Bilingual Control Mechanisms and Influencing Factors in Voluntary Language Switching

ZHUANG Binyuan¹, YANG Jing¹
(¹ School of International Studies, Zhejiang University, Hangzhou 310058, China)

Abstract

Voluntary language switching refers to language shifts that bilinguals spontaneously produce. Compared with forced language switching, the switching costs incurred in voluntary switching are significantly reduced—and may even disappear—when lexical access is high and language choice is unrestricted. Neuroimaging research indicates that voluntary language switching involves less activation of inhibitory control networks while additionally recruiting brain mechanisms associated with self-initiated intention and spontaneous behavior. The bilingual control mechanisms underlying voluntary switching are closely tied to second language proficiency, contextual factors, and individual executive control abilities. Future research should integrate individual and environmental factors to construct more refined dynamic models, combine real-world behavioral patterns with laboratory measures to deepen exploration of neural mechanisms, and conduct longitudinal studies examining the interaction between production and comprehension. Such efforts will facilitate the translation of voluntary language switching into cognitive advantages, communicative functions, and learning efficiency.

Keywords: voluntary language switching, forced language switching, inhibitory control, context, executive control

Bilinguals must select a language to express themselves and, in bilingual contexts, switch between languages to communicate with interlocutors from different linguistic backgrounds. Forced language switching is guided by external cues (such as the interlocutor), whereas voluntary language switching is spontaneously generated by bilinguals themselves. Although both are common in bilingual life, research has predominantly focused on forced language switching, employing language cues (e.g., colors, shapes, flags, facial images) to instruct bilinguals to respond in a designated language. Decades of research have revealed that forced language switching in both bilingual production (e.g., \cite{Costa & Santesteban, 2004}; \cite{Ma et al., 2016}; \cite{Zhuang et al., 2023}) and comprehension (e.g., \cite{Declerck & Grainger, 2017}; \cite{Olson, 2017}) incurs switching costs—longer naming latencies or lower accuracy compared to language repetition.

The Inhibitory Control Model \cite{Green, 1998} and the Bilingual Interactive Activation Plus (BIA+) Model \cite{Dijkstra & van Heuven, 2002} posit that bilingual language processing involves activation of both target and non-target languages. To accurately and efficiently select lexical items from the target language, bilinguals must inhibit the non-target language. Switching costs reflect a top-down inhibitory control mechanism employed to resolve language conflict \cite{Christoffels et al., 2007; Green,1998; Ma et al., 2016}. Many studies have also found that switching costs are typically asymmetrical in non-balanced bilinguals: switching from the dominant language (usually the first language, L1) to the non-dominant language (L2) is less costly than switching from L2 to L1 \cite{Costa & Santesteban, 2004; Meuter & Allport, 1999}. The Inhibitory Control Model explains this by proposing that the dominant language is activated by default to a higher degree in non-balanced bilinguals. Consequently, naming in the non-dominant language requires stronger inhibition to control automatic activation of L1. When switching back to L1, reactivating the previously more strongly inhibited language takes longer, resulting in greater switching costs.

Forced language switching tasks have greatly advanced our understanding of bilingual language switching mechanisms. However, spontaneous voluntary language switching in natural communication clearly differs from forced switching based on explicit language cues. This article systematically integrates recent advances in voluntary language switching research, elucidates the unique role of volition in bilingual control mechanisms, and highlights the complex regulatory effects of individual and contextual factors. Exploring these issues can deepen theoretical understanding of the cognitive and neural mechanisms of language switching and promote the evolution of bilingual processing models.

1.1 Experimental Paradigms for Voluntary Language Switching

Previous bilingual switching research has typically employed forced language switching tasks, such as picture or digit naming \cite{Costa & Santesteban, 2004; Ma et al., 2016; Meuter & Allport, 1999}, phrase naming \cite{Blanco-Elorrieta et al., 2018}, and sentence reading tasks \cite{Gollan & Goldrick, 2018; Li et al., 2024} to investigate bilingual language control mechanisms. Bilinguals must select the designated language based on task-provided cues (e.g., picture colors, flags, facial images), making language selection determined by external factors. This paradigm simulates scenarios where bilinguals switch languages according to their interlocutor's linguistic background. To examine how cultural-linguistic context influences switching, some researchers have further modified task formats by adding racial faces or cultural objects that match or mismatch the naming language \cite{Li et al., 2013; Liu et al., 2021; Zhuang et al., 2023}. Although these modifications enhance ecological validity by considering naturalistic cues, they do not alter the essential nature of forced language switching.

