found to discriminate between sounding objects in the dark presented up close within reach versus further away out of reach (Clifton, Perris, & Bullinger, 1991; Litovsky & Clifton, 1992), and to discriminate the location of a sounding toy among five randomly varying positions in the azimuth (Perris & Clifton, 1988). Infants have also been found to vary their reaching patterns
Rachel K. Clifton, Department of Psychology, University of Massachusetts, Amherst, MA 01003; Tel: (413) 545-2655; Fax: (413) 545-0996; e-mail: [email protected].
●
INFANT BEHAVIOR & DEVELOPMENT 22 (3), 1999, 297–302 Copyright © 2000 Elsevier Science Inc.
ISSN 0163-6383 All rights of reproduction in any form reserved.
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INFANT BEHAVIOR & DEVELOPMENT
according to size of the object (Clifton, Rochat, Litovsky, & Perris, 1991). In this study a particular sound specified whether the object was a large ring, appropriate for a twohanded grasp, versus a small “doughnut-sized” ring appropriate for grasping in one hand. In the dark 6-month-old infants reached differentially for the two sounding objects. More recently Goubet and Clifton (1998) found that 6 1⁄2-month-old infants would search for a toy in the dark, basing their search on memory of auditory information contained in a complex auditory event that began and ended before reaching could be initiated. In all of this work infants had an opportunity to see, to hear, and to reach for the object in the light before being given the opportunity to reach for it in the dark. The role of the infant’s experience with the object in the light appears to be critical in understanding the infant’s behavior in the dark. In the more elaborate situations Clifton and colleagues (Clifton, Rochat et al., 1991; Goubet & Clifton, 1998) have claimed that 6-month-olds based their actions in the dark on representations of the object. However, some researchers would not accept these data as evidence for mental representation. In a clear statement of this issue, Mandler (1998) maintains that representation and memory for events requires a situation in which behavior is based on memory of the past that is uncontaminated by previously learned motor routines. Reaching for the object in the light, even though the experience was brief, might be sufficient to establish sensorimotor recognition or conditioning. (If this is in fact the case, one could still be impressed with the infant’s rapid learning because typically only 2 to 6 trials in the light preceded trials in the dark.) If one is to satisfy Mandler’s requirements for true recall and representation of events, the infant would have to reach in the dark with no prior reaching experience in the light for that object in that position. Furthermore, if the infant has seen the object in position in the light, a delay must be interposed before a reach in the dark is allowed. For this reason,
Vol. 22, No. 3, 1999
Mandler (1998) does not accept data from Hood and Willatts (1986) as evidence for representation. Hood and Willatts presented 5-month-olds with an object in the light on either the right or left, while mothers held the infants’ hands to prevent them from reaching. Lights were then turned out and infants reached more often to the side where they had last seen the object. Because the infant could immediately reach for the object, Mandler ruled this situation out as showing evidence for representation. Even though reaching in the light was prohibited, there was little or no delay between seeing the object in position in the light, then reaching for that location in the dark. We presented infants with varying experiences in the light in an effort to determine what aspects of this experience might be critical in determining their subsequent behavior with an unseen object in the dark. Five different experiences varied along a continuum that ranged from an almost identical visual, auditory, and motor experience in the light and dark, to no experience with the object in the light. Every group experienced the same set of dark trials in which an object was made to sound at various locations. These trials were preceded by variable conditions in the light. Two groups had reaching experience for the sounding object in either the same or a different location than in the dark. A third group reached for a silent object in the light, so they had no opportunity to associate sound with the object. The final two groups had no reaching experience in the light, but one group saw and heard the toy, while the last group had no experience with the object at all prior to dark trials. Note that no group saw the object in its target position immediately before the dark test trials, as in Hood and Willatts (1986). Thus, in the dark the infants were not recalling the object’s position in the light and reaching to this remembered position. Finally, no infant could simply reach back to a familiar location and be consistently correct. It was predicted that if reaching for objects in the dark is based on primitive sensorimotor recognition of the situation, only the first two
OBJECT REPRESENTATION
