4. Discussion
Language control exhibits its adaptability in two primary ways: firstly, it can flexibly implement a set of cognitive control processes to meet the demands of varying interactive contexts; secondly, it can dynamically adjust to subsequent cognitive control processes, a phenomenon commonly referred to as the cross-task conflict adaptation effect. However, the available evidence is mostly incomplete and far from conclusive. Manipulating switching contexts that approximates switching situations in bilinguals’ daily lives, the present study aims to investigate how the varying switching contexts modulate cross-task conflict adaptation effect. Our findings yielded the following insights: 1) natural switching contexts caused a smaller N1 effect in switch-incongruent trials compared to forced and voluntary switching contexts; 2) the cross-domain interaction on the P3 effect, however, revealed an atypical flanker effect in forced switching contexts only, and P3 amplitude of incongruent trials in forced switching contexts was smaller than both natural and voluntary switching contexts; 3) further, the prediction models revealed robust brain-brain relationships between language control and cognitive control in the forced switching context only for both congruent and incongruent trials; 4) specification curve analysis confirmed this predictability generalized to the behavioral performance (i.e., RTs of congruent trials). Collectively, our findings indicate the modulation effect of switching contexts on cross-task adaptation effects. Especially, the forced switching context profoundly exhibited a predictive effect on the domain-general control processing in both conflict and non-conflict situations.
Our first research question concerned whether cross-task conflict adaptation effect varied among distinct switching contexts. Although no behavioral differences between the three contexts were found, the results of the N1 and P3 components confirmed the differential effects of the three switching contexts on domain-general cognitive control, especially the forced and natural switching contexts. More specifically, for N1 effect, switch-incongruent trials in the natural switching context showed a smaller N1 compared to both the forced switching context and voluntary switching context. However, there was no significant difference between voluntary and forced switching context. Instead of the N2 effect observed in previous studies (Bartholow et al., 2005; Folstein & Van Petten, 2008; Jiao et al., 2019; Kopp & Mattler, 1996; Van Veen & Carter, 2002), a negative ongoing waveform in the early time window of 120-220 ms was elicited in our study, which we preferred to define as N1 effect. The N1 effect is assumed to be indicative of persistent covert visual attention control and sensitive to the level of visual attention intensity on the target stimuli (Di Russo). The greater N1 amplitude for incongruent trials in forced and voluntary switching contexts relative to corresponding condition in natural switching context implied more attention allocated to the conflict situation in the contexts of forced and voluntary switching (Luck et al., 2000). This concentrated attention on conflict situation may be indicated as an efficient harbinger of conflict adaptation (Liu et al., 2022b; Yuan et al., 2021). The reduced N1 effect in switch-incongruent trials in natural switching contexts compared to forced and voluntary switching contexts suggests that brain activity in the former indeed stays at a lower functional level.
Furthermore, the P3 amplitude of incongruent trials in the forced switching context was smaller than both natural and voluntary switching contexts. Importantly, an atypical flanker effect was detected on the P3 effect in the forced switching context, whereas such effect was not found in the other two switching contexts. This pattern is consistent with previous studies (Hsieh & Lin, 2014; Jiao et al., 2020a; Wu & Thierry, 2013). In the pure flanker task, incongruent trials induced a larger P3 effect compared to congruent trials, which has been interpreted as a ‘leakage’ of response inhibition ability on the trials involving conflict (Groom & Cragg, 2015). Nevertheless, our findings indicated that constant monitoring of the appropriate language and the local inhibition of the non-target language in forced switching context resulted in a functional generalization of heightened inhibition and monitoring capacities engaged in subsequent flanker trials. Unlike the typical flanker effect, which indicates response competition in response selection, the reversed flanker effect mirrors the enhancement of perceptual processes (Hsieh & Lin, 2014; Rouder & King, 2003). These findings suggest that forced switching may alleviate some cognitive demands introduced by incongruent trials by strengthening the perceptual differentiation to incongruent trials relative to congruent trials. Bilingual participants were able to adaptatively generalize their conflict monitoring and language control to domain-general cognitive control, facilitated by more recruitment of inhibition and conflict monitoring in the switching context. Similarly, in a dual-EEG study by Liu et al. (2022), a reversed flanker effect was revealed in mixed-language contexts, but not in single-L1 contexts. The authors thus argued that the adaptation of language control was limited to a situation involving similar control mechanisms.
