Sorting between dimensions: Conditions of cognitive flexibility in preschoolers

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1 Available online at Journal of Experimental Child Psychology xxx (2008) xxx xxx Sorting between dimensions: Conditions of cognitive flexibility in preschoolers Daniela Kloo *, Josef Perner, Angelika Kerschhuber, Sandra Dabernig, Markus Aichhorn Department of Psychology, University of Salzburg, A-5020 Salzburg, Austria Received 30 July 2007; revised 14 December 2007 Abstract The Dimensional Change Card Sorting task frequently is used to measure extradimensional shifting abilities in preschool children. In two studies, we investigated what makes this extradimensional shifting task difficult. In Study 1 with to 4-year-olds, we showed that extradimensional shifts from one dimension to another are more difficult than reversal shifts within a dimension (even with irrelevant variation on a second dimension). Study 2 with and 4-year-olds further confirmed this finding using a computerized paradigm and showed that sorting instructions are critical for 3- year-olds difficulties with extradimensional shifts. This finding is taken to suggest that 3-year-olds have particular problems with spontaneously classifying one object in two different ways. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Executive functions; Shifting; Flexibility; Extradimensional; Reversal; Sorting Introduction Flexibility in thinking is a major competency in our everyday lives. It allows us to think of alternatives and to adapt to changes in our environment. One kind of flexible thinking that has attracted many research efforts is our ability to switch between different relevant dimensions, that is, extradimensional shifting. In children, this ability seems to emerge at * Corresponding author. Fax: address: daniela.kloo@sbg.ac.at (D. Kloo) /$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi: /j.jecp

2 2 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx around 4 years of age. The prototypical task used to measure extradimensional shifting abilities in children is the Dimensional Change Card Sorting (DCCS) task (Zelazo, Frye, & Rapus, 1996; Zelazo, Müller, Frye, & Marcovitch, 2003). In the standard version of the DCCS task, children are required to sort first according to one dimension (e.g., color) and then according to another dimension (e.g., shape). Two target cards, each affixed to one of two sorting boxes, are used. They depict, for example, a red apple and a blue pear. The test cards (red pears and blue apples) match one target card on one dimension and the other target card on the other dimension. During the preswitch phase, children are told a pair of rules, for example, the color rules; they are asked to sort all the blue ones into the box portraying something blue and to sort all the red ones into the box displaying something red. Typically, 3-year-olds have no problems when sorting the cards according to one dimension, but they usually have problems during the postswitch phase when the cards should be sorted according to the contrasting dimension. It is not until around 4 years of age that children master this extradimensional shift. In this study, we investigated further what makes this extradimensional shifting task so difficult. In the first experiment, we obtained evidence that only extradimensional shifts from one dimension to another are difficult for 3-year-olds and that reversal shifts within a dimension are not (even with irrelevant variation on a second dimension). In the second experiment, we replicated this finding using a computerized paradigm and, more important, showed that sorting instructions are important for 3-year-olds difficulties with extradimensional shifts. This finding suggests that 3-year-olds have particular difficulty in spontaneously and simultaneously confronting two descriptions of one object. Experiment 1 In contrast to extradimensional shift tasks, within-dimension reversal shift tasks, where rules change only within one dimension, have been found to be easy for 3-year-olds (Kloo & Perner, 2003, Experiment 1; Perner & Lang, 2002). In these reversal shift conditions, children needed to shift from a normal shape game to a silly shape game. For example, in the normal shape game, the cars went to the car target and the suns went to the sun target, but in the reversed or silly shape game, the cars went to the sun target and the suns went to the car target. Perner and Lang s explanation for their finding was that withindimension reversal shift tasks are easier because they do not require children to redescribe the objects on the cards (e.g., from a blue thing during the preswitch phase to an apple during the postswitch phase). However, in these reversal shift tasks, cards varied only in shape, whereas in the extradimensional shift tasks (standard DCCS), cards varied in shape and color. Consequently, it is not clear whether the reversal shift tasks were easier than the standard DCCS task because the stimuli were less complex or because no extradimensional shift was required. Evidence that stimulus complexity may play a role came from Brooks, Hanauer, Padowska, and Rosman (2003). In their Experiment 1, they replicated the finding that 3-yearolds have no difficulties with a reversal shift task (shifting either from same to silly rules or from silly to same rules) using unidimensional stimuli that varied only with respect to the shape dimension (black and white line drawings of airplanes and dogs). In their Experiments 2 and 3, however, children had some difficulty in following silly rules (e.g., In the silly game, if the card has an airplane we place it next to the dog because it is

