When Stroop helps Piaget: An inter-task positive priming paradigm in 9-year-old children

Journal of Experimental Child Psychology 139 (2015) 71–82

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When Stroop helps Piaget: An inter-task positive priming paradigm in 9-year-old children A. Linzarini a,b,c, O. Houdé a,b,c,d, G. Borst a,b,c,⇑ a

Laboratory for the Psychology of Child Development and Education (LaPsyDÉ), CNRS Unit 8240, 75005 Paris, France Institut de Psychologie, University Paris Descartes, 75006 Paris, France University of Caen Basse-Normandie, 14032 Caen, France d Institut Universitaire de France, 75005 Paris, France b c

a r t i c l e

i n f o

Article history: Received 17 February 2015 Revised 22 May 2015 Available online 15 June 2015 Keywords: Inhibitory control Number conservation Piaget Stroop task Domain general Children

a b s t r a c t To determine whether inhibitory control is domain general or domain specific in school children, we asked 40 9-year-old children to perform an inter-task priming paradigm in which they responded to Stroop items on the primes and to Piaget number conservation items on the probes. The children were more efficient in the inhibition of a misleading ‘‘length-equals-number’’ heuristic in the number conservation task if they had successfully inhibited a previous prepotent reading response in the Stroop task. This study provides evidence that the inhibitory control ability of school children generalizes to distinct cognitive domains, that is, verbal for the Stroop task and logico-mathematical for Piaget’s number conservation task. Ó 2015 Elsevier Inc. All rights reserved.

Introduction Piaget’s model postulates that cognitive abilities develop linearly in successive stages from the sensorimotor (babies) to formal operational (adolescents and adults) stages, which occurs through an interaction between the organism and its environment (Piaget, 1984). Piaget designed a number of tasks to determine the stage of cognitive development in children. One of the most emblematic tasks from the concrete operational stage is the number conservation task. In this task, the child is presented ⇑ Corresponding author. Fax: +33 1 40 46 29 93. E-mail address: [email protected] (G. Borst). http://dx.doi.org/10.1016/j.jecp.2015.05.010 0022-0965/Ó 2015 Elsevier Inc. All rights reserved.

