There is no coherent evidence for a bilingual advantage in executive processing
Highlights
► Bilinguals are compared to monolinguals on 19 indicators of executive processing. ► We report no bilingual advantages and one bilingual disadvantage. ► Indicators of the same component of executive processing show no convergent validity. ► Bilingualism does not enhance a domain-independent ability to engage in executive processing.
Introduction
Fluent bilinguals have extensive experience in language switching that involves monitoring the situation to select the appropriate language, activating the selected language, and inhibiting the other language. This extensive practice may lead to an enhanced ability in cognitive control that is general and not language specific. Indeed several investigators have reported a bilingual advantage in tasks that seem to require executive processing (EP), that is, the ability to monitor goal-setting cues, to switch attention to goal-relevant sources of information, and to inhibit those that are irrelevant or competing (Bialystok, 2006, Bialystok et al., 2004; Bialystok, Craik, & Luk, 2008; Costa, Hernández, & Sebastián-Gallés, 2008). However, as Hilchey and Klein (2011) enumerate in their recent comprehensive review, there have also been many failures to observe a bilingual advantage.
Although the construct of EP continues to evolve, it is often viewed as a set of interrelated component processes all involving the prefrontal cortex (PFC) with each component recruiting other constellations of cortical function. This componential framework allows for the possibility that the related components have some degree of anatomical and functional independence. Ideally an investigation of bilingual advantages in EP should be grounded in a specific conceptual framework that elucidates the nature of executive processes and guides operational definitions for manipulating and measuring them.
As discussed in Section 1.1.1 there is very little evidence that the measures and tasks typically used to test for differences between bilinguals and monolinguals in inhibitory control are tapping into the same general ability. This is somewhat surprising because the seminal work of Miyake and Friedman (Miyake et al., 2000, Friedman et al., 2008, Miyake and Friedman, 2012) shows evidence for three components of EP: updating, switching1, and inhibition. In their large-scale studies these investigators conducted confirmatory factor analyses (CFA) using measures from three different tasks for each of the three hypothesized components. For each latent variable (viz., updating, switching, inhibition) the three observed variables (e.g., color, number, category) linked to the same latent variable (e.g., switching), are correlated with one another, and result in standardized factor loading ranging from .40 to .71. However, it is instructive to note that three of the four lowest factor loadings are for the inhibition tasks. At the higher level the three latent variables (updating, switching, inhibition) correlate with one another and this is consistent with the assumption that each contributes to a common EP. When the same data are reanalyzed with a second order CFA where the three latent variables (updating, switching and inhibition) are nested under a common EP latent variable, the nine observed measures all load on common EP with two of the nested components (updating and shifting) still making unique contributions. In summary, these studies support the theory of a general EP ability with separable updating and switching components and an inhibition component that is not separable and that is weakly linked to the general EP ability.
Most tests for a bilingual advantage in EP in adults have focused on only two of three components studied by Miyake and Friedman (viz., switching and inhibition) and frequently employ tasks not tested by Miyake and Friedman. As discussed in the next section this different mix of specific tasks has resulted in less convergent validity.
In Bialystok et al.’s seminal article primary focus was placed on the proposition that bilinguals are better at selecting goal-relevant information and suppressing competing and distracting information. Bilinguals exercise this type of control at two levels: (1) at a high level of goal setting when one language is selected and the other is inhibited and (2) at a lower level where the lexical forms of the goal relevant language are activated and the competing translation equivalents are inhibited (e.g., Green, 1998). If this extensive practice hones a general ability, not specific to language, then bilinguals should be less vulnerable to interference in nonlinguistic tasks.
The standard marker of inhibitory control is the difference in mean response time between trials that require conflict resolution compared to those that do not. In the Stroop, Simon, and the Eriksen flanker tasks conflict occurs on a subset of trials because a potent but task-irrelevant stimulus is often paired in an incongruent manner with the task-relevant stimulus. Performance can be enhanced by boosting the influence of the goal-relevant information relative to that of the competing information. The effectiveness of this control can be inferred from differences in response time between congruent trials and incongruent trials with smaller interference effects implying superior ability. In a recent and comprehensive review of bilingual advantages in EP Hilchey and Klein (2011) conclude that evidence for a bilingual advantage in inhibitory control is rare in both children and young adults.
In the present studies inhibitory control should play a role in the Simon task (Studies 1–3), antisaccade task (Study 1), and Eriksen flanker task (Study 3). Thus, the critical test for a bilingual advantage in inhibitory control is the presence of a significant Group (bilingual versus monolingual) × Trial Type (congruent versus incongruent) interaction with the pattern of interaction showing a larger interference effect for the monolingual group. Compelling evidence for a bilingual advantage in inhibitory control would demonstrate significant advantages in two or more tasks requiring inhibitory control and further show that the interference effects correlate with each other as one would expect if each task includes a common component associated with a general ability to exercise inhibitory control.
