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Abstract We examined differences in attentional control among school-age children who were monolingual English speakers, early Spanish-English bilinguals (who began speaking both languages by age 3), and later Spanish-English bilingual children (who began speaking English after age 3). Children's attentional control was tested using the Attention Network Test (ANT). All language groups performed equally on ANT networks; however, when controlling for age and verbal ability, groups differed significantly on reaction time. Early bilingual children responded faster on the ANT compared to both monolingual and later bilingual children, suggesting an attentional monitoring advantage for early bilinguals. These results add to evidence of advantaged cognitive functioning among bilinguals and are consistent with the possibility that children who begin speaking a second language earlier in childhood have greater advantages, due either to effects of acquiring a second language earlier or to longer duration of bilingual experience.

. Introduction A growing body of research suggests that bilingual individuals outperform monolinguals on a variety of cognitive measures (Bialystok, 1999, Bialystok et al., 2006a, Bialystok et al., 2006b, Carlson and Meltzoff, 2008 and Costa et al., 2008). These advantages, which have been characterized as advantages in cognitive control, have been documented across the lifespan. Improved cognitive control among bilinguals has been observed in bilingual-exposed infants (Kovàcs and Mehler, 2009a and Kovàcs and Mehler, 2009b), toddlers (Poulin-Dubois, Blaye, Coutya, & Bialystok, 2011) bilingual preschool children (Bialystok, 1999, Bialystok and Martin, 2004, Carlson and Meltzoff, 2008 and Yoshida et al., 2011), young adults (Costa et al., 2009, Costa et al., 2008 and Prior and MacWhinney, 2010) and older adults (Bialystok et al., 2007 and Bialystok et al., 2004). Further, cognitive control advantages of bilingualism have been demonstrated using multiple cognitive tasks and have been found among bilinguals speaking a variety of language pairs, suggesting that these effects are not limited to a single task or particular language pairing (see Adesope, Lavin, Thompson, & Ungerleider, 2010, for recent meta-analysis). 1.1. Bilingualism and cognitive control Although bilinguals have outperformed monolinguals in a variety of cognitive skills, two types of cognitive control skills have been consistently reported to be advantaged among bilinguals: attentional inhibition and attentional monitoring. Attentional inhibition is the ability to ignore distracting or conflicting information in order to focus attention on relevant information. Tasks that measure attentional inhibition commonly include distracting information that participants must ignore in order to respond successfully. For example, in classic flanker tasks the use of attentional inhibition is necessary on incongruent trials in which the target arrow is oriented in the opposite direction of flanker arrows (→ → ← → →). On such incongruent trials, successful responding requires participants to ignore the flankers to focus only on the target arrow. Attentional inhibition is not required on congruent trials in which all flankers are oriented in the same direction (→ → → → →), as there is no conflicting information to ignore. Typically, responses on congruent trials are faster and more accurate than responses on incongruent trials. The difference in reaction time or accuracy between congruent and incongruent trials provides an index of participants’ attentional inhibition abilities, with smaller differences between congruent and incongruent trials representing more efficient attentional inhibition, or in other words, less cost of ignoring conflicting information. Bilingual adults and children have been previously reported to show smaller differences between congruent and incongruent trials (i.e., more efficient attentional inhibition) in flanker tasks (Costa et al., 2008, Costa et al., 2009, Luk et al., 2011, Toa et al., 2011, Yang and Lust, 2004 and Yoshida et al., 2011), the Simon task (Bialystok et al., 2005a, Bialystok et al., 2004 and Bialystok et al., 2005b), and antisaccade tasks (Bialystok et al., 2006a, Bialystok et al., 2006b and Bialystok and Viswanathan, 2009). Such bilingual advantages in attentional inhibition are frequently explained as resulting from the need for bilinguals to keep their two language systems separate. In order to maintain this separation, it is hypothesized that bilinguals must employ attentional inhibition to avoid accessing the non-target language and instead access the target language (Green, 1998). However, recent evidence from bilingual-exposed infants