In contrast, voluntary switching tasks typically present stimuli without cues, instructing participants to freely choose the naming language, or simultaneously display bilingual cues representing both languages before each stimulus, allowing bilinguals to freely select their intended language on any given trial \cite{Blanco-Elorrieta & Pylkkänen, 2017; de Bruin et al., 2018; de Bruin & Martin, 2022; Kleinman & Gollan, 2016; Zhu et al., 2022}. This paradigm simulates bilingual language switching behavior that occurs spontaneously without interlocutor constraints—when both conversation partners share a bilingual background, they can freely alternate between languages \cite{Myers-Scotton & Lake, 2020; Poplack, 1980} without relying on external language cues. For example, \cite{Blanco-Elorrieta and Pylkkanen (2017)} created a free bilingual dialogue environment where participants first viewed an image of a video call with a conversation partner, then performed a picture-naming task. Participants were explicitly informed before the experiment that the interlocutor was an Arabic-English bilingual. Given this bilingual background, participants could freely choose between Arabic and English for naming. The results showed that highly proficient Arabic-English bilinguals exhibited no switching costs during naming, demonstrating that voluntary language switching fundamentally differs from forced switching.

1.2 Language Switching Costs in Voluntary Language Switching

\cite{Gollan and Ferreira (2009)} conducted the first study on voluntary language switching in bilinguals, introducing the voluntary task-switching paradigm \cite{Arrington & Logan, 2004} into language switching research. They asked Spanish-English bilinguals to freely choose either language for picture naming under unrestricted-choice instructions, examining whether voluntary switching would produce costs or benefits. The study found that voluntary switching incurred costs for both younger and older bilinguals. Unlike the pattern observed in forced switching, voluntary switching costs were symmetrical across languages and were not modulated by participants' L2 proficiency. Additionally, \cite{Kleinman and Gollan (2016)} created a bottom-up voluntary switching condition by simultaneously presenting flags representing Spanish and English, allowing participants to freely choose a language when each picture first appeared and maintain that choice for subsequent repetitions of the same picture. The results showed that Spanish-English bilinguals exhibited no significant switching costs in this bottom-up voluntary task, suggesting that consistently selecting more accessible lexical items in voluntary switching is cost-free.

To further reveal the unique characteristics of bilingual control mechanisms in voluntary switching, recent studies have directly compared voluntary and forced switching patterns and costs. For example, \cite{Jevtović et al. (2020)} included both voluntary and forced language switching tasks to investigate the mechanisms of voluntary switching in highly proficient Spanish-Basque bilinguals. The results showed that switching costs persisted in the voluntary switching task alone; however, when voluntary and forced switching occurred concurrently, costs for switching to Basque were smaller under voluntary than forced conditions, and overall reaction times were shorter for voluntary switching. Thus, although many studies report switching costs in voluntary switching, the cognitive load appears reduced compared to forced switching, manifesting as diminished switching costs. Furthermore, task settings may influence the difficulty of lexical access in voluntary switching, thereby modulating switching costs.

1.3 Theoretical Explanations of Voluntary Switching Costs

The Inhibitory Control Model provides a theoretical foundation for switching costs: because bilinguals activate both languages during processing, they must inhibit the non-target language when selecting the target language. When switching back to a previously inhibited language, overcoming this inhibition requires additional time—the switching cost. However, most studies finding switching costs have used forced switching tasks with language-specifying cues \cite{Christoffels et al., 2007; Declerck & Philipp, 2015; Ma et al., 2016; Meuter & Allport, 1999; Zhuang et al., 2023}. Many studies have found significant switching costs in forced language switching \cite{de Bruin et al., 2018; Gollan et al., 2014; Jevtovi et al., 2020; Liu et al., 2020; Mooijman et al., 2024; Sánchez et al., 2022; but see Mosca et al., 2022}, whereas in voluntary switching, these costs are significantly reduced or eliminated entirely \cite{Blanco-Elorrieta & Pylkkänen, 2017; Jevtović et al., 2020; Kleinman & Gollan, 2016; Zhu et al., 2022; but see Geng et al., 2024; Zhang et al., 2024}.