groups would reach in the dark. In keeping with Mandler’s criteria for representation, such behavior could be based on conditioned associations between reaching experience in the light and the sound cue in the dark. If all three groups who reached in the light also reached in the dark, this would be strong evidence that reaching for objects in the light lowered the threshold for reaching behavior generally, making it a likely response in the situation. However, if all groups reached in the dark for the unseen object, this would be evidence for representation based on either the infant’s memory of the object seen and heard in the light or for more general knowledge of objects in which a sound indicates the presence of an object, even when that object has not been seen previously and cannot be identified. Fifty infants (26 males and 24 females), ranging in age from 26 to 30 weeks (M ⫽ 28 weeks), completed the session. An additional 21 infants were tested but eliminated from the sample due to failure to reach for the object in the light (N ⫽ 3), fussing so that testing was not completed (N ⫽ 12), receiving medication on the test day (N ⫽ 2), experimenter error (N ⫽ 3), and mother’s interference by talking or holding the infant improperly (N ⫽ 1). The stimulus and apparatus were identical to that used by Perris and Clifton (1988). Sound was produced by a manually shaken rattle (plastic container 1/3 filled with popcorn kernels). A plastic finger puppet (replica of Big Bird) was backed with velcro and attached to the front of the rattle. This brightly colored toy was small enough that the infant could easily grasp and detach it from the velcro strip on the rattle. Five rattles mounted on rods were positioned at midline (0°), 30° and 60° right and left of midline in a semi-circle concave to the infant. The stimulus object was presented at shoulder height so that a completely extended arm could bring the hand into contact with the rattle. When the rattles were not being activated, the rods were weighted so that they rested in an extreme upward position, putting the rattle well above the infant’s head
299
out of reach. The base of the apparatus was marked off into eight 15° sectors (each sector spanned 8 cm) radiating out from the infant’s body. The session was conducted in a doublewalled, sound-deadened chamber, and videotaped by two infrared-sensitive cameras, one providing an overhead view and one a side view of the infant’s arm movements. An infrared light source was positioned directly overhead to illuminate the scene during dark trials. Before each trial the experimenter centered the infant’s attention at midline by calling out his/her name. The experimenter lowered the rattle to the infant’s shoulder level and shook it intermittently for 20 s. An outside observer who operated the video equipment communicated with the experimenter via a microphone connected to a headset worn by the experimenter. The outside observer instructed the experimenter on the condition, trial type (light or dark), and the rattle’s position for that trial, and indicated when 20 s had elapsed. For dark trials, the experimenter switched the room lights off and was then instructed by the observer about the rattle’s position to avoid possible cueing in the light about the upcoming trial. All trials ended after 20 s or when the infant grasped and removed the toy from the rattle. If the infant had grasped and pulled off the toy in the dark, he/she was allowed to play with it for a few seconds in the light. If the infant had not grasped the toy, the experimenter spoke to the infant in a playful manner for a few seconds then initiated the next trial. The session was divided into a light phase and a dark phase. All infants received identical dark trials, consisting of eight trials randomly presented at 30° left and 30° right, with four trials at each position. This dark experience was preceded by one of five conditions in the light. Infants were randomly assigned to one of the conditions described below. Each session began with three trials in the light and proceeded to the dark phase without interruption. Group R-Same, in which infants were given the opportunity to reach for the sounding ob-
300
INFANT BEHAVIOR & DEVELOPMENT
ject in one of the two test dark positions, either 30° left or 30° right. The same location was presented for all light trials for a particular infant, with this target light position counterbalanced across infants. Group R-Different, in which infants could reach to different positions in the light and in the dark; the object was presented at three positions in the light (midline and 60° right and left) and at the usual 30° left and right in the dark. Group R-No Sound, in which the procedure was the same as for Group R-Different, except that there was no sound (the plastic rattle was empty). This condition provided an opportunity for the infant to reach, but did not establish an association between the toy and the sound. Group Observe, in which the infant observed the experimenter manipulating the sounding object out of reach at one of three locations (midline, and 60° left and right). During each trial the experimenter shook the rattle, removed and replaced Big Bird twice at the location. Parents were asked to hold the infant’s arms to prevent them from reaching. These light trials familiarized the infant with the toy and the