who demonstrate cognitive advantages (Kovàcs & Mehler, 2009a) suggests that lexical access alone cannot account for these advantages, as pre-verbal infants demonstrate bilingual advantages over monolingual peers. The source of bilingual advantages in attentional inhibition thus remains under debate, but available evidence suggests that both exposure to and production of two languages may underlie these cognitive advantages. The second advantaged skill, attentional monitoring, refers to the ability to attend and respond to changing task demands. For example, in the previously described flanker task, congruent and incongruent trials are intermixed, resulting in the need for participants to switch back and forth between responding to incongruent trials that require attentional inhibition and responding to congruent trials requiring no attentional inhibition. Attentional monitoring is indexed by the overall reaction time to both congruent and incongruent trials, with a faster reaction time indicating better attentional monitoring (i.e., less cost of switching between responses). Based on average reaction time, bilinguals have outperformed monolinguals on attentional monitoring measures including flanker tasks (Costa et al., 2008, Costa et al., 2009, Toa et al., 2011 and Yang and Lust, 2004), the Simon task (Bialystok et al., 2005a, Bialystok et al., 2005b and Martin-Rhee and Bialystok, 2008), dual dimension classification tasks (Barac & Bialystok, 2012) and antisaccade tasks (Bialystok et al., 2006a, Bialystok et al., 2006b and Bialystok and Viswanathan, 2009). Although bilinguals respond faster to both congruent and incongruent trials when these tasks included mixed trial blocks, this reaction time advantage does not exist on trial blocks of a single trial type that do not require switching between responses, and therefore, do not tax attentional monitoring (Bialystok, 2006, Costa et al., 2009 and Martin-Rhee and Bialystok, 2008). The source of bilinguals’ advantage in attentional monitoring is proposed to be the need to monitor their linguistic environment in order to select the appropriate language to use with interlocutors (Costa et al., 2009), which occurs both when bilinguals must select one of their languages to use with monolinguals and when they are switching between languages (code-mixing) with other bilinguals. 1.2. Bilingual characteristics that affect attentional advantages Although a growing research literature supports the presence of attentional advantages among bilinguals, less is known about the characteristics of bilingual individuals who demonstrate these advantages. The studies that have addressed this issue have focused on the level of second language (L2) proficiency and balance between first language (L1) and L2 proficiency as potential variables influencing bilingual cognitive control. Such studies have produced somewhat equivocal findings. For example, Bialystok (1988) found that “partially bilingual” children with low L2 proficiency outperformed monolinguals on tasks requiring controlled metalinguistic processing (e.g., sun/moon), suggesting that high L2 proficiency is not a prerequisite for cognitive advantages. However, Carlson and Meltzoff (2008) reported that the performance of children with 6 months of L2 immersion experience was equal to their monolingual peers’ performance on a battery of attentional inhibition measures, whereas bilingual children outperformed children in both the monolingual and immersion groups. These results suggest that a certain level of L2 proficiency is necessary before cognitive advantages emerge. In addition to L2 proficiency, research has also examined the effect of relative balance between L1 and L2 proficiency on the bilingual advantage in cognitive function. Although few studies have compared bilinguals with varying levels of language balance, converging results suggest that greater balance between a bilingual's languages (i.e., similar proficiency levels in both languages) is associated with larger advantages in cognitive functioning compared to less balance (Bialystok et al., 2006a, Bialystok et al., 2006b and Vega and Ferdandez, 2011). 