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A comparison of between- and within-subjects imitation designs
Infant Behavior & Development 29 (2006) 564–573
A comparison of between- and within-subjects imitation designs Regina A. Kressley ∗ , Monika Knopf Johann Wolfgang Goethe-University, Department of Developmental Psychology, Georg-Voigt-Str. 8, D-60325 Frankfurt am Main, Germany Received 16 February 2006; received in revised form 23 May 2006; accepted 13 July 2006
Abstract Two experimental methods, which have dominated the study of declarative memory in preverbal children with imitation tasks, namely the deferred imitation and elicited imitation paradigm, differ in the amount of physical contact with test stimuli afforded infants prior to a test for long-term recall. The current study assessed effects of pre- and post-demonstration contact with test stimuli on deferred imitation of novel, single-step unrelated actions with multiple objects by 8½- and 10½-month-old infants (N = 50). The rate of target action completion after a delay remained consistent at both ages across different conditions of prior contact with test stimuli. This study shows that a within-subjects baseline appraisal is valid within certain experimental parameters and offers a more economical alternative. The results show furthermore that different experimental designs utilized to assess deferred imitation are highly comparable for the first year despite differences in determining baseline. © 2006 Elsevier Inc. All rights reserved. Keywords: Declarative memory; Deferred imitation; Infant cognition; Memory development
Imitation after a delay has been increasingly studied in recent years to investigate key aspects of declarative memory in preverbal children including the formation of representations and application of these representations in novel situations. Imitation in this context refers to the reproduction of an observed action which is absent during recall, thereby requiring infants to access an internal representation to guide his or her present behavior (Meltzoff, 1985; Piaget, 1951). Imitation is thought to provide a measure of nonverbal cued or free recall (McDonough, Mandler, McKee, & Squire, 1995) because infants must “reconstruct” what they have seen, not merely recognize that a current scene is related to an older one (Meltzoff, 1985). Studying delayed imitation within the second half of the first year is particularly interesting, because this is the timeframe in which it is thought that declarative memory emerges in humans (cf. Carver & Bauer, 1999, 2001; Nelson, 1995; Schacter & Moscovitch, 1984). The bulk of research regarding deferred imitation of novel, single-step actions with multiple, free-standing objects points to this capacity emerging around 9 months of age (Devouche, 1998; Heimann & Meltzoff, 1996; Herbert, Gross, & Hayne, 2006; Meltzoff, 1988b), whereas deferred imitation of a single action embedded in an enabling action sequence emerges by 6 months of age (Barr, Dowden, & Hayne, 1996; Kressley-Mba, Lurg, & Knopf, 2005). In order to conclude that infants have a memory of demonstrated actions, their performance in a memory test is compared to the spontaneous completion of target actions. Baseline completion of target actions is also a good index
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of action novelty (Meltzoff, 1988a). Among imitation studies there are two alternatives for assessing baseline: namely, employing a within-subjects baseline assessment, whereby participants serve as their own baseline (Abravanel, 1991; Bauer & Mandler, 1992; Bauer & Wewerka, 1995; Bjorklund, Yunger, Bering, & Ragan, 2002; Collie & Hayne, 1999; Eskritt, Donald, & Muir, 1998; Lukowski, Wiebe, Haight, DeBoer, Nelson, & Bauer, 2005), or incorporating a between-subjects age-matched control group (Barr et al., 1996; Hayne, Barr, & Herbert, 2003; Meltzoff, 1985, 1988a, 1988b). Experimental designs employing a separate age-matched control group to measure spontaneous production of target actions are thought to prevent any potential memory-enhancing effects due to prior handling of the target objects before a demonstration (1988b). Two different imitation research paradigms can be distinguished that have been used frequently to study the development of declarative memory in preverbal infants and which differ in their assessment of baseline. The experimental design that is frequently referred to as the method of deferred imitation typically employs a between-subjects design for the assessment of baseline target action completion (cf. Barr et al., 1996; Hayne, 2004; Hayne et al., 2003; Meltzoff, 1985, 1988a, 1988b). In this design the memory effect is determined by using a separate age-matched control group to measure spontaneous production of target actions. Infant responses occur within a fixed time interval ranging generally from 20 to 90 s and verbal interaction is minimized to encouraging the child to play and maintain rapport during the sessions. Patricia Bauer and Jean Mandler pioneered the method of elicited imitation, which allows researchers to investigate the temporal recall of demonstrated sequences and gauges spontaneous production of actions by allowing subjects to handle target objects during an infant-controlled baseline period prior to the demonstration (Bauer, Van Abbema, & de Haan, 1999; Mandler & McDonough, 1995). In these studies different types of multi-step events are used as memory material (familiar events; novel events enabling events in which one action in a sequence is temporally prior to and necessary for a goal or an outcome; novel events lacking a-priori relations between actions). During a test for recall children receive all props for a demonstrated event sequence simultaneously and are permitted to reproduce the steps of a sequence in any order they choose. Furthermore, in this design there is narration of the action sequences during the demonstration and children are verbally encouraged to reproduce modeled actions. Separate control groups appear to be regarded by some researchers as a more stringent control measure for baseline performance because they preclude any possibility of enhanced long-term recall through prior exposure to target objects or, according to Meltzoff (1988b), because they prevent the possibility that infants’ completion of target actions during a test for deferred imitation is based on a memory of the infants’ own prior actions. Furthermore, there is the potential that simply by seeing an experimenter handle a toy an infant may become particularly interested in the toy and more actively engage in nonspecific manipulations, which could increase the probability that the target behavior is discovered and repeated by chance (Meltzoff, 1985). Thus, target action completion after a demonstration could be more the result of trial-and-error learning due to stimulus enhancement during a baseline phase than due to memory of a modelled event. The question, which arises in conjunction with these two baseline methods, is their equivalence. Being able to demonstrate that different experimental designs are comparable with respect to delayed imitation despite different methods of baseline assessment would help provide convergent validity to this area of research. Demonstrating the equivalence of both methods has practical value as well. A within-subjects design is a much more economical alternative for determining baseline level of performance. Meltzoff (1985, 1988a, 1988b) examined variations of between-subject control conditions. In the activity-control condition Meltzoff demonstrated an alternative action with test materials prior to a test for immediate completion of target actions. In the adult-manipulation control the experimenter simply manipulated objects in an unspecific manner prior to handing children the objects in order to assess target action completion immediately afterwards. In none of the studies, which included infants ranging from 9- to 24 months old, did Meltzoff observe a difference between the standard baseline control and the two alternative control conditions. These studies provide evidence that within certain experimental parameters a period of object exploration prior to a test for immediate completion of target actions as an assessment of spontaneous action completion does not lead to subsequent target action completion above chance levels (Meltzoff, 1985, 1988a, 1988b). Because Meltzoff employed a between-subjects design to compare test action completion in the three control conditions described above to performance in a test for deferred imitation, no final conclusions can be drawn about possible effects of stimulus enhancement incurred prior to object exploration on a subsequent test for imitation after a demonstration. Research from the elicited imitation paradigm provides more information on effects of within-subjects control conditions. Bauer and Mandler (1992) altered their procedure for obtaining a within-subjects baseline with 13½-
