In the search for a locus of selection effects in bilingual word processing, we and others have turned to methods that can better elucidate the time-course and locus of cognitive processes involved in selecting and producing words in the both languages. The different accounts of language selection that we have already discussed would seem to make contrasting hypotheses about the time at which these effects should reveal themselves. Accounts which claim that language cues, operating at a conceptual level, can guide the selection process might suggest that effects of language selection should be seen early in the time course of processing, at least in a language production task. In contrast, accounts that propose an inhibitory process in response to competition from alternatives in both languages would be more consistent with a later locus of selection following activation and competition among within and between-language alternatives. While behavioral methods have been used to examine time-course issues by varying stimulus onset asynchronies and manipulating task constraints, behavioral approaches have the distinct disadvantage of relying on a discrete measure which may reflect the combined result of many stages and loci of processing. Response times on the order of hundreds of milliseconds may obscure the fine-grained series of events which underlie fluent language processing.
Unlike response time measures, event-related potentials (ERPs) can allow for evaluation of neurocognitive processes with millisecond resolution. This sensitivity to time-course, coupled with the fact that the ERP method is non-invasive, relatively inexpensive, and well-suited for use with a variety of populations, makes it the cognitive neuroscience method of choice when questions of time-course of processing are at issue3
. The ERP technique provides an invaluable opportunity to “observe” on-line processing of stimuli without requiring overt responses or additional decision processes needed for most behavioral measures. However, ERP recording can also be performed concurrently with many behavioral measures to allow for a direct comparison. This feature of the method is particularly useful, since recent evidence has suggested that ERPs may reveal aspects of L2 acquisition that are obscured in behavioral measures (e.g., McLaughlin, Osterhout, & Kim, 2004
; Tokowicz & MacWhinney, 2005
In the section below we focus on the literature pertaining to tasks involving language switching or mixing, primarily in contexts that should require processing to the phonological level, either because language production is required or because a task involves a decision based on the name of the stimulus. Thus, we will not review the growing ERP literature examining reading in the L2 (e.g., Alvarez, Holcomb, & Grainger, 2003
; De Bruijn, Dijkstra, Chwilla, & Schriefers, 2001
; Kotz, 2001
; Kotz & Elston-Güttler, 2004
; Kotz & Hernandez, 2004; Weber-Fox & Neville, 1996
) or reading of code-switched sentences (Moreno, Federmeier, & Kutas, 2002
; Proverbio, Cok, & Zani, 2004
). In addition, we have focused on studies in which the stimuli themselves should not cue the language choice (i.e., numerals or pictures, rather than words), but where task demands have been made explicit by the experimenter through external language cues. These studies most closely mirror the logic described in the behavioral approaches described above.
Previous ERP studies have suggested that both languages are activated even when bilinguals intended to speak only one of their languages, and that the time course and magnitude of nontarget language activation might be modulated by the relative proficiency of their two languages (Guo & Peng, 2006
). However, results concerning how far into processing both languages are active have been somewhat inconsistent. Similar to behavioral studies, ERP studies using cognates (Rodriguez-Fornells et al., 2005
; Christoffels, Firk, & Schiller, 2007
) have found evidence that phonological information of the nontarget language was activated in tasks which involved overt or tacit picture naming. In contrast, when using non-cognates in a picture-word interference paradigm, Guo and Peng (2005)
did not obtain significant activation of the L1 phonology when Chinese-English bilinguals spoke words in L2 although Guo and Peng (2006)
reported significant activation of the L1 translation in L2 production. It’s not clear how the different scripts for Chinese and English may account for the observed differences with respect to the activation of cross-language phonology. Despite the somewhat conflicting results, these ERP studies show consistent evidence that both languages are activated during speech planning and that activation of the non-target language may even spread to the phonological level at least for certain tasks or language pairings. Thus, a further concern of current ERP studies is how bilinguals can select the correct words in the correct language and whether they have to inhibit activation of the nontarget language.