Why does voluntary switching show this differential pattern? Many researchers suggest that the Adaptive Control Hypothesis \cite{Green & Abutalebi, 2013} and its distinction between switching contexts (single-language, dual-language, and dense code-switching contexts) may provide insight \cite{de Bruin et al., 2018; Jiang et al., 2024; Lai & O'Brien, 2020}. In single-language and dual-language contexts, bilinguals typically select the appropriate language based on their interlocutor's linguistic background (e.g., flexibly switching when communicating with monolingual work partners or family members who use different languages). To effectively avoid interference from the non-target language, bilinguals must strengthen language control mechanisms, placing higher demands on cognitive processes such as interference suppression, goal maintenance, and conflict monitoring. In dense code-switching contexts, bilinguals typically communicate with interlocutors who are also bilingual and can seamlessly alternate between languages. Therefore, compared to the first two contexts, bilinguals engage in timely language planning but involve less complex cognitive control processes such as conflict monitoring and interference suppression. Clearly, voluntary language switching tasks closely resemble the dense code-switching context described in the Adaptive Control Hypothesis, whereas forced switching tasks more closely approximate dual-language contexts \cite{de Bruin et al., 2018, Jevtovi et al., 2020; Jiang et al., 2024}. Forced mixing of two languages in dual-language contexts increases production difficulty, whereas free mixing in dense code-switching contexts makes language production more fluent \cite{de Bruin et al., 2018; de Bruin & Xu, 2023}.

According to the Adaptive Control Hypothesis, language control demands in dense code-switching contexts are lower not only than in dual-language contexts but also than in single-language contexts where language switching occurs infrequently. This difference is reflected in the mixing cost index (i.e., poorer performance on repeat trials in mixed-language blocks compared to single-language trials in pure blocks). Mixing costs frequently appear in forced language switching, reflecting higher control demands in dual-language contexts relative to single-language contexts \cite{Ma et al., 2016}. However, many studies have found that in voluntary switching, bilinguals name repeated trials in mixed-language blocks faster than single-language trials in pure blocks, showing a mixing benefit \cite{de Bruin et al., 2020; de Bruin & Xu, 2023; Jevtović et al., 2020}. This suggests that freely mixing two languages may be even easier than consistently using one language.

1.4 Cognitive and Neural Mechanisms of Voluntary Language Switching

Recent electrophysiological and neuroimaging studies have provided neural-level evidence for the cognitive control mechanisms of voluntary language switching. Some studies have found that voluntary switching activates inhibitory control networks similarly to forced switching \cite{Geng et al., 2024; Jiao et al., 2022b}, with both eliciting activation in language control-related brain regions (e.g., bilateral caudate nucleus and left supramarginal gyrus) and ERP components (N2 and LPC, late positive component), though with lower magnitude in voluntary switching. However, other evidence suggests that voluntary switching involves distinct neural mechanisms, particularly during the language selection stage. For instance, \cite{Zhang et al. (2024)} found that language selection in voluntary switching was associated with bilateral frontoparietal activation, which may involve bilinguals' naming intentions and regulation of spontaneous behavior \cite{Desmurget et al., 2009; Lau et al., 2004}, whereas language selection in forced switching activated left frontal regions associated with inhibitory control.

Similarly, \cite{Reverberi et al. (2018)} used multivariate pattern analysis to investigate neural activity during language selection and naming execution stages in voluntary switching. The results showed that encoding the target language during the selection stage activated the medial prefrontal cortex, a region closely associated with free choice and decision-making processes. The language control network activated during naming execution overlapped substantially with forced switching mechanisms \cite{Reverberi et al., 2015}, primarily including left inferior frontal gyrus, basal ganglia, bilateral angular gyrus, and inferior parietal lobule—regions collectively involved in target language selection and non-target language inhibition. Thus, voluntary switching may recruit additional neural modules related to self-intention and goal-directed behavior beyond the inhibitory control network.

In summary, although many voluntary switching studies report switching costs similar to forced switching, neural evidence suggests differences in the timing and intensity of inhibitory control. Compared to forced switching, voluntary switching shows reduced activation in language control-related brain regions, particularly during later naming execution stages. During the initial language selection stage, voluntary switching mechanisms more strongly reflect free decision-making and self-intention regulation. These findings suggest that previous bilingual control models may have overestimated the generality of inhibitory networks, while bilateral frontoparietal and medial prefrontal cortices may play central roles in intention formation rather than inhibitory control. Free choice may involve more intention generation and prospective planning than reactive inhibition.

1.5 Dual Advantages of Voluntary Language Switching: From Inhibitory Control to Lexical Access

(1) Voluntary Language Switching Relies Less on Top-Down Inhibitory Control

According to the Adaptive Control Hypothesis, language selection in dense code-switching or high-language-freedom contexts depends less on control processes such as conflict monitoring and interference suppression. This suggests that voluntary switching may require less inhibitory control than forced switching. In voluntary switching tasks, participants can spontaneously choose either language at any time without memorizing language cues. This task design eliminates the process of checking whether lexical items match expected languages, thereby reducing reliance on top-down control (such as inhibiting the dominant language or activating the non-dominant language) \cite{Kleinman & Gollan, 2016}. \cite{Blanco-Elorrieta and Pylkkanen (2017)} supported this hypothesis by examining brain activation patterns associated with language control in bilingual production and comprehension across contexts with varying ecological validity. They found that the lowest-ecology forced switching condition significantly activated prefrontal and anterior cingulate control networks, whereas these language control-related regions showed no comparable activation when bilinguals switched languages completely spontaneously during both production and comprehension. These results suggest that externally instructed language switching may impose additional neural resource demands.