Although researchers investigating the bilingual advantage have employed several different tasks that should require inhibitory control the same set of matched bilinguals and monolinguals typically participate in only a single task and, hence, provide only a single measure of this component. This is less than satisfactory because the three most-frequently used nonlinguistic interference tasks do not correlate with one another. Stins, Polderman, Boomsma, and de Geus (2005) tested a group of 12-year old children using the flanker, Simon, and Stroop interference tasks. The correlations between these tasks were all smaller than +0.20 and nonsignificant. Fan, Flombaum, McCandliss, Thomas, and Posner (2003) reported that the flanker, Simon, and Stroop tasks all activated the AC and the left PFC, but again the interference scores were uncorrelated. Each task also activated areas unique to that task. They conclude that “The behavioral and fMRI results taken together seem to argue against a single unified network for processing conflict, but instead support either distinct networks for each conflict task or a single network that monitors conflict with different sites used to resolve the conflict” p. 42. Likewise, Kousaie and Phillips (2012a) using a sample of 51 young adults report no significant correlations between the Stroop, Simon, and Eriksen flanker tasks. Keye, Wilhelm, Oberauer, and van Ravenzwaaij (2009) report a structural-equation analysis of the data obtained from 150 adults who participated in both the flanker and Simon task. There was no association between the two interference tasks. Likewise, Humphrey and Valian (2012) using a sample of 208 young adults report no significant correlation between the Simon and flanker effect. In contrast to all of the above comparisons involving the Simon, flanker, or Stroop tasks; Unsworth and Spillers (2010) in a study using college students do report a significant correlation (r = +0.17, p < .05) between a flanker and Stroop task. The weak correlation achieves statistical significance because the n of 181 is very large.
The fact that Friedman et al. (2008) found evidence for an inhibition component is at odds with the absence of significant correlations reported above for the Simon, Stroop, and flanker tasks. At first look this may appear puzzling since the nature of the inhibition required in the three tasks used by Friedman et al. (antisaccade, stop signal, and Stroop) appears to be as varied, if not more so, than the type of inhibition required across the Simon, flanker, and Stroop tasks. Although these tasks always produce robust interference effects, individual differences seem to be another matter. Friedman et al. (2008) point out that the individual difference correlations in interference tasks are usually low and seem sensitive to task variations. The present studies will provide additional tests of convergent validity between the Simon and antisaccade tasks (Study 1) and between the Simon and flanker tasks (Study 3).2
Given that cognitive control dynamically changes in response to changing goals and changing affordances another important type of executive process is monitoring one’s performance, internal states, and current environment.3 This possibility was first proposed by Bialystok et al. (2004) in the context of the Simon task: “The advantage for bilinguals, therefore, may be not in the enhanced ability to inhibit the misleading spatial cue but in the ability to manage attention to a complex set of rapidly changing task demands” p. 292. Costa, Hernández, Costa-Faidella, and Sebastián-Gallés (2009) have also focused more on monitoring than inhibitory control: “The bilingual advantage in overall RTs may reveal the better ability of bilinguals to handle tasks that involve mixing trials of different types: bilinguals would be more efficient at going back and forth between trials that require implementing conflict resolution and those that are free of conflict” (p. 136). Costa et al. have also updated the underlying rationale for why managing two languages should enhance conflict monitoring: “This better functioning of the monitoring system may come about because of the bilinguals’ need to continuously monitor the appropriate language for each communicative interaction. That is, proper communication in bilingual settings involves the monitoring of the language to be used depending on the interlocutor(s) language knowledge”. (Costa et al., 2009, p. 136).
Differences in monitoring between bilinguals and monolinguals have been inferred from two different measures. The arguably better indicator is the difference in mean RT between a pure block of easy choice RT trials that involve no conflict and the mean RT on the congruent trials in a block that mixes both congruent and incongruent trials together. This difference should reflect the cost of having to monitor for the presence of conflict, apart from the need to actually resolve the conflict on incongruent trials. Ideally, the two groups would show comparable RTs on the pure blocks of easy trials showing that they are matched in terms of speed when there is no need to monitor for potential conflict. A control condition of no-conflict trials is not always included in the experimental design. Under these circumstances a bilingual advantage in monitoring has been inferred if bilinguals are faster than monolinguals on the congruent trials.