3 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx 3 not the same ) when bidimensional stimuli (varying in shape and color) were used irrespective of whether children started with the same rules or with the silly rules. To account for the finding that children had difficulties with their silly card sorting game when an irrelevant dimension was added, Brooks and colleagues (2003) suggested that preschoolers difficulties with selective attention, rather than the presence of an extradimensional shift, are crucial for children s difficulties with the card sorting task. The researchers characterized selective attention as the ability to focus on a particular aspect of a stimulus while defocusing other aspects of the same stimulus (p. 199). In fact, it has been shown repeatedly that performance in speeded classification tasks slows down when cards to be sorted differ not only with respect to the relevant dimension but also with respect to one or more irrelevant dimensions and that the magnitude of interference declines with age (Garner & Felfoldy, 1970; Strutt, Anderson, & Well, 1975; Well, Lorch, & Anderson, 1980). Furthermore, Hanauer and Brooks (2003) reported severe difficulties with selective attention in young preschoolers, with 3-year-olds (but not 4-yearolds) being unable to perform a cross-modal Stroop task where they needed to name color patches while hearing distractor words (color and noncolor words). Therefore, Brooks and colleagues (2003) argued that Perner and Lang s (2002) unidimensional reversal shift task was easier than the standard DCCS task because no irrelevant dimension needed to be ignored and not because no extradimensional shift was required. Brooks and colleagues concluded that the 3-year-olds appeared to have limitations in selective attention that restricted their ability to focus on the shape dimension (p. 210). However, the bidimensional reversal shift tasks with cards varying on two dimensions used by Brooks and colleagues (2003) differ from the standard DCCS task in three other aspects. First, during the preswitch phase, correct sorting was not demonstrated (and no feedback was given). Indeed, children had difficulty in following silly rules irrespective of whether it was before or after the switch in sorting rules. Second, the researchers used four different test and target cards (e.g., green and yellow socks as well as green and yellow cups) instead of two. In their Experiment 2, half of the test cards matched the target cards exactly, whereas the others matched each target card in only one dimension. 1 In their Experiment 3, test and target cards depicted the same shapes, but the objects on the four different test cards were red or blue and the objects on the target cards were green or yellow. Third, new target cards were introduced at the start of each phase (Experiment 3), or the experimenter exchanged the positions of the target cards at the beginning of the postswitch phase (Experiment 1). Therefore, the objective of our first experiment was to clarify whether children are able to follow silly rules during the postswitch phase if the procedure matches the standard DCCS task. 2 That is, to make the bidimensional reversal shift task directly comparable 1 However, it should be noted that Brooks and colleagues (2003) mentioned in a footnote that they replicated the results of Experiment 2 in a follow-up experiment using only two test cards, with each test card matching each target card in only one dimension. 2 The use of feedback during the preswitch phase differs from the standard DCCS task protocol as described by Zelazo (2006). Although during recent years hardly any feedback is given during the preswitch phase (e.g., Diamond, Carlson, & Beck, 2005; Kirkham, Cruess, & Diamond, 2003), in their early work, Zelazo and colleagues (e.g., Frye, Zelazo, & Palfai, 1995, Experiment 2; Zelazo et al., 1996, Experiment 1; see also Perner & Lang, 2002; Perner, Lang, & Kloo, 2002) sometimes gave feedback during the preswitch phase. In our study, we adhered to this earlier method, although it might be preferable to give no feedback during the preswitch phase to make the two phases of the DCCS task more comparable.

4 4 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx to the standard DCCS task, correct sorting was demonstrated and feedback was given during the preswitch phase, only two different test cards were used, and the target cards remained on their boxes throughout the task. Children s performance on a bidimensional reversal shift task was compared with their performance on a unidimensional reversal shift and the standard DCCS task. Participants Method A total of 61 children (27 girls and 34 boys) from four nursery schools in Carinthia and two day care facilities in Salzburg, Austria, participated in this experiment. Children were predominantly from middle-class backgrounds, but this information was not recorded systematically. Their ages ranged from 2 years 6 months to 4 years 10 months (M = 3 years 6 months, SD = 7.77 months). To analyze and display age trends, we divided the children into four approximately equal-sized age groups: 21 children ranging in age from 2 years 6 months to 3 years 0 months (M = 2 years 10 months, SD = 2.29 months), 14 children ranging in age from 3 years 1 month to 3 years 6 months (M = 3 years 3 months, SD = 1.73 months), 13 children ranging in age from 3 years 7 months to 3 years 11 months (M = 3 years 9 months, SD = 1.50 months), and 13 children ranging in age from 4 years 2 months to 4 years 10 months (M = 4 years 5 months, SD = 2.83 months). Design Each child was tested individually in one session lasting approximately 15 min. Children received two card sorting tasks: a standard extradimensional DCCS task and a reversal shift task. Children were randomly assigned to one of two reversal shift conditions: Of the 61 children, 31 were given a unidimensional reversal shift task and the other 30 received a bidimensional reversal shift task. These two groups of children did not differ significantly in age (unidimensional: M = months, SD = 7.66; bidimensional: M = months, SD = 7.85), t(59) = 1.13, p >.25. The order of the two card sorting tasks and the direction of shift in the standard DCCS task (from color to shape or from shape to color) were counterbalanced. Materials and procedure Three sets of cards (9 7 cm) were used. Each set consisted of 2 target cards and 12 test cards. In all conditions, each of the target cards was affixed to its target box ( cm). The test cards needed to be placed into one of these boxes through a slit. Each task involved two phases: a preswitch phase and a postswitch phase. Standard DCCS task In the standard DCCS task, the target cards displayed a red dog and a blue bird, whereas the test cards showed blue dogs and red birds. First, the experimenter pointed at the target cards and explained the two dimensions (shape and color). Next, while pointing at the appropriate boxes, she said, Now we are playing a game, the color game. In