One approach in the literature has been to evaluate the role of executive function as a possible locus to inhibit the activation of the non-target language. These studies have primarily used the switching paradigm or “language mixing” to examine this issue. The main finding of these studies has been that modulations of the N2 component, observed to be maximal over the frontal and central scalp, may reflect the cognitive control system in bilingual speech production. Effects on the N2 component have been interpreted as evidence for inhibitory effects in these tasks, since this component has also been found to be sensitive to response inhibition processes required for the performance of go/no-go tasks (e.g., Schmitt, Rodriguez-Fornells, Kutasm & Munte, 2001
). Jackson, Swainson, Cunnington, and Jackson (2001)
investigated executive control during language switching by recording ERPs during a visually cued naming task in which bilinguals named digits in either L1 or L2. Switch-related modulation of ERP components was observed on the N2 component, around 310 ms after stimulus onset over the parietal and frontal cortices. As illustrated in , switch trials were observed to increase this negative ERP component compared to non-switch trials. Importantly, this effect persists throughout the recording epoch, suggesting that it is not a transient effect, but rather reflects a process that remains active throughout the process of lexical selection. This effect over the frontal scalp was significant when switching from L1 to L2 but not when switching from L2 to L1, suggesting that switching into the non-dominant language from the dominant language required greater allocation of resources than making switches in the opposite direction. These results are consistent with claims that speaking in the L2 may require active inhibition of the L1. However, switches to the dominant L1 should not require such a demanding process.
Figure 2 Adapted data from Jackson et al. (2001) demonstrating the ERP language-switching effect observed in their digit-naming task. Bars on the x-axis indicate 100 ms intervals, and the component peaking just after 300 ms is described by these authors as the (more ...) Verhoef, Roelofs, and Chwilla (2006)
also examined switch costs using the ERP technique. In their study, highly proficient, but unbalanced, Dutch-English bilinguals were asked to perform a cued picture naming task. They manipulated the time available for preparing the picture’s name, allowing either a short interval of 500 ms or a long interval of 1250 ms. The behavioral data revealed a larger switch cost for L1 than for L2 at short intervals. However, for long intervals, switch costs were symmetrical for both languages. They argued that these results challenged the proficiency hypothesis proposed by Costa and Santesteban (2004)
who attributed symmetry differences in switching costs to language proficiency, since symmetrical switch costs could be observed in the very same unbalanced bilinguals when longer preparation intervals were provided. Furthermore, ERP data in this experiment also showed evidence for differential switching costs by preparation interval, reflected in modulations of the N2. However, these authors interpret the N2 to reflect attentional control mechanisms rather than response inhibition and suggest that the observed pattern reveals that more attentional resources were engaged for the long preparation trials, but this engagement was not fully maintained during long non-switch trials in the L1, thus contributing to the observed switch-cost patterns.
Christoffels, Firk, Schiller (2007)
recently examined bilingual language control using a language switching task. ERPs and naming latencies were recorded while unbalanced German–Dutch bilinguals named pictures. The bilinguals attended university in their L2 context and commonly switched between their languages in daily life. Picture names were trained in advance, and participants named pictures in both blocked and mixed language conditions. Additionally, Christoffels et al. manipulated cognate status between translation equivalents to examine phonological activation of the non-target language. Both behavioral results and ERP results revealed a cognate facilitation effect in both languages and for both blocked and mixed language naming, suggesting that phonological information from the non-target language was activated. However, in contrast to previous studies, equal switch costs were observed for both languages behaviorally, which the authors attribute to participants’ experience of commonly switching between languages in their daily life. In addition, a small switching effect in the ERP data was obtained for L1 but not for L2 during two windows interpreted to be consistent with the N2 (275–375 ms and 375–475 ms). However, in contrast to previous ERP studies on language switching, non-switch trials elicited more negative ERP waveforms than switch trials. Finally, both their behavioral and ERP data showed that the mixed language context had a strong effect on L1 and L2, as compared to the blocked language context. Specifically, both languages showed a greater negativity for non-switch trials in the mixed naming context as compared to trials in the blocked naming context in the earlier epoch, while in the later epoch a reversal of this effect was found for L1, but not for N2. Thus, blocked naming trials showed an enhanced negativity in the L1 in the later epoch. Taken together, Christoffels et al. argued that their results suggest that language control takes place via global inhibition of languages which acts specifically to change the availability of the L1.
In recent experiments (Guo, Misra, Bobb, & Kroll, 2007
; Misra, Guo, Bobb, & Kroll, 2007
), we have evaluated the time-course of lexical activation and the interaction of a bilingual’s languages during speech production ERPs. In two experiments unbalanced Chinese-English bilinguals named pictures while ERPs were recorded. Picture names were untrained and repetitions of each picture were carefully minimized and controlled. Participants named pictures in Chinese or English, depending on the picture’s background color. In addition, pictures were named at both short (250 ms) and long (1000 ms) delays, with ERPs evaluated only at the long delays to minimize artifact. The short delay naming trials were included to ensure the early preparation of responses and to allow for evaluation of immediate behavioral responses (using logic similar to that described by Jackson et al., 2001
). In one experiment, pictures to be named in Chinese and English alternated in a predictable fashion in a mixed naming paradigm. In another experiment, participants named pictures in one language in the first block and then named the same pictures in the other language in the second block. The effects of switching from one language to another were evaluated for each experiment, and mixed naming was compared to blocked naming between experiments. Results suggest that there is a processing cost associated with forcing both languages to be active, reflected in effects on the P200, N300 (consistent in latency with the “N2” described in other ERP language switching paradigms), and N400 (see ). However, in contrast to expectations based on the behavioral literature, in which costs to the first language are typically greater, processing costs were similar for both languages in most conditions. Also, similar to results from Jackson et al. (2001)
, effects of language mixing and language switching began early, but persisted throughout the recording epoch. Representative results from this paradigm are presented in .