Moreover, imposing additional inhibitory control in voluntary switching tasks may actually produce switching costs. \cite{Liu et al. (2020)} applied transcranial direct current stimulation (tDCS) to the right dorsolateral prefrontal cortex, a region responsible for inhibitory control, to observe whether behavioral or neural switching costs would emerge in Chinese-English non-balanced bilinguals during voluntary switching. Behavioral results showed switching costs under both anodal stimulation (which accelerates neural information transmission) and cathodal stimulation (which slows neural transmission), but not under sham stimulation. ERP data revealed that only cathodal stimulation elicited language switching costs, specifically a larger LPC for switching to L2 compared to L1. These findings indicate that inhibitory control is not necessarily beneficial in voluntary switching and may instead trigger switching costs. A plausible explanation is that inhibitory control may interfere with bottom-up lexical access processes in voluntary switching, hindering bilinguals' free choice between languages. Therefore, voluntary switching may rely less on inhibitory control.

(2) Faster Lexical Access in Voluntary Language Switching

Beyond top-down inhibitory control, bottom-up lexical access level is another crucial factor affecting switching costs. Lexical access level influences bilinguals' language preferences \cite{de Bruin et al., 2018; Gross & Kaushanskaya, 2015}: the higher the accessibility of a lexical item in a given language, the more likely bilinguals are to select that language. When language switching is completely driven by lexical access level, switching costs disappear \cite{Kleinman & Gollan, 2016; Zhu et al., 2022}. In voluntary switching tasks, lexical access level can be manipulated through task-driven language selection. Completely voluntary switching requiring random language choice on each trial frequently yields significant switching costs \cite{Gollan et al., 2014; de Bruin et al., 2018}. However, when tasks require participants to select a preferred language for each picture and maintain that choice throughout the naming task, switching costs are reduced or eliminated.

Similarly, \cite{Kleinman and Gollan (2016)} compared whether completely voluntary switching and bottom-up switching differed in switching costs. The bottom-up switching task required bilinguals to choose a preferred language for each picture and consistently use that language throughout the naming task. This language switching produced no costs because bilinguals' switching costs were offset by the lexical access advantage of their preferred language, thereby eliminating switching costs. In contrast, completely voluntary switching tasks require participants to evaluate which language would be easier to use on each trial and decide whether to switch or repeat languages. This higher-level decision-making process means that language switching is not entirely driven by bottom-up lexical access but is significantly influenced by top-down decision processes \cite{Gollan & Ferreira, 2009}. Additionally, because no one-to-one correspondence is established between languages and lexical items, using different languages to name the same picture may reduce difficulty differences across languages, thereby eliciting switching costs. Thus, whether task design permits consistent language use may be a key factor triggering voluntary switching costs. Fixed language selection (driven by lexical access level) can reduce inhibitory demands, whereas unpredictable cross-language competition in free switching may trigger top-down control mechanisms.

\cite{Zhu et al. (2022)} cleverly used stimuli with inherent language bias, such as pictures of "DNA" and "shuttlecock," in a voluntary switching task. These stimuli have much higher lexical accessibility in their corresponding languages than their translation equivalents, naturally creating a voluntary switching context that reduces control demands for task-driven language selection. The results showed that in this voluntary switching context, language switching did not elicit switching costs or activate language control-related brain regions. In contrast, a controlled forced switching task using stimuli without language bias produced significant switching costs and stronger activation in the right inferior frontal gyrus. Natural switching contexts not only eliminated switching costs but also reduced mixing costs and decreased prefrontal neural activity. This suggests that lexically-driven selection combined with language congruency reduces cognitive load, while laboratory task settings (e.g., random language cues and switching instructions) may overestimate bilingual control demands in real-world scenarios.