5 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx 5 this game, all the red ones go to the red one. And all the blue ones go to the blue one. The experimenter sorted one test card (one blue and one red) into each box. After these two demonstration trials, the children were required to sort five cards on their own. On each trial, the experimenter randomly selected a test card, labeled the card by the relevant dimension only (e.g., Here is a red one ), and asked children to sort the card into one of the boxes ( Where does this card go in the color game? ). On each trial, children were told whether they had sorted the card correctly. If a card had been sorted incorrectly, the preswitch rules were repeated. When the children had completed five trials, the rules changed and the postswitch phase began. Children were told, Okay, now we are going to play a new game, the animals game. The animals game is different. All the dogs go to the dog. And all the birds go to the bird. Again, children needed to sort five cards according to the new rules. Children were not told whether a card had been placed correctly. However, as during the preswitch phase, every time a card had been sorted incorrectly, the experimenter repeated the postswitch rules. Reversal shift tasks In the unidimensional reversal shift task, the 2 target cards and 12 test cards all had the same color (green) and differed only in shape (apple or banana). In the bidimensional reversal shift task, the same shapes were used but the fruit had different colors. The target cards displayed a yellow apple and a green banana, whereas the test cards showed green apples and yellow bananas. 3 In both reversal shift tasks, the procedure was the same as in the standard task except that, during the preswitch phase, the children were asked to play the correct fruit game: All the apples go to the apple. And all the bananas go to the banana. And during the postswitch phase, children were instructed to play the silly fruit game: Now all the apples go to the banana. And all the bananas go to the apple. Results During both the pre- and postswitch phases of each card sorting task, the children were given a score between 0 and 5 depending on the number of cards sorted correctly. During the preswitch phase, children were nearly perfect. In the standard DCCS task, three children sorted one card incorrectly and two children 4 sorted two cards incorrectly. Only one child made one error in a reversal shift task (bidimensional). In both experiments, alpha was set at.05. The variable of interest was the number of correct responses during the postswitch phase. In all four card sorting versions, most children sorted either all or none of the postswitch cards correctly; specifically, 60.7% of the children in the standard DCCS task, 58.0% of the children in the unidimensional reversal shift task, and 50.0% of the children in the bidimensional reversal shift task did so. 3 The reader might note that correct responding in this bidimensional reversal shift task was identical to correct responding in the standard DCCS task (matching by shape during the preswitch phase and matching by color during the postswitch phase). That is, the main difference between the two tasks was the type of instruction (reversal vs. extradimensional shift). 4 We also analyzed the data excluding the two children who sorted more than one card incorrectly, but the results remained unchanged.

6 6 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx Table 1 Mean numbers of postswitch cards sorted correctly on each card sorting task for the four age groups in Experiment 1 Task Age group DCCS Reversal unidimensional Reversal bidimensional (N = 61) (n = 31) (n = 30) 2;6 3;0 (n = 21) 2.28 (1.74) 3.00 (1.41) (n = 8) 3.15 (1.21) (n = 13) 3;1 3;6 (n = 14) 3.00 (2.00) 4.28 (1.11) (n = 7) 3.86 (1.57) (n = 7) 3;7 3;11 (n = 13) 3.69 (2.17) 4.75 (0.71) (n = 8) 5.00 (0.00) (n = 5) 4;2 4;10 (n = 13) 3.92 (1.85) 4.75 (0.46) (n = 8) 5.00 (0.00) (n = 5) All ages 3.10 (1.99) 4.19 (1.19) 3.93 (1.34) Note. Standard deviations are in parentheses immediately after the means. Maximum score = 5. In the age group column, the number before each semicolon represents years and the number after each semicolon represents months. Table 1 shows the developmental trend on the standard and reversal shift tasks. Postswitch performance was analyzed with a 2 (Task Version: standard vs. reversal [within participants]) 2 (Reversal Shift Group: unidimensional reversal shift task and standard DCCS task vs. bidimensional reversal shift task and standard DCCS task) 2 (Task Order) 4 (Age Group [between participants]) mixed design analysis of variance (ANOVA). This revealed a significant main effect of task version, with children performing significantly better on the reversal shift tasks (M = 4.06 correct, SD = 1.26) than on the standard DCCS task (M = 3.10 correct, SD = 1.99), F(1, 45) = 12.89, p =.001, partial g 2 =.22. There was also a significant main effect of age group, F(3, 45) = 7.77, p <.001, partial g 2 =.34, indicating that children s performance improved with age on both the standard DCCS and reversal shift tasks. Furthermore, there was a significant main effect of task order, F(1, 45) = 4.95, p =.031, partial g 2 =.10. This main effect was qualified by a Task Version Task Order interaction, F(1, 45) = 4.81, p =.034, partial g 2 =.10. This interaction was due to the fact that task order influenced children s performance on the standard DCCS task (first position: M = 2.59 correct, SD = 2.08; second position: M = 3.65 correct, SD = 1.76), t(59) = 2.14, p =.036, but not on the reversal shift tasks (first position: M = 4.10 correct, SD = 1.14; second position: M = 4.03 correct, SD = 1.38), t(59) = 0.22, p >.80. No other main effects or interactions reached statistical significance. A similar order effect was reported previously by Perner and Lang (2002). In their study, the standard DCCS task was difficult only when presented as the first task and not when given after a reversal shift task (but see Kloo & Perner, 2003, Experiment 1, and Lang, 2001, Experiment 4, for nonreplications of this effect). That is, there is some evidence that confrontation with a reversal shift task improves performance on a subsequent standard DCCS task. Possible explanations for this effect could be that (a) children treat the subsequent standard DCCS task as a reversal shift task (as suggested by Phil Zelazo [see Perner & Lang, 2002, p. 102]) and (b) doing a reversal shift task may familiarize children with the fact that the cards need to be treated differently during the postswitch phase (see Perner & Lang, 2002, p. 102, for a discussion of this issue). Because the slight but nonsignificant age difference between the two reversal shift groups might have affected the between-participants effect of reversal shift group, we also conducted an analysis of covariance (ANCOVA) with task version as

