American Journal of Psychology

Winter 2011, Vol. 124, No. 4 pp. 405–419 • © 2011 by the Board of Trustees of the University of Illinois

Classic Stroop Negative Priming Effects for Children and Adults Diverge With Less-Conflicting and Nonconflicting ConditionsVERENA E. PRITCHARD and EWALD NEUMANN University of Canterbury

Negative priming indexes an inhibition process that aids target selection by reducing distractor interference. To date, children have produced negative priming only in tasks where distractor response tendencies are consistently greater than or equal to targets and not in tasks contain-ing a substantial proportion of low-conflict distractors. To establish the exact parameters under which children’s negative priming attenuates relative to adults, we varied processing demands across 2 experiments involving children and adults. Negative priming was comparable when 100% high-conflict conditions were encountered (Experiment 1) and was intact in adults but not children when a ratio of 50:50 high- to low-conflict conditions was encountered (Experiment 2). Compared with adults, children seem induced to divide attention more generally when low-conflict attentional conditions are included, attenuating negative priming.

Without a basic ability to prevent a response to the most dominant stimulus input, behavior would be chaotic and unrelated to current goals. How the hu-man information processing system acts to overcome attentional competition generated by conflicting stim-uli has become a topic of increased focus in studies of adult cognition (Cerf et al., 2010). One prominent theory holds that the effective selection of goal-rele-vant or targeted information is achieved via an acti-vation-reducing inhibitory process that suppresses unwanted but concurrently competing information (Driver & Tipper, 1989; Grison, Tipper, & Hewitt, 2005; Houghton & Tipper, 1994; Neumann & De-Schepper, 1991). However, a precise understanding

of the effectiveness of a selective control mechanism in childhood remains elusive. Specifically, although studies are beginning to establish that young children show inhibition ef-fects in selective attention and memory comparable to those of adolescents and adults (Frings, Feix, Röthig, Brüser, & Junge, 2007; Lechunga, Moreno, Pelegrina, Gomez-Ariza, & Bajo, 2006; Pritchard & Neumann, 2009), instances of diminished selective attention effectiveness in middle childhood have been documented with the negative priming (NP) index (Tipper, Bourque, Anderson, & Brehaut, 1989). NP is defined as the slowed, or more error prone, re-sponse to a target stimulus in an ignored repetition

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(IR) condition in which a distractor on the prime trial becomes a target on the probe trial. The inhibition ac-count of NP (Houghton & Tipper, 1994; Neumann & DeSchepper, 1991, 1992) holds that the response costs associated with the IR condition relate to an inhibi-tory control mechanism involved in the processing of activated but task-irrelevant stimulus representa-tions. Establishing the experimental conditions and parameters under which children’s NP attenuates or improves relative to that of adults during a selective attention task would be useful for tracking typical or atypical changes in information processing ability across development. However, the extent and nature of age-related differences in the processing of task-irrelevant information is not yet clear. Therefore, the present study was designed to investigate potential developmental differences in conceptual NP that may relate to contextual sensitivity. Conceptual NP refers to inhibitory effects associ-ated with the identity or semantic properties of pre-viously ignored stimuli and has been interpreted as indicating that task-irrelevant information in competi-tion with concurrent target information is subject to an involuntary form of neural inhibitory activity to aid target selection (Grison & Strayer, 2001; Vuilleumier, Schwartz, Duhoux, Dolan, & Driver, 2005). Several studies show that the onset of this suppression process is often activation sensitive, with inhibitory activity maximal when the internal representation of the dis-tractor stimulus is highly activated and in a response competitive state (Grison & Strayer, 2001; Strayer & Grison, 1999). According to inhibitory theorists, the act of overcoming the residual inhibitory activity tied to the internal representation of a recently ignored stimulus produces the NP effect (Houghton & Tip-per, 1994; Neumann & DeSchepper, 1991, 1992). This effect is often thought to be transient, occurring only on an immediate IR probe trial.1

Although conceptual NP is a robust finding in adults and often cited as a hallmark of efficient se-lective attention (Macqueen, Galway, Goldberg, & Tipper, 2003; Sullivan, Faust, & Balota, 1995), studies with children indicate that they may not produce con-ceptual NP under some task contexts. For example, Tipper et al. (1989, Experiments 1 and 2) failed to find conceptual NP in children aged 7 to 8 years, relative to adults, when using distractor stimuli that varied by their degree of response competitiveness across

conditions. This study led to the widely accepted conclusion that the distractor inhibition process is not developed by middle childhood. By contrast, however, a follow-up study using only highly re-sponse-competitive distractor stimuli (high-conflict) across all conditions found intact conceptual NP in children aged 5 to 6 years (Pritchard & Neumann, 2004). To account for their finding, Pritchard and Neumann suggested that children might be more suc-cessful inhibitors in task situations where the degree of conflict between a target and concurrent distractor is high across all conditions (as in their 2004 experi-ments) rather than low or variable across 50% of con-ditions (i.e., Tipper et al., 1989 experiments). More specifically, their argument was that task situations designed to maximize attentional selectivity might be more critical for inducing or maintaining inhibitory processing in children than adults and that this could account for the discrepancy in the data between the two laboratories. There have been surprisingly few studies of conceptual NP in children. Although spatial NP, ob-tained when the probe target shares the same location and identity as the prime distractor, has been reliably demonstrated in infants and young children (Amso & Johnson, 2005; Simone & McCormick, 1999), most recent investigations of NP associated with stimulus identity in young children have been limited to the Dimensional Change Card Sort (DCCS) task (Müller, Dick, Gela, Overton, & Zelazo, 2006) and the Pre-school Attentional Switching Task (PAST; Chevalier & Blaye, 2008), paradigms that are designed to assess children’s cognitive flexibility or capacity to switch from one sorting dimension (i.e., color) to another (i.e., shape). Although children’s post-switch errors on the DCCS and the PAST are often interpreted as evidence for NP (Müller et al., 2006; Chevalier & Blaye, 2008), such tasks tend to rely more on conscious or effortful executive control rather than automatic suppression processes (see Harnishfeger & Bjorklund, 1994; Lechunga et al., 2006; Pritchard & Neumann, 2009, for discussion). This raises ques-tions about whether children’s post-switch errors are evidence for conventional conceptual NP. Importantly, there have been no tests of Pritchard and Neumann’s (2004) hypothesis that use the full set of conditions previously associated with absent conceptual NP in children (see Frings et al., 2007).