2.1 Second Language Proficiency Level

In forced language switching tasks, bilinguals' L2 proficiency is widely recognized as an important factor influencing switching costs \cite{Bonfieni et al., 2019; Green, 1998; Meuter & Allport, 1999}. Differences in proficiency between L1 and L2 typically manifest as asymmetrical switching costs. Among non-balanced bilinguals, researchers often observe asymmetrical costs, where switching from L1 to L2 is more costly than switching from L2 to L1. In contrast, balanced bilinguals with comparable L1 and L2 proficiency tend to show symmetrical switching costs \cite{Calabria et al., 2012; Costa & Santesteban, 2004}. However, although L2 proficiency affects L2 lexical access speed, voluntary switching studies less frequently report asymmetrical costs, regardless of whether participants are balanced or non-balanced bilinguals \cite{de Bruin et al., 2018, 2020; Gollan et al., 2014; Gollan & Ferreira, 2009; Gross & Kaushanskayade, 2015; Jevtović et al., 2020; cf. Mooijman et al.,2024}. Unlike forced switching, L2 proficiency in voluntary switching modulates bilinguals' language selection preferences, which may affect proactive language control applied at the global language level. Proactive control typically manifests as language mixing costs or reversed language dominance effects \cite{Declerck, 2020}, functioning to suppress activation of the non-target language (e.g., the dominant language) at the global level, thereby reducing cross-language interference.

\cite{Gross and Kaushanskayade (2015)} investigated voluntary language switching in English-Spanish bilingual children, finding that children selected objects to name in their non-dominant language that were high-frequency, early-acquired, and lacked alternative names in both languages. Similarly, \cite{Gollan and Ferreira (2009)} found that non-balanced English-Spanish bilinguals tended to select their non-dominant language (Spanish) for naming easier pictures and their dominant language (English) for more difficult pictures. This flexible language selection reduced the difficulty of using the non-dominant language in mixed-language tasks, resulting in a mixing benefit effect for English. In contrast, balanced bilinguals did not show mixing benefits but instead experienced mixing costs across both languages. Voluntary switching allows non-balanced bilinguals to leverage bilingual advantages to avoid weak-language lexical items, such as by selecting the most retrievable word from either L1 or L2 to reduce retrieval difficulty. This strategy reduces inhibitory demands on L2 and avoids the cognitive load of continuously inhibiting L1, leading to faster naming in mixed than single-language conditions (mixing benefits). As L2 proficiency increases and accessibility levels converge across languages, decision conflicts may arise instead. Even when lexical access is relatively balanced, language selection itself may require additional cognitive resources (e.g., inhibiting the current language or monitoring the target language), thereby producing mixing costs.

\cite{Mooijman et al. (2024)} further validated how L2 proficiency, particularly self-rated proficiency, modulates proactive control. The study found that non-balanced Dutch-English bilinguals with higher L2 proficiency applied stronger proactive control to their dominant language during voluntary switching. Compared to participants with lower self-rated L2 proficiency, those with higher self-ratings were more likely to use English for picture naming. To maintain consistent English production, they applied more inhibition to the stronger Dutch language, resulting in a reversed language dominance effect—overall slower naming in Dutch relative to English. This phenomenon reveals proactive control mechanisms in non-balanced bilinguals during voluntary switching. High self-raters are more inclined to actively use L2 but require more resources to maintain language balance, leading to mixing costs; low self-raters may underestimate their L2 ability and actually rely more on L1, thereby reducing mixing costs.

These findings indicate that although high L2 proficiency improves lexical access efficiency and reduces lexical search time, the additional decisions required in voluntary switching may be independent of language ability, such as choosing whether and when to switch \cite{Kleinman & Gollan, 2016}. \cite{Mooijman et al. (2024)} found that self-rated proficiency influences switching frequency through metacognitive strategies; for example, increased proficiency may enhance language confidence and motivation to use L2, but does not necessarily reduce the speed and complexity of individual decisions, such as evaluating lexical items and avoiding difficulty. Therefore, effects on local-level switching costs are minimal. The dissociation between voluntary switching costs and proficiency reveals limitations of inhibitory control theory: switching costs may stem not only from inhibiting conflicting languages but also from the cognitive load of intentional decision-making. This aligns with neuroimaging evidence showing that voluntary switching involves not only language competition but also intention formation and self-behavior regulation \cite{Zhang et al., 2024}.

2.2 Context

Beyond bilinguals' own L2 proficiency, context is another important factor influencing bilingual control in voluntary switching. Research shows that bilingual control mechanisms in voluntary switching undergo adaptive changes and adjustments at both global and local levels, whether in non-linguistic contexts (e.g., interlocutor context) or linguistic contexts (e.g., sentence context) \cite{Blanco-Elorrieta & Pylkkänen, 2017; de Bruin & Martin, 2022; Kapiley & Mishra, 2024; Sánchez et al., 2022}. Interlocutor context in experiments is typically presented through non-linguistic cues (e.g., avatars, flags) and is therefore considered a non-linguistic context, distinct from linguistic contexts that provide direct cues through language content (e.g., sentence text).