7 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx 7 within-participants factor, reversal shift version and task order as between-participants factors, and age in months as covariate. Again, neither the between-participants effect of reversal shift group (p >.80) nor any interaction with reversal shift group was significant. We also conducted separate paired-samples t tests for each reversal shift group. In both groups, performance on the reversal shift task was significantly better than performance on the standard DCCS task (unidimensional: t(30) = 2.60, p <.05; bidimensional: t(29) = 2.34, p <.05). An independent samples t test further confirmed that children s performance on the two reversal shift tasks (unidimensional: M = 4.19 correct, SD = 1.19; bidimensional: M = 3.93 correct, SD = 1.34) did not differ, t(59) = 0.80, p >.05. Because postswitch responses were not normally distributed, the children s performance was also evaluated categorically. Children were classified as passing the postswitch phase if they sorted four (or more) of the five items correctly. A McNemar s v 2 test indicated that children performed better on the reversal shift versions than on the standard DCCS task, v 2 (1, N = 61) = 6.00, p =.023. Children s performance on the two reversal shift tasks did not differ (Fisher s exact test, p =.255). Discussion Replicating Perner and Lang (2002) as well as Kloo and Perner (2003, Experiment 1), card sorting tasks involving a reversal shift were much easier than the traditional DCCS task, which involves an extradimensional shift. In contrast to Brooks and colleagues (2003), reversal shift tasks were easy irrespective of whether or not the cards varied on two dimensions. Evidently, the mere presence of a second irrelevant dimension does not make shifting more difficult. The extra difficulty of the standard DCCS task seems to be due to the extradimensional shift rather than to problems with selective attention. One explanation for the difficulties with extradimensional shifts for 3-year-olds is that they require children to redescribe the objects on the cards (Kloo & Perner, 2003; Kloo & Perner, 2005; Perner & Lang, 2002), for example, from an apple during the preswitch phase to a blue thing during the postswitch phase. In contrast, reversal shift tasks do not require such a redescription because during both phases the cards need to be sorted according to the same dimension (shape). Brooks and colleagues (2003) contrasting results may be attributable to their different procedure. For example, in contrast to the standard DCCS task, sorting was not demonstrated during the preswitch phase, four different test and target cards were used, and new target cards were introduced at the start of each phase (Experiment 3), or the experimenter exchanged the positions of the target cards at the beginning of the postswitch phase (Experiment 1). This might have complicated the procedure too much because even older 4-year-olds (who usually master the standard DCCS task) were not significantly above chance on the silly trials in Experiment 3. In line with our results, Carlson (2005) also found that most 3-year-olds, but not 2-yearolds, passed a reverse categorization task in which children needed to reverse a sorting rule (see also Carlson, Mandell, & Williams, 2004). In this task, pairs of toy animals (e.g., tigers, horses) consisting of large ( mommy ) and small ( baby ) exemplars were used. Children first needed to sort mommy animals into a mommy bucket and baby animals into a baby bucket. Next, in a silly game, they needed to sort the baby animals into the mommy bucket and the mommy animals into the baby bucket.