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month-olds after observing that the children were hesitant to imitate familiar actions they had already performed during baseline. This effect did not, however, apply to novel action sequences with this age group, or to younger children 11½ months old regardless of sequence type (Bauer & Mandler, 1992). These results provide evidence that a within-subjects appraisal of baseline performance may be more appropriate for younger infants. More recent research provides evidence that a within-subjects baseline appraisal may be appropriate for an even greater age range than previously thought. Herbert, Kell, and Jones (2005) tested effects of baseline interaction with the same and with physically different target objects prior to a test for immediate imitation with 18-month-olds. Children who had the opportunity to explore the same test objects prior to the demonstration exhibited equivalent levels of imitation compared to a ‘no baseline’ condition. In addition to the question of the equivalence of baseline assessments due to potential extra familiarity with test objects accrued over successive test phases in a within-subjects design is the issue whether a within-subjects assessment of immediate imitation prior to a test for long-term recall provides a valid measure of deferred imitation. Motor-enhancing effects of target object handling directly after a demonstration and prior to a test for long-term recall have been implicated in the opportunity for immediate imitation. One of the essential features of declarative memory is that long-term recall of past events is not the product of incremental, motor learning (McDonough et al., 1995). Immediate imitation has been equated with motor practice (Meltzoff, 1995). If immediate imitation consistently has an enhancing effect on subsequent delayed imitation, then the question arises as to whether experimental designs incorporating the opportunity for immediate imitation prior to a delay are really (solely) measuring declarative memory in a test for long-term recall. This is a critical issue regarding future experimental designs for investigating the nature and development of declarative memory as well as the interpretation of deferred imitation studies. Some studies have employed the opportunity for immediate imitation as an independent variable in order to study mediating effects of post-demonstration handling of objects on delayed imitation (Bauer, Wenner, Dropik, & Wewerka, 2000; Carver & Bauer, 2001; Meltzoff, 1995), hence necessitating a within-subjects design. However, when making explicit comparisons between direct and delayed imitation of target actions (immediate imitation as a dependent variable), the method of deferred imitation typically uses a between-subjects design (Barr et al., 1996; Meltzoff, 1985, 1988b), whereas the elicited imitation procedure frequently utilizes a within-subjects design (Bauer & Hertsgaard, 1993; Bauer, Hertsgaard, & Wewerka, 1995; Bauer & Mandler, 1989; de Haan, Bauer, Georgieff, & Nelson, 2000; Mandler & McDonough, 1995). As with the baseline appraisal of target action completion discussed above an analogous question can be made about the comparability of delayed imitation results from different experimental designs depending on whether or not there was an intervening opportunity for immediate imitation. In a study with 14- and 16-month-olds, Meltzoff (1995) allowed one-third of the subjects the opportunity for immediate imitation after the demonstration and prior to delays of 2 and 4 months. There was no significant difference in mean performance level between the subjects who had the opportunity for practice (immediate imitation) and those who did not. It is conceivable, however, that memory-enhancing effects of immediate imitation might not be observable after long delays (Bauer et al., 2000). However, in an earlier study by Abravanel (1991) examining the effects of immediate imitation (“direct imitation”) on long-term memory, the results demonstrated that immediate imitation as a learning phase prior to a memory test after a delay of 10 min did not significantly enhance imitation in infants 13–16 months of age or in a second group of older infants 17–20 months old. More recent research indicates that the opportunity for immediate imitation may enhance subsequent deferred imitation among younger infants. Lukowski et al. (2005) examined effects of immediate imitation on deferred imitation among 9½-month-olds. Compared to infants, who were not permitted to interact with the materials after sequence demonstration, infants given the opportunity to reproduce modeled sequences directly after the demonstration exhibited significantly more deferred imitation after a delay of 7 days (Lukowski et al., 2005). Learmonth, Lamberth, and RoveeCollier (2005) examined the effect of immediate imitation on deferred imitation in a novel social context operationalized by a change in experimenter between demonstration and recall phases. The opportunity for immediate imitation facilitated generalization to a novel social context after 24-h among 9-, 12-, 15-, and 18-month-olds (Learmonth et al., 2005). The two studies cited above indicate that young infants prosper from an opportunity to imitate immediately prior to a test for long-term recall. However, both the Lukowski et al. (2005) and Learmonth et al. (2005) studies employed enabling action sequences as test stimuli. Since prior research provides evidence that enabling event sequences enhance infants’ immediate and deferred imitation (Bauer, 1992; Bauer & Hertsgaard, 1993; Bauer, Hertsgaard, Dropik, & Daly, 1998; Mandler & McDonough, 1995), it is possible that enabling events may be particularly conducive to the facilitative
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effects of immediate imitation. The question remains open as to what extent young infants benefit from the opportunity for immediate imitation in a more difficult task constituted by novel, unrelated items. The current study examined the potential effects of pre- and post-demonstration handling of test objects on a subsequent test of delayed imitation in the first year. The equivalence of different research designs to assess baseline was examined here using a cross-validation procedure achieved by combining different experimental designs traditionally used separately in order to study deferred imitation. In Experiment 1a the potential impact of physical exploration of stimuli prior to a demonstration on subsequent deferred imitation among 8½- and 10½-month-olds after a delay of 15 min was examined. Deferred imitation involving delays of 10 min or longer is considered a measure of long-term memory (Barnat, Klein, & Meltzoff, 1996; Bauer et al., 1999; Hanna & Meltzoff, 1993; Heimann & Meltzoff, 1996). Based on evidence that older children might be more susceptible than younger children to negative effects incurred by a within-subjects baseline assessment (Bauer & Mandler, 1992), we expected that a within-subjects baseline assessment prior to a test for long-term recall with infants in the first year would yield results comparable to a sole test for deferred imitation. In Experiment 1b, effects of pre- and post-demonstration handling of target stimuli – implemented through a withinsubjects baseline and the opportunity for immediate imitation – were examined on a subsequent test for deferred imitation among 10½-month-olds after the same delay. Bauer et al. (2000) have suggested that potential practice effects through the opportunity for immediate imitation may be manifested over the short term, but dissipate over longer delays. Therefore, if practice effects are not observable in the current study with a relatively short delay, then that finding can be more likely generalized to the conclusion that the effect will not be observed with longer delays under similar experimental parameters. Based on earlier studies with analogous test items employed by Meltzoff (1995) and anecdotal observations of infants in our lab, we hypothesized that levels of immediate and deferred imitation would be congruent. In order to avoid potential confounding facilitative effects due to an enabling task type and potential memoryenhancing effects of additional object handling, the current study only employed novel, unrelated single-action events with multiple objects. Furthermore, because prior research shows that verbal reminding enhances long-term recall among infants (Bauer et al., 2000), the current study does not include narration during a demonstration, naming of actions and objects, or verbal encouragement during test phases in order to minimize a potential confounding of an enhancement effect in delayed imitation as a function of verbal encouragement with facilitation mediated by additional handling of target actions prior to a delayed test for imitation. 1. Experiment 1a 1.1. Method 1.1.1. Participants Forty infants (20 female, 20 male) 8- and 9-months old (n = 20) (M = 268.95 days, S.D. = 15.18) and 10 and 11months old (n = 20) (M = 333.25 days, S.D. = 17.94) were recruited through visits to parent–child recreational groups and notices to local pediatricians. The total sample consisted of healthy, full-term children based on birth weight and length of gestation by maternal report. Data from three additional infants was excluded from the final analysis because infants did not meet pre-established subject characteristics for admission into the study (n = 2), and due to procedural error (n = 1). Infants were principally of Western European descent. The participant pool primarily includes families of middle class status. Participation was voluntary and all children received a small gift for their participation. 1.1.2. Apparatus The stimuli used in this experiment were four objects either constructed in the laboratory or commercially available items specially modified for the experiments, which have been described in an earlier study (Knopf, Kraus, & KressleyMba, 2006). These are shown in Fig. 1. The first target action involved half of a bright red plastic nested barrel and a small wooden spatula. The second object was a small, blue metal tin can. The third object was a yellow battery-run plastic drum. There was one large red button on the top of the drum as well as three smaller buttons in different colors on the side. A blue drumstick was attached to the drum by a string. The fourth object was a pink stuffed animal the shape of a pig wearing a hand-constructed linen hat attached to the pig with Velcro. There were five different object
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Fig. 1. These four props (from left to right: barrel, can, pig, drum) were employed in the current study. They were selected to be age-appropriate (safe and attractive), provide maximum variability in their composition, and vary with regard to their respective target actions.