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Therefore, it remains an open empirical question as to whether exposure to 50% low-conflict conditions may affect children’s distractor inhibition more than that of adults. By comparing NP between children and adults across two experiments differing in the proportion of high-conflict conditions, the current study provides a more comprehensive test of the potential influence of variable target–distractor competition on children’s inhibitory processing. Establishing the basic task pa-rameters under which NP may be present and absent among children of a specific age range can help track differences in NP across development. Instances of effective and ineffective distractor inhibition in children, as indexed by conceptual NP procedures, have been documented with classic in-terference sensitive paradigms such as the Stroop color word task or variants of the Eriksen and Schultz (1979) flanker task. For example, the Stroop task typi-fies a class of interference whereby the introduction of stimuli incongruent with target stimuli slows re-sponse. In standard versions of this task, participants generally take longer to name the print colors of in-congruent color word stimuli (e.g., the word BLUE printed in red) than to name the print colors of neu-tral stimuli (e.g., the letters XXXX printed in red) or congruent stimuli (e.g., the word BLUE printed in blue). In NP variants of the Stroop task, inhibition is indexed by a further increase in response times on trials where the color to be named matches the identity of the immediately preceding color word. With children typically displaying greater Stroop interference than adults (Comalli, Wapner, & Werner, 1962; Friedman, 1971), the initial Stroop NP experi-ment of Tipper et al. (1989) was designed to test the prediction that children should therefore show less NP than adults. The results obtained supported their prediction to the extent that whereas intact NP emerged in adults, there was a complete absence of NP in children. This result was replicated in a second experiment with French–English bilingual children and adults who were required to name the colors of Stroop stimuli in their nondominant language. The third experiment by these authors, using pictorial stimuli, yielded similar results, although there was some evidence of a slight (albeit nonsignificant) trend toward NP in children. By contrast, Pritchard and Neumann (2004) found that children aged 5 to 12 years produced

intact conceptual NP in a Stroop NP task and in a newly devised flanker-like NP task, both of which used list-naming presentation formats similar to those used by Tipper et al. (1989). These results were also replicated in children aged 8 to 12 years who par-ticipated in a same–different matching NP task that used a sequential trial presentation format and novel three-dimensional shapes. More critically, however, in a recent follow-up NP experiment using their earlier Stroop and flanker NP tasks, Pritchard and Neumann (2009) found that NP in children aged 5 to 7 years was highly comparable with, if not identical to, NP in young adults. Although these findings challenge the notion that the inhibitory process involved with in-formation selection does not operate in children, the failure of Tipper et al. to detect Stroop NP in children and equivocal evidence for pictorial NP in children has implied that the “NP effect develops inconsis-tently in early childhood up to the first grade” (Nigg, 2000, p. 227). Alternatively, it may be that there are subtle developmental changes in the ability to main-tain selective processing when within-task variability in the degree of response conflict is a salient factor, as was the case in the Tipper et al. study.

The Present StudyPritchard and Neumann (2009) suggested that mixed findings concerning children’s conceptual NP (cf. Pritchard & Neumann, 2004; Tipper et al., 1989) might relate to between-study differences in the over-all consistency and extent of distractor competitive-ness encountered by participants. In the context of the Stroop task, for example, these authors found NP to be indistinguishable between children and adults when the distractor dimension of the Stroop stimulus was consistently highly response competitive across all conditions encountered in the experiment. In con-trast, in the Tipper et al. study (Experiments 1 and 2), an absence of NP in children but not in adults was apparent when the distractor dimension was highly response competitive in only 50% of the conditions. On the basis of these data patterns, Pritchard and Neumann (2009) argued that the inhibitory process underpinning NP is essentially intact in children but may be modulated as a consequence of encounter-ing certain task conditions in which target selection is easy. These claims stem from Tipper and Cran-ston’s (1985) hypothesis that selective inhibition may

