One of the best-known claims from language acquisition research is that the capacity to learn languages is constrained by maturational changes, with particular time windows (aka ‘critical’ or ‘sensitive’ periods) better suited for language learning than others. Evidence for the critical period hypothesis (CPH) comes from a number of sources demonstrating that age is a crucial predictor for language attainment and that the capacity to learn language diminishes with age. To take just one example, a recent study by Hartshorne, Tenenbaum and Pinker (2018) identified a ‘sharply-defined critical period’ for grammar learning, and a steady decline thereafter, based on a very large dataset (of 2/3 million English Speakers) that allowed them to disentangle critical-period effects from non-age factors (e.g., amount of experience) affecting grammatical performance. Other evidence for the CPH comes from research with individuals who were deprived of linguistic input during the critical period (Curtiss, 1977) and were consequently unable to acquire language properly. Moreover, neurobiological research has shown that critical periods affect the neurological substrate for language processing, specifically for grammar (Wartenburger, Heekeren, Abutalebi, Cappa, Villringer & Perani, 2003).

Speaking a late-acquired second language (L2) involves increased cognitive demands, as has been shown mainly in young and middle-aged adults. To investigate grammatical inflection in older L2 speakers, we acquired behavioral and functional magnetic resonance imaging data, while L1 and L2 speakers performed a grammaticality judgment task. L2 speakers showed higher error rates than native speakers, specifically when incorrect forms had to be rejected. Poorer performance in L2 speakers was accompanied by increased activity in the medial superior frontal gyrus (SFG), indicating the additional recruitment of executive control mechanisms. In addition, post-hoc within-group comparisons of behavioral and neural correlates provide evidence for dual-mechanism models in older adults, suggesting that language processing involves both procedural and declarative memory systems. Moreover, we demonstrated that speaking an L2 requires more executive control and relies to a lesser extent on the procedural memory system than speaking one's own native language.

The present thematic set of studies comprises five concise review articles on the use of priming paradigms in different areas of bilingualism research. Their aim is to provide readers with a quick overview of how priming paradigms can be employed in particular subfields of bilingualism research and to make readers aware of the methodological issues that need to be considered when using priming techniques.

Recent studies have revealed remarkable interactions between language and emotion. Here, we show that such interactions influence judgments made regarding cultural information. Balanced Welsh–English bilinguals categorized statements about their native Welsh culture as true or false. Whilst participants categorized positive statements as true when they were true, they were biased towards categorizing them as true also when they were false, irrespective of the language in which they read them. Surprisingly, participants were unbiased when categorizing negative statements presented in their native language Welsh, but showed a reverse bias - categorizing sentences as false, even when they were true - for negative statements when they read them in English. The locus of this behavior originated from online semantic evaluation of the statements, shown in corresponding modulations of the N400 peak of event-related brain potentials. These findings suggest that bilinguals perceive and react to cultural information in a language-dependent fashion.

This Special Issue (SI) is dedicated to early bilinguals, who acquire two languages during early childhood, before age 6, simultaneously (2L1 bilinguals) or sequentially (cL2 bilinguals). Recently, the notion of heritage speakers (HSs) − bilinguals who grow up speaking a minority language at home − has become prominent in this context. HS research has typically targeted bilinguals at a mature (adult) state, but early developing bilinguals may of course be HSs too, though not uniformly labelled as such. HSs of a moribund language or variety are another type of early bilingual, representing the final or penultimate, often 4th or 5th, generation of speakers. Unfortunately, ‘deficiency’ or ‘incompleteness’ is a common thread linking much HS research − despite a wealth of evidence demonstrating HSs’ maintenance of complexity in many grammatical domains along with differences to monolinguals (see, e.g., Kupisch & Rothman, 2016; Putnam & Sánchez, 2013). This SI brings together studies on how and why the morphosyntax and phonology of early bilinguals might differ from that of monolinguals.

This study investigates the acquisition of grammatical gender in French by German L1 children (age of onset of acquisition (AO) 2;8-4,0). The analysis of spontaneous production data of 24 children gathered longitudinally and a gender assignment test administered to 8 of these children at ages 6;7-8;3 and to 9 children (AO 2,11-3;8) at ages 3;2-5;1 revealed that some of them resembled L1 learners whereas others behaved like adult L2 learners. The turning point is at around AO 3;6. AO is thus a crucial factor determining successive language acquisition.

The hypothesis that children surpass adults in long-term second-language proficiency is accepted as evidence for a critical period for language. However, the scope and nature of a critical period for language has been the subject of considerable debate. The controversy centers on whether the age-related decline in ultimate second-language proficiency is evidence for a critical period or something else. Here we argue that age-onset effects for first vs. second language outcome are largely different. We show this by examining psycholinguistic studies of ultimate attainment in L2 vs. L1 learners, longitudinal studies of adolescent L1 acquisition, and neurolinguistic studies of late L2 and L1 learners. This research indicates that L1 acquisition arises from post-natal brain development interacting with environmental linguistic experience. By contrast, L2 learning after early childhood is scaffolded by prior childhood L1 acquisition, both linguistically and neurally, making it a less clear test of the critical period for language.