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with two rows of objects with the same number of objects and the same length and is asked whether the two rows contain the same number of objects. After the child provides a response, the objects in one row are spread apart. According to Piaget (1941/1952), if the child is able to determine that the two rows still contain the same number of objects after the length of one row is transformed, the child has acquired number conservation and reached the concrete operational stage of development (which occurs at approximately 7 years of age). If not, the child remains at the ‘‘preoperational’’ stage of development, which is a stage of cognitive development characterized by intuitive reasoning. However, using dependent variables such as infant gaze, rather than actions used by Piaget, to investigate the cognitive abilities of newborns and infants, researchers have demonstrated that infants already possess some knowledge of numbers (e.g., Antell & Keating, 1983; Dehaene, Dehaene-Lambertz, & Cohen, 1998; Gelman, 1972; Lipton & Spelke, 2003; Mehler & Bever, 1967; Van Loosbroek & Smitsman, 1990; Wynn, 1992; Wynn, 1998). Moreover, newborns and infants appear to understand that there is an invariance between length and number in a situation very similar to the condition designed by Piaget in the number conservation task (e.g., Antell & Keating, 1983). These findings raise the question as to why newborns and infants who have some understanding of the invariance between length and number will subsequently commit systematic errors in Piaget’s number conservation task, which supposedly tests the same knowledge (Houdé, 2000). According to neo-Piagetian researchers, this developmental paradox can be best accounted for by models of cognitive development in which the ability to solve a logico-mathematical problem, such as Piaget’s number conservation task, does not rely exclusively on the acquisition of knowledge of increasing complexity as Piaget (1954) claimed; it also relies, in part, on the ability to inhibit conflicting knowledge (Bjorklund & Harnishfeger, 1990; Dempster & Corkill, 1999; Houdé, 2000; Houdé & Borst, 2014). Inhibitory control is defined as the ability to control impulses and override prepotent responses or strategies (e.g., Diamond, 2013). In this theoretical framework, systematic errors occur in the number conservation task because children tend to rely on a misleading ‘‘length-equals-num ber’’ heuristic (a fast effortless strategy; e.g., Gilovich & Savitsky, 1996; Shah & Oppenheimer, 2008) rather than a logico-mathematical algorithm (i.e., the reversibility of operations; e.g., Borst, Simon, Vidal, & Houdé, 2013; Piaget, 1941/1952), which is a slow and cognitively costly strategy (Kahneman, 2011), to solve this problem. A number of studies have demonstrated that solving the number conservation task requires the inhibition of the length-equals-number heuristic to activate the reversibility-of-operations algorithm in not only children (Houdé & Guichart, 2001; Houdé et al., 2011; Poirel et al., 2012) but also adults (Borst, Simon, et al., 2013; Daurignac, Houdé, & Jouvent, 2006; Leroux et al., 2009). For example, Houdé and Guichart (2001) used a negative priming adaptation of the number conservation task to provide evidence that number conservation relies on inhibitory control. The authors demonstrated that 9-year-old children required more time to determine that two rows of different lengths contained a different number of objects (i.e., an item in which the length-equals-number heuristic is automatically triggered) when preceded by a Piaget-like number conservation item in which the two rows had different lengths but contained the same number of objects (an item in which the length-equals-number heuristic must be inhibited) than when preceded by an item in which the inhibition of the length-equals-number strategy was not required. In addition, a functional magnetic resonance imaging (fMRI) study of 60 children aged 5 to 10 years demonstrated that the acquisition of number conservation was supported by a fronto-parietal network and, in particular, the progressive recruitment of the right inferior frontal gyrus, which is a region critically involved in the inhibition of dominant responses (see Aron, Robbins, & Poldrack, 2004; Aron, Robbins, & Poldrack, 2014, for reviews), with age (Houdé et al., 2011). In a follow-up study, Poirel et al. (2012) provided evidence that the activation of the right inferior frontal gyrus of children who performed the number conservation task during fMRI was a result of the need to inhibit a misleading heuristic; the findings indicated that the level of activation within this area (BOLD [blood-oxygen-level-dependent] signal) was selectively related to the inhibitory control efficiency of school children measured by an animal Stroop task (Wright, Waterman, Prescott, & Murdoch-Eaton, 2003). School children’s ability to inhibit a misleading heuristic is necessary to succeed at not only the number conservation task but also Piaget’s class inclusion task that assesses the categorization domain (Borst, Poirel, Pineau, Cassotti, & Houdé, 2013; Perret, Paour, & Blaye, 2003), as well as