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be part of a selection state that is engaged without conscious awareness to cope with target selection dif-ficulty. They reasoned that if difficulty, or anticipa-tion of such, is not upheld across IR trials, the active suppression of distractor stimuli (and hence resul-tant NP) would dissipate. Consistent with this view, several studies have demonstrated that continuous anticipation of target selection difficulty across trials appears critical in eliciting NP in adults (Khurana, 2000; Moore, 1994; Schooler, Neumann, Caplan, & Roberts, 1997). Building on Tipper and Cranston’s (1985) con-cept of a selection state, Pritchard and Neumann (2004) argued that children’s NP might be particu-larly prone to elimination if target selection difficulty or anticipation of such is minimized or not consis-tently upheld throughout the duration of the task at hand. Under such task situations, attention may become less tightly selective. In support of this idea, these authors pointed toward seemingly minor but potentially pertinent differences in methods between the respective studies comparing Stroop NP between children and adults. For example, their 2004 study contained only control and IR conditions in which the distractor (i.e., word) dimension of the Stroop stimulus was consistently response competitive (i.e., 100% high-conflict conditions). By contrast, the 1989 Tipper et al. study, designed to test for concurrent interference and facilitation effects in addition to NP, contained 50% low-conflict conditions (i.e., a neutral condition in which distractors were meaningless let-ter strings and a repeated distractor [RD] condition where the same distractor color word was repeatedly presented across trials) and 50% high-conflict condi-tions. Because NP did not emerge in children under this task situation, Pritchard and Neumann argued that the inclusion of low-conflict conditions in the Tipper et al. study may have reduced selective pro-cessing demands to a point that was detrimental to obtaining NP in children. From this they proposed the hypothesis that operative selection state might have a particularly important bearing on the level (or even the presence) of inhibition elicited in children. The extent to which low-conflict conditions influ-ence NP in children is unclear. To date, only one study has attempted to test Pritchard and Neumann’s (2004) hypothesis. Specifically, Frings et al. (2007) compared NP between 6- to 11-year-old children who encoun-

tered one third RD and two thirds high-conflict NP conditions and age-matched control children who en-countered only high-conflict control and IR conditions in a flanker NP task. On finding that NP did not differ between the RD-exposed and control children, they argued that the inclusion of low-conflict conditions in an NP experiment could not explain the discordant results between the Pritchard and Neumann (2004) and Tipper et al. (1989) studies. However, Frings et al. (2007) did not include the full set of conditions previously associated with re-duced or absent NP in children in their study. For example, only the RD condition was included as a low-conflict condition, whereas in the Tipper et al. (1989, Experiments 1 and 2) study both the RD and the neutral conditions were included. Pritchard and Neumann’s (2004) modified selection state hypoth-esis specifically predicts that it is the combination of neutral and RD conditions that attenuates NP in children. Therefore, the results of Frings et al. may not be as problematic for this hypothesis as claimed. Additionally, the ratio of low- to high-conflict condi-tions encountered in the experimental context may also influence children’s NP levels. For instance, the ratio of high- to low-conflict conditions in the Frings et al. study was 65:35, whereas it was 50:50 in the Tip-per et al. (1989, Experiments 1 and 2) study, in which NP did not emerge to any extent in children. The potential relevance of ratio is further exemplified in the Tipper et al. Experiment 3, where a trend toward NP in children was observed after the RD condition was removed in favor of the neutral condition, which decreased the proportion of low-conflict conditions encountered. Experiment-wide ease-of-selection issues have been studied only in adults. These are invariably investigated in the context of concurrent Stroop in-terference tasks whereby proportions of congruent trials are manipulated and corresponding changes in response times on incongruent trials are monitored. Examination of the contribution of word reading to performance in adults across contexts that vary in the proportion of congruent trials has shown that contex-tual control over the Stroop effect is possible (Crump, Gong, & Milliken, 2006). The variable influences of facilitation and interference in the Stroop paradigm in response to encountering different proportions of conflicting trials and the task parameters under which

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they might interact to mediate children’s selective attention performance (as opposed to adults’) have not been investigated yet. Therefore, it is not known whether ease of selection manipulations would af-fect children differentially in such Stroop tasks. Here, for the first time, we attempt to examine whether the magnitude of Stroop NP effects is modulated in adults by varying the proportion of “easy selection” conditions. In comparison to the adults, we also seek to determine whether Stroop NP effects in children are differentially affected by such manipulations to contextual features. The primary goal of the present two experiments was to provide some experimental evidence address-ing the specific conditions under which age dissocia-tions in Stroop NP effects emerge or not. The specific aims of this study were as follows:

To examine the combined impact of neutral and RD conditions on children’s Stroop NP effects.

To compare NP between children and adults when neutral and RD conditions are either ex-cluded from (Experiment 1) or included in (Ex-periment 2) the experimental context in order to identify any age-specific dissociations in Stroop NP that relate to contextual features.

eXPeRiMent 1

Experiment 1 used a classic Stroop NP task (Dal-rymple-Alford & Budayr, 1966) and followed the experimental design of Pritchard and Neumann (2004, 2009). Participants in Experiment 1 therefore encountered only control and IR conditions in which the distractor dimension of each Stroop stimulus was consistently response competitive and incongruent with the target dimension (i.e., high-conflict condi-tions). As in the earlier experiments by Pritchard and Neumann, control and IR conditions appeared on separate cards that were presented in regular alter-nation. In case of an age-specific effect, participants in Experiment 1 were matched by age to those in the original study by Tipper et al. (1989).

METHOD

ParticipantsThere were two age groups in Experiment 1. One group consisted of 35 children (18 female) with a mean age of 7 years, 6 months; the other consisted

of 30 adults (21 female) with a mean age of 18 years, 5 months. Following ethics approval, children were selected from a local primary school that agreed to take part in the study. Adults were recruited from the university campus and a local tertiary business school through advertisements and word of mouth. Participa-tion was voluntary, with all participants giving written consent for their participation. Written consent was also obtained from the parents of children participating in the study. All participants had normal color vision and normal or corrected-to-normal visual acuity.

DesignA 2 × 2 mixed design was used. The between-partic-ipant variable was age group (children vs. adults) and the within-participant variable was priming condition (Stroop control vs. Stroop IR). Experimental condi-tions (Table 1) consisted of 50% control conditions (in which neither the print color nor color word of a Stroop stimulus related to the subsequent Stroop stimulus) and 50% IR conditions (in which the color word in the preceding Stroop stimulus always identi-fied the subsequent print color). NP was computed as the difference in naming latencies between the control and IR conditions.