8 8 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx The 3-year-olds good performance on these reversal shift tasks seems to conflict with their problems with other paradigms where they need to follow opposite rules, including the day night Stroop task (Gerstadt, Hong, & Diamond, 1994; see also Diamond, Kirkham, & Amso, 2002, who even used say the opposite instructions) and Luria s tapping task (Diamond & Taylor, 1996). For example, in the day night task, children are instructed to say day when shown a picture of the moon and stars and to say night when shown a picture of the sun. Young children often have difficulty in doing this and say day to the sun picture and night to the moon and stars picture. However, Gerstadt and colleagues (1994) found that, in principle, 3½to 4½-year-olds were able to follow these opposite rules ( 70% correct). These children performed well at the beginning of the task (81% correct on the first four trials), but their performance deteriorated over the course of testing (54.3% correct on the final four trials). This suggests that they had difficulty in sustaining a high level of executive control over an extended time course especially when they did not receive feedback. In contrast, on the DCCS task, they have difficulty in following the instructions for the switch even once. This indicates a basic difficulty in appreciating the importance of the switch instructions and not just an executive difficulty in maintaining an unusual rule (sun? night, moon? day) over time in the face of the competing more natural combination (sun? day, moon? night). Differences between intradimensional and extradimensional shifts have also been investigated in neuropsychological work. In adults as well, shifting between dimensions has been found to be more difficult than shifting within a dimension. For example, Hampshire and Owen (2006) found that participants were slower on extradimensional shifts than on intradimensional shifts. And Nagahama and colleagues (2001) reported switch costs in an extradimensional set-shifting task but not in a within-dimension reversal shift task. In their functional magnetic resonance imaging (fmri) analysis, Hampshire and Owen (2006) found that activation in the ventrolateral prefrontal cortex was associated with extradimensional shifts. In contrast, intradimensional reversal shifts were associated with activity in the orbitofrontal cortex but not with activity in the ventrolateral prefrontal cortex. Furthermore, Nakahara, Hayashi, Konishi, and Miyashita (2002) found that extradimensional shifts were related to activation in the ventrolateral prefrontal cortex in both humans and macaque monkeys. In addition, Rogers, Andrews, Grasby, Brooks, and Robbins (2000) reported that extradimensional shifts were also associated with activity in the dorsolateral prefrontal cortex, possibly due to the requirement to actively work out the relevant dimension in that study, as noted by Hampshire and Owen (2006). This concurs with lesion studies in monkeys that report difficulties with extradimensional shifting after lesions of the lateral prefrontal cortex and problems with reversal learning after orbital prefrontal lesions (Dias, Robbins, & Roberts, 1996; Dias, Robbins, & Roberts, 1997; Rolls, 2000). Robbins (1996) interpreted these results in the following way: Identical compound stimuli can be processed at more than one site in the prefrontal cortex, perhaps simultaneously. At one of these sites, processing allows a reward tag to be shifted from one stimulus to another ( affective shifting ). At the other site, shifts are effected between responding to different dimensions (e.g., shapes rather than colors) of complex stimuli, rather than to particular exemplars, a seemingly higher-order cognitive process. (pp )

9 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx 9 In sum, the lateral prefrontal cortex seems to be associated with extradimensional shifts and the orbitofrontal cortex seems to be associated with reversal shifts. In addition, anatomically, the orbitofrontal cortex develops prior to the lateral prefrontal cortex. This could explain why reversal shifts are mastered earlier than extradimensional shifts. However, because reversal shifts often have also been associated with a shift in rewards, future research will need to clarify the role of reward-related affective shifting in rule use so as to construe a more integrated model of the development of hot and cool executive functions (see also Bunge & Zelazo, 2006; Happaney, Zelazo, & Stuss, 2004; Zelazo & Müller, 2002). The relative difficulty of reversal (intradimensional) and nonreversal (extradimensional) shifts has also been investigated in research on discrimination shift learning (e.g., Kendler & Kendler, 1962; for reviews, see Bolton, 1972; Esposito, 1975). In his review, Esposito (1975) concluded that reversal and intradimensional shifts seem to be learned faster than nonreversal and extradimensional shifts when the irrelevant dimension is variable within trials in the post-shift task, when subjects preferred dimensions are relevant, and when subjects receive overtraining (p. 447). Better performance on reversal shift tasks than on extradimensional shift tasks seems to be consistently the case in older children and adults, whereas there are some findings to the contrary in younger preschoolers. However, in our study, there was no such interaction between age and type of shift. Even the youngest children performed better on the reversal shift task than on the extradimensional shift task. Apart from other task differences, this might be due to the fact that the card sorting tasks used in the current experiment, in contrast to discrimination shift learning tasks, did not require children to extract dimensions because the relevant dimension and the specific values are explicitly stated in the rules. Nevertheless, the extradimensional shift task, but not the reversal shift task, required children to switch dimensions and, consequently, to redescribe the objects on the cards. And this kind of redescription might be difficult for the younger children, in particular, to recognize the need for such redescription from the standard instructions. Experiment 2 According to Experiment 1, reversal shift tasks, irrespective of whether or not the cards vary on two dimensions, are easier than the standard DCCS task, which includes an extradimensional shift. One factor that makes extradimensional shift tasks difficult was found by Perner and Lang (2002; see also Kloo & Perner, 2003). They provided evidence that extradimensional versions using puppets instead of target cards are easy. Our second experiment set out to replicate this finding using computerized task versions and to investigate whether sorting instructions are critical for the difficulty of the standard DCCS task. In the easier puppets versions, the two target cards were replaced by a pair of target pictures displaying familiar characters such as Donald Duck. The switch from one dimension to the other dimension was described as a change of the puppet s preference. For example, during the preswitch phase, Donald Duck wanted all red things and was to be given red cars, whereas during the postswitch phase, he wanted all of the suns and, consequently, was to be given yellow suns. As in the reversal shift task, 3-year-olds had no serious problems with this task. To explain the finding that target cards are essential, Perner and Lang (2002; see also Kloo & Perner, 2003; Kloo & Perner, 2005) suggested in their redescription hypothesis