sound but did not allow reaching for and handling of the object. Group Social, in which the experimenter engaged in social interaction with the infant without involving the object or the sound. The experimenter played finger games and talked to the infant in order to provide equal exposure to the setting. During the dark phase, three light trials, which were identical to those in the light phase, were interspersed among the 8 dark trials in the following sequence: D-L-D-D-LD-D-L-D-D-D. We have found that if infants are in the dark for prolonged periods they become fussy, but tolerate it very well if the dark trials are broken up by light trials. A reach was scored as a forward extension of the hand away from the body in the direction of the apparatus. We analyzed only the first reach of every trial, and did not consider subsequent movements or contacts with the toy
Vol. 22, No. 3, 1999
in order to avoid accidental contacts. Reaches were counted as correct if the hand entered the target area, defined as the two 15° sectors flanking the activated rattle. If the hand entered any other sector of the apparatus, the reach was scored as incorrect. All tapes were scored by two independent scorers, with disagreement settled by a third naive scorer. Reliability was 91% agreement for number of reaches and what sector the hand entered first. When a sounding object was presented in the dark, prior experience with the object in the light did not affect either the frequency or the accuracy of infants’ reaching in the dark. This was true regardless of whether that experience included no interaction with the object whatsoever, or sight and sound of the object followed by reaching and handling it. An ANOVA was run on the total frequency of reaching in the dark for the object, with the data contributed being the sum of correct and incorrect reaches for each infant. The groups were not reliably different from one another (F (4, 45) ⫽ 2.27, p ⫽ .076). Group means and SEs are plotted in the top panel of Figure 1. In a planned follow-up test the two groups who had no reaching experience in the light (Group NRObserve and Group NR-Social) were compared to the three groups who had reached for the toy. There was no significant difference for this contrast (t (45) ⫽ 1.36, p ⫽ .18). Taken individually, Group NR-Observe had less total reaching than the three reaching groups (t (36) ⫽ 2.33, p ⬍ .03), whereas Group NRSocial was not different from the three reaching groups (t ⬍ 1.0). While it is not clear why Group Observe would have a lower level of reaching, this was the only group whose mothers held their hands to prevent movement during light trials. Accuracy of reaching was tested by comparing the proportion of target reaches to total reaches for each infant. These data are presented in the bottom panel of Figure 1. Reaching in all groups was quite accurate, with the first hand movement landing in the target zone on 75 to 92% of all reaches. There was no reliable difference among groups for accuracy (F ⬍ 1.0), using Tukey
OBJECT REPRESENTATION
301
FIGURE 1 The mean frequency of reaches, both correct and incorrect, for each group (top panel); the accuracy of reaching displayed as the proportion of target reaches to the total number of reaches for each group (bottom panel). Error bars represent the standard error of the mean.
302
INFANT BEHAVIOR & DEVELOPMENT
HSD contrasts to test differences between all groups. We conclude that neither visual or auditory experience with the object nor the opportunity to reach and handle it in the light are necessary for accurate reaching in the dark. Infants who reached to exactly the same position in light and dark were not different from infants who never saw the object. Infants do not have to be trained and reinforced in the light in order to reach and grasp objects in the dark, a conclusion also reached by Stack, Muir, Sherriff, and Roman (1989). Although this conclusion is based on not finding a group difference, which is a null result, it nevertheless seems warranted because all groups showed the positive result of reaching for the unseen object. An intriguing question is why infants in the social interaction group reached at all in the dark. We assume that they came into the lab with prior knowledge that objects make sounds; hence the presence of sound in the dark indicated the presence and location of an object, even when they could not identify it. Because reaching in the dark was not based on immediately prior sensory information or motor experience with the object, we conclude that infants had a conceptual representation of the object they reached for but could not see. If any group had failed to reach in the dark, we could conclude nothing about their possible representation of the object. However, the fact that infants who had no experience of reaching for the toy and handling it in the light did reach for it in the dark allows us to conclude that prior motor action with the toy did not cause the behavior, making it likely that a conceptual basis led to the behavior. Acknowledgments: This research was funded by National Institute of Mental Health Research Scientist Award MH 00332, and by National Institute of Child Health and Human Development research grant HD 27714.