Another special experience of bilinguals is that they get lots of experience in switching per se as they shift from one language to the other. Language switching is complicated by the common assumption that both lexicons are connected to the same conceptual system and that consequently switching leads to the need to inhibit the translation equivalents in the non-target language. Setting this complication aside, the ability to switch from one task to a completely different task is assumed to require an executive process (“switching”) that is functionally separable from inhibitory control and monitoring and involves unique areas in addition to the anterior cingulate (AC) and PFC (Collette et al., 2005).
Tests for bilingual advantages in switching include three critical conditions: (1) pure blocks in which participants perform the same task on every trial, (2) mixed-block trials that involve a switch from one task to the other, and (3) mixed-block trials that repeat the same decision. For example, in a mixed block Prior and MacWhinney (2010) presented a precue on every trial that signaled to the participant whether to make a binary color decision with two fingers of one hand or a binary shape decision with two fingers of the other hand. The difference between repeat trials and switch trials in the mixed block was used as an indicator of “switching costs” whereas the difference between repeat trials and pure trials was used as an indicator of “mixing costs”. The results showed a bilingual advantage in switching costs, but not mixing costs. These results, in isolation, are coherent if switching and monitoring are two separate EP components and bilingualism provides special and domain-independent experience only for switching.
The main purpose of this study is to use the framework and strategy presented above to determine if there is a coherent set of evidence for a bilingual advantage in EP. Three large samples of participants complete multiple tasks that require EP. Each task yields multiple comparisons that are typically assumed to be associated with specific components of EP. More specifically, the analyses of each sample, and the combined sample, enable multiple tests for bilingual advantages in inhibitory control, monitoring, and switching. Furthermore, by examining the correlations between performance variables one can assess the assumption that these markers are converging on domain general measures of cognitive control.
Section snippets
Methods
The data reported are drawn from three studies. Each study consisted of a series of seven or eight activities that required 1.5–2 h to complete. Table 1 shows the sequence of activities for each study.
Simon task
The data for one bilingual was eliminated in Study 3 because her mean RT was more than 300 ms longer than the next slowest participant. The trimmed correct RTs were analyzed using a mixed design ANOVA with group (bilingual versus monolingual) and congruency (congruent versus incongruent) as factors. In testing for bilingual advantages the main effect of congruency (i.e., the magnitude of the Simon effect) plays a less important role than the main effect of group and the Group × Congruency
Inhibitory control
In their 2011 review of 31 experiments Hilchey & Klein concluded that “The absence of a bilingual advantage in…” children and young adults… “is simply inconsistent with the proposal that bilingualism has a general positive effect on inhibitory control processes” p. 629. Two reports by Kousaie and Phillips, 2012a, Kousaie and Phillips, 2012b reinforce this conclusion. The 2012a study uses a multiple-task approach similar to ours and found no behavioral differences between groups of young adults
Correlations between indicators of the same executive process
The argument advanced in the introduction was that a coherent demonstration of a bilingual advantage in inhibitory control, or any other EP component, would show the advantage in two different tasks and that the markers for the two tasks would correlate with each other. If the two indicators did not correlate with one another, then the bilingual advantages were likely to be task specific rather than providing evidence of a shared and domain-general ability. Although the present set of three
Evaluation of empirical evidence
The empirical focus of this article is on bilingual advantages in inhibitory control, monitoring, and switching obtained with young adults14 engaged in nonlinguistic interference tasks. There are two perspectives on reconciling the reports of significant bilingual advantages with the many failures to replicate.
Why might there be no bilingual advantages in executive processing?
The idea that a bilingual’s language experience generalizes and enhances EP is very attractive. However, in the hindsight of the lack of coherent evidence favoring a bilingual advantage in EP, it is easy to see that there is a sequence of three assumptions that have to be sustained in order for this received story to be true.
Conclusions
The research findings testing for bilingual advantages in EP do not provide coherent and compelling support for the hypothesis that the bilingual experience causes improved EP. Because individual studies tend to use only one task and use only one indicator for each EP component there is usually no test of convergent validity. Those studies that have used multiple tasks show no bilingual advantages and little or no convergent validity. Matching language-groups on factors that influence the
Acknowledgments
We thank the following members of the LACE (Language, Attention, & Cognitive Engineering) laboratory for their individual contributions to this project: Edgar Alcaine, Nick Alvarez, Tavi Alvarez, Kirstin Anderson, Olimpia Andrade, Jack Darrow, Frank Du, Eddie Ferrero, Lynne Freeman, Jenesis Imai, James Keenan, Jessica Koernke, Yunyun Liu, Brenda Mejia, Sati Morgan, Lindsay Pellichia, Oliver Sawi, Monique Toledo, and Carlos Urtecho.
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