The Adaptive Control Hypothesis \cite{Green & Abutalebi, 2013} emphasizes that bilinguals' language control mechanisms adapt dynamically to communicative contexts. In interactive situations, the frequency and costs of voluntary switching are modulated by interlocutor characteristics (e.g., production patterns and proficiency levels). For example, in a picture-description dialogue task, \cite{Kootstra et al. (2020)} found that Dutch-English bilinguals' language switching behavior converged with their interlocutor's. During turn-taking picture descriptions, if the interlocutor had just switched languages, participants tended to voluntarily switch in the next round. \cite{Kapiley and Mishra (2024)} further found that both interlocutor and bilinguals' own language backgrounds influenced lexical selection and language control mechanisms in voluntary switching. High-proficiency Telugu-English bilinguals experienced L2 switching benefits (faster switching to L2 than repeating L1) when facing high-proficiency interlocutors who appeared frequently; when low-proficiency interlocutors appeared more frequently, they showed L1 switching benefits (faster switching to L1 than repeating L2). Unlike high-proficiency participants, low-proficiency participants showed symmetrical switching costs across different interlocutor contexts. Thus, bilinguals with higher L2 proficiency can more effectively predict interlocutor frequency and flexibly adjust activation levels of both languages according to interlocutor proficiency, making language switching more fluent. Bilinguals dynamically adjust language selection based on interlocutor proficiency, indicating that language switching is not only goal-driven but also involves real-time social intention inference, such as assessing speakers' linguistic habits and abilities. This supports the language activation selection model proposed by \cite{Blanco-Elorrieta and Caramazza (2021)}, which posits that language selection is guided by lexical activation levels and depends on dynamic integration of contextual cues rather than unidirectional inhibitory control.

Beyond interlocutor-centered interactive contexts, the conceptual framework of \cite{Hasson et al. (2018)} further broadens the notion of context, proposing that natural language processing involves complex interactions among different context types, including communicative-social context (e.g., interlocutor background characteristics), personal context (e.g., one's own emotional state or beliefs), and prior discourse context. \cite{Jiang et al. (2024)} examined Chinese-English bilinguals' voluntary switching performance in neutral, negative, and positive emotional states, finding that emotional states affected proactive control at the global language level, manifested as reversed language dominance effects. Negative emotional states impaired proactive control (reduced reversed language dominance effects and longer overall naming times), while positive states improved proactive control (shorter overall naming times). Emotions may influence language switching strategies and efficiency, reflecting complex adaptive bilingual control across emotional contexts and supplementing the Adaptive Control Hypothesis's classification of interactive contexts.

Additionally, bilingual control mechanisms in voluntary switching are modulated by sentence context. Sentence context provides more naturalistic settings and more ample preparation time for language use \cite{Declerck & Philipp, 2015}, which may facilitate language switching. \cite{Sánchez et al. (2022)} used an image path description paradigm requiring Spanish-English bilinguals to describe in sentences the trajectory of a red dot moving through a network of eight pictures. The results showed that switching costs disappeared when bilinguals voluntarily switched to L1, indicating that contexts approximating natural communication may substantially reduce language control demands. However, \cite{de Bruin and Shiron (2024)} found different switching cost patterns in picture naming with sentence context. Bulgarian-English bilinguals showed significantly reduced voluntary switching frequency and increased switching costs in sentence contexts compared to no-context conditions. Moreover, high-predictability sentence contexts elicited fewer language switches than low-predictability contexts. This may be because high-predictability contexts activate the non-target language less, thereby increasing difficulty for switching to the non-target language. The authors propose that sentence context may suppress co-activation of both languages \cite{Coumel et al., 2024; Van Hell & de Groot, 2008}, leading to increased switching costs and reduced switching frequency. These findings suggest that sentence context may exert dual modulatory effects on voluntary switching. Context is closely related to dynamic allocation of language resources and integration of semantic expectations. Although not constituting core lexical meaning, contextual features serve as important components of conceptual information that constrain lexical selection \cite{Blanco-Elorrieta & Caramazza, 2021}. For example, in highly predictable sentences, semantic expectations may reduce competition through top-down processing, meaning lexical selection may not rely solely on inhibitory mechanisms, challenging traditional inhibitory models \cite{Green, 1998}.