1.3. Age of second language acquisition One issue that remains largely unaddressed in this literature is the potential effect of L2 age of acquisition (AoA) on cognitive functioning among bilinguals. Throughout this paper, we consider children who began speaking two languages between birth and 3 years to be early childhood bilinguals, whereas children who acquired a second language beyond the age of three are classified as later childhood bilinguals ( Genesee et al., 2004 and McLaughlin, 1984). In the present study, we measure AoA based on the age at which parents reported their children began speaking two languages, but other classification systems exist. Some researchers refer to children exposed to two languages within the first year of life as simultaneous bilinguals and children who acquire a second language prior to school entry as early bilinguals (de Houwer, 2005). However, classifying children based on the age at which they began speaking an L2 (rather than age of initial exposure), using the age range employed by McLaughlin (1984) and Genesee et al. (2004) to define early bilingualism (i.e., speaking the language before age 3), was most appropriate for our purposes. Studies of bilingual cognitive advantages in childhood generally include only early childhood bilinguals growing up in a bilingual household. Evidence for a bilingual cognitive advantage comes largely from research including bilingual preschool children who are proficient in both their L1 and L2 at the time of testing (Bialystok, 1999, Bialystok and Martin, 2004, Carlson and Meltzoff, 2008 and Yoshida et al., 2011). In order to achieve the proficiency level in their L2 required for study participation by preschool age (i.e., 3–5 years), these children would presumably be categorized as early bilinguals based on McLaughlin's (1984) definition. Additional support for cognitive advantages of early bilingual language acquisition in childhood comes from studies finding advantages of pre-verbal infants exposed to two languages (Kovàcs and Mehler, 2009a and Kovàcs and Mehler, 2009b) and bilingual toddlers (Poulin-Dubois et al., 2011). Less is known about the effects of later L2 acquisition on cognitive functioning in childhood. The majority of research on adult bilinguals’ cognitive control ability has not specifically defined bilingual groups based on AoA; instead, the typical criterion for inclusion in adult bilingual groups is frequent use of two languages since adolescence or early adulthood (Bialystok et al., 2007, Bialystok et al., 2006a and Bialystok et al., 2006b). Thus, in this literature, both early and later bilinguals would fit within the participation criterion and have likely been pooled within adult bilingual samples. Two notable exceptions are studies by Toa et al. (2011) and Luk et al. (2011), which to our knowledge are the only existing studies that have systematically compared bilingual groups that differ in AoA in order to examine differences in cognitive control abilities. Toa et al. (2011) compared performance on the Lateralized Attention Network Test (LANT; Greene et al., 2008), a computerized flanker task, among adult bilinguals who had acquired an L2 before age 6 (early bilinguals), adult bilinguals who acquired an L2 between 12 and 19 years of age (late bilinguals) and monolinguals. Based on their performance on the LANT, Toa et al. (2011) report that early bilinguals responded significantly faster on all trial types (i.e., congruent and incongruent) compared to monolinguals, which the authors interpret as an attentional monitoring advantage. Late bilinguals did not respond significantly faster than monolinguals, suggesting that unlike early bilinguals, individuals who acquired an L2 after age 12 are not advantaged in attentional monitoring. Both early and late bilinguals had a smaller RT conflict network effect (i.e., less difference in RT to congruent versus incongruent trials) compared to monolinguals, supporting an advantage in attentional inhibition for both bilingual groups. Taken together, these results suggest that both early and late bilinguals have improved cognitive control compared to monolinguals, but only early bilinguals are advantaged in attentional monitoring. Toa et al. (2011) posit that advantages in attentional inhibition among late bilinguals may result from the need to avoid influence from their dominant L1 during L2 acquisition, whereas attentional monitoring advantages may be limited to early bilinguals who develop two languages simultaneously. Similarly, Luk et al. (2011) divided young adult bilinguals into early versus late groups based on the age at which participants reported becoming actively bilingual (i.e., speaking two languages daily). The group of early bilinguals included participants who reported regularly using two languages before 10 years of age, whereas the late group comprised individuals who began actively using two languages after 10 years of age. Both bilingual groups were compared to a group of age-matched monolinguals on a flanker task containing both congruent and incongruent trials. Luk et al. (2011) report that early bilinguals demonstrated a significantly smaller difference between congruent and incongruent trials (i.e., better attentional inhibition) compared to both late bilinguals and monolinguals, suggesting an attentional inhibition advantage among the early