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classroom problems such as arithmetic word problems (Lubin, Vidal, Lanoë, Houdé, & Borst, 2013). Thus, these findings suggest that the development of conceptual knowledge during childhood in various cognitive domains appears to be supported, in part, by the development of inhibitory control. These authors argue that inhibition is a domain-general process (e.g., Houdé & Borst, 2014) that allows an individual to resist temptations, automatisms, and heuristics and to adapt to cognitive and perceptual conflicts (see Diamond, 2013, for a discussion). One method to investigate the generality (or specificity) of a given cognitive process is to use a priming paradigm. Priming occurs when the processing of a stimulus is facilitated after processing the same stimulus or a related stimulus (for reviews, see Roediger & McDermott, 1993; Schacter, 1987; Tulving & Schacter, 1990). In seminal inhibitory control tasks, such as the Flanker (Eriksen & Eriksen, 1974), Stroop (Stroop, 1935), and Simon (Simon, 1969) tasks, many studies have reported that participants are faster in responding to an incongruent item (i.e., an item in which inhibition is required because irrelevant information interferes with relevant information) when it is preceded by an incongruent item compared with a congruent item (e.g., Botvinick, Nystrom, Fissell, Carter, & Cohen, 1999; Hommel, Proctor, & Vu, 2004; Larson, Kaufman, & Perlstein, 2009). These priming effects are generally interpreted as reflecting conflict adaptation; that is, resolving a conflict by inhibiting the interfering information facilitates the resolution of a conflict on the next item (Botvinick, Braver, Barch, Carter, & Cohen, 2001; Egner, 2007; Gratton, Coles, & Donchin, 1992; Ullsperger, Bylsma, & Botvinick, 2005). Two recent studies used this conflict adaptation priming paradigm to determine whether conflict adaptation occurs not only between incongruent items of the same inhibitory control task but also between two different tasks that require inhibitory control. This inter-task priming paradigm typically enables investigation into the degree of generalizability of inhibitory control. Borst, Poirel, Pineau, Cassotti, and Houdé (2012) demonstrated that 10-year-old children solve a Piaget number conservation problem faster after they have solved a Piaget class inclusion problem and vice versa. Because the ability to solve these problems at all ages is rooted in the ability to inhibit misleading heuristics (e.g., Borst et al., 2013; Houdé et al., 2011), the authors argued that this facilitation effect was due to a common ability to inhibit heuristics in these two Piagetian problems. In addition, using two inter-task priming paradigms, Kan et al. (2013) reported a facilitation effect between syntactic (i.e., ambiguous sentence reading) and non-syntactic (i.e., Stroop) tasks and between perceptual (i.e., Necker cubes) and verbal (i.e., Stroop) tasks in adults. According to Kan et al. (2013), these positive priming effects were presumably produced by an inter-task conflict adaptation effect and demonstrated domain-general inhibitory control in tasks with priming paradigms. January, Trueswell, and Thompson-Schill (2009) also provided convergent evidence for domain-general inhibitory control by demonstrating that similar frontal regions (primarily posterior left inferior frontal gyrus) were activated during conflict resolution in two different cognitive tasks, that is, syntactic and Stroop tasks. However, no study has directly assessed the generality or specificity of inhibitory control in children using an inter-task priming paradigm between a seminal inhibitory control task (i.e., color–word Stroop task; Stroop, 1935) and a seminal Piagetian task (i.e., number conservation task; Piaget, 1941/1952). Thus, in the current study, we investigated the degree of generality of inhibitory control in 9-year-old children using a chronometric inter-task priming paradigm between these two tasks. In the color–word Stroop task, children are asked to name the color of the ink of a word in a congruent condition, where the ink color matches the color denoted by the word (e.g., RED written in red), and in an incongruent condition, where the color denoted by the word interferes with the ink color (e.g., BLUE written in red). The incongruent condition typically requires inhibitory control to avoid reading the word (Friedman & Miyake, 2004; MacLeod, 1991; Miyake et al., 2000; Penner et al., 2012; see MacLeod, Dodd, & Sheard, 2003, for a discussion). A positive priming effect between the color–word Stroop task and the number conservation task could be observed only in children who accurately perform the number conservation task, especially incongruent items, and who have sufficiently automatized reading so that the color denoted by the word interferes with the identification of ink color in the color–word Stroop task. Thus, we tested 9-year-old children because they can accurately perform the number conservation task (Houdé & Guichart, 2001; Piaget, 1941/1952) and have automatized reading (e.g., Dehaene, 2009).

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Stroop task

Congruent/covarying stimuli

Incongruent/interfering stimuli

Correct response

RED

BLUE

Red

Same Number conservation task Different

Fig. 1. Materials used for the Stroop and Piaget-like number conservation tasks.