Our second research question examined how the adaptive patterns manifested in flanker trials. Concerning our predictive model results, however, it is plausible to demonstrate that the cross-task conflict adaptation effect is not limited to conflict situations. A set of RR models using coding activities of language switching tasks across the three contexts to predict processing in the flanker task revealed that language control processing engaged in forced switching contexts displayed the earliest and most stable (of all predictive models) positive relationships in congruent trials. The positive correlation emerged at -10ms before the congruent stimuli were presented, lasting until 500ms. This suggests that forced switching offers cognitive assistance for subsequent flanker tasks during early processing stages. In the models for incongruent trials, negative brain-brain predictions were detected starting at 272ms in forced switching context, suggesting the effective enhancement of forced switching on conflict resolution. The prediction outcomes indicate varying effects and strengths of forced switching contexts on congruent and incongruent trials. Although in this context, which relies on top-down control mechanisms such as conflict monitoring, resolution, and inhibitory control, the contextual influence differs notably between congruent and incongruent trials. The early emergence of predictive relationships when processing congruent trials suggests that the continuous monitoring of two activated languages in forced switching contexts is swiftly generalized to executive control processing, while inhibition exhibited a later effect. The specification curve results corroboratively showed significant positive relationship between the predicted values in the forced switching context and RTs of congruent trials. However, no significant relationships were detected for other specifications. It seems that the cross-task adaptation of language control to domain-general cognitive control is reasonably general and not limited to conflict situations.
As afore-introduced, previous studies have not reached a consensus. Some studies suggest that this adaptation shifting was limited to conflict situations (Liu et al., 2022b; Wu & Thierry, 2013), i.e., incongruent-specific facilitation, whereas other work has reported a wider advantage for both incongruent and congruent trials (Jiao et al., 2019; Liu et al., 2016). For example, in a behavioral study by Liu et al. (2016), the performance of a group of bilinguals in a nonlinguistic “faces” task was measured before and after completing a language-switching task. Results showed that participants’ response inhibition and interference suppression were both improved after language switching. Similar results have been shown in research on code-switching. For instance, Alder and Valdés Kroff (2020) used a cross-task conflict-adaptation paradigm to examine whether reading code-switched sentences would influence performance on subsequent incongruent trials, and confirmed a behavioral enhancement introduced by reading code-switched vs. non-code-switched sentences. The present study, on the basis of previous research efforts, causally provides predictive evidence for the cross-task adaptation of language control to domain-general cognitive control. However, considering the predictive effect in congruent trials were stronger and earlier than incongruent trials, our findings, further substantiated through SCA analysis (a significant brain-behavior relationship between predict value of congruent trials and RTs in forced switching context only), suggest that the promoting effect of forced switching on domain-general control varies between congruent and incongruent conditions. The cross-task adaptation from language control to domain-general control dynamically adjusts the engagement of executive functions to adapt to current task demands.
This finding is consistent with recent neuroimaging studies revealing the overlap between language control and executive control, such as conflict detection and monitoring, along with shifting and inhibitory control (Abutalebi et al., 2012; D’Souza, & D’Souza, 2016; Hwang et al., 2017; Wu et al., 2019; Yeo et al., 2016; Yuan et al., 2021). In a study comparing the performance of English monolinguals, unimodal bilinguals, and bimodal bilinguals (who acquired both English and American Sign Language) in a flanker task, Emmorey and colleagues (2008) found that unimodal bilinguals performed faster than monolinguals and bimodal bilinguals, and that performance of bimodal bilinguals did not differ from monolinguals. These findings traced the bilingual advantage in executive control to bilinguals’ experience in switching languages more frequently. Subsequent research has shown that a bilingual advantage only emerged when the two languages were intermittently used in mixed-language contexts compared to single-language contexts (Alder and Valdés Kroff, 2020; Jiao et al., 2019; Jiao et al., 2020a; 2020b; Liu et al., 2016; Liu et al., 2022b; Wu and Thierry, 2013; Yuan et al., 2021). The constructs of language control and cognitive control share multiple subsets, such as conflict monitoring and inhibition of prepotent interference (Abutalebi et al., 2012; Blanco-Elorrieta & Pylkkänen, 2016, 2017; Emmorey et al., 2008; Liu et al., 2022b). Bilinguals need to trigger a stricter top-down control process depending on the needs of forced switching contexts. Building upon these findings, our study further indicates that different switching contexts distinctly modulates the components engaged in a subsequent executive control task.