StimuliThe stimuli were presented on 26- × 18-cm cards and consisted of 11 different color words. The 12 cards

Table 1. All Priming Conditions, Experiment 1

Control Ignored repetition

PINK–r ORANGE–br

BLUE–g BLACK–y

RED–blk PURPLE–blk

WHITE–y GRAY–pur

ORANGE–pi GREEN–g

BROWN–pur PINK–gr

BLACK–w RED–pi

PURPLE–bl BROWN–r

GREEN–or WHITE–br

GRAY–br YELLOW–w

YELLOW–gr BLUE–y

Note. Lowercase letters depict the target print colors of Stroop stimuli: bl = blue, blk = black, br = brown, g = gray, gr = green, or = orange, pi = pink, pur = purple, r = red, w = white, y = yellow.

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used in the experiment consisted of 6 control cards and 6 IR cards with 11 trials on each. On each card all 11 Stroop stimuli were arranged as a vertical list and printed against a light gray background. The print of each word was set in one of the 11 colors with the constraint that the color word and print color were incongruent. Four additional control cards were used as practice trials. On IR cards, to minimize the potential saliency of the IR condition, the first two Stroop items were control trials. Presentation orders were counterbalanced so that half of the participants began with a control card and half with an IR card. Subsequent cards were presented in regular alterna-tion of the two conditions. A stopwatch was used to record color-naming times.

ProcedureA double-blind procedure was followed and all par-ticipants were tested individually. Before the experi-ment, each participant completed a preliminary color identification task to ensure familiarity with the 11 colors used in the experiment. Participants were then given verbal instructions to name as quickly and ac-curately as possible the ink color of each color word from the top to the bottom of the list on each card. They were asked not to stop color naming if an error was made but to continue until the color naming for the card was completed. Each participant completed four control practice cards before encountering the 12 experimental cards. For each card the experimenter said, “Ready” as a warning and on the word “Go” removed a blank sheet covering the test card. The stopwatch was started with the removal of the blank sheet and stopped in synchrony with the naming of the last color on a card. Errors, classified as either omissions or verbalizations of an absent or incorrect color, were recorded for each card.

RESULTS

Table 2 shows the mean naming latencies per Stroop item and percentage of error for control and IR condi-tions in relation to age group. Mean reaction times (RTs) and error rates were submitted to a 2 (age group: children vs. adults) × 2 (priming condition: control vs. IR) mixed ANOVA.

Response Time AnalysesThere was a significant main effect of age group, F(1, 63) = 129.30, p < .01, with overall naming times significantly longer for children than for adults. The main effect for priming condition was highly signifi-cant, indicating an NP effect with color naming RTs longer for IR conditions than control conditions, F(1, 63) = 29.51, p < .0001. The interaction between priming condition and age group fell just short of statistical significance, F(1, 63) = 2.71, p = .11, sug-gesting, if anything, that the NP effect was larger for children than for adults, possibly because of children’s longer overall response latencies (see Pritchard & Neumann, 2009, for further discus-sion). Additional follow-up single-sample t tests indicated that the NP cost scores (IR minus con-trol) were significantly greater than zero (indicating intact NP) for both adults, t(29) = 3.67, p < .01, and children, t(34) = 4.35, p < .01. An effect size analy-sis correcting for dependence between means using Morris and DeShon’s (2002) Equation 8 indicated that effect sizes were largely similar between chil-dren (–0.736) and adults (–0.805). Furthermore, the proportional magnitude of NP in children (6%) did not differ significantly from that of adults (7%) (Mann–Whitney U, p = .48).

table 2. Mean Reaction Times (ms) per Stroop Item and Error Percentage as a Function of Priming Condition for Chil-dren and Adults, Experiments 1 and 2

Children Adults

Experiment 1 Experiment 2 Experiment 1 Experiment 2

Priming condition M SD (error %) M SD (error %) M SD (error %) M SD (error %)

Neutral 1,034 196 (1.5) 633 133 (1.5)

Repeated distractor 1,288 185 (2) 754 145 (2)

Control 1,482 344 (4.0) 1,530 370 (2.5) 754 130 (2) 835 169 (3)

Ignored repetition 1,581 338 (4.5) 1,545 309 (4) 807 175 (2) 920 200 (4.5)

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Error AnalysesSimilar analyses were conducted for error scores. The main effect of age group was significant, F(1, 63) = 12.87, p < .01, indicating that children made more errors than adults. There were no other effects, ps > .26.

DISCUSSION

The goal of Experiment 1 was to replicate Pritchard and Neumann’s (2004, 2009) findings of intact and similar conceptual NP between children and adults in an experimental context containing 100% high-conflict conditions. This seemed important for confirming that this effect may depend specifically on high-conflict conditions being a consistent vari-able. A clear reproduction of the results obtained by Pritchard and Neumann was demonstrated, with children producing NP comparable to that of adults in an experimental context where the distractor di-mension of each Stroop stimulus was always highly incongruent with the target dimension across all task conditions.