3 Bilingual Control Mechanisms in Voluntary Language Switching and Domain-General Executive Control

Numerous studies have demonstrated close connections between bilinguals' language switching behavior and domain-general executive control abilities, with long-term language switching experience enhancing individuals' executive control \cite{DeLuca et al., 2020; Han et al., 2022; Gosselin & Sabourin, 2023; Yao et al., 2025}. However, research over the past three decades on language switching and executive control has predominantly used forced language switching tasks (e.g., \cite{Declerck et al., 2017; Hernandez, 2009; Jiao et al., 2022a; Jylkkä et al., 2021; for a review see Tao et al., 2021). For instance, \cite{Declerck et al. (2021)} found that switching costs in forced language switching and task switching showed high similarity in both reaction time and ERP measures. However, such experimentally manipulated language switching does not perfectly map onto bilinguals' self-directed switching phenomena. In recent years, some studies have begun using voluntary switching tasks that more closely approximate bilinguals' spontaneous switching behavior to investigate relationships between language switching and executive control.

\cite{Gollan et al. (2014)} compared Spanish-English bilinguals' performance in voluntary language switching and non-linguistic task switching (reading vs. addition tasks), finding significant switching costs in both language and task switching. Moreover, bilinguals who switched frequently in language tasks also tended to switch frequently in non-linguistic tasks, indicating that bilinguals employ similar cognitive mechanisms when spontaneously switching languages and non-linguistic tasks. \cite{de Bruin and Samuel (2020)} found that both young and older Basque-Spanish bilinguals exhibited switching costs in voluntary switching. Although voluntary switching reduces external interference and allows more flexible resource allocation, it still requires executive control resources to inhibit the non-target language. Notably, older bilinguals showed larger voluntary switching costs than younger bilinguals, possibly due to cognitive aging-related deficits in inhibitory release mechanisms that make within-language interference difficult to resolve. However, these studies primarily relied on laboratory tasks, neglecting the complex diversity of spontaneous switching in natural contexts.

To further investigate voluntary switching behavior and its control mechanisms in naturalistic scenarios, recent researchers have expanded the relationship between bilingual control in voluntary switching and cognitive control by using task designs that more closely approximate real language use and by refining language switching types while incorporating individual factors such as language use environment and switching frequency. For example, \cite{López-Penadés et al. (2020)} distinguished bilinguals' language switching experiences into conscious switching (e.g., actively switching from L1 to L2), context-driven switching, and unconscious switching. They found for the first time that active L2 switching positively correlated with working memory updating and task-switching efficiency, whereas unconscious switching correlated with poorer interference control, revealing the complex adaptability of language control. \cite{Han et al. (2024)} moved beyond lexical-level task designs, using natural conversation and story narration tasks to observe Chinese-English bilinguals' voluntary switching behavior. They found a significant positive correlation between higher-frequency intrasentential insertional switching and domain-general inhibitory control performance (spatial Stroop task), manifested as shorter reaction times on incongruent trials. Similarly, \cite{Lai and O’brien (2020)} found that intersentential switching ability in English-Chinese bilinguals during natural dialogue tasks correlated positively with goal maintenance and conflict monitoring efficiency. However, the authors argued that the static classification of the Adaptive Control Hypothesis cannot fully explain actual language behavior in multilingual societies, as real language use involves contextual overlap and dynamic switching—such as simultaneous intersentential and intrasentential switching within the same conversation that cannot be strictly categorized as a single context.

Overall, these findings suggest close connections between voluntary language switching and multiple domain-general executive control components, including inhibitory control, working memory, and conflict monitoring. Different types of voluntary switching (e.g., conscious vs. unconscious; intrasentential vs. intersentential) may rely on different cognitive control mechanisms, while individual difference factors such as age and language use patterns significantly affect these relationships. Therefore, the Adaptive Control Model needs further refinement and adjustment by incorporating dynamic contextual factors (e.g., individual language use habits, sociocultural norms, language policies) to accommodate real-world complexity. Meanwhile, current naturalistic voluntary switching research has focused primarily on behavioral levels, with insufficient exploration at the neural level. Future research urgently requires more neuroscientific evidence to investigate whether bilingual control mechanisms in natural contexts share neural substrates with domain-general executive control and to reveal how voluntary switching experience shapes executive control abilities.

4 Summary and Outlook

Research on voluntary language switching focuses on bilinguals' spontaneous language alternation in daily life. Such research reflects self-driven language switching mechanisms in natural bilingual communication and reveals how bilinguals efficiently manage and control two languages without external coercion. The field of voluntary switching research continues to advance, with more nuanced understanding of switching costs, increasingly refined neural mechanism distinctions, and more systematic investigation of influencing factors. Recent studies indicate that voluntary switching does not necessarily entail high cognitive load; under specific conditions (e.g., lexically-driven contexts), switching costs can be significantly reduced or eliminated entirely. Neuroimaging research preliminarily reveals that voluntary switching shows specific brain activation patterns, involving not only inhibitory control-related regions (e.g., right inferior frontal gyrus) but also additional activation in self-selection-related regions (e.g., medial prefrontal cortex). In natural contexts, voluntary switching may depend less on inhibitory control, promoting efficient allocation of neural resources. Furthermore, the efficiency, frequency, and patterns of voluntary switching are subject to complex regulation by individual difference factors (language proficiency, switching experience, executive control abilities) and multiple environmental factors (interlocutor background, emotional state, contextual cues), highlighting the adaptability and flexibility of the bilingual control system. These advances are driving theoretical evolution from inhibition-centered single mechanisms to comprehensive frameworks encompassing multi-factor dynamic interactions.