orders (barrel, pig, can, drum; pig, can, drum, barrel; can, drum, barrel, stuffed animal; drum, barrel, pig, can; and pig, barrel, can, drum), which were counterbalanced between conditions. 1.1.3. Procedure Infants participating in the study were randomly assigned to one of two conditions as they became available: one prior contact with action-related objects (Condition 1) or no prior contact with action-related objects (Condition 2). Children in Condition 1 had one contact opportunity with target objects prior to the test for delayed imitation, namely before the demonstration (baseline), while the children in Condition 2 had no opportunity for handling of target objects prior to a test for deferred imitation. All infants were tested at the university’s baby lab. An interview prior to the experiment was conducted to explain the purpose of the study, details of the procedure, and obtain informed consent for the videotaping of the session. A warm-up toy was given to the child. At the beginning of each session, the parents were seated at a table (70 cm × 70 cm), and the infant was placed on the caregiver’s lap. 1.1.4. Target actions The action “barrel” was demonstrated by inserting the spatula into the barrel-half. The action “tin can” was demonstrated by shaking the can up and down three times. The target action “drum” was demonstrated by pressing the large red button on the surface with the attached drumstick. For the target action “pig” the experimenter removed the linen hat attached with Velcro. All target actions were novel for the current age groups according to criteria provided by Meltzoff (1988a) in that none of the target actions had been put in relation with the particular objects employed in the current study. This was assessed by asking caregiver’s whether they had the same or similar test objects at home. None of the caregiver’s reported having the exact same test objects at home. Even in the few cases where the parents reported having a similar object at home (i.e., another stuffed animal, plastic cups), in none of the cases did caregivers report observing their children complete the target actions employed in the current study with those objects. 1.1.5. Test sessions After the infant appeared comfortable and interacted with the experimenter, infants in Condition 1 were given the first test object for baseline lasting 45 s starting when the toy was placed within the child’s reach. The experimenter then demonstrated the first target action four times within 30 s. This procedure was repeated for the remaining three objects. Following the demonstration there was a delay of 15 min. During the delay infants had the opportunity for intervening play with distracter toys that were markedly, physically different from the test objects (i.e. a wooden car, foam cubes with animal shapes printed on them) (cf. Heimann & Meltzoff, 1996). After the delay and a second warm-up phase, a test for delayed imitation was administered to all infants. Children were given each of the objects sequentially for 45 s in the order that they had initially been presented. To help ensure that target action completion was based on memory of the demonstration and not learned incidentally children received always received the same objects modified during the recall phases so that they did not emit any noise if a target act was completed. The experimenter refrained from calling the objects by name or saying any part of the target action. 1.2. Results Two independent scorers used operational definitions to score all infants independently of one another with a dichotomous yes/no code. Scorers were blind to the specific hypotheses under investigation. Discrepancies were resolved by consensus to 100% agreement. Both percent reliability and Cohen’s kappa were calculated for target action completion in each of the test phases yielding an inter-observer reliability of 92.5% (κ = .81). Preliminary
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Table 1 Average number of target actions completed during test phases across conditions and age groups Condition
Experiment 1a One prior contact Baseline Deferred imitation No prior contact Deferred imitation Experiment 1b Two prior contacts Baseline Immediate imitation Deferred imitation
Age group 10½-month-olds
8½-month-olds
Mean (S.D.)
Mean (S.D.)
0.60 (0.70) 2.30 (0.95)
1.00 (0.82) 1.70 (1.06)
2.50 (0.71)
1.60 (1.06)
1.00 (0.94) 2.60 (0.84) 2.30 (1.05)
Note: scores range from 0 to 4.
analyses indicated that there was no main effect of gender on the number of target actions completed or interactions involving gender. Therefore, the data were collapsed across gender for all subsequent analyses. A score for each test phase (baseline and delayed imitation) was calculated for each infant by summing the number of target behaviors he or she exhibited during the respective tests (range 0–4). The average number of target actions completed across test phases is shown in Table 1. In line with our hypotheses there is a consistent trend of more target actions completed after a demonstration compared to the baseline phase. In order to test for different rates of responding between test phases across age groups in Condition 1, a mixed 2 (Age Group) × 2 (Test Phase) ANOVA was conducted for the completion of target actions before a demonstration and after a delay yielding a main effect of Test Phase, F(1, 18) = 44.49, p < .000, as well as a statistically significant interaction between Test Phase and Age Group, F(1, 18) = 7.38, p < .01. To test this interaction a 2 (Age Group) × 2 (Condition) ANOVA for the completion of target actions after a delay was conducted revealing a main effect of Age Group, F(1, 36) = 6.12, p < .02, whereby older infants produced more target actions after a delay than the younger infants. This result is depicted in Fig. 2. To be sure that the rate of target actions completion reflected memory and not pre-existing knowledge, paired t-tests were conducted to compare the mean number of target actions completed in respective test phases (baseline, deferred imitation) for both age groups revealing that both 8½-month-olds, t(9) = 2.33, p < .03, and 10½-month-olds, t(9) = 7.97, p < .00, produced more target actions during a test for delayed imitation compared to during a within-subjects baseline phase. There was no age difference in the production of target actions during baseline in Condition 1, t(18) = 1.18, p < .25 (two-sided), indicating that target actions were equally difficult for both age groups. For Condition 2, the rate of deferred
Fig. 2. Shown are average levels of target action completion (±1 S.E.) by infants in Experiment 1a in both age groups across conditions. Children demonstrated a comparable level of deferred imitation regardless of interaction with test materials prior to a demonstration.
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imitation was therefore compared to the collapsed baseline obtained in Condition 1 (M = .80, S.D. = .60). Again, this analysis revealed a memory effect for 8½-month-olds, t(9) = 2.35, p < .02, and 10½-month-olds, t(9) = 7.60, p < .00. Having identified a significant memory effect in both conditions a further one-way ANOVA was conducted to determine if deferred imitation varied across conditions as a function of prior contact with test stimuli. This analysis confirmed our hypothesis that deferred imitation did not vary as a function of condition, F(1, 38) = .02, p < .88. 1.3. Discussion The finding of significant deferred imitation among 8½-month-olds and 10½-month-olds is consistent with prior studies demonstrating deferred imitation of single-step, unrelated events with multiple objects at 9 months of age (Heimann & Meltzoff, 1996; Herbert et al., 2006; Meltzoff, 1988b). The pattern of results in Experiment 1a shows that levels of deferred imitation remained constant at both ages despite differences in familiarity with test objects prior to a test for long-term recall. These results support the hypothesis that between- and within-subjects designs for assessing baseline with very young children are appropriate and equivalent within certain experimental parameters. 2. Experiment 1b Using a method similar to the elicited imitation paradigm, which traditionally incorporates a within-subjects baseline and frequently the opportunity for immediate imitation prior to a test for long-term recall, but with fixed response times for all three phases and no narration during demonstration or verbal encouragement during recall phases, the following experiment was designed to examine if there were cumulative effects generated from pre-demonstration practice, visual observation, and motor practice immediately following modeling of target acts on a test for deferred imitation. 2.1. Participants Ten infants (three female, seven male) 10- and 11 months old (M = 335.55 days, S.D. = 19.90) were recruited through visits to parent–child recreational groups and notices to local pediatricians based on the same inclusion criteria as in Experiment 1a. Data from one additional infant was excluded from the final analysis because the infant did not meet pre-established subject characteristics for admission into the study (n = 1). 2.2. Test environment, apparatus, procedure, and scoring The test environment, target actions, test stimuli, and scoring of the responses were identical to those described in Experiment 1a. In addition to a baseline phase, infants were given the opportunity for immediate imitation immediately after the demonstration and prior to a test for deferred imitation. The videotapes of the test sessions of Experiment 1b were again scored by two raters using the same procedure and the same operational definitions as in Experiment 1a. The inter-rater reliability was 98.3% (κ = .95). 2.3. Results Again, based on descriptive data, the infants’ completion of target actions after a demonstration was greater than beforehand (see Table 1). Results from Experiment 1a for a test of deferred imitation among 10½-month-olds without any prior contact with test stimuli were employed here to as a comparison. In order to test whether completion of target actions varied across test phases as a function of extra handling, a mixed 2 (Test Phase) × 2 (Condition) ANOVA was conducted with Test Phase as a within-subjects measure and Condition as a between-subject factor for the mean completion of target actions as a function of prior handling of test stimuli yielding a main effect of Test Phase, F(1, 18) = 55.48, p < .00. The Test Phase and Condition interaction was not statistically significant, F(1, 18) = .99, which is consistent with our hypothesis that deferred imitation remained constant after a delay regardless of prior interaction with test materials. Memory was assessed with a paired t-test, which revealed that infants produced more target actions after a demonstration compared to baseline during a test for immediate, t(9) = 7.24, p < .00, and deferred imitation, t(9) = 4.99, p < .001. There was no difference between immediate and deferred imitation, t(9) = 1.15, p < .28.