eXPeRiMent 2

Experiment 2 was based on the experimental design used by Tipper et al. (1989, Experiments 1 and 2). Stroop stimuli were used again, but participants en-countered both low- and high-conflict conditions and equal proportions of each. The low-conflict condition consisted of half neutral and half RD tri-als, whereas the high-conflict condition consisted of the same control and IR trials used in the current Ex-periment 1. Following the procedure of Tipper et al., neutral and RD conditions were presented separately and intermixed with control and IR conditions. The ratio of neutral and RD conditions to control and IR conditions was 50:50. To ensure that any potential differences between the two age groups would not hinge on the exact method of Tipper et al. (1989), the test cards used in Experiment 2 comprised 11 stimuli each (instead of 30), and as in Experiment 1, a more extensive range of colors (i.e., 11 vs. 5) was used. Keeping such variables constant across the cur-rent experiments meant that any potential modulation of children’s NP observed in Experiment 2 would probably relate to their exposure to 50% low-conflict conditions rather than to any other design factor spe-

cific to the Tipper et al. study. Deviating from the Tipper et al. method in such respects also meant that stimulus repetition was much less frequent in our experiment than in the Tipper et al. study, thus making the IR manipulation less salient and reducing children’s likelihood of being “captured” by it. Finally, following Pritchard and Neumann’s (2004, 2009) selection state hypothesis, it also seemed necessary to take into account the potential effect of any preset anticipation of encountering only high-conflict conditions that might have occurred as a result of participating in Experiment 1. To avoid this effect, Experiment 2 used different participants.

METHOD

ParticipantsExperiment 2 included 65 new participants recruited from the same sources as participants in Experiment 1. Participants were grouped by age to include 35 children (20 female) with a mean age of 7 years, 9 months and 30 adults (18 female) with a mean age of 19 years, 2 months.

DesignA 2 × 4 mixed design was used. The between-partic-ipant variable was age group (children vs. adults) and the within-participant variable was priming condi-tion (neutral vs. RD vs. control vs. IR). Experiment 2 contained 25% neutral trials, 25% RD trials, 25% control trials, and 25% IR trials.

Stimuli and ProcedureExperiment 2 used the 6 control and 6 IR cards that were used in Experiment 1, differing only in the ad-dition of 6 neutral cards and 6 RD cards (Table 3). Neutral and RD cards were similar in design to those used by Tipper et al. (1989). On each neutral card, the stimuli consisted of 11 rows of Xs ranging from three to six Xs per stimulus item, with the print of each appearing in one of 11 colors used in Experiment 1. On each RD card, the same color word was repeat-edly presented across 11 trials, and its print color dif-fered between trials (the color words BLUE, GREEN, and RED were each used on two of the cards for this condition). Four cards, with each representative of one of the four priming conditions, were used as practice trials. Experimental cards were presented in a fixed order so that half of the participants began with a control card, followed by a neutral card, fol-

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lowed by an IR card, followed by an RD card, and so forth, in regular alternation. The remaining par-ticipants began with an IR card, followed by an RD card, followed by a control card, followed by a neutral card, and so forth, in regular alternation. Thus the IR condition was always preceded by a low-conflict condition. As in Experiment 1, a stopwatch was used to record naming times for each card. Error scores were also tabulated for each card.

RESULTS

To first test for the presence of a three-way interac-tion between age group, experiment, and NP effect, participants’ mean control and IR RTs were entered into a 2 (age group: children vs. adults) × 2 (experi-ment: 1 vs. 2) × 2 (priming condition: control vs. IR) ANOVA. Results revealed a significant three-way in-teraction, F(1, 315) = 7.59, p < .01. This suggested that NP differed between the age groups as a function of experiment. To determine the nature of this interac-tion, a series of planned comparisons were performed for Experiment 2. Participants’ mean RTs and corresponding error rates were entered into a 2 (age group: children vs. adults) × 4 (priming condition: neutral vs. RD vs. control vs. IR) ANOVA. Mean RTs and error rates are shown in Table 2. The between-participant factor

of age group was highly significant, F(1, 63) = 100.24, p < .01, with the mean RT shorter for adults than children. There was also a significant main effect of priming condition (neutral vs. RD vs. control vs. IR), F(3, 189) = 27.58, p < .01. A visual inspection of the associated profile plot revealed that naming times in-creased in the following order (neutral < RD < con-trol < IR). More importantly, there was a significant interaction between age group and priming condi-tion, F(3, 189) = 27.58, p < .01.

Children Versus AdultsMean scores for the neutral (control minus neutral RT), RD (control minus RD RT), and NP (control minus IR RT) conditions were calculated for children and adults. Age group differences between neutral (p < .02) and RD (p < .01) scores indicated that chil-dren’s responses were significantly more facilitated in these conditions than adults relative to RT costs incurred in the control condition. This age group difference was evident even after a log transformation of naming time to control for possible overadditive effects due to the longer overall RTs of children, F(1, 63) = 119.35, p < .01. Paired-samples t tests (with Bon-ferroni correction .05/3 = .02) contrasting the neu-tral with the control condition for each age group revealed significantly shorter naming times in the

table 3. All Priming Conditions, Experiment 2

Neutral Stroop control Repeated distractor Ignored repetition

XXX–r PINK–r GREEN–pur ORANGE–br

XXXXXX–gr BLUE–g GREEN–y BLACK–y

XXXX–blk RED–blk GREEN–br PURPLE–blk

XXXXX–y WHITE–y GREEN–w GRAY–pur

XXXXXX–bl ORANGE–pi GREEN–gr GREEN–g

XXX–pi BROWN–pur GREEN–blk PINK–gr

XXXX–w BLACK–w GREEN–or RED–pi

XXXXX–g PURPLE–bl GREEN–pi BROWN–r

XXX–or GREEN–or GREEN–br WHITE–br

XXXXXX–br GRAY–br GREEN–g YELLOW–w

XXXX–pur YELLOW–gr GREEN–bl BLUE–y

Note. Lowercase letters depict the target print colors of the Stroop items: bl = blue, blk = black, br = brown, g = gray, gr = green, or = orange, pink = pi, pur = purple, r = red, w = white, y = yellow.