In our inter-task priming paradigm, children performed a Stroop item on the prime and performed a number conservation item on the probe. On the prime, the Stroop items could be congruent (e.g., RED written in red) or incongruent (e.g., BLUE written in red) (Fig. 1). On the probe, the length of the rows and the number of objects in the Piaget-like number conservation items could covary (i.e., the two rows had the same length and the same number of objects or the longer row had more objects than the shorter row) or interfere (i.e., the two rows had the same length but different numbers of objects or the two rows had different lengths but the same number of objects) (Fig. 1). The incongruent Stroop and Piaget-like number conservation items in which the length of the rows and the number of objects interfere required inhibitory control, that is, the inhibition of naming the color denoted by the word in the color–word Stroop task and the inhibition of the length-equals-number heuristic in the number conservation task. We reasoned that if the Stroop and Piaget’s number conservation tasks rely on a similar ability to inhibit a prepotent response (or heuristic), then children should be faster to resolve Piaget-like number conservation items in which the length of the rows and the number of objects in the rows interfere (but not the items in which the length of the rows and the number of objects covary) when they are preceded by incongruent Stroop items than when they are preceded by congruent Stroop items. Method Participants A sample of 40 children (27 boys and 13 girls; ages 8–10 years, average age = 9 years 5 months, SD = 7 months; 2 left-handed) from two elementary schools in Paris, France, were recruited for this study. All participants provided parental written consent and were tested in accordance with national and international norms that govern the use of human research participants. Data regarding the socioeconomic backgrounds of the participants were not collected. All participants reported normal or corrected-to-normal vision. The parents and teachers reported that all children were normally developing. Data from an additional 10 children were not included in the analyses because they performed at chance level in the Stroop task or number conservation task (n = 8) or because no Stroop effect was identified on their response times (n = 2). Materials For the Stroop task, we created 4 congruent items (e.g., RED written in red) and 12 incongruent items (e.g., BLUE written in red) via the combination of four different ink colors (blue, red, yellow, and green) and four words that denote the same four colors. The words were presented in the center of the screen on a black background in 36-point Arial font. The RGB color codes used were 0;0;255 (blue), 255;0;0 (red), 255;255;0 (yellow), and 0;255;0 (green).

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For the number conservation task, we designed 32 different stimuli (16 stimuli in which the length and number covaried and 16 stimuli in which they interfered; 1191 ⁄ 246 pixel jpeg images), which comprised two rows of white dots on a black background. In the number conservation items in which the length and number covaried, the two rows had the same length and the same number of dots (5, 6, 7, or 8) or the top (or bottom) row was longer than the other row and contained more dots (5 vs. 3, 6 vs. 3, 7 vs. 4, or 8 vs. 4). In the number conservation items in which the length and number interfered, the two rows had an identical length but different numbers of dots (5 vs. 3, 6 vs. 3, 7 vs. 4, or 8 vs. 4) or had the same number of dots but the top (or bottom) row was longer than the other row (5, 6, 7, or 8). Note that the number conservation items were static in our task, unlike the original Piagetian task, because of the chronometric inter-task paradigm used in this study. Procedure Three children were simultaneously tested using three separate laptop computers with a screen resolution of 1366 ⁄ 768 pixels. All visual stimuli were presented using E-Prime 2.0. Vocal responses were used in the color–word Stroop and number conservation tasks. All vocal responses were recorded. The three children were tested in the same room by three separate experimenters. Each child was equipped with ear protectors to avoid any possible auditory interference from the vocal responses of the other two children. For both tasks of each trial, the experimenter pressed one of the two mouse buttons as soon as the child provided a response. The response times (RTs) were recorded from the stimulus onset to the button press. The experimenter sat across from the computer screen and was not aware of the items presented on the screen. This approach was necessary to reduce the difficulty of the two tasks. The difficulty of the two tasks increased because in each trial the children needed to switch between two cognitively demanding tasks. In the color–word Stroop task, the children were instructed to name the ink color of the word presented as fast as possible. In the number conservation task, the children were asked to determine whether the two rows had the same number of dots. They were instructed to say ‘‘same’’ as fast as possible if they thought the two rows had the same number of dots or to say ‘‘different’’ if they thought the two rows had different numbers of dots. The children first performed two blocks of four practice trials for the color–word Stroop and number conservation tasks. In each practice trial, the children received feedback from the experimenter regarding the correctness of their answers. The order of the blocks was counterbalanced across the participants. The children subsequently performed two blocks of 16 experimental trials. Each experimental trial was initiated with the presentation of a fixation cross (1000 ms) followed by a color–word Stroop item (i.e., the prime). The color–word Stroop item remained on the screen until the children provided a response within a time limit of 5000 ms. Then, a fixation cross appeared for 1000 ms, which was followed by a number conservation item (i.e., the probe) that remained on the screen until the participants provided a response within a time limit of 5000 ms. After each trial, a 450 ⁄ 450 pixel jpeg visual mask was displayed on the center of the screen for 1500 ms to minimize the potential effect of the probe on the following prime (Fig. 2). We designed four different types of experimental trials that depended on the type of item presented on the prime and probe (Fig. 2). A congruent color–word Stroop item could serve as a prime for a number conservation item in which the length and number covaried or interfered. Similarly, an incongruent color–word Stroop item could serve as a prime for a number-conservation item in which the length and number covaried or interfered. In each block, four trials of each type were presented. In each of these four trials, a different association between the ink color in the color–word Stroop item and the number of dots in the number conservation task was used. We presented the trials in a random order with the exception that no more than two trials of the same type could occur in a row. Results Analyses of the RTs were conducted for only the trials in which the children performed accurately on both the prime and probe. On average, 6.8% of the trials were excluded from the RT analyses. Outliers, defined as RTs greater than 2 standard deviations from the mean of a given child on the