10 10 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx that the target cards trigger a general rule such as the following: Put each card with the target that has the same thing on it. Importantly, to follow such a rule, children need to describe the object on the card, such as a blue apple, exclusively as an apple (in the preswitch fruit game), and they must suppress possible alternative descriptions of it as a blue thing or a blue apple or else they would not know which of the targets is the same as the to-be-sorted item. Next, in the postswitch color game, children need to redescribe the object as a blue thing and must suppress the preswitch description. However, young children might not have a mature conceptual understanding that objects can be described or viewed differently under different perspectives. Therefore, they do not take the postswitch instructions as an explicit prompt to redescribe the objects (e.g., from an apple to a blue thing ). In fact, Diamond et al. (2005) and Kloo and Perner (2005) showed that separating the two dimensions (color and shape) as properties of a single object (e.g., a blue apple) and having them characterize two different objects (e.g., by displaying an outline of an apple next to a blue filled circle on the card) improves performance considerably. In these separated versions, children need not understand that one object can be described in two different ways; they just need to switch between objects, for example, switch from sorting the line drawings during the preswitch phase to sorting the colored circles during the postswitch phase. In sum, evidence indicates that an extradimensional shift and the use of target cards are critical for the difficulty of the DCCS task. If the switch in dimension is avoided by using a reversal shift instead of an extradimensional shift, or if the visual interference between target and test cards is avoided by replacing the target cards by puppets, children have no serious problems. One aim of Experiment 2 was to replicate this finding using computerized card sorting versions that allow a more standardized task administration. To our knowledge, there is only one study that has used a computerized DCCS task. Bialystok and Martin (2004) presented 4- and 5-year-olds with computerized card sorting versions. Their color game corresponded to a reversal shift task. Children were shown red squares and blue squares. First, children needed to press the X button when the red square appeared and press the O button when the blue square appeared. After 10 trials, these response contingencies were reversed. However, in contrast to previous research, this reversal shift task had no target boxes marked with target cards; it had only arbitrary response keys. In any case, this reversal shift version (without target cards and without sorting instructions) was easier than extradimensional shift tasks (using target cards and sorting instructions). However, as can be seen, there was a confound of three relevant factors (the kind of shift, the use of target cards, and the sorting instructions), leaving open the question of which of these factors was responsible for the difficulty of the extradimensional shift tasks. A more systematic investigation, separately manipulating each of these three factors, is needed. Although there is evidence that an extradimensional shift and target cards are critical for the difficulty of the task, no study has investigated whether children s difficulties are confined to sorting instructions or whether problems generalize to nonsorting paradigms. In Experiment 2, the influence of this factor (Response Mode: sorting vs. nonsorting) was examined. The 3- and 4-year-olds were given eight different versions of the DCCS task. Each child received two reversal shift versions and two extradimensional shift versions one with target items and one without target items. Because of our intention to replicate the Perner and Lang (2002) finding, we also used unidimensional reversal shift tasks. This seemed

11 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx 11 unproblematic because Experiment 1 had shown that reversal shift tasks are easier irrespective of the number of dimensions. Half of the children were given four DCCS task versions without sorting instructions. In these versions, the target items were affixed to response buttons and children were told, for example, If there is something red, then press the button with something red on it. The other half of the children received four DCCS task versions with sorting instructions. In these tasks, pressing the response button made the test item go into a box with a target item on it. Participants Method A total of 89 Caucasian children between the ages of 3 years 0 months and 4 years 3 months (40 girls and 49 boys, mean age = 3 years 9 months, SD = 4.16 months) from 10 nursery schools in predominantly middle-class areas in Upper Austria and in Salzburg participated in this experiment. Informed consent was obtained from all parents of children who participated in the experiment. Data from 8 children (2 in the sorting condition and 6 in the nonsorting condition) were lost due to computer failure, and 4 children (all in the nonsorting condition) were excluded because they made more than one error during the preswitch phase of one of the tasks. 5 The final sample consisted of 77 children (32 girls and 45 boys) ranging in age from 3 years 0 months to 4 years 3 months (M = 3 years 9 months, SD = 4.13 months). Initially, children had been equally apportioned to the two conditions (sorting and nonsorting). Finally, the two groups were unequal in numbers due to the exclusions described above, but they did not differ significantly in age (sorting group: n = 45 [19 girls and 26 boys], mean age = months, SD = 4.13; nonsorting group: n = 32 [13 girls and 19 boys], mean age = months, SD = 4.12), t(75) = 1.09, p >.20. To analyze age trends, we divided the children into three age groups: 26 children ranging in age from 3 years 0 months to 3 years 6 months (M = 3 years 4 months, SD = 1.93 months), 33 children ranging in age from 3 years 7 months to 4 years 0 months (M =3 years 10 months, SD = 1.60 months), and 18 children ranging in age from 4 years 1 month to 4 years 3 months (M = 4 years 2 months, SD = 0.70 months). Design Each child was tested individually in a separate room and participated in two sessions, each lasting approximately 15 min. All children received four different tasks: one reversal shift task with target cards and one with puppets as well as one extradimensional shift task with target cards (standard DCCS task) and one with puppets. The order of the card sorting tasks was counterbalanced according to a Latin square design so that each version was in each position equally often. As an additional constraint, within each session, the two card sorting tasks were chosen so that they used two different kinds of shift (extradimensional and reversal) and two different target items (target cards and puppets). In the extradimensional shift tasks, the direction of shift always was from color to shape. In the 5 Including these four children did not change the obtained pattern of results.