Figure 3 Top panel: Grand average ERP waveforms at electrode site Fz for blocked and mixed picture naming trials for L1 and L2. Bottom panel: Grand average ERP waveforms at electrode site Fz for non-switch and switch picture naming trials for L1 and L2 (adapted (more ...)
Although there is little doubt that executive control is involved in tasks where people have to change frequently from one language to the other, there remains a question as to whether bilinguals have to inhibit the non-target language when speaking in only one of their two languages. Rodriguez-Fornells et al. (2005)
examined ERP and neuroimaging evidence for interference of phonological information from the bilinguals' non-target language and inhibition of this interference by the frontal cortex in a task where responses could be based on access to only one of a bilingual’s two languages. In their study, in order to avoid vocalization artifacts during EEG and fMRI data acquisition, a variant of the go/no-go picture-naming task was employed. German-Spanish bilinguals were required to respond when the name of the picture began with a consonant and to withhold a response for words starting with a vowel. The target language was changed on every block, but responses within a block did not require switching between languages. Stimuli were selected such that on half of the trials the names in both languages (Spanish and German) would lead to the same response (coincidence condition, e.g., vowel coincidence Esel - asno “donkey” or consonant coincidence Spritze - jeringuilla “syringe”), whereas on the other half responses were different for the two languages (noncoincidence condition, e.g., Erdbeere-fresa “strawberry”). Interference was evident behaviorally by slower response times (RTs) for incongruent than congruent trials for bilinguals as compared to monolingual controls. For the ERPs, an enhanced negativity with a frontal maximum was found between 300 to 600 ms (similar to the N2 described elsewhere) for incongruent as compared to congruent trials. These results provide evidence for cross-language interference at the phonological level in bilinguals. In addition, the results of fMRI data collected in the same paradigm showed two regions associated with the noncoincidence effect in bilinguals when compared to monolinguals: the left dorso-lateral prefrontal cortex (DLPFC) and the supplementary motor area (SMA). These neural areas have been associated with executive function in a variety of other tasks, suggesting that bilinguals recruit “typical ‘executive function’ brain areas” (Rodriguez-Fornells et al., 2005
, p. 427).
In a recent review of the literature, Rodriguez-Fornells, De Diego Balaguer, and Münte (2006)
further suggested that cognitive control executed by the left dorso-lateral prefrontal cortex is required in bilinguals, and the degree of activation of this mechanism might be related to the similarity of languages in use at the lexical, grammatical, and phonological levels. Abutalebi and Green (2007)
also reviewed fMRI evidence on bilingual language production and claimed that there is a single network mediating the representation of a person’s L1 and L2 and that cortical and subcortical structures generally associated with executive function such as LPFC (left prefrontal cortex) and ACC (anterior cingulate cortex) are engaged by bilinguals to inhibit lexical competition between languages in order to successfully select the intended language. The implication is that as a bilingual’s proficiency in L2 is increased, a reduction in prefrontal activity should be observed due to changes in the internal structure that will mediate the way in which control mechanisms are used.
A recent fMRI study by Wang et al. (2007)
reported that both the frontal gyrus and the ACC are involved in language switching, providing further evidence for the neural mechanism of the inhibition in bilingual word production. Another recent fMRI study (Abutalebi, Annoni, Zimine, Pegna, Seghier, Lee-Jahnke, et al., in press
) investigated whether the neural network underlying language control in bilinguals differs from that involved in general executive functions that control switching between competing tasks within language. They found that language control processes engaged in contexts during which both languages must remain active recruited the left caudate and the ACC in a manner that could be distinguished from areas engaged in within-language task switching. Taken together, the evidence from both ERPs and other neuroimaging methods support a view in which brain areas associated with inhibitory processing function to aid bilinguals in selecting the appropriate language alternative.