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Finally, in order to assess whether additional pre-demonstration object exploration combining data from both experiments a mixed 3 (Condition) × 2 (Age Group) ANOVA was conducted for the completion of target actions after a delay yielding no main effect of Condition, F(2, 47) = .30, p < .74, confirming that neither pre- or post-demonstration handling of target objects prior to a test for long-term recall enhanced delayed imitation in itself or as a function of age. 2.4. Discussion The finding that immediate imitation was comparable to the rate of deferred imitation is consistent with other studies utilizing materials and procedures similar to those in the current experiment (Meltzoff, 1985, 1988b) and with children in this age range (Barr et al., 1996). The data demonstrate that there was no effect of pre-demonstration handling of test items on a test for immediate or delayed imitation. The data analysis also showed that there was no main effect of immediate imitation on a delayed memory test. This finding is consistent with another recent study testing prior exposure to test stimuli on imitation among older infants (Herbert et al., 2005). A comprehensive analysis comparing delayed recall from both within-subjects designs (Experiment 1a Condition 1 and Experiment 1b) with a betweensubjects measure of long-term memory (Experiment 1a Condition 2) across all infants in the current study reinforced the conclusion that memory measured in a test of deferred imitation reflects learning of an event based foremost on visual observation of the model and supports the hypothesis that between- and within-subjects baseline designs for assessing deferred imitation with infants in the first year are comparable. Even very young infants build up a stable memory trace of novel events based on observation of target actions. This finding was demonstrated in another study by Provasi, Dubon, and Bloch (2001) with 9- and 12-month-olds. Even if provided with the opportunity to do so, there is no need to intensify this memory trace via direct practice of the respective actions. Indeed, in reality infants often do not even have this option. 3. General discussion Two key mnemonic features that indicate that delayed imitation taps declarative instead of non-declarative memory include the cross-modal nature of the task, and the lack of incremental learning or sequential motor practice while acquiring the memory (Mandler & McDonough, 1995). Characteristics of a delayed imitation task mean that children rely on visual observation of modeled actions during encoding and then express recall after a delay by completing the target actions based on their mental representation of the modeled event. Despite these key attributes of declarative memory, the potential role of prior practice, whether incurred through a within-subjects baseline phase or the opportunity for immediate imitation remains a topic of interest in conjunction with delayed imitation. A more recent version of this question has been formulated by Meltzoff and Moore (1999, p. 13): do infants have a representation of a past visual event when exhibiting deferred imitation, or are they remembering and repeating their own past actions? The current study examined the potential effects of baseline manipulation of stimuli and potential effects of ‘prior practice’ on long-term recall as a function of age within the latter half of the first year. Prior practice was operationalized by the opportunity for handling target actions prior to a demonstration and the opportunity for immediate imitation after a demonstration and prior to a delay. Analysis of the results shows that imitation did not vary as a function of additional handling of target objects prior to a demonstration at any age for children in the present study, although the results should be interpreted with caution due to the small sample size involved. These results add support to the conclusion that a within-subjects appraisal of baseline target action completion within certain constraints provides a valid and economical alternative to separate age-matched control groups. It would appear that findings for delayed imitation with age-matched control groups (cf. Meltzoff, 1985, 1988a, 1988b) and within-subject baseline assessments of spontaneous completion of actions (cf. Bauer & Hertsgaard, 1993; Bauer & Mandler, 1989) are comparable for infants in the first year despite this methodological difference. This is true at least for deferred imitation of novel, unrelated target actions with multiple objects during the first year after a delay of 15 min. However, completion of target actions by the older infants in the current study was marginally greater in the condition without any additional handling of materials prior to a test for deferred imitation. A within-subjects design with very young infants may include certain limitations such as potentially inducing fatigue among infants over successive test phases or negating a kind of novelty effect which occurs when infants receive test materials for the first time. Further research is needed to examine these issues and to assess how different experimental parameters not examined here might interact with the opportunity for pre-demonstration handling of objects.
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Piaget’s (1951) observations on the hierarchical development of imitative ability, culminating with deferred imitation around the middle of the second year and preceded by immediate imitation, have frequently been cited as a reference point in deferred imitation studies (cf. Barr et al., 1996; Meltzoff, 1988b). Piaget’s observations help explain the results from previous studies that have failed to consistently demonstrate a significant difference between immediate and delayed imitation under identical experimental parameters (i.e., Bauer & Hertsgaard, 1993; Learmonth, Lamberth, & Rovee-Collier, 2004; Meltzoff, 1985, 1988b) or a consistent facilitative effect of immediate imitation on deferred imitation (Meltzoff, 1995). These tasks may be equally difficult for children within certain constraints because they are based on the same process of representative imitation, namely memory of an action, which is no longer perceptibly present. Imitation during the sixth stage of sensory-motor development differs from earlier imitation because it is based on a representative image of the modeled behavior and reflects ‘representative imitation’ (Piaget, 1951, p. 62). Hence, toward the last stage of sensory-motor intelligence, not only does the capacity for imitation after a delay emerge at this age, but so does the immediate imitation of complex new models (Piaget, 1951, p. 66). The immediate imitation of complex new models is qualitatively different from the child’s ability to “. . . imitate immediately movements and sounds already known to him . . .,” (Piaget, 1951, p. 66). Thus, Piaget (1951) viewed both deferred and immediate imitation of complex new models – such as are typically employed in deferred imitation experiments – as types of representative imitation equivalent in their degree of difficulty for a child and having some of the same cognitive processes in common. This is congruent with modern memory taxonomies that classify working (short-term) memory as a kind of declarative memory (Eichenbaum & Cohen, 2001, p. 471; Schacter & Tulving, 1994, p. 26). Does this mean that the age at which the last stage of sensory-motor intelligence manifests itself should be corrected to include a larger age range given that over the course of many studies infants provide evidence of representational imitation – or even other higher cognitive capacities such as object permanence (Needham & Baillergeon, 1993) – at a much younger age than proposed by Piaget? A reasonable solution to this apparent paradox, which is already being implemented is to continue using Piaget’s developmental stages and their behavioral manifestations where appropriate as valuable indices for the study of emerging cognitive capacities and to supplement his observations with more recent evidence won through different techniques. The data of the present study add further evidence to the fact that visual observation of a demonstration is most essential for the imitation of novel events. The fact that no motor practice is necessary for infants in the first year of life to have a stable memory trace in a test of long-term memory is a further indication that immediate and deferred imitation of novel, complex events taps declarative instead of non-declarative memory. Acknowledgments The author gratefully acknowledges the help of Uta Kraus and Antje Bockler with the data collection. Many thanks are extended to Mariana Stefanova, Irmgard Heimann, Doreen Heinicke, and Tanja Schmitter for scoring videotapes. Special thanks to all the infants and parents who participated in this research. References Abravanel, E. (1991). Does immediate imitation influence long-term memory for observed actions? Journal of Experimental Child Psychology, 51, 235–244. Barnat, S. B., Klein, P. J., & Meltzoff, A. N. (1996). Deferred imitation across changes in context and object: Memory and generalization in 14-month-old infants. Infant Behavior and Development, 19, 241–251. Barr, R., Dowden, A., & Hayne, H. (1996). Developmental changes in deferred imitation by 6- to 24-month-old infants. Infant Behavior and Development, 19, 159–170. Bauer, P. J. (1992). Holding it all together: How enabling relations facilitate young children’s event recall. Cognitive Development, 7, 1–28. Bauer, P. J., & Hertsgaard, L. A. (1993). Increasing steps in recall of events: Factors facilitating immediate and long-term memory in 13.5- and 16.5-month-old children. Child Development, 64, 1204–1223. Bauer, P. J., Hertsgaard, L. A., Dropik, P., & Daly, B. P. (1998). When even arbitrary order becomes important: Developments in reliable temporal sequencing of arbitrarily ordered events. Memory, 6, 165–198. Bauer, P. J., Hertsgaard, L. A., & Wewerka, S. S. (1995). Effects of experience and reminding on long-term recall in infancy: Remembering not to forget. Journal of Experimental Child Psychology, 9, 260–298. Bauer, P. J., & Mandler, J. M. (1992). Putting the horse before the cart: The use of temporal order in recall of events by one-year-old children. Developmental Psychology, 28, 441–452.