The multiple influencing factors of voluntary switching pose new requirements for theoretical model construction. Building upon the Inhibitory Control Model \cite{Green, 1998}, bilinguals' language selection and use depend not only on proficiency-based control of non-target languages but also on speakers' integration of multidimensional information in conversational contexts, such as recent language use, conceptual information, and interactive contexts \cite{Blanco-Elorrieta & Caramazza, 2021; Hasson et al., 2018}. The Adaptive Control Model \cite{Green & Abutalebi, 2013} provides a basic theoretical framework for bilingual control adaptation to different interactive contexts but has not integrated individual differences, sociocultural, and emotional factors, and its classification cannot explain differences in switching patterns, frequencies, and social functions across cultural backgrounds and sociolinguistic environments. Therefore, existing theoretical models need to incorporate individual difference factors (language use habits, emotional states, executive control abilities) and expand to more fine-grained context classifications while constructing multi-level control frameworks that distinguish lower-level control mechanisms related to lexical selection from higher-level regulation related to social intention and semantic expectation.

Although current research has preliminarily explored bilingual control mechanisms and influencing factors in voluntary switching, future studies require further innovative expansion in research approaches and methods. Recent voluntary switching research has begun employing natural conversation and narration tasks that more closely approximate real interactive scenarios \cite{Han et al., 2024}, addressing the ecological validity limitations of traditional laboratory research. Future studies can leverage advantages of cognitive neuroscience techniques to deeply investigate the neural mechanisms of voluntary switching. For example, combining near-infrared spectroscopy with online conversation analysis to probe bilingual control mechanisms and cognitive control resource recruitment patterns during real peer interactions. Using VR technology to construct multimodal conversational scenarios (e.g., simulated test situations or conversations with authority figures), combined with EEG and other physiological measures, to explore neural coupling mechanisms between emotion and bilingual control networks, capturing multiple modulatory effects of communicative situations and individual factors (e.g., language proficiency and emotional state) on bilingual control.

Furthermore, although existing research has revealed associations between bilingual control mechanisms in voluntary switching and domain-general executive control, the role of voluntary switching experience in shaping cognitive development and brain structure and function awaits verification through longitudinal studies, particularly in bilingual children undergoing brain development and older adults experiencing cognitive decline. Future research could collect voluntary switching data from bilinguals' social networks, use speech recognition and semantic analysis tools (e.g., BERT, GPT large language models) for annotation and classification, and employ portable devices (e.g., eye-tracking and EEG) to capture voluntary switching behavior and neurophysiological indices in more natural environments. Combining complex behavioral patterns in social contexts with standardized laboratory measures will help parse the essence of bilingual control in voluntary switching, reveal the mutual facilitation between bilingual control and domain-general executive control functions such as memory, switching, and interference suppression, and investigate long-term adaptation of cognitive resources.

Finally, current voluntary switching research has focused primarily on language production, while the bilingual control mechanisms underlying comprehension and their relationship with production remain underexplored. Natural language comprehension and production recruit similar brain regions and share linguistic representations \cite{Patel et al., 2023; Wu et al., 2022}. Future research should compare bilingual control mechanisms in voluntary switching between comprehension and production and investigate their synergistic shaping effects on cognitive control. Examining the interaction between comprehension and production mechanisms in voluntary switching may also provide insights for second language learning and bilingual education. Previous research has found that the strength of neural coupling between speakers and listeners correlates with communication quality \cite{Zada et al., 2024}. During conversation, reduced production and comprehension costs for voluntary switching may lower mutual understanding and communication costs for both parties. For example, comprehending frequently occurring language switches in natural contexts imposes minimal burden on bilinguals \cite{Salig et al., 2024}. In classroom or group communication contexts, allowing speakers to voluntarily switch languages according to expressive needs may not only enhance their fluency but also reduce learners' cognitive load, thereby improving learning outcomes. Under the trend of interdisciplinary and technological integration, future voluntary switching research should combine theories and approaches from education and neuroscience to enhance the applied value of research findings.

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