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neutral condition for both children, t(34) = 13.20, p < .01, and adults, t(29) = 14.07, p < .01. However, and in keeping with previous findings (Comalli et al., 1962; Tipper, et al., 1989), further analyses revealed that the proportional size of decreased naming times in the neutral condition was larger in children (32%) than in adults (24%) (Mann–Whitney U, p < .01). A series of similar analyses were conducted for the RD condition, revealing significant RD effects for chil-dren, t(34) = 7.92, p < .01, and adults, t(29) = 6.76, p < .01. An analysis of the proportional size of the RD score indicated that it was larger in children (16%) than adults (10%) (Mann–Whitney U, p < .01). More critically, the opposite result was found for contrasts between IR and control conditions, with NP significant in adults, t(29) = 7.34, p < .01, but not in children, t(34) = .53, p = .62. These age group dif-ferences were confirmed by an ANOVA revealing a significant interaction between priming condition and age group, F(1, 63) = 4.94, p < .03. Furthermore, the proportional size of NP was greater in adults (9%) than in children (1%) (Mann–Whitney U, p < .04). Effect sizes also appeared substantially smaller for children (–0.096) than adults (–1.557). Collectively, these results suggest that children’s NP appears to be modulated by the proportion of low-conflict trials encountered in the experimental context, whereas adults’ NP does not.

Supplementary AnalysesTo rule out any potential effect of fatigue on chil-dren’s NP performance in Experiment 2, given the addition of 72 extra trials, we also compared NP in this age group by experimental half (i.e., first test block vs. second test block). Results fell short of sta-tistical significance, p = .12, indicating, if anything, a trend toward emerging NP in the second test block. Although a comparison experiment, containing the same number of total trials as Experiment 2 but in-cluding only control and IR conditions with matched blocks used for analysis, might have been more desir-able in ruling out the potential effect of fatigue, the results of the current analysis suggest that fatigue was not a factor. Rather, it appears that children pay a heavy cost in terms of selective processing efficiency when exposed to variable degrees of response conflict in a selective attention task. Finally, analyses investi-gating the potential influence of gender × age group

on NP were also conducted for both experiments but revealed no differences, ps > .55.

Error AnalysesThe between-participant factor of age group was not significant, F(1, 63) = .03, p = .95. There was a signifi-cant main effect of priming condition (neutral vs. RD vs. control vs. IR), F(3, 189) = 23.18, p < .01, with the mean error rate for the four priming conditions in-creasing in the following order (neutral < RD < con-trol < IR). No other error effects approached signifi-cance.

DISCUSSION

Experiment 2 appeared to reproduce the age group differences in NP first reported by Tipper et al. (1989, Experiments 1 and 2), with NP emerg-ing intact in adults but not in children when 50% low-conflict neutral and RD conditions and 50% high-conflict control and IR conditions were en-countered. The results of Experiment 2 strongly contrast with those obtained for Experiment 1, where NP was nearly indistinguishable between children and adults when only high-conflict con-ditions were encountered. This finding suggests that encountering a substantial proportion of neu-tral and RD conditions in a selective attention task significantly impaired children’s ability to maintain an active selection state. Also consistent with the resulting implications for selective inhibitory pro-cessing, subsequent analyses comparing naming times for control and IR conditions by experiment showed that children’s lack of NP in Experiment 2 was coupled with slower responses in the control condition, suggesting an increase in distractor in-trusion. No such effects were obtained for adults. Another notable result was that children’s responses were significantly more facilitated on neutral and RD trials than adults in relation to the RT costs incurred on the control condition (369 ms vs. 151 ms per item). This implies that although children incurred larger RT costs on the control condition than adults, their responses were substantially fa-cilitated on the neutral and RD conditions, possibly indicating that they experienced a more salient ease in selection difficulty (see also Tipper et al., 1989, Experiments 1 and 2, for similar findings).

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geneRal Discussion

The central goal of this study was to test whether the magnitude of Stroop NP is affected by exposure to “easy selection” or low-conflict conditions and to determine any age-specific dissociations, in an effort to resolve contradictory findings about NP in chil-dren relative to adults (cf. Pritchard & Neumann, 2004, 2009; Tipper et al., 1989). A further aim was to determine more exactly whether a ratio of 50:50 low- to high-conflict conditions may be necessary to eliminate NP in children, given that this age group has previously produced intact NP effects in an experi-mental context containing 35% low-conflict and 65% high-conflict conditions (i.e., Frings et al., 2007). It was hoped that the results of the current experiments might begin to establish the task parameters under which comparable and incomparable NP occurs for children and adults. The results of the study were clear. Experiment 1 reproduced Pritchard and Neumann’s (2009) find-ings of intact and comparable NP in children and adults. Experiment 2 reproduced the Tipper et al. (1989, Experiments 1 and 2) findings by showing intact NP in adults but not in children when 50% low-conflict neutral and RD conditions and 50% high-conflict conditions were encountered. Because our experiments used a wider range of color stimuli and shorter naming lists than those of Tipper et al., it seems certain that the absence of children’s NP in the Tipper et al. study and in the current Experiment 2 probably related to encountering a 50:50 ratio of high- to low-conflict conditions rather than to any other specific design factor. In light of the results obtained, it appears that between-study differences in the overall consistency and extent of distractor competitiveness may be responsible for the mixed outcomes concerning children’s NP to date (see also Christie & Klein, 2008; Levin & Neumann, 1999, for other experiment design factors to consider). Finally, and with respect to the context control phenomena that are commonly observed in adults in Stroop ex-periments in which the proportion of congruent trials is manipulated (Crump et al., 2006; Lindsay & Jacoby, 1994; Schmidt & Besner, 2008), it is noteworthy that adults’ exposure to neutral and RD conditions in the current study appeared to have no such modulatory influences on their Stroop NP. Further research on

forms of cognitive control in the Stroop task, in con-trast to the Stroop NP task, seem especially warranted because the absence of NP modulation in adults here suggests that context may play a stronger role in the implementation of conflict detection and adaption than in the initiation and maintenance of attentional set. To date, no research has been aimed at extending context control phenomena to children.