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Bauer, P. J., & Mandler, J. M. (1989). One thing follows another: Effects of temporal structure on 1- and 2-year-olds’ recall of events. Developmental Psychology, 25, 197–206. Bauer, P. J., & Wewerka, S. S. (1995). One- to two-year-olds’ recall of events: The more expressed, the more impressed. Journal of Experimental Child Psychology, 59, 475–496. Bauer, P. J., Van Abbema, D. L., & de Haan, M. (1999). In for the short haul: Immediate and short-term remembering and forgetting by 20-month-old children. Infant Behavior and Development, 22, 321–343. Bauer, P. J., Wenner, J. A., Dropik, P. L., & Wewerka, S. S. (2000). Parameters of remembering and forgetting in the transition from infancy to early childhood. Monographs of the Society for Research in Child Development (Vol. 65(4)). Malden, MA: Blackwell Publishing. Bjorklund, D. F., Yunger, J. L., Bering, J. M., & Ragan, P. (2002). The generalization of deferred imitation in encultured chimpanzees (Pan troglodytes). Animal Cognition, 5, 49–58. Carver, L. J., & Bauer, P. J. (1999). When the event is more than the sum of its parts: 9-month-olds’ long-term ordered recall. Memory, 7, 147–174. Carver, L. J., & Bauer, P. J. (2001). The dawning of a past: The emergence of long-term explicit memory in infancy. Journal of Experimental Psychology: General, 130, 726–745. Collie, R., & Hayne, H. (1999). Deferred imitation by 6- and 9-month-old infants: More evidence for declarative memory. Developmental Psychobiology, 35, 83–90. de Haan, M., Bauer, P. J., Georgieff, M., & Nelson, C. A. (2000). Explicit memory in low-risk infants aged 19 months born between 27 and 42 weeks of gestation. Developmental Medicine and Child Neurology, 42, 304–312. Devouche, E. (1998). Imitation across changes in object affordances and social context in 9-month-old infants. Developmental Science, 1, 65–70. Eichenbaum, H., & Cohen, N. J. (2001). From conditioning to conscious recollection: Memory systems of the brain. NY: Oxford University Press. Eskritt, M., Donald, M., & Muir, D. W. (1998). Delayed imitation of complex behavioural sequences by 14- to 16-month-olds. Early Development and Parenting, 7, 171–180. Hanna, E., & Meltzoff, A. N. (1993). Peer imitation by toddlers in laboratory, home, and day-care contexts: Implications for social learning and memory. Developmental Psychology, 29, 701–710. Hayne, H., Barr, R., & Herbert, J. (2003). The effect of prior practice on memory reactivation and generalization. Child Development, 74, 1615–1627. Hayne, H. (2004). Infant memory development: Implications for childhood amnesia. Developmental Review, 24, 33–73. Heimann, M., & Meltzoff, A. N. (1996). Deferred imitation in 9- and 14-month-old infants: A longitudinal study of a Swedish sample. British Journal of Developmental Psychology, 14, 55–64. Herbert, J., Gross, J., & Hayne, H. (2006). Age-related changes in deferred imitation between 6 and 9 months of age. Infant Behavior and Development, 29, 136–139. Herbert, J., Kell, H., & Jones, E. (April, 2005). How important are first impressions? Prior exposure to stimuli can impair 18-month-olds imitation and generalisation. Poster presented at the biennial meeting of the Society for Research in Child Development. Knopf, M., Kraus, U., & Kressley-Mba, R. (2006). Relational information processing of novel unrelated actions by infants. Infant Behavior and Development, 29, 44–53. Kressley-Mba, R., Lurg, S., & Knopf, M. (2005). Testing for deferred imitation of 2- and 3-step action sequences in 6-month-olds. Infant Behavior and Development, 28, 82–86. Learmonth, A. E., Lamberth, R., & Rovee-Collier, C. (2004). Generalization of deferred imitation during the first year of life. Journal of Experimental Child Psychology, 88, 297–318. Learmonth, A. E., Lamberth, R., & Rovee-Collier, C. (2005). The social context of imitation in infancy. Journal of Experimental Child Psychology, 91, 297–314. Lukowski, A., Wiebe, S., Haight, J., DeBoer, T., Nelson, Ch., & Bauer, P. (2005). Forming a stable memory representation in the first year of life: Why imitation is more than child’s play. Developmental Science, 8, 279–298. Mandler, J., & McDonough, L. (1995). Long-term recall of event sequences in infancy. Journal of Experimental Child Psychology, 59, 457–474. McDonough, L., Mandler, J., McKee, R., & Squire, L. (1995). The deferred imitation task as a nonverbal measure of declarative memory. Proceedings of the National Academy of Science, 92, 7580–7584. Meltzoff, A. N. (1985). Immediate and deferred imitation in fourteen- and twenty-four-month-old infants. Child Development, 56, 62–72. Meltzoff, A. N. (1988a). Infant imitation after a 1-week delay: Long-term memory for novel actions and multiple stimuli. Developmental Psychology, 24, 470–476. Meltzoff, A. N. (1988b). Infant imitation and memory: Nine-month-olds in immediate and deferred tests. Child Development, 59, 217–225. 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Infants, amnesiacs, and dissociable memory systems. In M. Moscovitch (Ed.), Infant memory (pp. 173–216). New York: Plenum Press. Schacter, D. L., & Tulving, E. (1994). What are the memory systems of 1994? In D. L. Schacter, & E. Tulving (Eds.), Memory systems (pp. 1–38). Cambridge, Mass: MIT Press.
A comparison of between- and within-subjects imitation designs Regina A. Kressley ∗ , Monika Knopf Johann Wolfgang Goethe-University, Department of Developmental Psychology, Georg-Voigt-Str. 8, D-60325 Frankfurt am Main, Germany Received 16 February 2006; received in revised form 23 May 2006; accepted 13 July 2006
Abstract Two experimental methods, which have dominated the study of declarative memory in preverbal children with imitation tasks, namely the deferred imitation and elicited imitation paradigm, differ in the amount of physical contact with test stimuli afforded infants prior to a test for long-term recall. The current study assessed effects of pre- and post-demonstration contact with test stimuli on deferred imitation of novel, single-step unrelated actions with multiple objects by 8½- and 10½-month-old infants (N = 50). The rate of target action completion after a delay remained consistent at both ages across different conditions of prior contact with test stimuli. This study shows that a within-subjects baseline appraisal is valid within certain experimental parameters and offers a more economical alternative. The results show furthermore that different experimental designs utilized to assess deferred imitation are highly comparable for the first year despite differences in determining baseline. © 2006 Elsevier Inc. All rights reserved. Keywords: Declarative memory; Deferred imitation; Infant cognition; Memory development
Imitation after a delay has been increasingly studied in recent years to investigate key aspects of declarative memory in preverbal children including the formation of representations and application of these representations in novel situations. Imitation in this context refers to the reproduction of an observed action which is absent during recall, thereby requiring infants to access an internal representation to guide his or her present behavior (Meltzoff, 1985; Piaget, 1951). Imitation is thought to provide a measure of nonverbal cued or free recall (McDonough, Mandler, McKee, & Squire, 1995) because infants must “reconstruct” what they have seen, not merely recognize that a current scene is related to an older one (Meltzoff, 1985). Studying delayed imitation within the second half of the first year is particularly interesting, because this is the timeframe in which it is thought that declarative memory emerges in humans (cf. Carver & Bauer, 1999, 2001; Nelson, 1995; Schacter & Moscovitch, 1984). The bulk of research regarding deferred imitation of novel, single-step actions with multiple, free-standing objects points to this capacity emerging around 9 months of age (Devouche, 1998; Heimann & Meltzoff, 1996; Herbert, Gross, & Hayne, 2006; Meltzoff, 1988b), whereas deferred imitation of a single action embedded in an enabling action sequence emerges by 6 months of age (Barr, Dowden, & Hayne, 1996; Kressley-Mba, Lurg, & Knopf, 2005). In order to conclude that infants have a memory of demonstrated actions, their performance in a memory test is compared to the spontaneous completion of target actions. Baseline completion of target actions is also a good index
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of action novelty (Meltzoff, 1988a). Among imitation studies there are two alternatives for assessing baseline: namely, employing a within-subjects baseline assessment, whereby participants serve as their own baseline (Abravanel, 1991; Bauer & Mandler, 1992; Bauer & Wewerka, 1995; Bjorklund, Yunger, Bering, & Ragan, 2002; Collie & Hayne, 1999; Eskritt, Donald, & Muir, 1998; Lukowski, Wiebe, Haight, DeBoer, Nelson, & Bauer, 2005), or incorporating a between-subjects age-matched control group (Barr et al., 1996; Hayne, Barr, & Herbert, 2003; Meltzoff, 1985, 1988a, 1988b). Experimental designs employing a separate age-matched control group to measure spontaneous production of target actions are thought to prevent any potential memory-enhancing effects due to prior handling of the target objects before a demonstration (1988b). Two different imitation research paradigms can be distinguished that have been used frequently to study the development of declarative memory in preverbal infants and which differ in their assessment of baseline. The experimental design that is frequently referred to as the method of deferred imitation typically employs a between-subjects design for the assessment of baseline target action completion (cf. Barr et al., 1996; Hayne, 2004; Hayne et al., 2003; Meltzoff, 1985, 1988a, 1988b). In this design the memory effect is determined by using a separate age-matched control group to measure spontaneous production of target actions. Infant responses occur within a fixed time interval ranging generally from 20 to 90 s and verbal interaction is minimized to encouraging the child to play and maintain rapport during the sessions. Patricia Bauer and Jean Mandler pioneered the method of elicited imitation, which allows researchers to investigate the temporal recall of demonstrated sequences and gauges spontaneous production of actions by allowing subjects to handle target objects during an infant-controlled baseline period prior to the demonstration (Bauer, Van Abbema, & de Haan, 1999; Mandler & McDonough, 1995). In these studies different types of multi-step events are used as memory material (familiar events; novel events enabling events in which one action in a sequence is temporally prior to and necessary for a goal or an outcome; novel events lacking a-priori relations between actions). During a test for recall children receive all props for a demonstrated event sequence simultaneously and are permitted to reproduce the steps of a sequence in any order they choose. Furthermore, in this design there is narration of the action sequences during the demonstration and children are verbally encouraged to reproduce modeled actions. Separate control groups appear to be regarded by some researchers as a more stringent control measure for baseline performance because they preclude any possibility of enhanced long-term recall through prior exposure to target objects or, according to Meltzoff (1988b), because they prevent the possibility that infants’ completion of target actions during a test for deferred imitation is based on a memory of the infants’ own prior actions. Furthermore, there is the potential that simply by seeing an experimenter handle a toy an infant may become particularly interested in the toy and more actively engage in nonspecific manipulations, which could increase the probability that the target behavior is discovered and repeated by chance (Meltzoff, 1985). Thus, target action completion after a demonstration could be more the result of trial-and-error learning due to stimulus enhancement during a baseline phase than due to memory of a modelled event. The question, which arises in conjunction with these two baseline methods, is their equivalence. Being able to demonstrate that different experimental designs are comparable with respect to delayed imitation despite different methods of baseline assessment would help provide convergent validity to this area of research. Demonstrating the equivalence of both methods has practical value as well. A within-subjects design is a much more economical alternative for determining baseline level of performance. Meltzoff (1985, 1988a, 1988b) examined variations of between-subject control conditions. In the activity-control condition Meltzoff demonstrated an alternative action with test materials prior to a test for immediate completion of target actions. In the adult-manipulation control the experimenter simply manipulated objects in an unspecific manner prior to handing children the objects in order to assess target action completion immediately afterwards. In none of the studies, which included infants ranging from 9- to 24 months old, did Meltzoff observe a difference between the standard baseline control and the two alternative control conditions. These studies provide evidence that within certain experimental parameters a period of object exploration prior to a test for immediate completion of target actions as an assessment of spontaneous action completion does not lead to subsequent target action completion above chance levels (Meltzoff, 1985, 1988a, 1988b). Because Meltzoff employed a between-subjects design to compare test action completion in the three control conditions described above to performance in a test for deferred imitation, no final conclusions can be drawn about possible effects of stimulus enhancement incurred prior to object exploration on a subsequent test for imitation after a demonstration. Research from the elicited imitation paradigm provides more information on effects of within-subjects control conditions. Bauer and Mandler (1992) altered their procedure for obtaining a within-subjects baseline with 13½-