Vol. 22, No. 3, 1999
REFERENCES Bower, T. G. R., & Wishart, J. (1972). The effect of motor skill on object permanence. Cognition, 20, 191–208. Clifton, R., Muir D., Ashmead, D., & Clarkson, M. (1993). Is visually guided reaching in early infancy a myth? Child Development, 64, 1099 – 1110. Clifton, R., Perris, E., & Bullinger, A. (1991). Infants’ perception of auditory space. Developmental Psychology, 27, 187–197. Clifton, R., Rochat, P., Litovsky, R., & Perris, E. (1991). Object representation guides infants’ reaching in the dark. Journal of Experimental Psychology: Human Perception and Performance, 17, 323–329. Goubet, N., & Clifton, R. (1998). Object and event representation in 6 1⁄2-month-old infants. Developmental Psychology, 34, 63–76. Hood, B., & Willatts, P. (1986). Reaching in the dark to an object’s remembered position: Evidence for object permanence in 5-month-old infants. British Journal of Developmental Psychology, 4, 57– 65. Litovsky, R., & Clifton, R. (1992). Use of sound pressure level in auditory distance discrimination. Journal of Acoustical Society of America, 92, 794 – 802. Mandler, J. (1998). Representation. In D. Kuhn & R. Siegler (Eds.), Cognition, perception, and language: Handbook of child psychology 5th ed. (Vol. 2, pp. 255–308). New York: Wiley. Perris, E., & Clifton, R. (1988). Reaching in the dark toward a sound as a measure of auditory localization in infants. Infant Behavior and Development, 11, 473– 491. Stack, D. M., Muir, D. W., Sherriff, F., & Roman, J. (1989). Development of infant reaching in the dark to luminous objects and ‘invisible sounds.’ Perception, 18, 69 – 82. Wishart, J. G., Bower, T. G. R., & Dunkeld, J. (1978). Reaching in the dark. Perception, 7, 507–512.
15 February 1999; Accepted 08 September 1999 䡲
Five groups of 7-month-old infants had varying amounts of experience with a sounding object in the light before being presented with it in the dark. Neither visual nor auditory experience was necessary for accurate reaching in the dark, suggesting that infants represented the unseen object.
representation
cognitive development
A wealth of evidence confirms early reports (Bower & Wishart, 1972; Wishart, Bower, & Dunkeld, 1978) that infants will reach out and grasp objects they cannot see, when presented to them in the dark. This behavior can be seen from the onset of reaching around 16 weeks of age. Clifton, Muir, Ashmead, and Clarkson (1993) tested infants longitudinally prior to the onset of reaching (8 to 10 weeks of age) through the age of onset, and about one month beyond onset. They found that infants reached in the dark for sounding or glowing objects just as early as they reached for objects in the
object permanence
reaching
light. Hood and Willatts (1986) reported that 5-month-olds would reach in the dark for the remembered position of an object seen in the light. In more elaborate situations, 6-to-7month olds have been found to discriminate between sounding objects in the dark presented up close within reach versus further away out of reach (Clifton, Perris, & Bullinger, 1991; Litovsky & Clifton, 1992), and to discriminate the location of a sounding toy among five randomly varying positions in the azimuth (Perris & Clifton, 1988). Infants have also been found to vary their reaching patterns
Rachel K. Clifton, Department of Psychology, University of Massachusetts, Amherst, MA 01003; Tel: (413) 545-2655; Fax: (413) 545-0996; e-mail: [email protected].