12 12 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx reversal shift tasks, all cards needed to be sorted according to shape (consistent with the targets or inconsistent with the targets). In the sorting condition, test cards needed to be sorted into the appropriate box by pressing the response button corresponding to the box. In the nonsorting condition, target items were directly affixed to the response buttons and children were told to press the corresponding response button. After each session, children received candy for doing so well. Procedure and materials The experiment was conducted using a laptop computer equipped with the software package Presentation 0.71 (Neurobehavioral Systems, Children were seated in front of the computer monitor. Stimuli were presented against a black background. Each task consisted of two demonstration trials, five preswitch trials, and five postswitch trials. Two cardboard response buttons were affixed to the fourth key from the left and to the third key from the right in the second row from the bottom of the keyboard. In the sorting condition, the buttons consisted of blue circles (diameter 6.5 cm), and one was marked by a white cross to make the buttons distinguishable from one another. In the nonsorting condition, the response buttons ( cm) displayed the target items/ puppets. Sorting condition Two sorting boxes displaying target cards/puppets were positioned at the bottom of the screen throughout the task. The test cards appeared centrally located in the top half of the screen. By pressing a response button, children made the test card move into the corresponding box. Response buttons were assigned by space (e.g., the left response button belonged to the left box). In the extradimensional shift task with target cards (standard DCCS task), the target cards showed a yellow banana and a red apple, whereas the test cards displayed red bananas and yellow apples. First, the experimenter pointed at the target cards at the bottom of the screen and explained the two dimensions (shape and color). Next, she said, Now we are playing a game, the color game. In this game, all the yellow ones go to the yellow one. And all the red ones go to the red one. Next, two demonstration trials were conducted. Test cards (one of each kind) appeared on the screen and the experimenter stated, for example, Here is a yellow one. This card goes into this box. Look, this button belongs to this box. If you press this button, the card goes into this box. After two correct responses, the preswitch phase started and children were required to sort five cards according to color. On each trial, a test card appeared in the top half of the screen and the experimenter said, for example, Here is a yellow one. Where does this card go in the color game? On each trial, children were told whether they had sorted the card correctly. When the children had completed five trials, the postswitch phase began. Children were told, Okay, now we are going to play a new game, the fruit game. The fruit game is different. All the apples go to the apple. And all the bananas go to the banana. Next, children were required to sort five cards according to shape. Children were not told whether a card had been sorted correctly. However, every time a card had been sorted incorrectly, the experimenter repeated the rules. In the extradimensional shift task with puppets, each target box was marked by a picture of either a girl or a boy. The test cards displayed yellow birds and red mice. Next, the

13 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx 13 procedure was the same as in the standard DCCS task except that, during the preswitch phase, the boy was described as wanting all red things and the girl was described as wanting all yellow things. During the postswitch phase, the switch to the new dimension was explained as a change of preference (e.g., The girl now wants all the mice ). This resulted in a reversal of preferred cards. For example, the girl who wanted yellow things and got yellow birds now gets red mice because she wants mice. In the reversal shift task with target cards, the target and test cards all had the same color (green) and differed only in shape (pigs or fish). The procedure was the same as in the standard task except that, during the preswitch phase, children were asked to play the correct animals game: All the pigs go to the pig. And all the fish go to the fish. And during the postswitch phase, they were instructed to play the silly animals game: Now, all the pigs go to the fish. And all the fish go to the pig. In the reversal shift task with puppets, each target box was marked by a picture of either a prince or a princess. The test cards displayed blue strawberries and blue pears. During the preswitch phase, the prince was described as wanting pears and the princess was described as wanting strawberries. During the postswitch phase, children were told that the preference of the puppets had changed, with the prince now wanting the strawberries and the princess now wanting the pears. Nonsorting condition On each trial, a test card appeared centrally located on the screen. No sorting boxes were displayed, and the target pictures used in the sorting condition were affixed to the response keys. Test and target items displayed the same color/shape combinations as in the corresponding sorting condition. The procedure was the same as in the sorting conditions except for the wording of the rules. Instead of instructing children to sort the cards, during the preswitch phase they were told, for example, If there is something red, then press the button with something red on it. And if there is something yellow, then press the button with something yellow on it. At the beginning of the postswitch phase, children were told, Okay, now we are going to play a new game, the fruit game. The fruit game is different: If there is an apple, then press the button with the apple on it. And if there is a banana, then press the button with the banana on it. Results During both the pre- and postswitch phases of each card sorting task, children were given a score between 0 and 5 depending on the number of cards sorted correctly. Children (n = 4) who made more than one error during the preswitch phase of one of the card sorting tasks were excluded. The variable of interest was the number of correct responses during the postswitch phase. In all four card sorting versions, the majority of children sorted either five times correctly or five times incorrectly; specifically, 65.0% of the children in the extradimensional shift task with target cards, 80.5% of the children in the extradimensional shift task with puppets, 67.5% of the children in the reversal shift task with target cards, and 61.0% of the children in the reversal shift task with puppets did so. Fig. 1 shows children s performance in the four different card sorting versions in the sorting and nonsorting conditions. Postswitch performance was analyzed with a 4 (Task Version [within participants]) 2 (Condition: sorting vs. nonsorting) 4 (Task Order) 3 (Age Group [between participants]) mixed design ANOVA. This revealed a significant