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month-olds after observing that the children were hesitant to imitate familiar actions they had already performed during baseline. This effect did not, however, apply to novel action sequences with this age group, or to younger children 11½ months old regardless of sequence type (Bauer & Mandler, 1992). These results provide evidence that a within-subjects appraisal of baseline performance may be more appropriate for younger infants. More recent research provides evidence that a within-subjects baseline appraisal may be appropriate for an even greater age range than previously thought. Herbert, Kell, and Jones (2005) tested effects of baseline interaction with the same and with physically different target objects prior to a test for immediate imitation with 18-month-olds. Children who had the opportunity to explore the same test objects prior to the demonstration exhibited equivalent levels of imitation compared to a ‘no baseline’ condition. In addition to the question of the equivalence of baseline assessments due to potential extra familiarity with test objects accrued over successive test phases in a within-subjects design is the issue whether a within-subjects assessment of immediate imitation prior to a test for long-term recall provides a valid measure of deferred imitation. Motor-enhancing effects of target object handling directly after a demonstration and prior to a test for long-term recall have been implicated in the opportunity for immediate imitation. One of the essential features of declarative memory is that long-term recall of past events is not the product of incremental, motor learning (McDonough et al., 1995). Immediate imitation has been equated with motor practice (Meltzoff, 1995). If immediate imitation consistently has an enhancing effect on subsequent delayed imitation, then the question arises as to whether experimental designs incorporating the opportunity for immediate imitation prior to a delay are really (solely) measuring declarative memory in a test for long-term recall. This is a critical issue regarding future experimental designs for investigating the nature and development of declarative memory as well as the interpretation of deferred imitation studies. Some studies have employed the opportunity for immediate imitation as an independent variable in order to study mediating effects of post-demonstration handling of objects on delayed imitation (Bauer, Wenner, Dropik, & Wewerka, 2000; Carver & Bauer, 2001; Meltzoff, 1995), hence necessitating a within-subjects design. However, when making explicit comparisons between direct and delayed imitation of target actions (immediate imitation as a dependent variable), the method of deferred imitation typically uses a between-subjects design (Barr et al., 1996; Meltzoff, 1985, 1988b), whereas the elicited imitation procedure frequently utilizes a within-subjects design (Bauer & Hertsgaard, 1993; Bauer, Hertsgaard, & Wewerka, 1995; Bauer & Mandler, 1989; de Haan, Bauer, Georgieff, & Nelson, 2000; Mandler & McDonough, 1995). As with the baseline appraisal of target action completion discussed above an analogous question can be made about the comparability of delayed imitation results from different experimental designs depending on whether or not there was an intervening opportunity for immediate imitation. In a study with 14- and 16-month-olds, Meltzoff (1995) allowed one-third of the subjects the opportunity for immediate imitation after the demonstration and prior to delays of 2 and 4 months. There was no significant difference in mean performance level between the subjects who had the opportunity for practice (immediate imitation) and those who did not. It is conceivable, however, that memory-enhancing effects of immediate imitation might not be observable after long delays (Bauer et al., 2000). However, in an earlier study by Abravanel (1991) examining the effects of immediate imitation (“direct imitation”) on long-term memory, the results demonstrated that immediate imitation as a learning phase prior to a memory test after a delay of 10 min did not significantly enhance imitation in infants 13–16 months of age or in a second group of older infants 17–20 months old. More recent research indicates that the opportunity for immediate imitation may enhance subsequent deferred imitation among younger infants. Lukowski et al. (2005) examined effects of immediate imitation on deferred imitation among 9½-month-olds. Compared to infants, who were not permitted to interact with the materials after sequence demonstration, infants given the opportunity to reproduce modeled sequences directly after the demonstration exhibited significantly more deferred imitation after a delay of 7 days (Lukowski et al., 2005). Learmonth, Lamberth, and RoveeCollier (2005) examined the effect of immediate imitation on deferred imitation in a novel social context operationalized by a change in experimenter between demonstration and recall phases. The opportunity for immediate imitation facilitated generalization to a novel social context after 24-h among 9-, 12-, 15-, and 18-month-olds (Learmonth et al., 2005). The two studies cited above indicate that young infants prosper from an opportunity to imitate immediately prior to a test for long-term recall. However, both the Lukowski et al. (2005) and Learmonth et al. (2005) studies employed enabling action sequences as test stimuli. Since prior research provides evidence that enabling event sequences enhance infants’ immediate and deferred imitation (Bauer, 1992; Bauer & Hertsgaard, 1993; Bauer, Hertsgaard, Dropik, & Daly, 1998; Mandler & McDonough, 1995), it is possible that enabling events may be particularly conducive to the facilitative
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effects of immediate imitation. The question remains open as to what extent young infants benefit from the opportunity for immediate imitation in a more difficult task constituted by novel, unrelated items. The current study examined the potential effects of pre- and post-demonstration handling of test objects on a subsequent test of delayed imitation in the first year. The equivalence of different research designs to assess baseline was examined here using a cross-validation procedure achieved by combining different experimental designs traditionally used separately in order to study deferred imitation. In Experiment 1a the potential impact of physical exploration of stimuli prior to a demonstration on subsequent deferred imitation among 8½- and 10½-month-olds after a delay of 15 min was examined. Deferred imitation involving delays of 10 min or longer is considered a measure of long-term memory (Barnat, Klein, & Meltzoff, 1996; Bauer et al., 1999; Hanna & Meltzoff, 1993; Heimann & Meltzoff, 1996). Based on evidence that older children might be more susceptible than younger children to negative effects incurred by a within-subjects baseline assessment (Bauer & Mandler, 1992), we expected that a within-subjects baseline assessment prior to a test for long-term recall with infants in the first year would yield results comparable to a sole test for deferred imitation. In Experiment 1b, effects of pre- and post-demonstration handling of target stimuli – implemented through a withinsubjects baseline and the opportunity for immediate imitation – were examined on a subsequent test for deferred imitation among 10½-month-olds after the same delay. Bauer et al. (2000) have suggested that potential practice effects through the opportunity for immediate imitation may be manifested over the short term, but dissipate over longer delays. Therefore, if practice effects are not observable in the current study with a relatively short delay, then that finding can be more likely generalized to the conclusion that the effect will not be observed with longer delays under similar experimental parameters. Based on earlier studies with analogous test items employed by Meltzoff (1995) and anecdotal observations of infants in our lab, we hypothesized that levels of immediate and deferred imitation would be congruent. In order to avoid potential confounding facilitative effects due to an enabling task type and potential memoryenhancing effects of additional object handling, the current study only employed novel, unrelated single-action events with multiple objects. Furthermore, because prior research shows that verbal reminding enhances long-term recall among infants (Bauer et al., 2000), the current study does not include narration during a demonstration, naming of actions and objects, or verbal encouragement during test phases in order to minimize a potential confounding of an enhancement effect in delayed imitation as a function of verbal encouragement with facilitation mediated by additional handling of target actions prior to a delayed test for imitation. 1. Experiment 1a 1.1. Method 1.1.1. Participants Forty infants (20 female, 20 male) 8- and 9-months old (n = 20) (M = 268.95 days, S.D. = 15.18) and 10 and 11months old (n = 20) (M = 333.25 days, S.D. = 17.94) were recruited through visits to parent–child recreational groups and notices to local pediatricians. The total sample consisted of healthy, full-term children based on birth weight and length of gestation by maternal report. Data from three additional infants was excluded from the final analysis because infants did not meet pre-established subject characteristics for admission into the study (n = 2), and due to procedural error (n = 1). Infants were principally of Western European descent. The participant pool primarily includes families of middle class status. Participation was voluntary and all children received a small gift for their participation. 1.1.2. Apparatus The stimuli used in this experiment were four objects either constructed in the laboratory or commercially available items specially modified for the experiments, which have been described in an earlier study (Knopf, Kraus, & KressleyMba, 2006). These are shown in Fig. 1. The first target action involved half of a bright red plastic nested barrel and a small wooden spatula. The second object was a small, blue metal tin can. The third object was a yellow battery-run plastic drum. There was one large red button on the top of the drum as well as three smaller buttons in different colors on the side. A blue drumstick was attached to the drum by a string. The fourth object was a pink stuffed animal the shape of a pig wearing a hand-constructed linen hat attached to the pig with Velcro. There were five different object
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Fig. 1. These four props (from left to right: barrel, can, pig, drum) were employed in the current study. They were selected to be age-appropriate (safe and attractive), provide maximum variability in their composition, and vary with regard to their respective target actions.