●
INFANT BEHAVIOR & DEVELOPMENT 22 (3), 1999, 297–302 Copyright © 2000 Elsevier Science Inc.
ISSN 0163-6383 All rights of reproduction in any form reserved.
298
INFANT BEHAVIOR & DEVELOPMENT
according to size of the object (Clifton, Rochat, Litovsky, & Perris, 1991). In this study a particular sound specified whether the object was a large ring, appropriate for a twohanded grasp, versus a small “doughnut-sized” ring appropriate for grasping in one hand. In the dark 6-month-old infants reached differentially for the two sounding objects. More recently Goubet and Clifton (1998) found that 6 1⁄2-month-old infants would search for a toy in the dark, basing their search on memory of auditory information contained in a complex auditory event that began and ended before reaching could be initiated. In all of this work infants had an opportunity to see, to hear, and to reach for the object in the light before being given the opportunity to reach for it in the dark. The role of the infant’s experience with the object in the light appears to be critical in understanding the infant’s behavior in the dark. In the more elaborate situations Clifton and colleagues (Clifton, Rochat et al., 1991; Goubet & Clifton, 1998) have claimed that 6-month-olds based their actions in the dark on representations of the object. However, some researchers would not accept these data as evidence for mental representation. In a clear statement of this issue, Mandler (1998) maintains that representation and memory for events requires a situation in which behavior is based on memory of the past that is uncontaminated by previously learned motor routines. Reaching for the object in the light, even though the experience was brief, might be sufficient to establish sensorimotor recognition or conditioning. (If this is in fact the case, one could still be impressed with the infant’s rapid learning because typically only 2 to 6 trials in the light preceded trials in the dark.) If one is to satisfy Mandler’s requirements for true recall and representation of events, the infant would have to reach in the dark with no prior reaching experience in the light for that object in that position. Furthermore, if the infant has seen the object in position in the light, a delay must be interposed before a reach in the dark is allowed. For this reason,
Vol. 22, No. 3, 1999
Mandler (1998) does not accept data from Hood and Willatts (1986) as evidence for representation. Hood and Willatts presented 5-month-olds with an object in the light on either the right or left, while mothers held the infants’ hands to prevent them from reaching. Lights were then turned out and infants reached more often to the side where they had last seen the object. Because the infant could immediately reach for the object, Mandler ruled this situation out as showing evidence for representation. Even though reaching in the light was prohibited, there was little or no delay between seeing the object in position in the light, then reaching for that location in the dark. We presented infants with varying experiences in the light in an effort to determine what aspects of this experience might be critical in determining their subsequent behavior with an unseen object in the dark. Five different experiences varied along a continuum that ranged from an almost identical visual, auditory, and motor experience in the light and dark, to no experience with the object in the light. Every group experienced the same set of dark trials in which an object was made to sound at various locations. These trials were preceded by variable conditions in the light. Two groups had reaching experience for the sounding object in either the same or a different location than in the dark. A third group reached for a silent object in the light, so they had no opportunity to associate sound with the object. The final two groups had no reaching experience in the light, but one group saw and heard the toy, while the last group had no experience with the object at all prior to dark trials. Note that no group saw the object in its target position immediately before the dark test trials, as in Hood and Willatts (1986). Thus, in the dark the infants were not recalling the object’s position in the light and reaching to this remembered position. Finally, no infant could simply reach back to a familiar location and be consistently correct. It was predicted that if reaching for objects in the dark is based on primitive sensorimotor recognition of the situation, only the first two
OBJECT REPRESENTATION