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Fig. 2. Examples of prime–probe sequences for each type of experimental trial. (A, B) Congruent color–word Stroop item as the prime and number conservation item in which the length and number covaried (A) or interfered (B) as the probe. (C, D) Incongruent color–word Stroop item as the prime and number conservation item in which the length and number covaried (C) or interfered (D) as the probe. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

primes or probes in each of the four types of trials, were deleted (4.9%). Then, the RTs and error rates were independently averaged for the primes and probes in the four types of trials for each child. Separate 2 (Type of Color–Word Stroop Items: congruent vs. incongruent)  2 (Type of Number Conservation Items: length–number covariation vs. length–number interference) two-way repeated measure analyses of variance (ANOVAs) were conducted on the RTs and error rates for the primes and probes. When the two means were compared, simple effect analyses were performed in accordance with our hypotheses. The effect size is reported for each ANOVA as the partial eta-squared (g2p). Primes As indicated by a two-way ANOVA for the prime RTs, the children were significantly slower in their performance on the incongruent Stroop items (M = 1823 ± 346 ms) than in their performance on the

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Table 1 RTs (ms) and error rates (%) for the congruent and incongruent color–word Stroop items followed by the number conservation items in which the length and number covaried or interfered. Congruent Stroop items

Incongruent Stroop items

RT

Error rate

RT

Error rate

0.1 (3.3) 0.1 (3.3)

1813 (355) 1833 (337)

1.9 (5.3) 2.8 (6.6)

Followed by number conservation items in which: Length and number covary 1520 (234) Length and number interfere 1501 (246) Note. Standard deviations are in parentheses.

Fig. 3. Mean RTs (ms) (left) and error rates (%) (right) on congruent and incongruent Stroop items (primes) that preceded number conservation items in which length and number either covaried or interfered (congruent and incongruent probes). Error bars represent standard errors of the mean. *p < .05, NS, nonsignificant.

congruent items (M = 1510 ± 241 ms), F(1, 39) = 113.44, p < .001, g2p = .74. The effect of the probe type and the interaction between the Stroop item types and the subsequent number conservation item types were not significant, F < 1 (Table 1 and Fig. 3). Similar results were identified for the prime error rates. The children made more mistakes on the incongruent Stroop items (M = 2.3 ± 6.0%) than on the congruent items (M = 0.9 ± 3.3%), F(1, 39) = 6.09, p = .018, g2p = .14; however, the effect of the probe type and the interaction between the types of prime and probe were not significant, Fs < 1 (Table 1 and Fig. 3). To test for a speed–accuracy trade-off in the Stroop task, we computed the coefficient of correlation between the difference in RTs and the difference in error rates between congruent and incongruent Stroop items. We found no speed–accuracy trade-off in the Stroop task, as revealed by the lack of correlation between the Stroop effect for RTs and for error rates, r(38) = .01, p = .91. Probes The two-way ANOVA on the probe RTs identified a significant main effect of the type of number conservation items; the children took more time to perform the number conservation items in which the length and number interfered those in which the length and number covaried, F(1, 39) = 42.87, p < .001, g2p = .52; however, no main effect of the type of color–word Stroop items was identified, F(1, 39) = 1.40, p = .24. In addition, we identified a significant interaction between the type of number conservation item performed on the probe and the type of color–word Stroop item performed on the prime, F(1, 39) = 5.33, p = .026, g2p = .12. Importantly, the children required less time to perform a number conservation item in which the length and number interfered when it was preceded by an incongruent Stroop item (M = 2237 ± 406 ms) compared with a congruent item (M = 2339 ± 503 ms), F(1, 39) = 6.94, p = .012, g2p = .15; this positive priming effect was not observed when Stroop items

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Table 2 RTs (ms) and error rates (%) for number conservation items with covarying and interfering length and number preceded by congruent and incongruent color–word Stroop items.