orders (barrel, pig, can, drum; pig, can, drum, barrel; can, drum, barrel, stuffed animal; drum, barrel, pig, can; and pig, barrel, can, drum), which were counterbalanced between conditions. 1.1.3. Procedure Infants participating in the study were randomly assigned to one of two conditions as they became available: one prior contact with action-related objects (Condition 1) or no prior contact with action-related objects (Condition 2). Children in Condition 1 had one contact opportunity with target objects prior to the test for delayed imitation, namely before the demonstration (baseline), while the children in Condition 2 had no opportunity for handling of target objects prior to a test for deferred imitation. All infants were tested at the university’s baby lab. An interview prior to the experiment was conducted to explain the purpose of the study, details of the procedure, and obtain informed consent for the videotaping of the session. A warm-up toy was given to the child. At the beginning of each session, the parents were seated at a table (70 cm × 70 cm), and the infant was placed on the caregiver’s lap. 1.1.4. Target actions The action “barrel” was demonstrated by inserting the spatula into the barrel-half. The action “tin can” was demonstrated by shaking the can up and down three times. The target action “drum” was demonstrated by pressing the large red button on the surface with the attached drumstick. For the target action “pig” the experimenter removed the linen hat attached with Velcro. All target actions were novel for the current age groups according to criteria provided by Meltzoff (1988a) in that none of the target actions had been put in relation with the particular objects employed in the current study. This was assessed by asking caregiver’s whether they had the same or similar test objects at home. None of the caregiver’s reported having the exact same test objects at home. Even in the few cases where the parents reported having a similar object at home (i.e., another stuffed animal, plastic cups), in none of the cases did caregivers report observing their children complete the target actions employed in the current study with those objects. 1.1.5. Test sessions After the infant appeared comfortable and interacted with the experimenter, infants in Condition 1 were given the first test object for baseline lasting 45 s starting when the toy was placed within the child’s reach. The experimenter then demonstrated the first target action four times within 30 s. This procedure was repeated for the remaining three objects. Following the demonstration there was a delay of 15 min. During the delay infants had the opportunity for intervening play with distracter toys that were markedly, physically different from the test objects (i.e. a wooden car, foam cubes with animal shapes printed on them) (cf. Heimann & Meltzoff, 1996). After the delay and a second warm-up phase, a test for delayed imitation was administered to all infants. Children were given each of the objects sequentially for 45 s in the order that they had initially been presented. To help ensure that target action completion was based on memory of the demonstration and not learned incidentally children received always received the same objects modified during the recall phases so that they did not emit any noise if a target act was completed. The experimenter refrained from calling the objects by name or saying any part of the target action. 1.2. Results Two independent scorers used operational definitions to score all infants independently of one another with a dichotomous yes/no code. Scorers were blind to the specific hypotheses under investigation. Discrepancies were resolved by consensus to 100% agreement. Both percent reliability and Cohen’s kappa were calculated for target action completion in each of the test phases yielding an inter-observer reliability of 92.5% (κ = .81). Preliminary
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Table 1 Average number of target actions completed during test phases across conditions and age groups Condition
Experiment 1a One prior contact Baseline Deferred imitation No prior contact Deferred imitation Experiment 1b Two prior contacts Baseline Immediate imitation Deferred imitation
Age group 10½-month-olds
8½-month-olds
Mean (S.D.)
Mean (S.D.)
0.60 (0.70) 2.30 (0.95)
1.00 (0.82) 1.70 (1.06)
2.50 (0.71)
1.60 (1.06)
1.00 (0.94) 2.60 (0.84) 2.30 (1.05)
Note: scores range from 0 to 4.
analyses indicated that there was no main effect of gender on the number of target actions completed or interactions involving gender. Therefore, the data were collapsed across gender for all subsequent analyses. A score for each test phase (baseline and delayed imitation) was calculated for each infant by summing the number of target behaviors he or she exhibited during the respective tests (range 0–4). The average number of target actions completed across test phases is shown in Table 1. In line with our hypotheses there is a consistent trend of more target actions completed after a demonstration compared to the baseline phase. In order to test for different rates of responding between test phases across age groups in Condition 1, a mixed 2 (Age Group) × 2 (Test Phase) ANOVA was conducted for the completion of target actions before a demonstration and after a delay yielding a main effect of Test Phase, F(1, 18) = 44.49, p < .000, as well as a statistically significant interaction between Test Phase and Age Group, F(1, 18) = 7.38, p < .01. To test this interaction a 2 (Age Group) × 2 (Condition) ANOVA for the completion of target actions after a delay was conducted revealing a main effect of Age Group, F(1, 36) = 6.12, p < .02, whereby older infants produced more target actions after a delay than the younger infants. This result is depicted in Fig. 2. To be sure that the rate of target actions completion reflected memory and not pre-existing knowledge, paired t-tests were conducted to compare the mean number of target actions completed in respective test phases (baseline, deferred imitation) for both age groups revealing that both 8½-month-olds, t(9) = 2.33, p < .03, and 10½-month-olds, t(9) = 7.97, p < .00, produced more target actions during a test for delayed imitation compared to during a within-subjects baseline phase. There was no age difference in the production of target actions during baseline in Condition 1, t(18) = 1.18, p < .25 (two-sided), indicating that target actions were equally difficult for both age groups. For Condition 2, the rate of deferred
Fig. 2. Shown are average levels of target action completion (±1 S.E.) by infants in Experiment 1a in both age groups across conditions. Children demonstrated a comparable level of deferred imitation regardless of interaction with test materials prior to a demonstration.
