Electrophysiological explorations of the bilingual advantage: evidence from a Stroop task

Abstract

Bilinguals have been shown to exhibit a performance advantage on executive control tasks, outperforming their monolingual counterparts. Although a wealth of research has investigated this 'bilingual advantage' behaviourally, electrophysiological correlates are lacking. Using EEG with a Stroop task that manipulated the stimulus onset asynchrony (SOA) of word and colour presentation, the current study addressed two facets of the bilingual advantage. The possibility that bilinguals experience superior conflict processing relative to monolinguals (a 'conflict-specific advantage') was investigated by comparing behavioural interference effects as well as the amplitude of the Ninc, a conflict-related ERP component occurring from approximately 300-500 ms after the onset of conflict. In contrast, the hypothesis that bilinguals experience domain-general, conflict-independent enhancements in executive processing (a 'non-conflict-specific advantage') was evaluated by comparing the control condition (symbol strings) between groups. There was some significant, but inconsistent, evidence for a conflict-specific bilingual advantage. In contrast, strong evidence emerged for a non-conflict-specific advantage, with bilinguals demonstrating faster RTs and reduced ERP amplitudes on control trials compared to monolinguals. Importantly, when the control stimulus was presented before the colour, ERPs to control trials revealed group differences before the onset of conflict, suggesting differences in the ability to ignore or suppress distracting irrelevant information. This indicates that bilinguals experience superior executive processing even in the absence of conflict and semantic salience, and suggests that the advantage extends to more efficient proactive management of the environment.

Figure 1. Example trial sequence.

Example of the trial sequence for a) a −400ms SOA congruent trial; and b) a 0ms SOA control trial.

Figure 2. Mean RTs (ms) for each group and language.

Figure 3. Behavioural data.

Behavioural a) Stroop; b) interference; and c) facilitation effects, and d) the average RTs for control trials averaged over SOAs, for the monolinguals, bilingual L1 and bilingual L2.

Figure 4. Monolingual ERP data.

Monolingual English ERP waveforms at Cz and Pz for the a) 0 ms SOA; and b) −400 ms SOA. Significant effects from the running t-tests, within the N_inc windows (shaded area), are indicated in bars underneath. Topographic maps show the N_inc components (incongruent vs. congruent), with black dots indicating electrodes that show significant differences (p<0.05) between the average amplitudes of incongruent and congruent trials across the specified window.

Figure 5. Bilingual L1 ERP data.

Bilinguals L1 Chinese waveforms at Cz and Pz for the a) 0 ms SOA; and b) −400 ms SOA. Significant effects from the running t-tests, within the N_inc windows (shaded area), are indicated in bars underneath. Topographic maps show the N_inc components, plus electrodes showing significant differences (p<0.05) between incongruent and congruent trials across the specified window.

Figure 6. Bilingual L2 ERP data.

Bilinguals L2 English waveforms at Cz and Pz for the a) 0 ms SOA; and b) −400 ms SOA. Significant effects from the running t-tests, within the N_inc windows (shaded area), are indicated in bars underneath. Topographic maps show the N_inc components, plus electrodes showing significant differences (p<0.05) between incongruent and congruent trials across the specified window.

Figure 7. ERP difference waves.

Difference waves (incongruent minus congruent) at Cz and Pz for each group in the a) 0 ms; and b) −400 ms SOAs. Significant differences between the groups, as evaluated by running t-tests, are plotted in bars below. Shaded regions show the approximate N_inc windows for each SOA.

Figure 8. Comparison of ERP waveforms for control trials.

Waveforms of control trials at Cz and Pz for each group in the a) 0 ms SOA; and b) −400 ms SOA. Significant differences between the groups, based on running t-tests, are indicated in bars underneath. Shaded regions indicate the windows of significant group differences, and topographic maps show the bilingual – monolingual differences (bilinguals averaged over L1 and L2) averaged across these windows; electrodes showing statistically significant differences (p<0.05) between the bilinguals and monolinguals across the specified window are indicated by a black square.