Preceded by: Congruent Stroop items Incongruent Stroop items

Covarying length and number item

Interfering length and number item

RT

Error rate

RT

Error rate

1932 (278) 1957 (323)

3.4 (6.9) 3.4 (6.9)

2339 (503) 2237 (406)

19.1 (15.2) 21.6 (16.3)

Note. Standard deviations are in parentheses.

Fig. 4. Mean RTs (ms) and error rates (%) on number conservation items (probes) in which length and number covaried (congruent) or not (incongruent), preceded by either congruent or incongruent Stroop items (primes). Error bars represent standard errors of the mean. *p < .05, NS, nonsignificant.

preceded number conservation items in which the length and number covaried (M = 1957 ± 323 ms when preceded by incongruent Stroop items vs. M = 1932 ± 278 ms when preceded by congruent items), F < 1 (Table 2 and Fig. 4). The two-way ANOVA on the probe error rates indicated that more errors were made on the number conservation items in which the length and number interfered (M = 20.3 ± 15.7%) than on those in which they covaried (M = 3.4 ± 6.9%), F(1, 39) = 50.68, p < .001, g2p = .57. The effect of the type of Stroop items and the interaction effect were not significant, F(1, 39) = 1.14, p = .29 and F < 1, respectively (Table 2 and Fig. 4). As in the Stroop task, we found no evidence of a speed–accuracy trade-off in the number conservation task, as revealed by the lack of correlation between the difference in RTs and difference in error rates between number conservation items in which length and number covaried or interfered, r(38) = .14, p = .38.

Discussion The aim of the current study was to determine whether the Stroop task and Piaget’s number conservation task rely on the same general ability to inhibit a prepotent response (or heuristic) in 9-year-old children. Regarding the primes, our results indicate that children required more time to name the color of the ink of a word when the ink color interfered with the color denoted by the word (incongruent Stroop items; e.g., BLUE written in red) than when the color matched the color word (congruent Stroop items; e.g., RED written in red). Longer RTs for incongruent Stroop items are the hallmark of the inhibitory control needed to resolve the conflict between the ink color and the color denoted by the word in incongruent items (e.g., MacLeod, 1991). Given that children committed very few errors in the Stroop task, floor effects might have affected the results. However, a typical Stroop effect was reported even on the percentage of errors in the color–word Stroop task. Regarding the