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imitation was therefore compared to the collapsed baseline obtained in Condition 1 (M = .80, S.D. = .60). Again, this analysis revealed a memory effect for 8½-month-olds, t(9) = 2.35, p < .02, and 10½-month-olds, t(9) = 7.60, p < .00. Having identified a significant memory effect in both conditions a further one-way ANOVA was conducted to determine if deferred imitation varied across conditions as a function of prior contact with test stimuli. This analysis confirmed our hypothesis that deferred imitation did not vary as a function of condition, F(1, 38) = .02, p < .88. 1.3. Discussion The finding of significant deferred imitation among 8½-month-olds and 10½-month-olds is consistent with prior studies demonstrating deferred imitation of single-step, unrelated events with multiple objects at 9 months of age (Heimann & Meltzoff, 1996; Herbert et al., 2006; Meltzoff, 1988b). The pattern of results in Experiment 1a shows that levels of deferred imitation remained constant at both ages despite differences in familiarity with test objects prior to a test for long-term recall. These results support the hypothesis that between- and within-subjects designs for assessing baseline with very young children are appropriate and equivalent within certain experimental parameters. 2. Experiment 1b Using a method similar to the elicited imitation paradigm, which traditionally incorporates a within-subjects baseline and frequently the opportunity for immediate imitation prior to a test for long-term recall, but with fixed response times for all three phases and no narration during demonstration or verbal encouragement during recall phases, the following experiment was designed to examine if there were cumulative effects generated from pre-demonstration practice, visual observation, and motor practice immediately following modeling of target acts on a test for deferred imitation. 2.1. Participants Ten infants (three female, seven male) 10- and 11 months old (M = 335.55 days, S.D. = 19.90) were recruited through visits to parent–child recreational groups and notices to local pediatricians based on the same inclusion criteria as in Experiment 1a. Data from one additional infant was excluded from the final analysis because the infant did not meet pre-established subject characteristics for admission into the study (n = 1). 2.2. Test environment, apparatus, procedure, and scoring The test environment, target actions, test stimuli, and scoring of the responses were identical to those described in Experiment 1a. In addition to a baseline phase, infants were given the opportunity for immediate imitation immediately after the demonstration and prior to a test for deferred imitation. The videotapes of the test sessions of Experiment 1b were again scored by two raters using the same procedure and the same operational definitions as in Experiment 1a. The inter-rater reliability was 98.3% (κ = .95). 2.3. Results Again, based on descriptive data, the infants’ completion of target actions after a demonstration was greater than beforehand (see Table 1). Results from Experiment 1a for a test of deferred imitation among 10½-month-olds without any prior contact with test stimuli were employed here to as a comparison. In order to test whether completion of target actions varied across test phases as a function of extra handling, a mixed 2 (Test Phase) × 2 (Condition) ANOVA was conducted with Test Phase as a within-subjects measure and Condition as a between-subject factor for the mean completion of target actions as a function of prior handling of test stimuli yielding a main effect of Test Phase, F(1, 18) = 55.48, p < .00. The Test Phase and Condition interaction was not statistically significant, F(1, 18) = .99, which is consistent with our hypothesis that deferred imitation remained constant after a delay regardless of prior interaction with test materials. Memory was assessed with a paired t-test, which revealed that infants produced more target actions after a demonstration compared to baseline during a test for immediate, t(9) = 7.24, p < .00, and deferred imitation, t(9) = 4.99, p < .001. There was no difference between immediate and deferred imitation, t(9) = 1.15, p < .28.
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Finally, in order to assess whether additional pre-demonstration object exploration combining data from both experiments a mixed 3 (Condition) × 2 (Age Group) ANOVA was conducted for the completion of target actions after a delay yielding no main effect of Condition, F(2, 47) = .30, p < .74, confirming that neither pre- or post-demonstration handling of target objects prior to a test for long-term recall enhanced delayed imitation in itself or as a function of age. 2.4. Discussion The finding that immediate imitation was comparable to the rate of deferred imitation is consistent with other studies utilizing materials and procedures similar to those in the current experiment (Meltzoff, 1985, 1988b) and with children in this age range (Barr et al., 1996). The data demonstrate that there was no effect of pre-demonstration handling of test items on a test for immediate or delayed imitation. The data analysis also showed that there was no main effect of immediate imitation on a delayed memory test. This finding is consistent with another recent study testing prior exposure to test stimuli on imitation among older infants (Herbert et al., 2005). A comprehensive analysis comparing delayed recall from both within-subjects designs (Experiment 1a Condition 1 and Experiment 1b) with a betweensubjects measure of long-term memory (Experiment 1a Condition 2) across all infants in the current study reinforced the conclusion that memory measured in a test of deferred imitation reflects learning of an event based foremost on visual observation of the model and supports the hypothesis that between- and within-subjects baseline designs for assessing deferred imitation with infants in the first year are comparable. Even very young infants build up a stable memory trace of novel events based on observation of target actions. This finding was demonstrated in another study by Provasi, Dubon, and Bloch (2001) with 9- and 12-month-olds. Even if provided with the opportunity to do so, there is no need to intensify this memory trace via direct practice of the respective actions. Indeed, in reality infants often do not even have this option. 3. General discussion Two key mnemonic features that indicate that delayed imitation taps declarative instead of non-declarative memory include the cross-modal nature of the task, and the lack of incremental learning or sequential motor practice while acquiring the memory (Mandler & McDonough, 1995). Characteristics of a delayed imitation task mean that children rely on visual observation of modeled actions during encoding and then express recall after a delay by completing the target actions based on their mental representation of the modeled event. Despite these key attributes of declarative memory, the potential role of prior practice, whether incurred through a within-subjects baseline phase or the opportunity for immediate imitation remains a topic of interest in conjunction with delayed imitation. A more recent version of this question has been formulated by Meltzoff and Moore (1999, p. 13): do infants have a representation of a past visual event when exhibiting deferred imitation, or are they remembering and repeating their own past actions? The current study examined the potential effects of baseline manipulation of stimuli and potential effects of ‘prior practice’ on long-term recall as a function of age within the latter half of the first year. Prior practice was operationalized by the opportunity for handling target actions prior to a demonstration and the opportunity for immediate imitation after a demonstration and prior to a delay. Analysis of the results shows that imitation did not vary as a function of additional handling of target objects prior to a demonstration at any age for children in the present study, although the results should be interpreted with caution due to the small sample size involved. These results add support to the conclusion that a within-subjects appraisal of baseline target action completion within certain constraints provides a valid and economical alternative to separate age-matched control groups. It would appear that findings for delayed imitation with age-matched control groups (cf. Meltzoff, 1985, 1988a, 1988b) and within-subject baseline assessments of spontaneous completion of actions (cf. Bauer & Hertsgaard, 1993; Bauer & Mandler, 1989) are comparable for infants in the first year despite this methodological difference. This is true at least for deferred imitation of novel, unrelated target actions with multiple objects during the first year after a delay of 15 min. However, completion of target actions by the older infants in the current study was marginally greater in the condition without any additional handling of materials prior to a test for deferred imitation. A within-subjects design with very young infants may include certain limitations such as potentially inducing fatigue among infants over successive test phases or negating a kind of novelty effect which occurs when infants receive test materials for the first time. Further research is needed to examine these issues and to assess how different experimental parameters not examined here might interact with the opportunity for pre-demonstration handling of objects.
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Piaget’s (1951) observations on the hierarchical development of imitative ability, culminating with deferred imitation around the middle of the second year and preceded by immediate imitation, have frequently been cited as a reference point in deferred imitation studies (cf. Barr et al., 1996; Meltzoff, 1988b). Piaget’s observations help explain the results from previous studies that have failed to consistently demonstrate a significant difference between immediate and delayed imitation under identical experimental parameters (i.e., Bauer & Hertsgaard, 1993; Learmonth, Lamberth, & Rovee-Collier, 2004; Meltzoff, 1985, 1988b) or a consistent facilitative effect of immediate imitation on deferred imitation (Meltzoff, 1995). These tasks may be equally difficult for children within certain constraints because they are based on the same process of representative imitation, namely memory of an action, which is no longer perceptibly present. Imitation during the sixth stage of sensory-motor development differs from earlier imitation because it is based on a representative image of the modeled behavior and reflects ‘representative imitation’ (Piaget, 1951, p. 62). Hence, toward the last stage of sensory-motor intelligence, not only does the capacity for imitation after a delay emerge at this age, but so does the immediate imitation of complex new models (Piaget, 1951, p. 66). The immediate imitation of complex new models is qualitatively different from the child’s ability to “. . . imitate immediately movements and sounds already known to him . . .,” (Piaget, 1951, p. 66). Thus, Piaget (1951) viewed both deferred and immediate imitation of complex new models – such as are typically employed in deferred imitation experiments – as types of representative imitation equivalent in their degree of difficulty for a child and having some of the same cognitive processes in common. This is congruent with modern memory taxonomies that classify working (short-term) memory as a kind of declarative memory (Eichenbaum & Cohen, 2001, p. 471; Schacter & Tulving, 1994, p. 26). Does this mean that the age at which the last stage of sensory-motor intelligence manifests itself should be corrected to include a larger age range given that over the course of many studies infants provide evidence of representational imitation – or even other higher cognitive capacities such as object permanence (Needham & Baillergeon, 1993) – at a much younger age than proposed by Piaget? A reasonable solution to this apparent paradox, which is already being implemented is to continue using Piaget’s developmental stages and their behavioral manifestations where appropriate as valuable indices for the study of emerging cognitive capacities and to supplement his observations with more recent evidence won through different techniques. 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