14 14 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx Mean Post switch Number Correct Extra Extra Reversal Reversal Target Puppet Target Puppet Sorting Non-Sorting Fig. 1. Mean postswitch performance in each card sorting version in the sorting and nonsorting conditions of Experiment 2 (maximum score = 5). Vertical lines depict standard errors of the mean. Extra, extradimensional. main effect of task version, F(3, 159) = 16.31, p <.001, partial g 2 =.23. The Condition Task Version interaction was also significant, F(3, 159) = 3.22, p <.05, partial g 2 =.06. No other effects or interactions reached statistical significance. The significant interaction between condition and task version was further analyzed by paired t tests (with Bonferroni correction of significance levels for post hoc tests). In the sorting condition, the extradimensional shift task with target cards (standard DCCS task) was more difficult than the other three tasks (all ps <.001). The other three tasks did not differ significantly from one another. In the nonsorting condition, the only significant difference was between the extradimensional shift task with target cards and the extradimensional shift task with puppets (p =.024). Furthermore, on the extradimensional shift task with target cards, children performed significantly better in the nonsorting condition than in the sorting condition, t(73.92) = 2.64, p =.01. Because postswitch scores were not normally distributed, children s performance was also evaluated categorically. Children were classified as passing the postswitch phase if they sorted four or more of the five items correctly. Cochran s Q tests revealed a significant difference among the four task versions both in the sorting condition, Q(3) = 42.39, p <.001, and in the nonsorting condition, Q(3) = 10.81, p =.013. Separate McNemar s v 2 tests confirmed that, in the sorting condition, the extradimensional shift task with target cards was more difficult than the other three tasks (all ps <.001), with the other three tasks not differing significantly from one another. In the nonsorting condition, more children passed the extradimensional shift task with puppets than the extradimensional shift task with target cards (p =.021). More children passed the extradimensional shift task with target cards in the nonsorting condition than in the sorting condition, v 2 (1, N = 77) = 4.10, p <.05. It should be noted that the sorting and nonsorting conditions differed not only with respect to the type of instruction (sorting vs. nonsorting) but also with respect to the complexity of stimulus presentation. 6 In the sorting condition, target and test stimuli appeared on the screen and spatially aligned response buttons needed to be pressed to indicate the appropriate target. In contrast, in the nonsorting condition, only test stimuli appeared on the screen and children needed to press the response button marked with 6 We are grateful to one of the reviewers, Ulrich Müller, for pointing this out to us.

15 D. Kloo et al. / Journal of Experimental Child Psychology xxx (2008) xxx xxx 15 the corresponding target. This might have increased the processing load in the sorting condition. To check for this possibility, we also analyzed children s preswitch performance (varying between four and five correct). There were no significant differences between the sorting and nonsorting conditions (all ps = 1.00 except for the extradimensional shift task with puppets, p =.71; however, contrary to the processing load argument, in the extradimensional shift task with puppets, 8.9% of the children made an error in the sorting condition, whereas 12.5% of the children made an error in the nonsorting condition). Furthermore, all four children who were excluded because they made too many preswitch errors received the nonsorting version. Discussion This experiment has three main findings. First, it showed that the well-documented problems of young preschoolers during the postswitch phase of the standard DCCS task also occur in a computerized task version. This is a good starting point for future computerized studies that could also analyze reaction time data on the DCCS task. Although one might object by arguing that computerized task administration restricts the personal contact with the children, this did not lead to any motivational problems in the current experiment. Second, this experiment replicated Perner and Lang s (2002; see also Kloo & Perner, 2003, Experiment 1) finding using computerized task versions. An extradimensional shift and the use of target cards were critical for the difficulty of the standard DCCS task. Only the extradimensional shift task with target cards posed problems for children. Extradimensional shift tasks with puppets as well as reversal shift tasks with target cards or puppets were easy. Third, the sorting instructions seem to be important. The extradimensional shift task with target cards was significantly more difficult in the sorting condition than in the nonsorting condition. This is a new aspect that indicates something important about the nature of children s difficulties. We suggest that sorting instructions induce children to think of the sorting boxes in terms of designated locations: This is the box where the red ones belong. In fact, in several experiments in our laboratory, we have observed children stating the following during the postswitch phase: Now I have to put the cards in the wrong box or But the horses belong in the other box. Sorting usually involves allocating things to distinct locations where the same sort of things are kept. Therefore, sorting may enforce the creation of a general rule such as the following: Put each card with the target that has the same thing on it. Such a general matching rule necessitates two mutually exclusive descriptions according to the two dimensions. To switch between these two descriptions, children might need to understand that one and the same thing can be described or viewed differently at the same time. Of course, even 3-year-olds know that a red apple is an apple and a red thing or that a cat is a cat and an animal (e.g., Clark & Svaib, 1997). And they are also able to switch between these descriptions if explicitly induced to do so. But what they are not able to do is apply different descriptions to one thing at the same time (confront descriptions). They have difficulty in acknowledging explicitly that something can be two things at the same time, for example, that something can be a rabbit and a bunny or a rabbit and an animal (Perner, Stummer, Sprung, & Doherty, 2002); therefore, they cannot induce switches to another description on their own. In a similar vein, Flavell, Green,