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probes, children were faster at solving Piaget-like number conservation problems in which the length and number interfered after they performed incongruent Stroop items than after they performed congruent Stroop items. This positive priming effect suggests that performing an incongruent Stroop item facilitates the resolution of a number conservation problem. Because the incongruent items of the Stroop task (e.g., Friedman & Miyake, 2004; MacLeod, 1991; Miyake et al., 2000; Penner et al., 2012) and the number conservation problems in which the length and number interfere (Borst, Simon, et al., 2013; Daurignac et al., 2006; Houdé & Guichart, 2001; Houdé et al., 2011; Leroux et al., 2009; Poirel et al., 2012) both involve inhibitory control, our results provide the first evidence that these two famous neuropsychological and cognitive–developmental tasks may rely, in part, on the same ability to inhibit a prepotent response, that is, the color denoted by the word in the Stroop task and the length-equals-number heuristic in the number conservation task. Importantly, children were not faster to perform number conservation problems in which the length and number covaried, (i.e., problems that did not require inhibition of the length-equals-number heuristic) when preceded by incongruent Stroop items. Thus, it is unlikely that the priming effect identified for the number conservation problems in which the length and number interfered reflects a nonspecific post-conflict facilitation. In addition, there was no difference in the amplitude of the Stroop effect when the Stroop items preceded the number conservation items in which the length and number covaried versus interfered. Therefore, the lack of positive priming for the number conservation problems in which the length and number covaried is unlikely a consequence of a difference in the Stroop effects on the primes that preceded the two types of number conservation problems. We note, however, that the effect size of the priming effect was relatively modest. The small effect size might be due to testing children who performed well on the number conservation task. In addition, the priming effect was restricted to response times in our study, whereas a similar inter-task priming paradigm between an ambiguous sentence reading task and a Stroop task (Kan et al., 2013) reported priming effects on both response times and errors. A direct comparison of the priming effect reported in these studies is difficult because we evaluated priming effects on performance in the number conservation task, whereas Kan et al. (2013) reported performance on the color–word Stroop task. The lack of priming effect on the errors in the number conservation task might be due to 9-year-old children performing accurately on the number conservation task. An alternative explanation is that although the inhibitory control initiated on the color–word Stroop task facilitates the inhibition of the misleading length-equals-number heuristics in the number conservation task, this facilitation might not be sufficient to enable children to overcome errors in this task. Our findings that children’s abilities to inhibit prepotent responses can generalize across tasks that involve the resolution of conflicts of a different nature (i.e., verbal in the Stroop task vs. visuospatial in the number conservation task) are consistent with two previous inter-task priming studies in children (Borst et al., 2012) and adults (Kan et al., 2013). Taken together, these findings suggest that inhibitory control could be domain general in both children and adults. However, as noted by Kan et al. (2013), the inter-task priming paradigm does not allow the determination of whether inhibitory control is domain general only when the conflict occurs at the level of the stimulus (or the representation). Indeed, according to Egner and colleagues (e.g., Egner, 2008; Egner, Delano, & Hirsch, 2007), two types of conflict must be resolved: response-based conflict, which arises from incongruency between an irrelevant stimulus feature and the relevant response (such as in the Simon task when the location of the correct response button is incongruent with the location of the presented stimulus), and stimulus-based (or representational) conflict, which stems from a competition between internal representations of relevant and irrelevant stimulus features (such as in the Stroop task between the color denoted by the word and the ink color). However, some authors have proposed that conflict could also arise at the task level when the relevant and irrelevant information trigger two different cognitive processes (Steinhauser & Hübner, 2009). Although some researchers have claimed that the Stroop task primarily involves the resolution of a representational/stimulus-based conflict (see Egner, 2008), especially when responses are provided vocally such as in the current study, many studies have demonstrated that the Stroop task necessitates the resolution of not only a representational/stimulus-based conflict but also conflicts at the task and response levels (Caldas, Machado-Pinheiro, Souza, Motta-Ribeiro, & David, 2012; Chen, Bailey, Tiernan, & West, 2011; Monsell, Taylor, & Murphy, 2001; Van Veen & Carter, 2005; Zhao, Chen, Fu, & Maes,

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2015). Given the multiple types of conflict that might be involved in the Stroop task and the lack of clear characterization of the nature of the conflict to be resolved in the number conservation task, our study does not provide information on whether inhibitory control can generalize across different types of conflict resolution. Thus, future studies are needed to investigate whether inhibitory control generalizes over conflicts at the response, task, and stimulus levels. Although we argue that our results suggest that inhibitory control processes generalize over two different domains due to the different nature of the two tasks (in terms of goals, demands and stimuli characteristics), we cannot completely rule out that the priming effect might be partially due to the transfer of other executive processes from one task to the other. Future studies should investigate whether the priming effect could also rely on a domain-general ability to detect conflict. Finally, additional studies are also needed to determine whether the generalizability of inhibitory control varies with age and brain maturation. We speculate that the specificity of the inhibitory control processes increases with age due to progressive automatization of inhibitory control. Indeed, some studies have provided evidence that inhibitory control can be automatized after intense training (Jasinska, 2013) and that this type of inhibitory control might be mediated not by prefrontal cortex but rather by posterior brain areas such as the superior parietal cortex (e.g., Lechuga, Moreno, Pelegrina, Gómez-Ariza, & Bajo, 2006), which mature earlier than prefrontal areas. Our developmental hypothesis is also consistent with the interactive specialization framework of the functional maturation of the brain supported by Johnson (2011), which posits that networks devoted to a wide range of cognitive functions, including executive functions, become more specific with age. 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