bilingual group, whereas monolinguals and late bilinguals performed equally. Furthermore, Luk et al. (2011) reported a positive correlation between onset age of bilingualism and the flanker effect, such that individuals who began using two languages at younger ages demonstrated a smaller flanker effect (i.e., better inhibition) compared to individuals who began speaking a second language later. Thus, in agreement with the findings of Toa et al. (2011) and Luk et al. (2011) report larger advantages for bilinguals who began using an L2 at younger ages. However, whereas Toa et al. (2011) reported that earlier bilinguals had greater advantages in attentional monitoring, Luk et al. (2011) report that earlier bilinguals are advantaged in attentional inhibition. Unfortunately, because early and late bilinguals in both studies were the same age at the time of testing, neither study can isolate the effects of the maturational age at which individuals became bilingual versus differences in their duration of bilingual experience (i.e., early bilinguals have more bilingual experience at the time of testing than late bilinguals). Noting the confounding nature of these two variables, Luk et al. (2011) concluded that in light of their correlational findings, it is likely that both age of acquisition and duration of bilingual experience contribute to the increased cognitive advantages among early bilinguals. 1.4. Present study The goal of the present study was to assess the effect of L2 AoA on cognitive advantages associated with bilingualism in childhood by comparing attentional control abilities of bilingual children who differ on the age at which they began speaking an L2. The study included three groups of school-age children: monolingual (MON) children who spoke only English, later childhood bilingual (L-BIL) children who were L1 speakers of Spanish and who began speaking English beyond 3 years of age, and Spanish-English early childhood bilinguals (E-BIL) who began speaking both languages by age 3. Following Luk et al. (2011), we used the age at which participants began speaking two languages as the measure of AoA. The three groups were compared on their performance on the Attention Network Test (ANT) modified for children (Fan et al., 2002 and Rueda et al., 2004). The ANT, is a computerized flanker task that includes varied cues before each flanker trial and measures three attentional networks: conflict (attentional inhibition), alerting, and orienting. The ANT has been used to compare attentional control of bilingual and monolingual adults (Costa et al., 2008, Costa et al., 2009 and Toa et al., 2011) and children (Bialystok et al., 2010a, Bialystok et al., 2010b, Carlson and Meltzoff, 2008, Yang and Lust, 2004 and Yoshida et al., 2011). This research has shown bilingual advantages in the conflict network (Costa et al., 2008, Toa et al., 2011, Yang and Lust, 2004 and Yoshida et al., 2011), overall reaction time (Costa et al., 2008, Costa et al., 2009 and Toa et al., 2011), and the alerting network (Costa et al., 2008). We anticipated that we would find larger cognitive advantages among the E-BIL children (compared to the L-BIL group) due to their experience managing the production of two languages earlier during the development of the attentional system, which undergoes rapid growth during the infancy and preschool periods (for review, see Garon et al., 2008 and Zelazo et al., 2004). Also, E-BIL children may demonstrate larger attentional advantages because they have been controlling two language systems for a longer duration compared to their age-matched L-BIL peers. We thus anticipated that the E-BIL group would have a faster overall RT (i.e., improved attentional monitoring) and a smaller conflict network score (i.e., improved attentional inhibition) compared to both L-BIL and MON groups. We also hypothesized that L-BIL children would demonstrate the bilingual advantage in attentional control when compared to MON children, as evidenced by faster overall RT and lower conflict network scores on the ANT. Because of mixed findings in previous research using the ANT with bilinguals regarding the orienting and alerting networks, we did not propose specific hypotheses regarding group differences on these attentional networks.