groups would reach in the dark. In keeping with Mandler’s criteria for representation, such behavior could be based on conditioned associations between reaching experience in the light and the sound cue in the dark. If all three groups who reached in the light also reached in the dark, this would be strong evidence that reaching for objects in the light lowered the threshold for reaching behavior generally, making it a likely response in the situation. However, if all groups reached in the dark for the unseen object, this would be evidence for representation based on either the infant’s memory of the object seen and heard in the light or for more general knowledge of objects in which a sound indicates the presence of an object, even when that object has not been seen previously and cannot be identified. Fifty infants (26 males and 24 females), ranging in age from 26 to 30 weeks (M ⫽ 28 weeks), completed the session. An additional 21 infants were tested but eliminated from the sample due to failure to reach for the object in the light (N ⫽ 3), fussing so that testing was not completed (N ⫽ 12), receiving medication on the test day (N ⫽ 2), experimenter error (N ⫽ 3), and mother’s interference by talking or holding the infant improperly (N ⫽ 1). The stimulus and apparatus were identical to that used by Perris and Clifton (1988). Sound was produced by a manually shaken rattle (plastic container 1/3 filled with popcorn kernels). A plastic finger puppet (replica of Big Bird) was backed with velcro and attached to the front of the rattle. This brightly colored toy was small enough that the infant could easily grasp and detach it from the velcro strip on the rattle. Five rattles mounted on rods were positioned at midline (0°), 30° and 60° right and left of midline in a semi-circle concave to the infant. The stimulus object was presented at shoulder height so that a completely extended arm could bring the hand into contact with the rattle. When the rattles were not being activated, the rods were weighted so that they rested in an extreme upward position, putting the rattle well above the infant’s head
299
out of reach. The base of the apparatus was marked off into eight 15° sectors (each sector spanned 8 cm) radiating out from the infant’s body. The session was conducted in a doublewalled, sound-deadened chamber, and videotaped by two infrared-sensitive cameras, one providing an overhead view and one a side view of the infant’s arm movements. An infrared light source was positioned directly overhead to illuminate the scene during dark trials. Before each trial the experimenter centered the infant’s attention at midline by calling out his/her name. The experimenter lowered the rattle to the infant’s shoulder level and shook it intermittently for 20 s. An outside observer who operated the video equipment communicated with the experimenter via a microphone connected to a headset worn by the experimenter. The outside observer instructed the experimenter on the condition, trial type (light or dark), and the rattle’s position for that trial, and indicated when 20 s had elapsed. For dark trials, the experimenter switched the room lights off and was then instructed by the observer about the rattle’s position to avoid possible cueing in the light about the upcoming trial. All trials ended after 20 s or when the infant grasped and removed the toy from the rattle. If the infant had grasped and pulled off the toy in the dark, he/she was allowed to play with it for a few seconds in the light. If the infant had not grasped the toy, the experimenter spoke to the infant in a playful manner for a few seconds then initiated the next trial. The session was divided into a light phase and a dark phase. All infants received identical dark trials, consisting of eight trials randomly presented at 30° left and 30° right, with four trials at each position. This dark experience was preceded by one of five conditions in the light. Infants were randomly assigned to one of the conditions described below. Each session began with three trials in the light and proceeded to the dark phase without interruption. Group R-Same, in which infants were given the opportunity to reach for the sounding ob-
300
INFANT BEHAVIOR & DEVELOPMENT
ject in one of the two test dark positions, either 30° left or 30° right. The same location was presented for all light trials for a particular infant, with this target light position counterbalanced across infants. Group R-Different, in which infants could reach to different positions in the light and in the dark; the object was presented at three positions in the light (midline and 60° right and left) and at the usual 30° left and right in the dark. Group R-No Sound, in which the procedure was the same as for Group R-Different, except that there was no sound (the plastic rattle was empty). This condition provided an opportunity for the infant to reach, but did not establish an association between the toy and the sound. Group Observe, in which the infant observed the experimenter manipulating the sounding object out of reach at one of three locations (midline, and 60° left and right). During each trial the experimenter shook the rattle, removed and replaced Big Bird twice at the location. Parents were asked to hold the infant’s arms to prevent them from reaching. These light trials familiarized the infant with the toy and the sound but did not allow reaching for and handling of the object. Group Social, in which the experimenter engaged in social interaction with the infant without involving the object or the sound. The experimenter played finger games and talked to the infant in order to provide equal exposure to the setting. During the dark phase, three light trials, which were identical to those in the light phase, were interspersed among the 8 dark trials in the following sequence: D-L-D-D-LD-D-L-D-D-D. We have found that if infants are in the dark for prolonged periods they become fussy, but tolerate it very well if the dark trials are broken up by light trials. A reach was scored as a forward extension of the hand away from the body in the direction of the apparatus. We analyzed only the first reach of every trial, and did not consider subsequent movements or contacts with the toy