The study investigates the effects of grammatical gender on bilingual processing. Native speakers of Russian (a gendered language) learning English and monolingual English controls performed a self-paced reading task in English (a non-gendered language). As predicted, bilingual speakers showed delayed latencies to gendered pronouns (he or she) that were incongruent with the noun's grammatical gender in Russian, indicating that first language (L1) grammatical gender assignment can be interpreted as biological gender in nonnative (L2) processing. The L1 gender bias was only found in sentences containing animate, but not inanimate, nouns. These results speak against the syntactic mechanism being solely responsible for gender biases, but rather support a semantic transfer account due to coactivation of linguistic and conceptual features as proposed in the sex and gender hypothesis (SAGH, Vigliocco, Vinson, Paganelli & Dworzynski, 2005). Overall, the study provides clear evidence for the L1 grammatical gender bias in bilingual processing, albeit constrained by animacy.

Research on nonnative auditory word recognition makes use of a lexical decision task with phonological priming to explore the role of phonological form in nonnative lexical access. In a medium-lag lexical decision task with phonological priming, nonnative speakers treat minimal pairs of words differentiated by a difficult phonological contrast as a repetition of the same word. While native speakers show facilitation in medium-lag priming only for identical word pairs, nonnative speakers also show facilitation for minimal pairs. In short-lag phonological priming, when the prime and the target have phonologically overlapping onsets, nonnative speakers show facilitation, while native speakers show inhibition. This review discusses two possible reasons for facilitation in nonnative phonological priming: reduced sensitivity to nonnative phonological contrasts, and reduced lexical competition of nonnative words with underdifferentiated, or fuzzy phonolexical representations. Nonnative words may be processed sublexically, which leads to sublexical facilitation instead of the inhibition resulting from lexical competition.

Native speakers show rapid adjustment of their processing strategies and preferences on the basis of the structures they have recently encountered. The present study investigated the nature of priming and adaptation in second-language (L2) speakers and, more specifically, whether similar mechanisms underlie L2 and native language adaptation. Native English speakers and Korean L2 learners of English completed a written priming study probing the use of double object and prepositional phrase datives. Both groups showed cumulative adaptation effects for both types of dative, which was stronger for the structure that was initially less frequent to them (prepositional phrase datives for the native English speakers, and double object datives for the L2 learners). This supports models of priming that incorporate frequency-based modulation of long-lasting activation of structures. L2 learners and native speakers use similar processing mechanisms; differences in adaptation can be accounted for by differences in the relative frequency of structures.

This study investigates the effects of Age of Onset of Acquisition (AOA) and the quality and quantity of input on the longitudinal development of gender in the acquisition of French by simultaneous (2L1) and successive bilinguals (cL2). Three aspects of French gender are studied: the abstract GENDER feature, gender assignment and gender concord. The findings show that amount and quality of input correlate significantly with the rate of development of gender assignment. Group-level analyses on gender concord show that there is no significant difference between the L1 and the 2L1 groups and that error patterns are different in the 2L1 and cL2 groups. We conclude that while it is clear from the data that the development of gender assignment is primarily dependent on input conditions the question is more open with respect to gender concord. For concord a combination of AOA, the L1 acquisitional timetable, and input are important factors.

On the basis of their review of studies, Mayberry and Kluender (2017) propose that the human language learning ability becomes severely compromised if it is not developed in tandem with brain development in early childhood, but that it functions more or less flawlessly, even in adulthood, if language acquisition had at one time proceeded according to the maturational timetable. Mayberry and Kluender therefore suggest that the critical period hypothesis (CPH) for language is unambiguously tied to the timing of L1 acquisition, but that its relevance to L2 acquisition is less clear, the implication being that the well-documented AoA effects in the SLA literature are due to non-maturational (i.e., psychological, experiential, cross-linguistic, etc.) causes.

The present study investigates the automatization of second language (L2) sentence processing. It compares the extent to which a mere speedup (faster execution) and restructuring (more stable execution) of sentence processing contribute to L2 oral performance. The maze task is used to measure the speed (reaction time, RT) and processing stability (coefficient of variance, CV) of sentence processing. The elicited imitation (EI) task measures L2 oral proficiency (repetition accuracy and accuracy in plural and third person s). These tasks were performed by 110 English-as-a-foreign-language learners with Japanese as their L1. The results show that only RT, not CV, significantly predicts L2 oral proficiency. Even though a subgroup of learners, who previously stayed in an English-speaking country, demonstrated some indications of automatization, RT was a better predictor of L2 oral proficiency than CV, irrespective of immersion experience. These findings suggest that CV has little practical value in predicting L2 oral proficiency.