. Results 3.1. Preliminary measures Mean ages across the three language groups (L-BIL, E-BIL, MON) were not significantly different, F(2, 76) = 0.73, p = .49. An analysis of variance (ANOVA) of standardized PPVT-III scores indicated that the three groups differed significantly in levels of English receptive vocabulary, F(2, 76) = 10.34, p < .001, η2 = .26. Pairwise comparisons revealed that the MON group had significantly higher PPVT-III scores than the L-BIL (p < .001) and E-BIL groups (p = .004). The discrepancy in monolingual versus bilingual vocabulary is a common finding when standardized vocabulary tests are used to compare bilingual children's receptive vocabulary in a single language to the vocabulary of monolingual children ( Bialystok, 2009, Bialystok et al., 2010a and Bialystok et al., 2010b). However, these inequalities are not always present ( Umbel, Pearson, Fernandez, & Oller, 1992), particularly when bilingual children's total conceptual vocabularies (i.e., total number of unique vocabulary items across both languages) are measured instead of relying on standardized vocabulary measures ( Loyola et al., 1991 and Pena, 2007). The PPVT-III scores of the L-BIL and E-BIL groups were not significantly different (p = .24), indicating that the bilinguals groups had equivalent English receptive vocabularies. The L-BIL and E-BIL also had equivalent TVIP scores, t(55) = 1.08, p = .29, indicating that the two bilingual groups did not differ in receptive vocabulary knowledge in either of their two languages. The three groups did not vary on forward digit span scores, F(2, 75) = 1.25, p = .29, suggesting that any group differences on ANT were not the result of short-term memory differences. We anticipated differences in length of bilingual experience because the groups differed significantly in their age of L2 acquisition, t(55) = −12.50, p < .001, η2 = .74. Therefore, we expected that children with younger L2 AoAs would have had a longer duration of L2 experience at the time of testing. This was confirmed. The E-BIL children had a significantly longer period of bilingual experience (84.9 months) than the L-BIL children (59.1 months), t(55) = 3.35, p = .001, η2 = .17. Thus, because children in the two bilingual groups differ significantly in both the age at which they began speaking a second language and the duration of bilingualism at the time of testing, the effects of each of these variables on ANT performance cannot be isolated. Due to the non-parametric nature of the parent education measure, a Kruskal–Wallis test was used to compare parent education across groups. It showed group differences in parent education, χ2(2, N = 76) = 43.07, p < .001. Pairwise comparisons indicate that the group differences arise from significantly higher parent education among the MON group compared to both the L-BIL group, χ2(1, N = 57) = 38.07, p < .001, and E-BIL group, χ2(1, N = 42) = 23.40, p < .001. However, the bilingual groups were equal on the parent education measure, χ2(1, N = 54) = 2.19, p = .14. 3.2. ANT reaction time and accuracy Previous comparisons of bilingual and monolingual children in the U.S. (Carlson & Meltzoff, 2008) have included verbal ability and SES as covariates in analyses due to the influence of these variables on cognitive control (Mezzacappa, 2004, Noble et al., 2006 and Noble et al., 2005). Due to the range of ages represented in the current sample, and the significant correlation between age and ANT performance (r = −.55, p < .001), age is covaried in the following analysis. Additionally, because verbal ability as indexed by standardized PPVT-III scores is significantly different across language groups and significantly correlated to reaction time performance on the ANT (r = −.34, p = .002), z-transformed PPVT-III scores are included as a covariate in the following analyses. Although SES has been previously reported to be related to attention measures, in the current sample SES and ANT performance were not significantly correlated, and therefore this variable is not controlled in the reported analyses. However, due to the significant group differences in SES, the analyses were also conducted with parent education included as a covariate, and the same pattern of results reported here was found. Prior to conducting the following analyses, trials with RT faster than 250 ms were removed, as these were considered anticipatory responses. This resulted in removal of approximately 0.3% of trials. Only correct response trials were included in RT analyses. Response time and accuracy rates on the ANT were compared among the three language groups using ANCOVAs. The results of an ANCOVA comparing overall ANT RT (i.e., averaged across all flanker and cue conditions) across groups shows a main effect of language on RT, F(2, 74) = 3.17, p = .048, η2 = .08, when controlling for age and PPVT-III, which supports our hypothesis that groups would differ in attentional monitoring. To further test our hypothesis that bilinguals would outperform monolinguals, with the E-BIL group showing the largest advantage, pairwise comparisons were conducted and revealed that the E-BIL group's RT was significantly faster than the MON group (p = .02) and marginally faster than the L-BIL group (p = .08). However, the