Vol. 22, No. 3, 1999
in order to avoid accidental contacts. Reaches were counted as correct if the hand entered the target area, defined as the two 15° sectors flanking the activated rattle. If the hand entered any other sector of the apparatus, the reach was scored as incorrect. All tapes were scored by two independent scorers, with disagreement settled by a third naive scorer. Reliability was 91% agreement for number of reaches and what sector the hand entered first. When a sounding object was presented in the dark, prior experience with the object in the light did not affect either the frequency or the accuracy of infants’ reaching in the dark. This was true regardless of whether that experience included no interaction with the object whatsoever, or sight and sound of the object followed by reaching and handling it. An ANOVA was run on the total frequency of reaching in the dark for the object, with the data contributed being the sum of correct and incorrect reaches for each infant. The groups were not reliably different from one another (F (4, 45) ⫽ 2.27, p ⫽ .076). Group means and SEs are plotted in the top panel of Figure 1. In a planned follow-up test the two groups who had no reaching experience in the light (Group NRObserve and Group NR-Social) were compared to the three groups who had reached for the toy. There was no significant difference for this contrast (t (45) ⫽ 1.36, p ⫽ .18). Taken individually, Group NR-Observe had less total reaching than the three reaching groups (t (36) ⫽ 2.33, p ⬍ .03), whereas Group NRSocial was not different from the three reaching groups (t ⬍ 1.0). While it is not clear why Group Observe would have a lower level of reaching, this was the only group whose mothers held their hands to prevent movement during light trials. Accuracy of reaching was tested by comparing the proportion of target reaches to total reaches for each infant. These data are presented in the bottom panel of Figure 1. Reaching in all groups was quite accurate, with the first hand movement landing in the target zone on 75 to 92% of all reaches. There was no reliable difference among groups for accuracy (F ⬍ 1.0), using Tukey
OBJECT REPRESENTATION
301
FIGURE 1 The mean frequency of reaches, both correct and incorrect, for each group (top panel); the accuracy of reaching displayed as the proportion of target reaches to the total number of reaches for each group (bottom panel). Error bars represent the standard error of the mean.
302
INFANT BEHAVIOR & DEVELOPMENT
HSD contrasts to test differences between all groups. We conclude that neither visual or auditory experience with the object nor the opportunity to reach and handle it in the light are necessary for accurate reaching in the dark. Infants who reached to exactly the same position in light and dark were not different from infants who never saw the object. Infants do not have to be trained and reinforced in the light in order to reach and grasp objects in the dark, a conclusion also reached by Stack, Muir, Sherriff, and Roman (1989). Although this conclusion is based on not finding a group difference, which is a null result, it nevertheless seems warranted because all groups showed the positive result of reaching for the unseen object. An intriguing question is why infants in the social interaction group reached at all in the dark. We assume that they came into the lab with prior knowledge that objects make sounds; hence the presence of sound in the dark indicated the presence and location of an object, even when they could not identify it. Because reaching in the dark was not based on immediately prior sensory information or motor experience with the object, we conclude that infants had a conceptual representation of the object they reached for but could not see. If any group had failed to reach in the dark, we could conclude nothing about their possible representation of the object. However, the fact that infants who had no experience of reaching for the toy and handling it in the light did reach for it in the dark allows us to conclude that prior motor action with the toy did not cause the behavior, making it likely that a conceptual basis led to the behavior. Acknowledgments: This research was funded by National Institute of Mental Health Research Scientist Award MH 00332, and by National Institute of Child Health and Human Development research grant HD 27714.
Vol. 22, No. 3, 1999
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15 February 1999; Accepted 08 September 1999 䡲