Toward Establishing the Boundary Conditions Under Which NP Emerges in ChildrenThe observed pliability of children’s NP suggests that inconsistent reports of conceptual NP in children may reflect the greater sensitivity of this age group to con-textual features. That is, inconsistent NP in children relative to adults may best be understood in terms of age-related differences in the ability to maintain se-lection state in the face of varying levels of distractor interference in the experimental context. More specifi-cally, the results of the current study taken in combina-tion with those from the studies of Frings et al. (2007), Pritchard and Neumann (2004, 2009), and Tipper et al. (1989, Experiments 1–3) reveal a pattern of NP in children that is remarkably consistent with the idea that increasing inhibitory demand in the experimental context is critical for the maintenance of selection state in this age group. With one of four possible condition combinations used in each of these studies, the de-gree of demand on inhibitory processing ranges from low (neutral + RD + control + IR), medium (neutral + control + IR), high (RD + control + IR), to extreme (control + IR), with the results stemming from each combination seeming to conform to the predictions of Pritchard and Neumann’s (2004) modified selection state hypothesis. For example, in an experimental context where only high-conflict control and IR conditions are included and target selection difficulty is at 100%, children have been shown to produce robust and replicable NP across a number of different stimulus types, as well as trialwise sequential and list naming presentation formats (Pritchard & Neumann, 2004, 2009, current Experiment 1). The inclusion of an RD condition among high-conflict NP conditions to make up approximately one third of the conditions is not sufficient to significantly reduce NP in children (Frings et al., 2007, flanker NP task). In the RD con-dition, target selection difficulty is only somewhat

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lower than in control and IR conditions. However, replacing an RD condition with a neutral condition (the easiest selection condition of the four conditions under consideration) to make up one third of the con-ditions appears to create a less favorable context for obtaining NP in children, with only a nonsignificant trend emerging (Tipper et al., 1989, Experiment 3). In the neutral condition, distractors are mean-ingless stimuli and therefore less likely to conflict with target processing than those in the RD condi-tion, which though repeated are still meaningful and thus potentially conflicting. And finally, the context least likely to induce NP in children appears to be that where low-conflict conditions make up 50% of the conditions and include a combination of both RD and neutral conditions. In this context, in which target selection difficulty is reduced by the greatest amount overall, no NP is evident in children (Tip-per et al., 1989, Experiments 1 and 2; confirmed by the current Experiment 2). These data patterns appear to isolate the source of the discrepant results from three independent laboratories, in line with Pritchard and Neumann’s (2004) prediction that decreases in inhibitory demand in an experiment-wide manner can result in an elimination of chil-dren’s NP relative to adults. They also attest to the generalizability of this hypothesis across a variety of different NP tasks.

Are Children Less Concentrated Selectors Than Adults?The current study raises the possibility that ability to adapt to differing processing demands and select in a consistently effective manner may develop after the middle childhood period. In early childhood selec-tive attention may be particularly prone to diffusion under experimental conditions where processing demand is variable. Applied to control and IR con-ditions in the current study, a division of attention between target and distractor information should be evidenced by increased sensitivity toward distractor information on control trials and decreased NP on IR trials if there is a lessening of active inhibition toward distractors. This, in turn, could give rise to them be-ing processed inadvertently or even occasionally in a more conscious sense (Houghton & Tipper, 1994). A comparison of the RT data patterns for children in Experiments 1 and 2 is remarkably consistent with such predictions. Mean control RTs were larger for

children in Experiment 2 (1,581 ms) than Experiment 1 (1,482 ms), with the opposite trend for IR RTs. Al-though there was a significant interaction between experiment and priming condition for children, no such interaction was observed for adults. Collectively, this implies children’s slower responses on the con-trol condition, and their greater tendency to process distractor information, was due to the lower selective inhibition of the high-conflict distractors in Experi-ment 2, which also eliminated NP. The idea that children tend to process distractor information, in a more conscious sense than adults, may also help explain the greater RD facilitation ef-fects observed for this age group via the process of habituation. The habituation hypothesis (Sokolov, 1963) holds that the repetitive presentation of iden-tical stimuli results in the lessening of the orienting response (an automatic response typically elicited by novel stimuli). Lorch, Anderson, and Well (1984) argued that the repeated presentation of a stimulus in an experimental situation enables a person to form a short-term neuronal representation of this stimulus. An integration or match between stimulus and inter-nal representation reduces the orienting response. The evidence that children appeared to process color words to a greater degree than adults in Ex-periment 2 implies that the internal representations children form for color word stimuli on RD trials are established more readily than those of adults, resulting in a faster reduction of the orienting re-sponse and a greater speed-up than occurs in adults in RD responses relative to the control condition. Similar RT effects for children relating to distrac-tor intrusion have been observed with congruent Stroop stimuli. For instance, a study by Wright and Wanley (2003) found that children gained a larger facilitation effect than adults on Stroop trials where the ink color to be named matched the identity of the concurrent color word. These authors argued that because children are more prone to attend to the word dimension of the Stroop stimulus than adults, the semantic activation for color information facilitated children’s response to the word-congru-ent ink color more than that of adults.