In language switching, it is assumed that in order to produce a response in one language, the other language must be inhibited. In unimodal (spoken-spoken) language switching, the fact that the languages share the same primary output channel (the mouth) means that only one language can be produced at a time. In bimodal (spoken-signed) language switching, however, it is possible to produce both languages simultaneously. In our study, we examined modality effects in language switching using multilingual subjects (speaking German, English, and German Sign Language). Focusing on German vocal responses, since they are directly compatible across conditions, we found shorter reaction times, lower error rates, and smaller switch costs in bimodal vs. unimodal switching. This result suggests that there are different inhibitory mechanisms at work in unimodal and bimodal language switching. We propose that lexical inhibition is involved in unimodal switching, whereas output channel inhibition is involved in bimodal switching.

The review describes how morphological priming can be utilised to study the processing of morphologically complex words in bilinguals. The article starts with an overview of established experimental paradigms based on morphological priming, discusses a number of basic methodological pitfalls with regard to experimental design and materials, then reviews previous L2 morphological priming studies, and concludes with a brief discussion of recent developments in the field as well as possible future directions.

A cross-linguistic structural priming experiment explores the issue of whether parallel syntactic constructions of the two languages in bilinguals share a representation when the surface word orders of the constructions differ. The target population was early balanced bilinguals of Korean and English; the tested constructions were structures relevant to the subject-to-object raising (STOR) operation, which until this study have not been used for structural priming research in cross-linguistic contexts (e.g., STOR: Mary believes Jerry to be trustworthy; non-STOR: Mary believes that Jerry is trustworthy). These syntactic structures exist in both English and Korean, but with different surface word orders. The results show that cross-linguistic priming of the STOR construction occurred, suggesting that parallel syntactic constructions of the languages in bilinguals can share a representation independent of surface word order.

This study investigates the acquisition of grammatical gender in both languages of 21 simultaneous Greek–Dutch bilingual children living in the Netherlands. Greek and Dutch stand on the two opposite sides in terms of frequency and transparency of gender cues. Consequently, monolingual acquisition of gender in Greek is precocious with few overgeneralizations of the default value, neuter, in early stages. In contrast, monolingual acquisition of gender in Dutch is very late with errors in neuter nouns persisting up to the age of 7. Simultaneous Greek–Dutch bilingual children present an interesting test case of crosslinguistic influence in the form of acceleration (Greek affecting Dutch) or delay (Dutch affecting Greek). Children were tested on gender marking on determiners and adjectives in production and grammaticality judgment tasks. Input measures of Greek and Dutch and lexical skills were also considered. Results point to crosslinguistic influence in the form of acceleration of gender discovery in Dutch.

We used the visual world eye-tracking paradigm to investigate the effects of cognitive load on predictive eye movements in L1 (Experiment 1) and L2 (Experiment 2) speakers. Participants listened to sentences whose verb was predictive or non-predictive towards one of four objects they were viewing. They then clicked on a mentioned object. Half the participants additionally performed a working memory task of remembering words. Both L1 and L2 speakers looked more at the target object predictively in predictable- than in non-predictable sentences when they performed the listen-and-click task only. However, this predictability effect was delayed in those who performed the concurrent memory task. This pattern of results was similar in L1 and L2 speakers. L1 and L2 speakers make predictions, but cognitive resources are required for making predictive eye movements. The findings are compatible with the claim that L2 speakers use the same mechanisms as L1 speakers to make predictions.

The idea that there is a critical period (CP) for language acquisition that starts sometime after birth and ends sometime around puberty has been central to language studies since Chomsky's paradigm shift that embedded language acquisition into human biological development (for discussion, see Bialystok & Kroll, in press). From early on, the critical period argument was loosely extended to include second language (L2) acquisition, an idea buttressed largely by anecdote: “children learn foreign languages so much better than adults” in spite of evidence to the contrary (Goldberg, Paradis & Crago, 2008; Snow, 1987). The review by Mayberry and Kluender (2017) describes the existing literature on L2 acquisition and adds recent empirical evidence from investigations of individuals learning American Sign Language (ASL) at different ages, often as a first language (L1) to offer important clarifications and insights to this debate. Their argument rests on a clear distinction between processes used in L1 or L2 acquisition to re-evaluation of some of the more extreme claims of the CP view. In brief, they argue that L2 acquisition is not a good test of the CP argument, a point with which we agree. However, our view is that CP effects on language acquisition are even more limited than Mayberry and Kluender suggest and we offer two further arguments to support their own formidable evidence and conclusion.

In this review, we examine how structural priming has been used to investigate the representation of first and second language syntactic structures in bilinguals. Most experiments suggest that structures that are identical in the first and second language have a single, shared mental representation. The results from structures that are similar but not fully identical are less clear, but they may be explained by assuming that first and second language representations are merely connected rather than fully shared. Some research has also used structural priming to investigate the representation of cognate words. We will also consider whether cross-linguistic structural priming taps into long-term implicit learning effects. Finally, we discuss recent research that has investigated how second language syntactic representations develop as learners’ proficiency increases.