MON and L-BIL groups were not significantly different (p = .39). Although the language groups differed on RT, they were not significantly different in overall ANT accuracy, F(2, 74) = 0.51, p = .43. Thus, the E-BIL group does not display a time/accuracy tradeoff in their faster responding, which suggests that this group is responding most efficiently to the task. Unadjusted means for RT and accuracy for each ANT flanker and cue condition by group are presented in Table 2. Table 2. Means and standard deviations of RTs (correct trials only) and proportion correct trials for each trial type on the ANT by language group. Trial type RT Accuracy MON L-BIL E-BIL MON L-BIL E-BIL Incongruent flanker 940 (138) 940 (158) 896 (173) 0.94 (0.06) 0.94 (0.08) 0.92 (0.08) Congruent flanker 866 (124) 870 (143) 828 (173) 0.96 (0.06) 0.95 (0.07) 0.95 (0.07) Neutral flanker 838 (138) 831 (135) 779 (162) 0.96 (0.04) 0.96 (0.05) 0.94 (0.10) No cue 923 (141) 929 (152) 885 (170) 0.95 (0.05) 0.94 (0.08) 0.93 (0.10) Double cue 847 (116) 871 (145) 815 (166) 0.95 (0.04) 0.97 (0.05) 0.95 (0.05) Central cue 880 (125) 873 (138) 834 (170) 0.95 (0.05) 0.96 (0.07) 0.94 (0.06) Spatial cue 852 (122) 849 (146) 802 (177) 0.96 (0.06) 0.95 (0.06) 0.93 (0.12) MON: monolingual; L-BIL: late bilingual; E-BIL: early bilingual. Table options 3.3. ANT attentional networks 3.3.1. Alerting We conducted a repeated measures ANCOVA to compare the within-subject variable of RT (correct trials only) on no cue trials to RT on double cue trials and the between-subject variable of language group. This analysis served to compare the alerting network among the three language groups while controlling for age and English receptive vocabulary. Across all groups, participants responded faster to double cue trials than no cue trials, F(1, 74) = 18.62, p < .001, η2 = .20, supporting a significant effect of the alerting network in the expected direction (i.e., faster responding on cued trials). The interaction between alerting and language group was not significant, F(2, 74) = 0.059, p = .943, which indicates that all language groups demonstrated an equal alerting network effect. 3.3.2. Orienting Reaction time on center cue trials was compared to RT on spatial cue trials (a within-subject variable) in a repeated measures ANCOVA including language group as the between-subjects variable in order to measure the effect of the orienting network within each group. Again, these analyses controlled for effects of age and English receptive vocabulary. In all language groups, participants responded significantly faster to spatial cue trials than double trials, F(1, 74) = 5.45, p = .02, η2 = .07. Thus, children in all language groups demonstrated a significant effect of the orienting network, but there was no significant interaction between orienting and language group, F(2, 74) = 0.22, p = .80, suggesting that orienting effects were equal across language groups. 3.3.3. Conflict A third repeated measures ANCOVA was conducted to assess the conflict attentional network through a comparison of RT between incongruent and congruent flanker trials (within-subjects) among the three language groups (between-subjects) controlling for age and PPVT-III. Across groups, participants responded significantly faster to congruent trials than incongruent trials, F(1, 74) = 21.0, p < .001, η2 = .22, which is the expected effect of the conflict network. The crucial comparison for our hypothesis of bilingual advantages in attentional inhibition is a comparison across language groups, which revealed no significant interaction between conflict network and language group, F(2, 74) = 0.35, p = .71, indicating equal attentional inhibition across groups. 3.4. Early versus later bilingual comparisons Because the E-BIL and L-BIL group did not differ on PPVT-III scores, these groups were compared to one another in a series of ANCOVAs with only age included as a covariate. These analyses mirrored the previous ones, which included all three language groups and covaried both age and PPVT-III scores. Specifically, a comparison of overall ANT RT revealed that the E-BIL group responded significantly faster to ANT trials than the L-BIL group, F(2, 54) = 5.80, p = .019, η2 = .097, supporting a E-BIL advantage in attentional monitoring. Also in line with prior analyses, the E-BIL and L-BIL groups performed equivalently on ANT accuracy, F(2, 54) = 0.146, p = .704, and there were no significant interactions between group and any of the ANT network scores (all ps > .05). Thus, these analyses comparing only the E-BIL and L-BIL groups’ performance support the results from the three-group analyses that indicate an advantage in RT among the E-BIL children, but no group differences in ANT accuracy or network performance.