Implications for the Episodic Retrieval Account of NPA popular alternative to inhibition-based accounts of NP is the episodic retrieval account (Neill, 1997; Neill

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& Valdes, 1992). By this account, NP is caused by an incompatibility between past and present response information associated with a recently ignored stimu-lus during its processing on an IR probe trial. More specifically, the automatic retrieval of a memory trace on an IR probe trial contains prime response infor-mation (i.e., a “do not respond” tag) that conflicts with the current correct response (i.e., a “respond” tag). The resolution of the conflict between these two incompatible tags is purported to result in the NP effect. However, more recent theorizing holds that the retrieval of prime information also reactivates the prime response (Rothermund, Wentura, & De Hou-wer, 2005). By this account, NP would then result from both response conflict and the reactivated prime response. Although several authors agree that both inhibitory and retrieval processes contribute to NP (Grison et al., 2005; Groh-Bordin & Frings, 2009; Tipper, 2001), there remain a number of retrieval purists (e.g., Mayr & Buchner, 2007; Milliken et al., 1998). Retrieval-based accounts of NP have all relied on adult-based samples; however, data from the pres-ent child-based study are difficult to reconcile with the retrieval of a “do not respond” tag during probe target processing, because all study participants were exposed to identical IR prime and probe displays across both Experiments 1 and 2. Presumably, the exact same opportunities for the retrieval of identi-cal “do not respond” tags were offered to children and adults during the IR probe trials in both experi-ments. Yet whereas NP was obtained for children in Experiment 1, no NP was obtained for children in Experiment 2. Thus, without some modification, the episodic retrieval theory does not easily accom-modate our findings, even if the “do-not-respond” tag is understood as a metaphor for the retrieval of an inhibitory process affecting a conceptual identity (Tipper, 2001).

LimitationsLimitations of this study must be considered in in-terpreting the findings. First, Experiments 1 and 2 used different participant groups. Therefore, the extent to which NP results reflect individual differ-ences between the two groups of adults and children is not known. Second, it is possible that the inclusion of low-conflict conditions in Experiment 2 afforded some opportunity for goal neglect in children. For

example, a study of card-sorting ability in young chil-dren by Marcovitch, Boseovski, and Knapp (2007) found that children more often neglected the task goal when faced with a large number of cards that were irrelevant to the current sorting requirement (see also Kane & Engle, 2003). Applied to Experi-ment 2, the inclusion of low-conflict cards may have reduced children’s incentive to stay on task. Third, the condition order was fixed in Experiment 2 so that the IR condition was always preceded by a low-conflict condition. Therefore, it is not entirely clear whether the proportion of low-conflict condi-tions in the experimental context or the preceding of a low-conflict condition resulted in the different NP results for children. Fourth, the degree to which NP in children and adults increased, decreased, or remained constant over the course of either experi-ment was not examined. Therefore, it is not clear how much sway the inclusion of low-conflict conditions versus the amount of time spent on the task would have had over the degree of NP observed in the first versus second test block of either experiment, an is-sue deserving of future research. Finally, one further limitation concerned the use of the manual data col-lection procedures that did not allow RT measures for individual stimulus items and therefore reduced our ability to eliminate outliers.

ConclusionsThe present results have a number of implications for theories aimed at understanding the develop-ment of inhibitory processing and selective attention in general. First, they reinforce recent suggestions that the inhibitory processes involved in information selection develop to maturity early (Lechunga et al., 2006; Pritchard & Neumann, 2004, 2009). Second, they highlight the importance of considering task design when attempting to assess or understand developmental differences in attention processes (McDermott, Perez-Edgar, & Fox, 2007) and reveal how instances of apparent inhibitory ineffectiveness in children’s selective attention may result from devel-opmental differences in the maintenance of selection state rather than signaling incomplete development in the inhibitory component of selective attention. Third, the current findings indicate the usefulness of further investigations aimed at identifying the age at which children become more focused selectors

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and determining the associated neurodevelopmental changes (Jordan & Morton, 2008). For example, the approach recently promoted by Christie and Klein (2008), using the full set of all possible prime–probe target–distractor relationships, would probably be most promising in this regard. In fact, systematically varying the proportion of easy selection trials within the seven conditions advocated by Christie and Klein could help to isolate more precisely the points of di-vergence in NP between different ages of children and those of adults. Finally, these experiments demonstrate the vi-ability of using an experimental approach to estab-lish which NP conditions might be most useful for comparing distractor inhibition across different age groups and provide a new perspective from which to approach ostensible individual differences in NP.

NOTE

Address correspondence about this article to Verena Prit-chard, Department of Psychology, PB, 4800, Christchurch, New Zealand (e-mail: verena.pritchard@canterbury.ac.nz).

1. Although not the main focus of the present study, it is important to note that an alternative explanation for NP that does not rely on inhibition has also been proposed (Neill, 1997; Neill & Valdes, 1992). Most, if not all, theoretical con-clusions negating the role of inhibition in NP phenomena have been formed on adult-based research samples. Two other accounts of NP include the feature match–mismatch (Park & Kanwisher, 1994) and temporal discrimination (Milliken et al., 1998) theories. However, because feature match–mismatch theory is limited to explaining NP effects in localization tasks and temporal discrimination theory faces mounting empirical counterevidence (see Frings & Wühr, 2007), neither is considered in this article.

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